Structure And Pervaporation Applications Of Composite Silicone Rubber (PDMS) Membranes

Pervaporation (PV) is one of research focuses in the field of membrane technology. It realizes the separation of liquid mixtures by using differences between various components in solubilities into membrane and in diffusions through membrane. As a promising technology, PV is very suitable for azeotropic separation, dehydration of organic solvents, removal of dilute volatile organic compounds from aqueous streams and organic-organic mixture separation, and therefore, has great application potentials in petrochemical industry, pharmacy manufacture, food process, environmental protection, bioengineering and energy field.Polydimethylsiloxane (PDMS) is one of the most promising materials used to prepare the manbrane for separating organics from aqueous solution. In order to optimize the methods of membrane preparation and promote the industrial applications of membrane, probing into the membrane structure, separation performance and transfer mechanism is obvioursly of great theoretical and practical importance,In this dissertation, the structure and pervaporation applications of composite PDMS membranes were comprehensively investigated. The application areas includes the improvement of new-type China liquor by pervaporation, the production of apple brandy and apple acidic beverage by fermentation of apple juice with a PDMS membrane bioreactor and the production of fuel ethanol by continuous fermentation in a silicone rubber membrane bioreactor. The dissertation consists of four parts as follows.1. Effect of the binding interfaces of composite polydimethylsiloxane (PDMS) membranes on the pervaporation performanceThe membranes were made up of PDMS as active skin layer and polysulfone (PSF) or polyamide (PA) as supporting layer. PDMS-PSF membrane was numbered 1#, and PDMS-PA membrane numbered 2#. The pervaporation experiments were carried out by using the composite membranes and dilute ethanol-water mixture. The experimental measurements for the permeation performance under various operating conditions showed that the specific permeation rate of 2# membrane was over 1# membrane by seven times at least. A resistance-in-series model was applied to formularize the transport of the permeants. Influence of the binding interfaces between the active skin layer and support layers in these membranes on pervaporation performance was analyzed. The cross section morphology of the membranes and chemical element distribution along membrane thickness were examined by using SEM and EDS. It was found that the different structure was formed when PDMS as active top layer matched with different support material. Although the PDMS intrusion layer into PSF near the interface was only about 2μm, it gave significant effect on the permeation performance. This implies that the resistance produced by the intrusion layer into PSF is apparently larger than that of PDMS intruding PA and even over intrinsic PDMS resistance, and indicates that the physical structures and material formation of the binding interfaces are very important factors affecting membrane performance.2. Study on improvement of the quality of new-type China liquor by pervaporation with PDMS membraneA composite PDMS membrane was used to separate aroma compounds from new-type China liquor by pervaporation at 30℃, 35℃and 40℃and 10mmHg downstream pressure. The analysis of aroma compounds in the retentate and permeate showed PDMS membrane presented excellent separation performance. Permeation ratio for five kinds of esters (except ethyl lactate) and acetal in the original liquor were 100%, over 70% for alcohols (except methanol), above 87% for aldehyde. The average flux for ethanol reached 3539 g/m²Â·h at 40℃. The result of sensory evaluation to the separated ingredients indicated the sensory quality of the new liquor was significantly superior to that of the original liquor. The study suggests that pervaporation is a very promising technique for the improvement of new-type China liquor.3. Production of apple brandy and acidic beverage with low alcohol content by silicone rubber membrane bioreactorA membrane bioreactor comprised of a fermentor with 5L effective volume and a flat pervaporation module with 0.08 m² PDMS membrane was developed for fresh apple juice fermentation at 25℃. The steady operation mode of membrane bioreactor for fresh apple juice fermentation was determined. The experimental design was based on an idea of suspended growth and close-circulating renewal of the yeast cell and recovery in situ of the product. The system eased the product inhibition to yeast, and thus elevated sugar conversion to ethanol and made batch time shorter. With stepwise condensation of the permeate vapor, a permeate product with any ethanol concentration could be collected and blended with certain ratio to make a so-called apple brandy. Furthermore, the residual broth on upstream side of membrane, in which alcohols was already removed by pervaporation, containing natural organic acids and other nutrients, could accordingly be processed as acidic beverage with a low alcoholic content. The results provide the experimental foundation for producing higher value product by fermentation of fresh apple juice in the membrane bioreactor4. Scale-up of continuous ethanol fermentation in a silicone rubber membrane bioreactorA membrane bioreactor comprised of a fermentor of 5L and a pervaporation module of 0.08m² PDMS membrane was developed for ethanol production by continuous and close circulation fermentation. With dry Saccharomyces cerevisiae as microorganism and glucose as substrate, the kinetic properties of fermentation and membrane performances in the bioreactor were experimentally investigated under long-term continuous operation to the system. A permeate of 17-28wt% ethanol was extracted while maintaining a level of 30-50g·L^-1 ethanol in the stirred-tank fermentor. The system exhibited excellent running stability. During continuous operation, cell densities of 10-24.8g·L^-1 and ethanol productivity of 2.33-3.99g·L^-1·h^-1 were achieved with utilizing feed streams of 30-50 g·L^-1 glucose. Pervaporation flux and selectivity for ethanol were 800-1050g·m^-2·h^-1 and 5.1-8.6 respectively. During ferment of 269 hours, the total amount of ethanol produced was 1999g. Ethanol conversion was 87.2%, and the recovery efficiency of carbon was about 90%. The amount of waste water released through ferment was roughly equal to two times of ethanol product, which was about 22.2% of that discharged from the conventional ferment technology.Hybrid kinetics for the ethanol production by continuous fermentation of Saccharomyces cerevisiae and the product separation by pervaporation were investigated. Mathematic models of cell growth, ethanol formation and substrate consumption in a hybrid process were proposed and the kinetics parameters of the model equations were obtained by nonlinear regression based on the experimental data. The calculated values by the model were in good agreement with the experimental data. The model equation can be used to predict the performance parameters of the membrane bioreactor operated in steady state conditions and, therefore, provide valuable guidance for the design and operation of such membrane bioreactors.