Spray column reactors

The very high rate of permeation of oxygen in the 20 pm diameter cell having a very thin (1 pm) wall can be explained in a manner similar to that in Section 3.4.2.4 Spray Column Reactor (Li et al. 2013). It may be noted that this is despite a relatively very low value assigned to the diffusion coefficient of oxygen in the cell wall. 

Fig. 4. Multiphase fluid and fluid—solids reactors (a) bubble column, (b) spray column, (c) slurry reactor and auxiUaries, (d) fluidization unit, (e) gas—bquid—sobd fluidized reactor, (f) rotary kiln, and (g) traveling grate or belt drier.

Figure 8.11 Types of reactors for gas-liquid precipitation, (a) bubbling stirred tank, (b) fiat interface stirred tank, draft-tube bubble column, (d) spray column after Wachi and Jones, 1994)...

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...

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. 

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.

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. 

FIGURE 1 Selected reactor configurations (a) batch, (b) continuous stirred-tank reactor, (c) plug flow reactor, (d) fluidized bed, (e) packed bed, (f) spray column, and (g) bubble column. 

Figure 1. Slurry reactors classified by the contacting pattern and mechanical devices (a) slurry (bubble) column (b) countercurrent column (c) co-current upflow (d) co-current downflow (e) stirred vessel (C) draft tube reactor (g) tray column (h) rotating disc or multi-agitated column reactor (i) three-phase spray column — liquid flow —> gas flow.

A unified approach has been developed for the prediction of transition in multiphase reactors such as gas-hquid bubble columns, liquid-liquid spray columns, solid-liquid fluidized beds, gas-solid fluidized beds, and three-phase fluidized beds. 

Gas-liquid systems of particular interest to the chemical engineer are encountered in bubble columns, spray columns, air lift, falling film, and stirred tank reactors. Usually the form of these reactors corresponds to that of vessels or columns. From the perspective of the chemical engineer, who is concerned with the conversion and selectivity of chemical transformations, it is of utmost importance that an intensive contact between a gas and a liquid be achieved and therefore very often one phase is continuous whereas the other is disperse. Therefore, the interfacial area and the size of the disperse phase elements constitute very important aspects of CFD modeling of these types of systems. 

The liquid may be sprayed through the gas as drops or jets (spray columns, jets, ejector reactor, and venturi). 

Reactor type Sparged stirred Bubble column tank Spray column Packed column... 

The first major decision in the choice of a reactor for gas-liquid reactions taking place in the liquid phase is based on the optimal usage of the total reactor volume, i.e. the choice of the parameter P, which is the ratio of the liquid-phase volume to the volume of the diffusion layer (see Section 8.4.2). When reactions are slow compared to the mass transfer from the gas to the liquid, sparged stirred tanks and bubble columns are preferred, as these reactors have the largest bulk liquid volume. On the other hand, fast reactions for a large part take place in the diffusion layer, so in this case spray columns and packed columns are more suitable. 

For example, when mass transfer limits the overall reaction rate, reaction occurs near the gas-liquid interface to make significant liquid volume unnecessary. To minimize the value of jS, bulk liquid volume is minimized, while interfacial area is maximized. As intense mixing and turbulence throughout a large liquid volume would be of limited benefit, the gas-liquid contactor of choice is a packed bed or a spray column as in the earlier TPA hydrogenation reactor example. 

Figure 1.3 Multiphase reactors (a) packed-bed reactor, (b) moving-bed reactor, (c) flui-dized-bed reactor, (d) bubbling column reactor, (e) spray reactor, and (/) kiln reactor.

Brief discussion of a total of over 25 reactors of all categories, such as fixed-, fluidized- and moving-bed reactors, bubble columns, sectionalized bubble columns, loop reactors, stirred-tank reactors, film reactors, rotating disk reactors, jet reactors, plunging jet reactors, spray columns, surface aerators... 

