Separating Azeotropic Mixtures

The most widely used technologies for separahng azeotropic mixtures are extractive dishllahon, azeotropic dishllahon and pressure swing dishllahon. 

In a case of azeotropic distillation an additional component is introduced that forms a lower boiling binary or ternary azeotrope (e.g. a hetero-azeotrope), which is easier to separate than the original azeotrope. 

Extractive distillation uses a selective solvent (entrainer). Here the entrainer influences the ratio of the activity coefficients of the components in order to alter the separation factor far from unity. Often about 70% of the liquid phase inside the column consist of the entrainer. A typical extractive distillation process for separating aromatics (benzene) from aliphatics (cyclohexane) is show in Fig. 3.2-4. 

A good indication of whether or not a certain component is a suitable entrainer from the thermodynamic point of view is the selectivity at infinite dilution. On the other hand, from the point of view of green separation processes, the amount of entrainer needed has to be minimized. Hence, the entrainer should combine a high selectivity with a high capacity. To classify selectivity and capacity of an entrainer the selectivity and capacity at infinite dilution, S-and k are used. The selectivity at infinite dilution S - is defined as the ratio of the activity coefficients at infinite dilution y It can be shown that the capacity k is also related to yf  

Entrainers with high selectivities usually suffer from a low capacity and vice versa, so, mixtures of entrainers are often used in industrial practice. Adding a small amount of water to N-methyl pyrrolidone (NMP) for separating aromatics from ali-phatics increases the performance of the entrainer. Water shows a good selectivity but a poor capacity whereas NMP combines a moderate selectivity with a moderate capacity. 

---

When separating azeotropic mixtures, if possible, changes in the azeotropic composition with pressure should be exploited rather than using an extraneous mass-separating agent, since ... 

P. Aptel, N. Challard, J. Cuny and J. Neel, Application of the Pervaporation Process to Separate Azeotropic Mixtures, J. Membr. Sci. 1, 271 (1976). 

Trimethylchlorosilane reacts with alcohols or phenols much more slowly, and at certain quanitities of phenol or alcohol does not participate in the reaction altogether. Methyl alcohol cannot be used to separate azeotropic mixture, because in this case there is an active secondary reaction between methyl alcohol and liberated hydrogen chloride with the formation of methylchloride and water. Water hydrolyses the reaction prod-... 

Although reverse osmosis can be used to separate organic and aqueous-organic liquid mixtures, very high pressures are required. Alternatively, pervaporation can be used in which the species being absorbed by, and transported through, the non-porous membrane are evaporated. This method, which uses much lower pressures than reverse osmosis, but where the heat of vapourisation must be supplied, is used to separate azeotropic mixtures. 

Separation method Dilute separations Bulk separations Azeotropic mixtures Temperature sensitive... 

The composition of many azeotropes varies with the system pressure (Horsley, Azeotropic Data-Ill, American Chemical Society, Washington, 1983 Gmehling et ah. Azeotropic Data, VCH Publishers, Deerfield Beach, Fla., 1994). This effect can be exploited to separate azeotropic mixtures by so-called pressure-swing distillation if at some pressure the azeotrope simply disappears, such as does the ethanol-water azeotrope at pressures below 11.5 kPa. However, pressure sensitivity can still be exploited if the azeotropic composition and related distillation boundary change sufficiently over a moderate... 

Martin, N. Separating Azeotropic Mixtures, Sulzer Technical Review, 3 1998. 

Because pervaporation is suitable for separation azeotropic mixtures, such as dehydration of an azeotropic mixture of ethanol-water (ca. 94%), economic comparison of the process with distillation has been reported.228 Besides separation of azeotropic mixtures of organic solvents, dehydration of nitric acid (azeotropic point ca. 68 wt.%) has been tried using a perfluorocarbon ion exchange membrane for the chlor-alkali process nitric acid is concentrated up to... 

