rrkss
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Portable Oxygen Concentrators
Saw an ad on TV for a device called a portable oxygen concentrator that increases the oxygen concentration delivered to the patient by extracting it
from the air. Since it does not rely on bottled gas I'm curious how the device works. Is it some sort of membrane system or something else?
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IrC
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It has a pump, chambers, cycling valves, using a lithium based chemical to absorb N2 from the air, thus outputting O2 in higher concentration. The
system switches valves and starts a cycle where the N2 is driven off restarting the cycle. The main chamber stores O2 to take up the slack while N2
purging is occurring.
[Edited on 6-16-2010 by IrC]
"Science is the belief in the ignorance of the experts" Richard Feynman
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rrkss
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Is the Nitrogen chemically bound to the lithium or is it more of a zeolite type absorbtion?
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IrC
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"zeolite type absorbtion"
Yes. Most smaller home type units are built using this process. Have also seen hydrides used but unsure in those what all the materials are in the N2
and N2/CO2 processing system. Most models only process N2, ignoring the CO2. There is not a lot of good data around on these O2 concentrators and I
have yet to take the time to search patents.
I scrapped some AC sized units for the tanks and valves. In my vacuum system one of the large tanks is my vacuum storage. If you build cool things you
must salvage some concentrators they are a wealth of great parts for MadSci.
The pumps are not good enough for Lasers or whatever but I did build a vacuum de-soldering station with one.
"Science is the belief in the ignorance of the experts" Richard Feynman
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watson.fawkes
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These concentrators are pressure swing adsorption systems. They rely on a differential adsorption of a target gas over other gases in the input
stream. There are two outputs: the extracted gas (mostly what you want) and the raffinate gas, which is depleted of the extracted gas. The raffinate
is never free of the extracted gas; it just has less of it (a lot less). Whether you want to use the extract gas or the raffinate gas is a matter of
engineering. The basic principle of operation is that, under pressure, the extract gas preferentially adsorbs onto a substrate. Then the inlet valve
is closed, the outlet valve is opened, and when pressure is released, the adsorbed gas desorbs from the substrate.
The engineering of these is weirdly fascinating to me, as it's all about parameter tuning for adsorbent species, plant geometry, and operating
parameters. The absorption and desorption cycles must be precisely timed, since the adsorbent bed is neither fully charged during adsorption nor fully
discharged during desorption, so that there's an oscillating density gradient within the bed.
The monograph on the subject I've read is Pressure Swing Adsorption, by Ruthven, Farooq, and Knaebel, VCH Publishers, 1994.
Some early patents:
U.S.Pat. 2,944,627 Method and Apparatus for Fractionating Gaseous Mixtures by Adsorption (Skarstrom)
U.S.Pat. 3,237,377 Oxygen Concentration Process (Skarstrom)
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zed
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Portable Nitrogen concentration.....That would be nice.
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watson.fawkes
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"Portable" depends entirely on what purity of N2 you
need. If 98-99% is OK, then a PSA system can do that. The adsorbent used is a carbon molecular sieve. They're made by firing carbonaceous material
first to create a porous body and then to with a controlled amount of oxidizer (H2O or CO2) to burn out the pores. Sometimes there's another firing in
inert gas to desorb CO2 and CO species; I suspect this is useful for N2 separation, but I don't have anything definitive. Bituminous coal is one
typical feedstock of the adsorbent.
This is a kinetic separation; the pores have to be (on average) small enough so that the difference in diffusion between O2 and N2 is large enough.
The O2 diffuses faster through the pore; presumably this is some not-quite-bonding effect between O2 and carbon that causes a lowering of surface
tension and subsequently lower boundary-slip friction. So a lot depends on the adsorbent processing.
If you need higher purity N2, then what's typically done is a deoxygenation step, which is in three parts. The first is to introduce H2 into the
mostly-N2 gas stream. The second is catalytic oxidation of the H2; this is just a catalytic converter run "backwards", since we're removing O2 and not
a combustible from the output gas. The third is a dehydration step.
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peach
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It's zeolite. And the nitrogen it gives out isn't clean enough for an inert atmosphere, it'd need scrubbing through something that strongly absorbs
oxygen; the zeolite isn't good enough alone. I haven't looked into the economics of it, but it may be cheaper to go straight to the scrubber over
using the two together.
My cylinder N2 is 99.998% pure with traces of argon and it's dirt cheap. N2 is used extensively by industrial consumers (HVAC).
