Solvent Effects in Extractive Distillation

Solvent Effects in Extractive Distillation In the ordinaiy distillation of ideal or nonazeotropic mixtures, the component witn the lowest pure-component boiling point is always recovered primarily in the distillate, while the highest boiler is recovered primarily in the bottoms. The situation is not as straightforward for an extractive distillation operation. With some solvents, the key component with the lower pure-component boiling point in the original mixture will be recovered in the distillate as in ordinary distillation. For another solvent, the expected order is reversed, and the component with the higher pure-component boiling point will be recovered in the distillate. The possibility that the expected relative volatility may be reversed by the addition of solvent is entirely a function of the way the solvent interacts with and modifies the activity coefficients and, thus, the volatility of the components in the mixture. 

In addition to the applications in extractive distillation referred to above, there are other industrial examples where electrolytes in mixed solvents occur. In many industrial situations nonvolatile electrolytes are either added to effect the separation of multicomponent process streams (e.g., the complexing agents added to enhance distribution coefficients in solvent extraction) or are present as a result of the process itself. Ex-... 

The effect of salts on the vapor-liquid equilibrium of solvent mixtures has been of considerable interest in recent years. Introduction of a salt into a binary solvent mixture results in a change in the relative volatility of the solvents. This effect can be used to an advantage where the separation of the solvents is of interest. Furter and co-workers have demonstrated the potential importance of salts as separating agents in extractive distillation (J, 2, 3). 

Solvents used for extractive distillation vary considerably, but in almost all cases solvent selection presents a trade-off between its selectivity and solvency (194). The effectivity of the solvent can sometimes be improved by the addition of a salt (195). 

Acetone and chloroform form an azeotrope and cannot be separated by conventional distillation. In extractive distillation, the separation is enhanced by adding benzene as a solvent. In a preliminary evaluation of the effectiveness of the solvent, calculations are made on a single equilibrium stage. The main feed stream is at a rate of 100 kmol/h with 50% mole acetone and 50% mole chloroform. The equilibrium stage is controlled at 70°C and 110 kPa. Determine the effect of adding 45, 50, and 60 kmol/h benzene on the separation. 

The net effect of adding a solvent in extractive distillation is that new values emerge... 

FIGURE 10.7 Effect of solvent-to-feed ratio on separation in extractive distillation. 

An effective solvent for an extractive distillation is one which is attracted to one or more of the components. This attraction of the solvent for these components reduces the volatility of the solvent as well as the volatilities of the components to which it is attracted. It is desirable that the attraction occur in the natural direction, that is, that the solvent be attracted to the relatively heavy components. However, this is not a necessary condition for the behavior of the solvent. Many separations are carried out in which one of the relatively light components is attracted by the solvent and removed in the bottom product with the solvent. 

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 net effect of adding a solvent in extractive distillation is that new values emerge for the relative volatilities of the binary to be separated. If such data were available for a given system, the vapor and liquid compositions could be plotted on a solvent-free basis (Perry, 1973) to generate Y-X plots... 

Extractive distillation usually is preferred over azeotropic distillation, if both methods can be used to separate the feed components. The bulk of the solvent in extractive distillation is not vaporized in each cycle, as compared to azeotropic distillation where the entrainer is recovered from the overhead vapor stream. The energy input necessary to effect separation usually is lower for extractive distillation than for azeotropic distillation. Also, an extractive distillation column can operate over a wider range of pressures than an azeotropic distillation column, because the azeotropic composition is a function of pressure. Finally, there usually is a wider choice of solvents than entrainers, thus enabling the designer to minimize added component cost. 

In principle, extractive distillation is more useful than azeotropic distillation because the process does not depend on the accident of azeotrope formation, and thus a greater choice of mass-separating agent is, in principle, possible. In general, the solvent should have a chemical structure similar to that of the less volatile of the two components. It will then tend to form a near-ideal mixture with the less volatile component and a nonideal mixture with the more volatile component. This has the effect of increasing the volatility of the more volatile component. 

The variable that has the most significant impact on the economics of an extractive distillation is the solvent-to-feed (S/F) ratio. For closeboiling 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 volatihty 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 equihbrium stages. As an illustration, the total number of theoretical stages required as a function of S/F ratio is plotted in Fig. 13-75 7 for the separation of the nonazeotropic mixture of vinyl acetate and ethyl acetate using phenol as the solvent. 

The first stage does the bulk of the separation, and the second is used to remove other dienes and acetylenes from the isoprene. In the extractive distillation tower of each stage, the ACN solvent is introduced near the top, and being the highest boiling component, travels downward. The effective relative volatility of the less unsaturated hydrocarbons is increased with respect to isoprene. Thus most of the impurities go overhead and the isoprene is carried down with the solvent. 

The caprolactam obtained must meet die specifications of permanganate number, volatile bases, hazen color, UV transmittance, solidification point, and turbidity in order to be used for repolymerization alone or in combination witii virgin CL.5 Reported CL purification methods include recrystallization, solvent extraction, and fractional distillation. One solvent extraction technique involves membrane solvent extraction. Ion exchange resins have been shown to be effective in the purification of aqueous caprolactam solutions. In one such process,... 

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