Zeolite membranes for effective production of biofuels

Abstract: 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 large amounts of CO2 that must be removed before methanol synthesis. However, commercial processes for CO2 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, ZSM-5 membranes were prepared and evaluated for removal of CO2 and H2S from synthesis gas. Both synthesis gas prepared 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 CO2 fluxes were very high for CO2 separation from synthesis gas free from H2O and H2S prepared from gas cylinders. CO2 fluxes up to 657 kg m-2 h-1 were observed for a binary mixture (CO2 and H2). The high flux was a result of a thin membrane film, an open graded support, a high pressure drop, resulting in a high diffusivity. A CO2/H2 separation factor of 32.1 was observed at 275K and the selectivity was controlled by CO2 adsorption, blocking the transport of H2. It was also found that the CO2 flux and separation factor decreased substantially when CO2 and H2S was separated from synthesis gas, derived from black liquor, also containing H2O and H2S. This was probably an effect of competitive adsorption of H2S and H2O, which are probably blocking the other molecules from permeating through the membrane.Mathematical models of a traditional methanol synthesis process and two alternative membrane processes were developed. 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 however approach that of the MRP, to the price of higher complexity of the process.