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.