On-line Monitoring And Control Of Membrane-assisted Seeding For Cooling Crystallization

As a typical solution crystallization process,cooling crystallization has been widely applied in the fields of pharmaceutical intermediates,food,fine chemicals and petrochemical industry.The control of the solution supersaturation in the cooling crystallization is the key to obtain high-quality crystal products.In the industrial crystallization,adding a certain amount of seed crystals in metastable zone to reduce the initial nucleation rate is a common method.However,a lot of factors such as seeds size distribution,amount and addition timing need to be considered,and the control process is complex.In recent years,researchers have introduced membrane materials and highly integrated membrane module devices into the crystallization process to regulate and enhance the nucleation and crystal growth process.The membrane interface can realize high-efficient mass or heat transfer,and the transfer and flow processes that occur near the interface can complete the formation and transportation of crystal nuclei.Membrane crystallization can realize almost all solution crystallization processes.Thus,the membrane crystallization can realize almost all solution crystallization processes,such as evaporative crystallization,antisolvent crystallization and cooling crystallization.At present,there are few studies on the on-line monitoring and control strategies for the membrane crystallization,and systematic research is urgently needed to guide the process design of the membrane-assisted cooling crystallization.Firstly,ammonium persulfate(APS)was taken as the research object.The APS nucleation induced by the cooling interface of hollow fiber membrane and seed crystals transportation process were observed by a high-speed microscope camera.The width of the metastable zone in both direct cooling crystallization and membrane-assisted cooling crystallization(MACC)was compared,which revealed that the degree of supersaturation required for nucleation in the bulk solution was much lower in MACC.Further,the Particle View Microscopy(PVM)and Process Crystallization Monitoring system(PCM)were used to monitor the direct cooling crystallization,seeded cooling crystallization and MACC in real time.The membrane module involved solution temperature range was determined,and the seed crystals morphology evolution processes in different cooling processes were visually displayed.It was found that the seed crystals obtained when the membrane module was used at 18 ℃ had larger average sizes and regular morphologies.The crystallization kinetics of the membrane-assisted seeding process was analyzed,and the number of crystals,growth rate and crystal size trend were compared,and the appropriate membrane-involved duration was 30 min when the membrane-involved temperature was 18 ℃.The crystal size distributions(CSD)of seed crystals in different stages of direct cooling crystallization and MACC were calculated,and the seed crystals obtained at 18 ℃ possessed the largest mean crystal size(1181 μm),the most concentrated distribution and the highest purity(>99.0%),which demonstrated the advantage of the membrane-assisted seeding(MAS)in regulating solution nucleation and crystal growth.Finally,the membrane interface induced nucleation at low saturation was applied into the membrane-assisted static cooling crystallization of 6FDA.The solution was cooled and crystallized on the wall of the tubular crystallizer to form a dense crystal layer.Combined with the subsequent heating and sweating process,the content of impurities and metal ions in the crystal layer were reduced(<1 ppm),and the high-purity 6FDA(99.9%)was obtained.