Various Activity Of Oil Mill
Some of the oil seeds are crushed in oil expellers directly and some are not crushed. This decision is taken based on the oil percentage in the oil seed. In general if the oil percentage is more than 20% the seed is treated in expellers, otherwise. It is not economical to expel the seed.A
1. Raw Material Procurement
At the time of purchase the purchaser should consider moisture content, percentage of damaged seed, maturity of seeds, oil percentage and other foreign matter, The rates are decided based on market price considering the above factors. There are guidelines available in different markets and specifications are followed accordingly. Raw material procurement is a very critical factor affection productivity and profitability.
2. Storage of Oil Seeds
Normally the oil seeds are stored in godowns packed in gunney bags. Care should be taken of proper moisture content is seed and proper aeration. Sometimes the seeds get damaged due to self combustion, if they are not dry. The most modern method of storing of oil seeds is to stock them in ling vertical well ventilated bins or “silos” which may be constructed either with concrete, steel or wood.
Before actual crushing of the seed for obtaining oil, following important steps should be observed.
A. Pre-Cleaning of the Seed
Seeds usually contain impurities like sand, stone, stems of plant, earth sutli, leaf, grass etc. Sometimes they contain iron and metallic pieces also. These impurities should be removed in order to obtain high quality of the oil. These impurities inspite of injuring the quality of the cake, lower the oil yield by absorption of certain amount oil and tends to wear out of the machinery.
Pre-cleaning of the seed increases the life of expellers and minimises unwarranted breakdowns and keeps the better quality of finished products.Different types of seed cleansing machines are used in the oil mill industry:
- Hand cleaning machines
- Stationary seed cleaning machines
- Vibration seed cleaning machines
- Rotary seed cleaning machines
- Brushing machines
Before passing the seeds to decortications machines, it should pass through the de-stoner machines, which will remove the stones of the seed size, which might have passed through pre-cleaning machines.
Certain seeds like groundnut, castor and cotton seeds have outer covering known as shell or husk. This husk contains practically no oil and represents approximately 30% of the total weight of the seed. Its removal before crushing is important due to the following reasons:
- It increases the pressing capacity of the mill.
- If husk is allowed to be pressed, it forms the part of the cake and absorbs oil to an extent as would be lost in the cake. This, therefore results in the low yield of the oil.
- The presence of husk in the seeds also impart colour to the oil.
- The husk being of siliceous nature, have a detrimental effect on the expellers.
The decorticator machine setting needs checking very often to ensure that husk do not carry small kernel particles, which otherwise is a direct loss.
In these machines the seeds are passed through special rollers set up with cutting edges and ridges which cause break in their shells, while the seed remains untouched. Various types of machines are used to decorticate depending on the size and nature of the seed. Various makes of groundnut decorticators, castor seed shellers and cotton seed hullers are available in the market. After breaking the husk, the seeds are cleaned by blowing the husk carefully.
C. Seed Conditioning
In order to extract better quality of oil most efficiently, using minimum pressure and electricity, proper conditioning of the seeds is very essential. The following two factors ensures mainly most effective conditioning.
D. Seed Cracking
Before crushing of seed it is desirable to pass the seed in a seed cracker machine, wherein seed is allowed to pass through two rotating chilled rollers of steel. In some cases the rolling operation is performed by passing seed through a set of five heavy rolls, staged vertically above each other. Several types of machines like disintegrater, edge-runner, and reduction rolls high etc.are employed for the preliminary reduction of a variety of seeds like mahua, Copra, Rape, Sesame, Groundnuts etc. For Copra special cutters and breaking machines are used. The cracked seed facilitates efficient cooking and drying of seed later on in expeller kettles.
E. Cooking and Drying
This is the most important step in the extant step in the extraction of oil by expeller method. The cracked seeds are properly cooked with open steam. Following are the main objects of cooking:-
- To regulate the moisture content of the seed meal.
- To coagulate the proteins in the walls of the fat containing cells.
- To make the cell walls permeable to the flow to oil.
The oil droplets are almost ultramicroscopic in size and are distributed throughout the seed. One effect of cooking is to cause these very small droplets to unite in to drops large enough to flow from the seed. Second effect of cooking is drying of the seed to give the seed mass the proper plasticity for efficient pressing. Third effect of cooking is to decrease the affinity of the oil for the solid surface of the seed, so that the best possible, yield of oil may be obtained when the seeds are subsequently pressed. Other important effects of cooking are insolubilization of phosphatised and other undesirable impurities, destruction of molds and bacteria, increase of flu cotton seed, detoxification of gossypol or related substances. Very dry seeds cannot be expressed efficiently. Cooking operation regulates the moisture. The optimum moisture of cooked seeds varies widely accordingly to the variety of the seed and the method to be used for expression.
During the process, the cracked seed is cooked in steam kettles made right over the expeller machine. The temperature of the seed is raised and the moisture is again be brought down to about 5%by drying the cooked seed in a steam jacketed type kettle. Normally the real retention time in expeller kettle for cooking /drying operations is a considered40-45 minute form groundnut.However, this may very little from seed to seed.
