Page 104 - Chemical process engineering design and economics

Process Circuit Analysis 89

is frequently used to determine the surface area required, A, for the required heat
transferred, Q. In process circuit analysis, as discussed earlier, the stream proper-
ties of a process circuit can be determined by initially avoiding the complication of
considering rate equations by specifying an approach to equilibrium. Later, to
determine the size of the process units to achieve the required energy transfer,
chemical conversion, and degree of separation, requires using rate equations.
Equilibrium Relations

From the previous discussion, equilibrium relations required for process circuit
analysis are evidently important. To achieve equilibrium requires equipment infi-
nite in size, which is a physical and economical impossibility. We must be satis-
fied with an economical approach to equilibrium conditions. In some cases, be-
cause of rapid mass transfer or chemical reaction, the difference between actual
and equilibrium conditions is insignificant.
By assuming chemical equilibrium at the exit of a reactor, we can write a
relationship between the composition of the components in the exit stream. For
example, for the oxidation of SO 2 with O 2 to give SO 3

2 SO 2 + O 2 -> 2 SO 3 (3.12)

At equilibrium,
(Pso3 ) 2 (yso3) 2
K P = ————— = —————— (3.13)

We can write an equilibrium relation for each independent reaction.
Similarly, for a single stage separator, if we assume equilibrium between
phases leaving the separator, we may write a relationship between the composition
of a component in each phase leaving the separator. Consider a solution of meth-
ane and propane being flashed across a valve. Downstream of the valve, we may
write an equation to express the phase equilibrium of methane in a way that is
similar to chemical equilibrium

CH4(l)^CH4(g) (3.14)

The relationship between the composition of methane in the vapor and liquid
phases is

K M = ——— (3.15)
YLM

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