Vinyl acetate-ethyl acetate Propane-propylene

Ethanol-isopropanol Hydrochloric acid-water

Nitric acid-water

Close-boiling Close-boiling


Maximum-boiling azeotrope Maximum-boiling azeotrope

Phenol, aromatics Acrylonitrile

Methyl benzoate Sulfuric acid, calcium chloride for salt process Sulfuric acid, magnesium nitrate for salt process

Alternative to simple distillation Alternative to simple distillation, adsorption Alternative to simple distillation Sulfuric acid process relies heavily on boundary curvature Sulfuric acid process relies heavily on boundary curvature

Several methods are available for determining whether the lower-or higher-boiling pure component will be recovered in the distillate. For a series of solvent concentrations, the y-x phase diagram of the low-boiling and high-boiling keys can be plotted on a solvent-free basis. At a particular solvent concentration (dependent on the selected solvent and keys), the azeotropic point in the pseudobinary plot disappears at one of the pure component corners. The component corresponding to the corner where the azeotrope disappears is recovered in the distillate [Knapp and Doherty, in Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, Vol. 8, Wiley, New York (1993)]. LaRoche et al. [Can. J. Chem. Eng., 69, 1302 (1991)] present a related method in which the aLH = 1 line is plotted on the ternary composition diagram. If the aLH = 1 line intersects the lower-boiling pure component-solvent face, then the lower-boiling component will be recovered in the distillate and vice versa if the aLH = 1 line intersects the higher-boiling pure component-solvent face. A very simple method, if a rigorous residue curve map is available, is to examine the shape and inflection of the residue curves as they approach the pure solvent vertex. Whichever solvent-key component face the residue curves predominantly tend toward as they approach the solvent vertex is the key component that will be recovered in the bottoms with the solvent. In Fig. 13-73a, all residue curves approaching the water (solvent) vertex are inflected toward the methanol-water face, with the result that methanol will be recovered in the bottoms and acetone in the distillate. Alternatively, with MIPK as the solvent the residue curves (Fig. 13-73b), all residue curves show inflection toward the acetone-MIPK face, indicating that acetone will be recovered in the bottoms and methanol in the distillate.

Extractive Distillation Design and Optimization Extractive distillation column composition profiles have a very characteristic

Activity Coefficient For Methanol WaterAcetone Water Distillation

FIG. 13-72 Effect of solvent concentration on activity coefficients for acetone-methanol system. (a) water solvent. (b) MIPK solvent.

FIG. 13-72 Effect of solvent concentration on activity coefficients for acetone-methanol system. (a) water solvent. (b) MIPK solvent.


Alcohol Stripping Systems




Triangular Plot Acetone Water

Methyl Isopropyl Ketone


FIG. 13-73 Residue curve maps for acetone-methanol systems. (a) With water. (b) With MIPK.

shape on a ternary diagram. The composition profile for the separation of methanol-acetone with water is given in Fig. 13-74. Stripping and rectifying profiles start at the bottoms and distillate compositions respectively, track generally along the faces of the composition triangle, and then turn toward the high-boiling (solvent) node and low-boiling node, respectively. For a feasible single-feed design these profiles must cross at some point. However, in an extractive distillation they cannot cross. The extractive-section profile acts at the bridge between these two sections. Most of the key-component separation occurs in this section in the presence of high-solvent concentration.

The variable that has the most significant impact on the economics of an extractive distillation is the solvent-to-feed (S'F) ratio. For close-boiling or pinched nonazeotropic mixtures, no minimum-solvent flow rate is required to effect the separation, as the separation is always theoretically possible (if not economical) in the absence of the solvent. However, the extent of enhancement of the relative volatility is largely determined by the solvent concentration and hence the S'F ratio. The relative volatility tends to increase as the S'F ratio increases. Thus, a given separation can be accomplished in fewer equilibrium stages. As an illustration, the total number of theoretical stages required as a function of S'F ratio is plotted in Fig. 13-75a for the separation of the nonazeotropic mixture of vinyl acetate and ethyl acetate using phenol as the solvent.

For the separation of a minimum-boiling binary azeotrope by extractive distillation, there is clearly a minimum-solvent flow rate below which the separation is impossible (due to the azeotrope). For azeotropic separations, the number of equilibrium stages is infinite at or below (S'F)min and decreases rapidly with increasing solvent, and then may asymptote, or rise slowly. The relationship between the total number of stages and the S'F ratio for a given purity and recovery for the azeotropic acetone-methanol system with water as solvent is shown in Fig 13-75b. A rough idea of (S'F)min can be determined from a pseudobinary diagram or by plotting the aLH = 1 line on a ternary diagram. The solvent composition at which the azeotrope disappears in a corner of the pseudobinary diagram is an indication of (S'F)min [LaRoche et al., Can. J. Chem. Eng., 69,1302 (1991)]. Typically, operating S'F ratios for economically acceptable solvents is between two and five. Higher S'F ratios tend to increase the diameter of both the extractive column and the solvent-recovery columns, but reduce the required number of equilibrium stages and minimum-reflux ratio. Moreover, higher S'F ratios lead to higher reboiler temperatures, resulting in the use of higher-cost utilities, higher utility usages, and greater risk of degradation.

Knight and Doherty [Ind. Eng. Chem. Fundam., 28, 564 (1989)] have published rigorous methods for computing minimum reflux for extractive distillation, with an operating reflux of 1.2 to 1.5 times the minimum value usually acceptable. Interestingly, unlike other forms of distillation, in extractive distillation the distillate purity or recovery does not increase monotonically with increasing reflux ratio for a given number of stages. Above a maximum-reflux ratio the separation can

Acetone Water Distillation
FIG. 13-74 Extractive distillation column composition profile for the separation of acetone-methanol with water.
Acetone Water Distillation
12 3 4

Solvent/Feed Ratio

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