Special Distillation Processes

Mixtures exhibiting nonideal solution behavior present both challenges and opportunities in connection with separation processes. Azeotropes cannot be separated by ordinary distillation, yet the formation of azeotropes itself may be used as a means for carrying out certain separations. The formation of two liquid phases in a column may complicate the separation process however, the coexistence of liquid phases with distinct compositions provides one more separation tool. Chemical reactions concurrent with distillation may be used either to enhance the separation or to perform both the reaction and the separation in one process. 

For the separation of azeotropes or mixtures with relative volatilities that lie below about 1.4, which are difficult to separate, special distillation processes are available, such as two-pressure distillation and extractive and azeotropic rectification. 

Two-pressure distillation is especially promising when the azeotrope-forming components have very different heats of evaporation. The thus resulting differing slopes of the vapor-pressure curves leads to a pressure dependence of different azeotropic compositions. 

Extractive distillation exploits the differing solubilities of the azeotrope-forming components in a higher boiling extractant. Aids to the selection of suitable extractants (e.g., expert systems) are available [Erdmann 1986a, Trum 1986]. Recovery of the extractant requires an additional downstream column. 

In azeotropic distillation, mitures that are difficult to separate are separated by addition of a substance that forms an azeotrope with only one of the components of the mixture. This process is rarely used. A widely used special case is heteroazeotropic distillation, in which water is present as azeotrope component and forms a miscibility gap. 

Phthalic acid is discontinuousty esterijied with n-butand. The released water of reaction is distilled off together with n-hutanol, separated in a phase separator, and eliminated via a 

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Other separation processes can become advantageous, when separation problems such as unfavorable separation factors (0.95 < aj2 <1.05) or azeotropic points occur. In these cases, a special distillation process (extractive distillation) may be used. Extraction processes do not depend on a difference of vapor pressure between the compounds to he separated hut on the relative magnitudes of the activity coefficients of the compounds. As a result, extraction processes are particularly useful in separating the different aromatic compounds (Cg to C[2) from the different aliphatic compounds (Cg to C12). Absorption processes are ideally suited for the removal of undesired compounds from gas streams, e.g., sour gases (HjS, COj) from natural gas. 

For the separation, only the difference, a,j - 1, is used. In the case of azeotropic systems Oij=l), no separation can he achieved. For separation factors close to unity (e.g., 0.95 < a,2 < 1.05), a large number of theoretical stages are required and a special distillation process has to he used. 

B. Ding, L. Zhongwei and C. Zhigang, Special Distillation Processes, Elsevier Science, 2005. ISBN 9780444516480. 

In most cases, special distillation processes such as azeotropic or extractive distillation are applied to separate azeotropic systems by distillation, where a suitable solvent is added. While in the case of azeotropic distillation a solvent is required which forms a lower boiling azeotropic point, in the case of extractive distillation a high-boiling selective solvent is used which changes the separation factor in a way that it becomes distinctly different from unity. Both processes are shown in Figure 11.16 together with the column configuration. 

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