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ASME - Citrus Waste Pumped Peel Systems

March 12, 1998


It was five years ago at this conference that Carlos Odio presented a paper on pumped peel systems in Brazil. This was the first time many of us had heard of the concept.

The key advantages claimed were a reduced initial capital investment, along with reduced maintenance and horsepower requirements. These resulted from elimination of many screw conveyors and the peel bin.

I remember many doubts being expressed in discussions following the presentation. The principal concern was in regards to handling the week-end shutdown typical of many Florida processors. Also, the absence of a peel bin meant that juice extraction would have to stop whenever anything went wrong in the feedmill.

Today four of the twenty-five citrus feedmills in Florida use pumped peel. One of these was a greenfield installation that gained the advantage of reduced capital investment. The other three converted from a traditional reaction conveyor system. At least two of these three made the change because they were facing major maintenance and rebuilding costs in aged facilities.

The first two, at Florida Juice and Caulkins Indiantown, were installed by Gumaco using the system described in Odio's presentation. The third system, designed and constructed by Cook Machinery, demonstrated remarkable innovation and advancement in the technology. The latest, at Tropicana in Ft. Pierce, includes further evolution of the pumped peel concept.

Another key development in pumped peel technology came from Cutrale in Araraquara. A paper describing this was presented by Daniel Marques at the 1995 ASME conference.

At that conference I also presented a paper on feedmill technology. In it I mentioned that Dan Vincent's 1940 patent covering peel liming said that three to five minutes reaction time was required. Later in the paper I said that normal practice is to use eighteen minutes. I was aware of this contradiction, so you can imagine my surprise when Marques gave his paper: He said that only four minutes was required and he had a feedmill to prove it.

All four of the pumped peel feedmills in Florida now operate with the short reaction time that Marques described. In fact, Florida Juice and Caulkins Indiantown converted after extended periods of operating difficulties associated with excessive reaction time.

To illustrate the technical changes that have occurred, we would like to start with the flow chart in Figure I. This system calls for using recirculating press liquor and molasses as a pumping medium. The presence of this press liquor and molasses, in the ratio of two parts liquid to one part peel, was felt necessary in order to fluidize the peel sufficiently to be pumped.

Note the metal separator tank. This was a relatively large tank that served to mix the peel, lime, press liquor, and molasses. It had a sloped bottom so that tramp metal would not enter the pumps.

The pumps used were progressive cavity pumps. It is interesting to note that today all four Florida plants use progressive cavity pumps built by either Geremia or Netzsch, the two manufacturers referred to in Odio's paper.

The reaction tank was exceptionally large, assuring a reaction time in excess of forty minutes. (The Florida Juice tank was sized for over 60 minutes!) The design called for the peel to be pumped into the bottom of the tank, overflowing from the top. This required operating with a full tank at all times, even during periods when peel flow to the feedmill was reduced.

A major departure from this system was described in Marques' paper. Since only four minutes of reaction time were required, the reaction could be completed in the pipeline running from the metal separator tank to the dewatering screens. This meant that a reaction tank was no longer required. Marques mentioned that the existing Cutrale reaction tank had been converted into a lime silo.

Note that the system requires separation of the pumping medium (press liquor and molasses) from the peel, ahead of the screw presses. The typical Brazilian system used static screens, although some preferred rotating drum screens or shaker tables. A key item is that the pumping medium, already containing d-limonene, flows into the hammermill.

Let us now take a look at the Cook system (Figure II). This design was installed at the new SunPure feedmill in Avon Park. It was also installed by FMC, under technical license from Cook, at the Parmalat feedmill in Sicily.

One very important change was that only non-recirculating molasses, with absolutely no press liquor, was used as a pumping medium. Also, despite the fact that citrus molasses is more viscous than press liquor, the ratio of molasses to peel was reduced to 1:1.

One fact about the use of molasses instead of press liquor is that improved oil recovery is made possible. This is because press liquor contains all of the d-limonene oil that is recovered in the feedmill. If recirculating liquor is used to pump the peel, there is opportunity for the oil that is present to be absorbed into the albedo of the peel. It is agreed that once oil is absorbed by albedo, it stays there; the action in the screw presses does not remove this oil.

The consequence is that oil absorbed by the albedo goes with the press cake into the peel dryer. Not only is this oil lost, but it is likely to be released, with the exhaust gasses, to the atmosphere. Thus the oil becomes a Volatile Organic Compound (VOC), a focus of the Clean Air Act.

No one disputes this theory. However there are many who do dispute the quantity of oil lost by pumping with recirculating liquor. Comparing oil recovery rates would seem to be the logical way to resolve the question. Unfortunately, conditions at feedmills vary so much that no clear answer has emerged.

There is another subtle, but important reason for pumping with molasses instead of press liquor. Molasses is warm because it comes from the Waste Heat Evaporator (WHE), while press liquor is at ambient temperature, the same as the oranges. As a result, a system pumping with molasses is noticeably warmer than one using press liquor. This difference can amount to 10º C, which does not sound like much.

However, increasing the temperature of a chemical mixture by 10º C cuts in half the time required for a reaction to occur. This is true in the case of the reaction between hydrated lime and citrus peel. Consequently a much faster and more complete reaction occurs, everything else being equal, in a system pumping with warm molasses.

The Cook system had a great many innovations, and, as can be expected with such a system, there were problems to be resolved during start up. One of these led to a change in the design of the mixing tank. It was found that the original tank at SunPure had warm spots and cold spots. These were traced to peel accumulating in the poorly agitated corners of the tank. This peel spoiled and eventually would break loose and get pumped into the screw presses. Screw presses do not operate well with old peel.

A replacement mixing tank was developed by SunPure personnel, and the fabricator, Keller Sales and Engineering, made significant contributions to the final design. (Figure III) Note that the dead spots were eliminated by going to a conical bottom. Internal baffling assured violent agitation, and a clever suction design keeps tramp metal out of the pumps.

There is an interesting detail applicable to any peel processing system. The metal separator used in the Cook plants is the simplest and most effective available. Figure IV shows this device, which is arranged so that molasses is added to the peel by pumping it into the bottom of a small tank through which the peel must pass. The peel enters at the top at one end and exits at the other. Since the molasses is flowing upward, the peel is fluidized as it passes through. The result is that heavy tramp material, including stainless steel, glass, rocks and sand, separates and falls to the bottom of the tank.

Devices similar to this have been installed, with great success, at Orange-co and Cargill Frostproof. Both of these plants have traditional reaction conveyor systems (not pumped peel) where molasses was added either ahead of the shredders or directly into the reaction conveyor. A simple piping change with the addition of a drop-out box has reduced damage to the shredder screens and screw presses at these plants.

Initially the two Cook pumped peel plants (SunPure and Parmalat) used rotary positive displacement pumps. There is a lesson in industrial marketing in what came about. At Parmalat the Italian OMAC pumps have worked fine and are likely to be used in future installations. However at SunPure there were repeated rotor failures. Not only was the pump manufacturer unable to help the situation, but high prices were charged for replacement parts. As a result the pumps were discarded after the first season and replaced with proven Brazilian progressive cavity pumps.

The Cook system uses a different reaction tank design. The peel is pumped into the side of the tank and drawn off at the bottom. This means that an extra set of pumps is required, as compared to the Brazilian system. However it also means that the tank can be operated at any desired level, which translates into any desired reaction time.

Operating at a low level in the reaction tank was found to be advantageous. Florida systems operating with full reaction tanks all had difficulty with screw press operation. After long periods of frustration, the operators at Florida Juice and Caulkins Indiantown traced the difficulty to fermentation that was occurring in the tanks. This seems to occur if there is excessive reaction time, even with good agitation. The result is that today these two plants entirely by-pass their reaction tanks; SunPure operates with a minimal 20% level.

At SunPure, the system was sized for 2,500 boxes per hour (significantly higher throughput can be achieved). At the 2,500 rating there is six minutes reaction time in the pipeline from the hammermill to the feedmill. Additional reaction time is available in the mixing tank and in the reaction tank.

The reaction tank does have an important function at SunPure. Even though the peel goes straight from extraction to the feedmill, without any delay in a peel bin, there are still periods of upsets. When these upsets occur the peel can instantly become un-pressable. At SunPure this condition is overcome by filling the reaction tank until a suitable, pressable, mixture is achieved. Thus the reaction tank is better described as a surge tank.

It is noteworthy that at SunPure there is a provision to add lime solution directly to the reaction/surge tank. This feature is useful when insufficiently limed peel reaches the feedmill.

The point needs to made that pumped peel systems are very sensitive. You will recall that our original worries were about weekly start-ups and having to shut down extraction if problems occurred in the feedmill. It is fair to say that it is the sensitivity to change that has caused far more problems to operators than the problems we anticipated.

By sensitive we mean that the presses stop working and operations become near impossible. There are many normal upsets that cause immediate problems in the pumped peel systems: a slug of CIP water, a load of cull fruit, trash water in the molasses, peel over a few hours old on a hot day, a jump in molasses pH, a skip in the limer. All of these lead to wet peel going into the dryer, which has resulted in multiple cases of smoldering peel in the dryer.

Returning to the SunPure system, another innovation is evident in how the peel and pumping medium are separated ahead of the screw presses. Instead of open screening devices, Cook chose to use closed pre-presses. These machines have limited compressive characteristics, so they perform a "soft" press on the peel. These have proven noticeably more effective than screens in separating free liquid from the peel. As a result, the screw presses that follow give better performance.

One feature of the SunPure system is the use of spent caustic as a replacement for lime. In the 1950's, working at Citrus World, R. W. Kilburn showed that spent caustic could be substituted, up to 50%, for lime. In oversimplified chemistry, in order to prepare peel so that it can be pressed it is necessary to break down the pectins and to raise the pH: the spent caustic will raise the pH, while it takes lime to break down the pectin. Cook chose to take advantage of this, using 10% to 20% caustic in the lime slurry. Lime and spent caustic are mixed, in batches, and then pumped to the hammermill.

One advantage of this is that, since less lime is used, there is less lime going to the Waste Heat Evaporator with the press liquor. As a result the formation of calcium citrate is reduced and the WHE works more efficiently for longer periods.

One aspect of this technology is difficulty of control. In a reaction conveyor system the addition of lime to the peel is set by matching the speed of the limer screw to the speed of the peel bin take-out screw. The operators adjust this system by measuring pH. In a pumped peel system it is more difficult to match the lime flow to the peel flow since the peel flow is not evident to the feedmill operators. Thus the operators are all the more dependent on pH as a means of controlling lime addition. Unfortunately, pH meters tend to be unreliable, leaving the operator uncertain about how much lime is required at any given moment.

One concern frequently voiced about pumped peel systems is that, since there is no peel bin, extraction will have to be shut down if something goes wrong in the feedmill. In fact, some of the Brazilian and most of the Florida pumped peel plants do have peel bins. The more practical solution is seen at SunPure where there is no peel bin as such. Instead as a first alternative they have provision to divert peel to a dump pad. From there the peel (both their own and peel from other processors that they run on occasion) can be put back into the feedmill flow with a front end loader. Their second alternative is to divert the peel to a small loading silo from which it can be trucked off-site. This system has worked well.

There are other innovations at the SunPure feedmill unrelated to pumped peel. The principal one is found in the WHE which is a five stage, three effect evaporator with advanced technology in the oil stripping area. This 80 WHE is matched to a 40 dryer which has resulted in excellent thermal efficiency.

Figure V shows the system that went into operation at Tropicana Ft. Pierce last year. It can be described as a progression in pumped peel technology. It is characterized by gross simplification: the mixing tank is designed to provide surge capacity; the reaction tank is eliminated; and the pre-presses or dewatering screens are also eliminated.

A key to this simple system is pumping with a low ratio of liquid to peel. At Tropicana only one part molasses is added to three parts peel. To our surprise, the progressive cavity pump handles this thick mixture with no evident difficulty.

Flow through the pipeline is mostly laminar; there is little turbulence. Therefore the mixing of the lime with the peel must be thorough and complete before the peel enters the pump. At Tropicana the dry lime is added at the peel bin take-out screw. Molasses is added shortly afterwards, ahead of the hammermills. At times the mixing tank is operated at such a low level that very little mixing occurs in the tank. We were surprised that this relatively limited and simple mixing of peel and lime has proven quite adequate for achieving a proper reaction.

Operating with the peel from 3,000 boxes per hour, there is about one minute reaction time between the limer and the hammer mill. Six minutes are available if the mixing tank is full. Finally, there is one minute reaction time in the pipeline.

The mixing tank design is worthy of note. The Keller units at SunPure and Tropicana feature vertical internal baffles that work in conjunction with three tiers of center-mounted rotating paddles. Dead spots are minimal. The suction pipe requires the fluid to make a U-turn, assuring that tramp metal is effectively separated in the bottom, ahead of the pumps.

The original Tropicana system included the Vincent pre- presses shown in Figure VI. These were hard-piped directly from the peel pump. This means that the peel, from the time it enters the pump, is totally enclosed, completely preventing the emission of any VOC's.

As the ratio of molasses to peel was reduced, it was found that the pre-presses were not necessary. The main presses alone were found capable of removing the pumped fluid as well as doing their normal job of pressing the peel. The pre-presses were removed; the system as it is today is shown in Figure VII. Note that the peel is hard-piped to the presses.

It was found necessary to protect the hard-piped system from over-pressure. This has been achieved by the addition of a pressure relief valve, shown in Figure VIII.

Under steady full-load conditions, there are few problems controlling either traditional or pumped peel systems. However with pumped peel systems operators need to be more alert and learn new tricks to handle upsets and start-ups.

The upsets have been previously enumerated: a slug of CIP water, a load of cull fruit, trash water in the molasses, peel over a few hours old on a hot day, a drop in molasses pH, a skip in the limer.

In a traditional system these present a minor inconvenience for several reasons. There is usually the opportunity to mix some fresh peel from the peel bin with the bad peel. Peel flow is easily controllable by changing the speed of the peel bin take-out screw. Reacted peel color is readily observed. Lime flow is readily adjusted, sometimes by dumping bags directly into the reaction conveyor.

These are not characteristics of a pumped peel system. As a rule, the operator can observe only pH, Brix and the discharge cake from his peel presses. Thus the operator must learn to spot what is causing the upset and know what to do about it. His choices are largely limited to adjusting the lime flow and slowing his pumps so as to fill a surge tank. Filling the surge tank allows both mixing of new material with the material causing the problem as well as slowing the input to the presses.

One change to pumped peel systems that has been proposed is the addition of a weigh belt. A weigh belt would provide the operator with an indication of the tonnage of peel entering the system. This would be helpful in situations where extractor lines are started or stopped. It would give the operator knowledge of where his limer control should be set.

Originally there were serious concerns about start-up operations. Plants that shut down each weekend face a Monday start-up with water, sour press liquor, or cold molasses. However, many plants, both in Brazil and Florida, routinely get through their "Monday bad hair day." It takes careful attention by the operators, and efficiency is less than optimal, but the difficulties have proven readily surmountable.

Despite the difficulties of start-up and control, there are strong reasons favoring the pumped peel system. The simplicity of the Tropicana system has obvious advantages of reduced maintenance, space requirement, and capital investment.

This same simplicity has important implications from the standpoint of VOC control. In a pumped peel system the peel is fully enclosed and captured from the inlet to the hammer mill all of the way through the screw presses. This eliminates a host of potential VOC emission points; the equation is simplified.

