Zeolite Membranes for Production of Biofuels
Sammanfattning: To deal with the increasing demand of renewable fuels, more efficient processes for the production of biofuels are needed. Zeolite membranes have the potential to improve many existing processes that could be used for production of biofuels. Methanol is a potential biofuel that may be produced from synthesis gas in an equilibrium limited reaction. The production of methanol from synthesis gas could be improved by use of a membrane reactor, which could increase the conversion of synthesis gas to methanol per pass in the reactor. Methanol and several other biofuels can be prepared by first gasifying biomass to synthesis gas. Synthesis gas produced from biomass usually contains significant amounts of carbon dioxide that must be removed before methanol synthesis. However, commercial processes for carbon dioxide removal are very energy intense, and a membrane process could also improve this process and offer lower energy costs and less complicated and more compact equipment. In the present work, silicalite-1 and ZSM-5 membranes (NaZSM-5 and BaZSM-5) were successfully prepared on graded α-alumina supports and evaluated for removal of carbon dioxide and hydrogen sulfide from synthesis gas. Both synthesis gas prepared from pure gas from gas cylinders and synthesis gas obtained from a black liquor pilot plant gasifier were used. The separations were performed at industrial relevant conditions, i.e. high pressures. It was found that the carbon dioxide fluxes were very high for carbon dioxide separation from synthesis gas free from water and hydrogen sulfide prepared from gas cylinders. Carbon dioxide fluxes up to 657 kg m-2 h-1 were observed for a binary mixture of carbon dioxide and hydrogen. The high flux was a result of a thin membrane film, an open graded support, and a high pressure gradient over the membrane. A CO2/H2 separation factor of 32.1 was observed at 2 ˚C and the selectivity was controlled by carbon dioxide adsorption, blocking the transport of hydrogen. The differences in carbon dioxide separation performance, observed for the different evaluated membranes, were likely due to differences in the carbon dioxide adsorption isotherms. The silicalite-1 membrane had a more favourable adsorption isotherm compared to the ZSM-5 membranes at these conditions, which resulted in larger difference in fractional surface loading between feed and permeate side of the membrane. It was also found that the carbon dioxide flux and separation factor decreased substantially when carbon dioxide and hydrogen sulfide was separated from synthesis gas derived from black liquor also containing water and hydrogen sulfide. This was probably an effect of competitive adsorption of hydrogen sulfide and water, which are probably blocking carbon dioxide molecules from permeating through the membrane. Furthermore, all-zeolite membranes (membranes consisting of both zeolite film and zeolite support) were prepared and evaluated for removal of carbon dioxide from synthesis gasin the present work. The membranes were carbon dioxide selective, but quite brittle, which made testing difficult. The zeolite supports used for all-zeolite membranes were prepared by collaborating researchers as an attempt to reduce crack formation in zeolite membranes, since the thermal expansion mismatch between the zeolite film and the membrane support will be minimized using this approach. Mathematical models of a traditional methanol synthesis process and two alternative membrane processes were also developed in the present work. Recorded experimental permeation data for a ZSM-5 membrane was used as input to the models. The estimated performance of the traditional process was compared with a membrane reactor process (MRP) and a membrane module process (MMP). The mathematical model indicated that the MRP is the best alternative, since it enabled one pass operation, due to the highest conversion per pass. The MMP is however better from a practical point of view compared to the MRP since membrane and catalyst is separated and the membrane and reactor can be operated at their optimal respective temperatures and the membrane and catalyst can be replaced independently. By adding more membrane modules, the performance of the MMP will approach that of the MRP, to the price of higher complexity of the process.
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