Numerical study on multi-stage vacuum membrane distillation with economic evaluation

The productivity and specific heat energy consumption of vacuum membrane distillation (VMD) can be enhanced by the appropriate module length because conductive heat transfer through the membrane is negligible. Therefore, this paper suggests the use of a multi-stage vacuum membrane distillation (MVMD) system. The suggested stages in the MVMD system can be arranged in series or in parallel and/or mixed. The MVMD system is simulated using a one dimensional in-house code with energy and momentum balance and heat and mass transfer equations. The capital cost, maintenance cost, spares cost, and operation cost are considered to evaluate the water product cost. Comprehensive analysis of productivity, the water product cost, and the membrane wetting problem is carried out to find the best configuration in the systems studied. As a result, the mixed MVMD system (which has 20 stages) has the highest productivity (3.79 m3/day), lowest water product cost ($1.16/m3), and lowest maximum transmembrane pressure difference (93.8 kPa) in the studied configuration. If the MVMD system uses the waste heat source, the water product cost can be reduced to $0.52/m3. This value shows the feasibility of using the MVMD system compared with the standard water product cost of $0.63/m3 for reverse osmosis.