Effect of highly saturated superhydrophilic porous media on air gap diffusion distillation considering preferential flow

Abstract

Air gap diffusion distillation (AGDD) is a new and inexpensive desalination technology. As the hot stream channel and evaporation surface of AGDD, the superhydrophilic porous medium plays a decisive role in its desalination efficiency. In addition, we also found the preferential flow in the highly saturated superhydrophilic porous media in experiments, which also impairs the desalination efficiency of AGDD. However, these have not been investigated. A detailed characterization of the superhydrophilic porous media was conducted in this study, based on which a theoretical model of an AGDD process containing coupled heat and moisture transfer in highly saturated superhydrophilic porous media (SIM) was developed. The preferential flow in porous media was visualized using an infrared imaging method, and the results were incorporated into the theoretical model (SIMPF), which agreed well with the experimental data. The maximum prediction errors of SIMPF for temperature and permeate fluxes were reduced by 61.2 % and 81.8 %, compared with those of SIM. The AGDD is most affected by the preferential flow at small feed volume flow rates, with a maximum reduction in permeate flux of 24.3 %. Moreover, the effects of operating and porous media structure parameters on the AGDD performance were also investigated.