Older traditional feedmills experience a great deal of maintenance. This is reflected both in operating costs and, more importantly, downtime. Screw conveyors, and especially the reaction conveyor, are notorious for failing and interrupting feedmill operations. It is not uncommon for these situations to cause a halt in juice extraction operations. Because of this condition it becomes financially attractive to convert an older system to the pumped peel technology.

Nevertheless we should note that the presses, dryer, WHE, pellet mill, and pellet loading remain unchanged in a pumped peel feedmill. Consequently it cannot be said that fewer maintenance personnel and operators are characteristic of pumped peel feedmills.

The technical progress in Florida, in pumped peel systems, has come as much from the citrus processing community as it has from the equipment suppliers. The early adaptors were the CEO's of their organizations: Ron Grigsby at Florida Juice and Roger Beret at Caulkins Indiantown. It is notable that both men were relative newcomers at the time the key decisions were made. It was their decisions that encouraged other processors to install pumped peel systems.

At SunPure it was Hadi Lashkajani who made the choice to go with the many innovations offered by Cook Machinery. Vice President Donald Dawson and Feedmill Manager Mac Greene stand out for having spent the innumerable hours required to make the system the success it is today.

Dave Van Etten, Plant Manager, had the vision that resulted in the Tropicana system. He heard Marques' presentation, went to Araraquara to see it, and came back convinced it offered many advantages in his plant. He is quick to give credit to Chris Sutherland, Instrumentation Manager, who had the understanding of what it would take to make it work on a day-to-day basis.

To summarize the technical progression that has occurred over the last five years: The Cutrale work with short reaction time allowed improvements at Florida Juice and Caulkins Indiantown, while it led into the innovation at SunPure and Tropicana. Ralph Cook first introduced concepts that will be fundamental for years to come: the avoidance of recirculating pumping fluid; bottom discharge reaction tanks; spent caustic as a lime substitute; using pre-presses instead of dewatering screens. At Tropicana the key new features are pumping with a limited amount of molasses, eliminating the reaction tank, and hard-piping to the presses. It remains to be seen what the next system will be like.

A forecast of the future should take into consideration other technical innovations in feedmilling technology. Items of note include:

Work directed by Benedito Jorge at Citrosuco has led to the development of a single pre-press that will handle the pumped peel coming from fifty FMC extractors.

Vincent pre-presses have been added to the pumped peel systems at both Florida Juice and Caulkins Indiantown. These have improved the performance of the main presses at both plants.

At Cutrale's Leesburg feedmill, the use of twin screw superchargers, with overlapping flights, has led to improvements in press performance. This has important implications in double pressing operations. See Figure IX.

The Fiber Filter, shown in Figure X, is being developed to clean-up of press liquor. This will allow WHE's and pumps to operate with higher Brix molasses than was previously possible in many plants. This results in reduced WHE fouling and improved thermal efficiency.

The use of twin circuits in the WHE, to produce higher Brix molasses for second pressing, is likely to result in significant improvements in overall thermal efficiency. One Brazilian plant is using this principle with solid results.

Some Brazilian processors have found that eucalyptus stands are more effective for disposing of wastewater than orange groves. This can impact the demands made on the WHE.

The bagged peel system, described at the most recent Citrus Short Course, eliminates the feedmill dryer altogether. Air pollution regulations in California have led to the adoption of this bagging process. Either VOC regulations or the low price of pelleted citrus peel could bring it about in Florida.

Dreyfus in Winter Garden has now begun feedmill operations using a unique Vincent screw press. Their new press features a screw design originally developed for alcohol extraction in soybean plants. In addition, the press makes use of profile bar screens, as opposed to the standard 3/32" perforated screen material. Its performance on limed orange peel is being tested.

 Presented by Robert B. Johnston, P.E.

ASME - Screw Presses In Citrus Feedmills

View PDF of ASME Screw Presses in Citrus Feedmills

Citrus Feedmill Thermal Efficiency


Daily Load = 100,000 Boxes in 22 Hours = 4,550 boxes per hour
4,550 x 44 #/peel per box = 200,000 #/peel per hour
200,000# Peel x 82% peel moisture = 36,000# Bone Dry Solids
Which produces 40,000 #/hr of Feed @ 10% Moisture

Press Cake
% Moisture
Press Cake Dryer
Therms Waste Heat
75 216,000# 156,000# 1,754 84,000#
74 207,692# 147,692# 1,660 92,308#
73 200,000# 140,000# 1,573 100,000#
72 192,857# 132,857# 1,494 107,143#
71 186,206# 126,206# 1,418 113,794#
70 180,000# 120,000# 1,349 120,000#
69 174,193# 114,193# 1,284 125,807#
68 168,750# 108,750# 1,222 131,250#
67 163,636# 103,636# 1,165 136,364#
66 158,823# 98,823# 1,110 141,177#
65 154,285# 94,285# 1,060 145,715#
64 105,000# 90,000# 1,011 150,000#
63 145,945# 85,945# 966 154,055#
62 142,105# 82,105# 923 157,895#
61 138,461# 78,461# 882 161,539#
60 135,000# 75,000# 843 165,000#

Rotary Dryer: 1,500 BTU Required Per Pound of Water Evaporated
One Therm: 100,000 BTU
Bunker C: 145,000 BTU/Gallon
Oil House Water Not Included:
  • Brown: Assume one pound per box

  • FMC: Assume four pounds per box


Citrus Feedmills 101

October 4, 2005
Issue #165

Last month Vincent Corporation gave a presentation at the Citrus Short Course (now called the International Citrus & Beverage Conference). Entitled "Citrus Feedmills 101", the presentation reviewed the efficiencies and economics of various feedmill concepts. The important points were as follows:

1. Feedmill efficiency is measured by the therms of energy required to produce a ton of citrus animal feed. This was presented as the gallons of fuel oil required per ton of pellets. It was seen that a very efficient feedmill requires 29 gallons of fuel oil per ton of feed.

2. With the selling price of pellets being around $55 a ton, delivered to the port of Tampa, it is evident that feedmill operators face a serious cost problem. Thus variations of the California Feedmill #4 option, which does not use a dryer, are of particular interest.

3. The "dryer" ceases to be a dryer once a waste heat evaporator (WHE) is installed. Instead, it is a generator of nearly saturated, high wet bulb temperature, gasses. This point is overlooked by dryer companies, attempting to enter the market, without familiarity with the fundamentals of citrus feedmills and the WHE technology.

4. It was shown that the screw presses account for only 7% of the capital cost of a feedmill, and the dryer, 13%. The WHE accounts for 33% of the capital cost.

5. The WHE does wonders for feedmill thermal efficiency because it works under a vacuum. This allows it to evaporate water with very low heat input. Furthermore, the WHE makes its own vacuum by condensing the moisture in the gasses.

6. The Vincent presentation included a statistical industry report. It showed that the typical feedmill has an overall energy consumption of about 600 BTU's per pound of water evaporated. This is contrasted to the typical stand-alone dryer, which requires at least double that much. (A British Thermal Unit (BTU) was defined as an energy unit, 1000 of which are required to evaporate a pound of water at sea level atmospheric pressure.)

7. It was shown that it is of little use to press citrus waste to where it has less than 63% moisture. There are presses available that can do this. However it is of little value because the resulting volume of press liquor will be beyond the capability of the WHE.

8. Material balances were presented, although they are upper classman, not freshman, technology. These are expressed in simple "in equals out" equations and the concept of Brix. Reiteration of simultaneous equations in a spreadsheet gives the material balance.




Citrus Installations

1992 VP-16-P11 FMC Food Machinery Italy SPA (Iran) Citrus Peel 92015
1992 VP-22-H35 Citrus Hill (Rebuild) Cargill Citro Pure Frostproof, FL - Cargill Citrus Peel 92170
1993 VP-22-P51 Orange-co Pasco Processing Bartow, FL Citrus Peel 93102
1993 VP-22-K12 Citrus World Florida's Natural Growers Lake Wales, FL Citrus Peel 93125
1993 VP-22-K51(2) Florida Juice Lakeland, FL Citrus Peel 93152
1994 VP-22-K51(2) Cook Machinery (SunPure) Cargill Citro Pure Avon Park, FL Citrus Peel 94145
1994 VP-22-K51(2) FMC Italy (Parmalat) Palermo, Italy Citrus Peel 94095
1995 VP-22-P-11(3) Cambuhy (Citrovita) Matao, SP, Brazil Citrus Peel 95001
1995 VP-22-K11(1) Tropicana Products Ft. Pierce, FL Citrus Peel 95079
1995 VP-22-K Citrus World Florida's Natural Growers Lake Wales, FL Citrus Peel 95202
1996 VP-22-K11(1) Tropicana Products Ft. Pierce, FL Citrus Peel 96042-B
1996 VP-16-K-21(5) FMC Italy (Parmalat) Palermo, Italy Citrus Peel 96200
1996 VP-22-K32(3) SunPure Avon Park, FL Citrus Peel 96042-A
1997 KP-16 Cargill Citro Pure Frostproof, FL - Cargill Emulsion 96196
1997 VP-22-K11(2) Bascitrus Agro Mirassol, SP, Brazil Citrus Peel 97002
1997 VP-22-K11(4) Cambuhy (Citrovita) Matao, SP, Brazil Citrus Peel 97009
1997 VP-16 Laconia Citrus Amyclae Sparti, Greece Citrus Peel 97103
1997 VP-22(2) Tropicana Products Ft. Pierce, FL Citrus Peel 97120
1997 VP-22(2) Cutrale Citrus Juices Leesburg, FL Citrus Peel 97160
1997 VP-22 Louis Dreyfus Citrus Winter Garden, FL (#97235) Citrus Peel 95134-B
1997 KP-16 Caulkins Indiantown Citrus Indiantown, FL Citrus Peel 97049-C
1997 KP-16 Florida Juice Partners Lakeland, FL Citrus Peel 97233-A
1998 VP-16 Del Oro Costa Rica Pineapple s/n 915131 98129
1998 FF-12 Pasco Processing Bartow, FL Press liquor 98168-A 98168-C
1999 FF-12 Texas Citrus Exchange Mission, Texas Citrus Peel 98297-B
1999 FF-12 SunPure Avon Park, FL Citrus Cloud 98168-B
1999 KP-16 FF-12 Citrus Belle LaBelle, FL Citrus Peel Press Liquor 99081-A
1999 KP-6 Tropicana Ft. Pierce, FL Sweco Tailings 99133-A
1999 VP-6 Tropicana Bradenton, FL Tech Center 83806
1999 VP-22 Citrofrut San Rafael, Vera Cruz Mexico Citrus Peel 99302
2000 FF-12 Christodoulou Bros. Greece Juice Finishing 98033-A
2000 KP-16 Mildura Fruit Juices Victoria, Australia Pulp Wash 99081-B
2000 KP-16(2) Caulkins Indiantown Citrus Indiantown, FL Citrus Peel 00049-A,B


Citrus KP-16

December 13, 2007

One of Vincent's more popular screw presses is the Model KP-16. Since its introduction in 1996, over one hundred have been built. Originally intended for applications with high freeness and capacity requirements, the presses have grown into high-torque machines being used in tough applications. At the same time, most of the economies of the original design have been retained.

The first prototype KP-16 was used at Cargill's Frostproof citrus feedmill. It was used in the first pressing position, removing the "easy" liquid from orange peel. The press worked so well that the downstream "hard squeeze" presses tripped out on overload! Shortly afterward, two KP-16's were installed at Tropicana Ft. Pierce for first pressing duty. The generation of press liquor increased so much that their Waste Heat Evaporator (WHE), which makes the press liquor into citrus molasses, was overwhelmed. In both cases, the KP-16's had to be removed from service.

Since that time Louis Dreyfus and Citrus Belle have installed KP-16's for first pressing in their citrus feedmills. The Louis Dreyfus project, in Indiantown, Florida, has worked especially well. The dewatering capacity of their KP-16's allowed a Stord press to be removed from service, saving 250 hp.

The original prototype at Cargill was put into service dewatering pulp wash. This material is quite slippery. However the press was successful in removing pulp, thus reducing the load on downstream finishers. The finishers have finer screens, and they remove fiber from press liquor.

In recent years Vincent has quoted a number of citrus feedmill projects. We have recommended that Series KP presses be used for first pressing because of their economical construction and high capacity. At the same time, the traditional VP, or newer TSP, presses are recommended for second pressing.

With the season which has just started, a new citrus application is being developed. KP-16's have been installed in Florida for dewatering core wash. Two versions of the press are being used, and both are operating in full-time service. Changes made include adding notches to the screw to improve screen wiping characteristics. The rotating cone feature has increased capacity and reduced channeling. Also, wider screen slots have proven advantageous, although this allows more pulp into the press liquor. (Downstream finishers remove this pulp.)

Issue 194

Citrus Molasses

November 19, 1998

In the states of Florida and Sao Paulo in Brazil all citrus juice extraction plants have citrus peel feedmills that produce citrus molasses. This molasses is made from the press liquor that results from pressing orange peel. The molasses is produced in waste heat evaporators (WHE) that are driven (heated) by the exhaust gasses from the cake dryers. Typically the molasses are concentrated into the range of 40º to 50º Brix and they are either sold to a distillery or re- combined with the peel. If the molasses are to be stored for more than a few days, they are concentrated to 72º Brix as fermentation will not readily occur at this high a sugar content.

Many smaller citrus processors in other countries process their peel into animal feed without producing citrus molasses. The technology they use is to either (a) simply dry the peel from its natural 80% moisture to 12% in a rotary drum dryer, or (b) press the peel from 80% to about 72% moisture and then put the cake into the dryer.

Those processors who simply dry their fresh peel find themselves at an economic disadvantage because it takes a great deal of energy to evaporate all of the moisture. Using a WHE, larger firms will use as little as one third as much energy to produce a ton of citrus pellets.

Those processors who use a press in front of the dryer fare better. Pressing from 80% to 72% moisture separates over half of the water from the peel. (It also results in the loss of almost one third of total solids because of the dissolved sugars that are carried away with the press liquor.)

However, the processors using a screw press face a serious environmental problem. Most of these, in Greece and Panama for example, send the press liquor to the sewer. The high sugar content of the press liquor (10º to 12º Bx) puts a high load on the wastewater treatment system. Odor and pollution complaints result in government pressures.

Acquisition of a WHE is out of the question for most small processors. Typically the WHE is the single largest capital item in a citrus processing plant. Furthermore, the dryers existing at many small processors are not suitable for generating the high wet bulb temperature gasses necessary for the proper operation of a WHE.

An interesting alternative exists for these small processors. Instead of a WHE, they can use a steam evaporator to produce molasses from their press liquor. A vertical tube evaporator, using falling film heat transfer, represents less capital investment yet it can be very effective in making citrus molasses. For example, the steam ratio is approximately 5:1 for a six effect evaporator. This means that for every pound of steam used to drive the evaporator, five pounds of water are evaporated from the press liquor.

The citrus molasses produced can be used as cattle feed or sold to a distillery. As a cattle feed, the molasses can be supplied to the farm in a liquid form. It is more common to add the molasses back onto the peel. The solids are diffused into the moisture in the peel, and the moisture is ultimately evaporated in either the dryer or the evaporator.

Distilleries buy molasses in order to ferment it in the production of citrus alcohol.

