Spray column reactors towers

Figure 24.1 Types of tower or column reactors for gas-liquid reactions (a) packed tower, (b) plate tower, (c) spray tower, (d) falling-film tower, (e) bubble column...

This situation describes an emulsion reactor in which reacting drops (such as oil drops in water or water drops in oil) flow through the CSTR with stirring to make the residence time of each drop obey the CSTR equation. A spray tower (liquid drops in vapor) or bubble column or sparger (vapor bubbles in a continuous liquid phase) are also segregated-flow situations, but these are not always mixed. We wiU consider these and other multiphase reactors in Chapter 12. 

Figure 12-9 Bubble column and spray tower reactors. Large drop or bubble areas increase reactant mass transfer,...

Figure 12-10 Sketches of reactant concentration Ca around a spherical bubble or drop that reacts after migrating from ftie gas phase into the liquid phase in bubble column and spray tower reactors.

Figure 12-12 Sketches of possible flow patterns of bubbles rising through a liquid phase in a bubble column. Stirring of the continuous phase will cause the residence time distribution to be broadened, and coalescence and breakup of drops will cause mixing between bubbles. Both of these effects cause the residence time distribution in the bubble phase to approach that of a CSTR. For falling drops in a spray tower, the situation is similar but now the drops fall instead of rising in the reactor.

If we simply turn the drawing of the bubble column upside down, we have a spray tower reactor. Now we have dense liquid drops or solid particles in a less dense gas so we spray the liquid from the top and force the gas to rise. The same equations hold, but now the mass transfer resistance is usually within the hquid drop. 

The bubble column and spray tower depend on nozzles to disperse the drop or bubble phase and thus provide the high area and small particle size necessary for a high rate. Drop and bubble coalescence are therefore problems except in dilute systems because coalescence reduces the surface area. An option is to use an impeller, which continuously redisperses the drop or bubble phase. For gases this is called a sparger reactor, which might look as shown in Figure 12-16. 

A spray tower is a continuous gas-liquid reactor. Gases pass upward through a column and contact liquid reactant sprayed into the column. The spray tower represents the opposite extreme from a bubble tower. The spray tower has greater than 90% of the volume as gas. This allows for much reduced liquid-handling rates for highly soluble reactants. 

The spray tower is a heterogeneous gas-liquid reactor. The gas passing up the column obeys plug flow conditions, and the liquid sprayed into the column behaves either as plug flow or as batch for individual droplets falling down the tower. 

Fluid phase only Countercurrent flow Absorber Countercurrent flow Absorber Countercurrent flow Spray tower Co-current or countercurrent Bubble column Absorber or Reactor Venturi Static mixers Falling film, etc. 

Spray towers/column Usually treated as a gas-liquid reactor. Liquid is sprayed counter currently to gas flow. Used for corrosive and liquids containing substantial amount of solid materials. Higher energy usage and capital investment... 

Figure4.4.6 Influence of the Hinterland ratio (Hi) and Hatta number (Ha) on the degree of utilization ofthe liquid phase (ST spray tower, CP column with packing, CT column with trays, STR stirred tank reactor, BC bubble column region above dashed line more than 80% of conversion takes place in the bulk phase of the liquid). Adapted from Emig and Klemm (2005).

Figure 4.10.11 shows examples of gas-liquid reactors. Gas is usually dispersed in the liquid by a bubble column, tray column, or a stirring reactor with pressurized gas. Liquid is dispersed in the gas by means of a jet type washer or a spray tower. Liquid in the form of a thin film is exposed to a gas by a falling film reactor or a tricHe reactor with filter elements. Details of the interplay of chemical reaction and mass transfer are given in Section 4.4. 

An interesting situation arises in processes where the reaction product P evaporates and is taken out of the reactor with the gas phase (the supply phase). Let us assume that there are no chemical reactions in the gas phase, e.g., l ause the liquid phase reaction is catalysed. We consider the case of rapid reactions, so that all the desired product P is formed in the diffusion layer in the liquid phase, close to the interface. When P can undergo undesired reactions in the liquid phase it is essential to remove P as effectively as we can, e.g., by creating a large surface area and very high gas-phase mass transfer coefficients. At the same time it is essential that the volume of the liquid phase is minimized, since decomposition of P will occur just there. The obvious choice would then be a configuration where the liquid is the dispersed phase, such as in a spray tower or a spray cyclone, provided the heat removal rate is sufficient. Another suitable arrangement could be a gas/liquid packed bed or a wetted wall column. The latter reactor type is very suitable for heat removal (section 4.6.3.1)... 

A variety of different types of gas-liquid reactors exist. The choice of the reactor type is sometimes obvious and sometimes very difficult. A summary of the selection criteria is listed in Table 7.2. For slow reactions, a bubble column is preferred for fast reactions, a column, a scrubber, or a spray tower should be used. For absorption processes in which a high conversion of the gaseous reactant is the main goal, the self-evident reactor type is a packed bed or a plate column. 

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