Atmospheric pressure hydrogen storage experiments

[Rich Feldman]

Rex and I have expressed interest in mylar balloons for temporary storage of deuterium.
The topic came up in one marginally related forum thread recently: viewtopic.php?f=6&t=9435&start=20#p73876

For what it's worth, I just measured an air-filled, sealed mylar balloon. It had gathered dust for months, along with my PEM cell kits.

In that other thread I mentioned the "mylar balloon" geometric shape. It maximizes the volume for a given generatrix length, the amount of unstretchy film along a "meridian" from pole to equator.
Anyway, this specimen appears to be a nominal 9 inch balloon. Diameter of flat circle before inflation, including the seam allowance. As found today, the puffy shape has OD of about 6.5 inches and thickness of about 4 inches.

Volume is about 1.7 liters. Measured by water displacement in a plastic nursery pot, since one was handy with no holes at the bottom. Two different ways. Archimedes would surely be pleased.

1) Change in apparent weight of the partly filled pail, when floating balloon was pushed down until submerged.

(White colander-like object was used to spread out the pushing down force, but same result was obtained just pushing with fingers.) As expected, the answer was practically independent of the amount of water and the depth of submersion.

2) Mark the waterline when inflated balloon is submerged, then remove balloon and measure the water to refill pail to same level.

Empirically, the volume is about 27% of that of a sphere with same nominal diameter. Also about 1.14 times the cube of nominal radius, and about 5% greater than the volume of a sphere with the same axial circumference. Anyone want to compare those with the closed-form solution given in Wikipedia?

I bet it would be against rules to send a balloon full of deuterium by U. S. mail, or to take it on board an airplane. The latter might be done more or less overtly, by pretending that the buoyant balloon contained helium. But if you got caught, it would be a topic for national news. Especially if you aren't white! Last time the topic came up, we joked about large parcels with gross weight near zero because of helium or deuterium balloons. Enough hydrogen to fill the small balloon pictured above would have about the same fuel mass as one wooden match.

[Rex Allers]

Nice observations on balloon data.

The rest of this is not very scientific...

The balloons I bought were, I think, similar in size. After this recent nudge, I wanted to measure the volume by filling one with air from large calibrated syringes. Unfortunately, due to a mixture of failing short-term memory and way too much stuff, I haven't been able to remember or discover where I stashed these balloons yet.

Mine were a more professional plain silver in color. As all of us scientific types know, looks isn't everything, and "Happy Birthday Princess" balloons for gas storage should work just as well. Actually, I must concede, much better than ones that I can't find.

[Rex Allers]

I found the balloons I hid from myself. Purchased on eBay about a year ago.
Current listing seems to be item: 381306135647

Mine are smaller than the ones Rich described. I picked the smallest one I could find which was: silver, 5" heart. They didn't offer 5" round and the heart shape would probably have less volume, which seems like what we want.

Today, I measured some parameters. To measure the volume of the balloon, I used multiple injections of air from a 60 cc syringe I had on hand. I found a plastic bottle top for squirting its contents that had a tapered tip. The tapered end of this cap fit snugly into the filling tube of the balloon. I secured the balloon with a little piece of tape for this test. I had some 1/4" OD plastic tubing that fit nicely on the end of the syringe. I was able to jam this inside the taper of the cap to be secure enough for this test.

In this testing for volume, I discovered that balloon technology is more advanced than when I was a kid. These Mylar balloons have an internal one-way valve to hold the gas inside. This was good for my syringe filling process as I had to disconnect the syringe each time I refilled it with air, but the valve, for storing D2, needs to be defeated so we can get our stored gas back out.

Here's the configuration I used to fill and measure the volume of the balloon.

Filling measurement of volume

What I found is a tightly filled balloon has about 400 cc of gas, but this is a bit of positive pressure inside, above atmospheric. If the balloon is filled to just about atmospheric pressure, a full volume seems to be about 300 cc. If I did the math right, this would require electrolysis of about .25 g of D2O or about .22 ml.

Here is a picture with the dimensions of an inflated balloon.

Balloon dimensions

I measure the inside diameter of the inflation port of the balloon at 6 mm or .236".

I sacrificed one balloon by cutting it open so I could see what the one-way valve inside looks like. Here's a picture.

Internal one-way valve

Its a flat tube about 3" long inside the balloon formed by two flat pieces of plastic film, sealed along their longer edges. Any pressure inside pushes the two flat sides together, keeping it sealed for any gas wanting to pass outward. This was handy for my measurement inflation process, but not good for getting stored D2 back out.

