| L-lactic acid as an importantly green platform compounds,has great application prospect. It's greatly significant to solve the key technology in the separation of L-lactic acid for strengthening the comprehensive utilization of renewable resources and enhancing environmental protection.In view of membrane separation technology with non-phase, low energy consumption, does not destroy biological activity and the characteristics of easy operation, this paper adopts membrane combined process to deal with L-lactic acid fermentation broth. Membrane flux attenuation and membrane fouling are the major problems for the membrane separation process.L-lactic acid fermentation broth pretreatment processes were studied to reduce membrane flux attenuation and membrane fouling, such as flocculation-sedimentation and activated carbon decolorization. Ultrafiltration and reverse osmosis membranes operating conditions were optimized, the mathematical models between membrane flux and operating parameters were established, the mechanism of membrane fouling was initially explored,the best membrane cleaning conditions were determined.(1)The flocculation-sedimentation process conditions were optimized by the combination of single-factor and orthogonal experiments.The results were as following:the dosage of calcium hydroxide was 4.5‰,the dosage of activated carbon was 7.5‰,the dosage of magnesium sulfate was 3‰,flocculation temperature was 70℃and flocculation time was 45 min. Under these conditions, the removal rate of bacteria was 99.48%,the removal rate of protein was 84.42%, the removal rate of residual sugar was 39.80% and lactic acid loss rate was 1.88%.(2)Compared to granular activated carbon, the powdered activated carbon was better for L-lactic acid decolorization.The mathematical regression models between the discoloration total efficiency rate and its influencing factors were attained by the combination design of quadratic orthogonal rotation based on the results of the single-factor design. The optimum process parameters for discoloration total efficiency rate were obtained as the dosage of activated carbon 0.318%,discoloring temperature 43.18℃and discoloring time 18.83 min. An optimum range under 95% probability of the discoloration total efficiency rate more than 70% could be summarized for the dosage of activated carbon, discoloring temperature and discoloring time at 0.395～0.695%,56.88～69.84℃, 15.68-22.58 min, respectively by frequency analytical method. (3)The parameters of ultrafiltration process were optimized through orthogonal test.The best ultrafiltration condition was A1B3C2,namely the operating pressure was 0.9 MPa, initial concentration was 50 g/L,operation temperature was 35℃.Under these conditions,the permeate flux was 7.913 L·m-2·h-1,the protein retention ratio was 98.93% and the penetration of L-lactic acid was 97.52%.The reverse osmosis conditions were as following:operating pressure was 1.0 MPa, initial concentration was 40 g/L,operating temperature was 30℃.(4)The empirical formula for membrane flux attenuation could be good to simulate the quantitative relationships of membrane flux decaying with time in the course of ultrafiltration and reverse osmosis process.Resistance-in-series model and the preferential adsorption-capillary flow model were respectively used as the prototype to explore the mathematical models between membrane flux and operating parameters in the process of ultrafiltration and reverse osmosis.The mathematical models were as following: Ultrafiltration process: Reverse osmosis process: J=42.136·exp(-0.1455·P-0.0305·Cf)·[P-0.0043RCf]Test results showed that they can be used to describe the process of ultrafiltration and reverse osmosis.(5)A mixed cleaning agent contained EDTA·2Na, sodium polyphosphate and NaOH was screened to clean the fouled ultrafiltration and reverse osmosis membrane.Cleaning the fouled ultrafiltration and reverse osmosis membrane with a mixed cleaning agent which was composed of 0.75% EDTA·2Na,0.75% sodium polyphosphate at pH 11and 35℃for 50 minutes was suggested.
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