Gas Displacement in an enclosed space
Gas Displacement in an enclosed space
(OP)
Hi everybody.
I am working with a large (7000 gal) horizontal tank that has a single vent in the top. I am trying to solve the following problem:
Before opening the tank, conditions are STP with a mixture of hydrogen gas and air in the tank (say 50/50). I will then open the vent, which is a 24" diameter flange, and allow the the hydrogen in the tank to be displaced by air through natural circulation. I'm trying to figure out some sort of function that will allow me to estimate the hydrogen concentration in the tank with respect to time.
Your suggestions appreciated.
I am working with a large (7000 gal) horizontal tank that has a single vent in the top. I am trying to solve the following problem:
Before opening the tank, conditions are STP with a mixture of hydrogen gas and air in the tank (say 50/50). I will then open the vent, which is a 24" diameter flange, and allow the the hydrogen in the tank to be displaced by air through natural circulation. I'm trying to figure out some sort of function that will allow me to estimate the hydrogen concentration in the tank with respect to time.
Your suggestions appreciated.





RE: Gas Displacement in an enclosed space
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Gas Displacement in an enclosed space
From what was described in the OP I think that when you open the vent, very little will happen. What is the driving force for the circulation? The vent is big, but it sounds like there is only one so you can't get a syphon going. I've watched single-vent tanks several times and they tend to breathe in, then breathe out, but neither the intake or exhaust flow is very fast or very consistent. The best air exchange I've ever seen is a tank with two 3-inch vents, one in shadow and one in sun. There was always flow out the sunny vent and in the shaded vent--void-space samples showed that the void space was filled with air (the stuff we wanted out was mostly gone).
David
RE: Gas Displacement in an enclosed space
The gas inside will see a diffusion problem alone, and the concentration gradient will drive the diffusion. However, it'll be an awfully slow process.
TTFN
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RE: Gas Displacement in an enclosed space
Quickest way of getting hydrogen out would be to split the vent into two, and fit a magic one way valve to each.
Now work out the flow through an orifice with of that size with the pressure differential above.
Then allow for an increase in the tank gas density as mixing takes place.
That'll overestimate the rate of dilution, in reality the system has a series of burps rather than using magic one way valves.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Gas Displacement in an enclosed space
Greg, I started out on this problem before I made this post with your "magic valve" method, basically modeling the system as a U tube with the air on one side and Hydrogen on the other. I just don't think that it's a reasonable approximation.
I have a feeling I'm going to do all this work to find out that daily temperature cycling is still faster than diffusion...
RE: Gas Displacement in an enclosed space
RE: Gas Displacement in an enclosed space
The 'magic one way valve' could be accomplished by placing a tube of approximately half the cross-sectional area through the vent hole to the bottom of the tank. You could even run this vent to the other end to the tank for improvend venting.
If the tube stuck up into the air, then this could allow fresh air into the bottom of the tank, with the hydrogen rising and exiting via the remaining open area.
I feel that you will get combinations of diffusion, bouyancy, temperature effects etc that are at such a slow rate that any assumptions will have orders of magnitude errors compared to the real answer.
Possible easier would be to optain measuring equipment to measure hydrogen concentration in the tank. Measuring this a number of times and charting may be able to be extrapolated to give an indication of when you will reach the conditions you require.
I assume that forced ventilation is not an option. A blower into the furtherest end of the tank could possibly acheive what you require in a fraction of the time.
Craig Pretty
Tru-Design Plastics
RE: Gas Displacement in an enclosed space
The purpose of my post was to seek ideas on a strategy to evaluate the problem. I am performing this evaluation to determine the relative benefits of adding a purge system to the tank.
One of the problems is that hydrogen remains combustible in air down to about 4% concentration. At 50% the gasses might be relatively well separated, but at 4% there isn't going to be a nice "cream" layer of hydrogen above the air, or at least I don't believe there will be. I believe the air and hydrogen will be well mixed...
RE: Gas Displacement in an enclosed space
RE: Gas Displacement in an enclosed space
As for purging, use an inert gas like N2. Less chance of explosion and fire. And above all don't use O2 as the purging medium.
RE: Gas Displacement in an enclosed space
eadwine, I was probably just going to purge it with a pipe bomb...
RE: Gas Displacement in an enclosed space
RE: Gas Displacement in an enclosed space