An important by-product is d-limonene, an oil that comes from the citrus peel. This oil is recovered in the evaporator. d-Limonene adds to the revenue stream used to justify the acquisition of an evaporator.

Issue 86

Citrus Oils


September 12, 2015

Although unknown to most people, citrus oils are a significant industry to themselves. Historically, Vincent screw presses in citrus feedmills have been a key component in the recovery of d-limonene. That is the "lemon oil" (actually it comes from oranges) everyone has smelled in industrial hand cleaners. It is a valuable by-product produced in the WHE (waste heat evaporator) which is used in large feedmills to produce citrus molasses.

Our screw presses have found application in Mexican lime and lemon processing plants. These facilities squeeze fresh fruit or peel, without the use of hydrated lime, to separate an emulsion-like flow from the peel. This emulsion contains citrus oil which can be separated in a calandria (still). Forty years ago Vincent designed and sold calandrias. Today JBT offers a stand-alone, skid-mounted READYGo™ d-LIMONENE system to recover d-limonene from oil-rich emulsions generated in the citrus extraction process.

The Cook Machinery Company of Dunedin, Florida has pioneered and led in the development of d-limonene and citrus essence oil recovery for many decades. JBT has been a key partner in their activities. Brown International is another key player in the industry.

Major processors of citrus oils include Firmenich and Givaudan. The Coca Cola Company purchases over half of all the lemon oil produced worldwide.
An overview of citrus oils follows.

Higher quality oil is more valuable as it contains more flavor and fragrance components. Usually this oil is not heat treated and has minimal contact with the fruit and water. Globally, high quality oil is purchased by flavor and perfumer houses for further refinement. Lower quality oil is less valuable and usually sold on the secondary market.
There are several of different types of commercially produced citrus oil:

The first type is cold pressed oil, or peel oil. This oil is extracted, without heat, from citrus peel. Oil is expressed from the peel and captured in water. Usually either Brown or JBT (FMC) extractors are used to express the oil from the fruit, although other types of extractors can be used. The resulting oil and water emulsion is sent to a series of centrifuges to separate the oil and water. GEA Westfalia and Alfa Laval are usually the OEMs for this equipment.

Oil produced this way is eventually fractionated into flavor components and d-limonene, using a complex distillation column. The phase separation process is described as folding oils. The flavor fraction is the more valuable component, and usually it is between five percent of the oil for some types of non-Valencia oranges up to 45% to 55% of the oil for limes. The balance of cold pressed oil is d-limonene. D-limonene is less valuable, but it has several uses including as a solvent and as an ingredient in cleaners.

The next type is essence oil. This oil is entrained in juice extracted and finished by juice extractors and finishers. Again, JBT (FMC) and Brown are the main providers of the extraction equipment, although some smaller companies also provide machines. Essence oil is captured by essence units installed on juice evaporators. These essence units also recover a water phase essence. Both oil phase and water phase essences contain flavor components. Oil phase essence can be particularly valuable to flavor and fragrance companies. Water phase essence can also be used in flavor applications, but this phase is more unstable and can degrade rapidly.

An essence-type oil archaically known as Wheeler oil is recovered from juice after juice extraction and finishing. This oil is also called juice oil. Centrifuges are used to separate this oil from the juice. No heat is used. This type of oil is not commonly produced because there is not a lot of it, mass-fraction wise. Wheeler oil is unique and has valuable flavor components due to the oil's extended contact with the juice.

A fourth type of oil is d-limonene, which is made in citrus feed mills or in distillation processes such as JBT’s READYGo d-LIMONENE unit. D-limonene is not really an oil, but a component of oil. This product is made when liquid pressed from citrus peel is concentrated in a WHE. The d-limonene is stripped from the press liquor in the WHE. Any oil or water phase essence contained in the press liquor is flashed off in the evaporator and not recovered. D-limonene recovered in this traditional system requires a full-scale feed mill, including hammer mill, lime addition and reacting screw, screw presses, dryer, waste heat evaporator, and pellet mills.

Note that lime oils are a little different. Three types of oil can be produced from whole lime fruit crushed by a screw press. Both oil and juice are expressed from the press, and peel and membranes are expelled separately. Centrifuges are then used to separate Type A lime oil from this juice and oil slurry. No heat is used to produce Type A oil. Distilled lime oil is usually produced from the oil remaining in juice exiting the centrifuges, but juice and oil directly exiting the press can also be used. For distilled lime oil, processors use steam in a still, a condenser, and a separator. Type B lime oil is cold pressed oil, extracted and separated like other cold pressed citrus oils mentioned above.

Issue #276

Citrus Pectin


OCTOBER 15, 2015

Pectin is a food ingredient which has a great many food and commercial applications.  It gives firmness to products we consume on a daily basis:  jelly, yogurt, ice cream, gravy, salad dressing and many, many others.  Its characteristics include good palatability, and it blends without affecting the flavor of the base material.

Commercial markets include pharmaceutical gel caps, paints, toothpaste and shampoos,

Pectin can be made from apples and sugar beets, plus other minor crops.  However, most pectin comes from citrus peel.  The preferred peel is from limes (mostly Mexican and Persian), closely followed by lemons.  Citrus pectin comes 56% from lemons, 30% from limes, and 13% from orange peel.  Pectin sells for around $15 a kilo – there are many different grades.  55,000 tons a year are sold, plus more in other forms.  The market is up to a billion dollars per year.

The citrus peel from which pectin is extracted is purchased from citrus processors.  These are primarily in Argentina and Mexico, although it is also produced in Brazil, Peru, Spain, Italy, and even Bolivia.  After extracting the juice, these processors wash the peel to remove oil and dissolved sugars, dry it gently, and then bale it for transport to the facilities which produce the pectin.

The major pectin producers are CP Kelco, DuPont, Cargill, Yantai Andre Pectin, and Herbstreith & Fox.   CP Kelco has three plants, in Denmark, Germany, and Brazil.  DuPont has plants in Mexico and Europe.  Cargill's plants are in Germany, France, and, added very recently, Italy.  Andre Pectin has their plant in China, and Herbstreith and Fox operates in Germany.  This link gives a listing of these firms:

Pectin is extracted from the washed and dried citrus peel by first using acid to dissolve the pectin.  The spent peel is then removed, and the remaining solution is treated with alcohol.  The alcohol causes the pectin to precipitate.  The pectin thus formed is dried and sold in powder from.

There are three applications for Vincent's screw presses in the production of pectin.


After the acid treatment there is a residual product, spent pectin peel.  This waste material has some value as an animal feed.  The material is very hard to dewater, so it is sold with high moisture content.  CP Kelco has done the best marketing by assigning a trade name, Braspulpa, to their material.

Over the years Vincent has done development work in Sicily, seeking to dewater spent pectin peel in a screw press.  It was found that a dosing with hydrated lime and cellulose (ground wood) fiber allowed a significant amount of water to be removed.  Unfortunately, the water removed would have overloaded the wastewater treatment plant, so it became one more technical success but commercial failure.


The alcohol precipitation step involves alcohol washing.  This is done in a two stage counterflow wash, first with 60% alcohol and then 80% alcohol.  A screw press, vapor-tight of course, is used between the two wash stages.  This is a "soft squeeze" application.


Squeezing out the alcohol ahead of the pectin dryer is a "hard squeeze" application which requires a great more torque capability in the screw press. 

When presses are supplied for these applications in Europe, they are designed and built to meet the ATEX explosion-proof standards.  These are ATEX Certified presses.  In other countries, where certification is not required, a less expensive unit, meeting the same standards, is supplied.

Issue #277

Citrus Pectin Peel Preparation

One of the by-products of a citrus processing plant is known as "pectin peel".  Production of pectin peel involves washing the sugars and oils from the peel and then drying the peel at low temperature. The dried peel is shipped to companies which use an acid-alcohol precipitation process to extract the pectin.  The pectin produced in this manner is sold world-wide as a food ingredient.

This processing of pectin peel is an area of technical expertise of Vincent Corporation. For many years we designed the plants and manufactured the machinery that is required. Today we offer free technical assistance and only our applicable specialty machines, screw presses and shredders.

Pectin peel is generally made from lime or lemon peel, although it can also be made from grapefruit and orange peel.

The dried peel is sold to firms such as CP Kelco in Denmark and Brazil, Danisco in Mexico, Cargill in Germany, and ICHI (Pectine Industria) in Italy.  These firms, in turn, extract pectin from the peel, for sale as a food additive.

Pectin peel generally sells in the range of US$ 300 to $600 per short ton. [April, 2011:  It is now around $1,000 a ton!]  This can be compared to selling pelletized citrus peel, for animal feed, where the market is in the range of US$ 50 to $100 per ton (in the United States).

The process of producing pectin peel revolves around washing the peel in water so as to diffuse out the soluble sugars.  Normally about 3 kilos of fresh water are required to wash one kilo of peel.  In advanced systems, only 1.25 kilo of water is used per kilo of peel.

This contrasts to drying citrus waste to make animal feed, in which the first step is to react the peel with lime. Hydrated lime degrades the pectin, releases the bound juices, and thus permits efficient pressing and dehydrating. In contrast to this, the production of pectin peel must preserve the pectin; therefore it can not be limed.

As a consequence of the washing process, the peel is a lot more slippery and a lot more difficult to press (dehydrate).  Accordingly we de-rate the capacity of our screw presses by 50%, or more, in pectin peel applications.

Further, pectin peel must be dried very carefully at low temperatures and with carefully controlled humidity. The Vincent-design rotating drum drier was the norm for the industry; however, we withdrew from the dryer business in 2007.  That dryer permitted recirculating some of the partially dried peel and mixing it with the material coming from the screw press. The design was a triple pass dryer with a stationary outer shell, which contrasts to the single pass rotating drum dryer most commonly used in the production of animal feed.

Processing plants where Vincent, over 40 years, installed pectin operations include Ci Pro Sicilia and Cesap in Italy; Laconia, Paco Hellas, Nikopolis (ex-Esperis) and Greek Juice Processing in Greece; Citrex and San Miguel in Argentina; Quimica Hercules, Productos Esenciales, Industrial Citricola, and Industriales Limonera in Mexico; Jn-Jacques and Moscoso in Haiti; Priman Canning and Yahkin in Israel; Avante in Brazil; and Unipectin in Morocco.

Currently there are no processors producing pectin peel in the United States: Ventura Coastal and Sunkist in California and Parman Kendall in Florida have all discontinued their peel washing operations.  Environmental considerations, especially disposal of the sugar laden, used wash water, were driving factors.

Typically we look at peel coming from juice extraction in the range of four tons of peel per hour on the low side and sixteen on the high side. More recently flows of 26 MTPH have been of interest.

The following equipment is key to the process:

Vincent VS-180 Shredder. This machine slices the peel in order to permit proper washing and pressing without creating excessive fines. It is of the thin, rigid blade design, as contrasted to the hammer mill concept.

Pulp Wash Conveyors or Tanks. These are used for diffusing the sugar from the peel. We recommend either three or four counterflow wash stages, in either vertical or horizontal configuration.

Pulp Wash Sumps. Stainless steel sump tanks, possibly with progressive cavity peel transfer pumps.

Dewatering Between Stages. We recommend the use of either static screens or rotary drum screens for dewatering between wash stages.  Water usage is reduced by pressing the solids from these screens with Vincent Series KP "soft squeeze" presses,

Vincent Screw Press. Traditionally the Series VP presses have been used to remove moisture from the peel prior to further dehydration in the dryer.  Oversize, low speed KP presses are proving to be a more economical alternative.  The Series TSP Twin Screw presses have also proven successful.

The press is a horizontal, all stainless machine featuring an interrupted flight design. The unit is equipped with an air cushioned cone, complete with pneumatic controls.  Rotating cones are used on the single screw machines.

The pectin peel is made from the cake, which usually comes out in the range of 84% to 86% moisture, depending if we are talking about lemon peel, or Persian, Limon Mexicano, Tahitian, and Key limes.

Furnace. The dryer comes with its burner and refractory lined furnace for burning natural gas, light weight fuel oil, or a combination of these fuels. A very low gas temperature, in the neighborhood of 1200º F or less, must enter the dryer. A low wet bulb temperature is important in the production of high quality pectin peel, whereas a feedmill dryer must produce high wet bulb temperature in order to drive the Waste Heat Evaporator.

Dryer Feeder. This is a stainless steel screw conveyor and feeder with a companion flange matched to the dryer throat. It has a variable speed drive.

Dryer Drum.  A triple pass dehydration unit with a stationary outer drum works best.  The unit is equipped with recycle extractor conveyors so that partially dried material is extracted at the end of the second pass and mixed with the incoming press cake.  Also important is a 180º elbow between the furnace and the inlet to the drum.  The selection of this dryer must be made using the appropriate de-rating associated with (low temperature) pectin peel production.

Exhaust System. This separation system features a low level entry cyclone separator that has been proven in producing pectin peel. The expansion chamber is complete with an air lock screw conveyor product discharge.

Exhaust Fan. A radial blade fan complete with inlet elbow and exhaust stack.

Standard Instruments. A solid state programmable controller modulates combustion through a sensor mounted at the inlet to the third pass of the dryer. This is required for precise control of product quality.

Cooling Reel. The cooling reel uses ambient air for final drying.  It is complete with a fan, dust collector, ductwork, supports, and electric motors. This type of cooler is not used in modern feedmills because of the need for a pellet cooler (which is not used in pectin peel production).

Product Elevating Screw and Surge Hopper. A carbon steel conveyor and hopper, leading to the bagging or baling equipment, are used.

Sewing Head and Bagging Scale.  Most pectin peel is baled, so that a maximum amount can be loaded into a cargo container.  If bagging is used, a Fischbein sewing head and conveyor with a bagging scale are typically included.

Despite the length of this letter, there are a great many details that have been left out.  We would be glad to work with your specific requirements.

Robert B. Johnston, P.E.


PS  The purchasing contacts are as follows:

CP Kelco
Paul van Wagernen
Skensved, Denmark
011-45-56-165 616

Renato Rodriguez
Tecoman, Mexico
011-52-332-40940 or 42155

Bernard Cerles
Wayzata MN 55391  USA
Tel 1 952 742 0291

Raymund Asmussen
Neuenbuerg, Germany
Tel +49 7082 7913 400

Citrus Pressing

September 25, 1996

We recently sent out a newsletter, Four Kinds of Water, that helps explain why it is not possible to take potato waste down to 50% moisture in a screw press. The problem is that potato has a lot of water that is chemically bound to other molecules. The water cannot be released without the addition of heat, as occurs in a rotating drum dryer.

If a screw press could reduce the moisture content to a low number, then there would be no need for a dryer. Yet the opposite is true: dryers are more common than screw presses.

Citrus peel presents a good illustration. The peel starts out at 82% moisture, and it is taken down to 10% in making by- products. If the plant is making cattle feed, they first add caustic lime (calcined calcium carbonate) to the peel to break down the pectin molecules. With this procedure it is possible to press citrus peel to as low as 62% moisture. The rest of the moisture reduction is achieved by adding heat in the form of fuel burned in a dryer.

If we try to press to less than 62% moisture, the peel just extrudes through the screen in the form of frit. Potato behaves very similarly; it comes through the screen looking like mashed potatoes. It is not possible to separate the water from the solids with the mechanical action of a screw press.

