Oilseed extraction is a solid‑liquid extraction process, a mass‑transfer process in which oil contained in oilseeds dissolves in a selected solvent, so that oil transfers from the solid phase to the liquid phase.
Oilseed extraction methods can be classified by production operation mode into batch‑type and continuous‑type extraction. They may also be categorized by the contact mode between solvent and oilseeds into immersion extraction, percolation extraction, and combined immersion‑percolation extraction.
Pure batch‑type extraction involves repeatedly immersing oilseeds in solvent inside equipment until nearly all oil in the oilseeds is extracted. The miscella obtained from the first immersion has the highest concentration, and miscella concentrations decline in subsequent immersions. This extraction method consumes large quantities of solvent, yields low‑concentration miscella with a large miscella volume, which is unfavorable for industrial production.
Continuous‑type extraction uses progressively diluted miscella to extract continuously fed oilseeds inside the extractor. Finally, fresh solvent is applied to oilseeds with low residual oil content to extract nearly all oil. This method reduces solvent consumption and produces high‑concentration miscella. Suitable for large‑scale and high‑efficiency production, it is widely adopted in modern vegetable‑oil extraction industry.
The main factors affecting extraction performance are listed below:
When oil contacts solvent, mutual dissolution occurs following the like‑dissolves‑like principle: oil molecules migrate into the solvent and solvent molecules migrate into the oil. As the system tends toward thermodynamic equilibrium, oil and solvent molecules move in opposite directions from regions of higher concentration toward regions of lower concentration. Mass transfer reaches dynamic equilibrium only when solute concentrations are identical in both phases.
Oil extraction from raw materials generally proceeds in two stages. In the first stage, oil dissolves in solvent penetrating into the material to form miscella, which flows to the outer surface of the material. In the second stage, oil contained in the miscella transfers from the material’s outer surface into the bulk flowing miscella inside the extractor.
Oil transfer rates vary for different raw materials, as well as for the same raw material subjected to different pretreatment processes. Accordingly, different extraction times are required to achieve equivalent extraction results. Sufficient contact time between solvent and raw material must be guaranteed to extract as much oil as possible. Theoretically, longer extraction time improves extraction performance; nevertheless, extraction time cannot be extended infinitely in industrial production due to constraints from equipment and production efficiency.
The solvent‑to‑material ratio refers to the volume ratio of solvent to raw material used for oil extraction. A high solvent‑to‑material ratio produces low‑concentration miscella during extraction, creates a large concentration gradient for mass transfer between solvent and oil, accelerates mass‑transfer rate, and hence shortens required extraction time under identical conditions. Conversely, a low solvent‑to‑material ratio slows mass‑transfer between solvent and oil and prolongs extraction time.
During extraction, the percolation rate of miscella or solvent sprayed onto the seed bed should be neither excessively fast nor excessively slow; an appropriate percolation rate is required. It shall maintain concentration gradients of miscella across different spraying sections while ensuring adequate contact time between solvent and raw material.
An excessively fast percolation rate results in insufficient contact time between solvent and material and impedes mass‑transfer between solvent and oil. An excessively slow percolation rate, despite prolonged contact time, causes liquid accumulation on the seed bed surface, reduces miscella concentration gradients among spraying sections, and deteriorates extraction performance. Percolation rate is related to raw‑material pretreatment, moisture content and meal fineness.
Extraction temperature is determined by material temperature, solvent temperature and their respective mass proportions. Temperature exerts significant influence on extraction rate and performance. Higher temperature intensifies molecular random motion, lowers viscosity of solvent and oil, reduces mass‑transfer resistance and accelerates molecular diffusion.
Nevertheless, extraction temperature cannot be raised arbitrarily, as it is constrained by the initial distillation point of solvent and initial boiling point of miscella. Extraction temperature shall not exceed the boiling points of solvent and miscella. Otherwise, massive solvent vaporization will occur, lowering the effective solvent‑to‑material ratio and raising pressure inside extraction equipment.
Major pretreatment operations for extraction raw materials include crushing, flaking, pre‑pressing and extrusion, yielding crushed particles, raw flakes, pre‑pressed cakes and extruded pellets respectively. Oil in oilseeds mainly resides inside cell cytoplasm. Therefore, raw materials receiving different pretreatment processes deliver different extraction efficiencies.
Qualified extraction raw materials should have no intact undestroyed cell tissues, no secondary structures, and abundant internal pores. Intact cells or secondary structures hinder oil diffusion and prolong extraction time, whereas abundant internal pores reduce diffusion resistance and raise extraction rate. Extraction efficiencies of raw materials in descending order are: extruded pellets, pre‑pressed cakes, raw flakes, crushed particles and whole seeds.
Moisture content affects solvent wetting performance on raw materials and oil diffusion inside seed flakes. Excessively high moisture weakens solvent wetting, causes seed‑flake swelling, reduces internal porosity, and hinders solvent penetration inward and miscella diffusion outward. Moreover, high moisture triggers seed‑flake agglomeration, breaks continuous flow channels inside the seed bed and impairs bed permeability.
Excessively low moisture generates substantial fine meal particles during conveying and extraction. This also reduces seed‑bed permeability, increases meal fines in miscella, and raises difficulties for miscella evaporation and stripping. For this reason, every type of raw material shall maintain an optimal inlet moisture content before entering extraction.
