Analogous description of the hydrodynamics of gas-solid fluidized beds and bubble columns

Abstract

In this paper we stress analogies in the hydrodynamic behaviour of gas solid fluidized beds and bubble columns. Using published experimental data, it is demonstrated that the analogous hydrodynamic-behaviour is not only qualitative but also quantitative in nature. Specifically, we show the following. (1) The gas holdup in the homogeneous regimes of bubble columns and fluidized beds can be modelled in a unified way using V_slip = υ~(1 - ε_d)ⁿ⁻¹, where V_slip refers to the slip velocity between the dispersed (bubbles or particles) and continuous phases and ε_d the dispersed phase holdup. The Richardson-Zaki exponent n decreases with increasing gas density. (2) The transition from homogeneous to heterogeneous flow regimes in gas liquid bubble columns and gas solid fluid beds is delayed by increasing system pressure. Extrapolation of the influence of increased gas density allows us to consider liquid liquid dispersions and liquid solid fluid beds as limiting cases. (3) In the heterogeneous flow regime of operation the classic two-phase theory of fluidized beds can be applied with profit to also describe the hydrodynamics of gas liquid bubble columns provided that the "dilute" phase is identified with the fast-rising large bubbles and the "dense" phase is identified with the liquid phase containing entrained "small" bubbles. Tentative analogies can also be drawn for the interphase mass transfer processes. (4) The "dense" phase backmixing can be modelled in a unified manner. (5) The two-phase theory can be extended to describe slurry reactors. It is argued that, because of cross-fertilization of concepts and information, appreciation of analogies can be invaluable tool in scaling up.