Class 1 equipment are also called column-type equipment. Under this category, there are the various multiphase contactors. Gas-liquid contactors include bubble columns, packed bubble columns, internal-loop and external-loop air-lift reactors, sectionalized bubble columns, plate columns, and others. Solid-fluid (liquid or gas) contactors include static mixers, fixed beds, expanded beds, fluidized beds, transport reactors or contactors, and so forth. For instance, fixed-bed geometry is used in unit operations such as ion exchange, adsorptive and chromatographic separations, and drying and in catalytic reactors. Liquid-liquid contactors include spray columns, packed extraction... 

Spray column, packed extraction column, liquid-liquid adaptations of loop reactors, plate extraction column, static mixers. 

Liquid-liquid reactors are similar to gas-liquid reactors. In the former case, the dispersed phase is in the form of droplets as against bubbles in the latter. The motion of bubbles and drops can be described using a unified approach. A spray column (or a drop column) is the equivalent of a bubble column but with one difference. The dispersed gas phase is always lighter than the continuous liquid phase (p < Pl)- However, the dispersed liquid phase in spray columns may be lighter or heavier than the continuous immiscible liquid phase. Nevertheless, spray columns can be easily described similar to bubble columns. Furthermore, packed bubble columns and sectionalized bubble columns can be considered equivalent to packed extraction columns and plate extraction columns. External-loop and internal-loop reactors are also possible (for equivalent gas-liquid reactors, refer to Section 11.4.2.1.4). 

Multiple reactor constructions for gas-liquid reactors are available, because of the large number of different application areas. Spray columns, wetted wall columns, packed columns and plate columns are mainly used for absorption processes. The gas concentrations are low in the case of absorption processes, hence a large interfacial contact area between the gas and the liquid is important to enhance the absorption process. These column reactors usually operate in counter-current mode. Counter-current operation is the optimal operating mode, because at the gas outlet where the gaseous component concentration is lowest, the gas is in contact with a fresh absorption solution. The low concentration of the gaseous component can then partly be compensated by the high concentration of the liquid component. 

Efficiencies of several kinds of small-scale extractors are shown in Fig. 19-28. Larger-diameter equipment may have less than one-half these efficiencies. Spray columns are inefficient and are used only when other kinds of equipment may become clogged. Packed columns as liquid-liquid reactors are operated at 20 percent of flooding. Their height equivalent to theoretical stage (HETS) range is from... 

Here we will pay attention to the gas-liquid reactors. The reaction takes place usually in the liquid phase. Three main types of contact may be distinguished following the phase ratio (1) gas bubbles dispersed in liquid, (2) liquid drops dispersed in gas, and (3) gas and liquid in film contact. In the first category we may cite gas-liquid bubble columns, plate or packed absorption columns, agitated tanks, agitated columns, static mixer columns, pump-type reactors. As examples in the second class we may name spray columns or liquid injection systems. The third category can be used with very exothermic reactions or viscous liquids. 

When carrying out a gas-liquid reaction, the gas may be dispersed in the liquid, as in bubble-column reactors or stirred tanks, or the gas phase may be continuous, as in spray contactors or trickle-bed reactors. The fundamental kinetics are independent of the reactor type, but the reaction rate per unit volume and the selectivity may differ because of differences in surface area, mass transfer coefficient, and extent of mixing. In the following sections, gas holdup and mass transfer correlations and other performance data for gas liquid reactors are reviewed and some problems of scaleup are discussed. 

The reaction gas is cooled to ambient temperature in a water spray column, and the quench water that condenses is fed back in a closed cycle to the electric arc reactor. The gas is then compressed and subjected to a sequence of scrubbing operations with selective solvents [i.e., methanol, octane, and A/ -methyl-2-pyrrolidone (NMP) in that order]. The solvents are regenerated by low-pressure stripping with gas fractions from the production process. This removes mainly the polyunsaturated hydrocarbons. 

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