Mixtures 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... 

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. 

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. 

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. 

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. 

Figure 11.3-3 shows the vapor-liquid and liquid-liquid equilibrium behavior computed for the system of methanol and n-hexane at various temperatures. Note that two liquid phases coexist in equilibrium to temperatures of about 43°C. Since liquids are relatively incompressible, the species liquid-phase fugacities are almost independent of pressure (see Illustrations 7.4-8 and 7.4-9), so that the liquid-liquid behavior is essentially independent of pressure, unless the pressure is very high, or low enough for the mixture to vaporize (this possibility will be considered shortly). The vapor-liquid equilibrium curves for this system at various pressures are also shown in the figure. Note that since the fugacity of a species in a vapor-phase mixture is directly proportional to pressure, the VLE curves are a function of pressure, even though the LLE curves are not. Also, since the methanol-hexane mixture is quite nonideal, and the pure component vapor pressures are similar in value, this system exhibits azeotropic behavior. 

As an example, the separation of a butene/methanol/MTBE system is investigated. Methyl tertiary-butyl ether (MTBE) is the desired product, and hence needs to be efficiently recovered from a reactor output. Difficulty arises when separating such a mixture with conventional distillation processes, because of the binary azeotropes that exist between methanol and MTBE, as well as between methanol and butene. The driving force for separation in membrane processes differs from that in distillation, thus a membrane process will not exhibit the same azeotropic behavior. Thus, the limitations on distillation processes can be overcome by using a membrane unit. 

The azeotropic behavior of solvent mixtures (e.g., methyl acetate-acetone-methanol) can prevent distillative separation... 

For mixtures with azeotropic behavior, we can estimate the change in the entropy of vaporization at azeotropic temperature when the heat flow is known at azeotropic pressure ... 

CO2 is an important substance which is present in many processes in the chemical industry. In the following, a case study on the prediction of the Henry s law constant for CO2 in ethanol and the vapor-hquid equilibrium of the binary mixture CO2 + C2H6 is discussed. The aim is to explore the capabilities of force fields to predict the temperature dependence of gas solubility and to predict azeotropic behavior. 

Particularly when polar groups are present in liquid mixtures, azeotropes are often formed. For the design of separation processes like distillation, the knowledge of the azeotropic composition at different thermodynamic conditions is of critical importance. In this context, molecular simulation offers a powerful route to predict azeotropic behavior in mixtures. The prediction of the vapor-liquid equilibrium of the mixture CO2 + C2H6 is presented here as an example. 

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