Tubular reactors with both axial and radial temperature gradients

Tubular reactors with both axial and radial temperature gradients In many exothermic processes the reactor temperature has to be controlled within much narrower limits. This is particularly true for many catalytic gas phase reactions. The reasons are usu y that undesired side reactions have to be avoided, or that the catalyst has to be protected against sintering. There are two reactor types for solid/gas-reactions that make good temperature control possible  

Both reactor types are quite complicated firom a construction tube bundles in both reactor types have to be constructed to temperature variations that occur when the process is started, reactor requires a special distributor, and multi-cyclones catalyst in the bed. The tubular reactor requires a support each pipe. Nevertheless both reactor types find wide application Here we shall consider the fixed bed reactor with wall cooling, cooling causes radial temperature gradients, that in its 

In the boundary conditions (8.16a) and (8.16b) the concentrations have to be averaged over the cross section of the reactor. 

The mechanism for radial heat transport is complicated. It consists of a combination of heat conduction through the solid particles, solid/gas heat transfer, and radial mixing. The combined process is usually describ by an effective heat conductivity, that is a function of solid and gas properties, and of flow conditions. The radid heat flux q is expressed as 

Since this effective conductivity depends on several different transport mechanisms that take place both in parallel and in series, it is difficult to give reliable predictions. Despite a lot of research, no simple correlations have been found. The reader is referred to the literature, e.g. the paper by Zehner and Schliinder (1975) (see also Van Wijngaarden and Westerterp, 1993). First, the effective conductivity is considered to consist of two terms, the conductivity of the bed without flow, and the contribution of the gas flow (u)  

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