Chain reaction or degradation reaction

　　The reaction scheme we are considering is A-X-Y. Here, the desired product is substance X, while substances like Y are undesirable. The transformation of X into Y is a “degradation reaction,” a phenomenon that is quite common in organic chemistry. For example, such reactions often occur during the production of nitrobenzene from benzene, where there is a tendency to form di-substituted and even tri-substituted compounds. An important example of a non-substituted degradation reaction is the oxidation of methanol to formaldehyde: the useful product formed here can further react to yield CO2: CH3OH → HCHO → CO2. If we consider a batch reactor operating under high pressure, and start by adding substance A, it becomes clear that at some optimal time t, the concentration of the desired product will reach a maximum—meaning that when the reaction time equals t, a certain number of A molecules will have been converted into X molecules. Therefore, relative to the amount of reactant initially introduced into the system, in order to achieve the maximum yield of X, the reaction process should last neither too long nor too short. In any type of continuous-flow reactor, there will inevitably be a residence-time distribution. Thus, even if the average residence time can be set equal to t, the residence times experienced by individual fluid elements within the system will vary—some will be longer than the optimal time, while others will be shorter. As we have seen, the residence-time distribution alone is generally insufficient to determine the conversion rate; however, equations can still be used to approximate the influence of the residence-time distribution. Such a distribution typically reduces the maximum yield that can be obtained for the intermediate product X. Clearly, in tubular reactors that closely approximate plug flow, the yields achieved will not be significantly lower than those obtained in batch reactors. Indeed, tubular reactors of this type are widely used in practice for many degradation reactions, such as the oxidation of methanol catalyzed by silver and the production of chloroethane.