The filtration process, also known as continuous extraction process, is based on the principle of uninterrupted moisture of the extraction material. Here, the solvent liquids go beyond the material of the extraction, allowing interrupted exchanges between it and the free-flowing solvent that extracts it. The maintenance of a constant solvent flow ensures that the locally saturated solvent slides and is replaced by the unsaturated solvent. This process requires that the extraction agent flows freely within the extraction material.
The advantage of this procedure is that the material of the extraction is not exposed to mechanical stress. This process is used mainly when “free” oil must be extracted. Any fine particles, such as those inevitably formed during extraction, are filtered by the particles of the seeds and thus are prevented from entering the micelle.
The immersion process is used when oil must be extracted from a hard extraction matrix, as well as in cases where there are large amounts of unrefined fiber in the extraction material.
In the immersion process, the total amount of seeds from which the material is extracted in solvent is submerged. The design of the system means that there are no forced movements, which ensures that the saturated oil solvent (micelle) is constantly exchanging with the new solvent. Therefore, the static system needs to be agitated, to balance any difference in concentration. When agitating, abrasion of the extraction material is inevitably caused, so the micelle must be filtered afterwards.
The two extraction processes are influenced by a wide range of factors. However, in general, there are only five factors that have real meaning.
a) Water content of the seed
Because it is a polar substance, water interferes by soaking the surface of the seed and penetrating solvent into the seed. In addition, it reduces diffusion. What is necessary, however, is some degree of residual moisture to maintain the elasticity of the seed scale and to prevent it from flaking, which would make it difficult for the solvent to penetrate the seed.
b) Size and shape of the particle
First of all, the shape of the particles in the extraction material must be sufficient to allow the solvent to flow freely, without any great resistance. Secondly, the size of the particle must allow the best possible extraction of each individual particle, minimizing diffusion. The seed should not be presented in the form of flour, since it would make the filtration of the solvent impossible.
c) Amount of solvent
The quantitative ratio of the solvent to the extraction material will depend on the composition of the seed. Generally, the amount of the extraction solvent increases proportionally with the crude fiber content in the seed. The concentration of the micelle also plays another role. As a general rule, the greater the concentration of this, the less energy is needed to remove the solvent at the end of the process.
d) Extraction temperature
High temperatures reduce the solvent viscosity and increase the solubility of the extract in the solvent. The reduced viscosity of the solvent and the enhanced function of the solvent at elevated temperatures cause the extraction to improve. While there are no big differences, it is better to use heated extraction agents. The increase in oil production compensates for the cost of heating the solvent.
e) Extraction time
The extraction time depends on the level of extraction and the type of nature and structure of the extraction material.