Combination of mass transfer and reaction steps

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.) 

Choosing a clearly defined basis is an all important first stage in the treatment of three-phase reactors. In this case the overall reaction rate 91, will be based 

Eq - volume fraction of gas, i.e. bubbles, eL = volume fraction of liquid, and eP = volume fraction of particles. 

For the particles, the external surface area is most conveniently defined as  

For both bubbles and particles, there will be a distribution of sizes in the dispersion. The above quantities can be related to the volume/surface or Sauter mean bubble and particle diameters 4 and dp (see Volume 2, Chapter 1). 

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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... 

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