28.1.01 Directly following the digesting is the actual extraction of the oil, achieved usually by a pressing action.
Presses have evolved from hand operated ones to hand operated hydraulic ram types, to automatic hydraulic ram types and to the presently almost universally used twin screw type presses.
28.1.02 The change over from hydraulic ram type presses to screw type presses was partly as a result of changes in the fruit composition as a result of "improved" planting material with a higher yield of oil.
In general it can be stated that the "Dura" type f.f.b. suited the hydraulic ram type extraction equipment, but created problems if processed by screw presses, whilst the later developed "Tenera" type f.f.b. proved to be problematic with the hydraulic ram type presses, but suited the screw type presses.
28.1.03 This is partially due to the composition of the press cake from the different types of fruit.
To illustrate the difference between the average "Dura" and the average "Tenera" type fruit, it is useful to compare the main parameters:
"DURA" "TENERA"
M.P.D. : F.F.B. 65 % 70 %
Bunch trash : F.F.B.
(Calyx leaves,spikelets, abortive fruitlets etc.) 7 % 10 %
Fibre : Nuts 65 : 35 80 : 20
Clean sterilized fruit : F.F.B. 60 % 60 %
Nuts : F.F.B. 25 % 13 %
Oil in M.P.D. : F.F.B. 18 % 24 %
Oil extraction rate : F.F.B. 17.5 % 24 % Kernel extraction rate : F.F.B. (clean, no dirt) 4.5 % 6 %
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It must be noted however, that even within the specific type of fruit, the composition can vary greatly and the figures shown above are by no means absolute, but are only to show the differences.
Between the two types and are not decisive for the types.
28.1.04 With the exception of the type of press which utilizes a feed screw, the shape and confirmation of these screw presses varies as a result of manufacturer's ideology, but their operating principles and action is virtually the same. The objective of the machinery used in the pressing process is to extract the maximum quantity of good quality palm products.
28.1.05 Palm produce is the sum of the products C.P.O. and P.K and in order to achieve the objective as stated in 28.1.04 above, it will be necessary to strike a balance between the two, since occasionally the required actions conflict with each other.
In general the aim of a palm oil mill is to maximize the profitability of its operation, thus an optimum balance between the efficiency of the extraction of good quality products and the cost of operation of the mill is required.
28.2 The pressing operation is only part of this overall process and its efficiency depends to a large extend on the correctness of the processes prior to this pressing stage.
28.2.01 The pressing has the following objectives:
General aim: Specific aim: Efficiency measured by:
High oil recovery % O.L.D.B.
% O.L. on nuts
High kernel recovery % kernel in fibre
C.P.O. suitable for efficient clarification Clarification station results (see note 1 , below) High extraction
efficiency
Press cake suitable for efficient
depericarping % fibre in nuts for kernel extraction % nuts lost in (fuel) fibre Minimum deterioration of oil extracted Laboratory tests
(see note) High quality
products
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Note :
It will be obvious that the pressing cannot correct errors made in the previous processes of sterilization threshing and digesting; whilst pressing only partly controls these measures.
28.2.02 There are a number of known reasons / situations in and out side of the mill which affect press performance and all of these can vary considerably, for example:
VARIABLE AFFECTS a) F.F.B.: harvesting efficiency harvesting interval handling contamination sterilization
Throughput, oil & kernel recovery % ditto
ditto
Throughput, quality of oil produced Throughput, oil & kernel recovery and quality
b) Digesting: condition filling
temperature
Throughput, oil recovery
Throughput, oil & kernel recovery % Throughput, oil & kernel recovery % and quality
c) Pressing: screw speeds cone pressure condition
Throughput, oil & kernel recovery % Oil & kernel recovery and quality Oil & kernel recovery %
28.2.03 Virtually all variables have a direct effect on the through put and the extraction efficiency, because:
a) F.F.B.:
Prolonged harvest interval usually result in the harvesting of "over ripe" bunches from which too much fruits have already detached, or will detach on cutting this fruit which will also have a higher f.f.a. %.
