Page 105 - Chemical process engineering design and economics

90 Chapter 3

We can write a similar relationship for propane. One equilibrium relation-
ship can be written for each component in a mixture.

System Properties

After writing mass balances, energy balances, and equilibrium relations, we need
system property data to complete the formulation of the problem. Here, we divide
the system property data into thermodynamic, transport, transfer, reaction proper-
ties, and economic data. Examples of thermodynamic properties are heat capacity,
vapor pressure, and latent heat of vaporization. Transport properties include vis-
cosity, thermal conductivity, and diffusivity. Corresponding to transport proper-
ties are the transfer coefficients, which are friction factor and heat and mass trans-
fer coefficients. Chemical reaction properties are the reaction rate constant and
activation energy. Finally, economic data are equipment costs, utility costs, infla-
tion index, and other data, which were discussed in Chapter 2.
There frequently seems to be insufficient system property data. We may ob-
tain accurate system property data from laboratory measurements, which are ex-
pensive. To avoid making measurements, we must rely on correlations or empiri-
cal equations for estimating these data. Reid et al. [2] have compiled many useful
methods for estimating thermodynamic as well as transport properties. In most
cases, these methods are empirical or at best semi-empirical with limited accuracy.
The accuracy of system property data may limit the accuracy of process calcula-
tions. Without experimental data, we can attempt to estimate the thermodynamic
property from a knowledge of the molecular structure of a molecule. For example,
if we know the molecular structure of a pure organic compound, its heat capacity
may be estimated by adding the contribution to the heat capacity made by various
functional groups, such as —CH 3, —OH, —O—, etc., as illustrated by Reid et al.
[2]. We can estimate other properties by these "group methods." An ultimate goal
of physical property research is to be able to calculate accurately any physical
property of a compound from its basic molecular properties. Thus, we can reduce
the need for costly property measurements.
Temperature and composition affect physical properties, but the effect of
pressure is generally small and we can neglect it. One exception is gas density. A
well known example of the effect of temperature is the variation of heat capacity
of a gas with temperature, which is generally curve fitted in the form of a polyno-
mial.

2
Cp = a + bT + cT + dT 3 (3.16)
An equation of state describes the variation of molar density of a gas with
pressure and temperature. For a gas at high temperature and low pressure, the
ideal gas law,
p = P/RT (3.17)

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