(474a) Global Minimization of Multicomponent Distillation Configurations’ Total Cost

Separation is the most universal process in the chemical industries for separation of multicomponent mixtures. Multicomponent mixtures are separated in sequential steps in multiple distillation columns. The sequence that a feed mixture is separated into products, referred to as distillation configuration, is considered as a candidate configuration for achieving the separation task. As the number of components in the feed mixture increases, the number of candidate configurations increases rapidly, thus, it is challenging for practitioners in the early stages of the process design to find the most economic configuration(s) among the vast search space of configurations.

Shah and Agrawal¹ proposed a method to enumerate the basic configurations for separation of a given n-component feed mixture into n nearly-pure product streams using n-1 columns. Although for a given number of components the configurations have the same feed and final products, installation and operation costs vary depending on the vapor duties, vapor and component flows, etc. Nallasivam et al.²developed a global minimization algorithm (GMA) to find the optimal flows that result in the minimum vapor duty for each configuration, and proposed several strategies to improve the convergence and speed of the algorithm. However, the vapor duty as an objective function is loosely proportional to the capital cost of a given configuration and a reliable formulation is needed to identify the optimal configuration with minimum total cost.

In this work, we propose a new objective function formulation and extend the GMA method to minimization of the total cost. The new objective function includes calculation of column height and diameter, tray size and diameter, and heat exchanger capital and annualized operating costs. The proposed method identifies the globally optimal configuration from a complete ranklist of all configurations, including partially and completely thermally coupled basic configurations. Finally, the efficacy of the method is demonstrated by considering three case-studies where the effect of separation difficulty and balancing weight of capital and operating costs are studied for five-component feed mixture.

1. Shah, V. H., Agrawal, R. A Matrix Method for Multicomponent Distillation Sequences. AIChE J., 56(7), 1759-1775, 2010.
2. Nallasivam, U., Shah, V. H., Shenvi, A. A., Tawarmalani, M., Agrawal, R. Global Optimization of Multicomponent Distillation Configurations: 2. Enumeration based Global Minimization Algorithm. AIChE J., 62(6), 2071-2086, 2016.