| With the constant consumption of petroleum resources,people pay more and more attention to biobase plastics.Polylactic acid（PLA）as a biomass-based plastic is one of the hot materials in current research.Lactic acid is considered to be the largest application field of polylactic acid in the future.At present,it is mainly prepared by microbial fermentation in industry,which has a high yield of lactic acid and fewer by-products.However,waste lactic acid produced in the process of lactic acid extraction brings great pollution to the environment.In this paper,a series of studies have been conducted on the value-added utilization of waste lactic acid.（1）Lactic acid extraction and propionic acid preparation from waste lactic acid:Propionic acid was synthesized from waste lactic acid by purifying waste lactic acid because the yield of propionic acid was low and the product was difficult to separate.In this paper,the method of extracting lactic acid from waste lactic acid was discussed.The extraction method of anhydrous ethyl ether and anhydrous ethanol was used（the volume ratio of anhydrous ethyl ether and anhydrous ethanol was 90:10）.The lactic acid recovery rate was 79.1%.The purity of lactic acid was more than 99.0%by HPLC after distillation and purification.In addition,the designed continuous extraction device for waste lactic acid not only improves the treatment efficiency of waste lactic acid,but also realizes the recycling of extraction agent.With purified lactic acid as raw material and NaI as catalyst,the highest yield of propionic acid was 96.7%at 220℃,2.8 MPa H2 and 10 h.（2）Lactide preparation from lactic acid:Lactide is usually quantified by gas chromatography（GC）,liquid chromatography（LC）or hydrogen nuclear magnetic resonance spectroscopy（1H NMR）.However,gas chromatography is not suitable for quantitative analysis of lactide from lactic acid,and the yield is always much higher than LC.Detailed analysis showed that some lactic acid oligomers which were not completely transformed in the reaction process were depolymerized at high temperature to produce lactide,which made the yield of lactide higher than the actual one.Therefore,LC is a relatively accurate quantitative analysis method of lactide.Based on this phenomenon,we used a fixed bed reactor to simulate the reaction conditions of gas chromatography analysis.The batch reaction liquid was placed in a high temperature fixed bed reactor filled with quartz sand without catalyst.The optimized reaction conditions can make the maximum yield of lactide reach 100%,which is of great significance for the industrial production of lactide.Under these conditions,the yield of lactide from purified lactic acid can reach 100%. |
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