Efficient separation of CO2 from methane at high pressures by modified microporous titanosilicates; design and breakthrough studies

Abstract

In this work, type-4 titanosilicate (TS-4) having a micro-pore size of 3–4 Å, was synthesized from inexpensive source materials, such as commercial silicon dioxide, with the template-free hydrothermal method. The XRD analysis confirmed formation of the TS-4 crystalline phase. Then, strontium cation modification was performed using SrCl2 solution to adjust the pore structure and increase the basicity of the surface to improve CO2 selectivity versus methane. In this study, the adsorption isotherms of methane and CO2 were measured in the range of 0 to 50 bars, at two different temperatures of 298 K and 308 K, then the ideal selectivities of CO2/CH4 were calculated at different pressures. The ideal CO2/CH4 selectivity was determined at 19.04 and 38.49 for TS-4 and Sr-TS-4, respectively, at the pressure of 50 bar and the temperature of 298 K. The isosteric heat of adsorption of CO2 was calculated by Van’t Hoff equation as −29.63 kJ/mol and −26.22 kJ/mol, respectively. Low CO2 heat of adsorption of Sr-TS-4 and high selectivity of CO2/CH4 at high pressures, against zeolites, offer Sr-TS-4 as an economic and efficient adsorbent for cyclic adsorption processes. To study the dynamic selectivity of CO2/CH4, dynamic adsorption experiments were conducted at the temperature of 298 K, pressures of 7 and 15 bars, flow rates of 10 and 20 Nml/min in a bed of 2.7 gr adsorbent, with the inlet molar fraction of 5–95 % CO2-CH4 mixture. Comparison of the breakthrough curves indicated that Sr-TS-4 had a longer breakpoint and a larger used ratio of the length at any feed flow rate. The maximum relative dynamic selectivity of CO2/CH4 at 15 bar, by the Sr-TS-4, was obtained 95.29 at CO2 breakpoint of 40 min, whereas maximum relative dynamic selectivity, by the TS-4, was obtained 63.54 at CO2 breakpoint of 30 min. It is concluded that Sr-TS-4 would be an efficient adsorbent for separation of CO2 from natural gas in commercial scales at high pressures with high methane recovery.