During the course of gathering material for this publication the author visited Benin, Cameroon, Ghana and Nigeria. It was observed that a steady evolutionary development had taken place in machinery and equipment required to process palm fruit bunches to meet changing circumstances of the small-scale palm oil processing industry. These innovations have progressed from the development of individual machines to carry out particular operations to machines that combine several operations in the process.
Mechanical extraction
Pounding (digestion) and oil extraction are the most tedious
and essential operations in traditional palm fruit processing; therefore early
efforts concentrated on these tasks. In small-scale processing, digestion, the
breaking up of the oil-bearing cells of the palm fruit’s mesocarp, is the
most labour intensive.
Two methods of fruit maceration common in traditional
processing:
· pounding cooked/soaked fruits in large wooden or concrete mortars with a wooden pestle;
· foot trampling the cooked but cold fruits in canoes or specially constructed wooden troughs.
Direct screw-pressing
Mechanisation was introduced to Cameroon in the 1930s through
the importation of Colin palm oil expellers. The Colin is a low-pressure,
continuous-feed expeller made in France. It has two 6’ (2 m) diameter
coaxial counter-rotating screws that turn horizontally or vertically in a
perforated cage. The discharge end is fitted with a backpressure cone. As the
cooked palm fruit is fed into the expeller it is pushed forward by the spiral
flights (worms) against the backpressure of the end cone. The oil is forced out
through the perforated sides of the cage. The remaining fibre and nut are
released at the end of the cage through the gap between the end cone and cage
body. The ability to simultaneously de-pulp and press is a major advantage of
this type of press. Small expellers may be manually operated or motorised. These
expellers have been the dominant - if not the exclusive equipment - used by
small-scale palm oil processors in Cameroon. In Ghana and Nigeria the earliest equipment introduced was the
Stork manual hydraulic press. The impression was created that, for economic
reasons, the only operation that needed mechanisation was oil pressing. In
colonial days farm labour was cheap and easily available. Hence there was no
attempt to mechanise the digestion operation. Thus, in the British colonies,
early attempts at mechanisation had to focus on complementing the presses with
mechanical digesters. Two types of digesters were developed: horizontal
digesters based on the dry process technique; and the vertical digester, which
adopts the wet process technique.
In the wet system, sterilized fruits are poured into the
digester. As the fruits are being macerated, hot water is continuously poured
into the digester (at a regulated rate) to wash off the released oil. The
resultant mixture of water and oil is filtered and then clarified. Another attempt at mechanising the maceration process resulted
in the development of the manual digester for women. This digester consists
principally of a large wheel (connected to the differential system of a car
axle), and a vertical shaft carrying some beater arms that rotate inside a
conical shaped metal trough. The ratio of rotation of the wheel to the vertical
shaft is 1:7. It takes between 12 and 15 minutes to digest a 30 kg load of
fruit. The mechanical digesters currently in use consist of a
cylindrical shell and a system of beater-arms driven by a 6 hp. diesel engine
through a speed reducer (where necessary). The speed reducer steps down the
speed of the motor (engine) to 125 rpm - the running speed of the digester. The
digester is capable of macerating over 250 kg of fruits per hour and has the
singular attribute of macerating thoroughly either the Dura or Tenera fruit or a
combination of both without breaking any nut.
Pressing
The traditional method of oil extraction consists
of:
· steeping the pounded fruit mash in hot or cold water;
· removing fibre and nuts in small baskets and hand squeezing;
· filtering out residual fibre from the oil/water emulsion in perforated metal colanders or baskets;
· boiling and skimming palm oil from the oil/water mixture;
· drying the recovered oil.
Standing by the open fire during this operating period is not
only a health hazard but is inefficient, as a lot of oil is left trapped in the
mixture as an emulsion.
