Recovery of hydrochloric acid and glycerol from aqueous solutions in chloralkali and chemical process industries by membrane distillation technique

Membrane distillation (MD) is a rapidly advancing process for separation of azeotropic and close boiling liquid mixtures besides dewatering of high boiling solvents. Recovery of hydrochloric acid (HCl) from a chloralkali industrial effluent and dehydration of glycerol/water mixture was performed using MD technique. HCl was recovered using chemically resistant polytetrafluoroethylene (PTFE) membrane of 0.22 μm pore size and 78% porosity. The effluent feed contained 32.8 wt.% of aqueous HCl with color forming Fe compounds and heavy hydrocarbon (C9-C14) impurities that gave an oily appearance. Permeate obtained was colorless aqueous HCl (33 wt.%) at a high flux with negligible impurity levels. An increase in permeate pressure from 5 to 15 mmHg resulted in a gradual reduction in flux from a high value of 8.57 kg/m2/h to a moderate 1.02 kg/m2/h at ambient temperature of 28 °C. Effect of feed composition in terms of acid and inorganic salt contents besides feed temperature (25-60 °C) on flux and separation efficiency was demonstrated at a constant downstream vacuum of 8-10 mmHg. Dehydration of glycerol was performed using novel indigenously synthesized ultraporous hydrophobic polystyrene (PS) membrane of 0.72 μm pore size. Permeate was found to contain pure water due to the low vapor pressure and larger molecular size of glycerol which cannot penetrate PS as it does not get wetted by water. This indicated a selectivity of infinity (∞) which is associated with a reasonable water flux in the range 0.56-0.02 kg/m2/h at a vacuum of 5 mmHg for feed glycerol concentration varying from 10 to 90 wt.%. PS membrane was characterized by SEM, FTIR, XRD and TGA to assess surface and cross-sectional morphologies, structural elucidation, crystallinity and thermal stability of the membrane, respectively. A detailed economic estimation of HCl recovery for a feed effluent capacity of 2 m3/h is presented. The study showed that commercial grade HCl and glycerol could be recovered from aqueous streams at a reasonable price by employing MD technique.