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3.7 Three-Phase Fixed Beds: Trickle-Bed and Ped Bubble-Bed Reactors ack 169

The packed-bed reactor is particularly useful in two cases:

v
(a) if a relatiely small amount of gas is to be treated with a large amount of liquid, and
(b) if a large residence time is desired (Ramachandran and Chaudhari, 1984).

In packed-bed reactors, the catalyst is fully wetted, whereas the heat and mass transfer
efficiency is higher than that observed in trickle-bed reactors. However, low operation effi-
ciency may appear due to backmixing of the liquid phase. Moreo high liquid-phase v
er
,
residence times can result in the occurrence of homogeneous side reactions.
Some examples of bubble fed-bed reactors application are the hydrogenation of nitro ix
compounds, amination of alcohols, and ethylnylation of formaldehyde to b utynediol
(Ramachandran and Chaudhari, 1984).

3.7.2 Modeling of trickle-bed reactors

The employment of three-phase reactors is mostly desirable when there are some reactants
that are too volatile to liquefy, whereas some others are too nonvolatile to vaporize. Hence,
the situation where a gaseous component reacts with another reactant in the liquid-phase
is of great interest. The following reaction represents this case (Smith, 1981):
a A (gas) + B (liquid) → products

Characteristic examples of this form are the hydrogenation reactions, e.g. hydrodesul-
furization of petroleum fractions, hydrogenation of oils, and oxidation reactions, e.g. oxi-
dation of pollutants dissolved in liquids. In a few cases such as the Fischer–T ropsch
synthesis, the liquid is inert and acts as a heat-transfer medium.
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 cov-
ers the outer surface particles ( f = 1 or a LS = a ) so that the reaction can tak e place solely by u
w
the mass transfer of the reactant through the liquid–particle interface. Generally, the assump-
tion 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 liq-
uid 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).
In most applications, the axial dispersion in both phases can be considered to be ne gli-
gible (Smith, 1981). Moreo no radial gradients of concentration and velocity exist for
,
v
er
the gas or liquid.
h oac Conservation equations for each flowing phase—Smith’ s appr
For reactant A in the gas phase, at steady state, and for a volume element that e xtends
, across the reactor

d F AG ( Ka )  C  AG C   0 (3.360)
L
d V LG A  H A AL 