Comparison of volume- and surface-detention-time distributions

The main process parameter for flow reactors (and in many cases also storage reservoirs) is the detention time. In technical practice the mean value of this parameter is commonly used. This version of the detention time results from the simplified plug-flow-model, which describes the real course of considered fluid-flow-phenomena only to a low degree. In order to obtain more precise characteristics of the system, one should make use of the so-called extrinsic distribution of detention time. Unfortunately, determination of such a curve for the whole reservoir (volume-detention-time) is rather expensive and time-consuming. Preceding this, a very attractive alternative is given by the surface-detention-time, which is determined for the upper layer of the liquid, close to the free surface. Investigation of the equivalence of both these possibilities is the subject of this paper.