Abstract
The effective medium approach to radial fixed bed dispersion models, in which radial dispersion of mass is superimposed on axial plug flow, is based on a constant effective dispersion coefficient, DT. For packed beds of a small tube-to-particle diameter ratio (N), the experimentally-observed decrease in this parameter near the tube wall is accounted for by a lumped resistance located at the tube wall, the wall mass transfer coefficient km. This work presents validated computational fluid dynamics (CFD) simulations to obtain detailed radial velocity and concentration profiles for eight different computer-generated packed tubes of spheres in the range 5.04 ≤ N ≤ 9.3 and over a range of flow rates 87 ≤ Re ≤ 870 where Re is based on superficial velocity and the particle diameter dp. Initial runs with pure air gave axial velocity profiles vz(r) averaged over the length of the packing. Then, simulations with the tube wall coated with methane yielded radial concentration profiles. A model with only DT could not describe the radial concentration profiles. The two-parameter model with DT and km agreed better with the bed-center concentration profiles, but not with the sharp decreases in concentration close to the tube wall. A three-parameter model based on classical two-layer mixing length theory, with a wall-function for the decrease in transverse radial convective transport in the near-wall region, showed greatly improved ability to reproduce the near-wall concentration profiles.
Highlights
Fixed bed reactors are tubes filled with catalytic packing material, on which a chemical reaction may occur
Fixed bed reactors are preferred for their simplified technology and operation, and the design of such beds has been based on effective medium models coupled with correlation-based transport parameters
The use of larger particles within a tube of relatively small diameter results in a reduced pressure drop across the bed and, a more economically-favorable operation. This results in a higher void fraction near the wall of the bed, which induces significant flow and transport effects that are difficult to account for in conventional packed bed models. In this near wall area, boundary layers develop in which heat and mass transfer are dominated by molecular transport, and a high resistance to transport is present, causing strong changes near the wall
Summary
Fixed bed reactors are tubes filled with catalytic packing material, on which a chemical reaction may occur. The use of larger particles within a tube of relatively small diameter results in a reduced pressure drop across the bed and, a more economically-favorable operation. This results in a higher void fraction near the wall of the bed, which induces significant flow and transport effects that are difficult to account for in conventional packed bed models. In this near wall area, boundary layers develop in which heat and mass transfer are dominated by molecular transport, and a high resistance to transport is present, causing strong changes near the wall
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