SEGREGATION AND MAXIMUM MIXEDNESS

The difference between complete segregation and maximum mixedness is largest when the reactor is a stirred tank and is zero when the reactor is a PFR. Even for the stirred tank case, it has been difficult to find experimental evidence of segregation for single-phase reactions. Real CSTRs approximate perfect mixing when observed on the time and distance scales appropriate to industrial reactions, provided that the feed is premixed. Even with unmixed... 

Only scattered and inconclusive results have been obtained on the relative performances of the different models as converters. In problems P5.08.13 and 22, dispersion gives higher conversion than segregation in problems P5.08.17 and 21 they are about the same in problem P5.08.20, dispersion falls in between segregation and maximum mixedness. 

To answer this question we will model a real reactor in a number of ways. We shall classify each model according to the number of adjustable parameters that are extracted from the RTD data (see Table 13-1). In this chapter we discuss only the segregation and maximum mixedness models. Other models are discussed in Chapter 14. 

The bounds on the conversion are found by calculating conversions under conditions of complete segregation and maximum mixedness. 

If tracer tests are carried out isothermally and then used to predict nonisothermal conditions, one must couple the segregation and maximum mixedness models with the energy balance to account for variations in the specific reaction rate. For adiabatic operation and ACf> = 0,... 

Assuming that E(t) is unaffected by temperature variations in the reactor, one simply solves the segregation and maximum mixedness models, accounting for the variation of k with temperature [i.e., conversion see Problem P13-2(h)]. 

Comparison cf Conversionfor Segregation and Maximum Mixedness Model fbr Conversion for Reaction Orders Between 0 and 1... 

Tanks-iti-Series Versus Segregation for a First-Order Reaction We have already stated that the segregation and maximum mixedness models are equivalent for a first-order reaction. The proof of this statement was left as an exercise in Problem P13-3b, Wc now show the tanks-in-series model and the segregation models are equivalent for a first-order reaction. 

The states of complete segregation and maximum mixedness represent limits on the extent of micromixing that is possible with a given RTD. In complete segregation, molecules that enter together stay together. They are surrounded by molecules having... 

The difference between complete segregation and maximum mixedness is largest when the reactor is a stirred tank and is zero when the reactor is a PER. Even for the... 

Comparison of the asymmetric distribution results with the bimodal distribution results for both the segregation and maximum mixedness models yield similar outcomes except the values are slightly different. 

For the segregation and maximum mixedness models, Sef is much lower than for the CSTR but still far greater than for the PFR. The CSTR and PFR values are unchanged as they do not depend on E(t). 

One of these approaches is to use the exit-age distribution directly. For ideal reactors this will allow determination of limits between which the actual conversion must lie. These two limits are, of course, those of complete segregation and maximum mixedness as described in Chapter 4. For nonideal reactors one follows the same procedure employing the experimentally determined exit-age distribution. 

The residence time distribution measures features of ideal or nonideal flows associated with the bulk flow patterns or macromixing in a reactor or other process vessel. The term micromixing, as used in this chapter, applies to spatial mixing at the molecular scale that is bounded but not determined uniquely by the residence time distribution. The bounds are extreme conditions known as complete segregation and maximum mixedness. They represent, respectively, the least and most molecular-level mixing that is possible for a given residence time distribution. 

---