(22a) Redefining the Upper Bound Curve for Process Intensification in Natural Gas Processing

The use of membrane for acid gas (such as CO₂) separation has attracted significant attention since its ranking as the twelfth out of the 38 research topics constituting the research needs in the membrane separation industry. Though, membrane application has witnessed about tenfold growth afterwards, however, solvent-based absorption still has about 90% of the natural gas (NG) processing market. This is because today’s membranes are not good enough to displace the use of absorption. High pressure operations have been suggested as one of the means to improve the competitiveness of membrane for separation of acid gases. Unfortunately, the penetrant-induced plasticization phenomenon has limited the use of membrane for high-pressure CO₂ separation. A number of studies on plasticization resistant membrane material and the optimization of membrane-based gas separation processes have been published. Most of these studies have employed models with constant CO₂permeability coefficients while evaluating high-pressure gas separation.

The present paper proposes the use of a process intensification approach that takes into account the variation of gas permeability with upstream pressure. Therefore, an approximate model based on plasticization pressure and permeability parameters at plasticization was proposed for defining the Robeson upper bound curve of membranes used for high pressure gas separation. The model which was derived by applying the partial immobilization assumption to the fundamental model of solution - diffusion was used for selecting the best membrane material which was used in this work. A comparative study of the traditional solvent-based CO₂ separation and membrane-based gas separation techniques was carried out on a gas separation plant using the process intensification approach. The study was performed using process intensification metrics including mass intensity; waste intensity; productivity/size ratio; productivity/weight ratio; flexibility and modularity and a multi-objective optimization method. The results obtained indicated that huge saving is possible by installing a plasticization resistant membrane in an area which involves high pressure CO₂ separation. Furthermore, this idea is justified since natural gas is often obtained directly from gas wells at pressure ranging from 20 to 83 bar and compositions between 4 and 50% CO₂.