Page 15 - Applied Process Design For Chemical And Petrochemical Plants Volume II

4 Applied Process Design for Chen nical and Petrochemical Plants

sure, because it is only a valid concept in the stage of infi- Y = activity coefficient
nite dilution. It is equal to the reference fugacity only at Q = fugacity coefficient
infinite dilution. From [82]:
Strict Henry’s Law The ideal concept is usually a good approximation for
close boiling components of a system, wherein the com-
xi Hij = yi P (8 - 6) ponents are all of the same “family” of hydrocarbons or
chemicals; for example paraffin hydrocarbons. When
for restrictions of: 3 < 0.01 and P < 200 kPa “odd” or non-family components are present, the possibil-
Simple Henry’s Law ity of deviations from non-ideality becomes greater, or if
the system is a wide boiling range of components.
Often for preliminary calculation, the ideal conditions
are assumed, followed by more rigorous design methods.
for restrictions of: 3 < 0.01, yj - 0, and P < 200 kPa The first approximation ideal basis calculations may be
* completely satisfactory, particularly when the activities of
K=E=& the individual components are 1.0 or nearly so.
Xi P Although it is not the intent of this chapter to evaluate
the methods and techniques for establishing the equilibri-
where Hy = Henry’s constant um relationships, selected references will be given for the
xi = mol fraction of solute component, i, in liquid benefit of the designer’s pursuit of more detail. This sub-
P = pressure, absolute
yi = mol fraction of solute component, i, in vapor ject is so detailed as to require specialized books for ade-
fi = mol fraction solvent component, j, in vapor quate reference such as Prausnitz [54].
kPa = metric pressure Many process components do not conform to the ideal
gas laws for pressure, volume and temperature relation-
Care must be exercised that the appropriate assump ships. Therefore, when ideal concepts are applied by cal-
tions are made, which may require experience and/or culation, erroneous results are obtained-some not seri-
experimentation. ous when the deviation from ideal is not significant, but
Carroll [83] presents Henry’s Law constant evaluation some can be quite serious. Therefore, when data are avail-
for several multicomponent mixtures, i.e., (1) a non- able to confirm the ideality or non-ideality of a system,
volatile substance (such as a solid) dissolved in a solvent, then the choice of approach is much more straightfor-
(2) solubility of a gas in solution of aqueous electrolytes, ward and can proceed with a high degree of confidence.
(3) mixed electrolytes, (4) mixed solvents, i.e., a gas in
equilibrium with a solvent composed of two or more com- K-Factor Hydrocarbon Equilibrium Charts
ponents, (5) two or more gaseous solutes in equilibrium
with a single solvent, (6) complex, simultaneous phase K-factors for vapor-liquid equilibrium ratios are usually
and chemical equilibrium. associated with various hydrocarbons and some common
Values of K-equilibrium factors are usually associated impurities as nitrogen, carbon dioxide, and hydrogen sul-
with hydrocarbon systems for which most data have been fide [48]. The K-factor is the equilibrium ratio of the mole
developed. See following paragraph on K-factor charts. fraction of a component in the vapor phase divided by the
For systems of chemical components where such factors mole fraction of the same component in the liquid phase.
are not developed, the basic relation is: K is generally considered a function of the mixture com-
position in which a specific component occurs, plus the
temperature and pressure of the system at equilibrium.
(8 - 9) The Gas Processors Suppliers Association [ 791 provides
a more detailed background development of the K-factors
and the use of convergence pessure. Convergence pressure
For ideal systems: vi = Mi
alone does not represent a system’s composition effects in
where I(1 = mol fraction of component, i, in vapor phase in hydrocarbon mixtures, but the concept does provide a
equilibrium divided by mol fraction of component, rather rapid approach for systems calculations and is used
i, in liquid phase in equilibrium for many industrial calculations. These are not well adapt-
& = equilibrium distribution coefficient for system’s ed for very low temperature separation systems.
component, i The charts of reference [79] are for binary systems
pi* = vapor pressure of component, i, at temperature unless noted otherwise. Within a reasonable degree of
p = total pressure of system = x accuracy the convergence can usually represent the com-