Studv On The Sedaraiton Of Glycine And NH₄Cl By Electrodialysis

Glycine is an important chemical intermediate product, which is widely used in theproduction of pesticides, medicines and animal foods, and more attention has been paid on itssynthesis and separation technology. The glycine is generally synthesized through theammoniation of chloroacetic acid, Strecker method and Hydantion method. The key problemproblem facing the method of ammoniation of chloroacetic acid is the separation of glycine fromammonium chloride. The separation of glycine from ammonium now is mainly through diluentscrystallization with methanol, which has the disadvantages of high cost of methanol, highconsumption of energy and reactant, and thereby the high production cost and the weak marketcompetitiveness.In considering the problems facing the method of ammoniation of chloroacetic acid, thisstudy mainly focused on the study of the separation of glycine and NH4Cl. Electrodialysis (ED),a green and environmental separation technology, was adopted to separate glycine and NH4Cl.Through this innovative separation process of ED, the quality of glycine and by-product NH4Clcould be effectively improved, and the catalyst HA could be recycled. Parameters such aslimiting current, feed concentration, ion-exchange membrane stack were studied in terms ofseparation efficiency and the economic of the ED process was also evaluated and compared withthe conventional method. The main conclusions obtained from the experimental results are asfollows:(1) The study of limiting current revealed that, under the same operating conditions,limiting current density of membrane stack increased with the increasing concentration andflow rate of feed solution in the diluted chamber. The influence of concentration on limitingcurrent was larger than that of the flow rate. Additionally, the limiting current was also affectedby the ion concentration in the electrode chamber.(2) The investigation of the effects of operating conditions on the separation efficiency ofthe electrodialysis process showed that, with the increasing of solution concentration in thediluted chamber, the yield of glycine reduced and energy consumption increased. Under the pHof5.97, the IP point of Gly, the ED process exhibited lower yield of Gly and higher energy consumption compared with others pHs. The desalting efficieny increased with the increasingcurrent density. With tht increasing of density, the yield of Gly remained constant and the theenergy consumption decreased firstly and then the yield of Gly declined and the the energyconsumption ascended when the current density was larger than29.30mA/cm2. The optimumcurrent density for the separation of aqueous solution containing13%Gly and20%NH4Cl was29.30mA/cm2. Under the same operating conditions, with the increasing of circulation flow rate,the desalination rate and current efficiency increased, and the energy consumption decreased.The optimum circulation flow rate was150L/h.(3) The results of the experiments using different membrane stackes illustrated that, underthe same operating conditions, the performance of ED processes employing different IEMs wereas follows: IEM1#: yield of glycine=92%，current efficiency=88.62%, energy consumption=0.12kWh/kg, final glycine concentration=19.30%; IEM2#: yield of glycine=87%, currentefficiency=84.87%, energy consumption=0.137kWh/kg, final glycine concentration=16.40%;IEM3#: yield of glycine=83.4%, current efficiency=80.4%, energy consumption=0.143kWh/kg,final glycine concentration=15.80%. The performance of different IEMs followed the order ofIEM1#> IEM2#> IEM3#. Membrane IEM1#was preferable in treatment of glycine and NH4Clmixture.(4) The economic evaluation of the separation of glycine/NH4Cl mixture throughelectrodialysis process (IEM1#) indicated that, under the optimum operating conditions, the costfor the production of1.0t pure glycine was about1198.6yuan, which was relatively lower thanthat of the traditional technique (about1550yuan). Therefore, the ED process adopted in thisstudy was an economic and environment-friendly technology for the separation ofglycine/NH4Cl mixture.