Reversible-gas-membranes (RGM) Process For Separating And Concentrating Bromine From Brine And Removal Of Ammonia From Wastewater Streams

Although supported-gas-membrane?SGM?technology has been widely studied and is industrialized,there are still problems such that microporous hydrophobic membrane is susceptible to pollutants,that the long-term operational stability is poor,that the by-product obtained is not the target one,that the absorbent consumption is considerable and that the regeneration and reuse of absorbent cannot be realized.Polytetrafluoroethylene?PTFE?is the most suitable material to fabricate porous hydrophobic membrane because of its excellent hydrophobic performance and its strong resistance to all kinds of acid,alkali and antioxidant corrosion.A regenerative absorption-based SGM?RSGM?process to separate bromine from brine or concentrated seawater,and to remove ammonia from wastewater streams,was studied in which NaBr and NH₄H₂PO₄ were used as reversible absorbent respectively.In order to avoid leakage and thus pollution between feed and stripping-solution,types of dual-hollow and single-hollow membrane modules fabricated with PTFE hollow fibers were chosen as the membrane contactor.It was studied that the influence of the operating conditions including the flow rate of feed and stripping solution,feed-in concentration and absorbent concentration,feed temperature on the mass transfer performance,the feasibility and long-term operational stability of the type of PTFE modules in RSGM process for extracting bromine from the brine or concentrated seawater and removal of ammonia from wastewaters.Mathematical model to describe the mass transfer process in the RSGM process was applied and the feasibility to obtain highly concentrated and purified bromine and ammonia liquor by means of regeneration of the used striping solution.The experimental results demonstrated that,when RSGM process was used to extract and concentrate bromine from concentrated seawater,the overall mass transfer coefficient K and the recovery rate of bromine ? significantly increased with the increase of feed temperature,and the value of K and ? could be up to 1.10×10-5m/s and 46.18% at 25?,a recovery rate of 95% can get when several modules installed in series,while feed-in flow rate,feed-in bromine concentration,absorbent concentration and flow rate of stripping solution had little influence on K within certain operating conditions.Analysis of the used absorption solution indicate that bromine is enriched in more than 33 times in the stripping solution compared with bromine content in the feed brine.When distillation was used to regenerate the absorbent,bromine wasenriched in the overhead distillate in which concentration of bromine could be >75%,and that it is almost avoided the consumption of acid and alkali,the thermal steam consumption was less than 20% of the traditional bromine extracting?steam stripping?process,solution of sodium bromide from the column bottom could be regenerated and reused as an absorbent for bromine extracting.The long-term operation stability test of RSGM process demonstrates that the microporous hydrophobic PTFE membrane has both high mass transfer performance and good operational stability.The experimental results of ammonia removal from wastewater showed that while the mass transfer coefficient K and ammonia removal rate of traditional single-hollow fiber membrane module ? was 1.37×10-5m/s and 97.5% respectively at 25?,K and ? of new type of dual-hollow membrane module could be up to 9.6×10-6m/s and 93.5% respectively.Although the value of K of dual-fiber module was less than that of the traditional single-fiber module,the new module configuration was helpful to maintain a useful negative temperature difference between the feed and stripping solution,thus effectively inhibited osmotic distillation which lead to the dilution of the stripping solution,and that the value of K could still maintain higher when the concentration of acid in the stripping solution was low when compared with single-fiber module.Ammonia content in the aqueous feed could be reduced to<5mg/L by using RSGM process.The used stripping solution could be regenerated by using distillation,and then a concentrated aqueous ammonia solution of 5%¹8% was obtained as a by-product,NH₄H₂PO₄ was recovered and reused,and the thermal steam consumption was less than 1/3 of conventional distillation process.The long-term operation stability test of RSGM process demonstrates that the mass transfer performance is stable and porous hydrophobic PTFE membrane could satisfy this requirement.Reversible supported-gas-membrane process can obtain a highly concentrated and purified by-product in which the volatile is in the original form as it is in the feed,and while the absorbent can be regenerated and reused.This study offers a potential method and technical basis for the industrialized application of extracting and concentrating bromine from brine or concentrated seawater and removal of ammonia from wastewater with reversible supported-gas-membranes process.