SRS SOLVENT EXTRACTION PROCESS

Introduction:

Solvent Extraction is a process to extract oil from oil-bearing materials by treating it with a low boiler solvent as opposed to extracting the oils by mechanical pressing methods (such as expellers, hydraulic presses, etc.) The solvent extraction method recovers almost all the oils and leaves behind only 0.5% to 0.7% residual oil in the raw material. In the case of mechanical pressing the residual oil left in the oil cake may be anywhere from 6% to 14%. The solvent extraction method can be applied directly to any low oil content raw materials. It can also be used to extract pre-pressed oil cakes obtained from high oil content materials. Because of the high percentage of recovered oil, solvent extraction has become the most popular method of extraction of oils and fats. Fat is one of the essential components of the human diet, therefore the demand for oils and fats are increasing with the in-crease in population and standards of living. Today large quantities of oil cakes such as peanut, cottonseed, linseed, kardiseed, neem, castor, mowha, copra, sunflower, etc. are extracted. Direct extraction of rice bran, salseed and soybean is also used.

The Process:

Solvent Extraction is basically a process of diffusion of a solvent into oil-bearing cells of the raw material resulting in a solution of the oil in solvent. Various solvents can be used for extraction. However, after extensive research and considera-tion of various factors, such as commercial economics, edibility of the various products obtained from extraction, physical prop-erties of the solvent especially its low boiling point etc. Hexane is considered to be the best and it is exclusively used for the purpose.

In a nutshell, the extraction process consists of treating the raw material with hexane and recovering the oil by distillation of the resulting solution of oil in hexane called miscella. Evaporation and condensation from the distillation of miscella recovers the hexane absorbed in the material. The hexane thus recovered is reused for extraction. The low boiling point of hexane (67°C / 152°F) and the high solubility of oils and fats in it are the properties exploited in the solvent extraction process.

The entire extraction process can be divided into the following stages.

1. Preparation of raw material
2. Process of extraction
3. De-solventisation of extracted material
4. Distillation of miscella
5. Solvent recovery by absorption
6. Meal finishing and bagging

Because of the highly inflammable character of the normal hexane, those stages of process which involve high speed machineries such as material preparation, finishing and bagging are carried out at least 50 feet away from the main extraction plant wherein the remaining processing stages involving handling of the solvent are carried out. The typical flow-chart below illustrates the various processing steps.

Preparation of Raw Material:

For thorough and efficient extraction, it is necessary that each and every oil-bearing cell of the material be brought in contact with the solvent. Therefore, proper preparation of materials prior to extraction is very important to ensure this contact. The smaller the material size, the better is the penetration of the solvent into the oil-bearing cells; but too fine a size will prevent the solvent form percolating through the mass. Therefore an optimum size is to be maintained for best extraction. Hence material preparation methods vary from material to material depending on its oil content, size and physical properties. For high oil content materials (oil content of 15% or more), the following steps of preparation are recommended to make the mate-rial suitable for penetration of the solvent into the oil cells as well as for best percolation.

1. Passing the seeds through corrugated roller mills with 3 mm flutes to reduce the size to about 3mm.
2. Heating the broken material to about 80°C with open steam in temperor and humidifying the material to raise the moisture content to about 11 to 12%.
3. Flaking of the humidified material between a pair of plain rolls to 0.25 mm thickness or lower.
4. Conveying the flakes to the extraction system after crisping them firm.

Rice bran is a fine floury material and therefore is bound to obstruct the percolation. The best preparation of rice bran for extraction is found to be pelletising the same after tempering with open steam. The pelletised bran is then crisped in a current of air while conveying to the extractor.

Some oilseeds such as cottonseed and soybean can be directly extracted but they are to be decorticated by special equipment to separate the oil-bearing meats from the hulls. The decorticat-ing equipment varies from seed to seed. The decorticated meats are tem-pered, flaked and the flakes are then sent to extractor after crisping.

Process of Extraction:

The prepared material enters the extractor through the rotary air seal. The extractor consists mainly of a very slow moving articu-lated band conveyor inside a totally enclosed chamber. The band is lined with perforated sheets and porous stainless steel cloth. The mass of the material moving on this band forms a slow moving bed. During the movement of the bed through the extractor it is washed continuously at various points with miscella of decreasing concentrations and finally with a fresh solvent in a counter current manner by means of sprayers kept in a line over the meal bed. The miscella percolates through the perforated bottom and collects in various hoppers kept below the bed. The miscella from the last hopper, which is concentrated, is taken off for distilla-tion.

