Extraction And Separation Of L-Arabinose From Gum Arabic

As a functional monosaccharide,L-arabinose has an excellent property of inhibiting sucrose absorption,showing a great market potential in pharmaceutical and food industries.Among a large number of raw materials for L-arabinose preparation,gum arabic is readily available and cost-efficient because of its high content in L-arabinose.However,most traditional methods for L-arabinose preparation are deficient because of inpractical operation,cost-inefficience and large consumption of organic solvents.In this dissertation,a feasible separation process was established,which was inclusive of extraction of L-arabinose from gum arabic followed by the purification steps including catalytical hydrolysis reaction,decolorization,neutralization,desalination,crystallization and recrystallization.Experiments on the optimization of processes including catalytical hydrolysis reaction,adsorption separation and crystallization were performed.Meanwhile some foundamental data were determined for the furture process design and industrilization.The process propsed in this dissertation shows a promising application in the industry.Firstly,catalytical hydrolysis reaction by sulfuric acid,as the initial step of the process,plays an important role in the preparation of L-arabinose from gum Arabic.The reaction significantly affects the robustness of subsequent purification process,energy consumption and product quality.In this work,single-factor experiments were performed to investigate the effects of reaction time,temperature,acid concentration and solid-to-liquid ratio on hydrolysis.The results showed that the hydrolysis degree was as high as 81%and the concentration of L-arabinose in the hydrolysate was around 34 g/L with the absolute purity of L-arabinose up to 55%(based on the average content of L-arabinose in arabic gum of 42%)under an optimal hydrolysis conditions:reaction for 200 minutes at 90℃ with 0.1 mol/L H2SO4 and solid-to-liquid ratio of 1:10(m/V).Secondly,feasibility of purifying L-arabinose from D-galactose and L-rhamnose by adsorption on activated carbon was investigated.Adsorption isotherms at different temperatures of L-arabinose,D-galactose and L-rhamnose on activated carbon were determined.Meanwhile,breakthrough curves for adsorption of L-arabinose,D-galactose and L-rhamnose on a fixed bed were measured with the initial concentration of L-arabinose,D-galactose and L-rhamnose of 28.5 mg/mL,1.3 mg/mL and 1.2 mg/mL,respectively.The interaction strength between activated carbon and monosaccharide follows the decreased order of L-rhamnose,D-galactose and L-arabinose.The saturated adsorption amount based on the dynamic adsorption on activated carbon column was 22.4 mg/g,14.4 mg/g and 220.3 mg/g for L-rhamnose,D-galactose and L-arabinose,respectively.Thee fixed bed can be completely regenerated with three bed volumes of aqueous ethanol solution(20 V%).Because of small gap between the breakthrough point of L-arabinose and D-galactose,the fixed-bed adsorption process was inefficient for L-arabinose separation but was found to be useful for hydrolysate decolorization.Afterwards,neutralization and desalination of declored liquor were necessary prior to separation of monosaccharide by crystallization.Finally,crystallization is chosen for the purification process,while selection of solvent and temperature is a critical task during development of the crystallization process.In this work,the solubility data of L-arabinose and D-galactose in binary mixed solvents containing ethanol/methanol and water were measured,and the parameters of crystallization and recrystallization were optimized based on the solubility data.The results showed that the insoluble impurities could be removed by adding aqueous ethanol solution(85 V%)until solid-to-liquid ratio approaching 1:8(m/V).The absolute purity of L-arabinose up to 90%could be obtained,and the average recovery was 63%.Recrystallization was operated under the same condition,with the absolute purity of L-arabinose up to 98%and the recovery as high as 72%.In the whole process,the average recovery of L-arabinose was 30%based on the gum arabic containing average 42%of L-arabinose.The mother liquor of crystallization could be recycled to the previous separation unit in order to improve the yield of L-arabinose.