Design and Control of Heterogeneous Azeotropic Column System for the Separation of Pyridine and Water

The design and control of two types of separation systems using heterogeneous azeotropic distillation have been studied in the literature. One type (e.g., isopropyl alcohol dehydration) is to add a light entrainer (cyclohexane) into the system so that a minimum-boiling ternary azeotrope is formed which can split into two liquid phases in a decanter. This type of system also introduces two additional azeotropes (isopropyl alcohol-cyclohexane and water-cyclohexane), thus dividing the ternary system into three distillation regions. Another type (e.g., acetic acid dehydration) does not contain azeotrope in the original system. However, due to a tangent pinch near the pure water end ail entrainer (isobutyl acetate) is added into the system to aid the separation. This type of system has only one binary heterogeneous azeotrope (water-isobutyl acetate); thus there is only one distillation region. In this paper, design and control of a different residue curve map (RCM) type of separation system utilizing heterogeneous azeotropic distillation will be studied. An example of this RCM type is pyridine and water separation using toluene as entrainer. Adding toluene into this system introduces two additional azeotropes, one is minimum-boiling binary heterogeneous azeotrope (water-toluene) and the other one is a binary homogeneous azeotrope (pyridine-toluene). There is no ternary azeotrope for this system. Two alternative design flow sheets are compared in this paper to find the one which is most economical and also meet stringent product purity specifications. A simple overall control strategy of this process has also been developed which requires only one temperature control loop in each column.