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Process Circuit Analysis 95

with an inert gas - such as combustion gases, nitrogen or carbon dioxide - until
the mixture composition is below its lower flammability limit. Thus, there is no
possibility of ignition. Carbon dioxide cannot be produced on site at a low cost,
and it is not inert in some applications. Nitrogen is usually the preferred purge gas.

Process Analysis
To illustrate the purging operation, consider the operation of filling a storage tank
with liquefied natural gas (LNG). In 1965, Exxon contracted to build two storage
3
tanks, each with a capacity of 40,000 m , in Barcelona, Spain [9]. A liquefaction
plant built at Marsa el Brega in Libya supplied the storage facility with LNG by
ship. Before filling with LNG, the oxygen concentration in the tanks must be at a
safe level. The tanks were purged of oxygen using nitrogen, delivered from a liq-
uid-nitrogen storage tank, at 180 1/s (at 20 °C, 1 arm). The liquid nitrogen is vapor-
ized before flowing into the LNG tank. Samples of the gas taken during purging
at various heights in the LNG storage tank showed that the oxygen content in the
tank was essentially the same. Calculate how long it takes to purge the LNG stor-
age tank.
As stated earlier, formulate or define the problem first before attempting to
obtain a numerical solution. At this point there may not be enough information.
After defining the problem, the information required will be evident. If we refer to
the list of available relationships, the first step is to make a mole balance. Since
there are two components, we can make two component balances or the total bal-
ance and one of the component balances. Also, the oxygen analysis shows that the
gas in the tank is well mixed. Thus, the gas composition in the tank and in the exit
stream are equal. Figure 3.1.2 is the flow diagram for the process. The first sub-
script, either 1 and 2, identifies the stream number. The second subscript, either
oxygen or nitrogen, identifies the component.
This problem isan unsteady state problem because the oxygen concemtra-
tion will change with time. On the left side of Equation 3.2 - discussed at the be-
ginning of the chapter - the rates of oxygen flow into the tank and formation of
oxygen by chemical reaction are zero. On the right side of Equation 3.2, the rates
of accumulation and disappearance of oxygen by chemical reaction are also zero.
Thus, Equation 3.2 reduces to

The rate of depletion is expressed by

d(y 2 ,iN)
rate of depletion = - ————— (3.1.2)
dt

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