Page 95 - Separation process principles 2

60 Chapter 2 Thermodynamics of Separation Operations

all pressures and noncryogenic temperatures, the Soave- coefficient method is selected as follows. If the binary inter-
Redlich-Kwong equation is suitable. For all temperatures, action coefficients are not available, select the UNIFAC
but not pressures in the critical region, the Benedict-Webb- method, which should be considered as only a first approxi-
Rubin-Starling [5,66,67] method is suitable. mation. If the binary interaction coefficients are available
If the mixture contains (PC), the selection depends on and splitting in two liquid phases will not occur, select the
whether (LG) are present. If they are, the PSRK method is Wilson or NRTL equation. Otherwise, if phase splitting is
recommended. If not, then a suitable liquid-phase activity- probable, select the NRTL or UNIQUAC equation.

SUMMARY

1. Separation processes are often energy-intensive. Energy 5. For nonideal vapor and liquid mixtures containing nonpolar
requirements are determined by applying the first law of thermody- components, certain P-V-T equation-of-state models such as
namics. Estimates of minimum energy needs can be made by S-R-K, P-R, and L-K-P can be used to estimate density, enthalpy,
applying the second law of thermodynamics with an entropy bal- entropy, fugacity coefficients, and K-values.
ance or an availability balance. 6. For nonideal liquid solutions containing nonpolar and/or polar
2. Phase equilibrium is expressed in terms of vapor-liquid and components, certain free-energy models such as Margules, van
liquid-liquid K-values, which are formulated in terms of fugacity Laar, Wilson, NRTL, UNIQUAC, and UNIFAC can be used to es-
and activity coefficients. timate activity coefficients, volume and enthalpy of mixing, excess
3. For separation systems involving an ideal-gas mixture and an entropy of mixing, and K-values.
ideal-liquid solution, all necessary thermodynamic properties can 7. Special models are available for polymer solutions, electrolyte
be estimated from the ideal-gas law, a vapor heat-capacity equa- solutions, and mixtures of polar and supercritical components.
tion, a vapor-pressure equation, and an equation for the liquid den-
sity as a function of temperature.
4. Graphical correlations of pure-component thermodynamic
properties are widely available and useful for making rapid, manual
calculations at near-ambient pressure for an ideal solution.

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