Azeotropic mixtures definition

The first application of pervaporation was the removal of water from an azeotropic mixture of water and ethanol. By definition, the evaporative separation term /3evap for an azeotropic mixture is 1 because, at the azeotropic concentration, the vapor and the liquid phases have the same composition. Thus, the 200- to 500-fold separation achieved by pervaporation membranes in ethanol dehydration is due entirely to the selectivity of the membrane, which is much more permeable to water than to ethanol. This ability to achieve a large separation where distillation fails is why pervaporation is also being considered for the separation of aromatic/aliphatic mixtures in oil refinery applications. The evaporation separation term in these closely boiling mixtures is again close to 1, but a substantial separation is achieved due to the greater permeability of the membrane to the aromatic components. 

Not all liquids form ideal solutions and conform to Raoult s law. Ethanol and water are such liquids. Because of molecular interaction, a mixture of 95.5% (by weight) of ethanol and 4.5% of water boils below (78.15°C) the boiling point of pure ethanol (78.3°C). Thus, no matter how efficient the distilling apparatus, 100% ethanol cannot be obtained by distillation of a mixture of, say, 75% water and 25% ethanol. A mixture of liquids of a certain definite composition that distills at a constant temperature without change in composition is called an azeotrope 95% ethanol is such an azeotrope. The boiling point-composition curve for the ethanol-water mixture is seen in Fig. 4. To prepare 100% ethanol the water can be removed chemically (reaction with calcium oxide) or by removal of the water as an azeotrope (with still another liquid). An azeotropic mixture of 32.4% ethanol and 67.6% benzene (bp 80.1 °C) boils at 68.2°C. A ternary azeotrope (bp 64.9°C) contains 74.1% benzene, 18.5% ethanol, and 7.4% water. Absolute alcohol (100% ethanol) is made by addition of benzene to 95% alcohol and removal of the water in the volatile benzene-water-alcohol azeotrope. 

As mentioned previously, many systems show significant deviations from Raoult s law. There are even some solutions, such as azeotropic mixtures, that are not even qualitatively described by Raoult s law. By rearranging the definition of the activity coefficient (see Eq. (6.2)), Raoult s law can be generalized to apply to a wider class of mixtures ... 

Categories that cause problems for this definition of chemical substance include (1) enantiomers (species containing equal amounts of two optical isomers, like I- and d-tartaric acid) (2) azeotropic mixtures (3) dissociative compounds in equilibrium (4) certain types of mixed crystals or other polymorphic compounds (e.g,d- and /-camphoroxime) (5) synthetic polymers (6) many biochemical compounds (7) systems that are not in "pure" thermodynamic equilibrium and (8) isotopes. In each case, pragmatic decisions have to be made, as the notion of pure substance cannot be essen-tialized. There are no competing definitions of "pure substance" that can avoid the need for "inspired adhoccery" to deal with difficult cases. 

An example where pratm is significant is the removal of water from an azeotropic mixture of water and alcohol. By definition, 0evap for an azeotropic mixture is 1. Thus, the 200- to 500-fold separation achieved is due entirely to the selectivity of the membrane. 

In certain foods a bitter taste is definitely not desirable, therefore different debittering methods have been developed. Methods for removing the bitter taste of enzymatic protein hydrolysates (such as casein hydrolysates) are described in Section 2.3.2.2. These methods are mainly based on controlled proteolysis, plastein reaction, extraction with azeotropic mixtures of alcohols and masking of bitter substances. 

Phase transition is a fundamental defining characteristic of this approach to pure substances. Hence substances that exist in one phase only, easily decompose, only occur in solution, etc. can only be included by analogy. Timmermans [1928, 23-53] lists the following potentially difficult cases for the molar approach of substance definition as summarised in this section azeotropic mixtures, dissociative compounds in equilibrium, enantiomers and racemates ( 18), certain types of mixed crystals or other polymorphic compounds, polymers ( 16), many biochemical compounds, and systems that are not in thermodynamic equilibrium. 

