Purging is an activity used to remove oxygen in piping or vessels to certain concentration, for example 2%-mole. It uses inert gas, usually nitrogen, to do that.
In this post, I want to share you how to calculate inert gas for purging activity. There are three different calculation methods used.
In dilution purging method, the inert gas simply flows through the vessel and reduces the concentration of unwanted component. It is used for tanks, reactors, and other vessels. Figure below is used to determine requirement of inert gas.
Let say, you want to purge a tank full of air (21%-mole oxygen) to 1%-mole oxygen. Then:
- Ratio of initial to final concentration of oxygen is 21
- From figure above, at ratio of 21, you need 3 vessel volume of inert gas
In this method, the vessel at 1 atm is alternately pressured with inert gas and vented. It is used for vessel that can withstand 30 psig (about 2 barg) or more, vessel with only one port, or vessels with coils or baffles inside. I also used this method for leak test in piping system.
The dilution ratio is (1/P)n. Where P is in atm and n is number of cycle.
The quantity of inert gas required for each cycle is P-1 vessel volumes.
Let say, you want to purge a vessel from 21%-mole oxygen to 1%-mole oxygen. You use inert gas to pressure to 5 atm.
- Guess number of cycle. We guess 2 cycles for started.
- (1-P)n = (1/5)2 = 0.04
- 0.04 x 21%-mole = 0.84%-mole. The results is satisfied. If the result is bigger than requirement, try to increase number of cycle.
- Purge requirement (P-1) = 5-1 = 4 vessel volumes
Based on its name, this method can only be used for vessels capable to withstand a vacuum. In this method, the vessel is alternately evacuated and fed with inert gas to 1 atm.
Concentration of unwanted component is reduced from C to CPn.
C = concentration (%)
P = pressure (atm)
n = number of cycles
The quantity of inert gas required for each cycle is 1-P vessel volumes.
Let say, you want to purge from 21%-mole oxygen to 1%-mole oxygen. You use vacuum-cycle purging to 0.5 atm.
- C = 21%
- CPn = 1%, (21%)(0.5)n = 1%. We get n = 4.4, so use 5 cycles
- Purged gas required = 5 x (1-0.5) = 2.5 vessel volumes
Reference: Rules of Thumb for Chemical Engineers, Carl Branan