Page 85 - Separation process principles 2

50 Chapter 2 Thermodynamics of Separation Operations

Table 2.7 Classification of Molecules Based on Potential for Forming Hydrogen Bonds
Class Description Example

I Molecules capable of forming three-dimensional networks Water, glycols, glycerol, amino alcohols, hydroxylamines,
of strong H-bonds hydroxyacids, polyphenols, and amides
I1 Other molecules containing both active hydrogen atoms Alcohols, acids, phenols, primary and secondary amines,
and donor atoms (0, N, and F) oximes, nitro and nitrile compounds with a-hydrogen atoms,
ammonia, hydrazine, hydrogen fluoride, and hydrogen cyanide
I11 Molecules containing donor atoms but no active Ethers, ketones, aldehydes, esters, tertiary amines (including
hydrogen atoms pyridine type), and nitro and nitrile compounds without
a-hydrogen atoms
IV Molecules containing active hydrogen atoms but no donor CHC13, CH2C12, CH3CHC12, CH2ClCH2Cl, CH2C1CHC1CH2Cl,
atoms that have two or three chlorine atoms on the same and CH2C1CHCl2
carbon atom as a hydrogen or one chlorine on the
carbon atom and one or more chlorine atoms on
adjacent carbon atoms
V All other molecules having neither active hydrogen Hydrocarbons, carbon disulfide, sulfides, mercaptans, and
atoms nor donor atoms halohydrocarbons not in class IV

Nonideal-solution effects can be incorporated into When polar species are present, mixing rules can be modi-
K-value formulations in two different ways. We have fied to include binary interaction parameters, kij, as in (2-5 1).
already described the use of 6, the partial fugacity coeffi- The other technique for handling solution nonidealities is
cient, in conjunction with an equation of state and adequate to retain &,v in the K-value formulation, but replace &,L by
mixing rules. This is the method most frequently used for the product of Y~L and bL, where the former quantity ac-
handling nonidealities in the vapor phase. However, & re- counts for deviations from nonideal solutions. Equation (2-26)
flects the combined effects of a nonideal gas and a nonideal- then becomes
gas solution. At low pressures, both effects are negligible. At
moderate pressures, a vapor solution may still be ideal even
though the gas mixture does not follow the ideal-gas law.
Nonidealities in the liquid phase, however, can be severe
even at low pressures. Earlier in this section, &L was used which was derived previously as (2-27). At low pressures,
to express liquid-phase nonidealities for nonpolar species. from Table 2.2, bL = P,S/P and &v = 1.0, so (2-68)

Table 2.8 Molecule Interactions Causing Deviations from Raoult's Law
Type of Deviation Classes Effect on Hydrogen Bonding

Always negative I11 + IV H-bonds formed only
Quasi-ideal; always positive or ideal I11 + I11 No H-bonds involved
I11 + V
IV + IV
IV + V
v+v
Usually positive, but some negative I+I H-bonds broken and formed
I + I1
I + I11
I1 + I1
I1 + I11
Always positive I+IV H-bonds broken and formed, but dissociation
(frequently limited of Class I or I1 is more important effect
solubility)
I1 + IV
Always positive I + V H-bonds broken only
I1 t V