3. PALM OIL PROCESSING



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3. 1 General processing
description

Research and development work in many disciplines -
biochemistry, chemical and mechanical engineering – and the establishment of
plantations, which provided the opportunity for large-scale fully mechanised
processing, resulted in the evolution of a sequence of processing steps designed
to extract, from a harvested oil palm bunch, a high yield of a product of
acceptable quality for the international edible oil trade. The oil winning
process, in summary, 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 for storage and export.

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 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.

Conversion of crude palm oil to refined oil involves removal
of the products of hydrolysis and oxidation, colour and flavour. After refining,
the oil may be separated (fractionated) into liquid and solid phases by
thermo-mechanical means (controlled cooling, crystallization, and filtering),
and the liquid fraction (olein) is used extensively as a liquid cooking oil in
tropical climates, competing successfully with the more expensive groundnut,
corn, and sunflower oils.

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.

Alongside the development of these large-scale fully
mechanised oil palm mills and their installation in plantations supplying the
international edible oil refining industry, small-scale village and artisanal
processing has continued in Africa. Ventures range in throughput from a few
hundred kilograms up to 8 tonnes FFB per day and supply crude oil to the
domestic market.

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 processors of all sizes go through these unit
operational stages. They differ in the level of mechanisation of each unit
operation and the interconnecting materials transfer mechanisms that make the
system batch or continuous. The scale of operations differs at the level of
process and product quality control that may be achieved by the method of
mechanisation adopted. The technical terms referred to in the diagram above will
be described later.

The general flow diagram is as follows:

PALM OIL PROCESSING UNIT
OPERATIONS

Harvesting technique and handling
effects

In the early stages of fruit formation, the oil content of the
fruit is very low. As the fruit approaches maturity the formation of oil
increases rapidly to about 50 percent of mesocarp weigh. In a fresh ripe,
un-bruised fruit the free fatty acid (FFA) content of the oil is below 0.3
percent. However, in the ripe fruit the exocarp becomes soft and is more easily
attacked by lipolytic enzymes, especially at the base when the fruit becomes
detached from the bunch. The enzymatic attack results in an increase in the FFA
of the oil through hydrolysis. Research has shown that if the fruit is bruised,
the FFA in the damaged part of the fruit increases rapidly to 60 percent in an
hour. There is therefore great variation in the composition and quality within
the bunch, depending on how much the bunch has been bruised.

Harvesting involves the cutting of the bunch from the tree and
allowing it to fall to the ground by gravity. Fruits may be damaged in the
process of pruning palm fronds to expose the bunch base to facilitate bunch
cutting. As the bunch (weighing about 25 kg) falls to the ground the impact
bruises the fruit. During loading and unloading of bunches into and out of
transport containers there are further opportunities for the fruit to be
bruised.

In Africa most bunches are conveyed to the processing site in
baskets carried on the head. To dismount the load, the tendency is to dump
contents of the basket onto the ground. This results in more bruises. Sometimes
trucks and push carts, unable to set bunches down gently, convey the cargo from
the villages to the processing site. Again, tumbling the fruit bunches from the
carriers is rough, resulting in bruising of the soft exocarp. In any case care
should be exercised in handling the fruit to avoid excessive bruising.

One answer to the many ways in which harvesting,
transportation and handling of bunches can cause fruit to be damaged is to
process the fruit as early as possible after harvest, say within 48 hours.
However the author believes it is better to leave the fruit to ferment for a few
days before processing. Connoisseurs of good edible palm oil know that the
increased FFA only adds ‘bite’ to the oil flavour. At worst, the high
FFA content oil has good laxative effects. The free fatty acid content is not a
quality issue for those who consume the crude oil directly, although it is for
oil refiners, who have a problem with neutralization of high FFA content palm
oil.

3.1.1 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.

3.1.2 Threshing (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.

In a mechanised system a rotating drum or fixed drum equipped
with rotary beater bars detach the fruit from the bunch, leaving the spikelets
on the stem (Fig. 3).

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.

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.

3.1.3 Sterilization of
bunches

Sterilization or cooking 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.

Fig. 3 Bunch thresher (Centre de
Formation Technique Steinmetz-Benin)

Fig. 4 Fruit sterilizer (Centre de
Formation Technique Steinmetz-Benin)

3.1.4 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.

3.1.5 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.

3.1.5.1 Batch presses

In batch operations, material is placed in a heavy metal
‘cage’ and a metal plunger is used to press the material. The main
differences in batch press designs are as follows: a) the method used to move
the plunger and apply the pressure; b) the amount of pressure in the press; and
c) the size of the cage.

