Azeotropic distillation azeotropes

Azeotropic distillation. Azeotropes could exist in the initial mixture or created by means of an entrainer. The synthesis of separation sequences involving azeotropes is a complicated matter, but systematic methods based on Residue Curve Maps are available. These methods will be developed in larger extent in Chapter 9. 

Azeotropic distillation Azeotropic distillation Microwave drying Supercritical drying... 

The absence of an interphase compositional differential makes the separation of azeotropes into their constituent components impossible by conventional vapor-hquid separation processes. The azeotropic pattern may be altered by adding an external component— an endainer—that breaks the original azeotrope while forming new ones with the feed components. The process may be designed so that the new azeotropes may be separated from individual compouents or from other azeotropes by what is known as azeotropic distillation. Azeotropic distillation is not limited to the separation of azeotropes it is also used for separating close boilers that are difBcult to separate by conventional distillation. 

Mixtures with low relative volatility or which exhibit azeotropic behavior. The most common means of dealing with the separation of low-relative-volatility and azeotropic mixtures is to use extractive or azeotropic distillation. These processes are considered in detail later. Crystallization and liquid-liquid extraction also can be used. 

Distillation of Mixtures Which Exhibit Azeotropic Behavior or Have Low Relative Volatility... 

If the light and heavy key components form an azeotrope, then something more sophisticated than simple distillation is required. The first option to consider when separating an azeotrope is exploiting change in azeotropic composition with pressure. If the composition of the azeotrope is sensitive to pressure and it is possible to operate the distillation over a range of pressures without any material decomposition occurring, then this property can be used to... 

This technique is useful not only when the mixture is impossible to separate by conventional distillation because of an azeotrope but also when the mixture is difficult to separate because of a particularly low relative volatility. Such distillation operations in which an extraneous mass-separating agent is used can be divided into two broad classes. 

In the first class, azeotropic distillation, the extraneous mass-separating agent is relatively volatile and is known as an entrainer. This entrainer forms either a low-boiling binary azeotrope with one of the keys or, more often, a ternary azeotrope containing both keys. The latter kind of operation is feasible only if condensation of the overhead vapor results in two liquid phases, one of which contains the bulk of one of the key components and the other contains the bulk of the entrainer. A t3q)ical scheme is shown in Fig. 3.10. The mixture (A -I- B) is fed to the column, and relatively pure A is taken from the column bottoms. A ternary azeotrope distilled overhead is condensed and separated into two liquid layers in the decanter. One layer contains a mixture of A -I- entrainer which is returned as reflux. The other layer contains relatively pure B. If the B layer contains a significant amount of entrainer, then this layer may need to be fed to an additional column to separate and recycle the entrainer and produce pure B. 

Figure 3.10 A typical azeotropic distillation using an entrainer.

The second class of distillation operation using an extraneous mass-separating agent is extractive distillation. Here, the extraneous mass-separating agent is relatively involatile and is known as a solvent. This operation is quite different from azeotropic distillation in that the solvent is withdrawn from the column bottoms and does not form an azeotrope with any of the components. A typical extractive distillation process is shown in Fig. 3.11. ... 

As with azeotropic distillation, the separation is possible in extractive distillation because the extraneous mass-separating agent interacts more strongly with one of the components than the other. This in turn alters in a favorable way the relative volatility between the key components. 

In principle, extractive distillation is more useful than azeotropic distillation because the process does not depend on the accident of azeotrope formation, and thus a greater choice of mass-separating agent is, in principle, possible. In general, the solvent should have a chemical structure similar to that of the less volatile of the two components. It will then tend to form a near-ideal mixture with the less volatile component and a nonideal mixture with the more volatile component. This has the effect of increasing the volatility of the more volatile component. 

If an azeotropic mixture is to be separated by distillation, then use of pressure change to alter the azeotropic composition should be considered before use of an extraneous mass-separating agent. Avoiding the use of extraneous materials often can prevent environmental problems later in the design. 

Wastewater leaves the process from the bottom of the second column and the decanter of the azeotropic distillation column. Although both these streams are essentially pure water, they will nevertheless contain small quantities of organics and must be treated before final discharge. This treatment can be avoided altogether by recycling the wastewater to the reactor inlet to substitute part of the freshwater feed (see Fig. 10.36). 

At z in the curve, however (the minimum of vapour pressure), the solution and vapour are in equilibrium and the liquid at this point will distil without any change in composition. The mixture at z is said to be azeotropic or a constant boiling mixture. The composition of the azeotropic mixture does vary with pressure. 

Absolute Ethanol.. Supplies ot iibsolutc cthnnol, which is frec ucntly required in <>rj> mic chemiciil work, nrc now freely iiwiihible commercially as a result of azeotropic distillation methods. If however it should be... 

The breaking up of azeotropic mixtures. The behaviour of constant boiling point mixtures simulates that of a pure compound, because the composition of the liquid phase is identical with that of the vapour phase. The composition, however, depends upon the pressure at which the distillation is conducted and also rarely corresponds to stoichiometric proportions. The methods adopted in practice will of necessity depend upon the nature of the components of the binary azeotropic mixture, and include —... 

The formation of ethyl isopropylidene cyanoacetate is an example of the Knoevenagel reaction (see Discussion before Section IV,123). With higher ketones a mixture of ammonium acetate and acetic acid is an effective catalyst the water formed is removed by azeotropic distillation with benzene. The essential step in the reaction with aqueous potassium cyanide is the addition of the cyanide ion to the p-end of the ap-double bond ... 

Nitromethane is a very common material. Just go down to your local drag strip and pick up a gallon or two for doping your high performance cars fuel. It s also available up to 40% pure in RC model fuels. Simply fractionally distill the nitromethane (bp 101°C) out of the model fuel mixture and you re ready to go. If methanol Is present in the fuel formulation, some will azeotropically distill over with the nitromethane lowering its boiling point slightly, but this does not present a problem. 

By far the best method I have tried to produce benzodioxole in terms of yields and simplicity. In comparison to other processes, this is in fact quite fun and I ll explain It in a fashion that can be followed by a complete novice, like I was when I started a while ago. What we do is react and reflux the ingredients first, then use a simple distillation procedure to extract the product with water as an azeotrope. Once extracted we wash until the product is clear, and then separate. From start to finish it will take about six hours. 

The first binary mixture quantitatively studied was the water-thiazole system, thiazole being a very hygroscopic compound (104), Determining the purity of thiazole sample obtained by distillation, Metzger and Distel-dorf (287) observed the existence of a stable azeotropic mixture, the characteristics of which are the following ... 

An azeotropic mixture con tains two or more substances that distill together at a con stant boiling point The benzene-water azeotrope contains 9% water and boils at 69 C... 

Another way to shift the position of equilibrium to favor the formation of ester is by removing water from the reaction mixture This can be accomplished by adding benzene as a cosolvent and distilling the azeotropic mixture of benzene and water... 

Diols that bear two hydroxyl groups m a 1 2 or 1 3 relationship to each other yield cyclic acetals on reaction with either aldehydes or ketones The five membered cyclic acetals derived from ethylene glycol (12 ethanediol) are the most commonly encoun tered examples Often the position of equilibrium is made more favorable by removing the water formed m the reaction by azeotropic distillation with benzene or toluene... 

TABLE 5.11 Binary Azeotropic (Constant-Boiling) Mixtures An azeotrope is a mixture that cannot be separated by distillation. 

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