1 Introduction

Separation processes are not only of great importance in refineries, but also in the chemical, petrochemical, gas processing, and pharmaceutical industries. Although the reactor can be regarded as the heart of a chemical plant, in most cases, 60-80% of the total cost is taken up by the separation step. This step involves one or more thermal separation processes such as distillation, extraction, absorption, crystallization, adsorption, membrane processes, etc., which are used to obtain the products at the required purity.

In the various separation processes, concentration differences between two different streams (in most cases, two different phases) are employed to carry out the separation. The second stream is generated with the help of energy or a mass separating agent (solvent, membrane, adsorbent, etc.). Because of the various advantages of the distillation process (energy as separating agent, large density difference of the coexisting fluid phases (vapor, liquid)), 90% of all separations involve distillation. ^[1]

Other separation processes can become advantageous, when separation problems such as unfavorable separation factors (0.95 < ? ₁₂ <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 be separated but 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 (C ₆ to C ₁₂) from the different aliphatic compounds (C