
Supercritical CO2 is heavy like a liquid, with weird and useful properties - firstbase
https://popula.com/2019/01/29/saving-the-planet-with-co2/
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neltnerb
Couldn't comment to the author without paying for an account there, but as
someone who has actually built sCO2 reactors -- you can get away with a much
lower cost HPLC pump to pressurize the CO2 if you can force it to be a liquid
at the inlet side using an eductor tube and slightly heating the tank to
guarantee that the slightly cooler HPLC pump head won't cavitate.

It worked pretty well, though I had to stick a TEC on the pump head. $5k
instead of $100k+ for a high pressure syringe pump.

My application was to use the sCO2 to solubilize organometallic precursors to
deposit thin films on particles in a fluidized bed, and one of the best
reasons for sCO2 was that the only likely contamination would be carbon (sCO2
is sometimes used to form carbides intentionally).

~~~
Roritharr
>My application was to use the sCO2 to solubilize organometallic precursors to
deposit thin films on particles in a fluidized bed, and one of the best
reasons for sCO2 was that the only likely contamination would be carbon (sCO2
is sometimes used to form carbides intentionally).

I understand some of these words.

Kidding aside, what was the goal of this endeavor? What is the end product?

~~~
jcims
Let's guess. I'm going to say they are dissolving the 'precursors' into the
co2, pumping it through the bed of particles with sufficient vigor to achieve
'fluidized' [1] state. As the supercritical co2 rises through the bed, the
particles are dumping heat into it, ultimately flashing to vapor and leaving
the solute (or at least the metallic part of it) behind as a thin film on the
particle that did the trick.

This of course leaves the particle itself cooler, making it less likely to
receive additional deposition and resulting in a more uniform thickness across
the batch.

These coated particles are probably then going to be used as a catalyst in
some other process.

[1]
[https://www.youtube.com/watch?v=My4RA5I0FKs](https://www.youtube.com/watch?v=My4RA5I0FKs)

(alternate is that the the precursors themselves are the fluidized bed and the
co2 is used to help move the coating from the donor to the recipient. If GP
called them parts and not particles i would be more inclined to this option)

~~~
neltnerb
Almost! Except for the flashing CO2 to vapor, I actually just had a filter at
the top to keep particles from flying out but the reactor itself was at
5000PSI and I just used a backpressure regulator for the exhaust stream which
was very little since basically every byproduct was a gas. But in backstory, I
invented a process, particle supercricial fluid deposition (UMass Amherst
previously demonstrated deposition onto wafers and I worked with them on it
through STTR), which lets you put down conformal thin films onto particles as
a protective coating.

[https://www.brianneltner.com/thin-films](https://www.brianneltner.com/thin-
films)

The key is conformal films and aspect ratio. If you're familiar with CVD, you
might have encountered "duning" where film growth can produce unstable
interfaces or find that a trench will get filled in at the top and leave a
void underneath.

One common option is using ALD which uses alternating precursors so you can
fully coat every surface of an arbitrarily high surface area material with a
monolayer of your precursor, and then a second step converts that precursor to
metal so you get atomically thick layers.

The downside of ALD is that it's quite slow in terms of growth rate because
you can only grow one atom thick layer at a time. Additionally, the precursor
has to have a vapor pressure high enough that the often solid precursors can
actually be flowed through a reactor under vacuum. But it can get super thin
films that can be conformal and which have incredible aspect ratios.

What I did was use sCO2 to _dissolve_ chemicals like those ALD or CVD
precursors in an "organic solvent" (sCO2 itself) to deliver it at super high
concentrations to the reactor (at like 5000PSI) instead of at super low
concentrations under vacuum. It stays supercritical through the full fluidized
bed. But the reactor itself is hot so the precursors spontaneously decompose
and do so preferentially on surfaces. But! The key difference is that because
of the high precursor concentration, instead of the film growth rate being
dictated by how fast precursor can get to the particle surface (diffusion
limited) it's limited by how fast the precursor reacts on the surface
(kinetically limited). The prior is notorious for unstable interfaces, the
latter is much better because it's just growing at max speed at all locations
simultaneously.

So the real difference is that it let me deposit thin films from a high
concentration precursor stream.

My actual application was trying to coat copper flake with chromium metal to
increase the oxidation temperature for use as a replacement to silver
conductive pastes used in solar panel manufacturing. Silver was (might still
be) like 10% of the total cost of making one, so this was of keen interest and
would be pretty awesome. Ultimately, I was able to make it so that copper
showed no oxidation at 300C over a minute while uncoated copper did show a lot
of oxidation.

However, the amount of coating I needed to keep the oxygen away from the
copper was too thick (or at least I couldn't make it thinner with that tech)
and the chromium was expensive enough that it'd have ended up more expensive
than just using silver.

So I moved on =) A project worth doing for sure, everyone I talked to about it
simply could not guess if it would work. Sadly it worked but wasn't
commercially interesting in the end.

