Nowadays, there are about 20 countries in the world producing palm oil. And the main producing countries are Malaysia, Indonesia, and Nigeria. The output of these three countries accounts for 88% of the world’s total palm oil production. And since 2004, the total amount of palm oil and palm kernel oil has been more than soybean oil, the world’s major edible oil, for the first time. While there exists a great difference between palm oil and palm kernel oil, here we would talk about the palm kernel oil extraction.
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;
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 oil press, 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 break 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, revolving at 200-300 rpm. The thickness of kernel cakes is progressively reduced as it travels from the top roller to the bottom. 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 separate the oil from protein materials.
The important factors 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 oil press, which consists of a worm that 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 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 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 choose complete pre-treatment and the equipment is usually imported from manufacturers who 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. The solvent extraction process is an alternative for high capacity mills. However, the process is not recommended for small enterprises.
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