Synthesis of small pore zeolite membranes for nitrogen/methane separation

Abstract

The natural gas consumption has grown from 5.0 trillion cubic feet (TCF) in 1949 to 27.0 TCF in 2014, and is expected to rise to 31.6 TCF in 2040. This large demand requires an effective technology to process and purify natural gas, especially those inert gases, such as N2. Most of the N2 reject plants in US apply a cryogenic distillation process. However, this technology needs high-energy requirement, high operation cost and has high tendency to block equipment. Membrane separation offers the advantage of energy saving and flexibility for small N2-contaminated gas wells because of lack of suitable N2 rejection technology. Porous crystalline molecular sieve membranes are ideal candidates to remove natural gas impurities because of their exceptional molecular sieving effects, high thermal and chemical resistance. This work aims at developing molecular sieve membranes that can effectively and economically separate N2 from CH4. Specifically we focused on three types of porous crystalline molecular sieve membranes: SAPO-34, AlPO-18 and DNL-6. The best resultant SAPO-34 membranes were prepared at 230 °C for 6 h in a stainless steel autoclave and a Teflon-lined autoclave, which could separate N2 from CH4 with N2 permeance as high as 2,600 Gas Permeation Unit (GPU) and a N2/CH4 selectivity of 7.4 and 2,100 GPU and a selectivity of 8.6 at 23 °C for a 50/50 premixed N2/CH4 mixture, respectively. This observed separation performance is superior to those of the state-of-the-art membranes. If scaled-up, these SAPO-34 membranes are potentially viable for economic N2 rejection in natural gas processing. The resultant AlPO-18 membranes display N2/CH4 mixtures separation with unprecedented N2 permeance as high as 3,076 GPU and separation selectivities as high as 4.6. The resultant DNL-6 membranes were successfully prepared, but these membranes possess large concentration of defects, resulting in non-continuous membranes, or at best resulting in membranes displaying limited separation performance for N2/CH4 mixture. We studied the separation mechanisms for SAPO-34 and AlPO-18 membranes for N2/CH4 gas mixtures. Difference in diffusivities in both cases was the dominant separation mechanism.