Another citrus peel by-product is pectin peel. This is dry peel in which the pectins have not been broken down. It is not permissible to add lime to peel that is used in this process, so instead water is used to wash the sugars out of the raw peel. This drives up the moisture content from 82% to over 90% (after screening). Next the peel is run through a screw press, and the best we can do is get down to 86% moisture. Thus the pectin peel goes into the dryer at 86% after pressing, which is wetter than it started.

Processors would love to run the moisture down to 50% in their screw presses because it would save them a fortune on their fuel bills. But it just can not be done.

Issue 50

Citrus Waste Disposal

September 30, 1998

In was in the 1930's that citrus canneries first addressed the problem of disposing of waste. This waste consists mostly of peel, along with other elements such as seed, rag, core, pulp, and others. As early as 1931 Dan Vincent was operating a peel dryer in order to produce a feed for dairy cows.

The investment required for a modern citrus feedmill is in the millions of dollars. And, at today's low grain prices, it is a poor investment. As a result the citrus processors in small countries cannot justify erecting a feedmill.

At the same time environmental laws are coming into play that limit the options for disposing of citrus waste. Basically, there are the following alternatives to consider:

    • In Sicily, Spain, Mexico and California the small plants give the peel to nearby farmers for livestock feed. The animals eat it fresh, within a couple days. No energy is required. The peel must be eaten before it starts to ferment.
    • There are medium sized processors in Belize, Sicily and Mexico that have too much peel to give away, even for free. Their options are to either landspread or landfill the waste. This practice is being challenged on the basis of groundwater contamination.
    • Many medium sized processors in Mexico have gone one step further. They react the peel with lime and then dry it in rotating drum dryers. The dry peel is then sold either as bulk dry feed or it is pelleted. About 1,500 BTU per pound of water evaporated are required to evaporate the moisture in the peel, so the fuel cost is apt to exceed the value of the livestock feed produced.
    • In California some medium sized plants react the peel with lime and press it into press liquor and press cake. They concentrate the press liquor into citrus molasses (in steam evaporators) which is sold, either to distilleries or as a liquid animal feed. The press cake is sold as livestock feed which has a shelf life of a couple weeks. Some farmers store the peel press cake in gigantic plastic bags (10' diameter, up to 300' long) for extended storage. The only energy required is the steam for the molasses evaporator. This system is quite economic, but it is practical only in California where immense dairies and feedlots are located near citrus processors.
    • In Florida and Brazil the citrus feedmills all have waste heat evaporators (WHE) in use. These greatly improve the thermal efficiency of the feedmill by driving the evaporator with the flue gasses from the dryer. The process is to react the peel with lime and press the peel into press liquor and press cake. The press liquor is made into molasses in the WHE, and this molasses is added back to the peel, usually in the reaction conveyor. The press cake is dried in a rotary drum dryer and subsequently pelleted. The process requires about 500 BTU per pound of water evaporated.
    • The new option is to burn the peel as fuel. The heat value of the solids in the peel is excellent. The heat released by combustion is used to dry the peel and to generate steam. The steam can be run through a generator in order to supply more than enough electrical energy to run the entire citrus processing plant. In addition, the steam can be extracted from the turbine at 35 psi so as to supply the steam required by the juice evaporators.

Vincent Corporation gave a presentation at the recent 1998 Citrus Processing Short Course. This paper details the process of burning citrus waste in order to make the processing plant energy self-sufficient. A summary will be released in a future issue of Pressing News.

Issue 84

Double Pressing Basics

February 23, 1999
Rev. 2008

Many people assume that a second press is used because the first press fails to remove all the free moisture. This is not the case; presses are designed to remove almost all of the moisture that can be removed in a single pressing. However there are two applications where double pressing is technically sound.

In the production of juices for human consumption, double or even triple pressing is common. This is because the moisture in the press cake from first pressing contains many of the dissolved solids which are the essence of the juice. To capture these solids, water is added to the press cake. The dissolved solids in the cake diffuse into the water. The water is separated in the second pressing. This water carries with it the dissolved solids from the previous press cake. These are valuable solids which would otherwise be lost. Production of apples juice and coconut cream are two applications where double pressing is recommended.

The second application where double pressing is sound is in the production of animal feed from citrus waste.

In 1970 Dan Vincent was awarded a patent covering this double pressing concept. The system described pressing citrus waste in two consecutive presses, positioned in series. The idea was to reduce the moisture content of the press cake going into the peel dryer, thus reducing the amount of fuel required to dry the peel.

The key to this double pressing is to diffuse dissolved solids into the peel. This is done in a diffusion conveyor located between the first and second presses. High Brix (solids) molasses is added to the press cake from the first pressing. After a couple minutes of stirring in the diffusion conveyor, some of the solids in the molasses diffuse into the moisture in the press cake. Equilibrium is reached at a medium dissolved solids content, a point which is between the low Brix press cake and the high Brix molasses. When this cake is run through a second pressing, the cake resulting will have a higher Brix and, consequently, lower moisture content.

With a lower moisture content it takes less fuel energy to dry the cake into cattle feed. The result is a lower fuel cost per ton of pellets produced.

A material balance shows us how this works. We start with peel at 80% moisture and 10º Bx. In a 100# pound sample this is 20# total solids and 80# water. The 80# of water, at 10º Bx, has 8.9# of dissolved solids (mostly sugars). The rest of the solids, 11.1#, are suspended (insoluble) solids.

If we press this peel, the cake will still have 10º Bx. By diffusing 50º Bx molasses into the cake, the Brix will equalize at around 20º Bx. When this mass is run through the second pressing, the resultant cake will still have 20º Bx. In effect water in the first press cake has been displaced with dissolved solids from the molasses.

Some people argue against double pressing because "the second pressing only squeezes out the molasses added in the diffusion conveyor." This is not an accurate evaluation. Solids from the molasses will have been diffused into the peel. This is seen in the fact that the press liquor from the second pressing will have a lower Brix than the molasses that was added in the diffusion conveyor.

Copies of the 1970 patent are available upon request.



Issue 91

Fiber Filter I

July 21, 1998

Vincent Corporation is introducing an exciting new product, the Fiber Filter. Featuring fine filtration of low consistency, high gpm throughputs, the Fiber Filter is a unique machine. There is nothing quite like it available in the market. It operates continuously with a fabric filter that is vibrated clean by the process flow, requiring only occasional back-flush cycles.

Liquid flows with fiber contents ranging from 100 ppm to 4.0% are thickened to a range of 2% to 14% solids with the Fiber Filter. The filtrate liquid is remarkably free of suspended solids. The Fiber Filter can replace equipment ranging from pre-thickening screens to centrifuges. Fiber Filters can be used both to thicken flow ahead of a screw press and to remove fiber from press liquor.

Mechanically the Fiber Filter consists of a rotating paddle impeller that whirls and pulses the incoming fluid against the inside of a cylindrical filter screen. The filter screen, held taught in a frame, is made of woven polymer fabric. The fabric is available in meshes ranging from 600 (25 microns or .001") to 70 (200 microns or .008"). The angle of inclination of the machine is adjusted to optimize flow rate and solids concentration.

External Fabric Tensioning is an important innovation. The springs that hold the fabric sleeves tight are tensioned at the discharge of the machine. This allows adjustment of the fabric tension with the machine in operation, an important operator convenience. Also, it eliminates springs, turnbuckles and threaded rod from the inside. This is important where the application involves products for human consumption because the machine becomes much more sanitary. There is a patent pending on this feature.

To date, only the engineering prototype and a short production run of FF-12's have been produced. These units are being placed in the rental fleet so that they can be used for field testing. Our key target applications for testing are: press liquor in food processing plants, breweries, and citrus feedmills; whey in the cheese industry; residual fiber in wet corn milling ethanol plants; screen rejects, black liquor and clarifier underflow in the pulp and paper industry; press water in manure dewatering operations; and juices.

Rental units are available on a first come, first served basis. The machines are all-stainless.

Issue 80

Fiber Filter II

February 5, 1999

After a year of testing, Vincent Corporation has gained a great deal of confidence in the Fiber Filter. It is a unique filtering machine that has broader market application than our traditional screw presses, dryers, and shredders. Once the newness is overcome, it is easy to understand.

The best place to test a Fiber Filter is where a centrifuge is being used. If the Fiber Filter works, it is a sure sale because its total cost is less than the routine maintenance of a centrifuge. There are applications in both fruits and meats where the performance of the Fiber Filter beats that of a centrifuge.

Another ready market for Fiber Filter is where the flow to be filtered tends to blind existing static or rotary drum screens. These traditional screens are very economical to acquire and operate. However they have been put into many applications where they are only marginal performers. The Fiber Filter not only features non-blinding characteristics but it also offers finer filtration.

Here is how a Fiber Filter works. A flow containing suspended solids is pumped into a fabric sleeve. The sleeves offered have hole sizes ranging from 0.001 to 0.006", which is finer than is available in metal screens. Normally a fabric with holes this small would rapidly become blinded by the solids being filtered from the liquid stream. This problem is overcome in the Fiber Filter by vibrating the fabric at high frequency. The vibration is induced by (a) tensioning the filter sleeves with a pair of springs and (b) a high speed rotor inside the sleeve induces pulse waves in the fluid being filtered. These pulses of liquid make the filter sleeve vibrate.

The whirling rotor also has ribbon flights to push the solids toward the sludge discharge end of the machine. At the same time the pulsing forces filtered liquid through the fabric.

Applications which promise success include:

    • Cleaning press liquor from a screw press.
    • Filtering waste water ahead of a treatment plant at vegetable, meat and fish processors.
    • Concentrating a dilute flow of suspended solids so that the solids can be further dewatered in a screw press.
    • Filtering a flow ahead of an evaporator so as to reduce evaporator fouling and to improve heat transfer efficiency.
    • Removing water from spent grain at breweries, distilleries, and ethanol plants.
    • Finishing fruit juices.
    • Thickening good fiber by filtering out the ink, ash (clay) and water in a paper recycling operation.
    • Removing fiber from black liquor ahead of the evaporators in a virgin fiber paper mill.

Because of the newness of this technology, we do not expect a firm to buy a Fiber Filter without first testing it. For that purpose a rental fleet is available. The small FF-6 rents for $200/week; the FF-12 for $350. The customer must also pay the freight to and from the test site.

Issue 90

Fiber Filter for Pectin Recovery

July 26, 2005

An interesting application for the Fiber Filter has evolved over the last few years.  The project started as a rental at a plant whose wastewater treatment plant (WWTP) was causing excessive odor. 

The customer's operation involves receiving fresh lemon peel as a raw material, washing the dissolved solids (sugars) from the peel, and then extracting pectin from the washed peel.  The precipitated pectin is dried and sold, in powder form, as a food ingredient.

The WWTP load came principally from the dirty water from the peel washing system.  This water contains both dissolved solids and suspended particles of lemon peel.  The initial project involved using a Model FF-12, with coarse 190 micron sleeves, to filter this wastewater.

The project was a success.  Odor from the WWTP noticeably decreased after filtering out the suspended solids.  This is true even though most of the solids are dissolved, so they pass through the Fiber Filter.

At first, the solids sludge from the Fiber Filter were trucked to a remote site.  Soon, however, it was determined that this sludge contained valuable pectin of good quality.

For the next season, a second Fiber Filter was added, and the sludge from the machines was pumped back to the peel wash system.  In this manner the pectin was recovered.

Today, a third Fiber Filter has been added.  The system has been further refined by pumping the sludge from the three machines directly to the acid/alcohol plant.  There it is mixed with the washed peel, and the pectin is extracted.  (They found that if this sludge was added back to the washing system, the screw presses did not seem to work correctly.)

Issue 163


IFT - Burning Citrus Waste

The two processing plant examples used in the September 17 presentation both are based on using existing rotating drum peel dryers. For the greenfield site, there exists a better investment. Instead of installing a dryer, the use of a screw press in combination with a steam driven evaporator is recommended. This will allow a significant improvement in efficiency over the use of a dryer. 

With a steam driven evaporator, the press liquor from the screw press can be made into 72º Brix or greater molasses. Diffusing the solids of this molasses into the citrus waste will allow the screw press to produce press cake with a moisture content below 60%. Such press cake can be burned directly in the fluid bed combustor. Blending the press cake with drier material is not required.

In order to minimize capital investment, a four effect steam evaporator has been selected for the calculations in Exhibit V. This evaporator will evaporate approximately three pounds of water for every pound of steam that it uses.

Exhibit V shows that the citrus processor can be entirely energy self-sufficient. A small turbine with 35 psi exhaust steam will be adequate to generate all of the process steam and electricity that is required. There is enough excess steam that the possibility of using an absorption refrigeration system should be considered.

From the standpoint of capital investment, the use of an induction generator is recommended. This is less efficient than alternative equipment; however, it does not require electrical switchgear to synchronize the electric current that is generated.

Addendum II, September 2010

Alex Andreassen of Danisco has pointed out that  burning citrus pellets will lead to slagging of the boiler tubes because of the alkali-metals (Na, K) content of the peel.  Thus applications would be limited to using pellets as dryer fuel.


1998 Citrus Processing Short Course
Energy Self-sufficiency: Using Citrus Peel as Fuel 

September 14, 1998


A citrus processing plant can be energy self-sufficient. The energy value of citrus waste, when used as a fuel, is sufficient to operate a citrus juicing and concentrating plant. That is, when burned in a steam generator, the BTU value of the peel is sufficient to prepare the peel for burning and to generate the steam required for process needs. With the added efficiency of a waste heat evaporator, there may be enough steam to drive an electric generator to supply the electrical needs of the plant.

Escalation of energy costs following the Oil Embargo in 1973 gave rise to theoretical analysis. Was the value of citrus peel greater as an animal feed or as a fuel? A paper by Kesterson, Crandall, and Braddock published in 1979 compared dried citrus pulp with Bunker "c" fuel as a source of energy. It was concluded that the peel was more valuable as a livestock feed.

My personal interest in using peel as a fuel came as a result of looking for alternative uses for citrus waste. The GATT agreements of about four years ago dictate that the tariffs on animal feedstuffs, such as wheat, corn and soy bean meal, will be reduced over the coming years. Since there is no tariff on citrus pellets, the concern was that this would reduce the relative attractiveness of citrus pellets as a dairy feed in Europe. As the great majority of the citrus peel in both Florida and Brasil is pelleted and exported to Europe, the future of the citrus feedmill was being questioned.

The situation came to a head as a result of a dioxin problem in Brazilian pellets. Minute traces of dioxin were found in German and Dutch milk products in April 1998. This was traced to improper fuel oil additives that were burned in some Brazilian orange peel dryers. An embargo went into effect as the citrus processing season was starting. Since the source of the dioxin was not known with certainty, there was urgent interest in disposing of citrus peel by burning. (A directive for individual European nations to control compliance remains in force.)

In Brasil processors such as Citrosuco, Cargill, Cutrale, Dreyfus and Bascitrus have fluid bed combustors that are used for burning sugar cane bagasse. Consequently the option of burning citrus peel in this existing equipment was given serious consideration.

Citrus peel has fuel value because carbon, hydrogen and nitrogen are present in the dry solids. Peel will burn if dried sufficiently. The spontaneous combustion that occurs in citrus pellets and dried pulp give evidence to this fuel value. A quick analysis showed that the energy released is more than enough to dry wet (fresh) peel so that it can be burned.

