Modelling of enrichment of natural gas wells by membranes

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.