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chief
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Quote: Originally posted by bquirky  | wow. that had never crossed my mind. so if you preheat air and propane (say to 800 degrees) the flame temprature go's up 800degrees acordingly ?
is there an upper limit to that ? so chould you use your preheated flame to preheat some more gas to get another 800degrees ?
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Of course. This is used in the steel-industry, where the exhaust-gasesd heat up alternatingly 2 heat-exchangers:
==> Every now and then one of the heat-exchangers is hot: Then it's switched, so that the incoming air goes through that hot exchanger, while the
exhaust gases go through the other one, heting it up ...
That's how steel can be melted with coal ..., up to 1700 Cels or even hotter ...
======================
The amateur can eg. get himself an electric hot-air-gun and burn charcoal in th 600-Cels-Air of it, reaching temperatures beyond 1400 or 1500 Cels ...
======================
The pre-heating can go as far as the raectants allow; like in a pyramid-construction there are even multi-stage processes possible, depending on what
the materials used can withstand ...; it just costs more fuel, so a lot of fuel would form the basement of the temperature-pyramid, for a quit hot
flame on the top ...
=====================
Since cheap materials exist for up to 2000 Cels: Those 2000 Cels can somewhat easily be set onto the top of eg. a Hydrogen-Oxygen flame ...
[Edited on 11-4-2010 by chief]
[Edited on 11-4-2010 by chief]
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bquirky
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very interesting...
I must meditate on the possibility's, I might be able to do things with air that I had assumed i neaded oxygen for.
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hissingnoise
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| Quote: |
If I'm wrong then fair enough, but don't just say I'm wrong; say why. |
Well, I try to avoid exerting myself (simple laziness!) but I came up with this; an explosive uncompressed will detonate with an appreciably slower
velocity than the same explosive compressed to high density.
It seems safe to assume that the reaction taking place in a smaller timeframe will produce an appreciably hotter flame.
An awkward analogy but you see my point?
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unionised
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"It seems safe to assume that the reaction taking place in a smaller timeframe will produce an appreciably hotter flame."
Not to me it doesn't.
You still end up with the same energy shared between the same molecules so you get the same final temperature.
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hissingnoise
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Well then, we'll have to agree to disagree.
I'd like to go into it more deeply but I haven't time right now. . .
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unionised
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Does anyone else out there have any ideas about this?
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chief
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Higher pressure in a detonation can be calculated to be some sort of equivalent with higher Temperature ...
==> ... but the calories per mole are still the same ...
So it depends on the conditions the temperatures are given for ...
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hissingnoise
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Yes, but I believe the faster reaction will attain a higher temperature, however momentarily.
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Formatik
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That's what I was pointing at with the cyanogen example. Explosions are hotter than respective combustions. The energy of explosions tend to be less
than combustions (compare enthalpies to see the difference).
It has to do with the kinetics of explosion as to why explosions are hotter (not more energetic). Like the pressure generated in an explosion, this
wouldn't be able to be so high if the reaction were not so instantaneous. The velocity and pressure are also directly dependent on loading density. It
seems like this is also the case for temperature.
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unionised
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There may be a bit of talking at crossed purposes here.
Initially in this thread the conditions were not specified for any of these flame temperatures. In those circumstances the usual value quoted is the
adiabatic flame temperature. (It's the hottest the reaction can get and it assumes a start at room temp and pressure)
On the other hand if you have a relatively small quantity of material in a container of some sort then the temperature rise will depend, among other
things, on how much energy goes into heating the container.
It seems to me that the temperature measured for TNT etc depends on the packing density because the more of it you pack in the more heat is available
to warm up the container.
In that case you are not really talking about "what combination of materials gives the highest temperature" but "what design of furnace / device gives
the higher temp."
In the limit of an infinitely large blob of premixed fuel/oxidant it doesn't matter- the stuff gets to the adiabatic limit.
That will be higher for gaseous ozone, rather than the liquid; if you are looking for a "world record" the gas phase wins for the reasons I gave
earlier.
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chief
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A faster detonation will set free more caloric power: More mass will be heated, but to the same temperature ...
Maybe at the high pressures some sort of phase-changes could come into account:
Such phase-changes would probably eat up energy, thereby lower the temperature ...
Molcular-dynamically Temperature halfway is defined via a medium-velocity-distribution of the particles, and pressure via the force exerted by the
particles onto a surface ... ...
