Recirculation Ratio a

Figure 16.7 Effect of recirculation ratio on the ability of a packed tower of fixed size to remove substrate. The recirculation of effluent is from a perfect clarifier. The values of the kinetic parameters, stoichiometric coefficients, and system variables are given in Table 16.1 unless otherwise specified.

Although the model results discussed above show that recirculation of clarified effluent will decrease substrate removal, circumstances exist in which recirculation could increase it. For example, if the feed substrate concentration was so high that oxygen transfer limited substrate removal, recirculation could decrease the problem by reducing the reaction rate and increasing the oxygen transfer rate. Furthermore, the presence of biomass in the recirculated flow can have an impact. The results in Figures 16.6 and 16.7 were obtained by assuming that the settler was perfect so that no biomass was present in the recirculation flow. It is possible, however, that if biomass had been present the reaction term for substrate removal by suspended organisms would have been large enough to make the effluent substrate concentration lower than it was without recirculation." Thus, while it is true that recirculation generally reduces substrate removal through packed towers, one must not conclude that the effects of recirculation are always negative. Rather, each situation must be evaluated on a case by case basis.

During design of a packed tower for a given feed flow rate, an engineer may choose any cross-sectional area that gives a THL that is acceptable for the media under consideration. However, because of the impacts of THL on tower performance, different cross-sectional areas will require towers of different depth. These effects are illustrated in Figure 16.8 for a tower without recirculation. An increase in the cross-sectional area results in an increase in the surface area of biofilm per unit length of tower, and thus it is apparent that an increase in cross-sectional area should decrease the depth of tower required to remove a given fraction of substrate. However, because the increase in cross-sectional area also decreases the THL, the decrease in depth is not in proportion to the increase in the area so that the net result is an increase in tower volume. This happens because the decrease in THL decreases the external mass transfer coefficient, thereby decreasing the overall effectiveness factor. The results in Figure 16.8 suggest that the total media volume will be minimized by choosing a tall, thin tower rather than a short, fat one. Because a similar conclusion has been reached with other models,"1217 as well as experimentally,1" 2" it appears to be general and would be expected to be true for other parameter values as well. It should be recognized, however, that the decrease in tower volume associated with increased tower depth may be less than shown in Figure 16.8 for other parameter values. Consequently, from a practical perspective, the effects of cross-sectional area and tower depth may not be significant in some situations.12

The model results presented herein are very useful for understanding the fundamental characteristics of packed towers. The complexity of such models and the difficulty of evaluating the parameters in them have prevented their widespread use in practice. Rather, most design of packed towers is based on empirical, rather than mechanistic, models. Consequently, in Section 16.4 we will briefly introduce such models. First, however, we need to consider several factors not included in the model presented here.

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