Alternating flow model for mass and heat dispersion in packed beds

Abstract

The alternating flow model (AFM) views dispersion in packed beds as a sequence of streamline plugs that must repeatedly split and merge as the bulk fluid traverses the vessel. Thus, the flow in the AFM is ordered, as opposed to the random flow implied by the Fickian analogy. For mass dispersion only, model parameters arise from a priori considerations of packing geometry. Steady state and transient data (5.6 < DJd, < 54.4, 100 < Re, < 1,000, gases and liquids) show the AFM to surpass the Fickian analogy (based on correlations for dispersion coefficients) in most cases. Further, it can describe well the radial velocity profile trends in packed beds. For heat dispersion, two additional parameters (heat transfer coefficients) arise that are not functions of packing geometry. Simple correlations for these parameters and the justifications are given. Most of the comparisons made with the literature experimental results show the AFM to be at least as good as the back-fit Fickian analogy. The AFM should be most useful for packed beds with a relatively small DJd,.

A New Approach to Fixed Bed Radial Heat Transfer Modeling Using Velocity Fields from Computational Fluid Dynamics Simulations

“…21,22 In parallel with the problems associated with heat transfer modeling in packed tubes, several authors have expressed dissatisfaction with the standard dispersion model (SDM) which uses effective diffusion to represent axial and radial dispersion 23 and has drawbacks including infinite speed of propagation and overestimation of back-mixing. Some different approaches to this problem have included the cross-flow model, 24 the alternating flow model, 25 and the wave model, first put forward by Stewart 26 and more recently strongly championed by Kronberg and his colleagues. 27,28 The application of the wave model to fixed bed heat transfer was demonstrated by Kronberg and Westerterp 27 whose work showed that this model also results in parameters that must be determined from experimental data.…”

CFD as a Design Tool for Fixed-Bed Reactors

“…Several recent attempts have been made to radically change the approach to fixed-bed reactor modeling. A revival of the cell model approach has been tried, , which is likely to meet the same problems as the original, as it must rely on idealized pictures of mixing in the interstices of the packing, and as it is extended to accommodate both heat and mass transfer only with difficulty. The approach to the dispersion of mass and heat through Fick's and Fourier's laws has been challenged, and a wave model first developed over 30 years ago is now being extended and revised …”

Influence of tube and particle diameter on heat transport in packed beds

“…But, since the large-scale introduction of the computer, continuum modeling became more popular. Although several investigators recently have switched to discrete modeling (for example, Klingman and Lee, 1987;Schnitzlein and Hofmann, 1987). We will focus on deterministic continuum models.…”