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)
(Fig. 4, 5, 6) or a hydraulic system (hydraulic press) (Fig. 7, 8, 9) 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.

3.1.5.2 Continuous systems

The early centrifuges and hydraulic presses have now given way
to specially designed screw-presses similar to those used for other oilseeds.
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.

3.1.6 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.

The diluted mixture is passed through a screen to remove
coarse fibre. 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. (Fig. 10, 11, 12)

Fig. 5 Spindle press (Luapula,
Zambia)

Fig. 6 Spindle press (Luapula,
Zambia)

Fig. 7 Another model of spindle
press (Nova Technologies Ltd., Nigeria)

Fig. 8 Hydraulic press
(manual)

The wastewater from the clarifier is drained off into nearby
sludge pits dug for the purpose. No further treatment of the sludge is
undertaken in small mills. The accumulated sludge is often collected in buckets
and used to kill weeds in the processing area.

3.1.7 Oil storage

In large-scale mills the purified and dried oil is transferred
to a tank for storage prior to dispatch from the mill. Since the rate of
oxidation of the oil increases with the temperature of storage the oil is
normally maintained around 50°C, using hot water or low-pressure
steam-heating coils, to prevent solidification and fractionation. Iron
contamination from the storage tank may occur if the tank is not lined with a
suitable protective coating.

Small-scale mills simply pack the dried oil in used petroleum
oil drums or plastic drums and store the drums at ambient temperature.

3.1.8 Kernel recovery

The residue from the press consists of a mixture of fibre and
palm nuts. The nuts are separated from the fibre by hand in the small-scale
operations. The sorted fibre is covered and allowed to heat, using its own
internal exothermic reactions, for about two or three days. The fibre is then
pressed in spindle presses to recover a second grade (technical) oil that is
used normally in soap-making. The nuts are usually dried and sold to other
operators who process them into palm kernel oil. The sorting operation is
usually reserved for the youth and elders in the village in a deliberate effort
to help them earn some income.

Large-scale mills use the recovered fibre and nutshells to
fire 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 (Fig. 13, 14, 15, 16). The kernels are normally separated from the
shells using a combination of winnowing and hydrocyclones. The kernels are then
dried in silos to a moisture content of about 7 percent before
packing.

During the nut cracking process some of the kernels are
broken. The rate of FFA increase is much faster in broken kernels than in whole
kernels. Breakage of kernels should therefore be kept as low as possible, given
other processing considerations.

Fig. 9 Manual vertical press
(O.P.C., Cameroon)

Fig. 10 Motorised horizontal screw
press (Centre Songhai, Benin)

Fig. 11 Combined digester and
motorised hydraulic press (Technoserve/Cort Engineering, Ghana)

Fig. 12 Flushing extractor (Cort
Engineering Services, Ghana)

Summary of Unit operations














Unit operation

Purpose

1.

Fruit fermentation

To loosen fruit base from spikelets and to allow ripening
processes to abate

2.

Bunch chopping

To facilitate manual removal of fruit

3.

Fruit sorting

To remove and sort fruit from spikelets

4.

Fruit boiling

To sterilize and stop enzymatic spoilage, coagulate protein
and expose microscopic oil cells

5

Fruit digestion

To rupture oil-bearing cells to allow oil flow during
extraction while separating fibre from nuts

6

Mash pressing

To release fluid palm oil using applied pressure on ruptured
cellular contents

7

Oil purification

To boil mixture of oil and water to remove water-soluble gums
and resins in the oil, dry decanted oil by further heating

8

Fibre-nut separation

To separate de-oiled fibre from palm nuts.

9

Second Pressing

To recover residual oil for use as soap stock

10

Nut drying

To sun dry nuts for later cracking

Fig. 13 Clarifier tank (O.P.C.,
Cameroon)

Fig. 14 Clarifier tank (Nova
Technologies Ltd., Nigeria)

Fig. 15 Oil filter (Faith
Engineering Workshop, Nigeria)

Fig. 16 Palm nut cracker (AGRICO,
Ghana)

Fig. 17 Palm nut cracker (NOVA,
Technologies, Nigeria)

Fig. 18 Palm nut cracker (Ogunoroke
Steele Construction Works Ltd, Nigeria)

Fig. 19 Palm nut cracker combined
with Kernel/Shell separator (Hormeku Engineering works, Ghana)


3.2 Process equipment design and
selection criteria

In designing equipment for small-scale oil extraction one of
the key factors to consider is the quality required. ‘Quality’ is
entirely subjective and depends on the demands of the ultimate consumer. For the
edible oil refining industry the most important quality criteria for crude oil
are:

  • low content of free fatty acids (which are costly to remove during oil
    refining);
  • low content of products of oxidation (which generate off-flavours);
  • readily removed colour.

