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.
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.
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.
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.
Bunch thresher (Centre de Formation Technique Steinmetz-Benin)
Fruit sterilizer (Centre de Formation Technique Steinmetz-Benin)
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
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.
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.
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.
Spindle press (Luapula, Zambia)
Spindle press (Luapula, Zambia)
Another model of spindle press (Nova Technologies Ltd., Nigeria)
Hydraulic press (manual)
The
waste water 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.
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.
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.
Manual vertical press (O.P.C., Cameroon)
Motorised horizontal screw press (Centre Songhai, Benin)
Combined digester and motorised hydraulic press (Technoserve/Cort Engineering, Ghana)
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 |
Clarifier tank (O.P.C., Cameroon)
Clarifier tank (Nova Technologies Ltd., Nigeria)
Oil filter (Faith Engineering Workshop, Nigeria)
Palm nut cracker (AGRICO, Ghana)
Palm nut cracker (NOVA, Technologies, Nigeria)
Palm nut cracker (Ogunoroke Steele Construction Works Ltd, Nigeria)
Palm nut cracker combined with Kernel/Shell separator (Hormeku Engineering works, Ghana)
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.
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.
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.
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.
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
|
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.
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.
Source: http://www.fao.org/DOCrEP/005/Y4355E/y4355e04.htm