EXTRACTIVE DISTILLATION OF THE ACETONE-METHANOL SYSTEM

In a final section of this chapter, we look at the use of other solvents beside water. The purpose is to see the effect of solvent selection on dynamic controUabihty. 

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There are many important industrial applications of azeotropic separations, which employ a variety of methods. In this book we discuss several of these chemical systems and demonstrate the application of alternative methods of separation. The methods presented include pressure-swing distillation, azeotropic distillation with a light entrainer, extractive distillation with a heavy entrainer (solvent), and pervaporation. The chemical systems used in the numerical case studies included ethanol-water tetrahydrofuran (THF)-water, isopropanol-water, acetone-methanol, isopentane-methanol, n-butanol-water, acetone-chloroform, and acetic acid-water. Economic and dynamic comparisons between alternative methods are presented for some of the chemical systems, for example azeotropic distillation versus extractive distillation for the isopropanol-water system. 

Knapp and Doherty studied heat-integration of binary homogeneous azeotropic systems using extractive distillation methods. One of their examples considered the acetone-methanol system with water as the solvent. They did not consider pressure-swing distillation, nor did they consider dynamics and control. 

Knapp and Doherty present the economic optimum design of an acetone-methanol separation using water as the extractive solvent. The design used in this chapter is based on their work. Kossack et al. presented a systematic synthesis framework for extractive distillation systems and the acetone-methanol system was considered. 

At this point it is useful to give a direct comparison of the dynamic performances of the three alternative solvent systems. Figure 11.30 provides this comparison. The sohd lines are for the water system, the dashed lines are for the DMSO system, and the dotted lines are for the chlorobenzene system. Control of the DMSO system is excellent. Control of the water system is good. Control of the chlorobenzene system is the poorest of the three. The transient deviations in the purity of the acetone product (the distillate from the solvent-recovery column) are the largest. This is due to the inherent tendency of methanol to drop out the bottom of the extractive column during an upset, and this methanol shows up in the distillate of the acetone column as impurity. 

In this chapter, the use of extractive distillation has been illustrated using the acetone-methanol system as a numerical example. Steady-state and dynamic comparisons have been presented between extractive distillation and a pressure-swing distillation, with and without heat integration. In addition, the effect of solvent selection on dynamic controllability has been investigated. 

The RCMs and the equivolatility curves of this chemical system ean be seen in Figure 13.1, where the numbers in the equivolatility emwes denote the relative volatility of acetone versus methanol in the presence of water. The RCM indicates that any mixture of acetone and methanol, even premixed with water, will produce the acetone-methanol azeotrope at the top of the column. However, by continuously adding water (a heavy entrai-ner) into the column, it can be seen from the equivolatility curves that the acetone is becoming more and more volatile than the methanol in the extractive section. Acetone and methanol can then be separated in the extractive section if the number of trays in this section is sufficient. Acetone will go toward the top of the column while methanol will be carried with the water toward the column bottom. In the rectifying section, owing to the lack of methanol in this section, only the separation of acetone and water is performed. Pure acetone will preferably go to the top of the batch extractive distillation column. After the draw-off of the acetone product and a slop-cut period, where the acetone in the column is completely depleted, the methanol product can be collected at the top of the column. The heavy entrainer (water) can be collected at the column bottom. 

Such a process depends upon the difference in departure from ideally between the solvent and the components of the binary mixture to be separated. In the example given, both toluene and isooctane separately form nonideal liquid solutions with phenol, but the extent of the nonideality with isooctane is greater than that with toluene. When all three substances are present, therefore, the toluene and isooctane themselves behave as a nonideal mixture and then-relative volatility becomes high. Considerations of this sort form the basis for the choice of an extractive-distillation solvent. If, for example, a mixture of acetone (bp = 56.4 C) and methanol (bp = 64.7°Q, which form a binary azeotrope, were to be separated by extractive distillation, a suitable solvent could probably be chosen from the group of aliphatic alcohols. Butanol (bp = 117.8 Q, since it is a member of the same homologous series but not far removed, forms substantially ideal solutions with methanol, which are themselves readily separated. It will form solutions of positive deviation from ideality with acetone, however, and the acetone-methanol vapor-liquid equilibria will therefore be substantially altered in ternary mixtures. If butanol forms no azeotrope with acetone, and if it alters the vapor-liquid equilibrium of acetone-methanol sufficiently to destroy the azeotrope in this system, it will serve as an extractive-distillation solvent. When both substances of the binary mixture to be separated are themselves chemically very similar, a solvent of an entirely different chemical nature will be necessary. Acetone and furfural, for example, are useful as extractive-distillation solvents for separating the hydrocarbons butene-2 and a-butane. 

So there are two vital design parameters that must be determined in extractive distillation the solvent-to-feed ratio and the reflux ratio. The three graphs given in Figures 5.8 and 5.9 show the effects of solvent-to-feed ratio and reflux ratio on the composition of the distillate from the extractive column solvent impurity (DMSO), heavy-key impurity (methanol), and light-key purity (acetone). They provide the basis for designing an extractive distillation system. 

The responses of the chlorobenzene system to feed flowrate disturbances are shown in Figure 11.29a. Responses for feed composition are shown in Figure 11.29. Product purities (xdi(M) and Xd2(A)) are held quite close to their specificalions for the feed flowrate changes, but the purity of the methanol distillate from the extractive column x i(m) drifts downward for the disturbance in feed composition ( Az) from 0.5 to 0.4 acetone in the feed. More methanol must come out the top of the extractive column in stream D. Since the solvent flow is unchanged, the increased methanol load produces a drop in purity. The dynamics of the chlorobenzene system are the slowest of the cases, taking over 2 h to come to a new steady state. 

Chapter 13 will illustrate the operations of batch distillation utilizing two previously mentioned separation methods for azeotropic mixtures extractive distillation and heterogeneous azeotropic distillation. In the first part of Chapter 13, operation of batch extractive distillation is studied for separating acetone and methanol using water as the entrainer and separating IPA and water using DMSO as the entrainer. In the latter part of that chapter, a heteroazeo-tropic batch distillation system for acetic acid dehydration will be studied. 

Batch distiUalion is commonly used in the fine chemicals industries, speeialty polymer, bioehemieal, pharmaceutieal, and food. In these types of applications, the production scale is usually small, whieh justifies rumiing the separation process in batch mode. When the mixture contains an azeotrope, the separation methods mentioned in previous chapters can also be operated in batch mode. We will start this chapter by studying the operation of batch extractive distillation for two systems. One is to separate acetone and methanol using water as the entrainer. The other system is to separate IPA and water using DMSO as the entrainer. 

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