Info

butyl acetate

Minimum boiling azeotrope

Self-entraining

Pinched, azeotropic system

Self-entraining

Minimum boiling azeotrope

Toluene, methyl isobutyl

ketone

Minimum-boiling azeotropes

Self-entraining

Minimum-boiling azeotrope

Benzene

Minimum-boiling azeotrope

Self-entraining

The choice of the appropriate azeotropic distillation method and the resulting flowsheet for the separation of a particular mixture are strong functions of the separation objective. For example, it may be desirable to recover all constituents of the original feed mixture as pure components, or only some as pure components and some as azeotropic mixtures suitable for recycle. Not every objective may be obtainable by azeotropic distillation for a given mixture and portfolio of candidate entrainers.

Exploitation of Homogeneous Azeotropes Homogeneous azeotropic distillation refers to a flowsheet structure in which azeotrope formation is exploited or avoided in order to accomplish the desired separation in one or more distillation columns. The azeotropes in the system either do not exhibit two-liquid-phase behavior or the liquid-phase behavior is not or cannot be exploited in the separation sequence. The structure of a particular sequence will depend on the geometry of the residue curve map or distillation region diagram for the feed mixture-entrainer system. Two approaches are possible:

1. Selection of an entrainer such that the desired products all lie within a single distillation region (the products may be pure components or azeotropic mixtures).

2. Selection of an entrainer such that although the desired products lie in different regions, some type of boundary-crossing mechanism is employed.

As mentioned previously, ternary mixtures can be represented by 125 different residue curve maps or distillation region diagrams. However, feasible distillation sequences using the first approach can be developed for breaking homogeneous binary azeotropes by the addition of a third component only for those more restricted systems that do not have a distillation boundary connected to the azeotrope and for which one of the original components is a node. For example, from

System

Type

Entrainer(s)

Remark

Exploitation of homogeneous azeotropes

No known industrial examples

Exploitation of pressure sensitivity

THF-water

Methyl acetate-methanol

Alcohol-ketone systems Ethanol-water

Hydrochloric acid-water Nitric acid-water

Exploitation of pressure sensitivity

Azeotropic Curve Ethanol Water

Exploitation of boundary curvature

Hydrochloric acid-water Nitric acid-water

Exploitation of boundary curvature

Alternative to salt extractive distillation Alternative to salt extractive distillation

Alternative to extractive distillation Element of recovery system for alternative to production of methyl acetate by reactive distillation; alternative to azeotropic, extractive distillation

Alternative to extractive distillation, salt extractive distillation, heterogeneous azeotropic distillation; must reduce pressure to less than 11.5kPa for azeotrope to disappear

Alternative to salt extractive distillation Alternative to salt extractive distillation

Exploitation of azeotropy and liquid phase immiscibility

Ethanol-water

Acetic acid-water

Butanol-water

Acetic acid-water-vinyl acetate Methyl acetate-methanol

Diethoxymethanol-water-ethanol

Pyridine-water

Hydrocarbon-water

Minimum boiling azeotrope Pinched system

Minimum boiling azeotrope Pinched, azeotropic system Minimum boiling azeotrope

Minimum-boiling azeotropes Minimum-boiling azeotrope Minimum-boiling azeotrope

Cyclohexane, benzene, heptane, hexane, toluene, gasolene, diethyl ether Ethyl acetate, propyl acetate, diethyl ether, dichloroethane, butyl acetate Self-entraining Self-entraining Toluene, methyl isobutyl ketone

Self-entraining

Benzene

Self-entraining

Alternative to extractive distillation, pressure-swing distillation

Element of recovery system for alternative to production of methyl acetate by reactive distillation; alternative to extractive pressure-swing distillation

Alternative to extractive distillation

Fig. 13-59, the following eight residue curve maps are suitable for breaking homogeneous minimum-boiling azeotropes: DRD 002, 027, 030, 040, 051, 056, 060, and 061 as collected in Fig. 13-64«. To produce the necessary distillation region diagrams, an entrainer must be found that is either: (1) an intermediate boiler that forms no azeo-tropes (DRD 002), or (2) lowest boiling or intermediate boiling and forms a maximum-boiling azeotrope with the lower-boiling original component. In these cases, the entrainer may also optionally form a minimum-boiling azeotrope with the higher boiling of the original components or a minimum-boiling ternary azeotrope. In all cases, after the addition of the entrainer, the higher-boiling original component is a node and is removed as bottoms product from a first column operated in the indirect mode with the lower-boiling original component recovered as distillate in a second column.

