Study On Separation And Purification Of Glycine In Tertiary Amine Type CO₂ Switching Solvents

Glycine is the basic building block of protein and plays an important role in the food,medical and pesticide industries.Usually methanol is used to separate glycine and ammonium chloride,and energy consumption is required for recovery of methanol.As a green solvent,CO2 switching solvent is used as a switchable trigger in the separation system to get glycine by salting out,and then the solvent is recovered by heating or nitrogen.In this paper,physical property data of aqueous solution[DBAEH][HCO3]and solubility data of glycine in tertiary amine-type CO2 switching solvent constructed by N,N-dibutylethanolamine(DBAE)were determined,providing basic data for separation and purification of glycine in tertiary aminetype CO2 switching solvent system.The following three aspects of work were mainly carried out:(1)The density,viscosity,conductivity and refraction of aqueous solution at different temperature and[DBAEH][HCO3]molar concentration were measured.The empirical equation is used to correlate the measured values of density,viscosity,conductivity and refraction,and it is found that the experimental results are in good agreement with those obtained by fitting the empirical equation.The apparent molar volume,ultimate partial molar volume and standard molar volume expansion rate of[DBAEH][HCO3]aqueous solution were calculated by the measured density values,and the relative viscosity and viscosity B coefficients were calculated according to the measured viscosity values.Walden product(κ0η0)and molar refraction index RM were calculated from the conductivity data and refraction data respectively.The results show that the interaction between solvent and solute weakens with increasing temperature and strengthens with increasing solution concentration.The apparent molar volume and relative viscosity of the solution at infinite dilution were used to calculate the hydration number of different CO2 switching solvents.The hydration number of[DBAEH][HCO3]aqueous solution was the largest,indicating that the salting-out effect of[DBAEH][HCO3]aqueous solution was better.The salting-out capacity of[DBAEH][HCO3]aqueous solution was stronger with the decrease of temperature.(2)The solubility of glycine in different mass concentrations of tertiary amine CO2 switching solvent was determined in the temperature range of 5℃～35℃.By comparing the solubility of glycine in CO2 switching solvent with that in water,it was found that the solubility of glycine in CO2 switching solvent was much smaller than that in water,and the solubility of glycine in water was the largest difference between the solubility of glycine in water and that in CO2 switching solvent constructed with N,N-dibutylethanolamine.The results showed that in N,N-dibutylethanolamine(DBAE),triethylamine(TEA),N,N-dimethylcyclohexylamine(DMCHA),1-ethylpiperidine(EP)and N,N-dimethylbutylamine(DMBA),the salting-out effect of CO2 switching solvent was better.The solubility data of glycine in different CO2 switching solvents were correlated by simplified Apelbat equation,λh equation,NRTL model and Wilison model,and the thermodynamic transfer properties were discussed.The results indicated that the migration of glycine from pure water to tertiary amine CO2 switching solvent was not a thermodynamic spontaneous process and could be salted out to obtain glycine.(3)The separation and purification of glycine in tertiary amine CO2 switching solvent were studied,and the effect of the type and concentration of CO2 switching solvent,temperature,initial concentration of glycine solution,salting-out time and other factors on glycine yield were discussed.The results showed that the separation and purification of glycine with N,N-dibutylethanolamine as CO2 switching solvent had better effect.The saltingout temperature was 10℃,the initial concentration of glycine was 3.2 mol·L-1,the volume ratio of tertiary amine to crude glycine was 2,and the salting-out time was 16 h.The yield of glycine reached 79.6%and the purity was 98.7%.After the separation and purification of glycine,the switching solvent was recovered,and the highest recovery rate of N,Ndibutylethanolamine was 85.3%.