Energy release by reaction

There may be radial temperature gradients in the reactor that arise from the interaction between the energy released by reaction, heat transfer through the walls of the tube, and convective transport of energy. This factor is the greatest potential source of disparities between the predictions of the model and what is observed for real systems. The deviations are most significant in nonisothermal packed bed reactors. 

It may be desirable to operate in semibatch fashion in order to enhance reaction selectivity or to control the rate of energy release by reaction through manipulation of the rate of addition of one reactant. Other situations in which semibatch operation is employed include a variety of biological fermentations where various nutrients may be added at predetermined rates to achieve optimum production capacity and cases where one reactant is a gas of limited solubility that can be fed only as fast as it will dissolve. 

For exothermic reactions AHR is negative, and this term will then represent the rate of energy release by reaction. 

We now consider operation of the batch reactor under adiabatic conditions. We will assume that we need not worry about reaching the boiling point of the liquid and that the rate of energy release by reaction does not become sufficiently great that an explosion ensues. 

The equipment requirements that we have determined are well within the realm of technical feasibility and practicality. The heat transfer requirements are easily attained in equipment of this size. The fact that some of the heat transfer requirements are positive and others negative indicates that one should probably consider the possibility of at least partial heat exchange between incoming cold feed and the effluent from the second or third reactors. The heat transfer calculations show that the sensible heat necessary to raise the cold feed to a temperature where the reaction rate is appreciable represents a substantial fraction of the energy released by reaction. These calculations also indicate that it would be advisable to investigate... 

Rate of energy release by reaction versus temperature for an irreversible exothermic reaction carried out in a CSTR. 

Other reactions will have somewhat different forms for the curve of Qq versus T. For example, in the case of a reversible exothermic reaction, the equilibrium yield decreases with increasing temperature. Since one cannot expect to exceed the equilibrium yield within a reactor, the fraction conversion obtained at high temperatures may be less than a subequilibrium value obtained at lower temperatures. Since the rate of energy release by reaction depends only on the fraction conversion attained and not on the position of equilibrium, the value of Qg will thus be lower at the higher temperature than it was at a lower temperature. Figure 10.2 indicates the general shape of a Qg versus T plot for a reversible exothermic reaction. For other reaction networks, different shaped plots of Qg versus T will exist. 

Output — input = energy released by reaction (by conduction)... 

Unfortunately, most of these experiments do not define the energy distribution between the products. In many cases it was possible to examine only one of the products. The problem has been to devise experiments in which it is possible to observe the spectra of both products simultaneously, preferably with the same technique. The present state of our knowledge is that there is ample evidence for excitation of the new-bond molecule up to levels corresponding to the energy released by reaction, and there are data, fewer in number but no less certain, showing some excitation in the old-bond product. These are considered below. 

Similarly, conservation of energy requires equating Eq. (10-25) to the expression, corresponding to Eq. (10-22), for the total energy released by reaction ... 

Respiration (oxidative breakdown and energy release by reaction with oxygen)... 

Many fixed-bed industrial reactors operate adiabatically, and the temperature profiles can be used to follow changes in catalytic activity and to optimize reactor performance. The temperature profile for an exothermic reaction is similar to the temperature-time curve for a batch reaction. The energy released by reaction is carried out by the fluid, since, except for startup, there is no accumulation in the catalyst. The conversion at any distance from the inlet can be calculated from the temperature rise relative to that for complete conversion. 

Consider an irreversible first-order exothermic reaction in a CSTR. The rate of thermal energy release by reaction can be plotted versus temperature, as shown by the curve in Figure 5.1. At low temperature, the reaction rate is low, and the slope of is slight. At high temperatures the reactor is operating at a high level of conversion (low reactant concentration) and additional... 

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