Fixed Bed Fluid Flow and Heat Transfer by CFD - 1

urrent fixed bed reactor models have been based on fairly radical simplifying assumptions driven by the complex structure of random packed tubes. However, even the most advanced models today cannot quantitatively predict reactor behavior if independently-determined kinetics and transport parameters are used. The effects of tube and catalyst pellet design changes are lost by the use of effective transport parameters and simplified models. There is a consensus among reaction engineers that the entire field has neglected the role of fluid flow in reactor modeling. Specifically for fixed beds, what is needed is a better understanding of fluid flow through arrays of realistic catalyst particle shapes, with special attention to the problematical wall region. The presence of the tube wall causes changes in bed structure, flow patterns, transport rates and the amount of catalyst per unit volume, and is usually the location of the limiting heat transfer resistance.

The main limitation in our understanding is the lack of resolution of the detailed flow picture in these beds. In reality, thermal energy is transported by strong radial convective flows as fluid is displaced around the packing elements, as well as conduction through the particles. Regions of stagnant flow, and possibly even reverse flow, have been identified by magnetic resonance imaging (MRI) experiments, which are limited to liquid flows at very low flow rates. These flow features may be strongly connected to the poor heat transfer performance near the wall. To understand them, and to quantify them for gas flows at the large flow rates of industrial practice, realistic three-dimensional simulation of the fluid flow in a fixed bed is necessary, such as that shown in the figure for pathlines in a tube of spheres with N = 4.

We have suggested that CFD is a powerful tool in building new approaches to fixed bed modeling [Dixon, A.G. and Nijemeisland, M., 2001]. In particular, it allows us to investigate aspects of heat transfer and the temperature field in a packed tube, that would be difficult or impossible to measure in an experiment. By solving for the energy balance on mesh volumes inside the catalyst pellets, we can link conduction through the solid particles to convective heat transfer around them. CFD can show us details of the temperature fields in both the fluid and solid phases in a fixed bed, as illustrated in this picture of the heating of gas flow as it moves through a tube with a constant wall temperature.