Mass transfer and reaction steps

The individual mass transfer and reaction steps occurring in a gas-liquid-solid reactor may be distinguished as shown in Fig. 4.15. As in the case of gas-liquid reactors, the description will be based on the film theory of mass transfer. For simplicity, the gas phase will be considered to consist of just the pure reactant A, with a second reactant B present in the liquid phase only. The case of hydro-desulphurisation by hydrogen (reactant A) reacting with an involatile sulphur compound (reactant B) can be taken as an illustration, applicable up to the stage where the product H2S starts to build up in the gas phase. (If the gas phase were not pure reactant, an additional gas-film resistance would need to be introduced, but for most three-phase reactors gas-film resistance, if not negligible, is likely to be small compared with the other resistances involved.) The reaction proceeds as follows  

Mass transfer and reaction steps in gas-liquid-solids reactors (solids completely wetted by liquid), showing concentration gradients for reactant A being transferred from the gas phase 

The final stage in the complete reaction process is the diffusion of the products of the reaction out of the particle and their eventual transfer back into the liquid or gas phase. However, only if the reaction is reversible will any build-up of products affect the rate of the reaction itself. For an irreversible reaction, generally the fate of the products is not so important and need not be taken into account in determining the rate of the forward transfer and reaction steps. 

Gas-Uquid-Solid Reactor Types Choosing a Reactor 

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Example 11.8 With highly reactive absorbents, the mass transfer resistance in the gas phase can be controlling. Determine the number of trays needed to reduce the CO2 concentration in a methane stream from 5% to 100 ppm (by volume), assuming the liquid mass transfer and reaction steps are fast. A 0.9-m diameter column is to be operated at 8 atm and 50°C with a gas feed rate of 0.2m /s. The trays are bubble caps operated with a 0.1-m liquid level. Literature correlations suggest = 0.002 m/s and A, = 20m per square meter of tray area. 

Describe the various mass transfer and reaction steps involved in a three-phase gas-liquid-solid reactor. Derive an expression for the overall rate of a catalytic hydrogenation process where the reaction is pseudo first-order with respect to the hydrogen with a rate constant k (based on unit volume of catalyst particles). 

Now put all the mass transfer and reaction steps into the same rate form and then combine. Thus... 

The individual mass transfer and reaction steps outlined in Fig. 4.15 will now be described quantitatively. The aim will be firstly to obtain an expression for the overall rate of transformation of the reactant, and then to examine each term in this expression to see whether any one step contributes a disproportionate resistance to the overall rate. For simplicity we shall consider the gas to consist of just a pure reactant A, typically hydrogen, and assume the reaction which takes place on the interior surface of the catalyst particles to be first order with respect to this reactant only, i.e. the reaction is pseudo first-order with rate constant A ,. In an agitated tank suspended-bed reactor, as shown in Fig. 4.20, the gas is dispersed as bubbles, and it will be assumed that the liquid phase is well-mixed , i.e. the concentration CAL of dissolved A is uniform throughout, except in the liquid films immediately surrounding the bubbles and the particles. (It will be assumed also that the particles are not so extremely small that some are present just beneath the surface of the liquid within the diffusion film and are thus able to catalyse the reaction before A reaches the bulk of the liquid.)... 

Fig. 4.20. Suspended-bed agitated-tank teactor Combination of mass transfer and reaction steps. Impeller used would typically be a pitched-blade turbine, pumping downwards as shown, serving both to suspend particles and to disperse gas...

Three-Phase Fluidised Suspended-Bed Reactor—Combination of Mass Transfer and Reaction Steps... 

Figure 12-1 Mass transfer and reaction steps for a catalyst pellet.

Figure 8.5 Mass transfer and reaction steps in the three-phase model.

If a gas-liquid reaction is carried out by passing gas continuously through a batch of liquid in a stirred tank, the concentration of liquid-phase reactant changes with time, and this usually changes the reaction rate and the relative importance of the mass transfer and reaction steps. The change in reaction rate also leads to gradual accumulation of gas A in the bulk liquid, which means that the rate of absorption of A is no longer equal to the rate of reaction, as was assumed in Eq. (7.7). 

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