==> A higher pressure can come from a higher number of particles ...
==> but a higher temperature must mean a higher medium velocity of the particles ...
There's the difference: Quicker detonation means more particles, at the same speed, pressing against the environment ... higher pressure ; but the
speed would be the same, so: Same Temperature ....
===============================
But on the other hand:
==> Detonations take place at velocities way above any usual molecular speeds ...
The medium molecular speed even for the lightest Atoms (Hydrogen) is only maybe 2-3 km/s (which limits the muzzle-velocity in guns)
==> Then again the speed of sound in eg. steel is around 5000 m/s, in other solids as well, which is often _higher_ than the molecular speed in
gases for any of the atoms involved ...
==> For compressed explosives this speed is even higher ... in the zone where the shockwave runs ...
So: If the velocity of sound in a shockwave, namely 8000 m/s or anything like that, could be directly converted to a temperature, than by molecular
velocity this would mean a _very_ high temperature ..., above the caloric possibilities ...
==> so either the molar heat-content, in calories per mol and Kelvin, is lower at high temperatures than at standard-conditions ...
==> or there is a mechanism in the shockwave that momentarily sucks heat ("temperature") away from most of the material and stores it in the
shockwave ..., i.e. overheats the shockfront at expense of already detonated material ... (then a necessary symptom for that would be that the
shockwave would have to build up speed, not start at full speed ... )
=====================
Well: I'm quite lazy now, lets see what anyone else thinks ...
[Edited on 11-4-2010 by chief]
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Panache
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i tend to side with hissingnoises' rational on this one, temperature is only relevant when measurable, ie the effect/change it bears upon on some
other matter, unionised is absolutely correct in asserting that the calorific value of the combined reactions remains constant, however this is not
temperature as we understand it practically, temperature to be relevant for lab use has to be defined in the context of the effect that reaction will
have on your measuring device. This is very related to surface area of the device and the area of effect (aoe lol) of the reaction. If you had 1kg of
reactant spread out in space over hundreds of km3 its ludicrous to believe it would raise the temperature of a small thermocouple very much at all,
even though it was within the area of effect, compress that same amount of reactants down into a cubic meatre and place the thermocouple within it and
the temperature rise will be appreciably larger than that for the former situation.
Detonation and comparisions with it i think confuse the discussion, as chief has already pointed out (actually he/she also pointed out everything else
i have said, i just dumbed it down, my god we're a committee, anyone else want to restate this information in another way, rofl)
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Panache
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| Quote: Originally posted by chief | [
That's how steel can be melted with coal ..., up to 1700 Cels or even hotter ...
======================
The amateur can eg. get himself an electric hot-air-gun and burn charcoal in th 600-Cels-Air of it, reaching temperatures beyond 1400 or 1500 Cels ...
======================
The pre-heating can go as far as the raectants allow; like in a pyramid-construction there are even multi-stage processes possible, depending on what
the materials used can withstand ...; it just costs more fuel, so a lot of fuel would form the basement of the temperature-pyramid, for a quit hot
flame on the top ...
=====================
Since cheap materials exist for up to 2000 Cels: Those 2000 Cels can somewhat easily be set onto the top of eg. a Hydrogen-Oxygen flame ...
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apparently back in the day (middle ages) it used to take a few days to setup the pyre for their blast furnaces, carefully cutting and stacking the
fuels to produce this effect exactly.
Nasa recently lit the first flame in space after all these years, its on utube (crap video though you really have to use your imagination). I think it
was butane, a small blob the size of a hazenut. When lit it produced a perfectly spherical flame that as it heats and the butane expands increases
then suddenly as no fuel remains it dies, strange, but obvious how the shape of our flames are completely determined by gravity.
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chief
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The Boltzmann-treatment of temperature is quite interesting; everything derived from molecular dynamics ...
==> I once read his original books, when I had access to them ... (still have) ; recommending this !
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unionised
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| Quote: Originally posted by Panache | i tend to side with hissingnoises' rational on this one, temperature is only relevant when measurable, ie the effect/change it bears upon on some
other matter, unionised is absolutely correct in asserting that the calorific value of the combined reactions remains constant, however this is not
temperature as we understand it practically, temperature to be relevant for lab use has to be defined in the context of the effect that reaction will
have on your measuring device. This is very related to surface area of the device and the area of effect (aoe lol) of the reaction. If you had 1kg of
reactant spread out in space over hundreds of km3 its ludicrous to believe it would raise the temperature of a small thermocouple very much at all,
even though it was within the area of effect, compress that same amount of reactants down into a cubic meatre and place the thermocouple within it and
the temperature rise will be appreciably larger than that for the former situation.
