Mechanical extraction processes are suitable for both small- and large- capacity operations. The three basic steps in these processes are (a) kernel pre-treatment, (b) screw-pressing, and (c) oil clarification.
Diagram 2: Mechanical extraction of palm kernel oil.
Line (A) is for direct screw-pressing without kernel pre-treatment; Line (B) is for partial kernel pre-treatment followed by screw-pressing; and Line C is for complete pre-treatment followed by screw-pressing.
Proper kernel pre-treatment is necessary to efficiently extract the oil from the kernels. The feed kernels must first be cleaned of foreign materials that may cause damage to the screw-presses, increasing maintenance costs and down time, and contamination of products. Magnetic separators commonly are installed to remove metal debris, while vibrating screens are used to sieve sand, stones or other undesirable materials.
A swinging hammer grinder, breaker rolls or a combination of both then breaks the kernels into small fragments. This process increases the surface area of the kernels, thus facilitating flaking. The kernel fragments subsequently are subjected to flaking in a roller mill. A large roller mill can consist of up to five rollers mounted vertically above one another, each revolving at 200-300 rpm. The thickness of kernel cakes is progressively reduced as it travels from the top roller to the bottom. This progressive rolling initiates rupturing of cell walls. The flakes that leave the bottom nip are from 0.25 to 0.4 mm thick.
The kernel flakes are then conveyed to a stack cooker for steam conditioning, the purpose of which is to:
· adjust the moisture content of the meal to an optimum level;
· rupture cell walls (initiated by rolling);
· reduce viscosity of oil;
· coagulate the protein in the meal to facilitate separation of the oil from protein materials.
The meal flows from the top compartment down to the fifth compartment in series. At each stage a mechanical stirrer agitates the meal. Steam trays heat the cookers, and live steam may be injected into each compartment when necessary. The important variables are temperature, retention time and moisture content. In the palm kernel, the meals are normally cooked to a moisture content of 3 percent at 104-110°C.
The properly cooked meal is then fed to the screw-press, which consists of an interrupted helical thread (worm) which revolves within a stationary perforated cylinder called the cage or barrel. The meal is forced through the barrel by the action of the revolving worms. The volume axially displaced by the worm diminishes from the feeding end to the discharge end, thus compressing the meal as it passes through the barrel.
The expelled oil drains through the perforation of the lining bars of the barrel, while the de-oiled cake is discharged through an annular orifice. In order to prevent extreme temperatures that could damage the oil and cake quality, the worm-shaft is always cooled with circulating water while the barrel is cooled externally by recycling some cooled oil.
The expelled oil invariably contains a certain quantity of fines and foots that need to be removed. The oil from the presses is drained to a reservoir. It is then either pumped to a decanter or revolving coarse screen to remove a large part of the solid impurities. The oil is then pumped to a filter press to remove the remaining solids and fines in order to produce clear oil prior to storage. The cakes discharged from the presses are conveyed for bagging or bulk storage.
As can be seen from Diagram 2, not all crushers use the same procedure for mechanical extraction of kernel oil. There are three variations: direct screw-pressing, partial pre-treatment, and complete pre-treatment.
Some mills crush the kernels directly in the presses without any pre-treatment. Double pressing usually is required to ensure efficient oil extraction. The screw-presses used normally are less than 10 tonnes per unit per day.
The kernels are first broken down to smaller fragments by grinding prior to screw-pressing. In some cases, cooking is also carried out.
The full pre-treatment processes described earlier are carried out prior to screw-pressing. Plants with larger capacities (50-500 tonnes per day) choose complete pre-treatment and the equipment is usually imported from Europe. FATECO and Faith Engineering now offer the complete line for small-scale operators.
Solvent extraction processes can be divided into three main unit operations: kernel pre-treatment, oil extraction, and solvent recovery from the oil and meal. For the purposes of small-scale operations it is sufficient to mention the solvent extraction process is an alternative for high capacity mills. However the process is not recommended for small enterprises.
Palm kernel extraction is a specialised operation undertaken by a completely different set of processors. They are usually better organized as a group and are not as dispersed as palm oil processors. The kernel processors have to go around the palm oil processors during the peak season, when prices are lowest, to purchase the nuts for drying. The nut processing and oil extraction is undertaken in the dry season when the pressure to obtain raw materials has subsided.
The traditional palm oil processing starts with the shelling of the palm nuts. The shelling used to be performed using two stones to crack each nut and separating the kernel and shell simultaneously. This manual operation has been largely superseded by the use of nut-cracking stations.
The mechanical nut-crackers deliver a mixture of kernels and shells that must be separated. The kernel/shell separation is usually performed in a clay-bath, which is a concentrated viscous mixture of clay and water. The density of the clay-bath is such that the shells sink while the lighter kernels float to the top of the mixture. The floating kernels are scooped in baskets, washed with clean water and dried. Periodically, the shells are scooped out of the bath and discarded.
The traditional oil extraction method is to fry palm kernels in old oil or simply heat the dried nuts. The fried kernels are then pounded or ground to a paste in a motorised grinder. The paste is mixed with a small quantity of water and heated to release the palm kernel oil. The released oil is periodically skimmed from the top.
Today, there are stations in villages that will accept well-dried kernels for direct extraction of the oil in mechanised, motorised expellers. (Fig. 20, 21)
Fig. 20 Whole palm kernel expeller (CAMEMEC, Benin)
Fig. 21 Palm kernel expeller (O.P.C., Cameroon)