It was in 1997 that I was introduced to the concept of an energy self-sufficient citrus plant. Gioacchino Sardisco of FMC do Brasil explained that he had studied the matter, with encouraging results. The data that he has supplied for the preparation of this paper have proved invaluable.

Analysis has shown that it is technically feasible to burn the peel generated by a citrus processing plant. The heat released by this combustion is sufficient to drive the existing feedmill dryers and waste heat evaporators (WHE). Additional heat released in burning the peel is sufficient to satisfy the steam needs of the TASTE evaporators of the processing plant. In fact there is enough additional steam generated that a steam turbine can be used to drive an electric generator. With the proper equipment the electricity generated will be more than enough to fill all of the needs of the citrus processing plant. With such equipment surplus "co-gen" electricity will be available for sale by the citrus processor.

The economic feasibility depends, most of all, on the relative value of dried citrus peel that can be sold as livestock feed, as compared to the cost of energy (fuel and electricity) purchased by the citrus processor. This comparison depends heavily on the relationship between the value of livestock feed and fuel in the country where the processor is located. Clearly the situations in Brasil, the United States, and a Central American country will present three quite different situations.

The capital cost will depend heavily on the existing investment. The cost to convert a US plant with an existing dryer and WHE will be more than required by a Brazilian plant that has these plus the fluid bed combustor with an attached boiler. However, a processor who has been landfilling the citrus waste (but can no longer do so for environmental reasons) faces an entirely different investment analysis.

The first step was to find the heat value of citrus peel. This was measured in laboratory studies that were part of Bob Braddock's 1979 paper. The heat of combustion of dried citrus pulp was found to be 7,470 +/-212 BTU per pound. This figure was very consistent with a variety of measurements made more recently in both Brasil and Florida.

A quick calculation is as follows:
A short ton of pellets typically has 200 pounds of moisture plus 1,800 pounds of dry solids. When burned the solids provide 7,470 x 1,800 = 13,400,000 BTU.

This can be compared to the high thermal value of Bunker "c" fuel oil, 154,000 BTU per US gallon. Based on 10% moisture in the pellets, there are 200 pounds of water which must be evaporated in the combustion process. Using typical citrus peel dryers, this will require about 1,350 BTU per pound of water evaporated, for a total of 200 x 1,350 = 270,000 BTU. Thus, burning a ton of dried peel will release 13,400,000 - 270,000 = 13,130,000 BTU. This is equivalent to about 85 US gallons of heavy fuel oil.

In therms, this is about 131 therms since there are 100,000 BTU's in a therm. The therm is a unit of measurement used in the calculations for this paper.

In the recent depressed market the value of either 85 gallons of fuel oil or 131 therms of natural gas approached what Florida citrus processors were getting for a ton of pellets. Thus, if a processor had the required combustion equipment, he could have been better off burning his peel than selling it as animal feed!

There is another quick analysis that sheds light on the subject. As many of you know, the Florida Citrus Processors Association in Winter Haven has compiled records for many years for their members. These typically show that an average of 50 to 60 therms per short ton are required to produce dried citrus peel (47 was the figure selected for calculations). This would be peel at approximately 10% moisture.

The 50 to 60 therms/ton figure is typical because in Florida both (a) feedmills dispose of (evaporate) large amounts of wastewater along with the peel and (b) most feedmills do not operate with a full 2:1 WHE/dryer ratio. On the other hand, some plants require less than 40 therms per ton. Calculations in Exhibit I show that 38 therms/ton is a realistic optimum figure for drying peel.

In any case, it is apparent that the energy released by burning a ton of dry peel, 131 therms, is considerably more than is required to dry the wet peel. Therefore a citrus processor will have ample peel-fuel to dry all of his wet peel. In fact, the processor will not want to burn all of his peel unless he has a boiler which will make use of the excess energy.

The steam generated by such a boiler can be used to drive the juice evaporators. Typically a citrus concentrate plant uses from 12 to 25 pounds of steam to process a 90 pound box of fruit. (Eighteen pounds were used in preparing this paper, further assuming that a box of oranges yields 45 pounds of peel with 22% solids, with 8 pounds of wastewater being added per box of fruit.)

Two mathematical analyses have been prepared. The first, in Exhibit II, is for a small 1,000 box per hour plant. It presumes no peel presses or WHE are available. The reacted peel is fed through the dryer to produce partially dried peel at 50% moisture. This peel is burned in a fluid bed combustor ahead of the boiler.

The second analysis, in Exhibit III, has calculations based on a plant that processes 15 million boxes in a season. It assumes there are existing dryers and WHE's which are used. About three quarters of the press cake is dried to 10% moisture in the dryer. The peel dried in this operation is mixed with the remaining quarter of the press cake that is not placed in the dryer. This combination makes a mixture of peel at 35% moisture, which is what is burned in the fluid bed combustor.

To summarize the figures in Exhibit III: The peel from 15 million boxes provides 3,846 therms per hour of energy. Of this 1,011 therms/hour will be required to dry the peel. This will leave enough heat energy to generate 181,098 pounds per hour of high pressure steam. This steam will be used to generate 16,787 KWH of electricity, well over the 7,280 KWH required to run the plant. 93,600 pounds/hour of the steam will be extracted from the turbine at 35 psi for use in the evaporators, while the rest, 87,498 pounds/hour, will go to a vacuum condenser.

Note that these calculations are based on running only three quarters of the peel through a dryer. The dried peel can be mixed with press cake (or fresh peel) so as to form a mixture with moisture in the range of 25% to 55%. This mixture is then burned in the fluid bed combustor. A material balance is shown in Exhibit IV that supports Exhibit III.

In both examples the flue gasses from the boiler are directed through the dryer so as to assist in drying the peel. Since a WHE operates on high wet bulb temperature gas, and boiler exhaust is normally low wet bulb, close attention to the design is important.

In the small plant shown in Exhibit II with no WHE, all of the energy available in the peel is used to dry the peel and to generate process steam. Excess steam will not be available to generate electricity. If the plant needs to generate its own electricity, supplemental fuel will be required in the fluid bed combustor.

The efficiency arising from the WHE in the larger plant allows use of an electric generator driven by a steam turbine. The turbine is arranged so that both the high pressure injector steam and low pressure process steam are drawn off for use in the evaporators. The rest of the steam would go to a vacuum condenser.

The calculations in Exhibit II and III are very detailed. This was done so that they can be used to calculate alternative scenarios. Besides fitting data to suit a specific processor, the underlying assumptions must be carefully reviewed by fluid bed combustor and boiler experts.

Checking with processors, we have seen examples in which 25,000,000 to 40,000,000 KWH are required to run an equivalent 15 million box plant for a season. The product (FCOJ or NFC) will have a major impact on electricity consumption. Clearly the figures presented herein are only a starting point, not a blueprint.

Burning citrus waste will reduce operating costs. The processing plant will save the money spent on oil or gas. If a WHE is available, additional savings will come from the money spent on electricity. It is even possible to realize incremental revenue from selling electricity. The normal revenue from the sale of citrus pellets (but not all the d- limonene) would be lost, partially offsetting these gains.

What investment is required? It can be examined in steps. As we have seen, a peel reaction system and dryer will be required, while peel presses and a WHE are not necessarily required. The minimum requirement is a fluid bed combustion chamber in which to burn the peel. This alone will reduce the purchased fuel required to operate the dryer. The second step would be to acquire a boiler suitable for use with the fluid bed combustion chamber. This would save the fuel being purchased to run the existing boilers at the citrus plant.

The third step is to acquire the steam turbine, electric generator, and switchgear required to generate steam with the excess peel. This would greatly reduce the electric bill at the citrus plant, plus it can result in revenue from the sale of surplus electricity.

Here is the rough order of magnitude of the investment in an 18,000,000 box per year facility: Two existing 60,000 pound per hour dryers, matched with two existing 100,000 pound per hour WHE's, represent an equipment cost of about $ 6,000,000. The site work, construction, buildings, utilities and auxiliary equipment (presses, pellet mills, conveyors) probably double this figure to $12,000,000. The addition of a Fluid Bed Combustor, with a 200,000 pound per hour boiler and a 20 megawatt turbine/generator set could be in the range of $20,000,000 to $40,000,000.

The small 1,000 box per hour plant would probably be in the range of $2,000,000 to $5,000,000 depending on options on electricity generation.

Clearly the electricity generating business is more capital intensive than what citrus processors are used to. There is so much capital in an electricity generating plant that it must be run year round. This will require alternative fuels during the off-season. In all likelihood, the business would be financed and operated by a co-gen partnership rather than the citrus processing company.

Burning peel at 10% moisture is very easy. It can be burned clean, with a minimum requirement for excess air, in many kinds of combustion equipment. The soot (ash and unburned fuel in the exhaust gases) will be minimal. However, to burn peel at 50% moisture is more challenging. A fluid bed combustor will be required.

At this point I would like to describe the fluidized bed combustion system. I am deeply indebted to Energy Products of Idaho for this part of my presentation. This company specializes in designing and installing fluidized bed energy systems. They have sold systems in California that have operated with citrus peel in the fuel mixture.

The arrangement they use is called a bubbling fluidized sand bed combustor. The chamber in which the peel is burned can be either rectangular or circular. There is a two foot deep bed of sand on the floor through which the air of combustion enters. The sand and fuel remain suspended in mid-air (fluidized) during operation. The chamber is usually refractory lined, although with some fuels the walls consist of boiler tubes.

With most fuels the burning occurs at 1800º F or less. This is the key to minimizing emissions. A principal emission concern is NOx (nitrogen oxides) which form at higher temperatures. Since citrus peel has 2% nitrogen, this is definitely a design consideration. EPI uses the non- catalytic system for controlling NOx. In severe cases ammonia, at the proper temperature, is added as an abatement agent.

Citrus peel contains around 8% ash, which is greater than the 2% common to sugar cane bagasse. At the same time this is less than the 50% ash that is typical of certain paper mill sludges. In this perspective, the ash in citrus peel becomes manageable.

Ash, in the form of slag, becomes a problem if it comes in contact with hot surfaces. It sticks to hot surfaces, so screening tubes may be used ahead of the boiler superheater. These tubes cool the slag so that it falls into the ash hopper before it has a chance to foul the superheater. In some applications limestone is added because it raises the melting temperature of the ash, reducing the sticky nature of the material. (Limestone also captures sulphur, another contaminant.)

Dioxins and furans are another emission worthy of note. They are grouped with other products of incomplete combustion (PIC's) such as VOC's and carbon monoxide. Chlorines give rise to dioxin when incomplete combustion occurs. This occurs when the concentration of carbon monoxide is in excess of 100 ppm.

When burning organic waste on a grate it is not uncommon to find carbon monoxide concentrations in excess of 1,000 ppm. In contrast, the concentration of carbon monoxide in a fluid bed combustor will rarely exceed 50 ppm. Consequently, prevention of the formation of dioxins and furans is possible with fluid bed combustion technology.

The action in the fluid bed combustor is such that auxiliary fuel is not required. That is, peel conveyed or blown in at 50% moisture will dry out and burn without additional heat being supplied by oil or gas burners. However, if additional steam is required, auxiliary fuel can be burned.

The fluid bed combustors being referred to are about 50' high. Generally they are 20' wide. The length varies according to capacity. The 18,000,000 box per year plant will have a unit about 45' long. The footprint, including the boiler and generator, will be about 30' by 150'.

The fluid bed combustor usually is refractory lined because this helps assure uniform operation despite swings in inbound fuel characteristics. The refractory radiates heat to the peel so as to help evaporate the moisture in the peel. It is noteworthy that some of the sugar cane bagasse burners in Brasil do not use such refractory.

There is one consideration that has a significant effect on overall thermal efficiency. In the process of burning citrus peel all of the moisture must be evaporated. Evaporating this moisture in a fluid bed combustion chamber takes about 1250 BTU per pound of water. On the other hand, the combination of a screw press, dryer and WHE is considerably more efficient: they require only 500 BTU per pound of water evaporation. Thus plants that have existing WHE capacity should use this to its maximum capacity. The practical result is that the peel admitted to the fluid bed combustor will have a lower moisture content and, therefore, will be easier to burn.

The manner in which this works is that the dry peel from the dryer is combined with wet peel (presumably press cake from the peel presses). Some Brazilians have reported the practicality of blending press cake at 67% moisture with some dried peel at 10% to produce a 50% to 55% fuel. (Citrus peel will support combustion with moisture contents as high as 70%.)

The d-limonene in the press cake peel adds fuel value to the peel. It is interesting to note that it causes flashing in the furnace.

One important question arises: How does this relate to the Clean Air Act? Most citrus peel dryer operators in Florida are addressing serious VOC problems. It is thought that evaporation of d-limonene from the press cake in the dryer is a major contributor to the VOC's being measured. The following scenario should be considered: The oil in the peel going into the fluid bed combustor will be burned and thus cause no VOC problems. Conversely, the oil evaporated in the dryer is a source of VOC's. Therefore, to minimize VOC's, it could prove advantageous to dry a minimum fraction of the peel and burn as wet a mixture as practical in the fluid bed combustor.

This analysis assumes that the citrus molasses, which is part of the dried animal feed, is burned. That is, the molasses is either put on the peel going into the dryer or burned directly in the fluid bed combustor. Note that molasses at 50º Brix (50% moisture) has more fuel value than press cake which typically is 62% to 70% moisture.

An interesting alternative would be to sell the citrus molasses separately. This could be of interest to a citrus processor who needs to dispose of all of the peel but does not have a boiler to go with a fluid bed combustor.

This discussion confirms Sardisco's point: a citrus processing factory can be energy self-sufficient if the peel is burned.

Environmental problems are challenging, but manageable. Even the stringent regulations of the United States can be met.

The future depends on two major forces outside the control of the citrus industry: (1) Where will the market for citrus pellets stabilize in light of dioxin scares and the GATT elimination of the tariff umbrella enjoyed by citrus pellets? (2) Will the declining price of fossil fuels reverse course and go into an upward trend?

In closing let me reiterate that the figures presented here must not be taken as gospel. They are theoretical figures, and they must be modified according to the practicalities of actually burning peel as well as the characteristics of each processing plant.

References and Acknowledgments

  • The Heat of Combustion of Dried Citrus Pulp, Journal of Food Process Engineering 3 (1979) 1-5, by J. W. Kesterson, P. G. Crandall, and R. J. Braddock.
  • Sugar Cane Bagasse: An Alternate Fuel in the Brazilian Citrus Industry, Food Technology, May 1988, by Jose Luiz Guerra and Elizabeth Steiger.

Invaluable and important assistance in the preparation of this report was given by:

  • Ralph Cook of Cook Machinery, Dunedin, Florida (727-796-1367)
  • Kent Pope of Energy Products of Idaho, Coeur d'Alene, Idaho (208-765-1611)
  • Gioacchino Sardisco, FMC FoodTech, Araraquara, Brasil, (011- 55-16-232-1300)



Biomass Burning System

January 3, 2012

Vincent Corporation has extensive experience with dewatering spent coffee.  With spent coffee the press cake comes out at around 58% moisture content, depending on the extraction process used.

Most soluble coffee producers burn their spent coffee because of its high BTU content.  Historically, either grate furnaces or fluidized bed boilers have been used.

