30.6.01 The cracked mixture expelled from the nut crackers contains kernel, the broken shell parts and fragments of kernel and shell.
From this mixture primarily the whole, clean kernel needs to be separated and recovered.
30.6.02 In principle there are two methods in use to achieve this, i.e. the "dry" separation method and the "wet" separation method.
30.6.03 Before either of the above two methods a variety of screening activities can take place, all with the aim to separate the various components of the mixture as effectively as possible.
30.6.04 The "dry separation" is by far the simplest and the most economical, but is not always the most effective method.
The principle of this pneumatic separation is similar to that of the nut / fibre separation, i.e. the difference in volume - weight ratio of the various components in the mixture.
process is known as "winnowing" and can be done either as a single stage operation, or in multiple stages.
30.6.05 Provided the losses can be kept within the required parameters, this method has the great
advantage that the kernel recovered from this process has already a low moisture content and kernel drying thus becomes a much less difficult matter.
Also the average quality of the palm kernel oil extracted from this kernel has proved to be generally lower in F.F.A.
It is thus well worth while to pursue with dry separation, even if the direct end result is only partially acceptable and the unacceptable portion of the resulting product has (separately) to be submitted to a further (possibly "wet") separation method.
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The volume of the material for this secondary "wet" separation will be greatly reduced, thus reducing the capacity and the equipment requirements, whilst improving the overall final quality of the recovered palm kernel.
30.6.06 The "wet" separation methods can in principle be divided into two methods, i.e.: the clay (or brine) bath separation and the separation with the help of a hydro cyclone unit.
30.7.07 The clay bath separator takes advantage of the difference in specific gravity of the not dried kernel (about 1.07) and the specific gravity of shell (about 1.2).
Thus, in a bath with suspended clay to maintain a specific gravity between these two (i.e. about 1.12) the kernel will float and the shell will sink.
30.6.08 The clay suspension is circulated continuously and the floating kernels are skimmed of the top continuously.
The shell is usually removed by a screw conveyor at the bottom.
30.6.09 The clay bath separator can be quite effective, provided the correct density level is maintained, i.e. a regular replacement of the clay removed with the kernel and the shell is required.
The process is however difficult to control and can be quite expensive, due to the cost of clay etc. and the now a days favoured option would be the separation with the use of the hydro cyclone separator.
30.6.10 The hydro-cyclone separator action is comparable to that of an air cyclone, but as the name implies, the medium is liquid, here water. Water enters the unit tangentially and the resulting circular motion causes the heavier particles to be deposited on the outer wall of the cylinder by centrifugal force, with a spiral path down and out through the bottom part of the cyclone.
This movement of water requires fairly large pumps and the power consumption of a hydro-cyclone unit is therefore quite high.
In a correctly adjusted hydro cyclone, most of the shell particles (high density) pass downward, with a small flow of water, while most of the
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water together with most of the kernels (lower density) move to the centre of the cylinder and upwards to leave the cyclone via the overflow at the top of the cylinder.
30.6.11 The pumps, cyclones, adjustable conical sections etc. are all subject to considerable wear and tear and regular adjustments and replacements are required to ensure a consistent effective operation and if losses are to be kept within the required parameters.
30.6.12 The shell fraction, separated from the mixture by whatever means or system is usually transported directly by the most suitable method to the boiler fuel storage area, to supplement the fibre fuel component. 30.7 KERNEL RECOVERY
30.2.01 The kernel recovered after the separation requires further treatment, the extend of this treatment depending mainly on the method of separation used. Kernel recovered from dry separation methods have already a relatively low moisture content ( about 12%) and the retention time in the traditional kernel drying silos can be shortened considerably to achieve the required final moisture content for storage (about 7%)
This type of kernel is also less susceptible to mould formation, which ensures a better quality of the final product of palm kernel oil.
30.7.02 Kernel recovered from the wet separation method can have an internal moisture content of over 20%.
This moisture is much more difficult to remove, since it must first diffuse to the kernel surface before if can evaporate.
Wet kernel (i.e. all kernel with a moisture content > 7%) can support the growth of a mould that leads to an increased rate of hydrolysis of the palm kernel oil extracted from this kernel, i.e. an increase in F.F.A. The enzyme causing this is produced by the mould growth and can withstand quite high temperatures, thus the drying after the mould has developed will only enhance the appearance of the kernel, but will not prevent the later quick deterioration of the kernel oil since the enzyme will remain active in the palm kernel oil.
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