25.01 F.F.B. sterilization
Sterilization is the first step in the process of extracting oil and kernel from the f.f.b.
Inadequate or incorrect sterilization will undoubtedly adversely effect the efficiency of the extraction process.
25.02 The usual method of sterilization is a batch process, and this "batch" process has to supply the feed needed to maintain the subsequent extraction processes, most of which are of a more "continuous" nature. A hold up in the supply of sterilized fruit results in the disruption of down stream processing, which not only leads to the loss of through put, but also to the loss of product resulting from a lower efficiency of the total operation.
It is for this reason that sterilization operation must be scheduled according to a precise timetable, geared to the through put of the mill. 25.03 The most important functions of the sterilization process are:
a) to inactivate the enzymes that promote the formation of free fatty acid.
(To ensure that these enzymes are destroyed the whole of the oil carrying fruit must reach a temperature of at least 55 degrees Celsius.)
b) to loosen the fruit in the bunch so that the maximum amount of fruit is recovered in subsequent threshing (stripping) process.
(This aim will be achieved provided the fruit reaches a temperature of 110oCelsius for a minimum period of 20 minutes and the heating
medium provides moisture. The sterilization with live steam of low pressure is therefore suitable.)
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Other functions of the sterilization can be stated to be:
c) To soften the fruit in the bunch so that the mesocarp and the nut can be detached from each other (in the digester).
(A "clean" separation of the nut and the fibre will facilitate the proper operation of the depericarper later on in the process.) d) To condition the mesocarp so that the oil bearing cells can be more
easily and effectively broken and the oil recovered.
(Unbroken oil cells have a density close to that of water and will not be recovered in the clarification process.)
e) To dehydrate the fruit, which appears to have two functions, i.e. the pre-treatment of nuts for kernel recovery and an apparent positive effect on the efficiency of the operation and through put of the screw presses used for the extraction of the oil.
f) Bio chemical changes also appear to take place during the process of sterilization, having a beneficial effect on the process of clarification.
25.04 Sterilization is commonly achieved by means of live steam admission into the sterilizing vessel loaded with f.f.b. in partly perforated steel cages.
The steam pressure is usually 3 kg/cm2 (±42 lbs/inch2) and a properly
controlled cycle with 30 minutes or more at this pressure will generally give satisfactory sterilization results.
25.05 As noted previously, the standard of ripeness of the f.f.b. delivered may necessitate the cycle times to be varied to suit the type of fruit to be sterilized.
Very ripe fruit can be sterilized in a shorter period, whilst when fruit is well set, under ripe and hard the sterilization times may have to be extended .
Double or triple peaks can be used to ensure proper sterilization and good stripping of the bunches, the latter is the most common.
Air release
25.1.01 When a sterilizer is loaded with fresh fruit and the door is first closed, the vessel is full of air.
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Air is a poor conductor of heat and must be removed from the vessel so that the heat transfer to the bunches will not be impaired.
25.1.02 There are two practical methods of air removal from sterilizers, i.e.: a) steam sweeping and
b) diffusion, followed by blow off of steam. a) Steam sweeping:
Steam is lighter than air and as the vessel fills with steam it will sweep the air downwards and force it through the de-aeration valves.
This action should be controlled so that there is as little turbulence as possible.
Strong turbulence will mix the steam and air and pockets of air will remain in the vessel; within these pockets there will be low temperatures.
An idea of the time needed to clear the vessel of air can be obtained by considering the volume of the vessel and the area of the air release valves.
Example:
A 10 x 2.5 tonne cage sterilizer is 2.1 meter in diameter and 30 meter long, i.e. has a volume of about 104 cubic meter.
If 4 x 75 mm outlets provided, these will have a combined open area of 177 square centimetre, (0.0177 m2 ).
The vessel should be cleared of air in say 10 minutes, thus the average speed out of the valves during de-aeration must be about 9.8 m/sec (35 km/hr).
25.1.03 The fruit and the cages have a volume so there is actually less then the volume of air calculated above, but inevitably air and steam will mix, the air will not necessarily be swept evenly to the outlet valves etc. so the estimate of time can be accepted as reasonably valid. To achieve an average speed of 9.8 m/sec through the outlet valves, the vessel must be under pressure during the steam sweeping, de- aeration process.
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At the start of the cycle the vessel and its contents will be cold and the steam first admitted will be condensed and there can be neither an increase in pressure nor an air displacement until the rate of the steam admission to the vessel exceeds the rate of condensation. To achieve the required de-aeration the steam admission must be relatively high and this will conflict with the need to reduce turbulence.
To keep turbulence within acceptable levels sterilizers are fitted with a steam distributor along the top of the vessel to ensure an even dispersion of steam.
This distributor is essential and must have a properly selected cross sectional area and openings correctly sized to ensure good steam distribution, evenly along the whole length of the sterilizer.
As the fruit is held in cages, the air must be removed from these cages as well as from the open volume of the vessel and to ensure that this is achieved it is essential that both the size and the number of holes in the cage sides and bottom plate are adequate not only for the purpose of admitting steam into the cage and fruit, but also allow air to escape.