Detonation and comparisions with it i think confuse the discussion, as chief has already pointed out (actually he/she also pointed out everything else
i have said, i just dumbed it down, my god we're a committee, anyone else want to restate this information in another way, rofl)
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The problem you mention, that the temperature reached depends on how much stuff you have, is precisely the reason why adiabatic flame temperatures are
used.
Otherwise it's simply meaningless to talk about the temperature of a flame, because it depends on how big the flame is.
Unless you are going to specify the details of the system very precisely there's only one meaningful way to answer the question "how hot will the
reaction of A and B get?" and that's to quote the upper limit which will be obtained for a large enough flame.
On that basis, starting with the stuff cold means it ends up cold (in accordance with common sense too, though I accept that's not always a good
guide).
Incidentally, if temperature were only relevant when it were measurable then, in the heart of a brutal explosion which will, among other things, alter
the electron density and, therefore, the thermoelectric coefficients of the metals in your thermocouple, how do you measure it?
It's impossible to measure it in those circumstances so, by your logic it's not relevant.
I take the view that a temperature exists (in some cases several temperatures exist simultaneously) whether you can measure it or not and also whether
or not you can get the ozone and charcoal properly mixed before it explodes anyway.
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hissingnoise
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As The WiZ said; carbon sub-nitride burning in ozone produces a continuous high-temperature flame - now imagine a hypothetical mixture of liquid ozone
and liquid carbon sub-nitride being detonated by shock.
Common sense(?) tells me that the temperature of the fireball will far exceed that of the continuous flame. . .
And the detonation of large quantities would be expected to produce higher temperatures than that of smaller quantities.
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unionised
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Why?
I keep asking where you think the energy comes from to raise the temperature further and you keep not answering.
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The WiZard is In
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| Quote: Originally posted by hissingnoise | As The WiZ said; carbon sub-nitride burning in ozone produces a continuous high-temperature flame - now imagine a hypothetical mixture of liquid ozone
and liquid carbon sub-nitride being detonated by shock.
Common sense(?) tells me that the temperature of the fireball will far exceed that of the continuous flame. . .
And the detonation of large quantities would be expected to produce higher temperatures than that of smaller quantities.
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Well delta Hf is delta Hf... so comes now the question is there
a difference in the Hf of ozone gas and ozone liquid?
I ran a quick check and cannot find the Hf of ozone liquid.
Any rate this would only yield the theoretical high temp. Real
world ....
djh
-----------
... Your sickness appears to be a specific illness
of chemists. One could call it Hysteria Chemikorum,
which originates from the combined damaging
influence of mental exertion, ambition, and the
vapors and fumes. Davy suffered from it,
Mitscherlich, I -- on the whole probable all
great chemists."
Friedrich Wöhler to Justus von Liebig
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hissingnoise
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Temperatures attainable by chemical reactions are limited primarily by the exothermicity of the reactions, not by some arbitrary 'theoretical limit of
chemistry'. . .
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The WiZard is In
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| Quote: Originally posted by hissingnoise | Temperatures attainable by chemical reactions are limited primarily by the exothermicity of the reactions, not by some arbitrary 'theoretical limit of
chemistry'. . .
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Exactly.
The way we obtain our heat is by breaking old and forming new chemical bonds. If the energy produced by the new bonds is greater than used to break
the old, we have a net gain in energy, and this gain is output as heat.
Shidlovskii points out that the maximum gain will be obtained by breaking weak bonds and forming strong new ones. He also says that the strongest
bonds are formed by the combination of elements of opposite properties. As a guide to bond strength we can compare electronegativity.
Electronegativity being the ability of an element to attract electrons. In this race there is one clear winner, fluorine, with a value of 3.98.
However, while there are fluorine compounds with weak bonds the majority are either gases [BF3, CIF3, NO3F, F2O, F2O3, FCI04] or low boiling point
liquids [BrF3, F202], most are more than a little toxic! Next in the electronegativity contest is, you guessed it! Oxygen, with a value of 3.44.
Followed by chlorine 3.16, and nitrogen 3.04.
