Controlling Regime in Simple Reactions

The exact rate equation to be used for a reaction at hand depends on the regime in which it occurs. It is this equation that must be used in the reactor design methods described in Chapter 16. The equations for various regimes along with the appropriate conditions for their validity, already briefly discussed, are summarized in Table 14.2. 

Depending on the regime of a reaction, the controlling rate constant will be either the reaction rate constant or the mass transfer coefficient. The overall effect of temperature on the reaction will be the resultant of its effects on the rate coefficient in the controlling regime of the reaction and on the equilibrium concentration of the gas in the liquid. Table 14.3 summarizes these effects for the different regimes. 

There are a number of important gas-liquid organic reactions which are complex. Because more than one reaction is involved in a complex scheme, each with its own value of the rate constant, it is possible that the reactions would be in different regimes. This makes the analysis quite complicated. Hence we restrict the treatment here to a relatively simple but important system, the series-parallel reaction. For more complicated reactions, reference should be made to the original publications (many of which have been listed by Doraiswamy and Sharma, 1984). 

Typically, the series-parallel reaction may be represented by (see Chapter 5) 

Case 1 Both steps are very slow or slow (regimes 1 and 2). 

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