Influence of hydrate structure on continuous separation of coal bed methane gas: A thermodynamic simulation study

Influence of hydrate crystallographic structures on continuous CH₄ + N₂ gas separation is studied using thermodynamic simulations. A continuous separation process with discharge of hydrate crystals only (not as hydrate slurry; without liquid phases) is modelled. 35 mol% CH₄/N₂ and 50 mol% CH₄/N₂ gas separation simulations are performed for simple sI system, and isobutane sII, methylcyclohexane sH and neohexane sH promoted systems at pressures 1.0 MPa, 4.0 MPa and 7.0 MPa depending on hydrate structures. At the steady state, under 7.0 MPa, CH₄ concentration in hydrate phase (x_j=CH4,hyd) and percentage of CH₄ recovery (S. Fr.) were in the increasing order of isobutane sII < neohexane sH ≈ methylcyclohexane sH < simple sI systems. In the sII and sH system studied, x_j=CH4,hyd and S. Fr. at the steady state were lower during gas separation at higher pressures. The maximum CH₄ concentration that can be recovered in hydrate for continuous 35 mol% CH₄/N₂ and 50 mol% CH₄/N₂ gas separations are calculated to be approximately 64 mol% CH₄/N₂ and 77 mol% CH₄/N₂ respectively.