From: "Barry L. Ornitz" <ornitz@nospam.org>
Newsgroups: rec.crafts.metalworking
Subject: Re: Acetone and dry ice question.
Date: Tue, 2 Nov 1999 17:00:18 -0500

George McDuffee <73504.1016@CompuServe.COM> wrote in message
<7vm0j9+ACQ-718+ACQ-1@ssauraab-i-1.production.compuserve.com>...
>Acetone works on dry ice the same way that salt works on regular
>ice.  Acetone vapors are VERY  flammable / explosive.


The first statement is incorrect.  The assertion that certain
baths containing solid carbon dioxide snow (dry ice), especially
the acetone bath, give a temperature lower than pure solid carbon
dioxide is a common one but wrong.  It achieves a temperature of
-72 F whereas the sublimation temperature of dry ice is -78.5 F.

The advantage of using the acetone/dry ice slush is one of heat
transfer.  It is difficult to get good heat transfer from solid
dry ice and its vapors.  Adding a liquid that does not freeze
promotes the heat transfer (the liquid heat transfer coefficient
is far higher than the gas phase heat transfer coefficient).

George is very correct about the flammability of the acetone.
However, at these temperatures its vapor pressure is low and it is
well below its flash paint.  Probably the best liquid to use here
would be trichloro-trifluoro-ethylene, Freon TF.  But since this
is an ozone destroying material, it is nearly impossible to get
any more.  It is non flammable and has a low toxicity.

>Before I would try this I would check to see if I could get some
>liquid nitrogen.  At least the vapors arent explosive.

I agree with this.  Liquid nitrogen at - 321 F is generally much
safer to use.  In bulk, it costs about the same as gasoline per
gallon.  Many welding supply places carry liquid nitrogen but
liability issues may prevent the sales to individuals in small
quantities.  When handling liquid nitrogen, wear non-porous rubber
gloves over thermal insulating gloves.  Many materials become
extremely brittle at liquid nitrogen temperatures so be careful.

        Barry L. Ornitz

From: REMOVE_THISdwilkins@means.net (Don Wilkins)
Newsgroups: rec.crafts.metalworking
Subject: Re: Acetone and dry ice question.
Date: Tue, 02 Nov 1999 17:56:06 GMT

On 02 Nov 1999 04:50:30 GMT, gunluvver2@aol.com (Gunluvver2) wrote:

>I seem to remember a method of using dry ice and acetone to achieve super low
>temperatures. Does anyone know  the proper way of doing this and how low the
>temperatures will get?

Not super low but that is relative term. We used liquid helium for
super low stuff.

We used acetone (plus a coolant) routinely to separate (freeze) water
vapor from hydrogen and carbon monoxide in a vacuum system.

Standard cold trap design with a Dewar flask. Acetone in the flask and
add dry ice and stir. Actually we used liquid nitrogen instead of dry
ice and poured it in and stirred until we got an acetone slush. This
puts you around minus 80 to 90 degrees centigrade range. CO2 sublimes
around minus 78 so you will be in that range.

Why did we use liquid nitrogen instead of dry ice? We had large tank
of it in the lab. The dry ice was in a freezer in the chem stockroom
about a city block away and then up five flights of stairs. Dry ice
was not allowed on the elevators.

Since acetone melts at minus ~95 it gives you a lower limit as long as
you keep liquid present.

Either LN or dry ice will cause a lot of splatter. Small droplets on
the arms sting. Large ones when held on the skin "burn". Safety
goggles are an absolute must. Now the temperature may be cold but
acetone vapor will ignite so treat it with respect.

Pick the temperature you want then find a liquid with a freezing point
near that temperature and you have a built in lower limit as long as
you don't let it freeze solid.

From: "Barry L. Ornitz" <ornitz@nospam.org>
Newsgroups: rec.crafts.metalworking
Subject: Re: Acetone and dry ice question.
Date: Wed, 3 Nov 1999 22:12:22 -0500

Paul Koning wrote in message <38207E1C.749ED81E@xedia.com>...
>"Barry L. Ornitz" wrote:
>> ...When handling liquid nitrogen, wear non-porous rubber
>> gloves over thermal insulating gloves.  Many materials become
>> extremely brittle at liquid nitrogen temperatures so be careful.
>
>Rubber is one of those materials.  At least, the kind
>of rubber used in rubber balls is -- I remember a demonstration
>where one was dunked in liquid nitrogen, dropped on a
>steel plate, and shattered.  So make sure that the
>"rubber" you use for the gloves is in fact some substance
>that remains flexible at 77 K.


The rubber really does not have to remain flexible at 77 K.  It is there
just to prevent fibrous thermal gloves from absorbing the liquid.  Any
liquid nitrogen spilled on the gloves should run off.  You do not want to
cup your hands and hold the liquid nitrogen there.  Yes the rubber may get
brittle, but it has already done its job of protecting you.

        Barry

From: "Barry L. Ornitz" <ornitz@nospam.org>
Newsgroups: rec.crafts.metalworking
Subject: Re: Acetone and dry ice question.
Date: Fri, 5 Nov 1999 21:32:00 -0500

Robert Nichols wrote in message ...

>The only danger would be touching the walls of the container.
>Liquid nitrogen has very little quenching ability.  The liquid is
>already right at its boiling temperature, and the phase change to
>vapor doesn't require much heat energy.

Incorrect answer.

The latent heat of vaporization does not come into play here.  In
fact, the phase change of a gram of liquid nitrogen to gaseous
nitrogen requires about half as much heat as does the same change
in a gram of Freon-114 refrigerant, or about 48 calories per gram.

