MATHEMATICAL-MODELING OF DIFFUSION-REACTION, AND SOLUTION ALGORITHM FOR COMPLEX-REACTION NETWORKS IN POROUS CATALYST PELLETS - STEAM REFORMING OF NATURAL-GAS

Three models of different degrees of rigor are developed for diffusion and reaction in porous catalyst pellets for the industrially important multicomponents' system with a multiple reversible reaction for the steam reforming of natural gas. The more rigorous model is based on the Stefan-Maxwell equation for multicomponents' diffusion, while the other less rigorous models are based on the simplified Fick's law. The resulting nonlinear two-point boundary value differential equations are solved using an efficient algorithm based on the global orthogonal collocation technique. A simple transformation is used, which shifts the collocation points towards the region close to the surface, where the profiles are steepest, without the need to use spline collocation. The algorithm is augmented with a technique to obtain suitable initial guesses that guarantees the convergence of the solution of the large system of nonlinear algebraic equations resulting from the application of the orthogonal collocation technique. The algorithm works very efficiently under industrial conditions characterized by high temperature, strong diffusional limitations and, therefore, very steep concentration profiles.