Owing to the non-ideality of binary or multicomponent mixtures, the liquid phase composition is often identical with the vapor phase composition. This point is called an azeotrope and the corresponding composition is called the azeotropic composition. An azeotrope can not be circumvented by conventional distillation since no enrichment of the low-boiHrig component can be achieved in the vapor phase. Separating azeotropic mixtures therefore requires special processes, e.g. azeotropic or extractive distillation or pressure swing distillation. Azeotropic information is available in literature (Gmehling et al., 2004). 

Except for pressure swing distillation, which makes use of the pressure dependence of the azeotropic composition, all technologies require an entrainer to separate azeotropic mixtures. For some systems the azeotrope vanishes at certain pressures. This means that ordinary distillation at a different pressure (pressure or vacuum distillation) may circumvent the azeotrope. Although pressure swing distillation does not require an additional component in the process, it is not necessarily environmentally advantageous when compared to the alternatives, since additional... 

V. Pressure effects—Azeotropes. Switch the system to isopropanol and water. Use NRTL or NRTL-2 as the VLB package. We want to look at the analysis at different pressures. To do this, you need to set up a column (dimensions and so forth are arbitrary). Run the simulation so that Aspen Plus will let you use Analysis. Look at the T-y,x and y,x diagrams at p = 1.0 atm, p = 10.0 atm, and p = 0.1 atm Notice how the concentration of the azeotrope shifts. (In the Binary Analysis Results Table the azeotrope occurs when Ki p = 1.000. Record the azeotrope mole fractions). This shift maybe large enough to develop a process to separate azeotropic mixtures (see Chapter 81. 

AlO. Some azeotropic mixtures can be separated by sending the vapor mixture to a gas permeation system—designated as vapor permeation if the mixture is easily condensed fHuang. 1991 Neel. 1991)—and some (probably different) azeotropic mixtures can be separated by sending a liquid mixture to an RO system Why is pervaporation a much more popular method of separating azeotropic mixtures Note In all cases a h5 rid membrane-distillation system will probably be used. 

PV has gained widespread acceptance in the chemical industry as an effective process for separating azeotropic mixtures, for example, separation of... 

Petlyuk, F. B. (1979). Structure of Concentration Space and Synthesis of Schemes for Separating Azeotropic Mixtures. Theor. Found. Chem. Eng, 683-9. 

Extractive distillation is used to increase the relative volatility of components being separated of nonideal mixtures and to separate azeotropic mixtures that cannot be separated by means of simple distillation. 

We examine separation of the mixtures, concentration space of which contains region of existence of two hquid phases and points of heteroazeotropes. It is considerably easier to separate such mixtures into pure components because one can use for separation the combination of distillation columns and decanters (i.e., heteroazeotropic and heteroextractive complexes). Such complexes are widely used now for separation of binary azeotropic mixtures (e.g., of ethanol and water) and of mixtures that form a tangential azeotrope (e.g., acetic acid and water), adding an entrainer that forms two liquid phases with one or both components of the separated azeotropic mixture. In a number of cases, the initial mixture itself contains a component that forms two liquid phases with one or several components of this mixture. Such a component is an autoentrainer, and it is the easiest to separate the mixture under consideration with the help of heteroazeotropic or heteroextractive complex. The example can be the mixture of acetone, butanol, and water, where butanol is autoentrainer. First, heteroazeotropic distillation of the mixture of ethanol and water with the help of benzene as an entrainer was offered in the work (Young, 1902) in the form of a periodical process and then in the form of a continuous process in the work (Kubierschky, 1915). 

The combination of distillation and membrane separation has been explored in the literature for many years. One of the applications is to separate azeotropic mixtures. A distillation column is used to make a preliminary separation with one of the product streams being one of the pure components. The other product stream has a composition fairly close to the azeotropic composition. This stream is fed to a membrane separation unit that achieves the separation not on the basis of VLE, but based on differences in diffusion rates through a solid membrane. The fluid passing through the membrane is called the permeate. The fluid that does not pass through the membrane is called the retentate. Composition differences are the driving forces for mass transfer. 

---