Using the press method, it is impossible to achieve complete degreasing of the material, because the thinnest layers of oil, held by greater surface forces, always remain on the surface of the cake.
The forces holding the oil in the surface layers are many times higher than the pressure developed by modern presses. The only method that allows for almost complete extraction of the oil is the extraction method, in which the forces that transfer the oil into the solution exceed the forces that hold the oil in the extracted material.
Losses of oil in production consist of losses with husks separated from seeds, losses with cake of final pressing (at a press plant) or with meal (at an extraction plant) and unaccounted oil losses.
The amount of unaccounted losses is caused by the discrepancy between the theoretical yield of oil, calculated by the factory laboratory on the basis of the selection of samples of the processed seeds, and the actual yield of oil in real production conditions.
Extraction in its physical basis is a diffusion process, therefore its driving force is the difference in concentrations of the medium – oil solutions in the solvent inside and outside the particles of the extracted material. Before the start of extraction, the solvent is not retained inside the extracted material in the oil, and the concentration of the mixture is 100% (pure oil), and the concentration outside the particle is 0% (pure solvent).
Penetrating inside, the solvent dissolves the oil that is there, forming a medium, the concentration of which is higher than outside the particle.
Under the influence of the difference in the concentration of the medium, the oil moves (transfers) from the middle of the particle to the external solution until the concentration of the oil in the particle and in the solvent outside the particle is equalized and the extraction stops. The transfer of oil into the solution can be carried out in the form of individual molecules – by molecular diffusion and in the form of groups of molecules or individual small volumes by convective diffusion. In real conditions, oil extraction is a combination of them. The speed of the extraction process is influenced by the size of the surface of contact (contact) between the solvent and the extraction oil, the temperature of the process, the size of the molecules of the extracted substance and the solvent, as well as its viscosity. The larger the contact surface of the solvent and oil, the higher their temperature and the smaller the radius of the molecules, the lower the viscosity of the solvent, the faster the extraction.