| Lactic acid and its derivatives, which are widely used in the chemical, food and pharmaceutical industries, are promising products. For food and pharmaceutical purposes, however, high optical purity L-lactic with lower content of impurities is required. Currently, L-lactic acid is mainly produced by fermentation method, which suffers some difficulties such as complex separation process and low efficiency.This thesis focused on the downstream purification process of lactic acid produced by Rhizopus oryzae 3.819. For main problems in the techniques of L-lactic acid clarification and purification, a series of technologies were introduced, the extracting process for traditional calcium salt was explored and improved. The design proposed the following technical process: fermentation broth→flocculation→ultrafiltration→adsorption→purify. The key factors in every process were further researched. Finally, the process of lactic acid clarification and purification and its parameters were optimized.The main conclusions of the experimental research are as follows:1,Chitosan is an ideal flocculent. When the chitosan concentration 20mg/L, the fermentation liquid temperature 40℃, stirring time 9min, stirring speed 110r/min and pH value 5, the removal rate of mycelium was 96.5% and the loss rate of calcium lactate was only 0.2%.2,In this trial ultrafiltration membrane UEOS 503 was chosen by comparing the protein-removing effects between UEOS503 and UPIS503. When transmembrane pressure 0.12MPa, pH value 4, temperature 45°C and concentration 40g/L, the protein-removing rate was 92.85% and the loss rate of calcium lactate was 3.14%.3,By the comparison of the effects on the removal of residual sugar and pigment in fermentation liquid between the powered activated charcoal and granular activated charcoal particles, the granular activated charcoal was chosen and temperature, the volume of adsorbent added, stirring speed, time and pH value were also taken into account. According to the orthogonal experimental design based on the single factor experiments, when the volume of adsorbent added 4%, temperature 40℃, pH value5.0, rotational speed 170r/min, time 12h, the rate of the removal of residual sugar was 91.22% , pigment 77.68% and the loss rate of calcium lactate was 4.80%.4,Technical conditions for the molecular distillation of lactic acid were studied. Response surface methodology (RSM) was employed in the present work and an equation for the molecular distillation of lactic acid was built. When the conditions for the yield of L-lactic acid were operation pressure 15Pa, feed temperature50°C distillation temperature 70°C, feed rate 100ml/h and rotating speed 100r/min, the yield of lactic acid was 62.5% and the purity was 95%. The correlation coefficient between the measured values and the expected value was 0.9936, which showed that the mathematical model built in this trial was effective.Techniques of separation and refinement of L-lactic acid through optimization grouping are reasonable and simple and have better continuity of working procedure, through which the lactic acid products with the high purity and fine stability can be got. |
