I built a pressure vessel from aluminum
and acrylic, and filled it by placing pieces of dry ice inside. The dry ice melts under high pressure, and
forms a liquid and gas phase. When the
vessel is
heated, the CO2 becomes supercritical -- meaning the liquid and gas
phases merge together into a new phase that has properties of a gas, but the
density of a liquid.
Supercritical CO2 is a good solvent, and
is used for decaffeinating coffee, dry cleaning clothes, and other situations where
avoiding a hydrocarbon solvent is desirable for environmental or health
reasons.
If you have a suggestion for what I
should do with the supercritical CO2, please leave a comment.
hey everyone I
thought I'd show you my
latest project this is a plastic chamber
that I built its holding liquid co2 so
first I'll show you how this thing works
and then I'll talk about how I built it
and when I plan to do with it so the
chamber is built from a piece of
aluminum with acrylic on the front in
the back and it's designed to be heated
slowly and I'm using this strip heater
this is just a flexible fiberglass
covered heater that's normally used for
bending acrylic reliefs if that's what I
normally use it for I guess they're also
used for heating pipes you can wrap it
around a pipe in this case I'm going to
wrap it around the chamber
like that and I think this heater is
about a couple hundred watts or
something on that order so we have
liquid co2 inside there the heater on
and I'm going to plug the heater in
right now the pressure gauge is reading
about 700 psi use a I drilled a tiny
hole in there just so I can put a
thermocouple in and get a more accurate
reading you can see the co2 is starting
to boil in there
you
okay now the boiling has pretty much
stopped there's there's you can still
see some density changes in the liquid
there but at this point there's no more
bubbles and the transition from the
liquid to the gas phase in there is
almost gone so let me give it another
shake and as you can see something
interesting has happened there's no more
dividing line between liquid and gas
inside there so I want to unplug the
heater and give it a few minutes for the
cloudiness to subside
as you can see what we're left with here
is one phase basically inside the
chamber the co2 has gone supercritical
meaning that the density between the
liquid phase and the gas phase
eventually became so similar that they
blended together so it's it's all one
continuous phase in there now now to
transition back what I'm going to use is
just a piece of dry ice and I'm going to
hold it against the the aluminum on the
outside here to bring the temperature
down quickly so check this out
surprisingly this actually works better
with just water ice since it melts and
makes a nice contact between the
aluminum and the ice cube as you can see
we're back to two phases so I built this
chamber by getting an aluminum disc with
a hole already in it and I turned it
down and faced it and then turned the
inside and then mounted it on the mill
and drilled the holes first for the bolt
circle and then since the there's a
groove that holds an o-ring in there and
the bolts are so close to the o-ring or
rather the part is so small it would
have been difficult to clamp to cut that
o-ring groove so what I did is I tapped
the the holes in the mounting surface on
the mill and then temporarily used some
some screws just to hold the piece down
and then cut the o-ring groove so same
on the other side just exactly the same
and then I also used the mill to cut out
these these plastic parts with the same
bolt circle and put an o-ring in there
and sandwiched it all together at one
interesting problem that I ran into is
that there's no valve on this chamber so
what I do to fill it is just you know
put half of it together drop a few
pieces of dry ice in there and then put
my own then screw the other side down
but this ends up being a problem because
the pressure builds before I have time
to actually get the chamber sealed up to
the proper torque so the o-ring is
severely deformed because it was under a
lot of pressure even as I was closing
the chamber you can see the o-ring is
kind of squeezed out into the space
between the plastic and the aluminum the
gland was actually carefully designed so
that the o-ring would crush down in
there and fill up the space in the gland
but it actually still managed to seep
out same on the back side I used actual
engineering formulas to determine the
thickness of acrylic that I would need
for this and I was anticipating
higher pressures so this the co2 starts
to go supercritical at around a thousand
or 1,100 psi but I designed this chamber
thinking that I would actually encounter
you know much higher pressures to three
or even four thousand psi if I raise the
temperature high enough obviously the
the pressure of the supercritical fluid
would go higher so initially when I
first set this up I was a little worried
about you know the thing rupturing
because you know who knows what's going
to happen so what I did is I set this up
in a corner of my shop with a camera on
it and plugged in the heater to a long
extension cord so I could you know watch
this thing on a TV monitor far away and
then turn the heater on and off and
monitor the temperature and pressure so
I got it up to one hundred and twenty or
thirty degrees Fahrenheit and the
pressure was still under 2,000 so I you
know I'm not sure maybe I'll try putting
more dry ice in so that there's more
volume inside there to see if the
pressure is going to be higher I don't
know but anyway so what are you going to
do is super supercritical co2 the first
thing I want to try is putting green
coffee beans in there and see if I can
extract the caffeine from them one of
the commercial uses of this process is
to decaffeinate coffee beans but of
course the caffeine doesn't get
destroyed it just gets pulled out so you
can refine it another interesting use
for supercritical co2 is dry cleaning so
you can put something in there that has
a solvent or that has a oily substance
on it and remove the oily substance
without without using water or a
hydrocarbon solvent okay well I hope you
enjoyed that let me know if you have any
suggestions for what I should do with
supercritical co2 all right see you next
time bye
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