Simulation and optimisation of direct contact membrane distillation for energy efficiency

This paper describes the formulation of a computational framework for simulating and optimising DCMD to minimise the consumed energy. A simulation procedure for DCMD was established on the basis of equating heat and mass fluxes through different domains in the process. Steady-state simulations for a wide range of operating conditions were carried out. It was revealed that the highest achievable energy efficiency of DCMD within the tested range was about 49.9%.A double loop optimisation problem was formulated in MATLAB to solve the highly nonlinear equations with unknown outlet and membrane surface conditions to implement the simulation procedure. An additional outer loop was also implemented to accommodate the dynamic condition of a real lab-scale DCMD system concentrating 1.5kg glucose solution from 30 to 60% w/w. A pseudo-real-time dynamic optimisation was performed to minimise the energy expenses for the DCMD process. This energy accounted for the heat exchanged between the feed and permeate streams within the membrane module and the power for their pumping, while maintaining a minimum mass flux of 0.5kgm-2h-1. The optimal operating conditions found in this study could save the total energy consumption by 26.3%.