(277d) Optimization of Heat-Integrated Multicomponent Distillation Sequences

Separation by distillation accounts for roughly 3% of the total energy consumption in the US. Almost all the above ambient distillation configurations are operated by heat. In multicomponent distillation, a common strategy to reduce the overall heat duty is to supply the rejected heat from a higher temperature condenser to a lower temperature reboiler [1]. Without heat integration of this sort, the Fully Thermally Coupled (FTC) configuration is known to be among the best in terms of minimum vapor duty, which serves as a proxy for heat duty. But with this mode of heat integration, even a simple sharp split configuration could perform better than the FTC configuration [2].

To check the feasibility of heat transfer, one would normally require relations to calculate the temperatures of condensing vapor and boiling liquid. Building upon the work of Agrawal and Herron (1997), we derive a condition that allows the comparison of the temperature levels of two streams, without calculating temperatures explicitly. This condition requires only information about the composition, component relative volatilities and pressure ratios of the streams.

The derived condition is extremely useful for optimization models for the identification of energy efficient distillation configurations. For sloppy-split configurations, the optimal vapor flows and component distributions are usually difficult to determine without solving an optimization problem [4]. Consequently, heat integration options are difficult to identify prior to optimization.

Our group has designed an optimization model incorporating Underwood’s shortcut method to minimize the vapor duty of a configuration [5]. This program can be run to quickly evaluate and rank list thousands of regular column configurations. We extend the framework by including the developed condition to optimize the vapor duty of heat-integrated configurations. In addition to the standard McCabe - Thiele assumptions, we assume that the changes in relative volatility with pressure are negligible, for small changes in pressure. The proposed framework predicts a 45% reduction in vapor duty for a four-component mixture of light alkanes, when the basic direct split configuration is heat integrated. Aspen Plus simulations show a good agreement with the results. Furthermore, our optimization model predicts that the vapor duty of this heat-integrated configuration is lower than that of the FTC configuration.

1. Agrawal, R. (2000). Multieffect distillation for thermally coupled configurations. AIChE Journal, 46(11), 2211-2224.
2. Rév, E., Emtir, M., Szitkai, Z., Mizsey, P., & Fonyó, Z. (2001). Energy savings of integrated and coupled distillation systems. Computers & Chemical Engineering, 25(1), 119-140.
3. Agrawal, R., & Herron, D. M. (1997). Optimal thermodynamic feed conditions for distillation of ideal binary mixtures. AIChE journal, 43(11), 2984-2996.
4. Nallasivam, U., Shah, V. H., Shenvi, A. A., Tawarmalani, M., & Agrawal, R. (2013). Global optimization of multicomponent distillation configurations: 1. Need for a reliable global optimization algorithm. AIChE Journal, 59(3), 971-981.
5. Nallasivam, U., Shah, V. H., Shenvi, A. A., Huff, J., Tawarmalani, M., & Agrawal, R. (2016). Global optimization of multicomponent distillation configurations: 2. Enumeration based global minimization algorithm. AIChE Journal, 62(6), 2071-2086.