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Process Circuit Analysis 97

Table 3.1.1 Summary of Equations for Calculating the Purging Time of a
Storage Tank________________________________

Subscripts: O 2 = 1, N 2 = 2

Mole Balances
d(y 2 ,,N)
- —————— = v 2>1 m 2 — at t = 0, y 2,, = 0.2 (3.1.4)
dt

m 1 = m 2 (3.1.5)
y2,i + y2,i (3-1-6)

m^^Q,' (3.17)
Thermodynamic Properties

p 2 = N/V' (3.1.8)

pi = PiR'T,' (3.1.9)
p 2' = p 2 R'T,' (3.1.10)

Design Specifications

Pl«P 2 (3.1.11)

T,' = T 2
(3.1.12)
Variables

Y2,i - y 2, 2 -1 - N - m, - m 2 - p, - p 2 - p, - T 2
Degrees of Freedom

F = V - R = 1 0 - 9 = 1

molar density equals the number of moles in the storage tank divided by the tank
volume. The only system property data needed is a relationship for the gas molar
density, given by Equations 3.1.9 and 3.1.10. At ambient conditions the ideal gas
law is adequate.
At this point, we have used all the relationships available. Now, determine
if we have completely defined the problem by calculating the degrees of freedom.

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