25.1.04 With a well designed system and if care is taken it should be possible to sweep the majority of the air from the open volume of the sterilizer and the cages, but the air trapped in the bunches is not swept out.
b) Diffusion:
Steam pressure will tend to compress this air into the bunch so that the fruit in the centre of the bunch will remain surrounded by air and thus not be subjected to heat.
After a period of time has elapsed, the steam will diffuse into the air and air will be displaced to allow heat penetration, but with very tightly knit fruit bunches there will not be sufficient time for the diffusion to be effective within the designed time of the sterilizing cycle.
This slow diffusion of steam into and the displacement of air from the bunches can partly be explained by the fact that steam entering the bunch will condense until the surfaces of the bunch exposed to the
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steam reach saturation temperature.
If fruit is well set then the diffusion process must be assisted by intermediate blow offs from the sterilizer.
25.1.05 Theoretically, if pressure is released from the sterilizer at 3 kg/cm2, one
quarter of the air left in the vessel should also be removed.
More importantly, the air trapped in the bunches will be released, so that on the next application of pressure steam will penetrate further into the bunch.
After steam sweeping and one blow off very little air will remain in the open volume of the sterilizer, but there will still be air trapped in the bunches.
25.1.06 The timing for subsequent blow offs (i.e. in multiple peak sterilization) could or should be delayed until there is some "heat" in the bunches. Since diffusion is assisted by turbulence in the gases steam admission after blow offs should be at the maximum rate attainable to give the best possible conditions for diffusion.
"Triple peak" sterilization has proved to be effective in assisting the best air release from the bunches during this sterilization.
Despite this and however careful the air sweeping is carried out, air will find its way to the bottom of the sterilizer vessel throughout the cycle and provision for the continual removal of this air (quite separate from the removal of condensate) must be made.
25.1.07 Temperature gauges or recorders fitted to the sterilizer provide a check on the temperature obtained in the vessel.
There will be no air in the top of the vessel, thus the temperature measured there will always be that of the corresponding steam pressure and does not necessarily reflect the temperature of the fruit that is to be sterilized.
A more accurate indication of that temperature can be gauged by placing the sensing device below the level of the fruit cages.
Any condensation droplets removed with the air release when the vessel is under pressure will flash off into steam under normal
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atmospheric pressure, thus the sign of steam emerging from the air release outlets does not indicate that all air is released and the valves can be closed.
25.1.08 Air release valves must be provided with a bypass without a valve to ensure a continuous and adequate release of air throughout the complete sterilizing cycle.
25.2 Condensate removal:
25.2.01 As steam is used in the sterilizer it condenses and this condensate has to be removed from the vessel for several reasons:
a) If it is not removed it will flood the bearings of the cage bogies, wash out the lubricating agent and ruin the bearings.
b) If the level is allowed to rise any further up to the level of fruit any "free" oil and oil out of the bunches will be washed out in excessive quantities.
c) The "free" oil on the surface of the fruit is a result of damage and bruising of the fruit and this oil has a high fatty acid content and is therefore quite corrosive.
The mixture with the condensate will thus be of a corrosive nature and attacks the steel work of the sterilizer.
This corrosion cannot be totally eliminated since condensate must flow out of the vessel, but should be minimized as much as possible by keeping the vessel as free of condensate as practicably possible.
d) At the end of the sterilizing cycle any free condensate still left in the vessel will flash off and thus increases the total blow off time of the sterilizer.
25.2.02 Condensate must be cleared when pressure in the vessel is still low to prevent a build up of condensate through the main condensate valves, whilst a "constant bleed" system with sample capacity to ensure continuous adequate removal of the condensate formed throughout the cycle must also be provided.
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valves should be used at least once during the cycle to make sure no condensate can build up in the vessel.
Hot condensate "blown" out of the sterilizer will flash off into steam and adequate provisions should be made for a condensate drainage system away from the operating area of the sterilizers.
Adequate in this sense must take into account that at the start of the cycle both air and condensate released are at their maximum and that these should (must) be provided with separate systems. (Pipes full of condensate cannot allow air to pass out.)
25.3 The sterilizing cycle
25.3.01 A full sterilizing cycle consists basically of three phases : i) Steam pressure build up
ii) Constant pressure phase iii) Blow off
i) Steam pressure build up
The pressure build up in the sterilizer must be at a rate that will allow the requirements of proper de-aeration as described before and the constant pressure phase of minimal 30 minutes at 3 kg/cm2 to be attained within the designed or preferred cycle
time.
The supply of steam and the piping transporting this steam has therefore to be calculated large enough to achieve this aim. The very large quantity of steam required at the start of the cycle is usually limited by the capacity of the back pressure system and if supplemented by live steam of reduced pressure direct from the boilers, by the capacity of the steam boilers supplying this steam.
