Optimal Design of Multistage Membrane Distillation Systems for Water Purification

In this paper, a novel optimal design strategy is proposed to produce pure water in a multistage membrane distillation (MD) system. It is a systematic approach to modeling and analyzing multistage MD processes. The proposed method can generate the optimal system structure and operation conditions and avoid poor integration and suboptimal configuration of multistage MD systems. In this approach, an innovative module model is first established, and then the cost function relating the capital and the operation cost to the design variables and the structural variables of the designed system is introduced in the objective function. A superstructure flow sheet is presented to describe all the possible flow routes for the cold stream. Then optimal design problem can be formulated as a mixed-integer nonlinear programming (MINLP) problem to minimize the total annualized cost. Based on our simulation studies, the mode of the commonly used countercurrent operation is the optimal flow route for the MD system. The design results would also determine the configuration of the optimal separation stage, the optimal operating mode, and the corresponding operating conditions.