Purpose:
The purpose of sterilising FFB (Fresh Fruit Bunches) is to treatment them for further processing.
What is sterilising?
Sterilising is a process where the fruit bunches are cooked under steam pressure. Cooking is normally carried out under 3 bar pressure (45 pounds per square inch – psi) for approximately 90 minutes.
During the cooking process the fruits become soft and the enzymes that produce free fatty acid are deactivated. The fruits are also easier to be detached from the bunch after the sterilising process. During the heating under pressure and the subsequent realising of pressure there is a moisture loss from the fruit bunch of about 10%. The loss of moisture from the stalk joining the fruit to the bunch enables the fruit to be easily separated from the bunch. The separation of the fruit from bunches is carried out in the next stage of the palm oil extraction process, called threshing.
Picture of sterilisation station
The sterilising Process.
The fruit bunches are loaded into the fruit cages. The cages with the fruits are pushed into the sterilisers. The movement of cages can be by hydraulic cylinders mounted on the floor in between the rail tracks or by winches.
Once the cages are inside the steriliser the doors are closed. The cooking is by steam. This is let in by opening of the inlet valves. In most palm oil mill the opening and the closing of steam valves is automated and controlled by a simple programmer.
The fruits are normally cooked under steam pressure for 90 minutes. Once the cooking is complete and the steam exhausted from the steriliser. The doors are opened and the fruit cages pulled out of the steriliser.
The cages are the emptied either by being lifted by a crane and rotated on to a thresher or rotated at ground level by a tipper.
The steriliser is a pressure vessel and as such it should be designed to pressure vessel design codes. The shell thickness of the steriliser is normally 12mm. The bottom half of the inside of the steriliser is lined with liners of mild steel or stainless steel so that when the wear they can be replaced. The liners protect the body of the steriliser from wear and extend the life of the steriliser. The doors and the dish end of the steriliser should be protected by full liners. The doors are expensive to replace. The body of the steriliser is insulated so that the heat loss is minimised.
The sterilising capacity for a palm oil mill depends on the design capacity of the mill. The sterilised fruit must be available at the right time in the right quantity so the mill can achieve the designed processing capacity.
As a sterilising cycle take approximately 2 hours -cooking time and door opening and closing.
Therefore for 30 tons per hour processing capacity the sterilising capacity should be 60 ton. If the mill is designed with cages of 10 tons the mill should have two sterilisers of 3 ten ton cages each. The fruit cage size normally varies from 2.5 tons to 20 tons depending on the design of the mill.
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