(569ak) Optimal Design of Thermal Membrane Distillation Systems and Heat Integration with Process Plants

This work presents a mathematical programing model for the optimal design of thermal membrane distillation (TMD) systems that are thermally coupled with processing facilities. TMD is an emerging technology that lies at the interface between thermal and membrane technologies. It is a non-isothermal separation process that is based on heating up the feed solution to be distilled to some moderate temperature to create partial evaporation of water, the vapor that permeates through the membrane is condensed and collected as a highly pure liquid on the permeate side. A superstructure representation and an optimization formulation are introduced to synthesize the TMD network and the heat-exchange network that integrates heating and cooling in the process facility. The superstructure and associated optimization formulation seek to identify the system configuration along with design and operating variables such as heat-exchanger areas, membrane area, extent of thermal coupling between the process and TMD, and the TMD feed-preheating temperature. The objective function maximizes the net annual profit which accounts for the revenues from the sales of purified water, the avoided cost of the treated wastewater, and the total annualized costs involving the capital investment of the added heat transfer units and the TMD network, the operating costs for the heating and cooling utilities and the operating expenses for the TMD system. Three scenarios are analyzed. The first one considers that there is not energy interaction between the TMD unit and the process facility. The second scenario considers only the heat integration among the process streams within the process facility (no heat integration with TMD). The third scenario involves the heat integration between the process facility and the TMD unit. The proposed optimization formulation is applied to a case study where a TMD system is integrated with a methanol plant and the results show significant economic benefits for the implementation of the proposed methodology.