Cottonseed, a by-product of the cotton ginning operation, is covered with fibrous lint. The cottonseed primarily consists of an inner kernel, usually called "meats." This meaty portion contains all the oil as well as proteins, and it is enclosed in the fibrous hull. The hulls protrude over the cotton fibers that escape the ginning process and are left on the seeds. These cotton fibers are referred to as "lint."
The inner kernel can be planted, fed to livestock, made into cottonseed oil for human consumption, used as fertilizer, or utilized in products like lotions and soap. The lint obtained from cotton delinting has wide applications in the cotton blanket, sweatshirt, paperboard industry, and cotton pulp, among others. Therefore, the cottonseed delinter is essential in cotton processing.
Cottonseed is first cleaned of foreign materials such as stones, shale, sticks, iron, and earth. The foreign matter in the seed can be anywhere from 2% to 5%, depending on how kapasis are handled in the fields. It is essential to remove foreign matter to protect the processing machinery from damage and excessive wear and tear. Seed cleaning is carried out using reciprocating type sieves to separate unwanted materials. The seeds are then pneumatically lifted to separate heavy materials such as stones and iron. The cleaned seeds are fed into a battery of delinting machines, where tightly packed rolls of seed are pressed against high-speed circular saws. The saws scrape off the lint from the seed, which is then pneumatically sucked into lint beating and cleaning devices.
The lint beating and cleaning processes are done to increase the cellulose content of the lint, and the product is bagged. Delinting is generally performed in two stages: in the first stage, a light cut is taken, and in the second stage, a more severe cut is made. The fiber length of the first lint is much longer than that of the second cut lint. Long fiber is preferred for use in the paper and artificial silk industries. The recovery of lint varies from seed to seed, depending on its original lint content, but generally, the recovery is about 1.9% to 2.2% for the first cut and 3.8% to 4.5% for the second cut.
The delinted seed is then fed into hullers, where the outer hull of the seed is cracked by knife edges mounted on high-speed rotors. The mixture of hulls and meats produced is passed over reciprocating sieves to separate the meats from the hulls. The hulls are processed through a series of separators and beaters to ensure that they are free of entrained oil and protein-bearing meats before being bagged. The meats, containing about 35% oil and 35% protein, are conveyed to the expelling section for crushing. A portion of the hulls is allowed to remain with the meats to adjust the protein content of the final meal to the desired level.
In the expelling section, the meats are cooked in a tempering apparatus and fed into a series of expellers consisting of heavy screws working in strong cages. The meats are subjected to pressure as high as 5 to 10 tonnes per square inch. At this pressure, the oil is squeezed out through slits in the cage, and the meats are pressed into oil cakes that contain about 10% to 12% oil and are discharged through the cage.
The crude oil obtained from the expellers is filtered through plate and frame-type filter presses and then sent to washing neutralizers before being sent to the refinery, where it is washed with caustic soda. This reaction with free fatty acids in the oil forms soap, which is separated from the oil. The oil is then pumped to the refinery for further processing.
The cake obtained in the expelling section is fed to the solvent extraction plant for further oil extraction.
Horizontal continuous extractors, used worldwide, operate on gravity percolation. The prepared material is transported to the extractor via a conveyor. Solvent sprayers spray the solvent across the raw material bed, moving it from the feed to the discharge end. The extractor bed, equipped with an efficient filtration system, holds the material and provides clean miscella, which then goes to heaters to separate hexane from oil in stages. Lighted sight glasses allow viewing of the spray distribution within the extractor.
The wet meal from the extractor is de-oiled but still contains absorbed solvent. It is transported by bulk flow conveyor to the desolventizing section for hexane recovery.
The desolventizer-toaster (DT) consists of a series of vertically mounted heating jackets. Each jacket has a double bottom for high-pressure steam, with open steam provision to ensure the material surface is fully treated.
De-oiled material from the extractor, containing 20-35% solvent, enters from the top of the desolventizer and passes through steam-heated stages while an agitator shaft with blades rotates it. The desolventization process involves direct and indirect heating to a temperature well above the solvent’s boiling point, ensuring no residual solvent remains in the de-oiled material. The material flows from one jacket to the next via discharge chutes, while solvent vapors are sent to condensers through a wet scrubber for washing.
The de-oiled, desolventized meal, known as de-oiled bran, is an excellent ingredient for cattle and poultry feed. It is conditioned and cooled to the desired moisture level before transport to the bagging section via conveyor
The miscella from the extractor contains approximately 12-18% solvent (for oil cakes or rice bran) and up to 25-35% for sunflower or rapeseed. These liquids are separated by evaporating the solvent, which has a lower boiling point (64-67°C), leaving pure oil.
The distillation occurs in three stages under vacuum (zero oxygen), retaining better oil characteristics. The first stage occurs in an economizer, followed by a flasher, which evaporates the solvent, leaving only oil. The oil is further treated with open steam to remove any residual solvent.
The produced solvent vapors pass through an oil vapor separator to capture any oil particles before moving to a condenser.
Hexane vapors from the desolventizing and distillation sections require condensation. Floating head-type condensers with tube bundles circulate cooling water through the tubes, with vapors outside. Solvent vapors cool and condense into liquid, which is then separated from water in a solvent-water separator and returned to the extractor. Uncondensed gases are directed to a contact cooler and washed with cold water spray.
Residual air in the system may contain trace solvent vapor. To recover this, a final vent air stripping column with an absorber is used, which provides ample contact surface. Mineral oil in the absorber captures solvent vapors from the air. Residual vapors are evacuated through a vent condenser cooled by water in a closed circuit. The residual air is aspirated through an absorption tower where mineral oil absorbs any remaining hexane before condensation.
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