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TL;DR: A liquid is circulated between a high vacuum
chamber, where it is chilled to dissolve gas molecules,
and a low vacuum chamber, where it is heated to drive
the molecules out. This should result in the high vacuum
chamber being highly evacuated.
(This has been on my list for years, but
[Frankx]'s closely
related CO2 direct air capture idea [link] reminded me to
write it up and post it.)
This is a type of high vacuum pump intended to lower
costs by not requiring much specialized hardware. It does
require a specialized liquid medium,
though, which might be expensive. Anyway, it consists of
a circuit for pumping this liquid (the solvent) around,
where the liquid is exposed to
the surroundings in two places: inside the high vacuum
chamber and inside the low vacuum chamber. The low
vacuum chamber is evacuated by
a conventional backing pump like a sliding-vane vacuum
pump (which is already inexpensive, and can easily be
bought used). The differential
solubility pump is therefore a substitute for other high
vacuum pumps like the turbomolecular pump and the
diffusion pump.
Anyway, as I said above, we (the general we) have
(hypothetically) a low vacuum chamber, and a high
vacuum chamber. Initially, both are at
low vacuum. The differential solubility pump has a part
of itself in each of the two chambers; these parts are
largely similar or even identical
and consist of trays for the solvent to have a high surface
area exposed to the chamber, as well as heat exchangers
or phase change
condenser/expander units. The two trays are connected
to each other by two pipes, so that the solvent can flow
continuously through both
trays in a loop. The pipes each have a positive-
displacement hydraulic pump/motor; these are geared to
each other and driven together by a
motor. This ensures that whenever solvent goes one way
through one pipe, an equal amount of solvent must go
the other way through the other
pipe. This prevents the solvent being sucked into one
chamber that has a lower air pressure in it. (The
requirements on that system are not
great, though, because there's not much difference in air
pressure between low and high vacuum, even though
there is a large difference in
vacuum. This is because pressure and vacuum are
inversely related, and inverse relationships are
nonlinear.)
The heat exchangers (in that version) are connected to a
heat pump, or the condenser and expander (in that
version) are connected to a
refrigerant compressor to make a heat pump out of
them. This is to make the solvent in the tray in the high
vacuum chamber cold, and the
solvent in the tray in the low vacuum chamber hot.
(Additional solvent-to-solvent heat exchangers may be
added to the solvent pipes at the
inlet/outlet of each tray, to conserve heat/cold in the
tray and thus increase efficiency.) The result of this is
that gas molecules that hit the
cold solvent in the high vacuum chamber are likely to be
dissolved in it, and gas molecules dissolved in the solvent
are driven out of it in the
low vacuum chamber. With the solvent being
continuously pumped between the two chambers, this
results in transport of gas molecules from
the high vacuum chamber to the low vacuum chamber,
producing the high vacuum.
To save space, this pump could also be made in a form
that doesn't need a low vacuum chamber, just a backing
pump, by substituting a (coiled?) long pipe
where the solvent and low vacuum can come into
contact with each other for the low vacuum chamber and
tray.
The solvent has to be some liquid with an extremely low
vapor pressure, and that can dissolve lots of different gas
molecules. Vacuum pump
oils meet the first requirement, but I don't know how
well they dissolve gases. In sliding-vane pumps, it causes
problems when the oil dissolves
process gas, which is why a sliding-vane vacuum pump
will usually have a "gas ballast valve" that just lets in a
bit of air to flush out the
dissolved process gas (at the expense of achieving a
worse vacuum on the suction side), which implies that
they do dissolve at least some gases
at least somewhat. Alternatively, an ionic liquid may be
applicable. Either way, though, this pump will probably
not pump all gases equally well,
which is a limitation that applies to the current options
as well—e.g. virtually all vacuum pumps are bad at
evacuating helium.
If you can find a liquid solvent that will dissolve the
gases you want to evacuate from your high vacuum
chamber, but won't dissolve some other gas, you
might be able to use that gas as an eluent to carry away
the evaporated process gases from the warm side. In
that case, you wouldn't need a low vacuum
backing pump at all, just a supply of eluent gas free of
any gases soluble in the solvent.
69/480 [between 2017-01-08 and late April 2017]
CO2 direct air capture
by [Frankx]. Closely related, but not inspiratory for this, because I had this idea two and a half years ago. It just reminded me that it was on my list. [notexactly, Oct 02 2019]
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Annotation:
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It's an interesting idea, but as you point out the critical factor is the solvent. |
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It needs to have extremely low volatility (like diff pump oil) in high vacuum, yet have an affinity for gases; that suggests a polar molecule. |
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How well would this pump noble gases ? |
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What's the advantage over a sorbtion pump, or a sputter ion pump ? Ion pumps are pretty compact, have no moving parts, and deliver high vacuum. |
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It would pump noble gases more or less well depending on their solubilities, and temperature coefficients of solubility, in your chosen solvent. I don't know if there
would actually be any solvent good for noble gases, though. |
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You could use a sorption pump as a roughing pump before turning this one on (though you couldn't use it as the backing pump to continuously evacuate the low
vacuum chamber, because it will saturate). I expect this will achieve better ultimate vacuum than a sorption pump. |
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Versus an ion pump, I expect you won't need as high a vacuum before you turn it on, and it's continuously disposing of the process gas molecules rather than just
trapping them in itself, so it should never saturate in an absolute sense. But I expect it won't produce as much ultimate vacuum either. I think you could use it as a
backing/roughing pump for an ion pump. Wikipedia says of ion pumps: |
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// Both the pumping rate and capacity of such capture methods are dependent on the specific gas species being collected and the cathode material absorbing it.
Some species, such as carbon monoxide, will chemically bind to the surface of a cathode material. Others, such as hydrogen, will diffuse into the metallic
structure. In the former example, the pump rate can drop as the cathode material becomes coated. In the latter, the rate remains fixed by the rate at which the
hydrogen diffuses. // |
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In this pump, the solvent doesn't have a constant surface to get saturated by CO. Of course, its volume can still get saturated if the gas can't be driven out fast
enough on the exhaust side, but this saturation will take a lot longer due to volumes being bigger than surfaces, which could make the difference when pumping
down or taking up a transient release of gas from the experiment. On the other hand, hydrogen should diffuse through it (and out of it on the exhaust side) much
faster than through a solid metal, though I hope not so fast that it can't be effectively pumped from one tray to the other. |
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