A much less costly, but equally efficient, method of burning spent coffee is common practice in Colombia.  They take the press cake from our screw presses and dry it down to 10% moisture content in a rotary drum dryer.  This material can be burned in a conventional boiler, without the cost and complexity of a fluidized bed unit.  In fact, the boiler is relatively inexpensive because it is deliberately undersized.  Being undersized, the chimney gasses come out quite hot.  Rather than put these flue gasses out the stack, they duct them to the drum dryer.  In this manner the flue gasses are used to dry the press cake down to a  required 10% moisture content.  At this low moisture, the cake burns in suspension, like pulverized coal or natural gas.  There is no auxiliary fuel required in the boiler (except for start-up).  There is no burner or furnace on the dryer, either. 

The boiler is an unusual design.  The furnace is a conventional water tube construction.  However, at the top exit of the boiler there is a fire tube section for pre-heating the feedwater coming into the boiler.  This seems an odd combination, but it must work since the Colmaquinas, Hurst and Cerrey boiler companies all offer it.

The chart showing this system is attached.

This same combustion system would work equally well in burning a variety of biomass by-products.

Burning Spent Coffee


Issue 241

IFT - Citrus Shredder Performance

August 3, 1996, Presented by Robert B. Johnston, P.E.

The aim of this study was to find some generalizations in regards to citrus oil in feedmill press cake. It was hoped that both useful guidelines could be developed and focus areas for further study could be identified. Peel samples were taken at sixteen different Florida citrus feedmills late in May 1996. These samples were quick snapshots, taken randomly at the time our engineers arrived at the processing plant. 

No definitive conclusions should be drawn because of the limited nature of the sample taking. While this study points out several interesting conditions, its purpose is to encourage further research.

An observation that gave initial impetus to this paper was made at the Cargill feedmill in Frostproof in January 1995. It happened when separate samples of press liquor were taken from the inlet hopper screen, main screen, and discharge cone screen of their horizontal screw press. It was noted that as the peel progressed through the press, both the pH and Brix of the liquor became progressively lower. This hinted that, as the peel was progressively squeezed, more cells were broken open and, possibly, more oil was being expelled with the press liquor. This is illustrated graphically in Exhibit I which compares mason jars of press liquor taken from the main screen and discharge cone of a press. In the end we concluded that a condition such as this is indicative of incomplete reaction between the peel and the lime.

It is uncertain which of the samples are representative of typical operation at each of the sixteen feedmills. There was room for error: upset conditions may have been occurring, especially because it was the end of the season and some plants were winding down; at least one plant was running sour peel; two plants were having press problems; molasses tank levels were not necessarily at equilibrium; some plants were running grapefruit along with their Valencias; etc. Rather than arbitrarily exclude data, we have attempted to present the full range of possible operating conditions.

The following are the tests that were conducted (Exhibit II):

  1. Oil content of the peel at the peel bin.
  2. Particle size distribution of the peel exiting the shredder.
  3. Particle size distribution of press cake leaving the screw press.
  4. Moisture content of press cake leaving the screw press (final pressing in plants with double pressing).
  5. Oil content of press cake leaving the screw press.
  6. Oil content of the press liquor.
  7. Brix of the press liquor and, where applicable, molasses.

The peel and press liquor samples were kept refrigerated from the time they were gathered until the tests were conducted.

Moisture tests were run with two different moisture balances as well as with a laboratory oven and scale. Several tests were run on each sample to verify the results; we feel that the readings presented herein are fair for analysis purposes. These readings are key because they were used to determine the proportion of press cake and press liquor, as a percentage of inbound peel.

Material Balances were found necessary to determine the proportion figures. These were run for each of the sixteen plants, taking into account single pressing, double pressing or pumped peel. It was during this step that something very interesting in regards to molasses diffusion was observed.

Almost half of the plants were running with press liquor Brix of 20º or higher. Despite many hours of computer time it was impossible to fit this high a Brix into a reasonable material balance for most of the plants. It possibly could be achieved by drawing down the molasses tank (using more molasses than was being produced), but this was not logically occurring at so many feedmills. Finally it was recognized that the most reasonable explanation for high press liquor Brix is incomplete diffusion of molasses into the peel.

This theory tied to the oft-repeated question, "Why add molasses in a delay conveyor between first and second pressing if it is just going to be pressed out again?" Our conclusion is that this is definitely what is taking place at many feedmills. Many years ago Dan Vincent did testing at Lykes Pasco that showed that it takes ten to twenty minutes for complete diffusion between peel and molasses. Yet most feedmills have only one to three minutes of delay between first and second pressing, thus explaining our data.

We feel that even a short diffusion period is better than none. Diffusing molasses into peel that is about to be pressed does improve the pressing action and ultimate thermal efficiency of a feedmill. We are anxious to see the results in the 1996-97 season at Southern Gardens: there a reaction conveyor has been relegated into service as a delay conveyor between first and second pressing. The size of the conveyor should allow around eight minutes for diffusion of the molasses. It will be interesting to see the extent to which pressing action is enhanced.

The press cake moisture readings of the samples taken for this presentation almost all read quite a bit higher than was expected for late season Valencias. We feel that this can be related to the fact that the Brix readings of peel coming from extraction were unusually low: our readings were almost all in the range of 7º to 9º, whereas normal readings are 10º to 11º Brix.

Oil analyses were made using the Scott method.

Peel samples were taken from four different models of shredders. The two most common were the Rietz RD-18 Disintegrator manufactured by Hosokawa Bepex and the 18" horizontal shredder manufactured by the former Gulf Machinery Company. Both models are 75 hp machines.

The other two brands are relatively new to Florida citrus: the Jacobson and Gumaco (Brazil) hammermills. These are larger machines, ranging from 150 to 300 hp. Exhibit III has photos of these various machines.

All four shredders can be operated with a variety of screen sizes. In fact, some were being operated with no discharge screens, and, being the end of the season, some were being operated with damaged screens and worn hammers.

An interesting technique was used to measure the particle size distribution of the shredded peel. The required equipment was loaned to us by CSC Scientific of Fairfax, Virginia. Their laboratory air lift dryer was used to gently dry the peel before vibrating it through a stack of sieves.

Samples from the shredder frequently were quite wet with molasses, and they took 25 minutes to dry. In contrast, samples of press cake dried out in 15 minutes. Appendix I shows and describes the apparatus that was used.

The photos in Exhibit IV show some typical peel samples before and after they were air dried and sieved. In the tests, sieve screen sizes ranging from 9.5 mm down to 0.212 mm were used. The photos illustrate the separations that were achieved.

The bar charts in Exhibit V illustrate the particle size distribution by weight. In general, one of two distributions was found: ether a bell shaped curve or a curve skewered to the larger sized particles. It is interesting to note that the same shredder will produce different particle profiles depending on the type of juice extractor that is used.

The results in the bar charts are in remarkably close agreement with measurements published by Dr. Bob Braddock in 1978 (slide).

Our preconception was that the best shredding resulted in (a) a minimum of fines and (b) a minimum of large pieces. The fines are undesirable because they are likely to either burn in the dryer or to be carried into the waste heat evaporator (WHE) where they result in problematic black water. At the same time the large pieces were thought to be undesirable because they seemed apt to contain oil that is not expressed from the peel in the pressing operation. (This proved to be not necessarily true.)

The reason that fines are likely to burn in the dryer is explained by the concept of latent heat of evaporation. As long as a particle of peel in the dryer contains moisture, it is cooled by evaporation and will not go much above 212º F. A small particle has much more surface area in proportion to its volume; evaporation is proportional to the surface area, so small particles become bone dry before the larger particles. Once bone dry, the particle increases in temperature until a point is reached where combustion occurs.

Fines leaving the dryer are very low in moisture, and some of these escape past the cyclone dust separators. Such particles either go to the WHE or they are recirculated back to the burner area. Being dry to start with, they are likely to burn upon being re-admitted to the hottest portion of the dryer.

In the study it was found that the percentage by weight of fine particles in the peel (the two smaller sieve sizes) varied significantly. The range was 1/2% to 6% fines in samples of peel coming from the shredder. If these are lost in the dryer, they can have a measurable effect in peel recovery (as measured by pounds of citrus pellets produced per box of fruit).

The percentage of fines could not be tied to any particular style of shredder or hammermill. As one would expect, the peel from FMC extractors did average somewhat more fines than that of Brown extractors, but this was not enough to explain the range of values that was measured. Possibly the disposition of extraction pulp would explain the differences we observed. I wish I could say more, but more study will be required to explain the wide range in the percentage of fines.

The percentage of fines did increase during the reaction and pressing operations. This increase of about 2-1/2 percentage points is shown in Exhibit VI.

The increase in the amount of fines was compared between the vertical and horizontal presses. They averaged virtually the same increase, right at 2-1/2 percentage points. Furthermore, the fines could not be tied to the moisture content (tightness of pressing) of the press cake from these presses.

It is noteworthy that in 1949 Dan Vincent was awarded US Patent 2,490,564 covering a "Vegetable Pulp Shredder Screen Having Cutter Blades". This patent dealt with using thin 1/16" blades to both reduce fines and improve the peel reaction. These machines were used in citrus for many years, and they gave good results compared to units with 1/2" and wider hammers. However, the design ultimately proved impractical due to its vulnerability to tramp metal.

We did not measure oil recovery in this study. Clearly the oil recovery systems used in both the juice extraction operation and in the WHE will govern oil recovery. Instead, we looked at the oil (mostly d-limonene) carried into the drier with the press cake. This oil is almost entirely evaporated in the dryer and released to the atmosphere. It exhausts as an unburned hydrocarbon.

Seeking a correlation between large pieces of peel and the oil going into the dryer with the press cake proved interesting. To begin with there are two key measurements: (1) the pounds of oil going into the dryer per ton of peel entering the feedmill, and (2) the percentage of oil going into the dryer relative to the quantity of oil in the peel entering the feedmill.

Note that the raw measurement of percent oil in the press cake is only part of the equation. A tricky part of the analysis is to recognize that the percentage of oil in the press cake must be adjusted for the pounds of press cake per one hundred pounds of peel. Because of this, a feedmill that presses very tight will have a little less oil going into the dryer. This is true even though the percentage of oil in the press cake may be higher than what is found in more moist press cake.

In general, as expected, the plants running Brown extractors had a higher proportion of large pieces of peel after the shredding operation than did those plants with FMC extractors. We were interested to see if the presence of large pieces of peel led either to increased oil in the press cake or to higher press cake moisture. Therefore a comparison was made separating samples from the extractor manufacturers. Each of these two groups were further split so that comparison could be made between shredders that were producing predominantly large pieces of peel and those that had a lesser proportion of large pieces.

The results were surprising, as shown in Exhibit VII. The pressing operation was definitely able to achieve dryer press cake when there were fewer large pieces of peel. The average was about 2-1/2 percentage points lower moisture content.

On the other hand, the pounds of oil in the press cake per ton of inbound peel went down only slightly. In the spreadsheet it is seen that generally the presses that pressed tighter had only a little less oil in the press cake than the pressing operations characterized by high press cake moisture.

In other words, shredding to reduce the fraction of large pieces of peel will reduce press cake moisture (and therefore improve thermal efficiency). However, the quantity of oil going into the dryer does not change appreciably.

Looking at it from another perspective, contrary to our expectations, we cannot say that press cake oil was significantly higher in samples that had a higher percentage of large pieces. Thus the data supports the postulate that fine shredding allows oil to be absorbed into the albedo, and that this oil does not press out of the peel.

The oil analysis brought attention to another condition. On the average, there was a noticeably higher oil content in peel from plants using Brown extractors as compared to FMC extractors. On an approximate basis, raw Brown peel had 1.0% oil, while FMC had 0.5%. The surprising thing is that about one third of the oil in the Brown peel was measured going into the dryer, as compared to two thirds of the oil in the FMC peel. The end result is that almost equal pounds of oil per one hundred pounds of peel were found in the press cake, regardless of which juice extraction system is employed!

Unfortunately our study of shredding and press cake had to ignore some very important considerations. Brown plants that made more use of the BOE (Brown Oil Extractor) did better than those that did not. Similarly, the FMC oil recovery system employed undoubtedly governed the results of the FMC plants. Other important factors which could have distorted our analysis are (1) the sufficiency of the peel reaction system and (2) the oil stripping characteristics of the WHE.

Before concluding this presentation I want to make mention of the new feedmill at SunPure. This processor takes pride in the high level of citrus oil recovery that they achieve. The day that we took the first samples there was an upset that led to a second set of samples being gathered.

Both sets showed a high level of oil recovery. In fact, the inclusion of the SunPure results are enough to distort the averages shown in Exhibit VII.

The minimal amount of oil going to the dryer at SunPure is helped by the fact that they are able to press the peel to the same low level of moisture content as the best feedmills in the State. However, we suspect that the outstanding low levels of press cake oil can be tied to the Cook Machinery Company technology used in the feedmill. This technology involves a combination of (1) improvements on the Brazilian pumped peel flow schematics, (2) using available heat to accelerate the peel reaction, and (3) WHE technology.

Recently a number of modifications have been made at the SunPure feedmill, so we are anxious to measure performance once again in the 1996-1997 season.


To summarize this presentation, let's look at citrus oil recovery in a broader sense. Clearly the two most important considerations are the oil recovery systems used (1) at extraction and (2) in the WHE. This paper has not examined either of these. Rather, we have focused narrowly on the shredding and pressing operations. It should be obvious that differences in extraction and WHE systems may have distorted our analysis.

At the same time some interesting points can be made:

  • A decline of press liquor pH between the inlet and outlet of the screw press is indicative of under-reacted peel.
  • High Brix press liquor is frequently an indication of incomplete diffusion of molasses into the peel.
  • Plants with Brown juice extraction systems will tend to have a higher proportion of larger pieces in their shredded peel. They should be sure their shredding equipment is doing a good job in order to achieve the best thermal efficiency.
  • There can be a significant fraction of peel fines going into the dryer, more so at plants using the FMC juice extraction system. Therefore dryer dust separation equipment is of importance.
  • Shredding the large pieces of peel into smaller pieces does not significantly improve oil recovery in the feedmill. However, it does improve pressing action and, consequently, thermal efficiency.

Let me conclude by warning against blindly accepting the results of this study. Our intent has not been to give the final word, but rather to point the way and to encourage additional and more thorough investigation.

This paper could not have been prepared without the assistance of a great many individuals and their firms. We want to extend our appreciation to the following:

    • CSC Scientific Company, 8315 Lee Highway, Fairfax, Virginia. Telephone 1-800-458-2558. The loan of their Test Sieves, High Performance Compact Shaker, and Fluid Bed Dryer proved invaluable.
    • Automatic Machinery and Electronics, Inc., 333 Avenue M, N.W., Winter Haven, Florida. Telephone 941-299-2111. Their laboratory took on the task of running oil content analysis on almost eighty samples of peel and liquor.
  • Lala Produce Inc., 1402 25th Street, Tampa, Florida, 33605. The generous use of their cold storage facilities was most helpful in controlling close to one thousand pounds of peel samples. 