>The result is that the layer of gas bubbles that form insulate
>the skin from the liquid.

This is true and is a result of heat transfer properties, not the
latent heat.  Because the liquid nitrogen is at -320 F and your
skin is close to 80 F, the large temperature difference leads to
film boiling.  This is a poor mechanism for heat transfer.
Physicists call the mechanism the Leidenfrost phenomena.  People
working in metal often see the same effect.  Heat a piece of iron
to 500 F (slightly below a dull red) and place a drop of water on
the surface.  Note that the droplet just dances around on the
surface, skittering about on a film of gas that reduces the heat
transfer.  Place a droplet of water on an iron piece at 250 F and
note that it evaporates instantly.  Nucleate boiling is very
efficient at transferring heat.

Touching your skin to the walls of the container, which are at the
liquid nitrogen temperature, will instantly freeze your skin to
the container if you grasp tightly.  In this case, you no longer
have the gaseous film to reduce the heat transfer.  You now
transfer heat by conduction which is far more efficient - causing
frozen fingers.

>A friend of mine encountered an instructor at school who would
>pour liquid nitrogen into his own mouth.  Now that's something I
>wouldn't care to try.

There was a chemical engineering professor at a midwest university
who would drink a very small quantity of liquid nitrogen too.  He
had perfected the ability to keep his hiatial sphincter open
allowing the gas to vent, blowing smoke rings.  He quit after a
student tried this and died.

Don't be stupid and try tricks like this.  Treat liquid nitrogen
with care, like any hazardous material, and you can use it safely.
Ignore the precautions and you will very likely get hurt.

One additional precaution when working with liquid nitrogen is to
never tightly cap a container containing it unless that container
has a proper pressure relief valve.  So don't place liquid
nitrogen in a Thermos and screw the cap down tight - it will
explode.

        Barry L. Ornitz

From: "Barry L. Ornitz" <ornitz@nospam.org>
Newsgroups: rec.crafts.metalworking
Subject: Re: Acetone and dry ice question.
Date: Fri, 5 Nov 1999 23:08:01 -0500

kenneth knaell wrote in message <80073n+ACQ-fe5+ACQ-1@autumn.news.rcn.net>...

>I've got an oil diffusion pump with a liquid nitrogen cold trap
>on top.  If I ever get around to using it I thought I would use
>dry ice and acetone if I can get it into the small opening.
>Other than the higher vapor pressure of the oil at this higher
>temperture, is there any experience that would dictate that this
>would be a bad idea?


The vapor pressure of the oil is a strong function of
temperature.  You are talking about approximately 225 F higher
operating temperature.  To a first approximation the logarithm of
the vapor pressure is inversely proportional to the absolute
temperature.  When operating at this higher temperature, expect
the oil to have several orders of magnitude higher vapor pressure.
You are likely to contaminate the evacuated system with pump oil.
Generally the cold trap ahead of a diffusion pump not only
condenses the oil, but it also freezes it.  It also removes many
materials that might contaminate the oil.  The liquid nitrogen is
not flammable either.  All my experience says this is a very bad
idea.

        Barry L. Ornitz

From: REMOVE_THISdwilkins@means.net (Don Wilkins)
Newsgroups: rec.crafts.metalworking
Subject: Re: Acetone and dry ice question.
Date: Sat, 06 Nov 1999 14:29:38 GMT

On Fri, 05 Nov 1999 22:13:45 GMT, mulligan@advinc.com wrote:

>In article <FKqCzs.5Aq.0.omega-3@interaccess.com>,
>  rnichols@interaccess.com (Robert Nichols) wrote:
>
>> The only danger would be touching the walls of the container.  Liquid
>> nitrogen has very little quenching ability.  The liquid is already
>> right at its boiling temperature, and the phase change to vapor doesn't
>> require much heat energy.  The result is that the layer of gas bubbles
>> that form insulate the skin from the liquid.  A friend of mine
>> encountered an instructor at school who would pour liquid nitrogen into
>> his own mouth.  Now that's something I wouldn't care to try.
>
>Honestly I feel bad now for bringing this up.  Yes, you can
>get a really bad frostbite from doing this, if the boundary layer
>of gas is disrupted (stirring you hand in the liquid, for example)
>so please do *not* do this.
>
>As far as cryogenic liquids go, LN2 can absorb a large amount
>of heat as it boils off.  And don't forget, the boil-off is still
>very, very cold and can easily cause frost burns.

198 KJ / kg, or 0.055 KWh / kg doesn't appear to be a whole lot of
"heat" but I guess what makes a large amount is relative. The
equivalent is a whole lot less than a penny's worth of electricity.

I used LN daily for many years and never got "burned". I never used
gloves of any kind when filling the Dewar. When I needed to grab
something that had been in the LN I used insulated gloves.  Usually
this was some glass ampoule which had been sealed under vacuum. Rubber
gloves over insulated would not be very practical handling these.

LN is one of those things which has a wealth of hazards but if you
know what you are doing is not likely to bite you. I have done the
demos for introducing kids to science. You just need to know that  it
is damned awful cold and if you have any part of you in contact with
anything at that temperature for very long it is gonna get you.

The other thing to keep in mind is that it is not a toy to play with.
If you don't know what you are doing don't do it.

>These can cause real problems so don't ever put yourself in the
>path of boil-off gas, or touch frosted fittings, valves or
>containers.  Not really a good idea.

I have never found the boil-off gases to be a particular problem. I
believe you would need to be in contact with the gas for an
exceedingly long time because of the low heat capacity of the gas.
Solids or liquids are another story. They can get you very quickly.