Azeotropic behavior

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... 

Acryhc esters may be saponified, converted to other esters (particularly of higher alcohols by acid catalyzed alcohol interchange), or converted to amides by aminolysis. Transesterification is comphcated by the azeotropic behavior of lower acrylates and alcohols but is useful in preparation of higher alkyl acrylates. 

Several enhanceci distihation-based separation techniques have been developed for close-boihng or low-relative-volatihty systems, and for systems exhibiting azeotropic behavior. All of these special techniques are ultimately based on the same differences in the vapor and liquid compositions as ordinaiy distillation, but, in addition, they rely on some additional mechanism to further modify the vapor-hquid... 

Introduction The term azeotropic distillation has been apphed to a broad class of fractional distillation-based separation techniques in that specific azeotropic behavior is exploited to effect a separation. The agent that causes the specific azeotropic behavior, often called the entrainer, may already be present in the feed mixture (a self-entraining mixture) or may be an added mass-separation agent. Azeotropic distillation techniques are used throughout the petro-... 

Mixtures with low relative volatility or which exhibit azeotropic behavior. Some homogeneous liquid mixtures... 

Pervaporation. Pervaporation differs from the other membrane processes described so far in that the phase-state on one side of the membrane is different from that on the other side. The term pervaporation is a combination of the words permselective and evaporation. The feed to the membrane module is a mixture (e.g. ethanol-water mixture) at a pressure high enough to maintain it in the liquid phase. The liquid mixture is contacted with a dense membrane. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, thus maintaining it in the vapor phase. The permeate side is often held under vacuum conditions. Pervaporation is potentially useful when separating mixtures that form azeotropes (e.g. ethanol-water mixture). One of the ways to change the vapor-liquid equilibrium to overcome azeotropic behavior is to place a membrane between the vapor and liquid phases. Temperatures are restricted to below 100°C, and as with other liquid membrane processes, feed pretreatment and membrane cleaning are necessary. 

Heuristic 1. Separations where the relative volatility of the key components is close to unity or that exhibit azeotropic behavior should be performed in the absence of nonkey components. In other words, do the most difficult separation last. 

In many cases, the azeotropic behavior is between two components, involving binary azeotropes. In other cases, azeotropes can involve more than two components, involving multicomponent azeotropes. 

All of the discussion in this chapter has so far related to binary or ternary systems. It will most often be the case that systems involving azeotropic behavior will also be multicomponent. The concepts developed here for ternary... 

When liquid mixtures exhibit azeotropic behavior, it presents special challenges for distillation sequencing. At the azeotropic composition, the vapor and liquid are both at the same composition for the mixture. The order of volatility of components changes, depending on which side of the azeotrope the composition occurs. There are three ways of overcoming the constraints imposed by an azeotrope. 

As also seen in Table I, the micellar composition can be a-f-fected substantially by nonideality. In -fact, azeotropic behavior in the monomer—micelle equilibrium is possible -for these nonideal systems i.e., as the monomer composition varies -from pure A to pure B, the micelle can vary -from Xn > y to Xn = y (azeotrope) to Xa < yA. This azeotrope -formation is illustrated -for the cationic/nonionic system in Figure 2, where an azeotrope -forms at Xa = yA = 0.3. The minimum CMC -for a mixture corresponds to the azeotropic composition i-f an azeotrope is present (32.37). For an ideal system, azeotropic behavior is not observed. 

The use of a dissolved salt in place of a liquid component as the separating agent in extractive distillation has strong advantages in certain systems with respect to both increased separation efficiency and reduced energy requirements. A principal reason why such a technique has not undergone more intensive development or seen more than specialized industrial use is that the solution thermodynamics of salt effect in vapor-liquid equilibrium are complex, and are still not well understood. However, even small amounts of certain salts present in the liquid phase of certain systems can exert profound effects on equilibrium vapor composition, hence on relative volatility, and on azeotropic behavior. Also extractive and azeotropic distillation is not the only important application for the effects of salts on vapor-liquid equilibrium while used as examples, other potential applications of equal importance exist as well. 

Solubility. H bonding plays several roles in determining solubility, just as it influenced the vapor pressure curves and azeotropic behavior. Actually, the same classification can be used for solubility. Thus, in the same order as in Table 2-X, the six types of mixtures vv ould be expected to have mutual solubilities that were (1) low, (2) high, (3) intermediate, depending on relative strength of bonds broken and formed, (4 and 5) intermediate, (6) not determined by H bonds variable depending on polarity and dispersion forces. 

TABLE 2-XI Comparison of Predicted and Actual Binary Azeotropic Behavior... 

COMPONENT 1 AZEOTROPIC BEHAVIOR WITH COMPONENT 2 OF RATING RATIO ... 

We will examine a quick method based on performing two flash computations to determine the existence of azeotropic behavior for binary mixtures. The hydrogen-bonding classes for the species in a mixture are also a clue that the mixture might exhibit liquidAiquid behavior. Indeed, we have used these classes to find mixtures that display nonideal behavior as illustrative examples. 

We can predict azeotropic behavior as follows from infinite-dilution /T-values. Using a flowsheeting system, we perform a bubble-point calculation for each species in the mixture. Assuming a mixture contains the species A, B, C, and D, we wish to compute the infinite-dilution L-values for three of the species in the remaining one. For example, we perform a flash calculation where A is dominant and B, C, and D are in trace amounts, using something like a feed composition of 0.99999, 0.000003333, 0.000003333, 0.000003334. It does not... 

Mixtures may also form two or more liquid phases at equilibrium. For example, a 50/50 mol% liquid mixture of toluene in water will partition into a water-rich liquid phase and a toluene-rich liquid phase. We just used infinite-dilution /f-values as a means to predict azeotropic behavior. We can argue that we should use infinite-dilution liquid activity coefficients to alert us to the potential for liquid/liquid behavior. We do so as follows. 

We predicted their behavior earlier using infinite-dilution /f-values, with the results at 1 atm shown in Table VIII. Only the acetone and chloroform appear to display azeotropic behavior. With this information and that for pure species boiling points at the pressure of interest, we can sketch the ternary diagram for this mixture. We can also use a computer code to generate it, which was done for Fig. 25. We see that there is one maximum-boiling azeotrope between acetone and chloroform. 

A distillation boundary exits. We deduce this when attempting to explain the azeotropic behavior determined using infinite-dilution Af-values. 

Next, we consider distillation but, with the exception of water and methanol, all other pairs exhibit azeotropic behavior. Also, water is present in very a small amount here. We should be trying to get rid of the pentane first. Thus, distillation does not seem an appropriate first step. 

Eliminate a phenomenon Eliminate an azeotropic behavior by adding a solvent in a distillation system... 

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