Residue Curves and Distillation Boundaries

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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... 

Fig. 24, Ethanol dehydration Residue curves and distillation boundaries.

Instead of a -model or a group contribution method like modified UN I FAC also an equation of state or a group contribution equation of state can be used for the calculation of residue curves and distillation boundaries. In Figure 11.15, the results are shown for the ternary system carbon dioxide-hydrogen sulfide-ethane at 266.5 K using VTPR. As can be seen, two binary azeotropes and one distillation boundary is observed. 

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 ... 

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

Identify the azeotropes. Initially, it is very helpful to obtain estimates of the temperature, pressure, and composition of the binary, ternary,..., azeotropes associated with the C-component mixture. For all of the ternary submixtures, these can be determined, as described above, by preparing residue curve or distillation-line maps. When it is necessary to estimate the quaternary and higher-component azeotropes, as well as the binary and ternary azeotropes, the methods of Fidkowski et al. (1993) and Eckert and Kubicek (1997) are recommended. When the C-component mixture is the effluent from a chemical reactor, it may be helpful to include the reacting chemicals, that is, to locate any azeotropes involving these chemicals as well as the existence of reactive azeotropes. This information may show the potential for using reactive distillation operations as a vehicle for crossing distillation boundaries that complicate the recovery of nearly pure species. 

For the system acetone-chloroform-methanol, even four distillation regions are found. This is illustrated in Figure 11.14, where besides the Tx-behavior, the lines with separation factors equal to one ( y = 1), the stable and unstable nodes and saddles, the residue curves, and the distillation boundaries at atmospheric pressure are shown. 

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. 

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 ... 

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. 

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