HOLDUP AND DISPERSION - TRACER RESIDENCE TIMES, MOMENTS AND INVENTORYMEASUREMENTS

The inventory function is the quantity of tracer remaining in a contin uous-flow system at elapsed time t when steady flow of the tracer is r eplaced by untraced flow at t = 0. The relations between residence-tim e distributions, moments and changes in inventory when a tracer is flu shed from a system are established. It is shown that inventory measure ments could be an attractive way of measuring moments. In particular, the mean residence time is given by the intercept on the baseline of t he initial tangent to the inventory curve, and the variance by the are a between the inventory curve, the initial tangent and the baseline. I t is proposed that dispersion be defined in terms of the variance of t he residence-time distribution. This would allow experimentalists to r ecord their results independently of models or theories in addition to comparing their results with the predictions of theories. Methods bas ed on inventory measurements are potentially more accurate than the tr aditional step- and pulse-response methods. Ways in which inventory me asurements might be made are suggested. It is timely that the theory s hould be presented now because tomographic methods that could be used to measure inventory are starting to appear.