Residue curve maps and distillation region diagrams are very powerful tools for understanding all types of batch and continuous distillation operations, particularly when combined with other information such as liquid-liquid binodal curves. Applications include:
1. System visualization. Location of distillation boundaries, azeotropes, distillation regions, feasible products, and liquid-liquid regions.
2. Evaluation of laboratory data. Location and confirmation of saddle ternary azeotropes and a check of thermodynamic consistency of data.
3. Process synthesis. Concept development, construction of flowsheets for new processes, and redesign or modification of existing process flowsheets.
4. Process modeling. Identification of infeasible or problematic column specifications that could cause simulation convergence difficulties or failure, and determination of initial estimates of column parameters including feed-stage location, number of stages in the stripping and enriching sections, reflux ratio, and product compositions.
5. Control analysis/design. Analysis of column balances and profiles to aid in control system design and operation.
6. Process trouble shooting. Analysis of separation system operation and malfunction, examination of composition profiles, and tracking of trace impurities with implications for corrosion and process specifications.
Material balances for mixing or continuous separation operations are represented graphically on triangular composition diagrams such as residue curve maps or distillation region diagrams by straight lines connecting pertinent compositions. Overall flow rates are found by the inverse-lever-arm rule. Distillation material balance lines are governed by two constraints:
1. The bottoms, distillate, and overall feed compositions must lie on the same straight line.
2. The bottoms and distillate compositions must lie (to a very close approximation) on the same residue curve.
Since residue curves do not by definition cross separatrices, 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.
For a given multicomponent mixture, a single-feed distillation column can be designed with sufficient stages, reflux, and material balance control to produce separations ranging from the direct mode of operation (low-boiling node taken as distillate) to the indirect mode (high-boiling node taken as bottoms). The bow-tie shaped set of reachable compositions for single-feed distillation is roughly bounded by the material balance lines corresponding to the sharpest direct separation and the sharpest indirect separation possible. The exact shape of the reachable composition space is further limited by the requirement that the distillate and bottoms lie on the same residue curve [Wahnschafft, et al., Ind. Eng. Chem. Res., 31, 2345 (1992)]. Since residue curves are deflected by saddles, it is generally not possible to obtain a saddle product (pure component or azeotrope) from a simple single-feed column.
Consider the recovery of MIPK from an MEK-MIPK-water mixture. The bow-tie approximation of reachable compositions for several feeds are shown in Fig 13-61a and the exact reachable compositions are shown in Fig. 13-61b. From Feed F3, which is situated in a different distillation region than the desired product, pure MIPK cannot be obtained at all. With the upper edge of the bow-tie region for Feed F1 along the MEK-MIPK (water-free) face of the composition triangle, and part of the lower edge along the MEK-water (MIPK-free) face, there are conditions under which both the water in the bottoms MIPK product can be driven to low levels (high-product purity) and MIPK in
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