Azeotropes limited separability

Even though the simple distillation process has no practical use as a method for separating mixtures, simple distillation residue curve maps have extremely usehil appHcations. These maps can be used to test the consistency of experimental azeotropic data (16,17,19) to predict the order and content of the cuts in batch distillation (20—22) and, in continuous distillation, to determine whether a given mixture is separable by distillation, identify feasible entrainers/solvents, predict the attainable product compositions, quaHtatively predict the composition profile shape, and synthesize the corresponding distillation sequences (16,23—30). By identifying the limited separations achievable by distillation, residue curve maps are also usehil in synthesizing separation sequences combining distillation with other methods. 

An azeotrope limits the separation that can be obtained between components by simple distillation. For the system described by cui ve B, the maximum overhead-product concentration that could be obtained from a feed with X = 0.25 is the azeotropic composition. Similarly, a feed with X = 0.9 could produce a bottom-product composition no lower than the azeotrope. 

The concept of minimum reflux is more complex in azeotropic distillation, because of the high non-ideal behaviour and distillation boundaries. For the special case of ternary distillation, the analysis may be simplified. It is useful to mention that the minimum reflux is linked with the concept of distillation pinch. This represents a zone of constant phase composition, so that the driving force becomes very small. Consequently, the number of necessary stages for separation goes to infinite. Similarly, there is a minimum reboil rate. In this respect, three classes of limiting separations may be distinguished (Stichlmair and Fair, 1999). Figures 9.36 to 9.38 present concentration profiles obtained by simulation with an ideal system benzene-toluene-ethyl-benzene. 

Azeotropes limit the separation that can be achieved by ordinary distillation techniques. It is possible, in some cases, to shift the equilibrium by changing the pressure sufficiently to break the azeotrope, or move it away from the region where the required separation must be made. Ternary azeotropes also occur, and these offer the same barrier to complete separation as binaries. 

In Section 2.8, we discuss several methods of azeotropic mixtures separation. Thus, the azeotropic mixtures are characterized by a limited separability, while processed in an individual distillation column, and complete separability of these mixtures will be achievable in the case of selection of special schemes consisting of several columns with recycles. 

The principle of azeotropic distillation depends on the abiHty of a chemically dissimilar compound to cause one or both components of a mixture to boil at a temperature other than the one expected. Thus, the addition of a nonindigenous component forms an azeotropic mixture with one of the components of the mixture, thereby lowering the boiling point and faciHtating separation by distillation. The separation of components of similar volatiHty may become economical if an entrainer can be found that effectively changes the relative volatiHty. It is also desirable that the entrainer be reasonably cheap, stable, nontoxic, and readily recoverable from the components. In practice, it is probably the ready recoverabiHty that limits the appHcation of extractive and azeotropic distillation. 

When the data for the X3 product were analyzed separately, the parameters estimated were significantly different in values from those for the other two products. Furthermore, the fit at higher conversion was as not as good as with the other products (E3,g3), This suggests that in the product with low levels of monomer A the phase equilibrium is different. It also indicates that the model may be limited to products containing concentrations less than the azeotropic composition. However, the model can still be applied as long as one recognizes that the model is semi-theoretical. 

There are two main issues concerning the chemistry of the reaction and the separation. One is how to separate the hydriodic acid and sulfuric acid produced by the Bunsen reaction. The other is how to carry out the hydrogen iodide (HI) decomposition section, where the presence of azeotrope in the vapor-liquid equilibrium of the hydriodic acid makes the energy-efficient separation of HI from its aqueous solution difficult, and also, the unfavorable reaction equilibrium limits the attainable conversion ratio of HI to a low level, around 20%. 

When the products are partially or totally miscible in the ionic liquid, the separation of the products can be more complicated. It is however possible to reduce the solubility of typical organic products in the ionic liquid by introducing a more polar solvent that can be separated by distillation afterward at a lower temperature (27). Because of the low vapor pressure of the ionic liquid, direct distillation can be applied without azeotrope formation (28). However, such operation is often limited to highly volatile or thermally labile products because of the general thermal instability of organometallic catalysts. 

