Bioconversion Of Glycerol Into1,3-propanediol And Salting-out Extraction Of Main Products From Fermentation Broths

1,3-Propanediol (1,3-PD) is an important industrial chemical, and can be widely used in the fields of food, medicine, texitile and cosmetic, especially as a monomer to synthesize a new polyester, polytrimethylene terephthalate (PTT). The production method of1,3-PD includes chemical synthesis and microbial fermentation. Compared with high-cost chemical synthesis, fermentation received widespread attention for its renewable resources, mild reaction conditions and little environment pollution. The relatively expensive industrial glycerol was always used as material for the microbial production of1,3-PD. The main production process is fed-batch fermentation in which the substrate concentration was difficult to be precisely controlled. In addition, the high hydrophility and boiling point of1,3-PD and the complex compositions of fermentation broth made the downstream processing more difficult. The fermentation and separation of1,3-PD were investigated from the following aspects in this paper.Firstly, the dynamic behavior was investigated in the anaerobic continuous fermentation of glycerol by Klebsiella pneumoniae. Fermentation process went into transient state when glycerol concentration was increased from2%to8%during steady state of fermentation. The concentration of3-hydroxypropylaldehyde (3-HPA) increased rapidly to19.8g/L and then decreased to1.45g/L immediately. And other fermentation products concentration decreased rapidly to minima. After a transient period of30h, the concentrations of all metabolic products increased gradually and the oscillation period started subsequently. Sustained oscillation disappeared when fermentation was over after78h, which amounted to about9continuous oscillating periods. It showed that3-hydroxypropylaldehyde might be the reason which triggered oscillation through the analysis on changes of its concentration.Secondly, a substrate feeding strategy was studied in the fed-batch fermentation of1,3-PD, which was distinctively coupled with cell growth and metabolism using glycerol as substrate. The functional relation between substrate consumption, CO2concentration and alkaline consumption was established by analysis of the cell growth kinetics during the logarithmic growth (F1=0.30C+1.01F2) and stationary phase (F1=0.11+1.37F2), respectively. The on-line fed-batch fermentation showed that the glycerol concentration was controlled at20±2g/L, and the final1,3-PD concentration reached80.3and78.8g/L, which increased25.0and23.5g/L compared with that of manual feedback strategy. Thirdly, raw glycerol, the main by-product from the biodiesel production, was used as substrate for bioconversion to1,3-PD by K. pneumoniae. The results showed that raw glycerol could be effectively used to produce1,3-PD by K. pneumoniae under anaerobic condition, and the concentration of1,3-PD was more than60g/L. Meanwhile, more lactic acid,30g/L or so, was also formed during fermentation process. The total concentration of1,3-PD and lactic acid could achieve94g/L, and the quality conversion of glycerol was56.8%. This made the coproduction of1,3-PD and lactic acid realized and the utilization rate of glycerol improved. Besides, the results of three batches fermentation experiments showed that fermentation broths during the early logarithmic phase could be used as seed.Finally, one-step salting-out extraction was adopted to separated1,3-PD from industrial glycerol fermentation broths. Two-step salting-out extraction was adopted to separate1,3-PD and lactic acid from the raw glycerol fermentation broths. During the one-step salting-out extraction, the optimal condition was ethanol (20%, w/w)/dipotassium hydrogen phosphate (24%, w/w). The partition coefficient and recovery ratio of1,3-PD were21.59and97.7%, and the removal ratios of cells and soluble proteins were99.6%and85.6%, respectively. During the two-step salting-out extraction of1,3-PD and lactic acid, the optimal extraction condition for the first step was isopropanol (30%, w/w)/potassium carbonate (30%, w/w). The partition coefficient and recovery ratio of1,3-PD were9.81and92.4%, and the partition coefficient and removal ratio of lactic acid were0.13and88.9%, respectively. The removal ratios of cells and soluble proteins were99.6%and85.5%, respectively. The optimum separation conditions for the second step were pH6.5and ethanol28%(w/w). The partition coefficient and removal ratio of lactic acid were1.27and73.8%, respectively. The removal ratios of cells and soluble proteins were100%and98.5%, respectively.