Efficient screening of novel adsorbents for coalbed methane recovery

Many adsorbents with outstanding methane (CH₄)/nitrogen (N₂) separation performance are reported recently. Some may have the potential for coalbed methane (CBM) recovery to resolve the current energy crisis. However, no systematic assessment method for evaluating these adsorbents is available. This study was performed for efficient comparison and assessment of 47 novel adsorbents that are suitable for CBM recovery and to guide further adsorbent development with a three-step simulation-based method. First, the adsorbents of interest were prescreened based on the CH₄/N₂ adsorption selectivity predicted from the ideal adsorption solution theory and a composite parameter S that incorporates both adsorption selectivity and working capacity. Then, the top 10 adsorbents from the prescreening step were tested in a simulated vacuum pressure swing adsorption process. The process performance of the adsorbents was evaluated by comparing their product purity, recovery and productivity at two base conditions. It was observed that Cu-MOF and NAPC-3-6 exhibited the highest product purity and OAC-1 showed the highest product recovery and productivity at the two base cases. The process performance indicators of various adsorbents were also correlated with their adsorption selectivities and capacities to investigate how these adsorption characteristics would affect the process performance. We find that the working capacities of the adsorbents are highly related with the product recovery while the adsorption selectivities are more related with the product purity. Finally, a process optimization study was performed employing the three adsorbents that exhibited the best performance in the previous evaluation. The objective of the optimization is to minimize the energy consumption of the process while meeting specified product purity (95% or 98%) and recovery rate (90%). The decision variables include the evacuation pressure, feed flow rate and adsorption pressure. The sensitivity of each variable was also examined through a parametric study. The optimization results indicate that the adsorbent selection will depend on the production scale and purity requirement. OAC-1 is the best candidate for a large scale CH₄ production with a regular purity grade while NAPC-3-6 is a better choice for a small scale CH₄ production with high purity requirement.