Study On The Mass Transfer Performance Of Separating HCl By Membrane Distillation And Osmotic Distillation In Acid-Washing Wastewater

Membrane distillation (MD) is a new type of membrane separation technology, the driving force is the vapor pressure gradient caused by temperature difference. Osmotic distillation (OD) is also called isothermal membrane distillation. It is a process driven by the vapor pressure gradient caused by concentration difference at the same temperature of each membrane. Both MD and OD can also be applied in separating volatile substances from various aqueous solutions. The mass transfer performance of HCl in separating water was studied by MD and OD, respectively. The mass transfer performances of HCl and H2O, the factors of inhibiting water transfer during the process of coupled MD and OD were also discussed in the paper.For the process of MD, mathematical models for evaluating total mass transfer coefficient and selectivity separation coefficient were derived, and the influence factors on mass transfer such as temperature, flow rate, packing density and so on, were investigated. It was found that the molar flux of HCl and total mass transfer coefficient increased, while selectivity decreased with improving the inlet temperature in the feed side. The phenomena of temperature polarization and concentration polarization can been reduced by improving flow rate, and the flow rate in the feed side influences on mass transfer more remarkable. For the hollow fiber membrane modules, the fluid distribution in the shell side was ameliorated and the channeling effect was reduced when improving packing density, so the molar flux of HCl increased.It is feasible to apply OD for separation of HCl, but the rate of trans-membrane mass transfer is lower than MD. By analyzing the electronic microscope photographs of PTFE, PVDF and PP hydrophobic microporous membrane, it was found that the hydrophobic membrane showed a good performance of acid-proof and alkalescency-proof, anti-corrosion, stable structure character. The total mass transfer coefficient and selectivity almost did not change with the time going.For the process of coupled MD and OD, temperature difference and concentration difference were taken into account simultaneously. The results showed that the activity coefficient of chlorine ion and relative volatility of HCl were enhanced owing to common-ion effect and salting-out effect when the feed contained chloride. Increasing the volatility of HCl, the molar flux of HCl could increase. It was because of hydration, which lowered the concentration of "free water" and the activity of H2O, so the flux of H2O decreased, inhibiting the mass transfer of vapor. The molar flux of HCl could also increase by raising the initial concentration of hydrochloric acid in the feed. Meanwhile, the mass transfer of vapor was inhibited. When the feed contained sulfuric acid, the total mass transfer coefficient, selectivity separation coefficient and the molar flux of HCl all increased, but the flux of H2O decreased.