There are however no hard and fast rules for the treatment of these seeds, and the operation is purely a matter of experience.
The theory involved in the process of cooking is that the combined heat and moisture break up the oil cells, soften their gelatinous coatings, increase the clarity of the oil and coagulate the aluminous sesediment forming particles in the seed, thus making it readily, possible for the oil to flow out easily when subjected to pressure.
F. Expelling the Seed
The expeller consists of a pressing box or cage surrounding a horizontal shaft mounted with a series of worms. At the feeding end of the cage there is an opening through which the meal is fed into the expeller. At the discharge end of the cage there is a cone which practically restricts the passage. The rotation of the horizontal shaft causes the meal to push forward by its screw action, thereby increasing the internal pressure and thus squeezing the oil. The internal pressure of the expeller is regulated by the adjustment of the cone. The extracted oil flows through the perforated box, while the cake passes out through the opening around the cone.
The expeller is a self contained unit which has its own heating kettle and an arrangement for automatic feeding. The expeller requires no attention after setting so long as proper temperature and moisture content of the seed meal is regulated. The meal as it passes through the expeller becomes hotter by liberation of frictional heat.
For the satisfactory extraction of different oil seeds the design of the worms plays a very important role. High grade steel worms possess great resisting power for pressure and abrasion, thus resulting in their long life.
Oil expellers are designed to operate at two pressures LOW and HIGH. Low pressure expellers remove only a portion of oil in the seed meal and generally have very high capacities ranging from 30-80 M.T.of seed per day of 24 hours. They are used in combination with their high pressure expellers or with solvent extraction plants or to extract the maximum oil from high oil containing seeds from which complete extraction at a stretch is not generally possible. High pressure expellers produce at one stretch all the possible extractable oil from the seed meal. High pressure expellers operate at pressure of 2.5-3.0 ton per square inch and have low capacities.
G. Filtration of Oil
The oil which comes out of the expeller contains lot of foots, suspended particles of seed and cake. This oil requires filtration before it is stored in storage tanks. This filtration takes place by pumping the oil to the filter press. Where it passes through a layer of the filter cloth. The liquid oil passes through the filter cloth and solid matter deposits on the filter cloth.
- There are two important types of filter presses, generally used in oil mill industry.
- Recessed plate or chamber type filters press.
- Plate and frame type filter press
- Recessed plate types Filter Press:
This consists primarily of a series of cast iron plates. These plates are recessed and have a raised rim round the periphery of each face, so that when two plates are brought together the dished centres from a chamber in between them. While the press is open a filter cloth is laid over each plate, hanging down on both sides and extending beyond the edges. Through the centre of each plate there is a circular feed hole and there are corresponding holes in the filter cloth. The filter clothes are secured round the central feed hole by special tightening nuts.
When all the plates and clothes are assembled together and tightened, the cloth forms a tight joint between the machined edges of the plate. The tightening of these plates is effected by means of rack and pinion arrangement. The pinion is rotated by means of a tommy bar, inserted in the holes round its flange.
Plate and Frame type Filter Press
In this type of filter press the edge of the plates are all finely machined and are almost in level with the filtering surface . An independent frame is used between these plates to form a chamber. The plate and the frames are arranged alternatively and the filter cloth is simply hung over. The clothes are nipped between the edges of the plates and the frames when they are closed together. The plates have special recess to form the fitting surface and for drainage. The filter oil is discharged by means of bib spout or cocks.
This type of filter press has an advantage over recessed plate type filter press incapacity. These can hold more quantity of sediments, so more oil can be filtered at a time without opening the filter press.
The Foots separated by the filter press are generally mixed with the oil seed fed to the expeller.
4. Storage of Oil and Cake
Storage of Oils
The filtered oil from the filter press is usually pumped on to the storage tanks. The capacity of oil storage necessary for a mill generally depends on its capacity and commercial considerations. Greater storage capacity is always considered better since it enables the crusher to continue its operations even when the oil market is unfavourable.
Storage of Cake
The storage of oil cake is also as important as the storage of oil seeds. The fresh cake is hot and contains superficial moisture. This cake is cooled by spreading it in well ventilated floors for a period of 24-36 hours. Thus the moisture and temperature are generally adjusted according to the atmospheric conditions. The cake now can be filled in gunney bags and stacked properly. Particular attention should be give to cake god owns for proper ventilation.
5. Quality Control
The oil mill managers should be q quality conscious. They should purchase the raw material of good quality for getting best yields and good profits. This rule applies to oilseeds as well as other inputs like gunney bags, fuel, spare parts etc. They should always have an eye of the oil content of the oil seeds purchased. Similarly they should check regularly the quality of the oil and cake produced in their mills.
They should always use best make, standard and repudiated spare parts in their machines to reduce the break downs and to increase the efficiency of the machine.