Sharp splits sections

Generally there is also a pinch in the stripping section of the tower. For sharp splits, the temperature at this second pinch will be the same. We can then derive a second equation like (103) by performing our balances about the bottom n stages of the column to obtain a second equation. 

Figure 18.5a illustrates an important consideration for sharp splits. A top section control tray (e.g., tray 15) will adequately control both top and bottom product compositions for plots 2 to 6. For plots 1 and 8 to 10, this control tray will retain the top product on-spec, but will permit wide variations of bottom product composition. Conversely, a control tray located in the bottom section (e.g., tray 30) will give good control of the bottom composition at all plots. It will also give good control... 

In sharp splits such as that shown in Fig. 18.5, neither a top section temperature controller nor a bottom section temperature controller will be capable of adequately a controlling both product purities over the entire operating range. In most cases, one of the two products is selected as the more important, and the control tray is located in the section from which this product exits. The other product purity is allowed to vary. Alternatively, an average temperature control scheme can be used (58, 59, 68) and effectively overcome the problem. This is described in the next section. 

Sharp splits. Boyd (58, 59) developed a double differential temperature control scheme, which is essentially an average temperature control scheme that compensates for pressure and differential pressure variations. The scheme overcame the problem described in the previous section and was demonstrated to maintain tight control on both... 

For the special case of sharp splits, scenario (c) is known as the double-feedpinch as this mutual pinch point will occur at the feed composition (illustrated in Section 5.4.1). These three cases are dependent on the product and feed compositions one chooses, and are illustrated in Figure 5.14a c. 

At this moment in time it may seem rather senseless to understand the behavior of these FP regions. It will however be shown in the sections that follow, specifically in Section 7.4 on sharp split Petlyuk design using TTs, that FP3 is the optimal FP. This may be different in other systems with nonideal behavior, but nevertheless, it is important to know how these behave and interact with one another under certain conditions. 

Figure 4.2. Location of reversible section trajectories of an ideal four-component mixture at sharp split and liquid-vapor tie-line of the feed point (xp - yp).

Now we can introduce the new important notion - sharp split region for column section at set sharp split (i.e., at fixed set of the components present i and absent jin... 

We call for the top section the region, in points of which phase equilibrium coefficients of the components present in the product (xdj > 0) are greater than those of the components absent xdj — 0) sharp split region Re l,.(KDi > Kdj), and vice versa for the bottom section (Kbi < Kbj). 

As far as Eqs. (5.15) and (5.16) should be valid for the points of trajectory located inside concentration simplex, the whole trajectory of the section from tear-off point to point of junction with the trajectory of the second section should be located in one region of section sharp split Reg or Reg. For each sharp split region, Reg or Reg consist of quite definite component-order regions... 

Regard-Therefore, for different splits section sharp regions, Reg and Reg are different. For splits without distributed components, sharp split regions of both sections coincide with each other Re J,. = Reg (Fig. 5.9). 

For splits with distributed components, sharp split regions of two sections are different (Fig. 5.10). Reversible distillation regions Regf and RegJ, which are discussed in Chapter 4, are a particular case of sharp split regions (in this case, component h is absent in overhead and component / is absent in bottom). 

Figure 5.10. Component-order regions Reg, trajectory tear-off regions Reg and Re , and sharp split regions Reg and Reg (hatched for bottom section and shaded for top section, hatched and shaded for two section) for the acetone(l)-chloroform(2)-methanol(3) mixture for splits (a)l,3 1,2 (Reg / and Reg j ) and (b) 2,3 1,2 (Reg ...

If the split without distributed components is set, then the mentioned trajectory tear-off regions of sections should be boundary elements of one sharp split region... 

Let s note that the top product point cannot be located at sides 2-3 and 1-3 because Eq. (5.15) is not vahd for these sides (i.e., Xo i [2-3] and xd [1-3]). Let s also note that ihc separatrix sharp split region of section trajectories bundle... 

Previously, we discussed sharp splits in the column section, which are important theoretical abstraction useful at the solution of the main questions of optimal designing of distillation units. 

Figure 5.24 shows the evolution of separatrix sharp split region of top section... 

L/ K) on D,2- Separatrix sharp split region for rectifying section Reg shaded. 

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