Zeolites are crystalline aluminosilicates with well-defined 3D porous structures consisting of tetrahedral units of aluminate (AlO45-) and silicate (SiO44-) ions. Zeolites can be classified by pore size, with small-pore zeolites featuring 8-membered rings and pore openings around 3-4.5 Å, medium-pore with 10-membered rings and pore sizes of 4.5-6 Å and large-pore with 12-membered rings and pore sizes between 6-8 Å. Zeolites are widely used in catalysis, adsorption, and separation processes in the form of pellets and membranes.

Small-pore zeolite membranes, such as CHA (0.37 nm pore size) and DDR (0.36 nm pore size), have been extensively evaluated for a variety of separations, due to their suitable pore sizes, which enable size-based separation, along with their excellent thermal stability and chemical resistance. Particularly in gas separation, these membranes have demonstrated exceptional performance for a range of industrially relevant gas pairs, such as CO2/CH4, CO2/N2, N2/CH4, and H2/CH4, highlighting their strong potential for biogas and natural gas upgrading. Nevertheless, further fundamental studies are needed in order to improve the membrane materials, deepen our understanding of the mass transfer mechanisms in zeolites, and optimize their performance in practical applications.

In this thesis, the adsorption isotherms of the common components of natural gas and biogas, CO2, CH4, N2, and He were experimentally measured over wide temperature ranges on all-silica CHA, DDR, and MFI zeolite large crystals. The Toth equation was fitted to the measured adsorption data, and adsorption parameters, such as adsorption capacity at saturation (Csat), affinity constant (b), Toth heterogeneity parameter (t), enthalpy of adsorption (ΔHads) and adsorption entropy (ΔSads) were estimated. The estimated adsorption parameters presented in this work are accurate, primarily due to the large crystals used for the adsorption measurements and the recording of low-temperature adsorption isotherms over broad temperature ranges. These data are invaluable for understanding adsorption and mass transfer in zeolite materials, as well as for advancing the development of zeolite materials for gas separation.

The second part of this thesis evaluates highly permeable DDR disc membranes under various conditions for the separation of CO2/CH4 and H2/CH4, gas pairs that are particularly relevant for natural gas and biogas upgrading. For CO2/CH4 separation, the exceptionally high selectivity of 2325 paired with a high CO2 permeance of 34 × 10-7 mol/(m2·s·Pa) was observed for an equimolar mixture at a feed pressure of 3 bar and a temperature of -30 ℃. The highest CO2 permeance was recorded at the same feed pressure and a temperature of +10 ℃ with a value of 44 × 10-7 mol/(m2·s·Pa), while the selectivity remained remarkably high at 1118. For H2/CH4 separation, a H2 permeance of 7.2×10-7 mol/(m2·s·Pa) was recorded for a feed of a 1/1 H2/CH4 mixture at room temperature and pressure of 3 bar. The high permeance was paired with a H2/CH4 selectivity of 207, markedly higher than previously reported for DDR membranes. Furthermore, a mass transfer model accounting for adsorption, surface barrier and surface diffusion was fitted to the experimental data and showed that the model could accurately describe the mass transfer in the zeolite pores, and that the surface barrier was the limiting mass transfer step. Based on the separation results, one-stage membrane processes for upgrading biogas to biomethane using DDR membranes, at three different operating pressures were designed and showed that in all cases a significantly low membrane area, methane slip, and electricity power was sufficient compared to the polymeric membranes processes.

The final part of this work focuses on upgrading synthetic natural gas mixtures with a composition that is typical after a Joule Thompsson process in the industry using CHA membranes. The membranes exhibited high flux at a feed pressure of 30 bar while the selectivities for the gas pairs of CO2/N2, CO2/CxHy, and N2/CxHy were also excellent. The optimal temperature for CO2 removal was found to be around 25 °C, where a great CO2 flux of 1.2 mol/(m²·s) was observed coupled with a CO2 permeance of 13×10-7 mol/(m²·s·Pa). Under these conditions, high selectivities for CO2/CH4, CO2/C2H6, and CO2/C3H8 of 68, 101, and 190, respectively, were observed. The optimal temperature for N2 removal was around 35 °C; at this temperature high N2 flux of 2.5×10-3 mol/(m²·s) was observed, with the N2 permeance reaching 1×10-7 mol/(m²·s·Pa). Finally, a membrane process, designed based on the separation data, showed that only 10.4 m2 membrane area is sufficient for the upgrading of 1000 Nm3/h natural gas to pipeline gas at a feed pressure of 30 bar, which is approximately 102 times smaller than the membrane area needed in polymeric membrane processes. 

Overall, the findings in the thesis suggest that small-pore zeolite membranes hold great potential for upgrading biogas and natural gas.