Productivity, selectivity, and energy consumption of pilot-scale vacuum assisted air-gap membrane distillation for the desalination of high-salinity streams

The implementation of air gap membrane distillation systems is limited by a lack of overall performance predictions which nowadays rely only on few available pilot-scale researches. This study evaluates the productivity, energy consumption, and selectivity of a pilot-scale air gap membrane distillation system by combining experiments and modeling activities. The effect of operating conditions, i.e., applied vacuum, feed flow rate, and feed stream salinity, was investigated to identify regulating factors and quantify dependencies. Response surface methodology was applied to model the phenomena and provide statistical analysis. Increasing flow rates produced a near linear increase of productivity within the investigated range. Working at higher applied vacuum also translated into enhanced productivity, even though the distillate flux increased of maximum 10 % when the vacuum value increased from -100 mbar to -500 mbar. The two operating variables also governed the observed salt flux, acting with a similar magnitude to increase it, since salt flux resulted mainly from liquid pore flow phenomena. On the other hand, the trans-membrane pressure regulated the membrane rejection: increasing the pressure difference led to a rejection decrease. Moreover, high feed stream salinity negatively affected both the productivity and the distillate quality. The productivity gains were typically achieved at the expense of an increase in specific thermal energy consumption; however, an interesting relation was observed with feed stream salinity, with a minimum of specific thermal energy consumption of roughly 300 KWh/m^3 identified in the treatment of a brine with a salinity of 150 g/L.