Principals of extraction
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Principles of Oil Extraction
The extraction of oil from an oil bearing commodity involves some or all of the steps shown in the flow diagram shown overleaf, depending on the particular commodity to be processed, the scale of operation and the technical options available to the processor.
The first stage of the process sometimes requires the separation of the oil bearing part of the plant be it a nut, fruit or seed. The process is generally referred to as decortication or dehulling. Seeds with thin testa similar to rapeseed or sesame seed can be processed without decortication.
In most cases the oil bearing material is then broken into smaller pieces by pounding in a pestle and mortar or by manual or motorised grinding. When motorised oil expellers are used the decorticated or undecorticated material can often be fed directly to the expeller.
The next step involves heating the oil bearing material, sometimes with the addition of a little water to assist in the rupture of oil bearing cells and in the liberation of oil. At small scale this is carried out in a pan over a fire but mechanised heaters or kettles are available and used in commercial plants small scale heater-Cecoco.
Figure 1 - Small Scale Heater (scorcher) Cecoco
The oil bearing material is now ready for oil extraction. Five basic technical options exist; hot water floatation, ghanis, manual presses, powered expellers and solvent extraction.
Traditional presses are now increasingly being replaced by better engineered, more efficient mechanical presses. Many different types of mechanical press are in use but they fall into two basic types, plate presses and ram presses. In the first type a plate or piston is forced into a perforated cylinder containing the oil bearing material by means of a worm. In some cases hydraulic jacks have been used, care is needed to make sure there is no leakage of hydraulic fluid that might contaminate the edible oil. In a ram press a piston forces the oilseed forward in a perforated cage fitted with an adjustable choke at the outlet, which controls the pressure. Ram presses provide a greater shearing action than simple screw presses and have been found to be considerably more efficient for some raw materials. It is important that when selecting a particular type of press its suitability for the raw material to be processed is confirmed.
Figure 4 -Plate Press
Figure 5 - Ram Press
To obtain greater throughputs and extraction efficiencies it becomes necessary to use powered devices and the oil expeller is the most common of these. A whole range of expellers are available with capacities ranging from a few kg/hr up to tons/hr. They all work on the same basic principle. The raw material, which may have been previously heated to aid in the release of oil, is fed continuously to the expeller where it is fed by the wormshaft into a horizontal cylinder. A controllable pressure is built up in the cylinder by means of an adjustable choke at the cylinder exit. The internal pressure ruptures oil cells in the material and oil flows out through perforations in the cylinder cage. Some care has to be taken when selecting an expeller for a particular commodity. Many Have been designed for particular applications, in terms of internal pressure, amount of shearing action etc. Certain types tend to be more adjustable and hence less product specific than others.
Small scale expellers c 40 kg/hr
Figure 6 - Cecoco
Figure 7 - Mini 40
None of the above extraction systems are able to remove all of the oil from a material. In most small scale rural situations this is of little or no importance as the cake, that remains after the oil has been removed, finds uses in local dishes, in the manufacture of secondary products or for animal feed. Some raw materials however do not release oil by simple expelling; the most notable being rice bran. In order to remove oil from commodities that do not respond to expelling or to extract the final traces of oil after expelling it is necessary to use solvent extraction. Solvent extraction is a high technology process that has to be carried out at comparatively large scale. Capital costs are high. Essentially the process is one of continuous countercurrent extraction with the raw material flowing in one direction against a solvent; usually hexane. After oil extraction the solvent passes to a recovery plant where the solvent is stripped off under vacuum. The crude oil then passes on for refining. Due to the large scale involved it would seem unlikely that solvent extraction would find much application in minor oil product processing.
The last stage in the processing of an oil is refining which includes some or all of the following treatments:- filtering, neutralisation, winterising, bleaching, deodorisation and degumming and filtering. In many cases refining is not a necessary stage in traditional processing systems as local palates are accustomed to the flavour of unrefined oils and in many parts of world these flavours are in fact preferred to the blandness of a fully refined oil. Many crude oils contain free fatty acids (FFA) which impart unpleasant odours and flavours. The FFA's are neutralised by treating the oil with a carefully controlled quantity of caustic soda solution. In larger oil refineries the caustic soda washes are sold on for soap manufacture and are known as soap stock.
Winterising involves allowing the oil to stand at low temperatures, during which time higher melting glycerides crystallise and are separated by filtration.
Some oils are rather dark in colour and are bleached by the addition of a small amount of bleaching earth or activated carbon prior to filtration. Many commercial plants in fact bleach crude oil as routine and then add a controlled amount of colour in order to produce a standard final product.
De-odourising involves sparging steam through the oil, usually under vacuum. The steam removes volatile odours, for example the coconut smell of coconut oil.
In some cases it is also necessary to treat oils with small amounts of water in order to remove gums and mucilages that are released along with oil as the plant cells rupture during extraction. These gums mainly consist of phospholipids. The above treatment, with heat, causes the gums to flocculate after which they may be removed by centrifugation or settling.
Oils produced at small scale in rural areas are rarely given all but a rudimentary level of refining, usually filtration. However clarity can be improved by filtration through cloth packed with fine sand or charcoal.
Vegetable food oils are of great importance in the diet, they are a concentrated form of energy vital to people all over the world. In developing countries poor people rarely have access to or can afford cooking oils from larger refineries and still rely on minor oil crops to meet their needs. In some cases viable and important local industry and trading is involved.
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