With the balloon fully inflated from my measurement, I was trying to figure out how to get the air back out. I have some 1/8" plastic tubing that is flexible but a bit stiff. I forced that through the internal valve to get a drain path. If that was done with the balloon empty it would have been easy, but with the pressure of a full balloon forcing the valve sides closed, it required some back-and-forth twisting while applying gentle insertion pressure to work the tube inside. For this balloon about 4" length of the 1/8" tubing into the filling port seems to be about right to keep it open.

My first thought to permanently defeat the valve was to insert a spring hook and pull the valve out to cut it. This might work but would probably be difficult and it would be easy to damage the balloon in the process. I think keeping a piece of the 1/8" tube inside the valve is probably best.

So my plan for a permanent storage is to turn a metal tube .236" in diameter to fit into the inflation port of the balloon. The inside of this will be drilled to accept the 1/8" tube about 4" long, to bypass the valve, and it will be glued to make the attachment. The outside end of the .236" tube will be soldered to some kind of plumbing fitting, possibly a 1/8" compression fitting. Once assembled, this adapter will be inserted into the balloon port and glued, probably with epoxy.

[Rich Feldman]

On another forum, I have talked about plans to re-levitate an 18" Mylar balloon which has lost its helium. How long will it hold hydrogen? In a practice run, I made 7 liters of regular hydrogen by chemical reaction of zinc and acid. There's at least one account here of extracting D2 from heavy water using sodium metal. (Drip water onto a surplus of the solid reactant, as in a carbide lamp.) Can't avoid the gas coming out humid.

The time had come to open a factory-sealed bag of desiccant granules.

Found pink indicator beads mixed in with the product. Package says its contents were silica gel and cobalt chloride. Internet says pink indicates humidity and blue indicates dryness. So the bag must have had a pinhole, or been permeable and old.

Blueness was restored by overnight baking at about 115 °C. Second batch was plenty blue after 2 hours this morning, at barely 100 °C.

Drierite, a competing desiccant product, is anhydrous calcium sulfate. Can also be regenerated many times, but I think it needs much higher temperatures.

The little annealing furnace in picture can get up to low red heat. No thermostat, so I plugged it into a variac and settled on 30 volts for the silica gel regeneration experiment. Takes more than an hour for the temperature to stabilize.

[Rich Feldman]

Got the balloon re-levitated today, after looking up humidity numbers. Moisture's especially significant if the aqueous reaction is allowed to get hot. For my estimated balloon volume, plus an allowance for flushing gas and spillage, there would be about 0.5 g of water at 30 degrees C or less than 0.1 g at 0 degrees C.

Reaction vessel and chemicals were same as reported before (on other forum), and once again immersed in a water bath outdoors.
New plumbing includes a bubbler filled with crushed ice and water, then a DIY column with a huge surplus of silica gel desiccant confined between wads of cotton, then an adapter to a plastic soda straw with which the old balloon had been catheterized.

It took about 1/2 hour from first mixing of chemicals to pinching off the balloon. 200 ml of acid in first batch, drained off and replaced with 250 ml more acid for batch 2. 50 g of die-cast scrap was reduced (not in the chemical sense) to 11 g, a loss of 0.6 mol if it were 100% Zn. Generally consistent with the 1/2 mol of H2 in old estimate. I didn't weigh the empty balloon, but the refilled one has a net buoyancy of about 2 grams.

The desiccant column indicator color changed slightly at the very bottom (inlet side).

Now we'll see how long the balloon floats with its H2 filling.

[Dan Knapp]

It occurred to me that the balloon could be useful even for those using deuterium from a cylinder. When one uses a closed rigid buffer volume with cylinder deuterium and bleeds it into the fusor, there is a constantly declining pressure in the buffer, which requires readjustment of the leak to maintain constant flow. Use of a balloon as a buffer volume, which would give a constant “one atmosphere” pressure, would be expected to give more stability.

[Rich Feldman]

Un-illustrated followup here.

The 18" mylar balloon filled with H2 floated for about a week. I think less time than it floated on original fill from party supply store.

Today, after about six months, the balloon is practically flat. Consistent with H2 diffusing out and air not diffusing in. Much flatter than it got in six months after the original fill. That's why I think Diddams was dispensing He-air mixture.

I believe the float times are controlled by gas permeation through the aluminized mylar film, much more than on ordinary leakage.
Permeation rate would depend strongly on gas type, and selectivity could be different for mylar and latex balloons. What's up with the liquid (?) additive offered at extra cost, to make helium balloons float longer?

It's not just about molecule size or weight. We know that latex balloons lose helium or carbon dioxide more than 10 times faster than they lose air. I've never tried argon, propane, or any hydrofluorocarbon gas. Anyone else out there got similar experiences to report?

p.s. the desiccant column in June picture now looks about the same near bottom port. But pinkness has crept into the top half, I guess because of some leak in the pipe thread connection or solidly plugged push-to-connect hose connection.