Trickle bed regime

Pressure Drop Some models regard trickle bed flow as analogous to gas/liquia flow in pipe lines. Various flow regimes may exist like those typified in Fig. 23-25/ but in a vertical direction. The two-phase APcl is related to the pressure drops of the individual phases on the assumptions that they are flowing alone. The relation proposed by Larkin et al. (AJChE Journal, 7, 231 [1961]) is APaj 5.0784... 

Fig. 5.2.1 Flow regimes in a trickle-bed reactor (after Sie and Krishna [2]). Typical conditions for research and industrial reactor operation are indicated. The black line indicates the boundary between the pulsed flow regime and the spray, trickle and bubble flow regimes.

Here, issues in relation to the trickle flow regime—isothermal operation and plug flow for the gas phase—will be dealt with. Also, it is assumed that the flowing liquid completely covers the outer surface particles (/w = 1 or aLS = au) so that the reaction can take place solely by the mass transfer of the reactant through the liquid-particle interface. Generally, the assumption of isothermal conditions and complete liquid coverage in trickle-bed processes is fully justified with the exception of very low liquid rates. Capillary forces normally draw the liquid into the pores of the particles. Therefore, the use of liquid-phase diffusivities is adequate in the evaluation of intraparticle mass transfer effects (effectiveness factors) (Smith, 1981). 

The second section presents a review of studies concerning counter-currently and co-currently down-flow conditions in fixed bed gas-liquid-solid reactors operating at elevated pressures. The various consequences induced by the presence of elevated pressures are detailed for Trickle Bed Reactors (TBR). Hydrodynamic parameters including flow regimes, two-phase pressure drop and liquid hold-up are examined. The scarce mass transfer data such gas-liquid interfacial area, liquid-side and gas-side mass transfer coefficients are reported. 

Here k is the permeability of the dry medium and J(S[) characterizes hysteresis behaviour in the trickling regime. It should be noted at this level that Eqn. (5.2-12) was derived by these authors from available data on two-phase imbibition and drainage curves, implicitly identified to the trickling flow regime in trickle-beds. The J function may be multi-valued and depends on the history of the flow, however Grosser et al. [22] as well as Dankworth et al. [23] assume it to be single-valued. 

Such relationships have been inspired from the work of Saez and Carbonell [26] to correlate macroscopically the pressure-drop data corresponding to the low-interaction regime in trickle-bed reactors. 

K.M. Ng, A model for flow regime transitions in cocurrent downflow trickle-bed reactors, AIChE Journal, 32 (1986) 115-122. 

W.J.A. Wammes, J. Middekamp, W.J. Huisman, C.M. Debaas and K.R. Westerterp, Hydrodynamics in a cocurrent gas-liquid trickle bed at elevated pressures, Part 2 liquid hold-up, pressure drop, flow regimes, AIChE Journal, 37, 12 (1991) 1855-1862. 

A. Attou and G. Ferschneider, A two-fluid model for flow regime transition in gas-liquid trickle-bed reactors, Chem. Engng. Science, 54 (1999) 5031-5037. 

Fig. 4.17. Flow regimes in three-phase fixed-bed reactors, (a) Gas and liquid in co-current downwards flow (trickle-bed operation). (b) Gas and liquid in co-current upwards flow (liquid floods bed), (c) Gas and liquid in countercurrent flow (not often used for catalytic reactors)...

The literature contains a number of references to other flow regime maps however, there is no clear advantage of using one map versus another. Wall effects can also have a major effect on the hydrodynamics of trickle bed reactors. Most of the data reported in the literature are for small laboratory units of 2-in diameter and under. 

FIG. 19-41 Trickle bed flow regime map. [From Gianetto et ai, AIChE J. 24(6) 1087-1104 (1978) reproduced with permission. ]... 

The existing data for dynamic saturation (dynamic holdup divided by bed porosity) in the trickle-flow regime (18, 20, 21, 24, 25) can be correlated by the following equation ... 

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