The most critical stages in the processing sequence for a
processor seeking to satisfy these criteria are: bunch sterilization as soon as
possible after harvest; and effective clarification and drying of the crude oil
after extraction.

By contrast, for the domestic consumer of crude palm oil,
flavour is the primary quality factor. This is boosted by the fermentation that
takes place within the fruit when the bunches are allowed to rest for three or
more days after harvesting. Thus sterilization immediately after harvesting is
not a crucial consideration. Herbs and spices for flavour are introduced during
the oil-drying phase of operations to mask off-flavours. Therefore rigid process
control during oil clarification need not be prescribed or incorporated in the
design.

The free fatty acids and the trace tocopherols contained in
the crude palm oil after natural fermentation also have a laxative effect, which
is desirable for African consumers for whom synthetic substitutes are a luxury.
The acidity imparts a ‘bite’ to the oil which some consumers prefer.
Thus the quality requirements of one market, leading to certain processing
imperatives, may conflict with those of another market.

The traditional manual methods are normally referred to as
‘low technology’ production. The mechanised units are likewise
referred to as ‘intermediate technology’ production.

The village traditional method of extracting palm oil involves
washing pounded fruit mash in warm water and hand squeezing to separate fibre
and nuts from the oil/water mixture. A colander, basket or a vessel with fine
perforated holes in the bottom is used to filter out fibre and nuts. The wet
mixture is then put on the fire and brought to a vigorous boil. After about one
or two hours, depending on the volume of material being boiled, the firewood is
taken out and the boiled mixture allowed to cool. Herbs may be added to the
mixture at this point just before reducing the heat. On cooling to around blood
temperature, a calabash or shallow bowl is used to skim off the palm oil.
Because of the large quantities of water used in washing the pulp this is called
the ‘wet’ method.

A mechanical improvement, based on the traditional wet method
process, is achieved by using a vertical digester with perforated bottom plate
(to discharge the aqueous phase) and a side chute for discharging the solid
phase components. The arrangement combines digestion, pressing and hot water
dilution into one mechanical unit operation.

The ‘dry’ method uses a digester to pound the boiled
fruit, which is a considerable labour-saving device. The oil in the digested or
pounded pulp is separated in a press that may be manual or mechanical. Motorised
mechanical presses are preferred, whether hydraulic or screw type.

Most medium- and large-scale processing operations adopt the
’dry’ method of oil extraction. This is because the fibre and nut
shells may immediately used to fire the boiler to generate steam for
sterilization and other operations, including electricity generation. If the
huge volumes of fibre and shells are not used as boiler fuel, serious
environmental pollution problems may result. Too much water in the fibre
increases the amount and cost of steam required to dry the fibre. Hence the
preference for the dry method in plants handling more than six tonnes FFB per
hour.

Processing machinery manufacturers tend to make machines to
fit individual processing operations. However, recent developments have been
toward the manufacture of integrated machines, combining several process
operations such as digestion, pressing and fibre/nut separation into one
assembly. It is found that these machines fit into two key process groupings:
batch and semi-continuous processes.

Schematic of processing models
and associated machinery

NB: NOS = Non -oily
solids entrained in oil such as coagulated protein, gums and resins,
etc.

The extraction of palm oil from boiled palm fruit can be
accomplished by handling successive batches of materials or continuously feeding
material to the machines.

3.2.1 Batch systems

The batch systems work directly on successive loads of boiled
fruit to extract oil in one operation for clarification. The ‘wet’
method uses a vertical digester (Fig. 11) with a perforated bottom plate to
pound a batch of fruit and then flush out the oil and other non-oil solids from
the mashed pulp with hot water. The direct screw-press is designed to pound a
batch of boiled fruit in the entry section of the machine while exerting
pressure on the mashed pulp in another section to expel the palm oil in one
operation.

The advantage of the wet system is that it is simple and
completely leaches all oil and non-oily solid substances that can be carried in
the fluid stream out of the digested mash to give clean and separated nuts and
fibre. The aqueous effluent from the vertical digester goes directly to the
clarification stage of processing. The amount of water needed to flush the pulp
is normally the same as that required for diluting the viscous oil that comes
from the mechanical press in preparation for clarification. An inexperienced
operator may use too much hot water to leach out the oil and thus consume
unnecessary wood fuel.

