Upgrading low-quality natural gas with H2S- and CO2-selective polymer membranes: Part I. Process design and economics of membrane stages without recycle streams

The upgrading of low-quality natural gas by membrane separation processes utilizing two different types of polymer membranes was investigated by process design, optimization, and economic assessment studies. The membranes considered for these processes exhibit high CO2/CH4 and H2S/CH4 selectivities, respectively. Low-quality natural gas was simulated by ternary CH4/CO2/H2S mixtures containing up to 40 mol% CO2 and 10 mol% H2S. The membrane processes were designed to reduce the concentrations of CO2 and H2S in the natural gas to US pipeline specifications (≤2 mol% CO2 and ≤4 ppm H2S). This study has identified the membrane process configurations (single stage or two stages connected in series) and types of membranes (CO2-selective, H2S-selective, or both) that would yield the most economical processes for the upgrading of natural gas containing different ranges of CO2 and H2S concentrations. The upgrading costs of process configurations without recycle streams are dominated by the cost of CH4 lost in the permeate stream. The effects of recycle streams, variations in feed flow rate and pressure, membrane module cost, and wellhead cost of natural gas on the upgrading costs will be reported elsewhere.