Study On Kinetics Of Continuous Ethanol Fermentation Using A Novel Silicone Rubber Bioreactor

Owing to its high selective ethanol permeability with the virtue of insulating other inorganic ions, the nonporous hydrophobic silicone rubber [polydimethysiloxane (PDMS)] membrane (SRM) has been used to construct the membrane bioreactor for continuous ethanol fermentation. For the last ten or more years, some researchers have suggested that the continuous ethanol fermentation with SRMBR is of more advantages over other technologies. However, obvious progress has not been achieved in study on process kinetics and other problems for continuous ethanol fermentation with SRMBR. In this work, a membrane bioreactor with a novel composite silicone rubber membrane was developed and used for continuous ethanol fermentation. The fermentation kinetics and mass transfer kinetics in the bioreactor were investigated.A kinetics model ethanol for continuous fermentation in a SRMBR without bleed stream was developed. With thermophilic alcohol active dry yeast (TH-AADY) as fermenting microorganism, continuous fermentation experiment was carried out. Some instructive conclusions about the effect factors for industrialization were achieved, which were that (1) for the ethanol production by continuous fermentation with microorganism TH-AADY, it is more profitable to keep the moderate temperature and to add intermittently glucose solution of different concentration into broth; (2) it is very important for the continuous fermentation to prevent cells from shearing damage by the high-speed rotation of circulating pump. The kinetics model for ethanol production may be described by Luedeking-Piret model. A continuous fermentation lasting 500h, without rinse and change of membrane, was firstlyachieved. The experimental data agreed well with the kinetics model developed for the coupled system, which revealed that long-term continuous operation of the system could be achieved by product recovery in situ by pervaporation to maintain long-term stabile cell concentration and activity while the feed was added for cell metabolizability. Stable and high membrane performance was realized through the experiment. The product with average ethanol concentration of 32% (w/w) up to concentration 38% (w/w) was collected, which was free of cells and other broth and could facilitate further downstream processing and greatly reduce energy consumption for final product ethanol recovery. The utilization rate of glucose was 98% and ethanol volume productivity was 4-6,8 g/h L.The mass transfer kinetics through SRM and the effect among the cell growth, substrate utilization and mass transfer were investigated based on the solution-diffusion and resistance-in-series model. The effects of flow rate, fermentation temperature and the flow pattern in the membrane module on pervaporation performance of silicone rubber membrane for real fermentation broth were studied. The increase of flow rate and fermentation temperature would improve the flux. The relationship between temperature and flux agreed with Arrhenius equation. It seems that the biomass on membrane surface could enhance the mass transport according to the performance comparison of the membrane module with different fluid flow and the performance comparison of the real fermentation broth with ethanol model solution. Owing to the efficient removal of ethanol from broth by silicone rubber membrane to alleviate the inhibition to cells, a continuous fermentation steadily lasted 500h with stable membrane flux and selectivity. The overall mass transfer coefficient was increased from 5.21 X 10-7m/s during the beginning 300h to 7.94 10"7m/s during the subsequent 200h, because the metabolic activities of the biomass on the membrane surface provokes the mass transfer. However, the membrane performance declined obviously while the biofilm on membrane was too thick. This discovery will be contributed to the design of membrane module.The operation model of membrane bioreactor for continuous ethanol fermentationwas analyzed based on the fermentation kinetics. For the SRMBR system without bleed stream, the relatively stead...