The ‘wet’ method yield of palm oil is severely
reduced when the wash water is cold. In the course of digesting the fruit mash,
in the presence of water, there is increased tendency to form an oil/water
emulsion that is difficult to separate from the fibre mass. The emulsified oil
loss in the fibre can be substantial if care is not taken to ensure full loading
of the digester. Vertical flushing digesters, requiring loading and discharging
of a specific amount of material, can thus only be used in a batch
operation.

3.2.2 Semi-continuous
systems

Continuous systems work sequentially, with one operation
feeding directly into another, related to the arrangement and timing of machine
operations. Careful engineering of unit operations is required to minimise
discontinuities in the feeding of one stage into another. Otherwise some
machines have to be stopped periodically for other stations to catch up. When
there are discontinuities in the flow of materials between process stations the
operations are known as semi-continuous. The dry extraction systems with
separate digestion and pressing stations are usually semi-continuous.

Also when digestion and pressing stations are combined into an
integrated unit and there is discontinuous feeding of boiled fruit to the
digester inlet the operation is termed “semi-continuous”. Once
operations have been integrated to attain full continuity the capital investment
capacity of small-scale operators has been surpassed, because both machinery and
working capital for raw material increases greatly with the increased level of
mechanisation.

The dry systems do not need much water for processing,
although they have the disadvantage of leaving substantial residual oil in the
press cake. The oil content of the press cake can be quite considerable (2-3
percent), depending on the type of press used and the strength of manual
operators.

The efficiency with which the various presses can extract oil
ranges from 60 to 70 percent for spindle presses, 80-87 percent for hydraulic
presses and 75-80 percent for the Caltech screw-presses. The first-pressing oil
extraction rates also range from 12 to 15 percent for the spindle-presses, 14-16
percent for hydraulic presses and 17-19 percent for the motorised screw-presses.
(Rouziere, 1995)

In many instances the first press cake is then sorted to
remove the nuts, and the fibre is subsequently subjected to a second pressing to
obtain more oil (an additional 3 to 4 percent on FFB). The second press oil is
generally of lower quality, in terms of free fatty acid content and rancidity.
Such low-grade oil is used in soap-making. Some village processors undertake the
traditional hot water washing of the entire press cake immediately after
pressing instead of sorting fibre and second pressing.

Local manufacturers have developed a wide range of machinery
and equipment for processing palm oil and palm kernel to fit any budget. All the
relevant unit operational machines can be produced to various degrees of finish
and quality in the Sub-Region. It is the combination of the unit operation into
an affordable process chain that distinguishes the manufacturers and their
supplies.

From traditional technologies that rely solely on manual
labour and simple cooking utensils, raising the level of mechanization depends
largely on a balance between the quantity of bunches available for processing in
a given locality and the money available for investment in machines.

The first consideration should be the availability of raw
materials and how to compute the processing scale. Knowing the optimum scale of
operations, it is then possible to consider the type of processing techniques.
The higher the technology, the more skilful operators will be required to handle
the machines. These technical considerations should lead to the equipment
selection and examination of the capital investments needed to acquire the
necessary machines.


3.3 Plant sizing

Assume a Village Group decides to plant oil palm and
establishes a program to plant a certain number of seedlings each year over a
seven-year period. In the third year the first set of trees begin to bear fruit.
The community wants to establish a processing mill and they call an expert. How
is the estimation made of the size and type of processing unit required by the
community?

Start by establishing the block of planted areas by year so
the age of the trees may be determined. The oil palm tree begins to bear fruit
from the third year and the yield per tree increases progressively with age
until it peaks around 20 years. The yield begins to decline from year 25 through
40 when the economic life of the tree ebbs.

Table 3 describes the potential yields of palm fruit bunches
(in metric tonnes) from the planted hectares per year. Estimates in Table 3 are
used to calculate the expected annual yield for each annual block. For example,
8 700 seedlings planted in 1998 began to yield fruit in 2000 at the rate of 3
tonnes per hectare to give 198 tonnes for the year. By Year 7 all planted areas
will be in production, at different yield rates. The estimated annual yield per
planting block is calculated and then the column for the year is added to give
the potential raw materials available for processing. For example, in Year 7,
when all planted blocks are yielding fruit, the total is 8 919 metric tonnes
(see the row designated ‘TOTAL’). How the annual yield is distributed
over the entire year needs to be determined in order to know which period
demands the attention of processors.

