High flux zeolite membranes for efficient production of biofuels

Sammanfattning: The greenhouse effect and the limited fossil oil resources have increased the demand for 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.Synthesis gas is a gas mixture containing mainly carbon monoxide, hydrogen and carbon dioxide. Synthesis gas produced from biomass usually contains more carbon dioxide than desired, and it also contains hydrogen sulphide. These components need to be removed prior to synthesis of e.g. methanol. Membrane separation is an energy efficient separation method, and zeolite membranes potentially offer high flux combined with high selectivity as well as high temperature and chemical stability. The multicomponent separation performance of a zeolite MFI membrane was in this work investigated for synthesis gas at industrially relevant pressures. The membrane displayed very high carbon dioxide flux for pure cylinder gas mixtures. Also, competitive adsorption was found to effectively block hydrogen permeance, resulting in a high carbon dioxide/hydrogen selectivity. For synthesis gas produced in pilot scale by black liquor gasification, the membrane selectively permeated carbon dioxide, hydrogen sulphide and water. The presence of hydrogen sulphide and water vapour in the feed was however found to reduce the carbon dioxide flux through the membrane as well as the carbon dioxide/hydrogen selectivity. Methanol synthesis from synthesis gas is equilibrium limited, and continuous removal of products 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 vapour. 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.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 removing carbon dioxide from bio-synthesis gas.