    Participating Citrus Processors
    • Alcoma Packing Company
    • Cargill Citro America Inc.
    • Citrus World, Inc.
    • Coca-Cola Foods, Auburndale
    • Coca-Cola Foods, Leesburg
    • Florida Juice Partners, Ltd.
    • Golden Gem Growers Inc.
    • Holly Hill Fruit Products
    • Indian River Foods Inc.
    • Orange-co of Florida Inc.
    • Peace River Citrus Products
    • Silver Springs Citrus Co-op.
    • Southern Gardens
    • SunPure, Ltd.
    • Tropicana Products, Inc., Bradenton
    • Tropicana Products, Inc., Ft. Pierce

    Technical Review

    • Dr. Robert J. Braddock, University of Florida, Lake Alfred
    • Messrs. John and Ralph Cook, Cook Machinery Company, Dunedin
    • Mr. Don Kimball, retired, Coca-Cola Foods, Winter Haven
    • Dr. Ashley Vincent, Savant-Vincent, Tampa



Material Balance

January 14, 2006                                                                                                                                                                                                  ISSUE #169

A material balance is a set of equations that express the flows of materials. The mathematics is all based on the simple premise that what comes in must equal what comes out. Variables are easily changed in order to gauge the impact on a system. Material balance is key to analyzing, and understanding, the flow of material in a processing plant. 

The concept of Brix is vital in a material balance done for fruit and vegetable processing facilities. Brix is much like a percentage, denoting the amount of sugar dissolved in water. It is measured in degrees, using an instrument called a refractometer. The equation for Brix is as follows: Bx = (Ds x 100)/(Ds + w), where Ds is the weight of dissolved solids and w is weight of water.

Note that suspended (or, insoluble) solids, which are almost always present, do not enter into the equation.

The beauty of Brix is that it holds constant as a flow of material is divided. That is, if a flow at 7° Bx is pumped into a screw press, the press liquor will have 7° Bx. Furthermore, if a drop of water is squeezed from the press cake, then it, too, will measure 7° Bx.

For example, if orange peel with 11° Bx and 9% suspended solids is fed into a screw press, both the press liquor and press cake will measure 11° Bx. (Naturally, the amount of suspended solids will be higher in the press cake than in the press liquor.)

Another important characteristic of dissolved sugar is that if flows of different Brix are mixed, diffusion occurs until a balance is achieved. Thus, if a pound of 50° Bx citrus molasses is added to two pounds of orange peel with 80 percent moisture and 11° Bx, the result will be three pounds of material with a moisture content of 70% and 25° Bx. When this material is pressed, the cake moisture content will be significantly lower than if the straight orange peel were pressed. This is simply because of the greater dissolved solids content in the water contained in the press cake.

Vincent has available a large number of material balances. These reflect single and double pressing, counter-flow diffusion, recirculation of press liquor, addition of oil house water, and many other options. These are transmitted by e-mail, in Excel.

The Excel spreadsheets contain a large number of simultaneous equations. Thus, Excel must be set on "reiterate" in order to find the common mathematical solution. Excel will freeze up if an illogical number is entered in error, so work must be saved frequently. In the event that a spreadsheet freezes, it must be closed without saving it.



Moisture Content

August 1, 2000

Everyone knows that screw presses can be used to reduce the moisture content of a material. Less appreciated are the limits that exist. Each material has a natural limit below which the moisture content cannot be reduced in a screw press.

These limits vary widely. For example, lumber sawdust can be pressed down to almost 50% moisture; fresh green alfalfa will go to 72%, while tomatos can be reduced to only 86%. These limits arise from the chemistry of the water in the material.

A screw press will remove the free and interstitial water in an organic material. However there are two forms of water that cannot be separated by mechanical force: (1) the water that is electrically attached by hydrogen bonding and (2) the hydrated water that is chemically bound to the molecules of the solids.

To remove these two forms of water the chemistry must be changed. In the case of citrus peel, this is achieved by adding lime, which reacts with the peel.

Most commonly the chemistry is changed with heat. Heating press cake through a dryer will reduce the moisture content right down to zero. In some cases, the heat is added in the form of steam: a steam dryer will do the same thing.

As recently as 1970 steam was added in Vincent screw presses used for pressing both citrus peel and fish. This was done by injecting the steam through holes drilled in the resistor teeth. About once a year we sell a press with provision for steam or water injection through the resistor teeth.

Some competitors direct steam through a hollow screw shaft. FKC and Dupps offer the heated shaft feature for paper mill sludges. It is not an energy efficient way to dewater sludge; but, the few points of moisture content reduction achieved can be sufficient to meet regulatory demands.

Some screw presses are designed to apply a great deal of friction to a material. The heat generated is sufficient to cook the moisture out of the cake. Low cake moisture is obtained; however, the electrical energy requirement is great. Stord uses this technique in pressing citrus waste.

Issue 108

New Citrus Feedmills

March 21, 2011

Two greenfield citrus feedmills have recently been commissioned in the Valencia area of eastern Spain.  Both turnkey projects were designed and constructed by FOMESA AGROINDUSTRIAL, a firm long based in Valencia.

ZUVAMESA with a 50 WHE was €8M, while CITROTECNO was €12M, each plus 18% VAT. The CITROTECNO feedmill with a 40 WHE was €7-7-1/2M; the rest was for the ethanol plant."

The CITROTECNO project is rated for 25-30 MTPH of citrus waste.  The waste comes from several citrus processors in the area as well as packing houses.  An ethanol plant at the site, using de-oiled and pasteurized press liquor, is entering the start-up stage.

Both local and national government entities have given financial support to the operation. 

The ZUVAMESA project is rated for 50 MTPH.  The waste comes from a juice extraction plant at the same site.  The company is owned by several hundred citrus growers in the area.  The local government has provided financial support.

The two feedmills are quite similar.  Both feature the use of a Waste Heat Evaporator (WHE) to achieve high thermal efficiency.  These WHE's are a three effect, vertical tube, falling fill design.  In the construction 304 stainless is used for vapor and water components while product contact is in 316L.  Durco pumps, imported from the States, were selected for their known reliability.

Single pass rotary drum dryers, standard of the citrus industry, are employed.  The CITROTECNO unit, rated at 30,000 pph of water evaporation, is paired with a 40,000 pph WHE.  At ZUVAMESA at 40,000 pph dryer is paired with a 50,000 pph WHE.

The dryers and WHE's were designed and constructed by FOMESA.

Both feedmills feature double pressing with Vincent screw presses with 24" diameter screws.  First pressing is done with a low torque Series KP press, while second pressing is done with a high torque Series VP press.  All presses use 50 hp motors.

The flexibility offered by double pressing has been advantageous where spoiled fruit is being processed.  This is quite common at CITROTECNO.

During start up a great deal of difficulty was encountered with the high viscosity and insoluble fiber content of the press liquor.  There is something different, possibly in the firmness, about this Mediterranean fruit as compared to fruit in Brazil and Florida.  As at FMC's Parmalat job in Sicily, it was necessary to add centrifuges to remove fiber from the press liquor.  Only after this was done was satisfactory WHE performance achieved. 

Both WHE's produce 45 Brix molasses on a consistent basis.  A 24 hour cleaning cycle is used.  An Alfa Laval centrifuge is used at ZUVAMESA while the unit at CITROTECNO is a GEA WESTFALIA.

The shredders feature hammers made of Duplex stainless steel, which is proving exceptionally durable.  The inlet housings are square, an innovation first introduced by Tegreene in Florida.  The blades are 180 degrees apart, and innovation introduced by CORENCO.  The discharge screens are exceptionally thick, 5/8", with square round holes.

The reaction conveyors are sized for 15 minutes reaction time, with 0.5% hydrated lime addition.  Inclined units are used to elevate the peel from the peel bin up to above the screw presses.  These use slow turning 24" diameter screw conveyors with half pitch flighting.

Pellet mills supplied by Van Aarsen are used at both feedmills.  These produce excellent pellets with a minimum of difficulty.

ADDENDUM: The CITROTECNO ethanol plant made 14,000 l of 90% ethanol the first season. They run on press liquor which is pasteurized at 90 C. They pump it to a flash tank to remove the d-limonene. When they had no press liquor, they kept the alcohol plant running by diluting molasses to 15-20 Bx."

Vincent Corporation is proud to have played an important role in the success of these feedmills.

PelletmillsPellet Mills


The photos and layout drawing show how Zuvamesa does first pressing with two Model KP-24 presses, and then second pressing with two VP-24's. After that the press cake goes to the dryer. In the photos the two KP-24's are on the right hand side, and the dryer is off to the left. You can see that they used screw conveyors to lift the cake up into a horizontal screw conveyor which runs from right to left. The peel comes into this conveyor on the right hand side. The peel can then fall into the two KP-24's, or it can by-pass to the two VP-24's, or it can by-pass all four presses and go to the dryer.

The cake from the two KP-24's is elevated in a screw conveyor which has a circular (cylindrical) trough; it is mounted at about 60 degrees. Similarly, the cake from the two VP-24's is elevated in a screw conveyor which has a circular (cylindrical) trough; it is mounted at about 60 degrees. The cake coming up in these two elevating conveyors drops into the long horizontal conveyor which runs from the far right to the dryer at the far left.

This is a very effective system of double pressing.



Issue 231



Orange Peel as Fuel

January 25, 1999
Rev. 2008

In September 1998 Vincent Corporation gave a presentation to a group of over five hundred citrus processors. The subject was using citrus waste as a boiler fuel.

It was years after this presentation that a fundamental observation was made. In order to burn biomass such as orange peel, the moisture content must first be reduced to zero. That is, as long as moisture is present, the temperature cannot rise above 212 F (at atmospheric pressure). Since combustion cannot occur until the mass reaches a much higher temperature, any burning system has an absolute requirement of first reducing the moisture content to 10%. If the system is frozen (on paper) at that point, it is easy to calculate if the mass now has more value as animal feed or as the fuel. Invariably, the feed value is greater than the fuel value.

There was interest in the paper for special reasons. The price of orange peel pellets, which are used as cattle feed, had been a very depressed $40/ton the previous season. Furthermore, dioxin had been found in Brazilian pellets, resulting in a European embargo. In the end the interest was academic because of the large capital requirements.

The price of pellets has currently (2008) gone to $170 per short ton, further reducing the financial justification of such a project.

On a dry solids basis, orange peel has a BTU content of 7,500 BTU/pound. This compares quite favorably with material like wood. It is enough energy value to satisfy several needs:

  1. Dry the wet peel sufficiently so that it will burn.
  2. Release energy to generate steam in a boiler.
  3. Generate enough steam to run the juice evaporators in an orange juice concentrate plant.
  4. Generate enough steam to produce electricity to run the citrus plant.

Several equipment alternatives were evaluated. A practical configuration recommended was to first use a screw press to remove moisture from the peel. Solids from high Brix citrus molasses could be diffused into the press cake until the moisture content was reduced to 60%. Press cake (peel) at this level of dryness will burn in either a fluid bed or stoke grate boiler. The high-pressure steam generated in the boiler would be used in a turbine to generate electricity. The low-pressure steam extracted from the steam turbine would be used both to (a) produce high Brix molasses from the press liquor and to (b) drive the TASTE evaporators that are used to produce orange juice concentrate.

The adoption of this system is attractive where fuel costs are high and the value of citrus pellet animal feed is low. Unfortunately, as is the case with all steam based electricity generators, the capital costs are extremely high. This high investment requires a long amortization period.

Copies of the paper that was presented are available from Vincent Corporation.

September 2010:  Alex Andreassen of Danisco has pointed out that  burning citrus pellets will lead to slagging of the boiler tubes because of the alkali-metals (Na, K) content of the peel.  Thus applications would be limited to using pellets as dryer fuel.

Issue 89

Pumped Citrus Peel

April 14, 1998

Last month Vincent presented a paper at the Citrus Engineering Conference of the ASME. This paper, Pumped Peel - Five Years Later, discusses advances in citrus peel processing technology.

In 1940 Dan Vincent was awarded a patent covering the mixing of hydrated lime with citrus peel. The lime attacks the peel and makes it possible to dewater it with a screw press. From that day forward the lime/peel chemical reaction was done in reaction conveyors.

It was Brazilian citrus companies that introduced a process whereby the orange peel is pumped and the reaction is completed in tanks. This has the advantages of using pipes and tanks instead of screw conveyors. This technology was the subject of a 1993 paper at the annual ASME conference.

Since then the Florida processors have leapfrogged the Brazilian technology. The greatest advances are attributable to Ralph Cook, best known as the inventor of the TASTE juice evaporator. Cook Machinery did two feedmills (both using Vincent presses!) improving on the pumped peel concept. The major changes were: (a) Better tramp metal separation; (b) Drastic improvement in the reaction tank; (c) The use of pre- presses ahead of the normal "hard" presses; (d) The use of spent caustic (a waste) as part of the liming system; and (e) Using molasses, with no press liquor, as a pumping medium.

In 1997 Vincent contracted to convert a Tropicana feedmill to our own pumped peel technology. The most notable feature is the elimination of the reaction tank: the reaction takes place in the mixing tank and in the pipeline. Another feature is direct (hard) piping the peel from the mixing tank directly to the presses.

It was found that progressive cavity pumps were suitable even when using a low ratio of molasses to peel. This eliminated the need for pre-presses at Tropicana.

The new systems represent a significant simplification of peel processing technology. Initial capital investment and operating maintenance costs have been reduced accordingly.

Issue 75

Pumped Peel

June 16, 1997

Traditionally screw conveyors have been used to convey citrus waste from the juice extraction building through the feedmill. The waste is mostly orange peel, and it is dehydrated and pelleted into animal feed in the feedmill.

About ten years ago Brazilian citrus juice concentrate producers developed a technology to replace the screw conveyors by pumping the peel from extraction to the feedmill. They used a ratio of one part peel to two parts press liquor to liquify the peel prior to pumping. Moyno progressive cavity pumps were used. (The press liquor is the liquid separated from the peel with screw presses.)

This technology was improved two years ago by the Cook Machinery Company. They designed and built a citrus feedmill (using Vincent VP-22 presses) that pumps the peel in the ratio of only one part peel with one part molasses. (The molasses are made from press liquor in a Cook evaporator.)

During the last month a Vincent designed and installed pumped peel system has undergone trials at a Tropicana feedmill. Here the peel is pumped in a ratio of two parts peel to one part molasses. A new design Geremia moyno pump was imported from Brazil to do the pumping.

The Tropicana project has proven remarkably successful. The mixing tank, pump, and pre-presses have performed with minor hitches under a wide range of adverse conditions. The 10" piping burst on occasions; however, the addition of a pressure relief by-pass has solved the problem.

Before peel can be pressed, it must be reacted with lime in order to break down the cell walls. At Tropicana this lime reaction is done in a Keller mixing tank and in the pipeline. The existing 4' diameter by 96' long reaction conveyor is bypassed.

Probably the most innovative feature of the Tropicana system is that the peel is hard piped (under pressure, without a vent) directly to the presses. Two pressing options are being tested. In one, the peel is pumped to a Model KP- 16 for pre-pressing the loose liquid from the peel. The cake from the KP-16 is then pressed again in a heavy duty Model VP-22.

The second option allows for the peel to be pumped directly into a Model VP-22. High capacity runs have been made with pressures up to 50 psi in the inlet hopper, producing press cake with 67% moisture content. The moisture content would have been even lower, but only 30º Brix molasses were available.

Both options work. The use of the KP pre-press appears to relieve problems when it is necessary to process bad (old, underreacted, or underlimed) peel. Further testing and evaluation will continue through June.

We hope to present a paper on this technology at next year's Citrus ASME meeting.

