An integrated approach to the retrofitting of mass exchange networks

Abstract

The synthesis of optimal mass exchange networks (MENs) deals with the identification of a cost-effective network of mass exchangers that preferentially transfer certain species from rich streams to lean streams. To date, MEN synthesis has dealt with grass-root design where the emphasis is on the maximization of process lean streams usage and the minimization of operating cost. Another important class of problems is the retrofit of MENs in which mass-exchange units already exist in the plant and the focus is to maximize the usage of the process units so as to minimize capital cost of newly added units and trade it off with operating cost. In this paper, we develop a systematic procedure for the retrofitting of MENs. First, we identify alternative structural configurations of interest through heuristics. Series and parallel structures are addressed. Next, we focus on two primary retrofitting strategies: those restricted by no capital constraints and those involving capital expenditure. The no-capital alternatives include enhancing performance of a current system as well as solvent substitution. The capital-based alternatives include the addition of new equipment. A new type of mass-pinch analysis is developed to maximize the utilization of existing capital while reconciling added capital with operating cost. The main concept in this novel pinch diagram is identifying maximum performance as dictated not only by thermodynamic limitations but also by physical size limitations. Different process alternatives are considered and screened to attain the optimum design. A case study is presented to demonstrate the broad applicability and potential benefits of the novel approach.