It was long realised that pressing is a bottleneck in
small-scale palm oil processing. The process is usually conducted slowly to
avoid the huge loss of oil that might result from inadequate pressing. The
economic importance of this process was therefore long recognised and has
received the greatest attention for mechanisation. Presses developed over the
years have included models such as:
· Manual vertical screw-press
· Stork hydraulic hand press
· Motor-jack press
· Motor-jack/cantilever press
· NIFOR hydraulic hand press
· Combined screw/hydraulic hand press
· mechanical screw-press
The manual vertical screw-press, the stock hydraulic hand
press and NIFOR hydraulic hand press enjoyed the highest patronage in Nigeria
for a long time, even though oil loss/fibre ratio for these presses range from
18-35 percent. This should be expected as the operation of these presses depends
on the strength of the operator.
In Ghana efforts to deliver a low-cost press to the smaller
village processors, led the Technology Transfer Centre (of the University of
Science and Technology, Kumasi) in the early seventies to come up with an
inexpensive manually-operated spindle press. The presses delivered low pressures
and relied on manual labour for pressure development. The throughput was about
50 kg per hour or 1.5 tonnes per day. For the really small-scale extractors in
villages with small patches of oil palm farms these screw-presses gained
widespread preference. Here, the traditional mortar and pestle was used to pound
(digest) fruits and then the mash was taken to a press operator who extracted
the oil for a fee.
The manual spindle-press was affordable and was bought by
individuals and groups. In the Kusi area of the Eastern Region the use of the
press was rented to the whole community. This was to signal the beginning of
community-based service palm oil milling.
Hydraulic presses
AGRICO introduced the use of manually operated hydraulic
presses into Ghana from India to complement the mechanical digesters. However,
these presses suffered from rapid wearing of the hydraulic cylinder pressure
seals, leading to poor pressure development. More importantly, the combined cost
of digester and manual hydraulic press, at the time, was more than most village
small-scale operators could afford. Indeed these mills were targeted at owners
of medium-sized plantations who wanted to process their fruits independently
rather than selling bunches to large-scale millers.
These hydraulic presses, although very popular with
small-scale processors, have two major weaknesses:
· they require human strength to operate;
· because of the disproportionate nut-to-fibre ratio in Tenera or Tenera-Dura combination, oil loss to fibre is high.
Combination digester and hydraulic press systems
TechnoServe Inc. brought the digester, hydraulic press and
spindle press into a rural community together with the business management
training to create small-scale palm oil processing enterprises.
However there were engineering problems with plant layout and
matching the throughput of machine components. For instance:
1. The press and digester stations were typically separated from each other. Extra labour was required to load the cages from material discharged from the digester. The extra labour added to production costs.
2. The digester works much faster than the press; therefore there is always digested material awaiting the press. The digested mash cools during the waiting period. The cooling process reduces oil extraction efficiency, reducing plant throughput. The digester discharge and press loading activities were performed too close to the floor from the viewpoint of hygiene.
3. The surface area of the press plates and cage diameters were too large and therefore reduced the transmitted pressure of the hydraulic presses. Reduced pressure meant reduced extraction efficiency. Operating pressure was measured at 30-40 psi in the hydraulic press cylinder.
4. The manual presses were not ‘women friendly’ since a great deal of muscle power was required to pump the hydraulic system all day. In the peak season the work was difficult for even two young, able-bodied men. The press cages were heavy and unyielding to manipulation by women.
5. The frequent start/stop operation was injurious to the engine and increased fuel consumption. There was idle power in the drive engine as the digester led the press in performance by about 30 minutes. The idle power could be used to drive the hydraulic system. There was, therefore, the opportunity to move to semi-continuous technology.
TechnoServe Inc. sought to address the above-mentioned defects
by producing a sturdy, hygienic, mechanically semi-continuous operation that can
be handled also by female processors. The aim was achieved through:
· Equipment layout design changes to link the digester and press stations through an operating table on which press cages can slide between stations so that the digested mash always remains above ground. The digester and press stations were arranged so that one operator could manipulate both units.
