A Multiple-effect Membrane Distillation Process For Deep Concentrating Aqueous Solution Of Hydrochloric Acid

Amounts of spent hydrochloric acid are generated in iron and steel, chemical,pharmaceutical industries and so on. Spent hydrochloric acid is of major concern dueto high acid content which renders it high corrosive and polluting in nature. Therefore,there is an urgent need to regenerate and recover hydrochloric acid to achieveeconomic and ecological benefits. Membrane distillation (MD), as a relatively newprocess, is a hybrid of thermal process and membrane process. MD process isgenerally considered to have advantages such as moderate operating conditions, highspecific evaporation area, high selectivity and corrosion-proof. Nonetheless, the highthermal energy consumption of the traditional MD process is one of the biggestbarriers in its industrialization. Therefore, it has a special significance in reduction ofenergy consumption to design a membrane distillation with the function of heatrecovery. This study provided experimental data support to seek and establish a newtreatment method of spent hydrochloric acid with energy efficient and no secondarypollution and also provided a technical support for a green recovery and enrichmenthydrochloric acid.Multiple-effect membrane distillation (MEMD) based on a hollow fiber-basedair-gap membrane distillation module was developed in this paper. This kind ofMEMD process can be operated in a strictly countercurrent mode and has capabilityof internal heat recovery in-situ, and thus achieves a high PR value and reduces thethermal energy consumption. The study was conducted with the dilute hydrochloricacid solution as a feed to investigate the possibility of concentrating dilutehydrochloric acid solution by MEMD process. The effects of cold feed-in temperature,hot feed-in temperature and feed-in volumetric flow rate on the performance ofMEMD process were also studied. The performance was evaluated based onperformance ratio (PR), the average selectivity of water over HCl (βavg) andpermeation flux (N). Two types of porous fibers made from polypropylene were usedto concentrate dilute HCl solution. The experimental data indicated that all PR, βavgand N decreased with the increase of feed concentration. When the feed concentrationwas below12wt%, PR could achieve6.0~9.6, and βavgwas about10~190. As theconcentration of HCl achieved18wt%, the values of PR and βavgwere still about4.4 and2.3, respectively. The maximum value of PR and N could reach11and4.3L·m-2·h-1respectively as aqueous solution of2wt%hydrochloric acid was used as afeed. And it was also found that there was a trade-off among PR, βavgand N. Thus, theappropriate operating parameters should be selected according to experimentalpurposes. During an operational stability test lasting for30days, the performance ofMEMD modules was kept in good condition.