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Oil winning process in general, involves, the reception of fresh fruit bunches from the plantations, sterilizing and threshing of the bunches to free the palm fruit, mashing the fruit and pressing out the crude palm oil. The crude oil is further treated to purify and dry it to storage and export.
The general flow diagram of the processing:
PALM OIL PROCESSING UNIT OPERATIONS
Fresh fruit arrives from the field as bunches or loose fruit. The fresh fruit is normally emptied into wooden boxes suitable for weighing on a scale so that quantities of fruit arriving at the processing site may be checked. Large installations use weighbridges to weigh materials in trucks.
The quality standard achieved is initially dependent on the quality of bunches arriving at the mill. The mill cannot improve upon this quality but can prevent or minimise further deterioration.
The field factors that affect the composition and final quality of palm oil are genetic, age of the tree, agronomic, environmental, harvesting technique, handling and transport. Many of these factors are beyond the control of a small-scale processor. Perhaps some control may be exercised over harvesting technique as well as post-harvest transport and handling.
Threshing ( removal of fruit from the bunches )
The fresh fruit bunches consist of fruit embedded in spikelet growing on a main stem. There are two ways to do threshing. Firstly, it can be done manually by cutting the fruit laden spikelet from the bunch stem with an axe or machete and then separating the fruits from the spikelet by hand.
Secondly, threshing can be done mechanically by mechanised system which rotating a drum or fixed drum equipped with rotary beater bar detaches the fruit from the brunch, leaving the spikelet on the stem.
Fig. 3 Bunch thresher (Centre de Formation Technique Steinmetz-Benin)
Most small-scale processors do not have the capacity to generate steam for sterilization. Therefore, the threshed fruits are cooked in water. Whole bunches which include spikelet absorb a lot of water in the cooking process. High-pressure steam is more effective in heating bunches without losing much water. Therefore, most small-scale operations thresh bunches before the fruits are cooked, while high-pressure sterilization systems thresh bunches after heating to loosen the fruits.
Small-scale operators use the bunch waste (empty bunches) as cooking fuel. In larger mills the bunch waste is incinerated and the ash, a rich source of potassium, is returned to the plantation as fertilizer.
Sterilization of bunches
Sterilization (cooking) means the use of high temperature wet-heat treatment of loose fruit. Sterilization normally uses the pressurize of steam. The several serves purpose for sterilization is:
Heat treatment destroys oil-splitting enzymes and arrests hydrolysis and autoxidation.
For large-scale installations, where bunches are cooked whole, the wet heat weakens the fruit stem and makes it easy to remove the fruit from bunches on shaking or tumbling in the threshing machine.
Heat helps to solidify proteins in which the oil-bearing cells are microscopically dispersed. The protein solidification (coagulation) allows the oil-bearing cells to come together and flow more easily on application of pressure.
Fruit cooking weakens the pulp structure, softening it and making it easier to detach the fibrous material and its contents during the digestion process. The high heat is enough to partially disrupt the oil-containing cells in the mesocarp and permits oil to be released more readily.
The moisture introduced by the steam acts chemically to break down gums and resins. The gums and resins cause the oil to foam during frying. Some of the gums and resins are soluble in water. Others can be made soluble in water, when broken down by wet steam (hydrolysis), so that they can be removed during oil clarification. Starches present in the fruit are hydrolyzed and removed in this way.
When high-pressure steam is used for sterilization, the heat causes the moisture in the nuts to expand. When the pressure is reduced the contraction of the nut leads to the detachment of the kernel from the shell wall, thus loosening the kernels within their shells. The detachment of the kernel from the shell wall greatly facilitates later nut cracking operations. From the foregoing, it is obvious that sterilization (cooking) is one of the most important operations in oil processing, ensuring the success of several other phases.
However, during sterilization it is important to ensure evacuation of air from the sterilizer. Air not only acts as a barrier to heat transfer, but oil oxidation increases considerably at high temperatures; hence oxidation risks are high during sterilization. Over-sterilization can also lead to poor bleach ability of the resultant oil. Sterilization is also the chief factor responsible for the discolouration of palm kernels, leading to poor bleach ability of the extracted oil and reduction of the protein value of the press cake.
