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Process Circuit Analysis 99

VW.

Air+ CH4 MixingLine

V V__\/ \/ \ / \ / \7
Vol.-*/. 80 70 60 50 4D 30 20 XI ON

25 °C and 1 atm

Figure 3.1.3 Flammability limits for oxygen-nitrogen-methane mixtures.
From Ref 10 with permission.

Next relate m 2 in Equation 3.1.14 to the tank volume and the volumetric flow
rate of nitrogen into the storage tank. From Equations 3.1.9 to 3.1.11, we find that
the gas density at the inlet and outlet of the tank is the same.
Solving Equations 3.1.5, 3.1.7, and 3.1.8 we find that

m 2 = NQ 1A^ (3.1.15)

Substitute Equation 3.1.15 into Equation 3.1.14. After solving for the purg-
ing time, we obtain

V yy
t = —In——— (3.1.16)
Q, 0.21

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