· Changing to a high-pressure motorised press developing about 70 tonnes (versus the current 30-40 tonnes) cylinder pressure. The higher cylinder pressure was to be transmitted to a narrower press cage with smaller (4 mm diameter) holes using a smaller diameter (270 mm-diameter) press plate. The existing cages are usually 460 mm in diameter with 10 mm drilled holes. The new cages conserve pressure better.
· The hydraulic fluid is pumped using a power take-off pulley connected to the continuously running digester shaft. Thus the prime mover engine supplies the pressing power. The press/release mechanism is a spindle-operated valve, which is held up or down. No real strength is required to hold down the valve handle to operate the press.
· The smaller press cages permit easy manipulation by women since movement is by sliding the cages on a metal table connecting the elevated digester chute and press station.
Combination mechanical digester and screw-press
The NIFOR mechanical screw-press is the latest used by the
small-scale palm oil processing industry in Nigeria. This consists of a
perforated tube inside which a transport screw rotates. The press outlet is more
or less closed by a cone that regulates the pressing pressure. The worm
transports and gradually compresses the macerated fruits. Released oil drains
through the perforations in the tube.
The press is mounted directly below a feed conveyor, which is
fed by gravity by the horizontal digester. The body of the feed conveyor is
perforated to allow oil released in the digester to drain away.
Preliminary trials have shown that the press can handle over 1
tonne FFB per hour with an average oil loss to fibre of 10.7 percent. The unit is sold together with the NIFOR sterilizer and
continuous clarifier as a standard set of machines for palm oil
processing. There are many artesanal fabricators of machinery and
equipment for small-scale palm oil processing that continue to supply individual
unit operational equipment. However, most established machinery designers and
manufacturers supply complete engineered sets of processing machinery comprising
the cooker/sterilizer, combination digester and press, along with a continuous
clarifier.
Typical
process unit performance and consumption per tonne of fresh bunches
Type
of unit
|
Key
machines
|
Rated
capacity
(kg
FFB/hr)
|
Extraction
efficiency
(%)
|
Extraction
rate
|
Consumption
per tonne of fresh fruit bunches (FFB)
|
Capital
investment
(US$)
|
|||
single
batch unit
|
Water
(litres)´
|
Elect
power
(kwh)
|
Fuel
(litres)
|
Wood
(kg)
|
|||||
Dry
|
Spindle
|
100-200
|
55
|
12-14.
|
282
|
0
|
0
|
88
|
150-200
|
Hydraulic
|
200-300
|
67--74
|
12-15
|
287
|
0
|
0
|
90
|
5 000-7
000
|
|
Screw
|
250-400
|
77.4-
|
16-18
|
718
|
12
|
7
|
73
|
1 500-6
000
|
|
Wet
|
Vertical
digester
|
500-800
|
80-90
|
19-20
|
750
|
0
|
2
|
70
|
1 500-2
500
|
Dry
|
Motorised
horizontal digester
(only)
|
500-1000
|
55
|
12-14
|
250
|
0
|
2.0-3.0
|
75
|
2 500-3
000
|
Dual separate units
|
|||||||||
Dry
|
Digester +spindle presses
|
200-300
|
60-70
|
16-18
|
380
|
0
|
1.0-1.5
|
84
|
3 000-5
000
|
Digester +hydraulic press
|
400-800
|
67-78
|
15-17
|
400-500
|
0
|
1.0-1.5
|
73
|
7 000-10
000
|
|
Semi-continuous combined units
|
|||||||||
Dry
|
Motorised digester +hydraulic +
spindle press
|
500-850
|
70-87
|
18-20
|
270
|
0
|
2.0-3.0
|
113
|
10
000-15 000
|
Digester + screw-press
|
500-850
|
76-90
|
18-20
|
267
|
0
|
2.0-3.0
|
146
|
12
000-15 000
|
|
Source: Compiled from various
sources: http://www.fao.org/DOCrEP/005/Y4355E/y4355e05.htm
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