Large-scale plants, featuring all stages required to produce palm
oil to international standards, are generally handling from 3 to 60 tonnes of
FFB/hr. The large installations have mechanical handling systems (bucket and
screw conveyers, pumps and pipelines) and operate continuously, depending on the
availability of FFB. Boilers, fuelled by fibre and shell, produce superheated
steam, used to generate electricity through turbine generators. The lower
pressure steam from the turbine is used for heating purposes throughout the
factory. Most processing operations are automatically controlled and routine
sampling and analysis by process control laboratories to ensure smooth,
efficient operation. Although such large installations are capital intensive,
extraction rates of 23 - 24 percent palm oil per bunch can be achieved from
good quality Tenera.
Extraction of oil from the palm kernels is generally separate from
palm oil extraction, and will often be carried out in mills that process other
oilseeds (such as groundnuts, rapeseed, cottonseed, shea nuts or copra). The
stages in this process comprise grinding the kernels into small particles,
heating (cooking), and extracting the oil using an oilseed expeller or
petroleum-derived solvent. The oil then requires clarification in a filter
press or by sedimentation. Extraction is a well-established industry, with
large numbers of international manufacturers able to offer equipment that can
process from 10 kg to several tonnes per hour.
Efforts to mechanise and improve traditional manual procedures
have been undertaken by research bodies, development agencies, and private
sector engineering companies, but these activities have been piecemeal and
uncoordinated. They have generally concentrated on removing the tedium and
drudgery from the mashing or pounding stage (digestion), and improving the
efficiency of oil extraction. Small mechanical, motorised digesters (mainly
scaled-down but unheated versions of the large-scale units described above),
have been developed in most oil palm cultivating African countries.
PALM OIL PROCESSING UNIT OPERATIONS
Bunch reception
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.
Removal of fruit from the bunches
The fresh fruit bunch consists of fruit embedded in spikelets
growing on a main stem. Manual threshing is achieved by cutting the fruit-laden
spikelets from the bunch stem with an axe or machete and then separating the
fruit from the spikelets by hand. Children and the elderly in the village earn
income as casual labourers performing this activity at the factory site.
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 spikelets 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.
Sterilization of bunches
Sterilization or cooking oil palm means the use of
high-temperature wet-heat treatment of loose fruit. Cooking normally uses hot
water; sterilization uses pressurized steam. The cooking action serves several
purposes.
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.
Digestion of the 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 (exocarp), 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 distinct methods of extracting oil from the digested
material. One system uses mechanical presses and is called the ‘dry’ method.
The other called the ‘wet’ method uses hot water to leach out the oil.
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.
Batch presses
The plunger can be moved manually or by a motor. The motorised
method is faster but more expensive.
Different designs use either a screw thread (spindle press) or a
hydraulic system (hydraulic press) to move the plunger. Higher pressures may be
attained using the hydraulic system but care should be taken to ensure that
poisonous hydraulic fluid does not contact the oil or raw material. Hydraulic
fluid can absorb moisture from the air and lose its effectiveness and the
plungers wear out and need frequent replacement. Spindle press screw threads
are made from hard steel and held by softer steel nuts so that the nuts wear
out faster than the screw. These are easier and cheaper to replace than the
screw.
The size of the cage varies from 5 kg to 30 kg with an average
size of 15 kg. The pressure should be increased gradually to allow time for the
oil to escape. If the depth of material is too great, oil will be trapped in
the centre. To prevent this, heavy plates’ can be inserted into the raw
material. The production rate of batch presses depends on the size of the cage
and the time needed to fill, press and empty each batch.
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.
Continuous systems
The early centrifuges and hydraulic presses have now given way to
specially designed screw-presses similar to those used for other oil seeds.
These consist of a cylindrical perforated cage through which runs a closely
fitting screw. 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.
Moderate metal wear occurs during the pressing operation, creating
a source of iron contamination. The rate of wear depends on the type of press,
method of pressing, nut-to-fibre ratio, etc. High pressing pressures are
reported to have an adverse effect on the bleach ability and oxidative
conservation of the extracted oil.
Clarification and drying of oil
The main point of clarification is to separate the oil from its
entrained impurities. The fluid coming out of the press is a mixture of palm
oil, water, cell debris, fibrous material and ‘non-oily solids’. Because of the
non-oily solids the mixture is very thick (viscous). Hot water is therefore
added to the press output mixture to thin it. The dilution (addition of water)
provides a barrier causing the heavy solids to fall to the bottom of the
container while the lighter oil droplets flow through the watery mixture to the
top when heat is applied to break the emulsion (oil suspended in water with the
aid of gums and resins). Water is added in a ratio of 3:1.
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