Question

A tank containing air at atmospheric pressure must be purged with pure nitrogen until the oxygen concentration is 5 ppm. The tank can withstand a vacuum, so vacuum purging is considered. The vacuum line used produces an absolute pressure of 6 mmHg. How many vacuum cycles are necessary?

Answer 1

Tank pressure - atmospheric i.e 1 atm

Initial O2 concentration C1= 21 mole %

Final O2 concentration C2= 5 ppm = 5* 10^ -4 mole %

Purge pressure= P= 6 mmhg= 0.008 atm

Now, Final concentration C2= C1(P)^n

5 x10^-4= 21 (0.008)^n

solving above n=2.20 so use 3 cycle

where is n= number of cycle

The quantity of inert gas required for each cycle is 1-P tank volume

purge gas ( i.e n2 required)= number of cycle * (1-P) tank volume

                                        =3* (1-0.008) tank volume

                                       =2.976 * tank volume

Reference: Rules of Thumb for Chemical Engineers, Carl Branan