Optimal Design of a Subambient Membrane Separation System with Work and Heat Integration for CO2 Capture

Abstract

Subambient membrane separation is proved more effective than ambient membrane separation for CO2 capture. To achieve the subambient temperature, it involves not only the membrane units but also heat exchange, compression, and expansion units that are energy intensive. Thus, energy efficiency became an issue for the application of subambient membrane systems. To design such a system more efficiently, the sequence of separation units should be considered together with heat and work integration. However, this will result in a complex mixed integer nonlinear programming (MINLP) problem that involves nonlinear state equations. We decompose the problem into two steps: first design the separation sequence and work exchange network and then design the remaining heat exchanger network. The two steps are bridged by involving heat integration within the first step. An initialization strategy is also proposed to solve the resulting MINLP model. Case studies from postcombustion CO2 capture are used to illustrate the proposed approach. Compared with the total annualized cost (TAC) of the ambient system, 21.5 $/(yr ton CO2), the subambient system first achieves a 16.14% reduction by optimizing the operating temperature of membranes and further deduced the TAC by 4.95% using the whole method. The proposed method could take full advantage of subambient membranes and significantly decline the total annual cost and energy demand.