Enthalpy management and temperature control

It is essential to look separately at heat effects on the scale of the volume elements and on the scale of the whole reactor. On the volume element scale we may have to deal with local temperature gradients in single phase systems, or with temperature differences between two phases. 

Examples of significant temperature gradients in single phase systems are found in flames, and also in very viscous liquids where rapid exothermic reactions take place. The effect of temperature gradients in viscous systems was briefly indicated in section 5.2.3. But in general, the problem of homogeneous reactors with two- or three-dimensional temperature and concentration gradients is considered too specialized for this book therefore the interested reader is referred to the relevant literature (see, e.g., the comprehensive book by Rosner, 1986). 

Heat transfer between two phases is a subject of general interest, and this is 

Effective control of the temperature of a continuous chemical reactor has several aspects. In the first place we wish to ensure that a certain maximum temperature is realized. This often requires continuous cooling. The average reactor temperature follows from heat balances. But we also have to attain a sufficient dynamic stability. In any continuous process small variations in feed conditions will occur. Dynamic stability requires automatic corrective actions, so that disturbances in the reactor concUtions are damped rapidly. Therefore, continuous reactors are usually connected with automatic control loops. An effective temperature control is particularly important in view of the strong dependency of reaction rate constants on temperature, which is shown by Arriienius law  

For gas-phase reactions, the adiabatic temperature rise at complete conversion may be on the order of 100 - 1000 K, but for liquid phase reactions it is usually lower. The latter approximation, in which average values of the physical constants are used, is therefore mostly only valid for liquid phase reactions. 

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