De-solventisation of Extracted Material:

After the fresh solvent wash the material is discharged from the band conveyor into an airtight chain conveyor, which conveys it to the Desolventiser. In the desolventiser the material is heated to about 100°C by jacketed steam, and thus the absorbed solvent is evaporated into vapors (boiling point of hexane 67-70°C). Finally, the material, which is now completely desolventised is continuously discharged through airtight seal into a pneumatic conveyor, which carries into the bagging section. The vapours evolved in the desolventiser are led through a dust catcher wherein they are washed with hot water to a condenser.

Some materials, such as cottonseed and soybean extractions are toasted after de-solventisation. In these cases both the steps of de-solventisation and toasting can be combined into one operation by the use of Desolventiser - Toaster (D. T.) Instead of the tubular jacketed desolventiser.

The D.T. consists of a vertical cylindrical vessel with horizon-tal jacketed compartments and a central rotating vertical shaft on which are mounted sweeps in each compartment. The Material to be desolventised and toasted is fed in to the top compartment of D.T and heated with open steam. Open steam condenses a lot of moisture in the material at the same time evaporating the solvent. The moisture up to 14 to 15% is condensed. The material then flows to lower compartment. In lower compartments the material is gradually heated to 115 to 120° C thus evaporating all the solvent, cooking the material and driving away extra moisture. The cooking in presence of moisture destroys undesirable enzymes.

The resulting high temperature toasts the material. The solvent and water vapours from various compartments are led first to a dust catcher wherein they are scrubbed with hot water spray to remove fine dust and then led to a condenser to condense the vapours. The de-soventised and toasted meal from bottom-most compartment discharges into a redler conveyor.

Distillation of Miscella:

The final miscella (solution of oil in hexane) obtained from the extractor is collected in a tank form where it is pumped to the distillation column kept under vacuum by means of a series of steam ejectors. The miscella is heated by jacket steam in the distillation column and thus the hexane is turned into vapour immediately. The vapours are led to another condenser through an entrainment separator.

The concentrated miscella from the evaporator is pumped into a similar secondary distillation unit to raise the temperature to about 100 - 110°C and then into the final stripper kept under high vacuum. Open steam is injected in the latter to strip the last traces of hexane from the oil. The vapour both from the secondary still and the stripper are condensed in a third condenser. The oil freed from solvent is pumped from the stripper to the storage.

Solvent Recovery by Condensation:

All the condensers are of floating head type with tube-bundles to carry the cooling water. The cooled water at 30°C or below is circulated inside the tubes and all the condensers and the vapours are passed outside the tubes. As a result the vapours are cooled and condensed into liquid. The uncondensed vapours from each condenser are sucked by a series of ejectors and pushed through the last condenser to a contact cooler where they are washed with cold-water spray. All the condensate liquid hexane water from these condensers and contact cooler is led to a solvent water separator where the pure solvent is separated from water by settling the difference in densities of water and the solvent and their immiscibility accomplishes complete separation. The fresh pure solvent from this tank is pumped to the extractor continuously for the final washing of the meal bed

Final Solvent Recovery by Absorption:

The vapour and gases from the contact cooler are led to absorber where they come into intimate contact with absorbing oil (vegetable oil or mineral oil). The solvent vapours if any, are absorbed in this oil and non-condensable gases are let out into the atmosphere. While theoretically these gases leaving the plant are expected to be free from hexane, in practice, a small amount of the solvent is lost with these gases.

The oil containing the absorbed solvent is led into an evaporator kept under vacuum and heated to 100°C. The solvent is vaporised and these vapours are led into one of condensers and recovered.

The hot oil from the evaporator is passed through a cooler to cool to room temperature, and having been freed from hexane it is sprayed back into the absorber.

Meal Finishing and Bagging:

The redler conveyor carries the desolventised meal form the DT to bagging section. The meal is not only conveyed but also cooled to about 45-50°C by means of cold air draft induced in the conveyor by a blower. The meal drops to a humidifier from the redler.  In the humidifier the meal is mixed with enough moisture to bring up the moisture content, thus replacing the amount of water lost during the extraction and de-solventisation steps. The humidified meal is then bagged at the discharge of the humidifier.

