Fractional Distillation, Azeotropes

Simple distillation, described in Technique 14, works well for most routine separation and purification procedures for organic compounds. When boiling-point differences of components to be separated are not large, however, fractional distillation must be used to achieve a good separation. 

The vapor-liquid composition curve for mixtures of benzene and toluene. 

Temperature-distillate plot for fractional distillation of a benzene-toluene mixture. 

Phase diagram for a fractional distillation of an ideal two-component system. 

The phase diagram relates the compositions of the boiling liquid (lower curve) and its vapor (upper curve) as a function of temperature. Any horizontal line drawn across the diagram (a constant-temperature line) intersects the diagram in two places. These intersections relate the vapor composition to the composition of the boiling liquid that produces that vapor. By convention, composition is expressed either in mole fraction or in mole percentage. The mole fraction is defined as follows  

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See destructive distillation batch distillation extractive distillation rectification dephlegmation flash distillation continuous distillation simple distillation reflux fractional distillation azeotropic distillation vacuum distillation molecular distillation hydrodistillation. 

Purification. Distillation is most commonly used for purifying solvents. Solvents with different vapor pressures can be separated from one another by fractional distillation. Azeotropic mixtures can be separated by extractive or azeotropic distillation (e.g., addition of benzene to a water-ethanol mixture), by chemical reaction of a component (e.g., addition of acetic anhydride to an ethanol-ethyl acetate mixture), or by altering the pressure during distillation. 

Sections 8.3 and 8.4 Physical Constants of Liquids Fractional Distillation, Azeotropes Ethanol and Fermentation Chemistry... 

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. 

In typical processes, the gaseous effluent from the second-stage oxidation is cooled and fed to an absorber to isolate the MAA as a 20—40% aqueous solution. The MAA may then be concentrated by extraction into a suitable organic solvent such as butyl acetate, toluene, or dibutyl ketone. Azeotropic dehydration and solvent recovery, followed by fractional distillation, is used to obtain the pure product. Water, solvent, and low boiling by-products are removed in a first-stage column. The column bottoms are then fed to a second column where MAA is taken overhead. Esterification to MMA or other esters is readily achieved using acid catalysis. 

Azeotropic and Extractive Distillations. Effective as they are for producing various Hquid fractions, distillation units generally do not produce specific fractions. In order to accommodate the demand for such products, refineries have incorporated azeotropic distillation and extractive distillation in their operations (see Distillation, azeotropic and extractive). 

The equihbrium shown in equation 3 normally ties far to the left. Usually the water formed is removed by azeotropic distillation with excess alcohol or a suitable azeotroping solvent such as benzene, toluene, or various petroleum distillate fractions. The procedure used depends on the specific ester desired. Preparation of methyl borate and ethyl borate is compHcated by the formation of low boiling azeotropes (Table 1) which are the lowest boiling constituents in these systems. Consequently, the ester—alcohol azeotrope must be prepared and then separated in another step. Some of the methods that have been used to separate methyl borate from the azeotrope are extraction with sulfuric acid and distillation of the enriched phase (18), treatment with calcium chloride or lithium chloride (19,20), washing with a hydrocarbon and distillation (21), fractional distillation at 709 kPa (7 atmospheres) (22), and addition of a third component that will form a low boiling methanol azeotrope (23). 

Distillation (qv) is the most widely used separation technique in the chemical and petroleum industries. Not aU. Hquid mixtures are amenable to ordinary fractional distillation, however. Close-boiling and low relative volatihty mixtures are difficult and often uneconomical to distill, and azeotropic mixtures are impossible to separate by ordinary distillation. Yet such mixtures are quite common (1) and many industrial processes depend on efficient methods for their separation (see also Separation systems synthesis). This article describes special distillation techniques for economically separating low relative volatihty and azeotropic mixtures. 

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-... 

Introduction Reactive distillation is a unit operation in which chemical reaction and distiUative separation are carried out simultaneously within a fractional distillation apparatus. Reactive distillation may be advantageous for liqiiid-phase reaction systems when the reaction must be carried out with a large excess of one or more of the reactants, when a reaction can be driven to completion by removal of one or more of the products as they are formed, or when the product recoveiy or by-product recycle scheme is complicated or macfe infeasible by azeotrope formation. 

In most cases, systems deviate to a greater or lesser extent from Raoult s law, and vapour pressures may be greater or less than the values calculated. In extreme cases (e.g. azeotropes), vapour pressure-composition curves pass through maxima or minima, so that attempts at fractional distillation lead finally to the separation of a constantboiling (azeotropic) mixture and one (but not both) of the pure species if either of the latter is present in excess. 

FIGURE 8.41 The temperature composition diagram of a minimum-boiling azeotrope (such as ethanol and benzene). When this mixture is fractionally distilled, the (more volatile) azeotropic mixture is obtained as the initial distillate. 

Alkyl borates. By heating boric acid with excess of the alcohol the water formed in the reaction is removed by fractional distillation as an azeotropic mixture with the alcohoi, for example ... 

Occasionally, you ll run across liquid mixtures that cannot be separated by fractional distillation. That s because the composition of the vapor coming off the liquid is the same as the liquid itself. You have an azeotrope, a liquid mixture with a constant boiling point. 

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