(617c) Nitrogen Rejection from Natural Gas Streams Using a Nitrogen Selective Metal Organic Framework

The adsorptive separation of CH₄ and N₂ is generally performed through a kinetic separation using a titanosilicate or carbon molecular sieve. However, a metal-organic organic framework (MOF) called V₂Cl_2.8(btdd) was recently discovered 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 Skarstrom cycle and a 6-step cycle with pressure equalization and light product pressurization steps. First, the Skarstrom 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), three particle densities (600.0, 775.0 and 961.7 kg/m³) and two high pressures (100 and 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 1000 samples for all possible combinations of these variables. These samples were trained using Gaussian process regression to perform process purity/recovery and energy/productivity optimizations. Artificial neural networks were used to predict axial bed profiles and to validate the machine learning predictions. From these simulations, 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 particle density did not seem to change the purity-recovery optimizations but did affect the energy-productivity optimizations. As the particle density decreased, the productivity also decreased. Most of these process conditions could yield the required CH₄ purity of 96 mol%, but often the recovery was poor. The maximum recovery at a purity of 96 mol% was about 80%. After the Skarstrom cycles process performance was determined, a more complicated 6-step cycle with pressure equalization and light product pressurization was used to improve the process recovery.