NUMERICAL-SIMULATION OF DIFFUSION-LIMITED PSA PROCESS MODELS BY FINITE-DIFFERENCE METHODS

A numerical method based on finite differences is proposed for simulat ing pressure swing adsorption processes with intraparticle diffusion c ontrolling. The problem has two important spatial coordinates: the axi al position in the bed and the location within a particle. The method proposed keeps the two coordinate scales separate. Diffusion equations with variable main- and cross-term diffusivities are solved implicitl y for the particles at each time step and the solution is expressed in terms of the bed-scale variables. The material balances on the bed ca n then be solved with the bed-scale variables as the only unknowns. Th e method is computationally efficient and validated by comparison with an exact solution derived under conditions of plug how, constant tota l pressure, and with a binary Langmuir isotherm. As a full example, we treat the kinetic separation of air to produce nitrogen using a fixed bed of carbon molecular sieve. Characteristics of the specific implem entation are incorporation of variable-step grids with identical volum es for the intraparticle diffusion equations and the use of the third- order QUICK scheme to discretize the convection terms in the material balances for the bed. Copyright (C) 1996 Elsevier Science Ltd