Counter-diffusion and mass transfer in continuous countercurrent extraction with dense carbon dioxide.

摘要

The actual behavior of continuous countercurrent extraction with dense gas has been the topic of many recent investigations. Most of these studies involved the extraction of an aqueous feed with supercritical carbon dioxide. Because of the low mutual solubility of carbon dioxide and water they were considered immiscible. For two immiscible phases, liquid-liquid extraction theory can be well adapted to describe mass transfer. Consequently, a textbook design method can be used to evaluate the performance of the column. When an edible oil comes in contact with dense carbon dioxide, a completely different situation arises. Compared to the aqueous system, a large amount of carbon dioxide dissolves in the liquid phase. Therefore, the physical properties of the liquid phase change dramatically with temperature and pressure, and the dilution effect due to the carbon dioxide dissolution decreases the concentration driving force in the liquid phase.; Experiments were performed with deodorization and deacidification of simulated unrefined peanut oil with dense carbon dioxide as the extraction solvent at various pressures, temperatures and extraction factors in a pilot-plant size, packed extraction column with an internal diameter of 2.86 cm and height of 161.6 cm. A differential flux-based mathematical model for dense gas-liquid extraction is presented. It was found that the physical properties change in the liquid phase had the greatest impact on mass transfer. Although supercritical fluid solvent was shown to be more effective in the extraction column, the increasing counter-diffusion effect in the liquid phase made it less efficient than liquid solvent extraction. The dissolution of carbon dioxide in the oil had no significant effect on the mass transfer efficiency. Counter-diffusion had an dramatic influence on the performance of the extraction column. In some cases only 16% of the column efficiency was realized due to the occurrence of counter-diffusion.