Effective Design of a Vacuum Pressure Swing Adsorption Process To Recover Dilute Helium from a Natural Gas Source in a Methane-Rich Mixture with Nitrogen

The conventional method of helium production is a cryogenic process, which is installed downstream of a liquefied natural gas (LNG) plant, where a helium extraction unit (HeXU) would be usually developed as the byproduct unit, due to the high cost of the process. In this study, pressure swing adsorption method accompanied by a relative vacuum condition for the bed regeneration (PVSA) was investigated as an alternative method for recovery of helium from natural gas source. The PVSA design was studied for a continuous product, utilizing zeolite 13X as the adsorbent, through a formerly validated model, for a dilute feed of 1.5 vol % He, mainly comprised of methane in 82.5 vol % accompanied by nitrogen. The impacts of repressurizing medium and equalization steps of the design conditions on the He purity and recovery were investigated. It was revealed that, generally, the bed repressurizing step should be performed by the feed as compared to the product. However, at high space velocities, a combination of the feed and product would be preferable to achieve higher He purity. Adding an equalization step is not advantageous to the process design at medium to high space velocities, but can be exploited only for recovery improvement when the purity is in an acceptable high range at low space velocities. Feasibility of the adsorption method and its performance was then analyzed in a basic 4bed-4step PVSA cycle design for different effective parameters including dimensionless space velocity (SV̅), bed pressure in adsorption step, and vacuum pressure in regeneration step and their interactions. However, due to the complex trends of the effective parameters and their interactions, response surface methodology (RSM) was implemented for optimization of the cycle performance. At the optimal design point with a feed pressure of 5.75 (bara), regeneration pressure of 0.05 (bara), and (SV̅) of 2.25, a helium purity of 99.9% and a recovery of 70% with productivity of 0.335 (mol/kg-adsorbent·h) would be achieved