Upgrading low-quality natural gas with H 2S- and CO 2-selective polymer membranes : Part I. Process design and economics of membrane stages without recycle streams

Abstract

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 CO 2/CH 4 and H 2S/CH 4 selectivities, respectively. Low-quality natural gas was simulated by ternary CH 4/CO 2/H 2S mixtures containing up to 40 mol% CO 2 and 10 mol% H 2S. The membrane processes were designed to reduce the concentrations of CO 2 and H 2S in the natural gas to US pipeline specifications (≤2 mol% CO 2 and ≤4 ppm H 2S). This study has identified the membrane process configurations (single stage or two stages connected in series) and types of membranes (CO 2-selective, H 2S-selective, or both) that would yield the most economical processes for the upgrading of natural gas containing different ranges of CO 2 and H 2S concentrations. The upgrading costs of process configurations without recycle streams are dominated by the cost of CH 4 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.