Multi-component adsorptive separation: use of lumping in PSA process simulation

Being a discrete-continuous process, approach to a cyclic steady state in computer simulation of Pressure Swing Adsorption is through iterative procedures and simulation itself is quite computation-intensive. Considering the fact that simulation based design itself is an iterative process, it is imperative that simulation be computationally very efficient and phenomenologically as close to the physics of adsorption-desorption as possible. Utility of lumping the components of a gas mixture into fewer pseudo-components was computationally examined in the simulation of a representative multi-step cycle of a pressure swing based adsorptive separation process applied to natural gas treatment. The actual feed had six components competing for adsorbent sites. Five different lumping alternatives were studied and compared with the simulation results for a full six-component simulation under identical equipment dimensions and operating conditions. Lumping could reduce the number of equations to be solved by more than half and the corresponding reduction in CPU time was about 90%. The six component mixture of Natural Gas was found to be sufficiently represented by two pseudo-components. The predicted recovery (in terms of Methane and Ethane) and quality (in terms of content of higher hydrocarbons) of the raffinate differed by not more than 0.8% and 0.02% respectively. The paper discusses possible heuristics for decision-making regarding appropriate lumping as verified by extensive simulation studies.