SAPO-34 Membranes for N2/CH4 separation: Preparation, characterization, separation performance and economic evaluation

SAPO-34 membranes were synthesized by several routes towards N2/CH4 separation. Membrane synthesis parameters including water content in the gel, crystallization time, support pore size, and aluminum source were investigated. High performance N2-selective membranes were obtained on 100-nm-pore alumina tubes by using Al(i-C3H7O)3 as aluminum source with a crystallization time of 6 h. These membranes separated N2 from CH4 with N2 permeance as high as 500 GPU with separation selectivity of 8 at 24 °C for a 50/50 N2/CH4 mixture. Nitrogen and CH4 adsorption isotherms were measured on SAPO-34 crystals. The N2 and CH4 heats of adsorption were 11 and 15 kJ/mol, respectively, which lead to a preferential adsorption of CH4 over N2 in the N2/CH4 mixture. Despite this, the SAPO-34 membranes were selective for N2 over CH4 in the mixture because N2 diffuses much faster than CH4 and differences in diffusivity played a more critical role than the competitive adsorption. Preliminary economic evaluation indicates that the required N2/CH4 selectivity would be 15 in order to maintain a CH4 loss below 10%. For small nitrogen-contaminated gas wells, our current SAPO-34 membranes have potential to compete with the benchmark technology cryogenic distillation for N2 rejection.