Temperature difference catalyst interior

It is often useful to quickly estimate the maximum possible temperature rise, also known as the adiabatic temperature rise, in a catalyst pellet. Since no heat is transferred to the surroundings in this case, all energy generated (or consumed) by the reaction goes to heat (or cool) the pellet. The temperature difference between the surface and the pellet interior is directly related to the concentration difference. Dividing the material balance by the energy balance eliminates the reaction rate ... 

The reactor consisted of a fixed bed of x --in. cylindrical pellets. The pressure was 790 mm Hg. The external area of catalyst particles was 5.12 ft /lb, and the platinum did not penetrate into the interior of the alumina particles. Calculate the partial-pressure difference between the bulk-gas phase and the surface of the catalyst for SOj at each mass velocity. What conclusions may be stated with regard to the importance of external diffusion Neglect temperature differences. 

The determination of the maximum temperature differences between bulk fluid, catalyst pellet surface, and catalyst pellet interior in terms of directly observable quantities is a veiy useful tool in the study of catalytic reactions. Only if these temperature differences are significant need one be concerned with further extensive analysis of the transport phenomena. 

To evaluate the potential of carbon formation in a steam reformer, it is therefore essential to have a rigorous computer model, which contains kinetic models for the process side (reactor), as well as heat transfer models for the combustion side (furnace). The process and combustion models must be coupled together to accurately calculate the process composition, pressure, and temperature profiles, which result from the complex interaction between reaction kinetics and heat transfer. There may also be a temperature difference between bulk fluid, catalyst surface, and catalyst interior. Lee and Luss (7) have derived formulas for this temperature difference in terms of directly observable quantities The Weisz modulus and the effective Sherwood and Nusselt numbers based on external values (8). 

In general, there will also be a temperature difference between the interior of the catalyst particle and the bulk fluid stream. Unless the reaction is thermally neutral, i.e., AHr = 0, heat wiU have to be transported into or out of the catalyst particle in order to keep the particle at steady state. The following figure shows the temperature profile for an exothermic reaction. 

To concentrate on the principles of catalytic reactor design, we will temporarily ignore the possible presence of concentration and temperature differences between the bulk fluid and the sites in the interior of the catalyst particle. For the time being, we will as5 /ne that the resistances to mass and heat transfer in the catalyst particle and through the boundary layer are very small. As a consequence, the concentration and temperature gradients will be very small. For this case, the concentration and temperature profiles will be as shown below. 

Phenolic resins are highly versatile, which has led to a broad range of applications in the aircraft, aerospace, automotive, electrical and electronic industries (Pilato et ah, 2008) as well as in the interiors of mass-transit cars and architectural and marine components (Lewark, 2007). Phenolic resins with diverse structures and properties can be obtained from different phenols, aldehydes and catalysts. In addition, resins with different properties can be prepared from a given set of parental or substituted phenol/aldehyde/catalysts by diversifying parameters sueh as the phenol/formaldehyde ratio and the reaction temperature and duration. 

We know that the reaction rate depends on temperature and concentration. If the temperature and concentration differences between the interior of the catalyst particles and the bulk fluid are significant, then these differences must be taken into account in solving the design equation. In essence, this would require simultaneously solving the design equation and equations that describe heat transport, mass transport, and reaction kinetics in the interior of the catalyst particle, using the equations for transport through the boundary layer as boundary conditions. 

The interactions among heat transfer, mass transfer, and chemical reaction in heterogeneous catalysts were discussed in Chapter 4, from a qualitative point of view. In all of our calculations up to this point, we ignored any temperature and concentration differences between die bulk fluid stream and the interior of the catalyst particle, where reaction actually takes place. To solve reactor sizing and analysis problems involving solid catalysts, we assumed that the concentrations and the temperature throughout the catalyst particle were identical to those in the bulk fluid. 

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