Thermal Behavior of Catalyst Particles and Pellet Runaway

In packed bubble columns the gas-liquid interfacial area also can be related to the external catalyst surface area, as in trickle flow reactors. However, in packed bubble columns channeling can occur with strongly reduced gas-liquid interfacial areas [11]. 

3 Thermal Behavior of Catalyst Particles and Pellet Runaway 

Consider a catalyst particle in a fixed bed through which gas is flowing, and assume that the reaction takes place at the catalyst surface and, accordingly, that the net heat of reaction is released at this surface. An example of such a system is the hydrogenation of acetylene in an ethylene stream, according to 

A pseudosteady state may be assumed to exist in the cataiyst bed, and the temperature of a catalyst particle may be thought to be constant. In such a case, all of the heat produced by the reaction must be transported from the catalyst surface to the surroundings by convection to the gas stream. In this way the feedback of heat is established from the reaction site to the gaseous reactant. Under the above conditions the molar flux of hydrogen (which we call reactant A) to the surface of the particle, multiplied by the heat liberated per molar unit of A converted (-A//r)A must be equal to the heat flux coming from the surface  

Here HPR stands for heat production rate and HWR for heat withdrawal rate. Usually, the value of Na depends on the texture of the particle and on the chemical and physical rate parameters. At a low value of Tt (the temperature of the solid), NA is mainly determined by the rate of the chemical surface reaction. As Tf increases, a situation is reached where the reactant is converted at the external particle surface at such a high rate that its concentration at the surface is zero and NA is entirely determined by mass transfer. According to the discussion in Section 4.1 we can write 

---