Edible oil technology can be grouped into two: mechanical pressing and solvent extraction. Sometimes the latter compliments the former. For oilseeds with high oil content such as ground nut, first mechanical pressing will be applied and over 85% of the oil will be extracted. The remaining oil in the expeller cake will then be extracted with solvent. For some other oilseed with low oil content, solvent extraction is generally considered as the best alternative. However, the initial investment cost of solvent extraction is much higher than mechanical pressing. In addition, solvent extraction is more appropriate for large scale processing than small scale edible oil plants. Therefore, in this study the mechanical pressing technology has been selected.
Ground nut oil production process, based on mechanical pressing technology, can be grouped into three stages: seed preparation, pressing and crude oil refining.
The seed requires undergoing a thorough cleaning process to remove sand, stalk, plant debris and any other foreign matters by rotary or table sieve. Usually, the screening process is assisted by air aspiration unit. After cleaning, the seeds have to be prepared for efficient oil recovery by pressing. The stages involved are size reduction of the seeds by breaking them and then conditioning the seeds by adjusting their moisture content and temperature, while keeping the seeds hot (say 90-95ºc) for a period of 30-60 minute. Then the prepared seed shall be conveyed to the screw pressing machine where it is pressed by the action of worm and outer shell. The crude oil so obtained from the pressing will be first clarified in a settling tank and then shall be pumped through the filter press.
The filtered crude ground nut oil will be pumped to the refinery where it shall pass through three stages of refining: neutralization, bleaching and deodorization.
To reduce the level of free fatty acid (FFA) in the oil, caustic soda will be mixed with the crude oil. The neutralized oil may have trace of soap which is a by-product of the neutralization process. Therefore, the oil will be washed with water. It will then be pumped to the bleacher in which it will be mixed with bleaching earth to improve the color of oil by the process called adsorption. The bleached oil, after being filtered, will be pumped to the deodorizer to avoid substances which are responsible for the odor of edible oil. In some very small plant the three stages of refining crude oil shall be executed in a single vessel. The plant requires a containment vessel for the collection and treatment of wastes to be generated in the process.