Zeolite Membrane Separation at Low Temperature

Sammanfattning: The energy consumption of separation processes accounts for a large part of the total energy consumption in chemical industry. Membrane separation processes require much less energy than the currently used thermally driven separation processes and could therefore reduce energy consumption in industry considerably. Today, most commercially available membranes are organic polymeric membranes. Inorganic zeolite membranes have several superiorities over polymeric membranes, e.g., higher flux and selectivity, higher chemical and thermal stability, and thus have great potential for a variety of gas and liquid separations. Whereas there have been extensive studies on zeolite membrane separation at high temperature during the past decades, scientific reports on the low temperature applications of zeolite membranes is extremely scarce and there are no reports at cryogenic temperature. This work is pioneering research on investigation of the performance of zeolite membranes for separation of various gas mixtures at unprecedentedly low temperature, down to cryogenic temperature. In the present work, zeolite membranes were, for the first time, evaluated for gas separation at cryogenic temperature. Air separation by ultra-thin MFI membranes was carried out at a feed pressure ranging from 100 mbar to 5 bar over the temperature range of 62–110 K. The membranes were found to be oxygen selective at all the conditions investigated. The observed results were well above the upper bound in the 2008 Robeson selectivity-permeability plot when the feed pressure was less than or equal to 1 bar. The O2/N2 separation factor reached 5.0 at 67 K and 100 mbar, with a high O2 permeance of 8.6 × 10-7 mol m-2 s-1 Pa-1. The performance of our membranes (in terms of selectivity) was comparable to that recently reported for promising polymeric membranes, but 100 times higher in terms of permeance and flux. The membrane selectivity was found to increase with decreasing temperature and feed pressure. The present work has therefore indicated the optimum conditions for air separation using MFI membranes, namely low feed pressures and cryogenic temperatures. A mathematical model showed that the selectivity to O2 emanated from O2/N2 adsorption selectivity. N2/He separation is essential for helium recovery from natural gas and helium reclamation for airships and submarines. Zeolite membranes were evaluated for this separation over the temperature range of 85–260 K, possessing high N2-selectivity at all the conditions investigated. When the feed pressure was 5 bar and the permeate pressure was 0.5 bar, a highest N2/He separation factor of 62 was observed at 124 K. The N2 permeance was rather high, up to 39 ×10−7 mol m−2 s−1 Pa−1. The separation was attributed to adsorption selectivity of the membranes to N2, effectively suppressing the transport of He in the zeolite pores and this effect was more significant at cryogenic temperature. A mathematical model showed that the largest difference of adsorbed loading over the film at ca. 120 K was probably the main reason for the observed maximum selectivity at this temperature. The model also indicated that the selectivity could even be increased by 2–3 times if the membrane was totally defect-free. This work demonstrates that a zeolite membrane process could be rather competitive for N2/He separation. Synthesis gas generated from biomass is a valuable, renewable resource that can be used for production of clean energy and various chemicals. It is mainly a mixture of CO, CO2, and H2. CO2 is an undesired component in the syngas and should, therefore, be removed. In this work, CO2 separation from H2 and CO using zeolite membranes was studied for at low temperatures, down to 235 K and at a feed pressure of 9 bar. The membrane performance in terms of both selectivity and flux was superior to that reported for the state-of-the-art polymeric and inorganic membranes. The highest separation factor was 202 for CO2/H2 separation at 235 K and 21 for CO2/CO separation at 258 K, significantly higher than that at room temperature. The observed CO2 flux was very high, i.e., 300-420 kg m-2 h-1, in the entire temperature range of 235–310 K. Initial cost estimation revealed that high flux zeolite membranes were economically competitive with the present commercial polymeric membranes. Moreover, the process relying on our zeolite membranes was shown to be appreciably more space-efficient. Efficient light olefins/N2 separation technologies are of great interest to recover monomers from N2 purge gas in polymer plants. C3H6/N2 and C2H4/ N2 separation were investigated using zeolite membranes in a temperature range of 258–356 K. The membranes were rather selective towards the hydrocarbons. For C3H6/N2 separation, a maximum separation factor of 43 was observed at room temperature with a C3H6 permeance of 22×10-7 mol m-2 s-1 Pa-1. For C2H4/N2 separation, the maximum separation factor was 6 at 277 K with a C2H4 permeance of 57×10-7 mol m-2 s-1 Pa-1. The findings reveal that zeolite membranes are promising candidates for light olefins/N2 separation in petrochemical processes. The adsorption properties dominate separation performance for systems studied in the present work. The high selectivity emanates from competitive adsorption, e.g., the strongly adsorbing components hinder the permeances of the weakly adsorbing ones and the effect was stronger at low temperature. In addition, gas permeances through zeolite membranes tend to decrease at low temperature most likely due to decreasing diffusivity, especially at cryogenic temperature. However, the permeances of our membranes even at low temperature were still one to two orders of magnitude higher than those reported for inorganic and polymeric membranes. Thus, the high-flux membranes have great superiority in this case. The fairly high permeance even at low temperatures was ascribed to the ultra-thin (< 1µm) film and highly permeable support used. We provide here a promising candidate, ultra-thin zeolite membranes, with high permeance and excellent selectivity for gas separation application at low temperature.