Separation And Purification Of L-lactic Acid From Fermentation Broth

L-lactic acid is biocompatible and therefore widely used in various industries.Currently,the lactic acid separation process is costly.China is rich in waste biomass,and waste biomass can be produced by microbial fermentation to produce L-lactic acid.In this study,the agricultural and forestry waste com stalks were pretreated and used as raw materials,and the L-calcium lactic acid fermentation broth prepared by fermentation of Bacillus coagulans was used for separation.The membrane process is used to remove the bacteria,proteins,and salts in the fermentation broth.After concentration,lactic acid with a purity greater than 85%can be obtained.The yield of a single batch process is 71.7%.The fermentation-membrane separation coupling system is also used.The waste liquid generated during the membrane separation process was reused and fermented,which reduced the pollution emission of the system to the outside world.Finally,6 batches of fermentation-separation were successfully carried out,and the total process yield reached 93.4%.First,the removal of the cells and proteins contained in the fermentation broth was studied.The microfiltration membrane can achieve a 98.9%removal rate of the bacteria in the fermentation broth,and a small portion of the remaining bacteria can be completely removed in the ultrafiltration.The US020 ultrafiltration membrane was used for protein removal,with a service life of more than 14 batches at a pressure of 20 psi and an average membrane flux of 9.48 L/(m2h)and an average protein removal rate of 87.78%.At the same time,the crystallization method was used to remove the protein in the fermentation broth,and the best 84%yield and 85.6%protein removal rate were obtained by crystallization of the 175 g/L lactate content fermentation broth at 4℃ for 24 hours.Subsequently,studies on the removal of pigments and salt ions in the fermentation broth were carried out.The effective decolorization of the fermentation broth was achieved by using NF245,NF270,DL and DK nanofiltration membranes,and the decolorization rate was above 94%.Subsequently,the fermentation broth was acidified to different pH values and the properties of the four nanofiltration membranes were investigated.It was found that the DK and NF270 nanofiltration membranes had the best overall performance when the fermentation broth was acidified to a pH of 2.45,including membrane flux and salt rejection.And lactic acid transmission rate.The desalting efficiency of NF270 remained good in the ten batch life test,which was better than the DK nanofiltration membrane,and NF270 was preferred as the nanofiltration membrane of the system.After the second nanofiltration,a colorless and transparent lactic acid solution can be obtained.Finally,the waste liquid generated in the membrane separation process is reused for fermentation.The nitrogen source was reused for the crystallization mother liquor and the ultrafiltration retentate.The results showed that the nitrogen source of the ultrafiltration retentate was the best.The recovery of cellulase used in the fermentation process was studied.The results showed that ultrafiltration can be used to intercept the cellulase in the fermentation supernatant,and Ca(OH)2 can be used to elute the cellulase adsorbed in the straw residue.Reach the most suitable system for recycling.Finally,a batch fermentation-separation experiment was carried out,comparing batch fermentation in 6 batches:84%of water was circulated in the system;the overall lactic acid yield was increased from 71.7%to 93.4%;the amount of water added to the system was reduced by 69%;wastewater discharge decreased by 83.6%;enzyme using is reduced 31.7%of the original process,90%lactic acid yield was obtained in the amplification experiment..