The Oretical Study On Separation And Purification Of Lactic Acid For High Purity Polymerization

Lactic acid is mainly used in food,cosmetics and pharmaceutical industries.With the improvement of quality of life,human demand for lactic acid has increased year by year.Industrial processes for producing lactic acid mainly include fermentation and chemical synthesis.By chemical synthesis,lactic acid having a configuration of DL can be obtained,but acetaldehyde and hydrocyanic acid must be used as raw materials for production,and chemical synthesis is difficult to be popularized due to high production cost.Fermentation method has a wide range of raw materials and sustainable production,but the separation and purification process is difficult,resulting in serious environmental pollution.The solvent extraction method has continuous operation,simple and efficient equipment,low operating cost,high quality of the lactic acid product,process cycle and environmental friendliness.In this paper,solvent extraction was used to separate and purify D-lactic acid fermentation broth to improve the purity of lactic acid.Firstly,with the extraction ability,selectivity,emulsification tendency and phase separation performance as the reference standards,and considering the stripping performance,water solubility and easy source,a variety of lacti c acid extractants were screened,and the organic phase composition was finally optimized.It is: N4423 with a mass fraction of 60% and a diluent 3# white oil with a mass fraction of 40% as a mixed extractant.The optimum process conditions for single-stage extraction of lactic acid were as follows: the extraction time was 5 min,the phase ratio was 3,the extraction temperature was 35?.Under optimal conditions,the saturated extraction capacity of N4423 for lactic acid was 60 mg/g.In order to increase the extraction rate of lactic acid,a multi-stage countercurrent extraction process was studied.The optimum conditions for multistage countercurrent extraction were as follows: the phase ratio was 3.5,the number of extraction stages was 14 and the extraction temperature was 35?.Under this condition,the extraction rate of lactic acid could reach 96.77%.After the countercurrent washing process,the secondary double distilled water was selected as the stripping agent,and the multi-stage countercurrent stripping was carried out to directly obtain the lactic acid product.The optimal multi-stage countercurrent stripping process conditions were optimized: the phase ratio was 3.5,the number of stripping stages was 14 and the stripping temperature was 70?.The stripping rate of lactic acid could reach 78.87%,the concentration of lactic acid in stripping solution reached 138.96 g/L,the separation factor between lactic and malic acids reached 1.3 and the purity of lactic acid product could reach 98.78%.In order to study the separation efficiency of N4423 on the lactic and malic acids,the extraction mechanism and thermodynamics of the lactic and malic acids were studied by slope method,the corresponding extraction coefficients and thermodynamic enthalpies were obtained.In addition,the organic phases before and after lactic acid extraction were analyzed by Fourier transform infrared spectroscopy.It was found that the intermolecular hydrogen bonding between the oxygen atom on N4423 carbonyl and the hydrogen atom on the lactic acid hydroxyl group was the main extraction reason of lactic acid by N4423.The method improves the production technology of separating and purifying lactic acid by solvent extraction,and is conducive to the development of the lactic acid i ndustry.