Wetted wall reactor

Falling film or wetted wall reactors can be used for very exothermic reactions. Furthermore, the limited and well defined interfacial area permits excellent control of very rapid reactions. 

Wetted wall reactor Verticalreactor with liquid phase entering at the top and flowing along its wall with gas flowing through its core. ... 

Laboratory reactors for studying gas-liquid processes can be classified as (1) reactors for which the hydrodynamics is well known or can easily be determined, i.e. reactors for which the interfacial area, a, and mass-transfer coefficients, ki and kc, are known (e.g. the laminar jet reactor, wetted wall-column, and rotating drum, see Fig. 5.4-21), and (2) those with a well-defined interfacial area and ill-determined hydrodynamics (e.g. the stirred-cell reactor, see Fig. 5.4-22). Reactors of these two types can be successfully used for studying intrinsic kinetics of gas-liquid processes. They can also be used for studying liquid-liquid and liquid-solid processes. 

Here, either both the fluids are spread over an existing surface or one of them is spread and the other is present in bulk. Usually, the walls of a column are utilized for this purpose by wetting them with one of the fluids. Wetted-wall columns and rotating film reactors (Kll) make use of this principle to... 

It is evident, however, that this problem can be much more comphcated than either the wetted wall column or the catalytic wall reactor, because it combines the complexities of both. In fact, there are numerous additional complexities with this reactor beyond those simplified cases. 

The problems discussed here are basic in the description of absorption in falling films, performance of wetted-wall towers, operation of tubular reactors, and fluid blending. 

We studied these phenomena experimentally in a wetted wall column and two stirred cell reactors and evaluated the results with both a penetration and a film model description of simultaneous mass transfer accompanied by complex liquid-phase reactions [5,6], The experimental results agree well with the calculations and the existence of the third regime with its desorption against overall driving force is demonstrated in practice (forced desorption or negative enhancement factor). 

Negligible and medium interaction regimes. Experiments were carried out with an aqueous 2.0 M DIPA solution at 25 °C in a stirred-cell reactor (see ref. [1]) and a 0.010 m diameter wetted wall column (used only in negligible interaction regime, see ref. [4,5]). Gas and liquid were continuously fed to the reactors mass transfer rates were obtained from gas-phase analyses except for CO2 in the wetted wall column where due to low C02 gas-phase conversion, a liquid-phase analysis had to be used [5]. In the negligible interaction regime some 27 experiments were carried out in both reactors. The selectivity factors were calculated from the measured H2S and CO2 mole fluxes and are plotted versus k... 

Figure 3. Selectivity factor S as a function of kgHts in the negligible interaction regime. Key O, stirred cell reactor +, wetted wall column, cocurrent and X. wetted wall column (countercurrent).

The major difference between this reactor and other gas-liquid reactors such as the wetted-wall column, the laminar-jet absorber, the disk contactor, and the stirred cell is that the experimenter has independent control of the physical factors, such as individual film resistances and interfacial area. 

Several other ICF reactor concepts use liquid metal walls (Ii W). These include the Los Alamos National Laboratory (LANL) wetted wall concept, the Bechtel concept called EAGLE (which uses a lithium spray in the chamber), the Lawrence Livermore National Laboratory (LLNL) concept called JADE (which uses a fiber-metal structure to control liquid metal flow), and the German/University of Wisconsin concept called HIBALL (which uses carbide "socks" to control liquid metal flow). 

Fickert et al. (1999) examined the production of Br2 and BrCl from the uptake of HOBr onto aqueous salt solutions in a wetted-wall flow tube reactor. The yield of Br2 and BrCl was found to depend on the Cl to Br ratio, with more than 90% yield of Br2 when [Cl ]/[Br ] (in molL ) was less than 1,000. With increasing [Cl ]/[Br ] BrCl was the main product (see Figure 2). They also found a pH dependence of the outgassing of Br2 and BrCl with greater release rates at lower pH. 

