Distillation boundary

The existence of azeotropes can introduce limits on the ability to separate components using distillation. These limitations are called distillation boundaries. They separate regions of feasible separations. Feed streams with compositions located in one region can produce certain products, while feed compositions in other regions will produce other products (different compositions of the distillate and bottoms streams). 

We return to this complex system in Chapter 5 and develop a separation scheme for producing high-purity ethanol (upper corner in Fig. 1.31). 

The basics of vapor-liquid phase equilibrium have been reviewed in this chapter. A good understanding of VLB is indispensable in the design and control of distillation systems. These basics will be used throughout this book. 

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Fig. 2. Methyl ethyl ketone (MEK)—methyl isopropyl ketone (MIPK)—water system where A1 and A2 represent two different a2eotropes FI, F2, and F3, different feed compositions and Y)n the corresponding bottoms and distillates, respectively (—), the distillation boundary and (B), the reachable compositions for the various feeds (a) approximate bow-tie and (b) exact reachable compositions.

Fig. 3. Combined residue curve and phase equilibria diagrams for the systems where Al—4 represent a2eotropes, (—) is the distillation boundary, the...

Bina System. The first task is to examine the characteristics of the 2-propanol-water-phase equiUbria (VLE, LLE, SEE) to determine the compositions of interest and any critical features. 2-Propanol forms a minimum boiling a2eotrope with water (80.4°C at 101.3 kPa (760 tort), 68 mol % 2-propanol). The a2eotrope is between the feed and the IPA product and is a distillation boundary, thus it is impossible to obtain both desired products from any single-feed... 

Table 3 contains strategic separations to be considered for crossing distillation boundaries. Many of these can be eliminated after examining the pertinent physical properties and equiUbrium behavior (see Table 4) and referring to the general separation considerations. The results are summarized in Table 5. 

Strategies from third section of Table 3, Overcoming distillation boundaries. 

The feed compositions and products of each of these strategic separations remain ill-defined. The unspecified 2-propanol—water mixture, the input to each strategic separation, could be but is not necessarily the original feed composition. The MSA composition (pure hexane in this case) is such that one of the products of the strategic separation is in region II, ie, the strategic separation crosses the distillation boundary. Two opportunistic distillations from... 

Selection of Fractionator 11 gives pure hexane, which can be recycled to Mixer 1. The distillate Dll, however, is a problem. It cannot be distilled because of its location next to a distillation boundary. It is outside of the two-phase region, so it cannot be decanted. In essence, no further separations are possible. However, using the Recycle heuristics, it can be mixed into the MSA recycle stream without changing the operation of Mixer 1 appreciably. However, as both outlet streams are mixed together. Fractionator 11 is not really needed. The mixture of hexane and isopropanol, 07, could have been used as the MSA composition in the first place. 

Fig. 3. Residue curve map for a ternary mixture with a distillation boundary mnning from pure component D to the binary azeotrope C.

Distillation boundaries for continuous distillation are approximated by simple distillation boundaries. This is a very good approximation for mixtures with nearly linear simple distillation boundaries. Although curved simple distillation boundaries can be crossed to some degree (16,25—30,32,33), the resulting distillation sequences are not normally economical. Mixtures such as nitric acid—water—sulfuric acid, that have extremely curved boundaries, are exceptions. Therefore, a good working assumption is that simple distillation boundaries should not be crossed by continuous distillation. In other words, for a separation to be feasible by distillation it is sufficient that the distillate and bottoms compositions He in the same distillation region. 

As an example, consider the residue curve map for the nonazeotropic mixture shown in Eigure 2. It has no distillation boundary so the mixture can be separated into pure components by either the dkect or indkect sequence (Eig. 4). In the dkect sequence the unstable node (light component, L) is taken overhead in the first column and the bottom stream is essentially a binary mixture of the intermediate, I, and heavy, H, components. In the binary I—H mixture, I has the lowest boiling temperature (an unstable node) so it is recovered as the distillate in the second column and the stable node, H, is the corresponding bottoms stream. The indkect sequence removes the stable node (heavy component) from the bottom of the first column and the overhead stream is an essentially binary L—I mixture. Then in the second column the unstable node, L, is taken overhead and I is recovered in the bottoms. 

Fig. 10. Residue curve map for separating a maximum boiling azeotrope using a high boiling solvent where (-----------------) represents the distillation boundary and...

Only a fraction of the known azeotropes are sufficientiy pressure-sensitive for the conventional pressure-swing distillation process to work. However, the concept can be extended to pressure-insensitive azeotropes by adding a separating agent which forms a pressure-sensitive azeotrope and distillation boundary. Then the pressure is varied to shift the location of the distillation boundary (85). 

Sy.stem visualization. Location of distillation boundaries, azeotropes, distillation regions, feasible products, and liquid-hquid regions. 

Exploitation of Boundary Curvature A second approach to boundaiy crossing exploits boundaiy curvature in order to produce compositions in different distillation regions. When distillation boundaries exhibit extreme curvature, it may be possible to design a column such that the distillate and bottoms are on the same residue curve in one distillation region, while the feed (which is not required to lie on the column-composition profile) is in another distillation region. In order for such a column to meet material-balance constraints (i.e., bottom, distillate, feed on a straight hne), the feed must be located in a region where the boundary is concave. 

FIG. 13-67 Separation of methanol-methyl acetate by exploitation of distillation boundary curvature. 

Figure 12.13 The residue curve maps can be divided into regions by distillation boundaries.

One extremely powerful feature of heterogeneous distillation is the ability to cross distillation boundaries. It was noted previously that distillation boundaries divide the compositions into two regions that cannot be accessed from each other. Decanters allow distillation boundaries to be crossed, as illustrated in Figure 12.32. The feed to the decanter at F is on one side of the distillation boundary. This splits in the decanter to two-liquid phases E and R. These two-liquid phases are now on opposite sides of the distillation boundary. Phase splitting in this way is not constrained by a distillation boundary, and exploiting a two-phase separation in this way is an extremely effective way to cross distillation boundaries. 

Figure 12.32 Phase splitting can be used to cross distillation boundaries.

The distillation boundary must be curved as shown in Figure 12.35. However, even if there is very significant curvature across the boundary, a column placed like the one in Figure 12.35 is going to be highly constrained in its operation. 

All of the discussions so far regarding distillation lines, residue curves and distillation boundaries have assumed equilibrium behavior. Real columns do not work at equilibrium, and stage efficiency must be accounted for. Each component will have its own stage efficiency, which means that each composition will deviate from equilibrium behavior differently. This means that if nonequilibrium behavior is taken into account, the shape of the distillation lines, residue curves and distillation... 

Thus, while it is possible in theory to cross a curved distillation boundary as shown in Figure 12.35, it is generally more straightforward to follow designs that will be feasible over a wide range of reflux ratios and in the presence of uncertainties. Such designs can be readily developed using distillation line and residue curve maps. 

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