Traditional Techniques and Innovations in Small-Scale Palm Oil Processing

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.

4.1 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.

4.2 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.

4.3 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.

4.4 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.

4.5 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