The oil palm tree yield is distributed over the entire year.
Most of Central and West Africa experience two rainfall seasons. The oil palm
bears fruit in response to the rainfall pattern and hence there are two peak
harvesting periods in these regions. Southern hemisphere tropical monsoon
regions such as Malawi, Zambia and South East Asia experience only one long
rainy season and therefore tend to have a single peak-harvesting
season.

For Central and West Africa the annual monthly distribution
pattern for produce is expected to show the following variations:















Month

Percent yield

Seasonal contribution

March

9


April

12


May

16

50 %

June

13


July

8


August

7


September

8

34 %

October

11


November

7


December

5


January

3

16

February

1


In the peak harvesting month it is estimated that 12 to 16
percent of the annual yield is generally available for processing. The plant
that is installed must be capable of processing the peak month output, which is
generally estimated as 15 percent of the annual output. Conservatively, it is
estimated that the plant will work two shifts during the peak season.

Table 3: Estimated annual yield per hectare (from year of
planting)




Year

1

2

3

4

5

6

7

8

9

10

11

12

15

20

Estimated yield

(Tonnes)

3.0

4.25

5.5

6.0

7.25

8.2

8.6

9.5

10.5

11.0

12.5

13.5

Table 4: Estimated FFB yields after planting and related
plant capacity

Year/yield in metric tonnes











Hectares

1

98

2

3

4

5

6

7

8

9

10

11

12

15

20

66

198

281

363

396

479

541

568

627

693

726

825

891

190


570

808

1 045

1 140

1 378

1 558

1 634

1 805

1 995

2 375

2 565

800



2 400

3 400

4 400

4 800

5 800

6 560

6 880

7 600

8 800

10 000

400




1 200

1 700

2 200

2 400

2 900

3 280

3 440

4 400

5 200

400





1 200

1 700

2 200

2 400

2 900

3 280

3 440

5 000

Total



198

851

3 571

6 041

8 919

10 619

12 526

14 121

15 558

17 041

19 840

23 656

Peak Month



29.7

128

536

906

1 338

1 593

1 879

2 118

2 334

2 556

2 976

3 548

Plant

Capacity/hr Plant



0.09

0.4

1.7

2.8

4.2

5.0

6.0

6.6

7.5

8.0

9.5

11.0

Source: Poku, K.
Feasibility study on Malawi palm oil mill establishment

In Year 3 there is the potential of processing 198 tonnes of
fresh fruit bunches. Assuming that the total quantity were to be processed in
one location over a 20-day period using 8 hours in the day, we would need a
processing unit that handles 186 kg per hour, or 93 kilos/hr if the choice was
made to operate 16-hours per day. Table 4 shows capacity based on a 16-hour
working day. For this capacity a wet type digester or the dry spindle-press
operation would be recommended. By Year 5 the community would require a fully
mechanised mill using motorised digesters and presses.

Before the sixth year the community would have to decide
whether they want to stay in the small-scale milling category or move up to a
medium-scale operation using a continuous system of machines. If the option is
to stay small-scale then the community will need to place orders for additional
small-scale processing modules. The new set of processing machines can be placed
to run alongside the existing facility or located in another village to minimise
bunch transportation costs.

The best plant size option for rural Africa is still unknown.
Large-scale operations normally require high-skilled labour and management
expertise. Most villages do not have such a pool of skilled labour. The villages
also lack the social infrastructure such as good accommodation, schools and
hospitals that would attract high-skilled labour. Thus, in order to establish a
large-scale processing operation, labour needs to be imported from other parts
of the country. To maintain these ‘alien’ workers and managers a
provision must be made in the capital investment for housing, schools and
clinics near the processing estate. Some of the schooling and medical services
must be extended to the whole community or there will be resentment towards the
‘alien’ workers.

Large-scale operations also require rapid transportation of
harvested bunches to the processing site, hence the need for investment in roads
and civil works. The establishment of large-scale operations creates an overhead
burden that is beyond the capacity of a village community.

Many of the large-scale operations established in the early
1970s have declined along with the national economies of African nations. The
cost structure of these establishments has rendered the output products
non-competitive on the international market.

Today decentralised small-scale processing operations are
preferred in most parts of Africa.


3.4 Process technology/capital
investment considerations

Once the required plant size has been determined, the next
item to consider is the amount of money required to buy the necessary machinery.
The more money available, the more units can be bought, to minimise the drudgery
of processors.