Bajoras and Makuska investigated the effect of hydrogen bonding complexes on the reactivities of (meth)acrylic and isotonic acids in a binary mixture of dimethyl sulfoxide and water using IR spectroscopy (Bajoras and Makuska, 1986). They demonstrated that by altering the solvent composition it was possible to carry out copolymerization in the azeotropic which resulted in the production of homogeneous copolymers of definite compositions at high conversions. Furthermore, it was shown that water solvent fraction determines the rate of copolymerization and the reactivity ratios of the comonomers. This in turn determines the copolymer composition. 

Based on the above information, the CAMD problem definition is revised as follows - The solvent can be acyclic hydrocarbons and ketones (aromatic compounds, chlorides, dioxanes are not considered for EH S concerns). The normal boiling point should be higher than that of chloroform (334 K), the molecular weight could be between 70-120, the solvent must not form azeotrope with either acetone or chloroform, and, must be totally miscible with the binary mixture of acetone and chloroform. 

The anhydrous acetates of the rare earths have recently been prepared. Moeller et al. [355] obtained them for La, Dy, Ho, Er, Yb and Y by the azeotropic distillation of a mixture of hydrated acetates with N,N -dimethylformamide (DMF) and benzene. In the case of Ce, Pr, Nd, Sm, Eu and Gd the same method gave a monosolvated acetate, M(C2Hs02)3 DMF. However, the anhydrous acetates of Ce, Pr, Nd, Sm, Eu and Gd can be prepared [355] by vacuum desolvation of the monosolvated compounds. A direct desolvation of the acetates in vacuum at —150° C was attempted by Witt and Onstott [389] after dissolution of the rare earth oxides in 50 per cent acetic acid, and anhydrous acetates of definite composition were obtained for La, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu and Y. 

Provide clear definition of the design problem. Collect sufficient engineering data. Get a comprehensive picture of chemistry and reaction conditions, thermal effects and chemical equilibrium, as well as about safety, toxicity and environmental problems. Examine the availability of physical properties for components and mixtures of significance, identify azeotropes and key binaries. Define the key constraints. 

Working with azeotropes - not all liquid mixtures can be separated by distillation. In some cases an azeotrope, a mixture of the liquids of definite composition, which boils at a constant temperature, is formed. For example, an azeotrope containing 95.5% ethanol and 4.5% water boils at 78.15 C, which is below the boiling point of pure ethanol (78.3 C). Therefore 100% ethanol cannot be obtained by distillation of ethanol-water mixtures, even though their boiling points are about 22 C apart. 

Binary systems are known that form solid solutions over the entire range of composition and which exhibit either a maximum or a minimum in the melting point. The Uquidus-solidus curves have an appearance similar to that of the liquid-vapor curves in systems which f orm azeotropes. The mixture having the composition at the maximum or minimum of the curve melts sharply and simulates a pure substance in this respect just as an azeotrope boils at a definite temperature and distills unchanged. Mixtures having a maximum in the melting-point curve are comparatively rare. 

An alternative method for removing water relies on the use of a solvent that forms an azeotrope with water. An azeotrope is defined as a constant boiling mixture that often boils at a temperature different from its components. To understand this definition, a zeotrope is defined as a mixture that can be separated by distillation. An azeotrope, therefore, is a mixtrue that cannot he separated by distillation. Ethanol, for example, forms an azeotrope with water that boils at 78.2°C, lower than the boiling point of ethanol (78.5°C) or water (100°C). This azeotrope is composed of 95.6% ethanol and 4.4% water. 

A rigorous, theoretical definition of the boiling range distribution of petroleum fiactions is not possible due to the complexity of the mixture as well as the unquantifiable interactions among the components (for example, azeotropic behavior). Any other means used to define the distribution would require the use of a physical process such as a conventional distillation or gas c omatographic characterization. This would therefore result in a method-dependent definition and would not constitute a true value from which bias can be calculated. 

Butadiene and acrylonitrile have different copolymerisation parameters. The azeotropic composition of the monomer mixture is about 38% acrylonitrile (at 25 °C). Where the mixture of monomers contains less than 38 % acrylonitrile the copolymers have acrylonitrile contents that are larger than those which would correspond to the acrylonitrile inputs. This results in continuous alteration of the monomer composition and hence in a lack of chemical uniformity. In industrial production additional acrylonitrile is therefore incorporated in a series of stages when definite degrees of polymerisation have been reached. 

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