This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 172085, “Purification of Natural Gases by Use of Carbon-Molecular-Sieve Membranes,” by Subrata Mondal and Kean Wang, The Petroleum Institute, Abu Dhabi, prepared for the 2014 SPE Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 10–13 November. The paper has not been peer reviewed.
Carbon-molecular-sieve membranes (CMSMs) are a promising candidate for natural-gas purification because of their excellent stability, permeation selectivity, and permeability. In this project, two CMSM samples were synthesized at different pyrolytic conditions and examined for separation of N2/CH4 gas pairs. Analysis revealed that both surface diffusion and molecular sieving play important roles in gas-permeation mechanisms, which results in abnormal behaviors in the selectivities of different gas molecules.
Introduction
Because of its abundance in natural reservoirs and relatively clean-burning nature, natural gas (NG) (CH4>70%) is becoming increasingly important to the global energy supply. Raw NG contains such impurities as carbon dioxide (CO2), hydrogen sulfide (H2S), H2O, and N2, which can result in health hazards, corrosion of equipment, and lower heating value. The concentration of these impurities varies considerably in NGs produced at different locations but should be reduced to certain levels before shipment.
Currently, large-scale NG- processing plants employ such technologies as adsorption (e.g., dehydration), absorption (e.g., amine wash for the removal of H2S and CO2), and cryogenic distillation (e.g., N2 removal). Although these technologies are mature and effective, they are expensive, energy-intensive, and environmentally unfriendly. For example, an N2- removal unit (NRU) generally operates at temperatures lower than –150°C, while the amine-wash unit requires constant maintenance and solvent replenishing.
Membrane technology offers great advantages for NG processing. It is compact, easy to install or scale up, and requires minimal space, energy input, labor, and maintenance resources. These attributes make it particularly attractive for offshore and remote gas fields. Currently, membrane modules for CO2 removal are commercially available and are the dominant technology in offshore applications even as they become more important in onshore applications. However, improvements are needed to compete with the current technologies in large-scale, inland NG-processing plants.
