Removal of sulfur compounds from natural gas for fuel cell applications using a sequential bed system

Abstract

The development of an adsorbent system and process conditions for the removal of sulfur-compounds from natural gas at ambient conditions of temperature and pressure to meet the purity specifications required for use in the fuel processor of a PEM fuel cell is described. The sulfur compounds to be removed from the natural gas were H2S, dimethyl sulfide (DMS), carbonyl sulfide (COS), ethyl mercaptan (EM) and tertiary butyl mercaptan (TBM). The water content of natural gas was 120ppmw. Metal-exchanged zeolites (Ca–X and Na–X) and mixed metal oxides (of Cu–Mn and Fe–Mn) supported on alumina were used as adsorbents. Even though some of the adsorbents were active for the removal of particular sulfur compounds (Ca–X, for example, for the removal of DMS), none of them could remove all the sulfur impurities simultaneously. Multiple adsorbents are necessary. The choice of the combination of adsorbents depends on the type and relative amounts of the various sulfur compounds. The latter vary with the origin and source of the natural gas feedstock. The sequential location and quantity of the individual adsorbents in the total adsorbent bed is also a critical parameter. A sequential bed system comprising of Ca–X followed by Fe–Mn oxides had significant desulfurization capacity at 38°C, 15psig, and a natural gas space velocity of 6000h−1. This combination of adsorbents was also tolerant to the presence of water, aromatics, CO2, higher hydrocarbons and other such impurities sometimes present in the pipeline natural gas.