Page 62 - Separation process principles 2

Chapter 2

Thermodynamics of Separation Operations

Thermodynamic properties and equations play a major role or density, enthalpy, entropy, availability, and fugacities and
in separation operations, particularly with respect to energy activities together with their coefficients, all as functions of
requirements, phase equilibria, and sizing equipment. This temperature, pressure, and phase composition. Methods for
chapter discusses applied thermodynamics for separation estimating properties for ideal and nonideal mixtures are
processes. Equations for energy balances, entropy and summarized. However, this chapter is not a substitute for
availability balances, and for determining phase densities and any of the excellent textbooks on chemical engineering
phase compositions at equilibrium are developed. These thermodynamics. Furthermore, emphasis here is on fluid
involve thermodynamic properties, including specific volume phases, with little consideration of solid phases.

2.0 INSTRUCTIONAL OBJECTIVES
After completing this chapter, you should be able to:
Make energy, entropy, and availability balances around a separation process using the first and second laws of
thermodynamics.
Calculate lost work and second-law efficiency of a separation process.
Explain the concept of phase equilibria in terms of Gibbs free energy, chemical potential, fugacity, fugacity
coefficients, activity, and activity coefficients.
* Understand the concept and usefulness of the equilibrium ratio (K-value) for problems involving liquid and/or
vapor phases at equilibrium.
Derive expressions for K-values in terms of fugacity coefficients and activity coefficients.
Write vapor-liquid K-value expressions for Raoult's law (ideal), a modified Raoult's law, and Henry's law.
Calculate density, enthalpy, and entropy of ideal mixtures.
Utilize graphical correlations to obtain thermodynamic properties of ideal and near-ideal mixtures.
Use nomographs to estimate vapor-liquid K-values of nonideal hydrocarbon and light-gas mixtures.
Explain how computer programs use equations of state (e.g., Soave-Redlich-Kwong or Peng-Robinson) to
compute thermodynamic properties of vapor and liquid mixtures, including K-values.
Explain how computer programs use liquid-phase activity-coefficient correlations (e.g., Wilson, NRTL,
UNIQUAC, or UNIFAC) to compute thermodynamic properties, including K-values, for nonideal vapor and
liquid mixtures at equilibrium.

2.1 ENERGY, ENTROPY, AND
average amount of crude oil processed by petroleum refiner-
AVAILABILITY BALANCES
ies in the United States in early 1991. At a crude oil price of
Most industrial separation operations utilize large quantities approximately $40/bbl, the energy consumption by distilla-
of energy in the form of heat and/or shaft work. A study by tion in the United States is approximately $20 trillion per
Mix et al. [I] reports that two quads (1 quad = 1015 Btu) of year. Thus, it is of considerable interest to know the extent of
energy were consumed by distillation separations in petro- energy consumption in a separation process, and to what
leum, chemical, and natural-gas processing plants in the degree energy requirements might be reduced. Such energy
United States in 1976. This amount of energy was 2.7% of estimates can be made by applying the first and second laws
the total U.S. energy consumption of 74.5 quads and is of thermodynamics.
equivalent to the energy obtained from approximately .I mil- Consider the continuous, steady-state, flow system for a
lion bbl of crude oil per day over a one-year period. This general separation process in Figure 2.1. One or more feed
amount of oil can be compared to 13 million bbllday, the streams flowing into the system are separated into two or