Trickle flow reactors

In the first class, the particles form a fixed bed, and the fluid phases may be in either cocurrent or countercurrent flow. Two different flow patterns are of interest, trickle flow and bubble flow. In trickle-flow reactors, the liquid flows as a film over the particle surface, and the gas forms a continuous phase. In bubble-flow reactors, the liquid holdup is higher, and the gas forms a discontinuous, bubbling phase. 

All these gas-liquid-particle operations are of industrial interest. For example, desulfurization of liquid petroleum fractions by catalytic hydrogenation is carried out, on the industrial scale, in trickle-flow reactors, in bubble-column slurry reactors, and in gas-liquid fluidized reactors. 

Fixed-bed reactors Trickle-flow reactor (TFR) This is a tubular flow reactor with a concurrent down-flow of gas and liquid over a fixed-bed of catalyst (Figure 3.10). Liquid trickles down whereas the gas phase is continuous. This reactor is mainly used in catalytic applications. Typical application examples of this reactor type are the following HDS of heavy oil fractions and catalytic hydrogenation of aqueous nitrate solutions. 

I. Iliuta, F. Larachi and B.P.A. Grandjean, Pressure drop and liquid hold-up in trickle flow reactors improved Ergun constants and slip correlations for the slit model, Ind. Engng. Chem. Res., 37 (1998) 4542-4550. 

F. Larachi, A. Laurent, G. Wild and N. Midoux, Pressure effects on gas-liquid interfacial areas in cocurrent trickle-flow reactors, Chem. Engng. Science, 47 (1992) 2325-2330. 

High pressure catalytic processes are developed and carried out in both preformed and powdered catalysts. Preformed catalyst are useful for fixed bed operation. Preformed catalyst pellets, are used as packing in multiphase trickling flow reactors. Trickling flow reactors have been described in detail in another part of this book (see Laurent). In this section we deal with slurry catalytic reactors, where the catalyst is used in powdered form. 

Another interesting example of reactive adsorption is the so-called gas-solid-solid trickle flow reactor, in which adsorbent trickles through the fixed bed of catalyst, removing selectively in situ one or more of the products from the reaction zone. In the case of methanol synthesis this led to conversions significantly exceeding the equilibrium conversions under the given conditions (67). 

Methanol synthesis 98,99 Gas-solid-solid trickle-flow reactor (GSSTFR)... 

Kuczynski M. The synthesis of methanol in a gas-solid-solid trickle-flow reactor. Ph.D. dissertation, University of Twente, Enschede, Netherlands, 1987. 

In ideal trickle flow reactors, all particles in the catalyst bed take part in the overall reactor performance, since each is surrounded (wetted) by the liquid phase that flows around it. Situations in which the liquid flows preferentially through a certain part of the bed, while the gas phase flows predominantly through another part, should be avoided [23]. In this case, part of the bed is not contacted by the liquid reactant at all and docs not contribute to the overall conversion. To avoid this maldistribution, Gierman [20] proposed the following criterion for the wetting number Wtx for co-current downflow operation ... 

Figure 8. Maximum allowable particle diameters as a function of the kinematic viscosity for complete wetting in trickle flow reactors.

Typical properties of slurry reactors, and of packed bed co-current downflow trickle flow reactors, are summarized in Table 1. Most properties indicated for slurry reactors also hold for three-phase fluidized beds. These properties can be advantageous or disadvantageous, depending on the application ... 

Property Slurry reactors Co-current downflow trickle flow reactors... 

The severity of the processing conditions depends on the feed for light petroleum fractions it will be milder than for heavy residues. Moreover, it is common practice to compensate for deactivation of the catalyst by increasing the temperature of the reactor. A simplified flow scheme involving a trickle flow reactor is shown in Fig. 2.5. 

Fig. 2.5. Simplified scheme of hydrotreatment involving a trickle flow reactor.

In packed bubble columns the gas-liquid interfacial area also can be related to the external catalyst surface area, as in trickle flow reactors. However, in packed bubble columns channeling can occur with strongly reduced gas-liquid interfacial areas [11]. 

Here Sh is the modified Sherwood number defined as Sh = /Csnsdp/fl,D and We is the modified Weber number defined as We = UoLpi.dr/hlci. A graphical illustration of the above correlation is shown in Fig. 6-20. The predictions of Eq. (6-67) also agree fairly well with the data of Lemay el al.so Specchia et al.9i showed that, in a trickle-flow reactor, KLaL and Ksas are essentially of the same order of the magnitudes. They also evaluated the conditions under which the mass-transfer (gas-liquid and liquid-solid) influences significantly the performance of a trickle-bed reactor. 

The above unsuccesful attempt at downscaling of a laboratory trickle-flow reactor which was made some 30 years ago, suggests that representative testing of practical catalysts for trickle-flow processes can only be done in relatively large pilot plant reactors. However, as will be clear from the discussion in earlier parts of this paper, the technique of catalyst bed dilution with fine inert particles has opened the way to small-scale testing of catalysts for these processes as well. 

Toye, D., Marchot, P., Crine, M and L Honune, G., Analaysis of liquid flow distribution in trickling flow reactor using computer assisted x-ray tomography. Trans. I. Chem. E. 73(Part A), 258 (1995). 

In most applications of trickle-flow reactors, the conversions generate heat that causes a temperature rise of the reactants, since the industrial reactors are generally operated adiabatically. In the cocurrent mode of operation, both the gas and the liquid rise in temperature as they accumulate heat, so there is a significant temperature profile in the axial direction, with the highest temperature at the exit end. When the total adiabatic temperature rise exceeds the allowable temperature span for the reaction, the total catalyst volume is generally split up between several adiabatic beds, with interbed cooling of the reactants. In the countercurrent mode of operation, heat is transported by gas and liquid in both directions, rather than in one direction only, and this may increase the possibility of obtaining a more desirable temperature profile over the reactor. 

Figure 13 also shows the range of liquid and gas velocities corresponding with the operation of industrial trickle-flow reactors. It follows that an internally finned tube of the present geometry with an outlet angle of 70 allows counterflow operation in the... 

H. Gierman, Design of Laboratory Hydrotreating Reactors. Scaling Down of Trickle-flow Reactors, Appl. Catal. 45 277-286 (1988). 

No maldistribution of gas or liquid in three-phase processes. Regarding application of the BSR concept to gas/liquid/solid processes, an important advantage of the BSR is that adjacent strings do not (necessarily) touch. Because of the liquid surface tension, liquid will not spill over from one BSR string to another. Consequently, the initial liquid distribution is maintained throughout the BSR module. This feature is especially advantageous when incomplete catalyst wetting (which results from liquid maldistribution in traditional, randomly packed trickle-flow reactors) would lead to hot spots and decreased selectivity. 

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