10.2.1 Step 1 Overview: Diffusion from the Bulk to the External Surface of the Catalyst

For the moment let’s assume that the transport of A from the bulk fluid to the external surface of the catalyst is the slowest step in the sequence. We lump all the resistance to transfer from the bulk fluid to the surface in the mass transfer boundary layer surrounding the pellet. In this step the reactant A at a bulk concentration CAb must travel (diffuse) through the boundary layer of thickness δ to the external surface of the pellet where the concentration is CAs, as shown in Figure 10-7. The rate of transfer (and hence rate of reaction, image) for this slowest step is

Rate = kC (CAbCAs)

where the mass transfer coefficient, kC, is a function of the hydrodynamic conditions, namely the fluid velocity, U, and the particle diameter, Dp.

External and internal mass transfer in catalysis are covered in detail in DVD-ROM Chapters 11 and 12.

Figure 10-7. Diffusion through the external boundary layer. [Also see Figure DVD-ROM E11-1.1.]

image

External Mass Transfer

As we see (DVD-ROM Chapter 11), the mass transfer coefficient is inversely proportional to the boundary layer thickness, δ, and directly proportional to the diffusion coefficient (i.e., the diffusivity DAB).

image

At low velocities of fluid flow over the pellet, the boundary layer across which A and B must diffuse is thick, and it takes a long time for A to travel to the surface, resulting in a small mass transfer coefficient kC. As a result, mass transfer across the boundary layer is slow and limits the rate of the overall reaction. As the velocity over the pellet is increased, the boundary layer becomes smaller and the mass transfer rate is increased. At very high velocities the boundary layer is so small it no longer offers any resistance to the diffusion across the boundary layer. As a result, external mass transfer no longer limits the rate of reaction. This external resistance also decreases as the particle size is decreased. As the fluid velocity increases and/or the particle diameter decreases, the mass transfer coefficient increases until a plateau is reached, as shown in Figure 10-8. On this plateau, CAbCAs, and one of the other steps in the sequence is the slowest step and limits the overall reaction rate. Further details on external mass transfer are discussed in DVD-ROM/Web Chapter 11.

Figure 10-8. Effect of particle size and fluid velocity on the overall reaction rate.

image

..................Content has been hidden....................

You can't read the all page of ebook, please click here login for view all page.
Reset
3.94.98.40