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 degrees 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 degrees 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. (c) 2018 Society of Chemical Industry