CFD simulation of transport and reaction in cylindrical catalyst particles

Taskin, Ertan M.

Etd

CFD simulation of transport and reaction in cylindrical catalyst particles

Public

Multitubular packed bed reactors with low tube-to-particle diameter ratios (N) are especially selected for strongly endothermic reactions such as steam reforming and propane dehydrogenation. For low N tubes, the presence of the wall causes changes in bed structure, flow patterns, transport rates and the amount of catalyst per unit volume. In particular, the particles close to the wall will behave differently to those inside the bed. The problem is that, due to the simplifying assumptions, such as uniform catalyst pellet surroundings, that are usual for the current pseudo-continuum reactor models, the effects of catalyst pellet design changes in the near-wall environment are lost. The challenge is to develop a better understanding of the interactions between flow patterns, species pellet diffusion, and the changes in catalyst activity due to the temperature fields in the near wall region for the modeling and design of these systems. To contribute to this improved understanding, Computational Fluid Dynamics (CFD) was used to obtain detailed flow, temperature, and species fields for near-wall catalyst particles under steam reformer and propane dehydrogenation reactor inlet conditions. As a first step, a reduced size model was generated by only considering a 120 degree segment of an N = 4 tube, and validated with a larger size complete bed model. In terms of the flow and temperature contours and profiles, the complete tubes can be represented well by the reduced size models, especially focusing on the center particles positioned in the middle of the near wall region. The methane steam reforming heat effects were implemented by a user-defined code with the temperature-dependent sinks in the catalyst particles, near to the pellet surfaces for different activity levels. For the sinks terms, bulk phase species concentrations were used in the reaction rates, and with the reaction heat effects inclusion, significant pellet sensitivity was observed with different activity levels. Furthermore, non-symmetric temperature fields in and around the near wall particles were noticed as contrary to the conventional approach. In order to focus on the 3D intra-pellet distributions of temperature and species, diffusion and reaction were coupled to the external flow and temperature fields by user-defined code. Strong deviations from uniformity and symmetry on the temperature and species distributions existed as a result of the strong wall heat-flux into the particles Additionally, the pseudo-continuum type of packed bed model was created, which considers the simplified environment for the reacting particles. The results obtained by the diffusion reaction application in the 3D discrete packing model could not be re-produced by the conventional simplified pseudo-continuum approach, no matter which parameter values were chosen for the latter. The significance of these observations is that, under the conventional assumption of symmetric particle surroundings, the tube wall temperature and reaction rates for catalyst particles can be incorrectly evaluated and important design considerations may not be well predicted, thus, negative consequences on the plant safety and efficiency may be observed.

Creator