Separation liquid mixtures, near ideal

Mixtures approximating curve (2), in which the critical locus is almost linear, usually are formed when the components have similar critical properties and form very nearly ideal mixtures. A minimum in the critical locus, as in curve (3), occurs when positive deviations from Raoult s law occur that are fairly large, but do not result in a (liquid + liquid) phase separation. Some (polar + nonpolar) mixtures and (aromatic + aliphatic) mixtures show this type, of behavior. 

When process streams contain several components, the specific enthalpies of each component must be determined separately and substituted in the energy balance equation when Ml is evaluated. For mixtures of near-ideal gases or of liquids with similar molecular structures (e.g., mixtures of paraffins), you may assume that H for a mixture component is the same as H for the pure substance at the same temperature and pressure. Procedures to follow for solutions of gases or solids in liquids and for mixtures of dissimilar liquids are outlined in Chapter 8. 

This tutorial paper is a review of recent advances in the synthesis of ideal and nonideal distillation-hased separation systems. We start hy showing that the space of alternative. separation processes is enormous. We discuss. simple methods to classify a mixture either as nearly ideal or as nonideal, in which case it displays azeotropic and possibly liquid/liquid behavior. 

A flash evaporator is a unit operation in which a mixture enters a chamber at a given temperature and pressure, and the vapor and liquid phases that result are separated. Suppose that a mixture of 40% furan and 60% carbon tetrachloride (which is a nearly ideal mixture enters a flash evaporator at 0.7 atm and 30oC. Assuming that the mixture follows Raoulf s Law, find the the compositions of the liquid and vapor streams that will exit. How much of the original mixture will be vaporized ... 

When a multicomponent mixture forms nearly ideal liquid and vapor solutions, and the ideal gas law holds, the K values and relative volatility can be readily estimated from vapor pressure data. Such K values are referred to as ideal or Raoult s law K values. Then, the SF for vapor-liquid separation operations employing an ESA (partial evaporation, partial condensation, or distillation) is given by... 

Now consider the more general case of the synthesis of all possible ordinary distillation sequences for a multicomponent feed that is to be separated into P final products, which are nearly pure components and/or multicomponent mixtures. The components in the feed are ordered by volatility, with the first component being the most volatile. This order is almost always consistent with that for normal boiling point if the mixture forms nearly ideal liquid solutions, such that Eq. (7.3) applies. Assume that the order of volatility of the components does not change as the sequence proceeds. Furthermore, assume that any multicomponent products contain only components that are adjacent in volatility. For example, suppose that the previously cited mixture of benzene, toluene, and biphenyl is to be separated into toluene and a multicomponent product of benzene and biphenyl. With ordinary distillation, it would be necessary first to produce products of benzene, toluene, and biphenyl, and then blend the benzene and biphenyl. 

A few experimental values of a are shown in Table 21.5-1. The values of a are frequently small, especially in dilute solution. They are largest for solutes of very different molecular weights or for highly nonideal solutions. They are more nearly constant for near-ideal solutions and are concentration-dependent in nonideal liquid mixtures. In short, they behave much like the ternary diffusion coefficients discussed in Chapter 7. They are usually of minor practical importance, even though they can be used to effect surprisingly good separations. 

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