Development of heat and mass transfer correlations and recovery calculation for HCl–water azeotropic separation using air gap membrane distillation

Abstract

In the present work, the air gap membrane distillation (AGMD) process has been experimentally studied for the separation of aqueous HCl azeotropic (20.2 wt% HCl) mixture. A test cell AGMD module equipped with polytetrafluoroethylene flat sheet membrane of 0.22 µm pore size was used to perform different experiments. The effect of operating parameters, mainly feed bulk temperature and air gap widths on the total permeate flux, selectivity and HCl concentration on permeate and retentate, was analyzed. Heat transfer correlation was developed from experimental data of heat transfer coefficient. Also, the effect of operating time on membrane morphology was investigated by FESEM (field emission scanning electron microscopy) and AFM (atomic force microscopy) analysis. The SEM image was further studied to determine the pore size distribution. A recovery model was developed to assess HCl recovery at several temperatures in the range of 40–50 °C. At 45 °C of feed temperature, 42% recovery was achieved by recycling the retentate to the feed tank resulting in 31 wt% of HCl in the feed tank for 80 h of continuous operation. The experimental results show that total permeate flux increases from 12.35 to 25.89 kg/m² h on decreasing air gap thickness from 11 to 3 mm at 1 lpm of feed flow rate. Also, it increases from 7.35 to 35 kg/m² h on increasing temperature from 30 to 50 °C at 3 mm air gap thickness. The maximum concentration of HCl in the permeate and retentate was found to be 16.09 wt% and 31 wt%, respectively. The HCl selectivity in the permeate was found to be lower than one, which reflects the high HCl concentration in the retentate as compared to the permeate.