SALIENT FEATURES:

The following are some of the salient features of the solvent extraction plant offered by SRS.

1. All the unit operations of the plant including extraction, distillation, de-solventisation, condensation, absorption etc. are continuous thus enhancing the operational efficiency.
2. The plant is automatic and as such after the manual feeding of the raw material at the feeding end in the preparatory section until the actual bagging of de-oiled material in the conditioning section, the processing is carried out mechanically. Therefore, the operation of the plant becomes very easy.
3. Most of the control instruments are located to the operation level to facilitate easy operation.
4. Thermostats installed in the distillation section and the wastewater ensures automatic control of temperature of oil and wastewater.
5. Level controllers and interlocking signalling system in the plant ensure smooth flow of the material through the entire system.
6. One of the factors responsible for solvent losses in an extraction plant is the amount of air passing into the system through the raw material inlet and de-oiled cake outlet. To minimise the air going in through the outlet, a material seal is maintained, thus in effect reducing the solvent losses.
7. Heat exchangers are provided to minimise the heat loss in the process thus resulting in substantial steam saving and increasing the profits.
8. For the final solvent recovery from vent gases a rotary oil absorber is provided to ensure high absorbing surface.
9. Layout of the equipment in this plant is arranged in such a way as to enable easy installation of balancing equipment to expand the capacity at a later date.
10. It is a general experience that the actual performance of SRS solvent extraction plant is better than what it is de-signed for.

OIL MILLING - AN ART (MECHANICAL EXTRACTION)

Oil milling is an art. Care has to be taken while expelling the oil to ensure maximum oil recovery with minimum power consumption; cake should not be burnt, protein should not be damaged, colour fixation of oil should not take place, minimum heat should be generated while expelling; moisture should not find its way with oil, filtration should be in such a manner that no cake particles remain in contact with oil, thus FFA of oil should not increase.

Seeds should be cooked properly and conveying should be provided to make operation totally mechanical. SRS design takes care of all this aspects, added is the decades of experience. Well-designed plant layouts, trouble free maintenance, ease of operation, low utility consumption makes SRS oil mill a viable proposition.

Oil milling (mechanical extraction) can be only prepressing in which case seeds are lightly pressed, and which leaves about 14% to 18% oil in the pressed cake. This method results in higher capacity; lower power consumption, lower wear & tear / maintenance. But it also lowers recovery of oil, it is recommended when the buyer has his own solvent extraction line, so that balance oil is recovered during solvent extraction process.

In the second method, when pressed cake is to be sold in the market, it is desirable to recover the maximum oil. Hence double pressing system is recommended. Here either in the same screw press - two stage pressing is carried out or prepressed cake from first stage screw press in sent for re - second pressing to other screw press. The oil left over after double pressing varies from 7% to 9% depending on the oil seed and extent of pressing.

Any oil-bearing seeds can be processed in an oil mill (mechanical extraction system), special preparatory equipment are recommended prior to expelling. For Cottonseed, Sunflower seed, Groundnut seed, decorticators are recommended. For copra shredders and for Palm kernel hammer mills are required as preparatory equipment.

SOLVENT EXTRACTION PLANTS:

• Solvent extraction plants of any type whether solid liquid or liquid liquid extraction system
• Capacities ranging from 1 MT per day to 1000 MT per day
• Extraction plant using various solvents polar or non polar or a combination of both as demanded by the process.
• Versatility for processing any kinds of seeds, bran or oil cakes. (Soya beans, sunflower, canola, cotton seed, ground nuts, rice bran, and any oil cakes).
• Other minor oil seeds processing such as Mango kernal, Mowha, sal seed, kokum, Dhupa and shea seeds.
• Unique distillation design for short contact low temperature design where the miscella is evaporated in a short period of 2 minutes and at a maximum temperature of 80°C
• Low temperature desolventisation and flash desolventising system to produce high PDI value meal.
• Unique feature like thermo syphon heating for controlled and efficient use of steam Inbuilt zero effluent discharge system designed without additional energy requirement.
• Instrumentation with PLC and SCADA software for total instrument control or partly with data logging and acquisition.

Note: The information contained herewith is issued as a guide and not intended to be definitive. All reference to technical and commercial information herein is subject to our Conditions of Sale.