Spent Grain

July 17, 1996
Rev. June 2006

Firms that produce alcoholic beverages from grain are an important market for dewatering screw presses. The industry firms consist mostly of beer brewers and distilleries. Their raw materials are barley, corn, wheat, rice, and other grains. They all produce a common by-product: spent grain.

Spent grain is the name given to material left after the grain is fermented and the alcoholic solution is drawn off. It is used as a cattle feed because of its protein content. The value to the farmer is not great, so smaller breweries are pleased to give the material away for free if the farmer will truck it.

Spent grain is normally wet, with 80% to 85% moisture content. In this state it is heavy to haul, and it is likely to drip while being transported. Furthermore, it will go sour in a few days. Therefore there is a need to dewater the spent grain if the brewer is of any significant size.

The practice is to run spent grain through a press. Typically a screw press will reduce the moisture content into the range of 64% to 70%. At this point the weight will have been reduced by a third to a half, and the "shelf life" will have been extended.

For many years it was common practice to use a rotating drum dryer to reduce the moisture content on down to 10%. In this condition the spent grain could be stored and marketed as a commodity. With the increase in energy costs, the value of the product has, by and large, not justified the final drying step.

In the past Vincent has supplied screw presses to Coors in Colorado, a brewer in Costa Rica, and a sake winery in Taiwan. Also, a distiller in Kentucky has used Vincent presses that he swears by.

This hit-or-miss picture ended in 1995 when Anheuser-Busch placed a major order with us to replace the spent grain presses in their St. Louis facility. This contract was awarded after extensive testing here in Tampa. We have an engineering video available that shows some of the trials.

Our main competitor for the contract was Stord, who has a number of their double screw machines in use in breweries. These are extremely heavy duty and press very tight. However they represented overkill and were not competitive.

At Anheuser-Busch our presses were selected to replace a group of very old Davenport V presses. These come in two sizes, 3' and 5'. They consist of two immense cast iron cone faced wheels. The cones are mounted so that they turn with a big gap at one side and a small gap at the other. Liquid drains through screening mounted on the faces of the cones, and it comes out at the bottom. Once the operator gets a plug established, the V press operates with intense pressure. The design is rarely sold today because (a) it is of limited throughput capacity, (b) it is expensive, and (c) it is even more expensive to maintain.

Initially it was thought that the use of a tall headbox above the inlet to the Vincent screw press would be valuable. This feature creates a hydraulic head at the inlet of the press. In practice, it appears that this pressure can plaster solids against the screen, causing blinding that reduces dewatering capacity.

One problem encountered was that, during idle periods, starch dries on the outside of the screens. This was solved with the use of spray washers.

The rotating cone option was not used, and the horsepower drawn by the presses is relatively modest. Nevertheless the pressing results have been excellent, especially at low screw rpm.

We now (1996) have in production a pair of CP-10 presses that have been sold to yet another rice winery in Taiwan. The rice is used to produce sake, and the presses will be used to dewater the rice mash.

More recently, the trend has been to offer Series KP presses for spent grain. These do not remove quite as much moisture as the Series CP and VP presses. However, if the grain is not being dried in a dryer, this does not matter. The KP presses are the most economical presses available.

Issue 46