What makes CO2 gas "sticky" in pipes?
There's some recieved wisdom where i work that when flowing CO2 gas in pipes it acts somewhat "sticky" in that it lingers around longer and takes more N2 or air to purge out than, say, Methane or N2.
Unfortunately no-one here seems to know the physical or chemical basis for this observed result. I do have some quantative data through various flow sensors & IR gas sensors that shows it really does stay around longer than N2 or CH4, so i don't think it's due to bad observation.
As you can imagine, googling "why is CO2 sticky" is not very helpful.
If anyone can explain to me why this happens i'd be very grateful, or even just point me in the right direction to do my own research.
Thanks.
EDIT: Thanks for your responses so far.
Here's some more info on our gas system: we buy in our CO2 in large bottles at ~400 psi. We then feed it from outside through a pressure regulator that regulates it down to 2 bar (~29psi) absolute pressure. All the pipes until this point are stainless steel.
It's then fed through a proportional valve and the pressure is held at 1030 mBar over a set of flow and gas sensors, then exhausted to atmosphere. The pipes in this section are a mix of Polyurethane tubes, and two proprietary rubber-like materials called Tygon and Viton tubing. 542
These are held together with nylon fittings. All of this is at ambient temperature.
In these pipes, only N2, CO2 and CH4 flow.
Here's some graphs i collected of the system: In this graph, the green & brown lines are each gas being flowed in the system. th two blueish lines are a rough analogue of how much CO2 is detected in the system. As you can see, even after 5 mins of methane and N2 purging, and continuous intermittent N2 purges, CO2 still creeps back into the system when no gas is flowing.
Here's a Graph of Gas Flow The [Flow In] sensor is just after the 1030 Prop valve, and the [Flow Out] sensor is just before the exhaust. They are both identical wheatstone bridge thermal flow sensors.
As you can see, there's a marked increase in flow loss (Flow In/Flow Out) from 2% to about 7% when CO2 is flowing, and some very strange transient spikes when I switch gasses to/from CO2.
That's most of the data I've collected. Anecdotally, i've also noticed that the CO2 valve is significantly harder to turn than the N2 or CH4 valves.
I hope this helps with the problem, and thank you again for your help so far. 542