Extraction agent selection criteria

The correct selection of extractant is the key factor to realize the extraction process. Based on the selection criteria proposed by several authors and the understanding gained through research practice, the following criteria are proposed:

① High extraction capacity. The extraction capacity is the concentration of the solute that can be reached in the extraction solvent, and the higher the extraction amount, the stronger the extraction capacity of the extraction solvent for the benzene component.

② High extraction selectivity. That is, good separation and purification effect can be obtained by fully extracting the components to be extracted and at the same time extracting no or little components to be separated.

③ Easy to recycle and reuse. In the case that the subsequent extractive step is back extraction, an appropriate stripping agent can be used for effective back extraction. Generally, the stronger the extraction capacity of the extractant, the more difficult it is to reverse extraction, so it is necessary to consider both when selecting the extractant, and if the extraction agent is recovered by distillation, the greater the relative volatility between the solvent and the extracted solute is required.

④ Fast mass transfer rate. To help reduce the extraction equipment volume, improve production efficiency.

⑤ Good physical and chemical properties. That is, it has the appropriate density, viscosity interfacial tension, etc., to ensure that the two phases can effectively flow, mix and quickly separate the phase and have the appropriate boiling point, flash point and ignition point, which not only ensures the safe operation of extraction, but also realizes the good connection with the subsequent external processing process. For example, in the case of recovery of extraction solvent by distillation method, the boiling point of the extraction agent cannot be too high. Because the extraction solvent needs to be recycled for a long time, its good stability (thermal stability, chemical stability, radiation stability, etc.) is very important. Otherwise, it not only increases the loss of solvent in the production process, but also its degradation products may pollute the extraction solvent, thus affecting its reuse performance.

⑥ Low water solubility. To reduce the loss of the extractant during the extraction process, or to remove the process step of distilling the extraction solvent from the raffinate, which is critical to the economy of the extraction process.

⑦ No emulsification or low emulsification trend to ensure the smooth progress of the extraction process.

⑧ Good safety and good environment. For this reason, the solvent is required to have good physical properties and non-toxic or very low toxicity.

This aspect is the need to ensure safe production, but also based on the limitation of solvent residues in the product, which is particularly important for pharmaceutical and food extraction processes.

⑨ Cheap, easy to obtain, industrial mass production.

It should be pointed out that the above requirements often cannot be met at the same time because some of the criteria themselves are contradictory, such as easy extraction is difficult to reverse extraction, high quality is high price, so in the selection of extractants must be considered according to the specific situation of the extraction object and extraction conditions, and strive to play the advantages of an extraction system, and then try to overcome its shortcomings.