Issue 62

Small Scale Citrus Feedmill

Citrus plants around the world are facing the same problem. The problem faced by these juicing plants is the disposal of orange peel. They are finding increased costs associated with landfill or with finding farmers willing to take the wet peel for animal feed. This is occurring at the same time as increased environmental regulations are being applied.


These plants are very small compared to the large scale operations in Florida and Brasil. That is, they generate a few tons per hour of peel, generally running only eight hours per day, as compared to 1,000 tons per day at the larger plants.

Because of the vast difference in scale the smaller plants cannot justify the investment associated with a highly efficient, full blown feedmill.

There is one solution that was popular among Florida's smaller processors in the 1960's. It is a "beginners" feedmill plant. It minimizes the capital expenditure at the expense of requiring more energy.

To understand this "beginners" plant, it is important to first understand how a large plant operates. In the large scale plants, peel moisture is removed in three independent operations:

  1. Firstly the peel is pressed to separate it into press cake and press liquor. This is a very energy efficient operation as the press horsepower is relatively low.
  2. The press liquor is evaporated in a waste heat evaporator. This heat exchanger evaporates water out of the liquor and leaves behind dissolved solids in a solution commonly called molasses. This water removal process is very economical: it requires no fuel because its energy source is the waste heat in the exhaust gasses leaving the dryer. The molasseses produced are then added back to the press cake.
  3. The press cake, with the added-back molasses, is dried in a rotating drum dryer. This is the least efficient of the three water removal devices. It requires about 1,600 BTU in the form of fuel oil or natural gas per pound of water removed from the peel.

In the process just described citrus peel, which starts at about 82% moisture, is dried down to 10 to 12% moisture. (This means that for every 100 pounds peel entering the peel bin, about 20 pounds of finished animal feed will result). As a final step the dried peel is pelletized in order to reduce its bulk. This is done in order to minimize transportation and storage costs.

The previously mentioned "beginners" process that may be economical for smaller processors calls for using only a dryer to remove all the moisture. This eliminates the need for investment in a dewatering press and a waste heat evaporator. In such a "beginners" plant we also recommend not investing in a pelleting mill and its associated pellet cooler.

To dry peel in this manner will require approximately 85 U.S. gallons of heavy fuel oil per short ton of feed produced. This compares to a range of 30 to 45 in Florida citrus plants. Thus we can see that the "beginners" plant has its lower capital investment being offset by higher operating costs.

The dried peel produced in this process is an excellent, palatable animal feed for dairy or cattle. Because of its low moisture content, it can be stored for prolonged periods with minimal spoilage.

The enclosed diagram shows the equipment required for drying the peel in this manner. One notable item is the reaction conveyor. It is necessary to add lime to the peel prior to drying; this is done in the reaction conveyor. The lime attacks the cells and releases the moisture so that it can rapidly and efficiently be evaporated in the dryer.

A frequent query has to do with incorporating a press into the cycle. The difficulty that arises involves what to do with the press liquor. The authorities will not accept it in the sewer system, and it can be used for irrigation purposes only if it is mixed with a great deal of water. (In heavy applications it kills the soil where land spreading is performed.)

Sometimes alcohol producers will buy press liquor at 8 to 12 Brix. It ferments readily and makes an excellent citrus alcohol. But how many small citrus processors have a nearby distillery?

Generally the only practical thing to do with press liquor is to run it through a steam or waste heat evaporator. A waste heat evaporator system generally costs almost as much as all of the rest of the feedmill put together. This investment becomes difficult to justify until all alternative methods of disposing of citrus peel have been exhausted.



Vincent Drawing C-91310 shows a citrus peel drying plant based on a Model 150 Dryer. The plant will dry approximately 19,000 pounds per hour of 82% moisture citrus peel, without the benefit of pressing or the use of a waste heat evaporator. The plant will produce approximately 3,800 pounds per hour of citrus peel dried to 10 to 12% moisture content.

The principal items, in their sequence in the production cycle, are as follows:

  1. Peel Bin. This vertical front peel bin, with hydraulically operated doors, is of carbon steel construction. A caged ladder to the top and a catwalk across the top are included. The unit is prefabricated and knocked down for shipment, to be welded together at the job site.
  2. Peel Bin Discharge Conveyor. Vincent will supply an ultra heavy duty conveyor, including three-bolt drilling, Gatke hanger bearings and a variable speed electric drive. This conveyor is of stainless steel construction, with a metering orifice plate. The peel bin is constructed with sufficient elevation such that the Discharge Conveyor can feed directly into the Peel Shredder.
  3. Liming System. A Vincent VL-450 Hydrated Lime Proportioning System, mounted on a fabricated steel base, is included. The lime hopper, sized to hold one and a half bags of lime, is installed adjacent to the shredder. An auger from this hopper adds approximately 1/2% by weight of hydrated lime to the peel as it leaves the peel bin.
  4. Peel Shredder. A Vincent VS-180 Shredder is included. This horizontal rotor machine reduces the peel mostly to a range of 1/4" to 3/4", with a minimum of fine material. It is of the thin, rigid blade design, as contrasted to the hammer mill concept. All contact parts are of stainless steel. The blades, which are fixed, cut the material before it is discharged through the perforated screen. The shredder housing is hinged so as to allow ease of washing, inspection, and changing the screens. In operation the housing fits folded onto the chute that feeds the shredder, assuring a tight fit. The rotor turns in only one direction; however, the blades can be reversed to give double life.
  5. Reaction Conveyor. Shredded peel drops directly into a slightly inclined Reaction Conveyor. This conveyor is sized to allow approximately 10 to 12 minutes dwell time. It is of carbon steel construction and features a notched blade screw. The chemical reaction between the lime and the peel that occurs in this conveyor is required in order to break down the cell structure of the peel so that moisture can be better removed in the drying operation.
  6. Elevating Conveyor. Limed peel from the Reaction Conveyor is elevated to the Dryer Feeder by this stainless steel conveyor. Also, recycled material from the second pass of the dryer is mixed with the limed peel in this conveyor. The design is such that, if required, material can be dropped back into the inlet of the reaction conveyor.
  7. Dryer Feeder. This screw conveyor receives peel from the Elevating Conveyor and feeds it into the Dryer. This is a stainless steel screw feeder fitted with a companion flange matched to the dryer throat. It has a variable speed drive to control the process feed rate.
  8. Burner. The burner will require up to 150 U.S. gallons per hour of fuel oil. The burner package is suppled with dampers and valves and a steam heater for the oil. The burner is capable of burning optional lighter fuel oils, or natural gas. It comes with the required combustion air blower.
  9. Furnace. Vincent will supply a Model VF-150 refractory lined furnace consisting of a carbon steel shell mounted on a fabricated steel base. It is designed to receive and mix recirculated exhaust gasses from the Dryer discharge in order to control the gas temperature entering the Dryer. The firebrick lining is supplied and installed by the customer.
  10. Return Elbow. There is a 180º Elbow between the Furnace and the Dryer so as to minimize the possibility of overheating peel in the Dryer.
  11. Controls. Vincent supplies a solid state programmable controller that modulates combustion and monitors the flame. Control of the combustion rate is through a sensor mounted at the inlet to the third pass of the dryer. This is required for precise control of product quality.
  12. Model 150 Dryer. This is a Vincent triple pass dehydration unit with an insulated, stationary outer drum. The unit includes recycle conveyors so that partially dried material can be extracted at the end of the second pass and mixed with the incoming material. This is especially important for the proper drying of unpressed citrus peel. The drum is carried on machined steel tires, mounted on an expansion type steel base with cast steel machined trunnions. The rotor is driven by a chain and sprocket system with a speed reducer. The base frame can be bolted to a 6" concrete slab without any special foundations; this saves installation costs.
  13. Separation System. The Vincent low level entry cyclone separator, ductwork, dampers, and stack are supplied. This system assures gentle handling of the dried peel. A motorized air lock and carbon steel screw conveyor are used to move the peel from the separation chamber to the cooling reel and bagger. A radial blade exhaust fan with its drive are also included.
  14. Cooling Reel. The Vincent Model 525 Cooling Reel gently cools the dried peel with an action similar to that of a large diameter clothes drier. Ambient air is drawn counter- current through the unit to achieve an evaporative cooling effect. Cooling is required in order to prevent the phenomena of re-heating of the stored peel. In the cooling process, evaporation results in a further reduction of moisture content of about 1%. The unit comes complete with a fan, dust collector, duct work, supports, and electric motors.
  15. Bagging System. This system includes an surge hopper and a semi-automatic weighing and bagging unit. This unit consists of a hold/weighing bin with an adjustable discharge, a weight indicator, a bag holder, and a compact Fischbein sewing head. The take-away belt, belt trays and motor and drive are included.



Spent Pectin Peel

February 17, 2016

Over a period of fifteen years Vincent participated in tests with spent pectin peel at four citrus pectin production facilities. The goal was to reduce the moisture content of the residue (spent pectin peel) which remains after pectin is extracted using the normal acid and alcohol precipitation process. Until 2013 none of these tests showed much promise.

Trials have shown that successful dewatering of spent pectin peel with a screw press requires two elements: (1) press aid (cellulose fiber) must be blended into the material, and (2) hydrated lime [calcium hydroxide, Ca(OH)2] must be mixed into the spent peel for a chemical reaction. Meeting these requirements involves quite a bit of equipment besides a screw press.

Alternatively, a belt press can be used to dewater spent pectin peel. Starting with 10% to 12% solids, this press removes free water, and a cake with about 16% solids can be produced.

In contrast, with the proper mix of lime and press aid, a screw press can increase the solids into the range of 30% to even 40%.

The advantage of dewatering the spent peel is that it becomes a saleable animal feed. Since large amounts of water no longer need to be transported, the geographic range in which the material can be sold is greatly expanded. One pectin producer has done very well by assigning a trade name to their spent pectin peel, giving it market identification.

An important consideration is that a large amount of wastewater is produced when the spent peel is dewatered. Since the flow of press liquor can be four times greater than that of the press cake, most of the dissolved solids go out with the press liquor. Although it can vary quite a bit, typically the press liquor has 5% solids, which translates into high BOD effluent. To overcome this, some pectin producers are considering the use of multiple effect evaporators to concentrate their wastewater into a molasses.

It is notable that, because of the dissolved solids, the solids capture rate in the press cake is only about 70% of the total solids entering the screw press. The rest go out with the press liquor.

Typically from 1% to 2.5% hydrated lime is added to the spent pectin peel. The lime is mixed with about ten parts water prior to blending with the spent peel.

We have tested a variety of press aids. By far the most successful is ground wood, sold commercially in both Europe and America. Suppliers include SCM in Sweden, International Fiber Corp. in North Tonawanda, NY, and Mat, Inc. in Floodwood, MN. The primary market for these press aids are factories engaged in the production of fruit juices such as apple juice. Typically 2.0% to 2.5% press aid works with spent pectin peel.

The main piece of equipment needed to blend press aid with spent pectin peel is a mixing tank commonly known as a hydrapulper. Bales of press aid are mixed with liquid in the hydrapulper. This fiber is pumped to be blended with the spent peel and the hydrated lime. This mixing can be done in a blender such as a Lodige or Khal. More economically, the mixing can be performed in a mixing paddle conveyor (known as a reaction conveyor in the citrus industry). The mixed and chemically reacted material is what is fed into the screw press.


Recently we tested dewatering the spent material after the pectin has been removed from apple pomace. We found that there was an excellent reaction with hydrated lime. The addition of press aid was not required. Once reacted with the lime, the material shows great promise in being dewatered with a screw press.

Steam Injection

July 5, 2002                                                                                                                                                                                                         ISSUE #129

Up until the Oil Embargo of the early 1970's it was common practice to inject steam into screw presses. The steam was injected directly, through hollow resistor teeth, into the material being pressed. This was done to reduce the moisture content of orange peel that was being made into cattle feed. The practice seems to have been abandoned for two reasons: the high cost of steam energy, and the fact that most citrus feedmills lacked the Waste Heat Evaporator capacity to dispose of all the press liquor being produced.


Driven by the need to reduce VOC (Volatile Organic Compound) emissions, interest has resumed in pressing as much liquid as possible from citrus waste. One solution has been to use high pressure, high horsepower screw presses. To evaluate an alternative, Vincent Corporation acquired a boiler and conducted tests with live steam injection.

Initially we were cool to the idea of using steam. We reasoned that it would be more efficient to evaporate moisture with a direct fired rotary drum dryer. However a California research firm, Altex Technologies, brought to our attention that steam addition in a press will drive out liquid water, whereas a drum dryer removes the moisture in the form of water vapor.

The difference is very important: by pressing out liquid water, there is a savings of about 1,000 BTU's per pound of water. This is because the drum dryer requires this energy to convert the water from the liquid to vapor state.

To prove the concept, a basic test was conducted. Drums of peel were brought to the Tampa works from two local feedmills. This peel had been reacted with lime and single pressed. The peel was collected at the inlets to rotary drum dryers. The samples from both feedmills had a respectable 18o Brix. However the moisture contents of both samples were high, 71% to 73%, because of special conditions existing when the peel was collected.

This cake was second pressed in a laboratory screw press in Tampa. Probably because of a delay of a few hours that occurred after the samples were collected, the resulting press cake was reduced (consistently) to about 59% moisture. Part of this moisture reduction was due to using the press electrical drive to heat the peel: cake temperature increased by 10° F to 15° F while passing though the press.

Addition of steam, at various flow rates and at pressures up to 45 psi, significantly improved the press cake moisture that could be achieved. Final moisture contents of 55% to 57% resulted. In the process the temperature of the discharge cake and liquor was increased to about 160° F.

Most importantly, the extra moisture separated by the screw press came out as a liquid, not as vapor. Therefore the old steam injection technology appears worthy of further investigation. We hope to participate in full scale testing during the next citrus season.



Twin Screw Press

October 24, 2000                                                                                                                                                                                                 ISSUE #110

Following field tests during the last six months, Vincent is introducing the Twin Screws Press. Designated the Series TSP, five models are being offered.

These presses have very positive feeding characteristics, making them well suited for slippery materials. The strong pressing action of overlapping screws, plus the use of seven stages of compression, result in the best dewatering action we have ever seen in a Vincent screw press.

The Twin Screw Press is well suited to many traditional markets: citrus peel, fruit and vegetable juicing, fish waste, spent brewers' grain, shrimp shells, and alfalfa. We do not anticipate application in the pulp and paper industry, but we think that sugar beet pulp should work well.

There are three principal advantages to the Vincent design:
Automatic Control. The use of an air cylinder operated discharge cone allows the press to work well with changing feed material conditions and throughput capacities. The press has above average turn-down characteristics.

Low Horsepower. To improve dewatering, material is sliced in the press. This is a characteristic of the interrupted screw with fixed resistor teeth that can be seen in the brochure photo. The press requires less horsepower than a continuous screw press where the action is more like mashing than slicing. Also, the result of this particle size reduction is that, if the press cake goes to a dryer, better dryer performance is achieved.

OEM Gearbox. We have carefully selected major OEM gearboxes for use with our press. A choice of Sumitomo and Foote- Jones/Illinois Gear drives are available.

Overall, Vincent is delighted with the Twin Screw Press. It marks a significant advance in screw press design because the pressing performance is equal to or better than anything achieved in the past. In financial terms, it is possible to guarantee a machine with double the capacity of a single screw press, but at less cost than two single screw presses of the same diameter.