The wide array of machinery options makes it possible for a
processor to start operations with a manual spindle-press used to pound the palm
fruit. Another may start with a single motorised vertical wet process digester.
Further up the investment scale are those who can afford the combination
horizontal digester and screw-press or combination horizontal digester and
hydraulic press along with the associated sterilizers, threshers, and oil
clarifiers. Another combination that is yet to be tried is the combination of a
horizontal motorised screw-press in combination with a second stage vertical
flushing digester for maximum palm oil extraction and fibre/nut
separation.















Type of unit

Key machines

Rated capacity

(k g FFB/hr)

Extraction efficiency

(%)

Capital investment

(US$)

Single batch unit

Dry

Spindle

100-200

55

150-200

Hydraulic

200-300

67-74

5 000-7 000

Screw

250-400

77.4

1 500-6 000

Wet

Vertical digester

500-800

80-90

1 500-2 500

Dry

Motorised horizontal digester

(only)

500-1000

55

2 500-3 000

Dual separate units

Dry

Digester + Spindle presses

200-300

60-70

3 000-5 000

Digester + hydraulic press

400-800

67-78

7 000-10 000

Semi-continuous combined units

Motorised digester +

500-850

70-87

10 000

Dry

hydraulic + spindle-press

-15 000

Digester + screw-press

500-850

76-90

12 000-15 000

Source: Compiled
from various sources

The extraction efficiency refers to the percentage of oil that
the machine can extract in relation to the total oil in the boiled fruit. The
type of fruit mix (Dura/Tenera) presented for processing greatly influences the
extraction efficiency of all units.

Many of the installations that use single spindle and manual
hydraulic press units require manual pounding with wooden mortars and pestles,
foot stomping, etc. Thus the throughput capacity of such a mill is determined by
the manual pounding rate. The presses are usually not mechanised and hence the
processing capacity of the press is also limited by the size of the press cage
and the operator’s energy level for turning the press screw or pumping the
hydraulic fluid mechanism.

Another limiting condition is the affordability of capital
equipment. Where the capital equipment cost exceeds a certain value villagers
will shy away from taking loans to purchase the combination of operations. The
designer must bear in mind that until the rural/urban migration of village youth
is reversed the villages will be mainly populated by the elderly. These elders
are naturally reluctant to take up long-term loans and the local banks are
reluctant to lend to a predominantly aged community group. In Ghana, for
instance, capital equipment costs should be around US$10 000 to be affordable to
village-based individuals or groups.

Because of the need to keep initial capital investment to a
bare minimum it is imperative that unnecessary mechanised unit operations are
eliminated. Work that can be done manually – without overly taxing profitability
– should be, thereby taking advantage of surplus labour and creating a stream of
wages and salaries in the local community. Operations that are usually
associated with drudgery by processors, such as fruit digestion and oil
extraction, can be mechanised. Other less strenuous tasks, such as fruit
separation and fibre/nut separation, can be contracted out to elderly women and
unemployed youth.

“Small-scale” does not necessarily mean a
significant decrease in efficiency. It does, however, mean a reduction in
working capital and operating costs. The small mills can be placed at the heart
of local communities, minimising reliance on vehicular transport that is
normally unavailable in rural communities, given the poor condition of road
networks and other infrastructure. This increased accessibility serves to
dramatically reduce fruit spoilage and consequent post-harvest losses.

Culturally, men cultivate or produce while women process and
sell. Traditionally, women decide the form in which the produce is to be traded
and hence determine the degree of processing they are willing to undertake.
These decisions form the basis of traditional technologies upon which
innovations are to be derived.

The operating philosophy for equipment innovation should
therefore be an attempt to develop machinery to alleviate the drudgery of female
processors while providing additional avenues for the employment of those
displaced by the improved technologies, keeping some operations
labour-intensive. It is therefore important to mechanise the key
drudgery-alleviation equipment that can be easily handled by women.

Prime mover power is also a major consideration. Most villages
do not have electricity and hence the diesel engine is the main source of power.
Thus, for cost reasons there cannot be a multiplicity of these engines to drive
the required unit operations. Where there is the need to drive several machines
the answer could be to use diesel power to generate electricity. The cost and
maintenance of this power source would eliminate most small-scale processors and
communities. The power source in such instances acts as a limitation to the
number of unit operations that can be mechanised and powered. Systems of pulleys
and gears to drive operational machines should be actively considered when
designing for village based groups.





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