Aqueous Two-phase Extraction Of 1, 3-Propanediol And 2, 3-Butanediol From Fermentation Broths

In the past thirty years, microbial route for manufacturing diols has attracted great attention, with significant progress achieved in the fermentations of 1,3-propanediol (1,3-PD) and 2,3-butanediol (2,3-BD), two important bulk chemicals. However, high boiling point and hydrophilicity as well as the complexity in the composition of the fermentation broths have restricted the development of efficient processes for their separation, and energy-intensive steam stripping and vacuum distillation are still dominated their industrial production. Currently, the cost associated with the downstream separation is as high as 30% of the total production cost. In this thesis, a novel aqueous two-phase system (ATPS) was developed for separating 1,3-PD and 2,3-BD from the fermentation broths. In addition, salt recovery was also studied for the ATPS system.Compared with hydrophobic organic solvent/salt extraction and salting-out, ATPS composed of multivalent anion salts and strong polar solvents was validated to be more efficient for the separation of 1,3-PD and 2,3-BD from the fermentation broths, and their partition coefficients of 11.4 and 31.9, respectively for the ethanol/potassium phosphate system. Furthermore, the ethanol/ammonium sulfate, methanol/potassium dihydrogen phosphate and ethanol/sodium carbonate systems were investigated, due to low costs of solvents and salts for commercial applications.The effects of salt, solvent and product concentrations and pH of the systems on the extraction of 1,3-PD and 2,3-BD were studied. It was found that the salt and solvent concentrations affected the extraction efficiency more significantly. Under optimum conditions, the partition coefficients and yields of the product recovery were over 4.5 and 90% for all three ATPSs. Meanwhile, most cells (79%) and proteins (99.5%) were concentrated at the interphase, which could be removed conveniently.Salt recovery is the major concern for the ATPSs. Experimental results indicated that the salts in these three ATPSs could be successfully recovered through methanol dilution crystallization for the ethanol/ammonium sulfate system, methanol dilution crystallization after adjusting pH for the methanol/potassium dihydrogen phosphate system, and crystallization by reacting with CO2 for the ethanol/sodium carbonate system, and more than 92% salt was recovered. The economic analysis performed by the program Proâ…¡showed that the ethanol/sodium carbonate system is more competitive, and thus it was recommended for commercial application.The multi-stage extraction of 1,3-PD from the fermentation broth was also performed, and the yield and enriched factor was increased to 98% and 3, respectively. In addition, more than 75% organic acids were enriched in the bottom phase, which greatly simplified the separation process.The bottom phase of the ethanol/sodium carbonate system was explored for adjusting the pH of the fermentation culture instead of sodium hydroxide, and experimental results indicated that the concentration and yield of 2,3-BD increased 24.8% and 6.5%, respectively, when the pH of the 2,3-BD fermentation was adjusted with the bottom phase. At the same time, the method not only increased 1,3-PD concentration by 16% when it was used in the 1, 3-PD fermentation, but also increased the concentration and yield of lactic acid (a by-product of great value) by 126% and 60%, respectively. The reason of this phenomenon was explored by the metabolic flux analysis, and it was found that carbonate was the main factor responsible for this change, since it controlled the metabolic fluxes of formic acid and acetic acid, and this in turn induced the accumulation of pyruvic acid, which promoted the generation of lactic acid.At the end, the process simulations in the ATPS, the steam stripping and the combination of ultra filtration and alcohol precipitation were performed by Pro II and Aspen. The results showed that the production costs of 1,3-PD and 2,3-BD in the ATPS were increased from 16.7% and 17.0%, respectively, due to the drastic decrease in the energy expenditure. Furthermore, the carbon dioxide emission of 1,3-PD and 2,3-BD from ATPS can be decreased by 75.6% and 77.4%, due to its absorption during the process of salt recovery and lower generation as a result of lesser energy consumption. Thus, ATPS would be a potential forefront technology for industrial application as it offers high substrate utilization ratio, lower cost and benefit for environment protection.