Section 8.8
CALCULATION OF FUGACITY (LIQUIDS)
±
8.8
CALCULATION OF FUGACITY (LIQUIDS)
Example S8.1
Vapor and Liquid Fugacities using the Virial Equation
Determine the fugacity (MPa) for acetylene at: (a) 250K and 10 bar; (b) 250K and 20 bar. Use the
virial equation and the shortcut vapor pressure equation.
Solution:
From the back flap of the text for acetylene:
T
c
= 308.3 K,
P
c
= 6.139,
ω
= 0.187,
Z
c
= 0.271.
For each part of the problem, the fluid state of aggregation is determined before the method of
solution is specified. At 250 K, using the shortcut vapor pressure equation, Eqn 8.11, the vapor
pressure is
P
sat
= 1.387 MPa.
We anticipate the need to calculate the virial coefficient at 250K using Eqns. 6.86.9:
T
r
= 250/308.3 = 0.810, B
o
=
−
0.5071, B
1
=
−
0.2758, B = 233.3 cm
3
/mol.
(a)
P
= 1 MPa <
P
sat
so the acetylene is vapor. Using Eqn 6.10 to evaluate the appropriateness
of the virial equation at 1 MPa,
P
r
= 1/6.139 = 0.163, and 0.686 + 0.439
P
r
= 0.76 and
T
r
=
0.810, so the correlation should be accurate.
Using Eqn 8.29,
f
=
ϕ
P
= 0.894 (1) = 0.894 MPa
(b)
P
= 2 MPa >
P
sat
so the acetylene is liquid. For a liquid phase, the only way to incorporate
the virial equation is to use the Poynting Method, Eqn 8.36. Using Eqn 6.10 to evaluate the
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 Spring '11
 Ku
 Vapor pressure

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