Optimization of a Pressure Swing Adsorption Process for Nitrogen Rejection from Natural Gas

Nitrogen rejection from natural gas is a capital intensive process. The most widely used technology, cryogenic distillation, is uneconomical for gas wells in remote areas or with small production rate. In this work, a detailed simulation and optimization is carried out to explore the feasibility of pressure-vacuum swing adsorption (PVSA) technology for on-site nitrogen rejection at remote/low-throughput gas wells. The designed PVSA unit possesses high mobility at a cost that scales well with low throughput. A nonisothermal and nonisobaric simulation model including detailed description of both micropore and barrier resistances in a kinetically selective carbon molecular sieve adsorbent has been used for process optimization. The process parameters constitute the decision variables of the optimization problem, with recovery and productivity as the objective functions, and industrial pipeline purity specification as an inequality constraint. Starting with a 6-step PVSA cycle, it is shown that a modified 9-step PVSA cycle promises to improve further the process performance. The process simulation model is the most comprehensive model for kinetically controlled PSA processes to date and the optimum results exceed the performances expected from the trends revealed in parametric studies published so far. Preliminary costing of the optimized process reveals that the modified PSA cycle has the potential to produce the desired product at a lower cost than the competing alternatives.