These elements to produce the largest amount of energy should be combined with elements with low values of electronegativity. Values for some of the
more useful elements are listed below:
K 0.82 Ti 1.54 Ba 0.89 Mn 1.55 Na 0.93
Al 1.61 Sr 0.95 Zn 1.65 Si 1.90 Li 0.98
Cr 1.66 Mg 1.31 Fe 1.83 Zr 1.33 Cu 1.90
No matter how good a chemist one do be, you do be limited .... delta Hf reactants vs delta Hf products.
Physics is different. Think Star-Trek's Matter/Antimatter.
Like you cannot boil water at 760mm Hg above 100o
(no microwaves please) I suspect the limit of chemistry
is where the gas produced, nitrogen for instance, disassociates, suckling up a lot of energy.
Where solids are produced the limit is the solid with the lowest boiling point. Assuming your reactants can produce that much heat.
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12AX7
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I expect the equilibrium temperature is the point where they become a soup of dissociated atoms, i.e. the temperature where the reaction could "go
either direction" and therefore is unable to proceed any further at that temperature due to the balance of energy gained in reaction vs. lost in
dissociation.
Incidentially, such a state of matter will have high apparent heat capacity, since the reactants basically haven't reacted; as the mixture cools, it
delivers significant amounts of heat, far more than the amount expected from an ideal gas at the same temperature and pressure.
The funny thing about physics, actually, is that it *isn't* different. The energy scale is higher, but the principle is the same. For instance, the
equilibrium temperature of a fusion reaction will be on the order of 7MeV, which is around the energy yield of D-D fusion, and also the binding energy
per nucleon of the byproduct, helium. Helium is not well known to fission or spall, and maybe it simply doesn't; but, as a reversible reaction, it
should be possible to add that amount of energy and smack it back into deuterons. And concerning matter-antimatter, a plasma of matter, antimatter
and photons at the same energy (~GeV for protons) will easily have self interactions where photon pairs produce matter just as well. Indeed, at high
energy scales, most matter turns into energy, and one of the early epochs in the Big Bang cosmology is the point when matter condensed out of energy
as it expanded and cooled. (The hard part is, how in the hell we apparently managed to get ordinary matter only, without antimatter!)
Tim
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JohnWW
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| Quote: Originally posted by 12AX7 | (cut)
The funny thing about physics, actually, is that it *isn't* different. The energy scale is higher, but the principle is the same. For instance, the
equilibrium temperature of a fusion reaction will be on the order of 7MeV, which is around the energy yield of D-D fusion, and also the binding energy
per nucleon of the byproduct, helium. Helium is not well known to fission or spall, and maybe it simply doesn't; but, as a reversible reaction, it
should be possible to add that amount of energy and smack it back into deuterons.(cut) |
Helium nuclei - alpha particles - are VERY difficult to spall by means of nuclear collisions, because it is a "doubly magic" nucleus, having "closed
shells" of 2 neutrons and 2 protons (which are spin-paired). This also means that fusion leading to it gives off a particularly large amount of
radiant energy. "Hydrogen bombs" contain LiD as the fusionable material.
This special stability of the He-4 nucleus or alpha particle also accounts for the mode of decay of many heavy nuclei, from Po-210 upwards to a limit
where spontaneous fission is more probable, and also a number of long-lived naturally-occurring alpha-emitters from about Pr to Re, being spallation
by means of alpha-emission. It also explains why the isotope Be-8, which one would think would be stable, decays almost instantly into two alpha
particles (the one stable isotope of Be being Be-9). It is also the reason why the stable isotopes of Li, Li-6 and 7, are so cosmically rare compared
to neighboring elements; while they are produced in large amounts in stars by fusion, the extra proton and neutrons are relatively loosely-held, and
both isotopes tend to be largely destroyed in supernova explosions in favor of either He-4 or heavier elements than Li.
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The WiZard is In
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To paraphrase Arthur C Clarke.
Sufficiently advanced chemistry is indistinguishable from physics.
djh
--------
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unionised
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| Quote: Originally posted by unionised | Why?
I keep asking where you think the energy comes from to raise the temperature further and you keep not answering. |
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hissingnoise
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Oh yeah, liquid ozone vs. gaseous ozone. . .
The liquid ozone will react faster than gaseous ozone.
The faster reaction will produce the higher temperature.
[Edited on 14-4-2010 by hissingnoise]
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