Figure 19. Global Bodenstein numbers reported for ALRs as a whole unit. See references in Table 4.

this combination for a particular reactor. This value thus has some limitations for extrapolation to other configurations or scales. Because each section has different mixing characteristics (it may have a different cross-section, flow configuration, etc.), from a strictly engineering point of view, the mixing in each of the sections of the ALR should be defined and considered separately.

One way of obtaining information on the mixing characteristics of each of the regions of the ALR is the simultaneous measurement of the response in the ALR at several points, so that after one single pulse injection the response of each section in the loop can be obtained (37,80). This method of measurement has the advantage that multiple measurements are made for the same tracer injection experiment. This enables us to check the consistency of the liquid velocity results obtained, since independent mea surements can obtained in the same run, as can be seen in Figure 20.

The results for Bo obtained by Verlaan (80) in an external-loop ALR are shown in Figure 21 (the much higher Bo in the downcomer was explained by the fact that the data were obtained under conditions of complete gas disengagement so that only liquid flowed). The results indicated that most of the mixing took place in the gas separator, and both riser and downcomer could be considered as plug-flow conduits. The same conclusions were drawn for internal-loop ALRs by Merchuk et al. (37) by analysis of mixing times: the shift in results for mixing times obtained with different probes in response to the same impulse perturbation indicated that the riser and the down-comer were introducing a pure delay in the response, that is, that they behaved as plug-flow sections.

Indeed, the simplest model of an ALR, from the point of view of fluid dynamics, is a combination of two plug-flow reactors, representing the riser and the downcomer, and a perfectly stirred tank, representing the gas separator (Fig.

22) (67,123). Merchuk and Yunger (67) showed that this simple model could take into account the mixing in the ALR. The validity of using this simple model as a first approximation is supported by experimental evidence that shows that the mixing time decreases when the separator volume increases (31,124). When the volume of the separator is increased substantially without changing the reactor diameter, the gas separator becomes tall and slender and may depart from total mixing behavior. It has been reported that under such conditions the circulation time measured from two successive peaks of a probe is not influenced by changes in the top clearance (42,43). This indicates that the gas separator acts as two interconnected well-mixed regions, one of them (the lower one in Figure

23) being the link between riser and downcomer, as shown schematically in Figure 23.

Brew Your Own Beer

Brew Your Own Beer

Discover How To Become Your Own Brew Master, With Brew Your Own Beer. It takes more than a recipe to make a great beer. Just using the right ingredients doesn't mean your beer will taste like it was meant to. Most of the time it’s the way a beer is made and served that makes it either an exceptional beer or one that gets dumped into the nearest flower pot.

Get My Free Ebook

Post a comment