Fig. 4 Fruit sterilizer (Centre de Formation Technique Steinmetz-Benin)
Digestion of fruit
Digestion is the process of releasing the palm oil in the fruit through the rupture or breaking down of the oil-bearing cells. The digester commonly used consists of a steam-heated cylindrical vessel fitted with a central rotating shaft carrying a number of beater (stirring) arms. Through the action of the rotating beater arms the fruit is pounded. Pounding, or digesting the fruit at high temperature, helps to reduce the viscosity of the oil, destroys the fruits' outer covering (excerpt), and completes the disruption of the oil cells already begun in the sterilization phase. Unfortunately, for reasons related to cost and maintenance, most small-scale digesters do not have the heat insulation and steam injections that help to maintain their contents at elevated temperatures during this operation.
Contamination from iron is greatest during digestion when the highest rate of metal wear is encountered in the milling process. Iron contamination increases the risk of oil oxidation and the onset of oil rancidity.
Pressing (Extracting the palm oil)
There are two methods to do the extracting of palm oil. Firstly, used a mechanical press which is called 'dry' method. Secondly, used a hot water which is called 'wet' method.
In the 'dry' method the objective of the extraction stage is to squeeze the oil out of a mixture of oil, moisture, fibre and nuts by applying mechanical pressure on the digested mash. There are a large number of different types of presses but the principle of operation is similar for each. The presses may be designed for batch (small amounts of material operated upon for a time period) or continuous operations.
The material is placed in heavy metal cage and a metal plunger is used to press the material. There are three differences in batch presses:
The method used to move the plunger and apply the pressure.
The amount of pressure in the press.
The size of the cage.
The plunger can be moved manually or motor. However, the motorised method is more expensive. There are two type of press machine to move plunger: spindle press and hydraulic press. Hydraulic presses are faster than spindle screw types and powered presses are faster than manual types. Some types of manual press require considerable effort to operate and do not alleviate drudgery.
Digested fruit is continuously conveyed through the cage towards an outlet restricted by a cone, which creates the pressure to expel the oil through the cage perforations (drilled holes). Oil-bearing cells that are not ruptured in the digester will remain unopened if a hydraulic or centrifugal extraction system is employed. Screw presses, due to the turbulence and kneading action exerted on the fruit mass in the press cage can effectively break open the unopened oil cells and release more oil. These presses act as an additional digester and are efficient in oil extraction.
Clarification and drying of oil
Clarification is to separate the oil from its entrained impurities. The fluid produced from the press is a mixture of palm oil, water, cell debris, fibrous material and non-oily solids. The non-oily solid make the mixture become very thick. However, the hot water is added into the mixture to thin it. The addition of water produced a barrier that cause the heavy solids to fall to the bottom of the container while the lighter oil droplets flow through the water mixture to the top when heat is applied. Water is added in ratio of 3:1.
To remove the coarse fibre, the diluted mixture must be passed through a screen. The screened mixture is boiled from one or two hours and then allowed to settle by gravity in the large tank so that the palm oil, being lighter than water, will separate and rise to the top. The clear oil is decanted into a reception tank. This clarified oil still contains traces of water and dirt. To prevent increasing FFA through autocatalytic hydrolysis of the oil, the moisture content of the oil must be reduced to 0.15 to 0.25 percent. Re-heating the decanted oil in a cooking pot and carefully skimming off the dried oil from any engrained dirt removes any residual moisture. Continuous clarifiers consist of three compartments to treat the crude mixture, dry decanted oil and hold finished oil in an outer shell as a heat exchanger.
The purified and dried oil is transferred to the tank for storage prior to dispatch from the mill. The oil is stored with a maintain temperature around 50° C by using hot water or low pressure steam heating coils to prevent from the solidification and fractionation.
The disposed from the press consists of mixture of fibre and palm nuts. The recovered fibre and nutshells put into the steam boilers. The super-heated steam is then used to drive turbines to generate electricity for the mill. For this reason it makes economic sense to recover the fibre and to shell the palm nuts. In the large-scale kernel recovery process, the nuts contained in the press cake are separated from the fibre in a depericarper. They are then dried and cracked in centrifugal crackers to release the kernels. The kernels are normally separated from the shells using a combination of winnowing and hydro cyclones. The kernels are then dried in silos to a moisture content of about 7 percent before packing.
Summary of unit operations
To loosen fruit base from spikelets and to allow ripening processes to abate
To facilitate manual removal of fruit
To remove and sort fruit from spikelets
To sterilize and stop enzymatic spoilage, coagulate protein and expose microscopic oil cells
To rupture oil-bearing cells to allow oil flow during extraction while separating fibre from nuts
To release fluid palm oil using applied pressure on ruptured cellular contents
To boil mixture of oil and water to remove water-soluble gums and resins in the oil, dry decanted oil by further heating
To separate de-oiled fibre from palm nuts.
To recover residual oil for use as soap stock
To sun dry nuts for later cracking