Page 106 - Chemical process engineering design and economics

Process Circuit Analysis 91

is sufficiently accurate, but we may use it at a high pressure if we are willing to
sacrifice some accuracy for simplicity. Accurate equations of state are more com-
plicated than the ideal gas law. For example, the Redlich-Kwong equation,

a
P + ————————— (v - b) = R T (3.18)
[T 1/2 v(v + b)]

a modification of Van der Waal's equation, is a more accurate equation of state
than the ideal gas law. Engineers always face "tradeoffs" between accuracy and
simplicity.
For mixtures, the problem is estimating a property of a mixture, given that
property for the pure components. Estimating thermodynamic properties of mix-
tures requires a "mixing rule" to calculate a property for a mixture from the pure-
component properties. If the solution is ideal, the mole fraction average,

(3.19)

of the property is sufficient. Reid et al. [2] shows that viscosity, a transport prop-
erty, has a more complex mixing rule than the mole-fraction average.
Transfer properties, the heat and mass transfer coefficient and friction factor,
depend not only on transport and thermodynamic properties but also on the hydro-
dynamic behavior of a fluid. The geometry of the system will influence the hydro-
dynamic behavior. By reducing the parameters by arranging them into dimen-
sionless groups, we can reduce the number of parameters that have to be varied to
correlate any of the transfer properties. For example, the friction factor equation,
0 25
f = 0. 1 [ (e / d) + (68 / Re)] ' (3 .20)
one of many correlations reviewed by Olyjic [3], has been correlated in terms of
the dimensionless roughness factor, s/d, and the Reynolds group.
Rates of reaction require rate constants and activation energies. These pa-
rameters are obtain from experiments.

Economic Relations

Usually, there is more than one solution to an engineering problem that is techni-
cally feasible, and socially, environmentally, and even esthetically acceptable. Among
these solutions, the engineer selects the solution that is the least costly and is fi-
nancially feasible. Even though a project may appear profitable, there may be
insufficient capital available to implement the project so that financial feasibility is
also an important consideration. Assuming that a particular solution meets all the

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