The kinetic parameter can be estimated in laboratory reactors. For solid-fluid systems, this subject was described in Section 11.3.1.6. For fluid-fluid reactions, the commonly employed laboratory reactors include stirred cell, wetted wall column, rotating drum, laminar jet, stirred contractor, and others. These are schematically shown in Figure 11.14. In practically all of these reactors, the value of the fluid-fluid interfacial area is known. These reactors have been described by Treybal (1980) and Doraiswamy and Sharma (1984). As an illustration, the stirred cell will be described first, followed by a comparison with other laboratory reactors. The discussion of the stirred cell is restricted to gas-liquid systems, but it is also applicable (with minor variations) to liquid-liquid systems. 

In addition to the stirred cell, other laboratory reactors commonly used include rotating drum contactor, wetted wall column, wetted sphere column, laminar jet, and stirred contactor. These equipments are shown schematically in Figures 11.14b-f. AU have several common features, the principal one being a weU defined gas-liquid interfacial area and the ability to vary the area per unit reactor volume a). In the stirred cell, it is achieved by varying the liquid height. As an alternative way, a solid circular baffle is placed at the gas-liquid interface. Holes are drilled on the baffle plate so that the hole opening area becomes the interfacial area. For varying a, baffle plates are made with different free (hole) areas. 

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. 

It must be clear from the various equations developed above that the gas-liquid interfacial area is a very important parameter in determining the rate of mass transfer. Any precise measurement of the mass transfer coefficient is possible only if the area is correctly known. This is best accomplished by using a stirred cell with a fixed gas-liquid interfacial area, although other experimental reactors such as the wetted wall column, laminar jet, and disk contactor can also be used (see Danckwerts, 1970 Doraiswamy and Sharma, 1984). The two commonly used cell designs are those of Danckwerts (1970) and Levenspiel and Godfrey (1974). 

The tubular multiphase hollow membrane wall reactor briefly described before and sketched in Figure 24.1 h is a multiphase reactor design very similar to the trickle-bed reactor. In a regular trickle-bed reactor, the liquid flows over a partially wetted pellet as a thin film and supplies the liquid-phase reactant to the catalyst pores. This action, however, has the effect of hindering pore access to... 

All boiling water reactors in Scandinavia are of Asea-Atom design. Containment is based on the pressure suppression (PS) principle, i.e. at a major pipe rupture the steam is led from the upper part, dry well, through a number of pipes to the lower part of the containment, wet wall, with a water pool, where condensation takes place. This principle of the volume of the containment has been kept small, about one-fifth of that of a dry containment with the same design pressure. 

Wetted wall colums, or falling film reactors... 

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)... 

An interesting example of a cooled tubular reactor is the falling film reactor (or wetted wall column), that was described in section 4,63.1. " en the film is sufficiently thin and the evolution of heat is not too excessive, a very uniform temperature can be obtained. 

The heat transfer coefficients at reactor walls can be highest in a parallel flow along a cooled wall (wetted wall column), and somewhat lower in a gas-in-liquid dispersion. 

When the reaction is exothermic with a very high heat effect, one should consider either the use of an evaporating solvent, or a very effective heat transfer to the reactor wall. Heat removal by an evaporating solvent is always practical in a gas-in-liquid dispersion. When for reasons of selectivity the liquid holdup has to be small, the wetted wall colunui with wall cooling is then the best alternative, (section 4.6.3.1). 

As for the uptake of OH on H20(l), Hanson et al. (1992a) gave y > 3.5 x 10 for pure water at 275 K by the measurement using wetted wall flow tube reactor. Takami et al. (1998) obtained y = (4.2 2.8) x 10 for pure water at 293 K by the impinging flow method, and reported that the value decreases with gas-liquid contact time and increases by a factor of 2-3 for the acidic and alkaline water with pH = 1 and 11. They estimated that the accommodation coefficient a is close to unity by the simulation using the rate constant of OH in the aqueous phase and Henry s law constants, which agrees with the result of a = 0.83 (300 K) by molecular dynamics calculation by Roeselova et al. (2004). The recommended values of the lUPAC subcommittee and the NASA/JPL panel are a >0.1 and 0.02, respectively (Wallington et al. 2012 Sander et al. 2011). 

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