Industrial equilibrium extraction behavior

The use of a dissolved salt in place of a liquid component as the separating agent in extractive distillation has strong advantages in certain systems with respect to both increased separation efficiency and reduced energy requirements. A principal reason why such a technique has not undergone more intensive development or seen more than specialized industrial use is that the solution thermodynamics of salt effect in vapor-liquid equilibrium are complex, and are still not well understood. However, even small amounts of certain salts present in the liquid phase of certain systems can exert profound effects on equilibrium vapor composition, hence on relative volatility, and on azeotropic behavior. Also extractive and azeotropic distillation is not the only important application for the effects of salts on vapor-liquid equilibrium while used as examples, other potential applications of equal importance exist as well. 

The potential of supercritical extraction, a separation process in which a gas above its critical temperature is used as a solvent, has been widely recognized in the recent years. The first proposed applications have involved mainly compounds of low volatility, and processes that utilize supercritical fluids for the separation of solids from natural matrices (such as caffeine from coffee beans) are already in industrial operation. The use of supercritical fluids for separation of liquid mixtures, although of wider applicability, has been less well studied as the minimum number of components for any such separation is three (the solvent, and a binary mixture of components to be separated). The experimental study of phase equilibrium in ternary mixtures at high pressures is complicated and theoretical methods to correlate the observed phase behavior are lacking. 

Solvent extraction systems often display nonlinear equilibrium behavior. Pratt (1983) and Pratt and Stevens (1991) have illustrated methods in which results of sufficient accuracy are obtained by representing the equilibrium curve by means of two or three straight segments for both continuous as well as multicompartment columns. Additional simple and shortcut procedures have been described by Von Stockar and Lu (1991). The extent of backmlxing In the continuous phase in Karr reciprocating-plate extraction columns (Figure 8.1.35(d)), widely used In the petroleum, pharmaceutical and hydro-metallurgical Industries, has been studied experimentally by Stella et al. (2006). 

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