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For mixtures, the pseudocritical properties of the mixture should be used to obtain the
compressibility factor. CHEMICAL ENGINEERING
P c,m D P c,a y a C P c,b y b C ÐÐÐ 8.6
T c,m D T c,a y a C T c,b y b CÐ Ð Ð 8.7
where P c D critical pressure,
T c D critical temperature,
y D mol fraction,
suffixes
m D mixture
a, b,etc. D components

8.7. VISCOSITY
Viscosity values will be needed for any design calculations involving the transport of fluids
or heat. Values for pure substances can usually be found in the literature; see Yaws (1993
1994). Liquid viscosities are given in Appendix C. Methods for the estimation of viscosity
are given below.

8.7.1. Liquids
A rough estimate of the viscosity of a pure liquid at its boiling point can be obtained
from the modified Arrhenius equation:
b D 0.01 0.5 8.8
b
2
where b D viscosity, mNs/m ,
3
b D density at boiling point, kg/m .
A more accurate value can be obtained if reliable values of density are available, or can
be estimated with sufficient accuracy, from Souders’ equation, Souders (1938):
I 3
log log 10 D ð 10 2.9 8.9
M
2
where D viscosity, mNs/m ,
M D molecular mass,
I D Souders’ index, estimated from the group contributions given in Table 8.1,
3
D density at the required temperature, kg/m .
Example 8.2

Ž
Estimate the viscosity of toluene at 20 C.
Solution
Toluene
CH 3