Leak from low pressure to high pressure???
Leak from low pressure to high pressure???
(OP)
I have a process in which liquified CO2 with less than 1ppm of entrained O2 stored at 240 psig on one side of a ball valve with air at a few psig on the other appears to not only leak CO2 through the valve (small amounts) but the CO2 appears to absorb O2 through it as well (very small amounts but measureable). The equipment with air in it doesn't normally have it (it is a condenser for the CO2). This only happens when air is introduced to that unit and goes away when the unit is purged. Is it possible that Henry's law is having some affect? I have noticed this affect in a number of processes where O2 has been removed from a liquid (notably water with O2 less than 10 ppb in a pressurized pipe with a leaking flange gasket will pick up O2 and when the flange is fixed the problem goes away).
I would greatly appreciate any help someone could provide in articulating this, I can explain henry's law but have difficulty explaining how the leak appears to occur from the low pressure to the high pressure side.
Thanks!
I would greatly appreciate any help someone could provide in articulating this, I can explain henry's law but have difficulty explaining how the leak appears to occur from the low pressure to the high pressure side.
Thanks!





RE: Leak from low pressure to high pressure???
Diffusion can happen down a concentration gradient, even when there's a pressure gradient in the opposite direction. Witness the loss of helium through the intact glass boretube of a helium-neon laser- in this case, the helium is diffusing from a high concentration of helium, under vacuum, into a lower concentration fo helium in the surrounding atmosphere. That the atmospheric pressure is higher than the vacuum inside the tube is immaterial- the diffusional concentration gradient remains.
If indeed it's occurring, diffusion in your case is happening against an advective gradient (the bulk leakage of fluid across the pressure differential). If the advective flow is high enough (the leak is big enough), diffusion will be overwhelmed- think fish swimming up river. If the river is flowing fast enough, the fish won't be able to move relative to the shore.
RE: Leak from low pressure to high pressure???
I like the fish analogy. It even goes a step farther--the mass flow rate of the fish is tiny in comparison to the mass flow rate of water in the river. If the fish was a lot bigger then the drag force would make it a lot harder to go against the flow. That kind of explains why volume of something like CO2 that can difuse is much smaller than the volume of He that can difuse.
David
RE: Leak from low pressure to high pressure???
This is especially common for liquified gas service.
The hole vents pressure if the ball happens to trap liquid. Otherwise trapped liquid would depressure through the stem packing.
In your case, the ball could be venting trapped air. Think about it.
RE: Leak from low pressure to high pressure???
We had the same problem with a high pressure hydrogenation process, O2 would diffuse into the high pressure H2 and make some very undesirable byproducts
You find this hard to believe that one of the first papers on the diffusion of O2 into a high pressure system, in this case Helium was published many years ago. The helium system was operating at 22,000 psig. The paper was published by the old Gulf Oil Research Corporation.
RE: Leak from low pressure to high pressure???
Do an "advanced search" on this site on the word "diffusion" (without the quotation marks. I think you will find some interesting reading as difussion has been discussed quite a few times before.
rmw