~~~
egdod
Super interesting. Comments like this are what keep me coming back to HN.

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gene-h
Venus' lower atmosphere is actually composed of supercritical CO2 at 90 atm
and ~450°C which makes corrosion interesting[0]. sCO2's property of acting
like a solvent works against us in this case. Copper grows a hair copper
sulphide from the trace amount of sulfur dioxide and other sulfur compounds
present. Hydrofluoric acid, even though it's present at part per billion
concentrations, cause corrosion problems too. While we now have interesting
silicon carbide integrated circuits like image sensors[1] and 555 timers[2]
capable of operating at Venusian temperatures, it's challenging to actually
use them on Venus because of corrosion of the bonding pads.

[0][https://www.researchgate.net/publication/325521557_Chemical_...](https://www.researchgate.net/publication/325521557_Chemical_Analysis_of_Materials_Exposed_to_Venus_Temperature_and_Surface_Atmosphere)

[1][http://kth.diva-
portal.org/smash/get/diva2:1303493/FULLTEXT0...](http://kth.diva-
portal.org/smash/get/diva2:1303493/FULLTEXT01.pdf)

[2][http://www.diva-
portal.org/smash/record.jsf?pid=diva2%3A1317...](http://www.diva-
portal.org/smash/record.jsf?pid=diva2%3A1317070&dswid=-1305)

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mikeyouse
It's amazing how wide the applications of supercitical CO2 can be -- we used
it as a solvent to extract high-value lipids from algal biomass at a biofuel
company I worked for:

[https://en.wikipedia.org/wiki/Supercritical_fluid_extraction](https://en.wikipedia.org/wiki/Supercritical_fluid_extraction)

But it can also be used as a 'solvent' in non-toxic and low/no-water laundry
systems:

[http://e3tnw.org/ItemDetail.aspx?id=512](http://e3tnw.org/ItemDetail.aspx?id=512)

In addition to all of the benefits it can provide for energy-related pursuits
as a working fluid.

~~~
grawprog
It's also being used as a solvent to make high quality cannabis concentrates
and extracts in place of butane or alcohol that's more commonly used now. I
haven't tried them, but I know the CO2 extracted products are being used more
in edibles as toxic solvents aren't used in the extraction process.

~~~
h2odragon
Such extracts are _too_ pure imo. Especially for edibles, I'd rather have an
ethanol extract that got a run of related chemicals and some other stuff that
came along for the ride... aka "flavor".

Super pure extracts that then have added back flavorings, or even blended
concertos of specific chemicals, miss the charm and much of the value of
taking the "light oils" off the plant. Flower is hard to fake, home extracts
are easy to do, and who knows what's in that commercial oil, really?

~~~
grawprog
Ah yeah, the terpenes in the strains do more than add flavour too. Different
terpenes in different strains have been found to have different effects in
combination with the cannabinoids in the strain. I doubt re-adding terpenes
and flavouring after the fact has the same effect. I suppose I have tried CO2
extract, I have tried the prefilled oil carts with the added flavour. I don't
like them. The high from them feels weird and the taste isn't great.

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BeefySwain
Applied Science has a great video (as all of his videos are) on supercritical
CO2.

[https://www.youtube.com/watch?v=-gCTKteN5Y4](https://www.youtube.com/watch?v=-gCTKteN5Y4)

~~~
geon
Codys Lab did a video as well; [https://youtu.be/4Z-KbcLs-
yo](https://youtu.be/4Z-KbcLs-yo)

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01100011
It seems like this link from the article deserves its own front page story:
[https://www.ge.com/reports/call-ecomagination-ge-building-
co...](https://www.ge.com/reports/call-ecomagination-ge-building-co2-powered-
turbine-generates-10-megawatts-fits-table/)

That's pretty neat.

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jchallis
Martyn Poliakoff of Periodic Videos fame has done a lot of work with
supercritical CO2, particularly in the context of more environmentally
friendly organic solvents.

This 5 minute video provides an excellent summary:
[https://www.youtube.com/watch?v=yBRdBrnIlTQ](https://www.youtube.com/watch?v=yBRdBrnIlTQ)

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Gravityloss
Some molten salt reactor designs were planned first with helium turbomachinery
and later with supercritical CO2.

