Hydrate-Based Mild Separation of Lean-CH4/CO2 Binary Gas at Constant Pressure

The product gas of natural gas hydrate production by CO2 replacement is the lean-CH4/CO2 binary gas which contains 85–95% CO2. In this work, the lean-CH4/CO2 (10 mol % CH4/90 mol % CO2) binary mixture was used to simulate the product gas of natural gas hydrate production via CO2 replacement, and a promising and mild (low pressure) hydrate-based gas separation (HBGS) technology was employed. This work innovatively proposes to inject the reaction liquid to maintain the constant pressure of the whole process, which makes hydrates form continuously to increase the gas consumption. First, the hydrate phase equilibrium of 10 mol % CH4/90 mol % CO2 + 5 wt % tetrabutylammonium bromide (TBAB, aq) was measured in the range of 280–287 K and 0.5–4 MPa. After that, hydrate-based mild separation of the lean-CH4/CO2 binary gas was conducted with the help of TBAB at a constant pressure in the range of 1–4 MPa. The results show that after HBGS in addition with 5 wt % TBAB­(aq) at 1 MPa and 278 K, the CH4 concentration could be increased from 17.2 to 66.2 mol % under constant pressure operation, while it was just slightly increased to 17.2 mol % under nonconstant pressure operation. HBGS with 5 wt % TBAB under constant pressure had the best separation performance. Subcooling has a greater effect on the separation kinetics. HBGS of 10 mol % CH4/90 mol % CO2 under a constant pressure operation of 2 MPa, subcooling of 6 K, and 5 wt % TBAB has the highest separation factor above 37. It is suggested that hydrate-based lean-CH4/CO2 separation experiments are carried out at a mild pressure with low G/L and, more importantly, in a short time. This work proposed a mild HBGS of lean-CH4/CO2 separation, which could benefit the separation of gas produced by natural gas hydrate replacement exploitation. This work also simultaneously provides insights into other HBGS for other gas mixtures (flue gas, coalbed methane, biogas, and shift gas) to achieve low energy consumption, high gas consumption, and high efficiency.