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The expeller (screw press) process

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The expeller (screw press) process

1 Operation principles

Continuous pressing by means of expellers (alsoknown as screw presses) is a widely applied process for theextraction of oil from oilseeds and nuts . It replaces the historical methodfor the batchwise extraction of oil by mechanical or hydraulic pressing. Theexpeller (Fig. 1) consists of a screw (or worm),rotating inside a cylindrical cage (barrel). The material to be pressedis fed between the screw and the barrel and propelled by the rotating screw ina direction parallel to the axis. The configuration of the screw and its shaftis such that the material is progressively compressed as it moves on, towardsthe discharge end of the cylinder. The compression effect can be achieved, forexample, by decreasing the clearance between the screw shaft and the cage (progressive or step-wise increase of the shaft diameter ) or by reducing thelength of the screw flight in the direction of the axial movement The graduallyincreasing pressure releases the oil which flows out of the press through theslots provided on the periphery of the barrel, while the press-cake continuesto move in the direction of the shaft, towards a discharge gate installed atthe other extremity of the machine.

Oil expeller
Fig1: Oil expeller (screw press) with cooker-dryer

Before entering the expeller, theoilseeds must be cleaned, dehulled (optional), flaked, cooked and dried.Flaking facilitates oil release in the press by decreasing the distance thatthe oil will have to travel to reach the particle surface. Cooking in thepresence of moisture is essential for the denaturation of the proteins and, tosome degree, for the coalescence of the oil droplets. Cooking plasticizes theflakes, renders them less brittle and thus reduces the extent of flakedisintegration as a result of shear in the press. Extensive flake disintegrationwould reduce oil yield and produce a crude oil with a high content of finesolid particles (foots). After cooking, excess moisture is removed in order toavoid the formation of muddy emulsions in the press. Cooking is usuallyachieved by mixing the flakes with live steam. Additional heat may be providedby indirect steam, while thoroughly mixing the mass.

2 Advantages anddisadvantages of the expeller process

Expellers can be used with almost any kindof oilseeds and nuts. Therefore, in a multi-purpose plant built to processdifferent types of raw materials and not only soybeans, the expeller processmay prove advantageous. The process is relatively simple and notcapital-intensive. While the smallest solvent extraction plant would have aprocessing capacity of 100-200 tons per day, expellers are available for muchsmaller capacities, from a few tons per day and up.

The main disadvantage of the screw-pressprocess is its relatively low yield of oil recovery. Even the most powerfulpresses cannot reduce the level of residual oil in the press-cake below 3 to5%. In the case of oil-rich seeds such as sesame or peanuts this may still beacceptable. Furthermore, most of the oil left in the cake can be recovered by astage of solvent extraction. Such two stage processes (pre-press/solventextraction) are now widely applied . In the case of soybeans, however, a 5%residual oil level in the cake represents an oil loss of about 25%. Solventextraction of the cake would not be economical, because of the bulk of materialwhich must be processed. Pre-press/solvent extraction processes are, therefore,not applied to soybeans.

In the case of soybeans, the commercialvalue of the meal is usually higher than the income from sales of thecorresponding quantity of oil. The quality of the meal is therefore a factor ofparticular importance in the selection of a processing method for soybeans. Inthis respect, the expeller process has several disadvantages. The first is thepoor storage stability of the press-cake, due to its high oil content.Furthermore,the extreme temperatures prevailing in the expeller may impair thenutritive value of the meal protein, mainly by reducing the biologicalavailability of the amino acid lysine. At any rate, expeller press-cake is notsuitable for applications requiring a meal with high protein solubility.

3 Equipment

Unlike solvent extraction equipment whichis supplied by a relatively small number of manufacturers, screw presses with awidely varying degree of sophistication are available from a multitude ofsources. Yet, considerable technical improvement and advanced features can befound in the models offered by the leading manufacturers. Such featuresinclude: multi-stage pressing to increase oil yield, better feed rate control,water cooled barrel and shaft, ease of maintenance and repair, improvements inthe drive and transmission, sanitary construction, safety features etc. Mostpress manufacturers also supply cooker-dryer units, designed to operate withthe press. Cooker-dryers may be horizontal (jacketed screw conveyor type), butthe most common types consist of vertical stacks of round chambers (rings)equipped with paddle stirrers. (Fig. 1)

This design is indeed very common inoperations for heat treating oilseed material and will be encountered in flakeconditioners, desolventizers, meal dryers and coolers.

3.1 COOKER–Rotary Dryer


The Anderson 72-Tube Rotary Steam Dryerdelivers faster drying at lower cost.  Extremely popular due to low airrequirement.  It is the most efficient equipment for accurately adjustingthe moisture content of poultry feathers, coagulated blood, grains, chemicals,minerals and other non-sticky granular materials.

Unique steam tube bundle arrangementallows expansion and contraction and reduced danger of leaks or strains. Electrical H.P. 5. Approximate shipping weight with motor drive – Domestic12,640 lbs.  Export 14,940 lbs (6791 Kgs) Cubic feet 440.

3.2 SCREW PRESS—Henan Huatai Achiever

Henan Huatai Achiever screw presses offeroutstanding features and reliability for full pressing to produce full presscake and crude oil from a wide variety of oil bearing seeds.  Available inmultiple sizes, depending on your processing capacity, the Achiever pressyields high quality oil and cake with residual oils among the lowest in the industrywhen using the single pressing process. The Achiever screw presses canalso be supplied for prepressing oilseeds to produce prepress cake prior tosolvent extraction.

Henan Huatai oil extracting screw presses haveseveral features which make the screw presses superior to others, including:

·huatai is the only company with a screw press designedto water-cool its main cages in addition to cooling the mainshaft.  When full pressing, this benefit reduces the temperature rise,reducing heat degradation of the press cake and crude oil, giving ourcustomers a lighter color cake and higher quality oil.

·Presses can have a life span over 50years when properly maintained.

·The Achiever is available with either a motorized conemechanism at the press discharge to allow full control of backpressure adjustmentwhile producing press cake or a die plate to produce porous pellets prior tothe solvent extraction process.

·The screw presses include an efficient force feederassembly to expel air from the feed stream and insure100% feed tothe feed hopper and main cage. The force feeder includes a vertical fixed speedscrew conveyor and a variable speed horizontal feed conveyorto convey the material from the conditioner discharge to the press forcefeeder.

·Hardcoated shaft parts and screen bars last twoto three times longer than case-hardened parts.

·Screw Presses include a parallel shaft helical gearreducer. Reducer has splash lubrication, a service factor of 1.75 or greater.

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