Zeolite membranes for efficient synthesis of biofuels
Abstract: The greenhouse effect and the limited fossil oil resources have increased the demand of renewable fuels. Zeolite membranes have potential applications in numerous separation processes, and could be useful in the development of efficient processes for renewable fuel production. Methanol synthesis from synthesis gas is equilibrium limited, and continuous removal of products in a zeolite membrane reactor could improve the productivity of a conventional methanol synthesis process. In this work, membranes of two types of zeolite structures, MFI and FAU, were synthesized and evaluated for the separation of methanol from synthesis gas. The synthesis gas was represented by a mixture of hydrogen, carbon dioxide and water. All evaluated membranes were found to display large permeances of methanol and water. At conditions where methanol and water were adsorbing, hydrogen and carbon dioxide were blocked from permeating through the membranes, and the membranes were hence selective. More polar membranes were found to be selective also at higher temperatures. Separation data for one of the membranes were used for modelling the performance of membrane processes for methanol synthesis. It was found that the one pass COx conversion would be much higher in a membrane process than in a traditional synthesis process, assuming that the same membrane performance would be obtained at reaction conditions as that observed at room temperature and atmospheric pressure. It was also found that the total membrane area needed in the membrane processes was practically reasonable. In the process of developing defect free zeolite membranes, it is important to have a tool to characterize flow-through defects. One such tool is permporometry. In this work, permporometry data was compared with SEM observations and mixture separation data. It was shown that permporometry can detect small defects, and that permporometry data correlate well with SEM observations and membrane separation performance. In summary, the present work has contributed to the knowledge of multicomponent separation processes in zeolite membranes. The work has also shown that zeolite membranes could be useful in the challenge of producing renewable fuels, for example by continuously removing products in equilibrium limited processes.
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