Surface barriers are influencing the mass transfer in nanopores, but their origin is unclear and can be quite different in different materials. For MFI and CHA membranes studied here, we show that the surface barrier may be a surface diffusion process with higher activation energy than the surface diffusion process in the pores, but other possible mechanisms such as pore blocking and pore narrowing has not been ruled out. The higher activation energy is probably a result of less interaction between adsorbed molecules at the pore mouth than inside the pores, i.e. the barrier is simply a geometrical effect in these materials. For pure components at low concentration in MFI zeolite, we found that barrier is proportional to the product of the molecular weight and heat of desorption. For MFI and CHA zeolite, we observed that the barrier is a function of concentration and approach zero at high concentration and that the barriers of the components become more similar due to interaction between the components in mixtures, which explains the high and selective mass transfer displayed by these nanoporous materials at high concentration.

The adsorption and mass transport of CO₂ and CH₄ in CHA zeolite were studied experimentally. First, large and well-defined CHA crystals with varying Si/Al ratios and morphologies ideal for adsorption studies were prepared. Then, adsorption isotherms were measured, and adsorption parameters were estimated from the data. In the next step, permeation experiments for pure components and mixtures were conducted for a defect-free CHA membrane with a Si/Al ratio of 80 and a thickness of 600 nm over a wide temperature range. A maximum selectivity of 243 in combination with a CO₂ permeance of 70 × 10⁻⁷ mol/(m² s Pa) was observed for a feed of an equimolar CO₂/CH₄ mixture at 273 K and 5.5 bar. Finally, a simple model accounting for adsorption and diffusion through the surface barriers and the interior of the pores of the membrane was fitted to the permeation data. The fitted model indicated that the surface barrier was a surface diffusion process at the pore mouth with higher activation energy than the diffusion process within the pores. The model also showed that the highly selective mass transport in the membrane was mostly a result of a selective surface barrier and, to a lesser extent, a result of adsorption selectivity.

In the present work, ultra-thin MFI zeolite membranes with a thickness of about 450 nm prepared in fluoride media were evaluated for separation of equimolar CO₂/H₂ gas mixtures. Both dry and humid mixtures were evaluated. For dry gas mixtures, the membrane selectivity increased from 13 to 45 when the temperature decreased from 318 to 285 K, and CO₂ flux was very high and almost constant at 4 mol m⁻² s⁻¹, corresponding to CO2 permeance of 100 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ (29851 GPU). For humid gas mixtures with a partial pressure of water of 3.5 kPa, the CO₂ flux decreased from 3.27 to 0.10 mol m⁻² s⁻¹ when the temperature decreased from 316 to 288 K. The corresponding CO₂ permeance decreased from 79 to 2.5 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ (from 23582 to 746 GPU). At the highest temperature 318 K, the CO₂/H₂ separation selectivity was slightly higher for the humid gas mixture. For the humid gas mixture, the CO₂ flux increased from 3.27 to 3.76 mol m⁻² s⁻¹ at 318 K and from 0.52 to 0.90 mol m⁻² s⁻¹ at 296 K when the permeate pressure was reduced from atmospheric to vacuum, respectively. The separation selectivity of CO₂/H₂ was almost not affected by the permeate pressure. The results show that ultra-thin fluoride MFI zeolite membranes are promising candidates for separation of CO₂ from dry or humid synthesis gas.

The development of more efficient technology for the production of helium from natural gas is pressing as the current resources are dwindling. In the present work, ultra-thin MFI membranes were evaluated for the separation of an equimolar CH4/N2/He mixture in a wide temperature range 120–293K. The membrane was highly selective towards CH4 and N2 at all the investigated conditions, which resulted in a helium rich retentate. The observed selectivity should be a result of selective adsorption of CH4 and N2. A maximum (CH4+N2)/He separation factor of 152 was observed at 153 K and a feed pressure of 3 bar and a permeate pressure of 0.2 bar. At these conditions, separation factors were 265 and 38 for CH4/He and N2/He, respectively, and the CH4 and N2 fluxes were 1.12 and 0.16 mol/(m2⋅s), respectively. To the best of our knowledge, these are the best results reported in the open literature for the recovery of helium from natural gas using membrane technology. The high selectivity and flux indicated that the ultra-thin MFI membranes are a promising candidate for efficient helium production from natural gas.

