Packed trickle flow reactor

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. 

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. 

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 ... 

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]. 

Fig. 5 Fixed-bed reactors with gas-liquid flow. (A) Trickle-bed reactor with cocurrent downflow (B) trickle-bed reactor with counter-current flow and (C) packed bubble-flow reactor with cocurrent upflow.

Gas-liquid mixtures are sometimes reacted in packed beds. The gas and the liquid usually flow cocurrently. Such trickle-bed reactors have the advantage that residence times of the liquid are shorter than in countercurrent operation. This can be useful in avoiding unwanted side reactions. 

Some contrasting characteristics of the main lands of three-phase reactors are summarized in Table 23-15. In trickle bed reactors both phases usually flow down, the liquid as a film over the packing. In flooded reactors, the gas and hquid flow upward through a fixed oed. Slurry reactors keep the solids in suspension mechanically the overflow may be a clear liquid or a slurry, and the gas disengages from the... 

Cas/Liquid Micro Flow Packed-bed or Trickle-bed Reactors... 

Laboratory reactor for studying three-phase processes can be divided in reactors with mobile and immobile catalyst particles. Bubble (suspension) column reactors, mechanically stirred tank reactors, ebullated-bed reactors and gas-lift reactors belong the class of reactors with mobile catalyst particles. Fixed-bed reactors with cocurrent (trickle-bed reactor and bubble columns, see Figs. 5.4-7 and 5.4-8 in Section 5.4.1) or countercurrent (packed column, see Fig. 5.4-8) flow of phases are reactors with immobile catalyst particles. A mobile catalyst is usually of the form of finely powdered particles, while coarser catalysts are studied when placing them in a fixed place (possibly moving as in mechanically agitated basket-type reactors). 

In continuous flow experiments, catalyst was packed into a downflow trickle-bed reactor of 30 cc bed volume. Hydrogen was passed slowly over the catalyst at atmospheric pressme and the temperature was slowly raised to the desired reduction/activation temperature and held for at least four hours. After activation, the reactor was cooled to the desired reaction temperature, the pressure was raised, and flow of an aqueous feed of glycerol and sodium hydroxide initiated along with a corresponding amonnt of hydrogen. A large set of reaction conditions was tested. 

Reactions involving gaseous and liquid reactants are carried out in various types of equipment. Packed columns, spray columns and bubble columns, as well as agitated tanks are all used (Fig. 2). Trickle-bed reactors are widely used in the petroleum industry for hydrodesulphurisation and related processes. In this type of reactor, liquid and gas both flow down through a bed of catalyst particles. The liquid flows around the particles as a thin film, thereby keeping the liquid residence time short and reducing undesirable side reactions. 

In multiphase reactors we frequently exploit the density differences between phases to produce relative motions between phases for better contacting and higher mass transfer rates. As an example, in trickle bed reactors (Chapter 12) liquids flow by gravity down a packed bed filled with catalyst, while gases are pumped up through the reactor in countercurrent flow so that they may react together on the catalyst surface. 

The analysis in this section is primarily dedicated to trickle-bed reactors. However, some basic aspects of packed bubble bed reactors will be presented as well. A bubble fixed-bed reactor is actually a tubular-flow reactor with concurrent upflow of gas and liquid. The catalyst bed is completely immersed in a continuous liquid flow, while gas rises as bubbles. 

In general, the material balances and the corresponding solutions for trickle and bubble bed reactors are the same, under the assumption that the plug-flow condition holds for both phases. Of course, the appropriate correlations should be used for the estimation of mass transfer coefficients. However, in packed bubble bed reactors, the liquid-phase is frequently found in a complete mixed state, and thus some adjustments have to be made to the aforementioned models. Two special cases will be presented here. 

In the following sections, the solutions of the models as well as examples will be presented for the case of trickle-bed reactors and packed bubble bed reactors. Plug flow and fust-order reaction will be assumed in order to present analytical solutions. Furthermore, the expansion factor is considered to be zero unless otherwise stated. Some solutions for other kinetics will be also given. The reactant A is gas and the B is liquid unless otherwise stated. 

Concerning packed bubble bed reactors, the evaluation of the Peclet number of the liquid phase is important in order to decide if we have to use a plug- or backmixed-flow model. For the specified Reynolds number, the Peclet number for the liquid phase using the Stiegel-Shah correlation (eq. (3.422)) is 0.15, much lower than in the trickle bed, which was expected as the backmixing in the liquid phase in packed bubble bed reactors is relatively high. The liquid phase can be considered to be well mixed if (Ramachandran, and Chaudhari, 1980) (eq. (3.423))... 

Operation of packed trickle-bed catalytic reactors is with liquid and gas flow downward together, and of packed mass transfer equipment with gas flow upward and liquid flow down. 

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