A pressure build up to 3 kg/cm2 in a time of maximum 10 minutes
would be ideal, but very few systems in C.P.O. mills are able to achieve this. (see under 25.4, Steam consumption)
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ii) Constant pressure phase
While careful attention must be given to the other parts of the cycle, it is in the constant pressure phase that the action aimed for in the sterilization process are accomplished.
Thus, if full pressure for the required length of time is not maintained, the careful control of the other phases will be wasted and of no value.
This is particularly so in multiple sterilizer installations where one sterilizer has reached full pressure and another vessel starts is pressure build up.
The tendency for the pressure in the first vessel to drop is to be watched and must be avoided.
If adequate steam supply cannot be obtained, the systems should be regulated so that this drop can be avoided and full pressure maintained.
iii) The blow off phase
Blow off must be completed as fast as possible, by fully opening the blow off exhaust valves, in order to achieve as much dehydration as possible and keep the non productive periods of the total sterilizing cycle to a minimum.
A complete blow off from full pressure to atmospheric pressure in about four minutes would be ideal.
Towards the end of the blow off, condensate main valves should be opened to ensure the removal of any condensate liquid still left inside the vessel.
Pipe lines and, if utilized, exhaust silencers should be adequately sized so as to not restrict the volume of the blow off and thus lengthen the time for this.
25.4 Steam consumption
25.4.01 The low pressure steam in the C.P.O. mill is usually provided by means of a back pressure system from the electrical power generating steam engines or turbine sets.
Such equipment can be, and for C.P.O. mills usually is, designed with an exhaust or back pressure steam at 3 kg/cm2.
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This provides sufficient temperature for sterilization, higher pressures and hence higher temperatures could result in decreased oil quality. Steam pressure and steam temperature are not proportionate, therefore an increase in pressure would not significantly improve the rate of heat transfer to the f.f.b. to be sterilized.
25.4.02 The steam consumption of the sterilizing relates almost entirely to the mass of metal and fruit that has to be brought up to temperature and to the losses through radiation and blow off.
25.4.02 The insulation of the sterilizer, provided it is in good condition, assists in reducing radiation losses as much as possible whilst keeping the time lag between (last) blow off and the steam admission for the next cycle as short as possible also assists considerably in keeping the sterilizer metal "hot" and thus reducing the consumption.
25.4.03 Theoretical calculation of STEAM DEMAND :
The sterilizer can be considered as a condenser with at the start of the cycle a very high condensing capacity and thereafter a quickly reducing capacity down to more moderate levels with a practically zero condensing capacity at the end of the cycle.
The quantity of condensed steam per unit of time (Q) equals:
Q = F x ∆ t x α
F = total surface in contact with the steam
∆t = temperature difference between the fruit surface and the surrounding steam.
Α = heat transmission co efficient
The total surface in contact with the steam at the start of the cycle has then the highest value, since the fruits are still hard and the bunches in the sterilizer cage are touching each other on only a few points and the total of these surfaces are rather small.
25.4.04 At the start of the cycle these surfaces are still dry and has been shown that droplet condensation will occur.
The heat transmission co efficient (1) then has a very high value of : 70.000 kcal/m2.hr.oCelsius.
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At the start of the cycle t has the highest value too, since: temperature of steam at atmospheric pressure = 100 o C
the temperature of the fruit surface say = 30 o C
————————— The temperature difference therefore = 70 o C
25.4.05 Towards the end of the cycle the situation is quite different, since in
the first place there is no droplet condensation any more, but a film condensation instead, with a heat transmission co efficient (2) of about : 6000 kcal/m2.hr.oC.
25.4.06 Secondly the surface in touch with the steam has been reduced as well since the fruits have become softer and the initial firmness has disappeared and the mass of bunches show a distinct shrinkage.
The surfaces of contact between the bunches is increased and consequently the fruit surface in contact with the steam has decreased.
It is rather difficult to establish the exact reduction in the free condensing surface, but an acceptable estimate would be that the reduced surface is 90% of the original surface. (F2=0.9F1)
25.4.07 Thirdly, the temperature difference (∆t) has been reduced to a great extend.
The aid of thermo couples the temperature difference between the surrounding steam and the layer of pericarp nearest to the nut has been measured and the tests have shown that the assumption of a temperature difference at the end of the cycle of 4o C is acceptable.
(t2 ) (The longer the cycle, the smaller this difference will be )
With the aid of the above, the condensing capacity at the start of the cycle can be calculated to be:
70.000 kcal/m2 . hr . o C x F1 m2 x 70 o C = 4.900.000 kcal x F1 / hr.
25.4.08 Towards the end of the cycle this can be calculated to be:
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In other words, the condensing capacity is about 200 times bigger at the start of the cycle then towards the end of the cycle.
25.4.09 The average steam consumption for a single peak sterilization operation is between 180 and 200 kg/ton f.f.b.
Figure1 shows the calculated steam consumption of a sterilizer, with no restriction in steam supply.
This figure shows that the peak consumption is about 7 times the average consumption.
Thus for example, for a sterilizer with a capacity of 15 tons f.f.b. and an actual steaming time of 60 minutes the average steam