Highly permeable and selective tubular zeolite CHA membranes with a thickness of about 450 nm and a length of 100 mm and an inner diameter of 7 mm were evaluated by single gas permeation experiments and for separation of an equimolar CO2/CH4 mixture. The membranes displayed high H2 and CO2 single gas permeances of 55 × 10−7 mol m−2 s−1 Pa−1 and 94 × 10−7 mol m−2 s−1 Pa−1, respectively, and a very low SF6 permeance of 3 × 10−9 mol m−2 s−1 Pa−1. The highest observed mixture separation factor was 99 with CO2 permeance of 60 × 10−7 mol m−2 s−1 Pa−1 at a feed pressure of 5 bar and permeate pressure of 0.12 bar. The corresponding CO2flux was 1.46 mol m−2 s−1. The highest observed flux was 1.98 mol m−2 s−1 with a separation factor of 52 at a feed pressure of 10 bar and permeate pressure of 0.12 bar at room temperature. To the best of our knowledge, this is the first report describing highly permeable and selective tubular CHA membranes. The results indicate that the membranes have a great potential for industrial separation of CO2from natural gas and biogas.

In the present work, single channel tubular zeolite CHA membranes with a length of 50 cm and a membrane area of 100 cm² were evaluated by SEM and permeation experiments with single components and industrially relevant humid mixtures. The membranes comprised well-intergrown and smooth CHA films with a thickness of <500 nm supported on the inside of a highly porous tube. For single component permeation, a very low SF₆ permeance of 4.5 × 10⁻¹⁰ mol/(m²‧s‧Pa) was observed, which indicated nearly defect free membranes. On the contrary, the membranes displayed a very high CO₂ permeance of 128 × 10⁻⁷ mol/(m²‧s‧Pa), which illustrated the very high permeability of the CHA pores. Finally, the membranes displayed excellent selectivity for separation of industrially relevant CO₂/CH₄/H₂O mixtures, which was attributed to selective interaction between the CO₂ molecules and the polar water molecules in the pores. The highest observed CO₂/CH₄ separation selectivity was as high as 198 in combination with a CO₂ permeance of 14 × 10⁻⁷ mol/(m²‧s‧Pa) at a feed pressure of 600 kPa (including 2.2 kPa water) and 293K. The corresponding CO₂ flux was 0.39 mol/(m²‧s) and the corresponding CO₂/CH₄ separation factor was 162. The observed membrane performance was reduced by concentration polarisation due to the limited feed flow in the experimental setup. The corresponding selectivity and CO₂ permeance corrected for concentration polarisation were as high as 236 and 16 × 10⁻⁷ mol/(m²‧s‧Pa), and the corrected CO₂ flux was 0.44 mol/(m²‧s) and corrected separation factor was 198. An estimate showed that even at a low feed pressure of 500 kPa, it would be sufficient with as few as 64 membranes for processing of a feed of 100 Nm³/h raw biogas, i.e. the capacity of a typical biogas plant at a large farm, to biomethane with high purity. These results illustrated that the membranes are promising candidates for industrial separation of CO₂ from e.g. natural gas and biogas.

Zeolite membrane processes were designed for biogas upgrading for feed pressures ranging from 5 to 20 bar and compared with corresponding polymeric membrane processes. The mass transfer through zeolite membranes was estimated by a model accounting for adsorption and diffusion through the surface barriers and the interior of the pores, while the mass transfer through polymeric membranes was estimated using reported permeances for commercial polymeric membranes. The zeolite membranes displayed approximately three orders of magnitude higher permeance and up to 7 times higher selectivity. To reach a low methane loss, two and three membrane stages were needed for zeolite and polymeric membranes, respectively, because of the differences in selectivity. Due to the higher selectivity, the electricity need for the zeolite membrane process was only 50–60% of that for the corresponding polymeric membrane process. As a result of the much higher permeability, the zeolite membrane processes were much more compact than the equivalent polymeric membrane processes. The estimated cost for zeolite membranes prepared in small scale including modules was much lower than the cost for industrially produced polymeric membranes including modules.