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The increased demand of fossil fuel along with the depletion of economical crude oil resources, environmental challenges such as the accumulation of CO2 and other greenhouse gases in the atmosphere and the reduction of the dependence on imported oil are some of the motivations for the huge interest in biofuels. Biobutanol produced from ABE fermentation has been considered to be a good partial replacement for fossil fuels. However, challenges such as the need for inexpensive feed-stocks, improved fermentation performance to achieve higher final butanol concentration and higher yield, an efficient method for solvent recovery, and water recycle are the main obstacles to make the production of this alcohol economically viable. Pervaporation, a membrane-based process, is considered to be an attractive separation method to remove butanol from ABE fermentation broth. Among the membranes used for butanol separation, PDMS membranes showed reasonable performance such as good permeability, and appropriate selectivity for butanol separation by pervaporation. However, PDMS membranes need to be improved in terms of performance to be applicable in large scale butanol production plants. In this study, activated carbon nanoparticles have been embedded into the matrix of the PDMS membrane to improve its separation performance and, in particular, the permeation flux and butanol selectivity. Result showed that the presence of nanoparticles improves the PDMS membrane performance up to a certain particle loading. Moreover, it was shown that the operating conditions have a major impact on the pervaporation membrane separation process. The best membrane for pervaporation separation of butanol from binary aqueous solutions was obtained for a 6 wt% particle concentration where the total permeation flux and butanol selectivity increased by 42.6% and 51.9%, respectively, compared to neat PDMS membranes. Moreover, the best performance for the separation of butanol from ABE model solutions was achieved for an 8 wt% nanoparticle loading. Both the selectivity for butanol and the total permeation flux more than doubled in comparison to neat PDMS membranes prepared in this study. Moreover, in order to compare the PDMS/AC mixed matrix membrane performance for pervaporation separation of butanol from binary and ABE model solutions with PDMS membranes available on the market, experiments were also performed with a commercial PDMS membrane. Result of butanol separation from ABE model solutions showed that mixed matrix membranes with 8 wt% nanoparticles loading had a higher permeation flux than that of the commercial membranes. It was clearly shown that the presence of activated carbon nanoparticles in the matrix of the PDMS would be beneficial for the pervaporation separation of butanol from ABE fermentation broths. To better comprehend how the presence of activated carbon nanoparticles in the polymeric membranes enhance the performance of the membranes, a series of numerical simulations were performed. A finite difference model was developed to simulate the mass transfer of permeating components through mixed matrix membranes by pervaporation for a wide range of relative permeability, nanoparticle loading, particle shape, particle size and different filler adsorption isotherms. Finally, an investigation has been performed to optimize the butanol pervaporation separation process from ABE fermentation broth at an industrial scale. |
