(683a) Carbon Formation and Catalyst Deactivation 3D Simulations of Hydrogen-Producing Reactions in a Fixed-Bed Reactor

To produce hydrogen as a clean fuel and a new source of energy, catalytic reactions are widely used in industry [1]. The deactivation of catalysts is an important problem in industry, such as steam reforming and the catalytic dehydrogenation of alkanes, which are both strongly endothermic reactions. Different publications reported on the deactivation of steam reforming of methane (SRM) as a significant source of hydrogen production [2] and propane dehydrogenation (PDH) as a commercial and interesting reaction [3] to understand the effect of deactivation on the reactor situations and performance generally. The local carbon deposition on catalysts can cause particle breakage and strongly decrease reaction rates. Catalyst deactivation in heated tubes removes the heat sink and can result in local hot spots that weaken the reactor tube. This is particularly a problem for a low tube-to-particle diameter ratio (N) fixed bed reactor, where a large fraction of the catalyst particles are located next to the heated tube wall. Computational fluid dynamics (CFD) has been reviewed as a suitable tool to simulate packed bed tubes [4]. Previous works [5] have introduced the coupling of gas flow and resolved species and temperature gradients inside pellets by CFD for SRM and PDH without considering deactivation. In this work, the focus is on using CFD to obtain better comprehension of the role of catalyst deactivation on reactions, mass and heat transfer of the gas-phase fluid flow around and inside of cylindrical catalyst particles. SRM and PDH reactions were simulated by CFD to study the interaction between local flow and reaction/deactivation. CFD simulations of flow, heat transfer, diffusion and reaction were carried out using the commercial CFD code Fluent 6.3 in a 3D 120-degree periodic wall segment of an N=4 tube. The mesh used boundary layer prism cells at both the inside and outside particle surfaces and at the tube wall, with tetrahedral cells in the main fluid volume. These reactions were represented in the solid particles using user-defined scalars to mimic species transport and reaction, with user-defined functions supplying reaction rates. Diffusion in the particles was modeled by Fick's law using an effective diffusivity, given by Hite and Jackson's approximation of the Dusty Gas Model [6]. Catalyst activity is related to the coke accumulation with a suitable initial coking expression and causes decrease of reaction rates with deposition of carbon. Carbon deposition can be calculated by first running a CFD simulation at base case (undeactivated) conditions, then accumulating carbon production rates over a 60-second interval to obtain the local carbon build-up. This is then inserted into the activity factor of the catalyst, which is used to modify the fresh catalyst reaction rates to give the reaction rates after 60 s. The CFD simulation is then run for a further 60-second time period to obtain increased values of carbon accumulation and the process repeated. The CFD simulations show local details of carbon laydown both on the surface of, and inside near-wall catalyst particles. The transient development of particle internal gradients and carbon accumulation were studied for the early stages of deactivation. Carbon concentration is initially strongest close to the surface and in the high temperature regions of the catalysts and affected by the wall heat flux. Deactivation of the endothermic reactions causes a slow increase in the average catalyst temperature. The results present a decrease of the product mole percentages at the outlet with time.

References:

[1] Chen, Z.; Yan, Y.; S.E.H. Elnashaie, S. Catalyst deactivation and engineering control for steam reforming of higher hydrocarbons in a novel membrane reformer. Chemical Engineering Science 59 (2004) 1965 ? 1978.

[2] Snoeck, J.; Froment, G.; Fowles, M. Kinetic Evaluation of Carbon Formation in Steam/CO2-Natural Gas Reformers. Influence of the Catalyst Activity and Alkalinity, International Journal of Chemical Reactor Engineering, Vol. 1 [2003], A7.

[3] Jackson, S. D.; Stitt, E. H. Propane dehydrogenation over chromia catalysts: micro-catalysis, deactivation, macro-kinetics, and reactor modelling. Current Topics in Catalysis 2002, 3, 245.

[4] Dixon, A. G.; Nijemeisland, M.; Stitt, E. H. Packed tubular reactor modeling and catalyst design using computational fluid dynamics. Advances in Chemical Engineering 2006, 31, 307.

[5] Taskin, M.E.; Troupel, A.; Dixon, A. G.; Nijemeisland, M; Stitt, E. H. Transport and Reaction Interactions for cylindrical particles with strongly endothermic reactions. Ind. Eng. Chem. Res., submitted.

[6] Hite, R.; Jackson, R. Pressure gradients in porous catalyst pellets in the intermediate diffusion regime. Chemical Engineering Science 1977, 32, 703.