I can understand generating reactive / harmful gases yourself, but nitrogen? Nah... it's not worth it at all! Cheap, easy to get, very pure, dry...
get a cylinder. Argon and helium are too expensive, and helium floats out of glassware. CO2 isn't inert enough. N2. Sign up, rent, use.
[Edited on 7-7-2010 by peach]
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JohnWW
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Highly electropositive metals such as Mg and Al and Li can be induced to burn in N2, while Na and K spontaneously react with it, forming nitrides. In
addition, some more refractory metals such as Ti form brittle nitrides if heated or welded in the presence of N2.
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peach
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Indeed.
I spent a long, long time reading the aerospace / reactor level theory of welding titanium. The normal kind of TIG welding with a cup shroud works
okay, but it's nowhere near good enough for nuclear work; you're still getting muck at the invisible scale.
Just about everything outside of argon and helium damages it in some way or another. For example, if you're welding things for nuclear applications,
you can't wash it with chlorinated solvents and you can't touch it with gloves containing halides.
Okay, nitrogen might not work for everything in chemistry, but I'm talking more about the idea of generating it yourself versus buying a cylinder. If
the others were talking about generating pure, dry argon or helium, cheaper than it comes in cylinders, there'd be a point to be made. But generating
N2 at home would only be worth it for the fun. It'd be economically retarded and likely perform nowhere near as well as bottled gas.
When it comes to the noble gases, a cylinder is the best route. They're used in huge quantities by industry, I was ordering O2 and acetylene cylinders
when I was about 14, inert gases are even easier, it's cheap, pure and dry.
I'd recommend starting with N2 then going to argon if you have problems, as it's more expensive. Helium is fearsomely expensive and will tend to float
away from where you need it.
Also, normal welding flow meters suck balls for chemistry. They can't regulate the flow slowly enough. I've tried, multiple times, with mine to just
barely make gas flow and it'll blow things like cyclohexane straight through a 0C coil condenser; in amounts large enough to wash the solvent out of
the flask in the course of an hour longer workup. With reactions that take hours to put together, warm, run, cool and process, the cylinder will be
empty after one or two runs. It needs to be a tiny, tiny, tiny flow rate; which a welding regulator can't handle. A needle valve might do it, that's
where I'm going next.
I have been wondering about renting cylinders of HCl(g) and such. The big names do them. I'm curious as to what kind of paperwork needs filling out
and if there are any courses or certificates I need to have. I'm VERY used to handling the cylinders and know about the contents, so I wouldn't mind
jumping through some hoops. It'd be nice to have lots of pure, dry HCl(g) to hand that I can tap into and know I'll get a constant stream for a decent
period of time. I bet they're really expensive, as the cylinders need routinely binning. I also suspect there'll be some form of security checks,
given that they could be opened in public and harm people.
[Edited on 8-7-2010 by peach]
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watson.fawkes
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The vapor pressure of dry HCl at room
temperature is about 300 psi (42.6 bar at 20 °C according to this reference sheet), so it's a candidate for liquid storage. I believe stainless steel tanks and valve are required. I seem to recall that Viton
gaskets are adequate. All these materials should be verified; I don't have materials compatibility charts handy at the moment.
I'm not sure if it's feasible to consider compressing/liquefying by the motivated amateur. Some combination of pump and refrigeration loop might work,
though.
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densest
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@peach - yes, needle valves are necessary for low flow rates. You may find that a 2-stage regulator is necessary as well to avoid surges from the
tanks. Flow meters (ball in a vertical tapered tube) are available down to very low flow rates (10 cc/sec or so).
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peach
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When I was trying to eek out the lowest flow rate possible from my welding regulator, I was sitting there and the regulator started oscillating,
making a banging noise in the cylinder. I nearly shit my pants until I realized what was happening.
In terms of liquid HCl storage, rather than compress it, it'd probably be easier to generate it and then blow it through a dewar style condenser,
feeding into the storage tank.
I suppose something suitable could be produced using high grade 316 stainless pipe and fittings. That is a worrying item to have hanging around the
home however. I'd certainly want it double walled with it venting through something that'd attempt to deal with the gas if it started leaking. Even if
I throw a single walled option outside, it'd still stink and probably have people nearby coughing.
[Edited on 15-7-2010 by peach]
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Fleaker
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Wet HCl and stainless do not get along.
Even dry HCl is quite aggressive. You'd be better off condensing it in a FEP tube after it's been dried.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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