Transient method isotopic tracers

A simple classification of the main macroscopic techniques is shown in Table 1, and this provides a useful framework for our review. Macroscopic measurements generally yield transport diffusivities, although variants of the techniques, using isotopically tagged tracers, can be devised to measure self-diffusivities. The large majority of the macroscopic techniques involve transient measurements. Steady-state or quasi-steady-state methods, notably membrane permeation and catalyst effectiveness measurements, have been demonstrated, but their application has been limited to a few systems. 

Equations (11) and (12) enable the generation of the total isotopic transient responses of a product species given (a) the transient response that characterises hypothesized catalyst-surface behaviour and (b) an inert-tracer transient response that characterises the gas-phase behaviour of the reactor system. Use of the linear-convolution relationships has been suggested as an iterative means to verify a model of the catalyst surface reaction pathway and kinetics. I This is attractive since the direct determination of the catalyst-surface transient response is especially problematic for non-ideal PFRs, since a method of complete gas-phase behaviour correction to obtain the catalyst-surface transient response is presently unavailable for such reactor systems.1 1 Unfortunately, there are also no corresponding analytical relationships to Eqs. (11) and (12) which permit explicit determination of the catalyst-surface transient response from the measured isotopic and inert-tracer transient responses, and hence, a model has to be assumed and tested. The better the model of the surface reaction pathway, the better the fit of the generated transient to the measured transient. 

---