Spray tower reactor

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. 

FIG. 23-25 Typ es of industrial gas/Hqiiid reactors, (a) Tray tower, (h) Packed, counter current, (c) Packed, parallel current, (d) Falling liquid film, (e) Spray tower, if) Bubble tower, (g) Venturi mixer, h) Static in line mixer, ( ) Tubular flow, (j) Stirred tank, (A,) Centrifugal pump, (/) Two-phase flow in horizontal tubes. 

Piston Flow in Contact with a CSTR. A liquid-phase reaction in a spray tower is conceptually similar to the transpired-wall reactors in Section 3.3. The liquid drops are in piston flow but absorb components from a well-mixed gas phase. The rate of absorption is a function of as it can be in a transpired-wall reactor. The component balance for the piston flow phase is... 

Other reactor types are also used for gas-liquid reactions, but they are not very common in fine chemicals manufacture. Spray towers and jet reactors are used when the liquid phase is to be dispersed. In spray towers the liquid is sprayed at the top of the reactor while the gas is flowing upward. The spray reactor is useful when a solid product, possibly suspended in the liquid, is formed, or if the gas-phase pressure drop must be minimized. In a jet reactor, the liquid is introduced to the reaction zone through a nozzle. The gas flows in, being sucked by the liquid. 

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 12-12 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.

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. 

Figure 17.11. Types of contactors for reacting gases with liquids many of these also are suitable for reacting immiscible liquids. Tanks (a) with a gas entraining impeller (b) with baffled impellers (c) with a draft tube (d) with gas input through a rotating hollow shaft, (e) Venturi mixer for rapid reactions, (f) Self-priming turbine pump as a mixer-reactor, (g) Multispray chamber. Towers (h) parallel flow falling film (i) spray tower with gas as continuous phase (j) parallel flow packed tower (k) counter flow tray tower. (1) A doublepipe heat exchanger used as a tubular reactor.

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. 

Fig. 2 Updated flash pyrolysis pilot plant for biomass (PDU-Scalc) (1 hopper, 2 vibration conveyor, 3 screw feeder. 4 fluidised bed reactor, 5 cyclone system. 6 monopump. 7 quench liquid reservoir, 8 heat exchanger, 9 spray tower, 10 electrostatic precipitators, 11 heat exchanger, 12 flare, 13 compressor, 14 gas preheater 1. 15 gas preheater 2, 16 overflow pipe, 17 char collection vessel).

The interfacial area is known accurately only in some systems used in laboratory studies falling laminar films, laminar cylindrical jets, undisturbed gas-liquid and liquid-liquid interfaces, and solid castings of known dimensions immersed in liquids. In all reactor systems used industrially such as packed towers, spray towers, and bubble trays, the interfacial area is relatively difficult to determine. Photographic, gamma-ray, light scattering and chemical methods have been used to determine a in bubble dispersions (5, 6, 7, 8, iO, 42). For an average bubble diameter dn, a superficial gas velocity Usa and a bubble rise velocity Un,... 

Rotary tube furnaces or spray towers are used as reactors. In the second step, the manganale(V) formed is oxidized at 180 to 220"C to manganate(Vl) after optional grinding ... 

Another important effect of SCR on downstream equipment was described by Gutberlet and Schallert [136]. Cooling of the flue gas at the exit of the SCR reactor leads the formation and deposition of aerosols. A position of these aerosols pass the spray tower scrubbers of the flue gas desulfurization unit and acid particles are emitted. 

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. 

Liquid-liquid spray towers are less likely to find application as chemical reactors. In this case the dispersed phase flows upward and the continuous downward. Such towers have been thoroughly investigated by Letan and Kehat [52, 53, 54]. 

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... 

Air pollution control APC units (cyclone, venturi scrubber, tangential spray scrubber, spray tower, ID fans, chimney) This is required to absorb the gases evolved from reactor and den for air pollution control HF will be released into atmosphere and cause air pollution Monitor operation of APC units and exit gases all the time... 

In the Swift process (Figure 12.14), phosphoric acid (50% P2O5) and liquid ammonia react in a two-fluid nozzle, which discharges into a reactor pipe. The mixture of finely divided MAP and steam is injected into a cooling tower where a countercurrent airstream carries away the water vapor formed by the heat of reaction and cools the product, The product moisture content is 3%-5%. The Nissan process is also a spray tower process, in which droplets of phosphoric acid react with ammonia in an ascending airstream. 

E-1 Annonia vaporizer R- Pressure reactor C-3 Spray tower BC-4 Belt conveyor... 

Although Li et al. do not explicitly mention it, the spray tower has no moving parts. This is a significant advantage over the stirred reactor since the impeller and sealing arrangement in the latter require careful maintenance. 

The foregoing discussion shows the superiority of the spray tower for oxidation of p-xylene to terephthalic acid. Li et al. (2013) suggest that this concept can be extended to replace existing stirred reactors for other similar gas-liquid mass transfer limited reactions. 

Dimiccoli, A., Di Serio, M., Santacesaria, E. 2000. Mass transfer and kinetics in spray tower loop absorbers and reactors. Ind. Eng. Ghem. Res. 39(11) 4082-4093. 

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