Industrial process development - Countercurrent multistage gas extraction (SFE) processes

Countercurrent separation using supercritical gases as solvents offers the opportunity of applying many theoretical stages in separation problems. While separating a binary system is comparably easy and well understood, separating compounds from multicomponent mixtures can only be demonstrated for real cases. In this paper, the methods for treating the separation problem for real mixtures are reviewed. First, analytical methods have to be developed to characterize the mixture. Then, phase equilibrium data are experimentally determined and tried to correlate with equations of state and/or empirical equations. Equilibrium solubilities, equilibrium distribution coefficients of individual components, and dependence of separation factors on concentration are determined. With the separation factor known, the separation problem is analyzed with chemical engineering methods, applying pseudo-components for characterizing the mixtures. The number of theoretical stages is calculated in dependence on reflux ratios, conditions of state, and separation factors. At this point, a decision must be made on the type of separation equipment. In this paper, countercurrently operated separation columns are considered for planning column experiments on laboratory and demonstration scales. Height of theoretical stages or transfer units and concentration profiles an determined experimentally. Separation factors are verified, and mixtures of different compositions from the feed mixture are produced for additional phase equilibrium experiments, product control and product introduction. Simulation of the multicomponent separation and the solvent cycle is carried out for optimization of number of stages and energy consumption. Capacity limits for packing materials and the individual mixtures must be determined by measuring or calculating pressure drop and flooding points. A cost analysis then provides the basis for further decisions. (C) 1998 Elsevier Science B.V.