Remixing section

The fractionation section of model uses the Peng-Robinson equation of state and the kinetic lumps directly as the fractionation lumps. The remixing section is a simple way to reconstruct the plant effluent since the reactor model produces separate streams for the hydrogen product and liquid product. Remixing the streams allow us to model the recontacting sections to predict compositions reported in the actual hydrogen product and liquid product streams. 

This remixing that occurs in both sequences of simple distillation columns is a source of inefficiency in the separation. By contrast, consider the prefractionator arrangement shown in Figure 11.9. In the prefractionator, a crude split is performed so that Component B is distributed between the top and bottom of the column. The upper section of the prefractionator separates AB from C, whilst the lower section separates BC from A. Thus, both sections remove only one component from the product of that column section and this is also true for all four sections of the main column. In this way, the remixing effects that are a feature of both simple column sequences are avoided4. 

However, the mass transfer coefficients found are clearly lower than those reported for distillation packings [26, 27]. This can be explained by the flow patterns in distillation packings, where the films constantly are disturbed and remixed, and therefore a completely developed laminar profile is never present. The mass transport is dominated by convection, not diffusion. It would be expected that remixing of the film layers, as accomplished by the stacking of monoliths (see Section 8.23) improves not only the RTD but also the mass transfer performance of monoliths. 

Parasitic Flow How often assumes a destructive role, despite efforts to prevent it. Convective currents frequently lead to the remixing of separated components. Electroosmotic flow (see Section 4.9) is destructive in some (not all) electrophoretic systems and is difficult to eliminate because of the ubiquitous presence of surface charges. These unintended and generally unproductive forms of flow can be termed parasitic flow. The following discussion serves to distinguish parasitic flows from nonparasitic flow processes. 

Papastathopoulou and Luyben, 1991). Basically, SSCs are convenient alternative schemes to sequences of simple columns when some limits on product purity are acceptable or when those limits correspond to a very low impurity content of light- and heavy-boiling components (Niesbach et al, 2013 Brambilla, 2014). The main idea comes from the composition profiles of the first column of the two sequences. In the direct sequence, the concentration of intermediate component B in the first column increases below the feed as that of the more volatile component A decreases (Brambilla, 2014). On the other hand, moving further down the column, the less volatile component increases in concentration. Therefore, the concentration of component B reaches a peak only to be remixed. At this highest point, one side stream is withdrawn from the vapor phase (Figure 9.3a) (Brambilla, 2014). The second column of the sequence is replaced by only one stripping section with limited separation capability between component B and less volatile component C. 

A further disadvantage is a certain transient of the positive vanadium ions from the compartment of the positive electrode to that of the negative electrode, caused by migration (cf. Section 1.3.3.2). It results in gradual concentration increase in the compartments of the negative electrodes and requires occasional remix of the two electrolytes. 

Figure 5.38 shows typical continuous catalyst regeneration (CCR) that we will use the build the model in question. We extensively discussed the features and operating issues associated with this type unit in Section 5.2. In the context of this chapter, we also build models for the remixing and hydrogen recontactor section of this flowsheet Tables 5.25 to 5.29 show some typical operating data for this unit... 

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