Hybrid processes for the removal of acid gases from natural gas

A process design study and an economic assessment were made of a hybrid process for the removal of up to 40 mole% CO2 and up to 1 mole% H2S from crude natural gas. The hybrid process combines membrane separation and gas absorption in diethanolamine. Membrane separation is to be used first for the bulk removal of acid gases, in particular of CO2, from the crude natural gas feed, while final purification to U.S. pipeline specifications (≤2 mole% CO2, ≤4 ppm H2S) is to be performed by gas absorption. The effects of the following factors on the cost of acid gas removal were examined: feed flow rate, feed pressure, acid gas concentration (CO2 and H2S) in feed, cost of lost methane, membrane properties (CO2 permeability, CO2CH4 selectivity, effective thickness), and membrane replacement cost. For feed streams containing only CO2, the cost of an independent membrane separation process is lower than that of either a hybrid process or of an independent gas absorption process under the conditions assumed in this study, except at pressures below about 500 psia (3,447 kPa). For feed streams containing also up to 1 mole% H2S, the cost of the hybrid process is lower than that of an independent membrane separation process in certain ranges of feed flow rates, pressures, and acid gas concentrations. An independent gas absorption process is not competitive with either the hybrid process or an independent membrane process under most of the conditions of this study.