Regions deadwater

The following definition of deadwater regions by Levenspiel (L12) overcomes these diflBculties while still maintaining a concept of these regions which is useful in matching models with real situations. 

Instead of considering fluid in deadwater regions to be completely stagnant, an alternate view considers that there is a slow interchange or cross flow between the fluid in these regions and the active fluid passing through the vessel. With this approach Adler and Hovorka (A2) treated the combined model shown in Fig. 20. This consists of j identical units... 

Select a reasonable cutoff point, say 6 = 2, then find the mean of the C curve up to that point. If no deadwater regions are present, then... 

The fraction of vessel consisting of deadwater regions is given by the deviation of 6q from unity. Hence with Ei as the volume of deadwater region, and with Va/v as the fraction of active fluid as given by the area of the C curve up to the cutoff point. 

Fig. 21. Particular features of age distribution curves for combined models which include deadwater regions. Adapted from (L13).

Using the completely stagnant interpretation of deadwater regions, Fig. 23 illustrates some simple combined models and their tracer-response curves. In these models Vh, Vp, and Va stand for the volume of backmix, plug flow, and deadwater regions. If V is the volume of vessel we then have... 

The minimum active volume depends on feed location but not on impeller size. This suggests that the maximum size of deadwater region depends on the geometry of the vessel. 

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