Study on the operation characteristic and transfer resistance of mechanical vapor recompression and vacuum membrane distillation system under multiple working conditions

Abstract

In order to alleviate the freshwater resource shortage worldwide, the multiple vacuum membrane distillation (VMD) modules were drawn into a mechanical vapor recompression (MVR) system to recover industrial wasterwater in this paper. An experimental platform was constructed and several experiments were conducted under multiple working conditions. The operation characteristic by varying membrane area was investigated to achieve stable freshwater production, and the influences of operation parameters on the energy efficiency were analyzed, the distribution characteristic of transfer resistance was also revealed. The experimental results showed that an excellent freshwater production performance with a separation efficiency of 99.9% was achieved, and increasing membrane area was effective to elevate the evaporation rate. Furthermore, lower feed concentration or higher feed temperature, feed flow rate and vacuum side pressure were helpful for reducing the specific heating energy consumption (SHEC) and improving the energy efficiency. The proposed system was superior to VMD in energy efficiency, and was superior to MVR in separation performance, but still exhibited a lower energy efficiency than that of MVR due to the transfer resistance. Accordingly, the involved transfer resistances including boundary layer resistance, fouling layer resistance and membrane resistance were obtained by the iterative calculation. Increasing the number of experiment from 1 to 10 increased the transfer resistance from 14.219 Pa·m2·h·kg−1 to 18.082 Pa·m2·h·kg−1, resulting in a rise of SHEC from 70.63 kWh·t−1 to 82.77 kWh·t−1. Finally, the methods to reduce transfer resistance and improve energy efficiency were suggested. The obtained results provides the basis for such system industrial application.