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 6h. 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 15kJ/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.