Figure 20 contains a typical ESR transient for the 2-nitropropane system already discussed. It may be seen that the decay is approximately exponential, as might be predicted for first-order decay. In fact, the curve is not a perfect exponential because there is a contribution to the decay from convection of the radical out of the cavity. Obviously, this is most noticeable at fast flow rates and for slowly decaying radicals. Theory has been presented which describes the transient shape under any conditions.Conveniently, however, it was found that for all but the most stable radicals (fcj <5s ), a simple method of analysis was applicable. This involves treating the measured transients as if they were exponentials and deducing the effective first-order rate constants at the flow rates employed. These may then be extrapolated [Pg.378]

Now we can really see why the CSTR operated at steady state is so different from the transient batch reactor. If the inlet feed flow rates and concentrations are fixed and set to be equal in sum to the outlet flow rate, then, because the volume of the reactor is constant, the concentrations at the exit are completely defined for fixed kinetic parameters. Or, in other words, if we need to evaluate kab and kd, we simply need to vary the flow rates and to collect the corresponding concentrations in order to fit the data to these equations to obtain their magnitudes. We do not need to do any integration in order to obtain the result. Significantly, we do not need to have fast analysis of the exit concentrations, even if the kinetics are very fast. We set up the reactor flows, let the system come to steady state, and then take as many measurements as we need of the steady-state concentration. Then we set up a new set of flows and repeat the process. We do this for as many points as necessary in order to obtain a statistically valid set of rate parameters. This is why the steady-state flow reactor is considered to be the best experimental reactor type to be used for gathering chemical kinetics. [Pg.390]

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