Modelling of enrichment of natural gas wells by membranes

Abstract

Models are developed to simulate on-site membrane enrichment of natural gas wells. The models consider transient behaviour of the enrichment process, the effects of permeability functions, flow patterns and separation of multicomponent systems. The gas mixture considered in the analysis includes CH4, CO2, N2 and H2S. Comparison of models shows that for either flow pattern, the presence of low permeating species reduces the final CH4 mole fraction in the enriched mixture. In addition, higher CH4 recoveries are predicted by the four-component model than by the binary model. This is a result of the lower driving force for permeation in the four-component model, caused by lower mole fractions. Similar behaviour is also predicted for the removal of CO2 and its final mole fraction in the gas mixture. The presence of other fast permeating species (H2S) resulted in lower permeation rates of CO2. As a result, higher CO2 mole fractions in the gas mixture and lower CO2 removal are predicted by the four-component model. The use of variable permeabilities versus constant permeability ratio showed similar predictions for some of the system variables with partial matching between model predictions at either low or high pressure.