Research On Ethanol Fermentation/Pervaporation Process In Catalytically Active Membrane Reactor

Fossil fuels are non-renewable resources,and the utilization lead to serious environmental problems.Therefore,new green renewable energy sources such as fuel ethanol have become a hotspot in many countries result from the comprehensive carbon emission.At present,95%ethanol in the world is produced by fermentation technology in industry,and more than 70%of which is conducted by batch fermentation with a low ethanol productivity of 1.0-2.5 g·L-1·h-1.Although the continuous fermentation could increase fermentation efficiency,it is limited by cell washing at high dilution rate,substrate loss and product inhibition.The immobilized cell technology and pervaporation are used in ethanol continuous fermentation process simultaneously,which could solve the cell washing problem and alleviate product inhibition.However,in the conventional ethanol fermentation-pervaporation process,the intracellular inhibitory ethanol first get into the bulk,and it must diffuse a long mass transfer distance(from the bulk to the membrane)before separation.More importantly,it was the intracellular ethanol that inhibit the yeast.In conventional ethantol fermentation-pervaporation process,ethanol in the bulk was removed,which promote the diffusion of intracellular ethanol to the extracellular space gradually.While it is difficult for the intracellular ethanol to diffuse from cell rapidly due to the limitation of mass transfer resistance,which still inhibit the growth and activity of yeast and lead to the gradual decline of ethanol productivity in continuous fermentation-pervaporation process.In this study,we proposed a method that could further alleviate the product inhibition by in situ removal of the ethanol from the vicinity of yeast as soon as it is excreted thus allowed a more rapid excretion of the intracellular inhibitory ethanol.The above process is mainly based on the immobilized cell technology to immobilize the yeast on the surface of the separation membrane.The yeast immobilized catalytically active membrane is prepared to construct the catalytically active membrane reactor for the ethanol fermentation process.The product inhibition could be further alleviated by this method and increase the growth and activity of yeast,promote the efficient and stable operation of the ethanol continuous fermentation process.Firstly,we have optimized the process conditions of ethanol continuous fermentation,and systematically research the inhibition of substrate,product and yeast on yeast growth.The kinetic model of ethanol fermentation process contained the above inhibitons.And established the kinetic model of ethanol continuous fermentation process based on the material balance calculation,which provided the basis for the subsequent research on ethanol continuous fermentation-PV coupling process.The optimal fermentation conditions were determined that initial glucose concentration is 100-300 g·L-1,fermentation temperature is 35℃;ratio of glucose and yeast is 50:1-100:1.Ethanol productivity increase with the increase of dilution rate but it is limited by the maximum specific growth rate.Ethanol productivity enhanced with the increase of initial glucose concentration.However,it can cause problems with product inhibition and substrate inhibition,and lead to a decrease in glucose utilization.Secondly,aiming at the product inhibition in ethanol continuous fermentation,the effect of ethanol on yeast activity was studied by adding ethanol,and the model of the effect of ethanol on yeast activity was established.The produced ethanol with different concentration could be obtained by the control of glucose concentration to further investigated its inhibition effect on yeast activity.In addition,effect of main by-products of fermentation on yeast activity were studied,and the main substances and mechanisms tha affecting yeast activity were systematically discussed.The results showed that the activity of yeast decreased as ethanol concentration increased,and the relationship between ethanol concentration and yeast mortality was exponential.The acidic substances in the fermentation products mainly affected the yeast activity through the H+,while glycerol didn’t influence the yeast activity.Then,a method that could further alleviate the product inhibition was proposed based on the conventiolal ethanol fermentation-pervaporation process to solve the long mass transfer distance of produced ethanol from the bulk to the membrane surface,and then further alleviating the inhibition of ethanol on the yeast.The PES/PDMS active catalytic membrane of immobilized yeast was prepared.The fermentation performance and separation ability were evaluated.By optimizing the membrane,the high porosity of the catalytic layer is as high as 79.1%,which ensures the comparable fermentation performance of the immobilized yeast with free yeast.And the immobilized yeast exhibits superior fermentation ability in high initial glucose concentration.The pervaporation performance of the catalyticlly active membrane is comparable to that of a conventional PDMS membrane.Aiming at the membrane degradation of ethanol fermentation-pervaporation coupling process,the influence of the main substances in the fermentation broth on the membrane performance was tested by the intermittent pervaporation experiment for 8000 h,indicating that the prepared PDMS membrane exhibited superior stability during long term utilization.Based on the above studies,the coupling properties of the active catalytic membrane were studied.The results show that compared with other fermentation processes such as batch fermentation and traditional fermentation-pervaporation coupling,the ethanol fermentation process in the active catalytic membrane reactor exhibited the lowest ethanol residual concentration.And the ethanol productivity of ethanol fermentation process in the active catalytic membrane reactor can reach 3.05 g·L-1·h-1 which was 35%increased when compared with batch fermentation(2.26 g·L-1·h-1).It indicated that the active catalytic membrane reactor technology can remove the ethanol the vicinity of yeast effectively,which is beneficial to the diffusion of the produced ethanol from the yeast.Therefore,ethanol inhibition could be further alleviated and increase ethanol productivity,and it has a better coupling effect.Finally,based on the above research on the active catalytic membrane reactor,a method for alleviating the ethanol yield gradually decreased due to the influence on the growth and activity of the yeast during ethanol fermentation was proposed.The variation of parameters of the ethanol continuous fermentation process in active catalytic membrane reactor was discussed.In the active catalytic membrane reactor,the yeast concentration and activity were higher than the traditional fermentation-PV coupling process,the ethanol instantaneous productivity didn’t decrease.The ethanol productivity increased by 21%when compared with the conventional ethanol continuous fermentation-pervaporation coupling process.Based on the continuous fermentation kinetics and product inhibition of ethanol,the kinetic model of the continuous fermentation-pervaporation coupling process of ethanol was established.The results show that the predicted values of the model are in good agreement with the experimental values,further indicating the advantages of the active catalytic membrane reactor on ethanol continuous fermentation.