Info

Non-Bonding (NB)

Paraffins Nonactive H chlorides

Nonactive H

fluorides Sulfides

Nonactive H

iodides Disulfides

Nonactive H

bromides Thiols

Deviations from Raoults Law

H-Bonding classes

Type of deviations

Comments

HBSA + NB HBAD + NB

Alway positive dev., HBSA + NB often limited miscibility

H-bonds broken by interactions

HBA + HBD

Always negative dev.

H-bonds formed by interactions

HBSA + HBD HBAD + HBD

Always positive deviations, HBSA + HBD often limited miscibility

H-bonds broken and formed; dissociation of HBSA or HBAD liquid most important effect

HBSA + HBSA HBSA + HBAD HBSA + HBA HBAD + HBAD HBAD + HBA

Usually positive deviations; some give maximum-boiling azeotropes

H-bonds broken and formed

HBA + HBA HBA + NB HBD + HBD HBD + NB NB + NB

Ideal, quasi-ideal systems; always positive or no deviations; azeotropes, if any, minimum-boiling

No H-bonding involved

note: n-HBA is enhanced version of HBA.

note: n-HBA is enhanced version of HBA.

vent is relatively nonvolatile and remains largely in the liquid phase. With this boiling point difference, the solvent should also not form azeotropes with the other components.

Selectivity at infinite dilution. Rank candidate solvents according to their selectivity at infinite dilution. The selectivity at infinite dilution is defined as the ratio of the activity coefficients at infinite dilution of the two key components in the solvent. Since solvent effects tend to increase as solvent concentration increases, the infinite-dilution selectivity gives an upper bound on the efficacy of a solvent. Infinite-dilution activity coefficients can be predicted using such methods as UNIFAC, ASOG, MOSCED (Reid et al., Properties of Gases and Liquids, Fourth Edition, McGraw-Hill, New York, 1987). They can be found experimentally using a rapid gas-liquid chromatography method based on relative retention times in candidate solvents (Tassios in Extractive and Azeotropic Distillation, Advances in Chemistry Series 115, American Chemical Society, Washington, 1972) and they can be correlated to bubble-point data [Kojima and Ochi, J. Chem. Eng. Japan, 7(2), 71 (1974)]. DECHEMA [Vapor-Liquid Equilibrium Data Collection, Frankfort (1977)], has also published a compilation of experimental infinite-dilution activity coefficients.

Experimental measurement of relative volatility. Rank candidate solvents by the increase in relative volatility caused by the addition of the solvent. One technique is to experimentally measure the relative volatility of a fixed-composition key component-solvent mixture (often a 1/1 ratio of each key, with a 1/1 to 3/1 solvent/key ratio) for various solvents. [Carlson et al., Ind. Eng. Chem., 46, 350 (1954)]. The Oth-

mer equilibrium still is the apparatus of choice for these measurements [Zudkevitch, Chem. Eng. Comm., 116, 41 (1992)].

Methanol and acetone boil at 64.5°C and 56.1°C, respectively and form a minimum-boiling azeotrope at 55.3°C. The natural volatility of the system is acetone > methanol, so the favored solvents most likely will be those that cause the acetone to be recovered in the distillate. However, for the purposes of the example, a solvent that reverses the natural volatility will also be identified. First, examining the polarity of

TABLE 13-21 Relative Polarities of Functional Groups

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