Distillation boundary curves

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

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...

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. 

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

Sketch the distillation line map (residue curve map) for the system ethanol-ethyl acetate-methanol at 1 atm and 5 atm from the data in Table 12.1. Does the system have a distillation boundary Is the position of the boundary sensitive to pressure ... 

Distillation into curved boundary, general separation heuristics for, 22 318 Distillation lines (residue curve maps), 8 790-793... 

To a first approximation, the composition of the distillate and bottoms of a single-feed continuous distillation column lies on the same residue curve. Therefore, for systems having separatrices and multiple regions, distillation composition profiles are also constrained to lie in specific regions. The precise boundaries of these distillation regions are a function of reflux ratio, but they are closely approximated by the RCM separatrices. If a separatrix exists in a system, a corresponding distillation boundary also exists. Also, mass balance constraints require that the distillate composition, the bottoms composition, and the net feed composition plotted on an RCM for any feasible distillation be collinear and spaced in relation to distillate and bottoms flows according to the well-known lever rule. 

The pure products A and B are separated by a distillation boundary and belong to different distillation fields. In this case, the distillation boundary must be sufficiently curved, such that one split can cross it and gives opportunities for separating the other pure component. 

Figure 3.14 Separation regions with a curved distillation boundary.

The selection of an entrainer with boundary crossing is based on the observation that in a RCM both A and B must be nodes, stable or unstable. By consequence the pure components are separated either as overhead or bottom products. Table 3.17 gives a list of recommended heuristics [13, 14]. In all cases, tbe distillation boundary must be highly curved, although how curved cannot be specified theoretically. In this case the simplest entrainer choice is a low-boiler entrainer for minimum AB azeotrope, and a high-boiler entrainer for maximum AB azeotrope, again not easy to meet in practice. 

The kinetics of a reaction rate has a substantial influence on residue curve maps. Distillation boundaries and physical azeotropes can vanish, while other singular points due to kinetic effects might appear. The influence of the kinetics on RCM can be studied by integrating Eq. (A. 10) for finite Da numbers. In addition, the singular points satisfy the relation ... 

Summing up, the influence of the kinetics of a chemical reaction on the vapor-liquid equilibrium is very complex. Physical distillation boundaries may disappear, while new kinetic stable and unstable nodes may appear. As result, the residue curve map with chemical reaction could look very different from the physical plots. As a consequence, evaluating the kinetic effects on residue curve maps is of great importance for conceptual design of reactive distillation systems. However, if the reaction rate is high enough such that the chemical equilibrium is reached quickly, the RCM simplifies considerably. But even in this case the analysis may be complicated by the occurrence of reactive azeotropes. 

Since residue curves do not cross simple batch distillation boundaries, the distillate and bottoms compositions must be in the same distillation region with the mass balance line intersecting a residue curve in two places. Mass balance lines for mixing and for other separations not involving vapor-liquid equilibria, such as extraction and decantation, are of course not limited by distillation boundaries. 

As an example. Van Dongen (Ph.D. Thesis, University of Massachusetts, 1983) considered the separation of a methanol-methyl acetate mixture, which forms a homogeneous azeotrope, using n-hexane as an entrainer. The distillation boundaries for this system (Fig. 13-87a) are somewhat curved. A separation sequence that exploits this boundary curvature is shown in Fig. 13-87b. Recycled methanol—methyl acetate binary azeotrope and methanol-methyl acetate—hexane ternary azeotrope are added to the original feed FO to produce a net feed com-... 

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