The seven residue curve maps suitable for breaking homogeneous maximum-boiling azeotropes (DRD 028, 031, 035, 073, 078, 088, 089) are shown in Fig. 13-64b. In this case, the entrainer must form a minimum-boiling azeotrope with the higher-boiling original component and either a maximum-boiling azeotrope or no azeotrope with the lower-boiling original component. In all cases, after the addition of the entrainer, the lower-boiling original component is a node and is removed as distillate from a first column operated in the direct mode with the higher-boiling original component recovered as bottoms product in a second column.

FIG. 13-64 Feasible distillation region diagrams for breaking homogeneous binary azeotrope A-B. («) Low-boiling entrances.

FIG. 13-64 Feasible distillation region diagrams for breaking homogeneous binary azeotrope A-B. («) Low-boiling entrances.

Extractive Distillation Schematic

FIG. 13-64 (Continued ) Feasible distillation region diagrams for breaking homogeneous binary azeotrope A-B. (b) Intermediate-boiling entrances.

FIG. 13-64 (Continued ) Feasible distillation region diagrams for breaking homogeneous binary azeotrope A-B. (b) Intermediate-boiling entrances.

In one sense, the restrictions on the boiling point and azeotrope formation of the entrainer act as efficient screening criteria for entrainer selection. Entrainers that do not show appropriate boiling-point characteristics can be discarded without detailed analysis. However, in another sense, although theoretically feasible, the above sequences suffer from serious drawbacks that limit their practical application. DRD 002 requires that the entrainer be an intermediate-boiling component that forms no azeotropes. Unfortunately these are often difficult criteria to meet, as any intermediate boiler will be closer-boiling to both of the original components and, therefore, will be more likely to be at least pinched or even form azeotropes. The remaining feasi ble distillation region diagrams require that the entrainer form a maximum-boiling azeotrope with the lower-boiling original component. Because maximum-boiling azeotropes are relatively rare, finding a suitable entrainer may be difficult.

For example, the dehydration of organics that form homogeneous azeotropes with water is a common industrial problem. It is extremely difficult to find an intermediate-boiling entrainer that also does not form an azeotrope with water. Furthermore, the resulting separation is likely to be close-boiling or pinched throughout most of the column, requiring a large number of stages. However, consider the separation of valeric acid (187.0°C) and water. This system exhibits an azeotrope

(99.8°C). Ignoring for the moment potentially severe corrosion problems, formic acid (100.7°C), which is an intermediate boiler and which forms a maximum-boiling azeotrope with water (107.1°C), is a candidate entrainer (DRD 030, Fig. 13-65a). In the conceptual sequence shown in Fig. 13-65b, a recycle of the formic acid-water maximum-boiling azeotrope is added to the original valeric acid-water feed, which may be of any composition. Using the indirect mode of operation, high-boiling node valeric acid is removed in high purity and high recovery as bottoms in a first column, which by mass balance produces a formic acid-water distillate. This binary mixture is fed to a second column that produces pure water as distillate and the formic acid-water azeotrope as bottoms for recycle to the first column. The inventory of formic acid is an important optimization variable in this theoretically feasible but difficult separation scheme.

Exploitation of Pressure Sensitivity The breaking of homogeneous azeotropes that are part of a distillation boundary (that is, into products in different distillation regions) requires that the boundary

Formic Acid

Valeric Acid

Valeric Acid

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