Studies on divided wall distillation column

Abstract

Distillation is the most employed, but also the most energy-intensive separation method being used in chemical process industry. Therefore, the efficient design of the distillation process is important so as to reduce the energy requirement without compromising with the product quality. Divided wall column (DWC) is an innovative design in this direction where two or more simple columns are thermally coupled into one column by providing a vertical wall, dividing the core in two parts that work as the pre- or post-fractionator column and the main column. Thermodynamically, a DWC can be considered as an equivalent of Petlyuk column; the only difference being that a DWC is built in only one shell, whereas a Petlyuk column consists of two or more columns. Simulation of a DWC is a challenging task, as it involves many connecting streams making the problem highly non-linear. Further, simulation of a DWC requires simultaneous solution of material, momentum and energy balance equations. For a simple distillation column, the momentum balance equations are important only at the mechanical design stage for the estimation of effects of pressure drop whereas these are usually neglected during theoretical design (simulation) stage as its impact on vapour-liquid equilibrium is not appreciable. The optimal design of a DWC is obtained by minimizing the energy duty, as it accounts for the major fraction of the operating cost. The design variables such as reflux ratio and split ratios interact with each other and need to be optimized simultaneously for optimal operation and design. The optimization of the DWC operation cannot be handled with the existing commercial simulation software without coupling them with an external sub-routine/ software for optimization. The structural and process parameters of a DWC are generally optimized separately and there are no reports in the open literature on the simultaneous optimization of the structural and operating parameters of a DWC.