A Stripping Cascade

In a stripping cascade, liquid is fed into the top. Liquid out the bottom of the cascade is partially vaporized and the vapor is recycled into the bottom of the cascade. The vapor stream at the top represents the second product stream. 

Substituting this result into (63), the operating line for a stripping cascade becomes ... 

Comment 1 Notice that the feed line for a stripping cascade is vertical, whereas the feed line for a rectifying cascade was horizontal (see page 75). This is because the feed... 

The combination of an absorption cascade topped by a condenser is referred to as an enricher. A partial reboiler topped by a stripping cascade is referred to as an exhauster. As shown in Fig. 12.25 stages for an enricher are numbered from the top down and the overhead product is distillate, while for an exhauster stages are numbered from the bottom up. Feed to an enricher is vapor, while feed to an exhauster is liquid. The recovery equations for an enricher are obtained from (12-64) by making the following substitutions, which are obtained from material balance and equilibrium considerations. 

Our example system has a flow-controlled feed, and the reboiler heat is controlled by cascade from a stripping section tray temperature. Steam is the heating medium, with the condensate pumped to condensate recovery. Bottom product is pumped to storage on column level control overhead pressure is controlled by varying level in the overhead condenser the balancing line assures sufficient receiver pressure at all times overhead product is pumped to storage on receiver level control and reflux is on flow control. 

Normally, a number of stages of extraction work in a countercurrent cascade, with mixer-settlers. A generalized flowsheet for the Am ex process is shown in Figure 5.34. Stripping may be implemented by any one of the reagents, as indicated in the figure. Alternatively,... 

Stripping can equally be done in single-stage, cross-flow, or counter-current systems. To illustrate how the overall concepts remain valid, the performance of a countercurrent cascade accepting as feed the scrubbed extract... 

A Two-stage stripping cascade B Two-stage enriching cascade... 

Beyond the expected changes in flowrates at the feed tray and at the top, there is a continuous drop in flowrate of both streams as we move down either the rectifying cascade or the stripping cascade. These changes in flowrates within a cascade are not consistent with the equiniolal overflow assumption. 

Some extraction systems are such that the solvent and diluent phases are almost completely immiscible in each other. Hence, separation yields an extract phase essentially free of diluent and a raffinate phase that is almost pure diluent. This greatly simplifies the characterization of the system. When partial miscibility for an extraction process is very low, the system may be considered immiscible and application of McCabe-Thiele analysis is appropriate. It is important to note that McCabe-Thiele analysis for immiscible extraction applies to a countercurrent cascade. The McCabe-Thiele analysis for immiscible extraction is analogous to the analysis for absorption and stripping processes. Consider the flow scheme shown in Figure 5.23,... 

This is the equation usually used to estimate the start-up time of a separation cascade. In most cases, it overestimates the time somewhat, because tw for a plant without stripping section is usually negative. 

Figure 6.39 Purification of sour gas by a two-stage stripping cascade.27...

The determination of the singular points appearing in these maps yields important information about the attainable bottom product compositions in real counter-current columns. However, as shown by Chadda et al. [3], both the distillate and the bottom product compositions can be better obtained as singular points of a reactive enriching flash cascade or a stripping flash cascade, respectively. As will be shown, singular point analysis can also provide valuable information about the role of interfacial mass-transfer resistances in RD processes. 

Fig. 6.7 A CO current flash cascades arrangement. The top half is the rectifying cascade and the bottom half is the stripping cascade...

Equation (6.14) can be solved recursively for given values of the parameters,

starting with the initial condition, a , = Ap. The solution is a trajectory of liquid compositions for the stripping cascade. 

Solutions of (6.14) and (6.15), the rectifying and stripping cascade flash trajectories, can be represented in mole fraction space (three dimensional for the IPOAc system). However, we represent the solutions in transformed composition space, which is two dimensional for IPOAc system (for a derivation and properties of these transformed variables [46]). This transformed composition space is a projection of a three dimension mole fraction space onto a two dimensional transformed composition subspace for the IPOAc system. Even though the correspondence between real compositions and transformed compositions is not one-to-one in the kinetic regime, we will make use of these transforms because of ease of visualization of the trajectories, and because overall mass balance for reactive systems (kinetically or equilibrium limited) can be represented with a lever rule in transformed compositions. We use this property to assess feasible splits for continuous RD. 

Fig. 6.8 Rectifying and stripping cascade tra- formed mole fraction space. For the stripping jectories for a saturated liquid equimolar reac- cascade, X, = Xhoac + tipoAo 2 = + Xipoac...

Note that calculating the flash trajectories at (f> = 0.5 does not provide the entire feasible product regions for continuous RD, but instead generates a subset of the feasible products. Selecting an iterate on the stripping cascade trajectory as a potential bottoms and an iterate on the rectifying cascade trajectory as a potential distillate does not guarantee that these products can also be obtained simultaneously from a RD column. This is simply because these product compositions may not simultaneously satisfy the overall mass balance for a reactive column. However, when the flash trajectories are used in conjunction with the lever rule for a continuous reactive column, the feasible splits for continuous RD can be quickly predicted. 

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