Harvesting efficiency includes the collection of the loose fruits, non or partial collection results in different ratios of M.P.D. to F.F.B. and lower extraction and/or recovery rates of both oil and kernel by the milling process.
F.F.B. handling, if incorrect, increases the bruising and damage to the fruit, decreasing the recovery of oil from M.P.D. and increases the oil losses on sterilizer condensate.
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F.F.B. sterilization affects the press results the most, since:
Under sterilized M.P.D. reduces the through put considerably and increases oil loss on fibre.
Over sterilized M.P.D. leads to higher oil losses due to the emulsifying effects in the digester and the breaking up of the fibrous material.
Pressing will generally take place with increased cone pressures, which will also lead to a higher kernel breakage.
F.F.B. contamination
Apart from the likely jamming or breakdown of the equipment, certain compounds will also effect the quality of the oil produced. b) The mechanical condition of the digester, i.e. badly worn stirring
and/or expelling arms leads to incomplete digesting and higher oil losses.
The level of filling of the digester has a direct influence on the efficiency of the digesting process (see 27.2.03 b) usually resulting in higher oil losses, kernel breakage and lower recovery rates. The temperature affects the through put and the recovery rates
(see 27.2.03 a)
c) The screw speed (for both the feed screw and the main screw) has a considerable effect on the overall performance of a screw press.
Usually the correct speed, producing on average the lowest oil and kernel losses, is found by trial and error and seldom changes during the process.
An exception to this is the feed screw, generally higher feed screw speeds appear to increase through put, but also increase the oil losses and the kernel breakage.
It is generally accepted that an increase in cone pressure does not greatly affect the through put, but reduces oil losses, whilst at the same time it increases kernel breakage and losses.
Again, the most suitable setting is usually determined by trial and error and the cone control (often automated) adjusted in such a
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way that an acceptable average is achieved.
The overall mechanical condition of the press, i.e. the wear on screws and press cages does not appear to have a marked effect on the press performance until a critical point is reached.
There after the recovery % of both oil and kernel reduces drastically and losses increase beyond acceptable limits.
28.3 Pressing (extraction) efficiency
28.3.01 It is common practice in the palm oil industry to consider the oil loss on fibre ex presses as the most important one.
It is however more appropriate and more accurate to take the overall oil losses into account and judge the overall performance by calculating the extraction efficiency of the overall palm produce , i.e. C.P.O. and P.K.
There exists a direct relation between the press cake composition, the oil loss in fibre, the percentage nut breakage, (the shell thickness) and the torque of the press, due to the setting of the cones at the exit side of the press.
Generally it can be observed that:
i) With constant torque, nut breakage increases in accordance with the percentage nuts in the press cake
ii) With constant torque, nut breakage increases in accordance with the ratio kernel to nut ( as a result of thinner shells)
iii) With improper feed, reducing the capacity of the press in relation to the main screw speed, the press cake tends to "slip" and nut breakage increases
iv) With a constant fruit composition the oil loss on fibre will decrease with an increase in torque, but at the same time nut breakage will increase.
28.3.02 Kernels from broken nut will not necessarily be lost, recovery can still be effected in the depericarper section, provided the kernel is not totally crushed and pulverized.
There are many factors involved in efficient extraction, starting with the actual type and composition of the fruit and continuing with the handling. the sterilization, threshing , digesting and pressing.
It is essential that all the operations involved are correctly executed and controlled.
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On average a particular mill has usually a particular type of f.f.b. to process and the most effective press settings are usually achieved by trial and error techniques.
Economic considerations are also of influence, i.e. if a high through put is not of a major importance it is usually preferred to operate on higher cone pressures, resulting in lower oil losses on fibre, which will usually outweigh the increased losses in kernel that may occur.
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