“The steam produced in cavitation is like putting water in a bell jar
and pulling a vacuum. If there is enough vacuum the water will boil,
converting it's temperature into energy for the phase change.
Eventually you end up with a chunk of ice in the bell jar.”
Not so! Not enough latent heat is lost. Sensible heat must be removed
also. In outer space, where there are huge radiative heat losses this
applies. If folks could make ice with simple vacuum pumps, who would
mess with refrigeration? It takes energy to evaporate water, not just
vacuum.
Plus, when a gas is adsorbed onto a sieve, energy is released. It
takes the same energy (and then some) to regenerate the sieves. No
free lunch; no where.
From: [email protected]
[mailto:[email protected]] On Behalf Of
[email protected]
Sent: Sunday, March 18, 2012 7:55 AM
To: [email protected]
Subject: Re: [Gasification] On the subject of H2 and O (was N2
removal)
Hi Greg,
Sure. Let's to a simple example so my feeble mind can follow it.
When I drop cold chips in my gasifier there is some moisture content
in the chips...this is liquid water in the wood cells. As the chips
burn down into the hearth they get hot and the water changes to steam.
This phase change absorbs some amount of energy. Then as the steam
goes through the char, some of it does the water gas shift if there is
enough heat. This absorbs even more energy. The remaining steam ends
up as condensate in the cooler.
The steam produced in cavitation is like putting water in a bell jar
and pulling a vacuum. If there is enough vacuum the water will boil,
converting it's temperature into energy for the phase change.
Eventually you end up with a chunk of ice in the bell jar.
The water gas shift will not happen in the phase change because there
isn't sufficient activation energy availble to make the reaction go.
If it did work that way, there would be hydrogen bubbles coming off
boat propellers. That would make a COOL rooster tail!
Gasifiers do not run a low enough pressure to vaporize the water, like
the bell jar. I can get the numbers if you want, but you need to be
in -13psi range. That's way more than we pull.
Any clearer or still muddy??
Stephen
-----Original Message-----
From: Greg Manning <[email protected]>
To: Discussion of biomass pyrolysis and gasification
<[email protected]>
Sent: Sun, Mar 18, 2012 10:09 am
Subject: Re: [Gasification] On the subject of H2 and O (was N2
removal)
Hi Stephen, thanks for the reply.
OK, I somewhat understand you, BUT, wouldn't water be doing a phase
change in the core of a gasifier as well ?
What I was getting at, is if steam is produced in cavitation in cold
water, then wouldn't the shift effect also happen within the core
during phase transition (from water as a liquid, to water as a vapor,
when heated by the core) within the same boundaries of effect as water
to steam in the trailing edge of a propeller ? (all of these
situations involve lower that normal pressure zones).
Aren't contrails produced in water vapor on the wing tips of an
airplane because of this same pressure drop phase shift, causing a
dew-point change?
The core of most gasifiers runs in a dynamic lower than atmospheric
pressure ( a very low internal barometric pressure) (suction based
units), so the same shifts should apply during phase transition,
shouldn't they ?
I understand that a pressure fed gasifier would behave differently
(and I've personally observed this) than a suction based one, I'm
speaking about suction based gasifiers.
Somewhat lost,
Greg
On Sun, Mar 18, 2012 at 8:26 AM, <[email protected]> wrote:
> Greg,
>
> The energy doesn't change. The bond energy in the water is constant
> regardless of pressure.
>
> The cavitation issue is different. It is a phase change phenomenon,
> not a
> chemical change one.
>
> Stephen
> -----Original Message-----
> From: Greg Manning <[email protected]>
> To: Discussion of biomass pyrolysis and gasification
> <[email protected]>
> Sent: Sun, Mar 18, 2012 9:02 am
> Subject: [Gasification] On the subject of H2 and O (was N2 removal)
>
> Greetings List.
> Since we are speaking input air, I thought I might ask a somewhat
> related question.
> Water gas shift. I know there are many that have talked about this,
> and I understand the basics.
> However, here is the question.
> At what negative pressure ( negative in/wc) does the shift move down
> the temperature scale, to the point of being within the 1000 - 1200 c
> area ?
> We all know that propeller cavitation produces steam in water that is
> 10 c (or there abouts), I have to assume (not being a chemist) that
> the same negative pressure effect would also apply to other principals
> when dealing with water.
> --
> Regards,
> Greg Manning,
> Brandon, Manitoba, Canada
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--
Regards,
Greg Manning,
Brandon, Manitoba, Canada
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