(406d) Nitrogen Rejection from Natural Gas Streams Using a Nitrogen Selective Metal Organic Framework

Recently a metal-organic organic framework (MOF) called V₂Cl_2.8(btdd) was discovered that is able to separate CH₄ and N₂ through an equilibrium separation, with CH₄ as the light product [1]. To determine feasibility of the vanadium MOF for nitrogen removal, a series of simulated process studies were performed. Two cycles were considered: the basic 3-step cycle and the Skarstrom cycle. First, the 3-step cycle was considered with a wide range of operating conditions. Three inlet compositions (55/45, 80/20 and 92/8 mol% CH₄/N₂), three process temperatures (30, 40 and 50°C) and a range of adsorption pressures (100 to 500 kPa) were considered. These conditions were chosen to span the range of possible landfill gas or natural gas well process conditions. The inlet conditions were used to generate at least 3500 samples for all possible combinations of these variables. These samples were trained using artificial neural networks to perform process purity/recovery optimizations. From the 3-step cycle optimizations, it was found that the cycle was often unable to meet the 96 mol% CH₄ purity requirement. The Skarstrom cycle was able to meet the 96 mol% CH₄ purity requirement, in all cases studied. It was determined that the material works best in vacuum swing adsorption cycles with the high pressure at 100 kPa. Due to the high isosteric heat of adsorption for N₂ on V₂Cl_2.8(btdd), the process performed better at higher temperatures. At 50°C, the best separations were achieved. The details of the process optimization study and the effectiveness of incorporating machine-learning based surrogate models will be discussed.

[1] Jaramillo, D. E., Reed, D. A., Jiang, H. Z., Oktawiec, J., Mara, M. W., Forse, A. C., ... & Long, J. R. (2020). Selective nitrogen adsorption via backbonding in a metal–organic framework with exposed vanadium sites. Nature materials, 19(5), 517-521.