The process of solvent extraction is basically leaching to take out the liquid (oil) from oilbearing raw materials (oil cakes, oilseeds) by using solvent (Hexane). Different types of solvents also can be used for this purpose, but among all those, hexane is extensively used due to its low boiling point.
Extraction Process: In this extraction process, the raw materials are treated with hexane and the oil is recovered, resulting into a solution of oil in hexane called Miscella. Condensation, evaporation and distillation of Miscella is the process through which the hexane in Miscella will be recovered & re-processed again. The reason due to which hexane is opted for the extraction process is its low boiling point (67 C) and high solubility of oils and fats in it. Hexane, being a highly inflammable compound requires utmost care during processing. Thus, all those stages that involve machineries for finishing, material preparation and bagging are carried out at least 50 ft. away from the main extraction plant.
Desolventising: The meal discharged from extractor contains 25% to 30% solvent. This solvent is removed from the meal in Desolventiser cum Toaster with the help of high-pressure jacket heating and low-pressure sparge steam. During meal toasting, the equipment pressure will be slightly below the atmospheric pressure. Thus, hexane vapours will be condensed and residual solvent in meal will be minimized.
Distillation: The Miscella has 20% to 25% content of oil. Firstly, the solvent is separated out from the mixture by sending it to distillation section by maintaining vacuum in Heat Exchangers. After distillation, the miscella is fed to Absorption Section to remove the entrapped air with hexane vapour to minimise hexane losses. The oil free from hexane will be cooled to room temperature by using Heat Exchanger before sending it to storage tanks.