Separation of the aromatics from each other and from other hydrocarbons by distillation is not economical because of the limited boiling-point differences and the formation of azeotropic mixtures. Instead, extractive or azeotropic distillation and liquid-liquid extraction are applied.234,235 The latter process is by far the most often used technique. The three processes are applied according to the aromatic content of the gasoline source. p-Xylene, the most valuable of the isomeric xylenes, is isolated by freezing (crystallization) or solid adsorption. 

Although the multicomponent Langmuir equations account qualitatively for competitive adsorption of the mixture components, few real systems conform quantitatively to this simple model. For example, in real systems the separation factor is generally concentration dependent, and azeotrope formation (a = 1.0) and selectivity reversal (a varying from less than 1.0 to more than 1.0 over the composition range) are relatively common. Such behavior may limit the product purity attainable in a particular adsorption separation. It is sometimes possible to avoid such problems by introducing an additional component into the system which will modify the equilibrium behavior and eliminate the selectivity reversal. 

The possibility of combining two different separation units into one, hybrid, process has not been considered in this chapter. Hybrid processes are quite novel and have only very recently been considered by industry and have, therefore, so far not made it into the standard textbooks. A hybrid process has the combined benefits of both of the component units and the benefits should theoretically outweigh the disadvantages. An example is a hybrid of a distillation column and a pervaporation unit for azeotropic separation, where the distillation unit alone is limited by the azeotropic point. Again, a lot of research is currently devoted to this type of operation and it is generally believed that it will become more widely used in the future. 

In separations limited by azeotrope formation under nonreactive conditions, the addition of a reaction (that is, usually adding catalyst) constantly changes the concentrations such that the separation continues beyond the azeotrope. In processes with coupled products (enantiomers, ortho-/meta-/para-substitution), the in-situ removal of the desired product will favor the reaction towards this product, and will therefore strongly increase the selectivity. 

Tran and Mujtaba (1997) and Mujtaba et al. (1997) highlighted the operating features and limitations of BED processes for close boiling and azeotropic mixtures. However, the works were limited to the separation of only one key component in the distillate without due regard to the recovery of solvent or the separation of other components in the feed mixture. 

An important property for process design is the limited reciprocal solubility of acrylonitrile in water. Table 11.3 shows the dependence against temperature. The solubility of AN in water is around 7% w/w, while water in AN about 3% at 20 °C. Therefore, liquid-liquid separation by decantation can be combined advantageously with azeotropic distillation for acrylonitrile purification. 

Because a large amount of water is entrained in the side stream, this is removed in the column C-3. Raw acetonitrile, namely a binary azeotrope with 20% water, separates in top. The bottom stream contains water with heavy impurities. Vacuum distillation at 0.5 bar is adequate to limit the bottom temperature. In the next step pure acetonitrile can be obtained by using pressure-swing distillation. 

Simple distillation cannot separate aromatics from noD -aromatic, because the relative volatilities are very low, and many azeotropes are formed. Azeotropic distillation is based on the formation of an azeotrope betu een the non-aromatic hydrocarbons and a low boiling polar solveat It is select among the hrst terms of the series of alcohols, ketones, aldehydes and nitriles, and is employed pure or mixed with water. If the solvent forms a hetero-azeotrope, its recovery is accordbgly facilitated. The )aeld is not limited in principle. The impurity content of the feedstock and the composition of the azeotrope determine the amount of solvent required. Cuts rich in aromatics can be treated in this way fairly economically. However, any variation in the type of impurity to be removed, and consequently in the composition of the azeotrope, may lead to less perfect purification. Furthermore, this method can be applied only to a narrow cut which contains... 

This subsection describes how to generate the feasible combinatorial possibilities of distillation column configurations for separation of mixtures that do not form azeotropes. Components are named A, B, C, D,. . . and they are listed in the order of decreasing volatility (or increasing boiling temperature). We limit our considerations to splits where the most volatile (lightest) component and the least volatile (heaviest) component do not distribute between the top and bottom product. For simplicity we consider only separations where final products are relatively pure components. Systems containing simultaneously simple and complex distillation columns are considered. Simple columns are the conventional columns with one feed stream and two product streams complex columns have multiple feeds and/or multiple product streams. 

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