43706-AC9
Connecting Catalytic Chemistry to External Particle Conditions via Computational Fluid Dynamics
The objective of this project is the more realistic treatment of catalyst particle internal transport and reaction processes by linking them to conditions in the fluid flowing past and surrounding the particle. The work is divided into two main tasks: 1) development and testing of computational fluid dynamics (CFD) approaches to coupling intraparticle chemistry and species transport to the external flow field; 2) evaluation of the behavior of catalyst particles near the wall of a steam reformer reactor tube, and the development of improved particle models.
Our original proposal for porous particles in the CFD simulations was to treat them as solid regions and write user-defined code to include species and reactions in the solid. This was very time-consuming, and we devised a different approach, defining the particles as porous regions for which species mass fractions were available. In the interim, however, Fluent released version 6.3, which provided user-defined scalars (uds) in solid regions. We were then able to take advantage of these new developments in the CFD code to re-visit our original approach of modeling particles with solid regions. Our motivation for doing this was to resolve a small problem that arose when we looked in detail at the fluxes into the porous particles. We found that with the porous region approach, there was a residual convective flux into the particle, which was an artifact of the discretisation method. Although small, this flux carried enough enthalpy to increase the particle temperature significantly, over what was expected compared to benchmark calculations.
We developed a uds solid particle approach, which uses species mass balances and energy inside the solid particles, and the standard CFD equations for uds in the gas phase. The gas-phase uds are then coupled to the mass fractions in the gas, which ensures that the change in moles in the gas phase due to reaction is properly captured. We have carried out a series of validation studies, in which the new method was used on simplified situations: a) gas phase reactions only, where we compared to the built-in reaction models of Fluent; b) solid sphere isothermal 1st-order reaction for which an analytical solution was available; c) constant known heat and mass sources inside the particle, to check that the correct particle-gas transfer took place. We have repeated our earlier work on simulation of wall segments of spheres and full cylinders, using the new validated method. Comparison to the porous region approach results that were reported previously, showed that all qualitative results and trends were unaffected, only the temperature profiles decreased in magnitude. We extended the analysis of these simulations to look more closely at regions of reactant depletion between particles.
Previously, we developed meshes, and conducted flow simulations, around single particles inclined at different angles to the oncoming flow, for complex particle shapes with multiple holes. In the present reporting period, we extended these simulations to include heat transfer, diffusion of species and reaction in the particle, using the user-defined scalar solid particle approach described above. This work has allowed us to evaluate the surface accessibility to flow, and thus reactants, of internal voids, and how this depends on angle of attack. We found that both methane (reactant) and heat uptakes were at a maximum at an angle of approximately 30° to the flow. Plots of heat and species fluxes for the individual surfaces of the particles showed complex interactions with flow. As the upstream flat and inner hole surfaces turned from being directly exposed to the flow, their effectiveness decreased. The outer curved surface changed from boundary layer flow over the whole area, to flow directly impacting on the upstream part of the surface, with a strong recirculation flow on the downstream part. The overall effectiveness of the surface increased, and the asymmetry of temperature and species in the particle also increased. It will be important to see to what extent these single particle results carry over to a bed of particles, as such strong variations within a particle would have implications for uneven deactivation through carbon deposition.
The funding provided by this ACS/PRF
grant partially supported Dr. Ertan Taskin in a postdoctoral stay following his Ph.D. defense
last year. Mr. Alexandre Troupel
arrived last September, was employed as a TA through the academic year by the
department, joined the research project in November, and was released by the
department to become an RA in May. The PI received a SUMR supplement, which was
used to support Ms. Winnie Nabea over the summer. Her
work has provided us with a heat transfer experimental facility for validation,
and she evaluated AUTOCAD, an alternative method of generating solid packing
models for meshing. The ACS/PRF funding continues to allow the PI to develop
the more chemistry-based aspects of the CFD research effort, which promises
well for future applications.
