CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 207
7.7.3 Estimates of Henry’s constant
Henry’s constant is usually determined experimentally from gas solubility measurements. If
the temperature of the system is below the critical temperature of the gaseous component, we
may estimate Henry’s constant using the following formulas.
By comparing (7.40) with (7.25) we obtain the relation
KH=γ∞ 2 f 2 • , (7.44)wheref 2 • (T, p) is the fugacity of pure liquid component 2 andγ 2 ∞ is the limiting activity
coefficient of component 2 in a liquid mixture in which this component is infinitely diluted. If
the gaseous phase behaves ideally, equation (7.44) becomes
KH=γ 2 ∞ps 2. (7.45)If, in addition, the liquid phase is an ideal solution (7.23), then
KH=ps 2. (7.46)Example- Estimate the solubility of propane(2) in liquid heptane at 25◦C and a partial pressure of
propanep 2 = 101 kPa. At this temperature, the saturated vapour pressure of propane is 969 kPa. - Estimateγ 2 ∞if you know that the experimental value of solubility expressed by the molar
fraction of propane is 0.117.
Solution- From relations (7.42) and (7.46) we get
x 2 =p 2
KH=
p 2
ps 2=
0. 101
0. 969
= 0. 104.
Agreement with the experimental value 0.117 is very good.- To estimate the limiting activity coefficient, we use relation (7.45) and the experimental value
of solubility
γ∞ 2 =
p 2
ps 2 ·(x 2 )exp