Hard screw pressing

Fig. 11.1. Depiction of hard screw pressing (provided by Anderson international, Cleveland, OH).

Fig. 11.7. Photograph of the interior of a hard-screw-press plant operating on soybeans.

In prepressing, the screw press is choked so that less pressure is developed than in hard screw pressing therefore, less press oil is recovered, and the capacity of the screw press is increased. The oil content of prepress cake is typically 14—16%, and the partially de-oiled cake is then extracted with solvent. The cake may be broken into pieces and even flaked to decrease particle size, to increase bulk density and extractor capacity, and to speed solvent extraction. The remaining steps are the same as for... 

Quality Trait Hard Screw Pressing Extruding- expelling Gas-assisted Screw Pressing Solvent Extracting ... 

Continuous screw presses are used (1) for extracting fats and oils in small operations where investment capital or supplies of raw materials are limited and installation of a solvent extraction plant is impractical (2) to partially defat high-oil content seeds for easier handling in subsequent solvent extraction or hard pressing and (3) for extraction of animal flesh and bones, fish, and fleshy-type oilseeds such as palm fruit, olives, and copra (dried coconut meat ), and oilseeds. These machines have been generically referred to as expellers, but the Expeller trademark belongs to Anderson International Corporation, Cleveland, OH, successor to the company founded by Valerius D. Anderson who patented the first continuous screw press in 1899. 

Some oilseeds are normally prepressed in screw-presses to mpture the oil cells and partially reduce the oil level. The cake from the screw-press is then cooled and either cmmbled and flaked or sent directly to a solvent extractor. The hot, tacky press-cake can also be formed into porous collets by extmsion. Enough steam is injected into the expander to raise the moisture 2-A percentage points. This allows the expander to transform the cake into porous collets. This must be done on stream, before the cake has had a chance to cool and harden. Once cool and hard, protein denatures, and the cake can no longer be transformed into a tacky, inflatable condition. 

Some oilseeds are mechanically screw-pressed to remove approximately half the oil and then solvent extracted to remove the rest. Expanders can transform the press-cake into porous collets. Steam injected into the expander raises cake moisture by 2 %. This must be done before the cake has cooled and hardened. Once cool and hard, the denatured protein in the cake can no longer be transformed into a gelatinous, inflatable condition. If oilseeds of high oil content are pre-pressed to 15-30% oil, they extmde similarly to flaked soybean (at 18% oil) and flaked cottonseed meats (at 30-33% oil). 

Figure 13. Laboratory sample (single firings coarse-grained quartz, 0.3% Na20, screw press, 1000 °C 27 h) coarse texture, continuous, hardness 2 Mohs, dark blue. Thefollounng phases can be identified copper oxide (white spots) in calcium silicate matrix (medium gray), Egyptian Blue (white/light gray), and quartz ( rk gray).

Fig. 8.10. Anderson Expeller Press, model 55" TMSD. Note elevated conditioner carrying product from left to right across top of press vertical screw press section (right side) second (higher pressure) screw press section carrying product from right to left. Other brands of hard presses have both stages on one shaft. By permission of Anderson International Corp., Cleveland, OH).

The A-A extruders are equipped with a flat perforated plate or screen, combined with two flat extrusion heads. The wet mass is pressed axially through the screen parallel to the feed screws. The extrusion forces are the highest, compared to other low pressure extruders, resulting in a hard and dense extrudate. It is commonly used in the extrusion of food, thermoplastics, and other industries, where a large pellet diameter and thermosetting properties are desired. 

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