Also GE tried to sell gas turbines for ship propulsion. You get back some
efficiency with a CO2 turbine running with the waste heat. Ship owners still
like their diesels. Really long time between overhauls.

~~~
DuskStar
> Also GE tried to sell gas turbines for ship propulsion.

Pretty common for warships though, or at least that's my understanding. The
Arleigh Burke class [0] is powered by GE gas turbines, for instance.

Though the requirements list for "large surface combatant engine" and "huge
cargo vessel engine" are probably more than a bit different.

0: [https://en.wikipedia.org/wiki/Arleigh_Burke-
class_destroyer](https://en.wikipedia.org/wiki/Arleigh_Burke-class_destroyer)

~~~
Gravityloss
Yes. They pay little for fuel. Also afaik they do not use the waste heat.
Though probably with the logistics train to some places the fuel can probably
be expensive at times.

There used to be passenger ships with turbines too. They changed the engines
to regular diesels at some point.
[https://en.m.wikipedia.org/wiki/GTS_Finnjet](https://en.m.wikipedia.org/wiki/GTS_Finnjet)

~~~
jabl
I think they actually do reuse the waste heat in a way, not via steam, but by
being recuperated.

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dv_dt
The other interesting thing is it can be used within a 100% carbon capture
power plant that happens to generate more liquid CO2 which is inherently
trapped within circuit of the plant. (and is presumably tapped off to
sequester in places - this is said to be better than trying post collect and
sequester the gas in a regular system).

[https://en.wikipedia.org/wiki/Allam_power_cycle](https://en.wikipedia.org/wiki/Allam_power_cycle)

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cperciva
Supercritical liquids are generally weird. H2O, for example, changes from
being a polar solvent to a nonpolar solvent -- you can dissolve oil in
supercritical water, and even burn it if you have a source of oxygen. It's
also more corrosive -- which is relevant if you're considering using
supercritical water in a nuclear reactor.

~~~
andrewflnr
> even burn it if you have a source of oxygen

How does that work? What are the combustion products? The closest reference I
could find is this, [https://science.nasa.gov/science-news/science-at-
nasa/2014/1...](https://science.nasa.gov/science-news/science-at-
nasa/2014/10jan_firewater) , which seems to be talking about something else.

~~~
cperciva
Easiest case of this: You have a tank of water and oil under supercritical
conditions, and you pump high pressure oxygen gas into it. You get a "flame"
around the intake, but unlike a regular flame (which is the boundary between
fuel and the surrounding oxidizer) this is the boundary between the incoming
oxidizer and the surrounding fuel.

The combustion products are the same as you would get in the absence of the
supercritical water -- typically H2O and CO2 -- but they're all contained
inside the supercritical water tank, which makes this very convenient for e.g.
disposing of chemical weapons.

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wwarner
I guess supercritical CO2 can be a decent refrigerant as well.

[https://www.greenbuildingadvisor.com/article/a-heat-pump-
usi...](https://www.greenbuildingadvisor.com/article/a-heat-pump-using-carbon-
dioxide-as-the-refrigerant)

~~~
jabl
AFAIU it's quite common in supermarket/warehouse refrigerators and freezers.
Apparently upfront cost is slightly higher, but the refrigerant is cheap and
there's no risk the next iteration of the F gas legislation will make your
investments a dead end. And of course the environment friendliness is
important to businesses that are concerned about their image.

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idlewords
I spent a summer in college working in a chem laboratory with no air
conditioning. Sometimes I would give the big standing CO2 cylinder a shake,
and if I didn't hear any sloshing, I knew we were in for a rough day.

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grenoire
Holy moly, until now I had absolutely no idea about the existence of
supercritical CO2. And now all the comments here are making me so incredibly
fascinated and excited about the potential utilities for stored CO2. I am
thinking not in terms of the science, but actually making a business case for
CO2 storage _beyond_ just CSR projects!

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intc
"GE has built a prototype which is small enough to fit on a desktop" \- The
linked page ([https://tinyurl.com/y2s9z69d](https://tinyurl.com/y2s9z69d))
claims 10 MW output - This is several orders of magnitude "north" from the
reality (~ 15 kW).

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at_a_remove
From what I can gather, the chambers are quite small due to the pressure
issues. This has the result of making aerogel, typically produced with
supercritical carbon dioxide, come in comparatively small chunks, especially
at the hobbyist level. It's a pity.

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yread
Nice! I was just wondering the other day with water having so many weird ice
forms and what not what do we know about other common chemicals and their
states of matter

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pixelpoet
Fantastic article, very well written and informative with lots of great links
(e.g. diamond anvil).

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wolfi1
sCO2 is also used for the production of aerogel:
[https://en.m.wikipedia.org/wiki/Aerogel](https://en.m.wikipedia.org/wiki/Aerogel)

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aj7
Link doesn’t display on mobile devices.

