Experimental and mathematical study of air gap membrane distillation for aqueous HCl azeotropic separation

Abstract

BACKGROUND: In the present study, the feasibility of air gap membrane distillation (AGMD) process is investigated for breaking an azeotropic mixture. A test cell AGMD module with hydrophobic PTFE membrane was fabricated and HCl/H2O azeotrope (20.2 wt% HCl) was taken as feed. A mathematical model based on Stefan diffusion multicomponent molecular-Knudsen mass transfer approach has been developed and validated with experimental data. The effects of different process parameters on total permeate flux, on selectivity and on breaking an HCl/H2O azeotrope mixture also have been studied. The feed side membrane surface temperature and membrane side condensing surface temperature were estimated by CFD modelling. RESULTS: The HCl selectivity obtained in the permeate was <1, which indicates that permeate flux is leveraged with water and a higher HCl concentration in the retentate was achieved relative to the permeate. The permeate flux decreased from 36 to 17 kg m−2 h−1 upon increasing the air gap from 3 mm to 11 mm at 50 °C feed temperature. The permeate flux increased from 4 to 28.5 kg m−2 h−1 upon increasing the feed temperature from 30 to 50 °C at 5 mm air gap. CONCLUSION: With azeotropic feed, the maximum concentration of HCl achieved in the retentate was 30.8 wt% HCl (i.e. a hyperazeotropic solution) and in permeate was found to be 15.29 wt% HCl (i.e. hypoazeotropic solution). This indicates a strong possibility of using AGMD for azeotropic mixture separation. It also was observed that the permeate flux is affected mainly by feed temperature and air gap width. © 2018 Society of Chemical Industry.