Microwaves

Your first question is easy to answer, yes! if the water is pressurised and will only boil at 400*C then the microwaves will continue heating the water until this temperature is reached. The water will need to be converted to steam to reach a higher temperature.

As for the power of the magnetron, in general if the magnetron's output is doubled the time taken to boil a volume of water will be halved - assuming the microwaves are all absorbed by the water etc...

When they heat water to 400c in an autoclave for the production of synthetic gemstones the water is not turned into steam, under the extreme pressures necessary the water remains liquid! The water will then disolve quartz (Si O2).

Spencer.

Interesting...

Doesn't water still follow the 4 phases though? solid - liquid - gaseous - plasma etc...?

I do seem to remember that water is a peculiar substance, with it having its highest density at +4*C etc...

Hi Electoman,

Hope you are fine?

As with almost all (if not all) liquids, enough pressure applied as it boils will provent the move to gas and on to plasma. ............... Can you get water plasma?.... Mmmm?

Take care.

That is not correct. At the critical point the liquid becomes a gas as the mean free path is that of a gas and it is compressible like a gas, not incompressible like a liquid.

Compressed nitrogen is a critical fluid, and not a liquid in a tank with free gas above is as the Tc is well below 0C.

Hello aurizon,

Thanks for your reply post my friend.

It has taken me a while to find proof of what I had in my mind but look at post #22 it will give the detail I knew but could not confirm.

I hope you and the OP find them useful. As far as I know the OP has not gotten back to give any more details of why he needs to get to 400 °C ?

Mind you, I doubt this pressure and temperature would make the water he is after usable? Unless he want to build a film covering machine or something?

Take care.

For water to be liquid at 400°C (aka 673K) so that it can boil will require considerable pressure. Looking at a water phase diagram the pressure required for this phase state change at this temperature is about 20 mega Pascals. Are you planning on doing this in a diamond anvil?

Since the question has been answered, what's the actual application out of interest?

(I have visions of disassembled microwave oven parts attached to a crazy looking tea kettle).

Our $85,000 microwave at work can do this...and it also re-heats a cold cup of coffee to scalding hot in less than 2 seconds Yes - I too want to know what the application is for this - I am guessing the gem stone business...

when water goes above the critical temperature it becomes a valor. Does it still dissipate microwave energy? or does it become transparent? I know it is not a plasma.

research has been done, look deeper into these search strings and see what you find

http://www.google.ca/#hl=en&source=hp&q=water+%2Bsupercritical+%2Bmicrowave&btnG=Google+Search&meta=&aq=f&oq=water+%2Bsupercritical+%2Bmicrowave&fp=11da210b3e3653dd

http://gow.epsrc.ac.uk/ViewGrant.aspx?GrantRef=EP/D012996/1

http://www.google.ca/#hl=en&safe=off&q=water+%2B%22critical+point%22%2Bmicrowave&meta=&fp=11da210b3e3653dd

WOW Mat Skywalker,

I have parted out about a dozen microwave ovens of asst sizes and powers. It seems that they all have capacitors rated at 2100 and or 2200 volts.

Just for the sake of it could you hazard a guess as to just how powerful the capacitor in an $85,000 microwave would be, and just what value could we expect the directional diode to be.

TMF

Since water heats by vibration of the molecular dipole, so long as microwaves are applied it will heat. At 400C water cannot exist as liquid as it is above the critical point. The time to heat is given by Q = mCp(DeltaT) If the 225 ml of water is in a shallow large dia vessel it should reach 400C in about 8 minutes at 1 kw delivered power. Since pressures will be high, it may not be possible to get a large cross section vessel. Time could then be longer.

Actually water can exist as a liquid at 400°C but only if the pressure is greater than 20 mega Pascals, 3000 PSI or 200 atmospheres. (Pick your pressure unit.) Look at the phase state link I provided earlier. Now how somebody contains this pressure while permitting microwave energy in, beats the hell out of me.

I am afraid not. The critical point for water is 374.2C and 218 Atm pressure. Above this temperature water is a gas.

Actually, it becomes a Supercritical_fluid

Are you saying that the Water Phase Diagram that I cited is wrong or that I'm reading it wrong? I'll grant you that the critical point is near these pressures and temperatures. Also some would consider a supercritical fluid a liquid but at these temperatures the diagram shows solid states of water at even more ridiculous pressures, notice the Ice VII, X, and XI regions. But these are insane pressures to be asking an open forum blog technical questions about. I'm very concerned about the OP's safety.

Under normal conditions, in a closed system as the temperature increases above the triple point, there is an equilibrium between water and its vapour. The vapour pressure keeps incresing till 374.2C, the critical temperature, at which the vapour pressure is 218 atm. Above this the separation between the liquid and vapour disappears and only vapour remains. This is said in the first para of the site. It also talks about a possible second triple point but also says that this is disputed. In the present case, since we are starting from cold and gradually heating, we should see the normal behaviour. The quasi states described will not be present. For the indicated volume if the chamber is spherical it will have a diameter of about 7.5cc with a surface area of about 200 sq.cm. The load at critical point will be about 20 tons, which though high is not alarming. I fully agree with you that this is not a problem to be approached casually.

Hi,

we are talking here about the critical point (not the triple point).

Vapor and liquid are separated by a boundary.

Above the critical point this boundary is no longer existing (as stated by bioramani),

naming the liquid a gas or liquid is in use but not really correct, superfluid is the adequate word.

Energy content is very high, if the pressure container fails, a complete building may be converted to pieces by less than one liter including any persons.

RHABE

Sorry that was an error. I was writing about the critical point only.

Hi RHABE, Yes as you point out, 374 °C --- 217.7 Atm --- 3200.19 psi./1454.5 Kpsi. /22.063 Mpa. Major pressure danger here, which is why the OP want to know about this specific detail. Actually they asked about getting water to 400°C. Very odd request?

Very odd request?

Hi babybear,

if they know what and how to do, this may be very useful.

At these conditions all hydrocarbons are quickly decomposing into CO2 and simple hydrocarbons and some hydrogen.

So this is one of the emerging methods of detoxifying any nasty stuff: poison gas, dioxines, PCBs, ? certainly some more.

Freed chlorine and oxygen may and will corrode the container walls, so only very few metallic materials are +- stable.

RHABE

Hi RHABE,

Yes I fully understand what you say. And at this state water is almost acting like something different. With, as you say molecules lasting a fraction of a second and or remixing with the main body of the water liquid which is barely a liquid anyway.

There is so many methods and splutter action for laying thing down in a layer of just a few molecules of a layer. One page I got back on research had 21000000 returns. I do not live enough to check that many!!!!!!

And we have yet to find out what the OP wants to do? Really makes me angry this kind of ignorance, after so much work by all on this thread. It is hardly the type of info' anyone would have in mind ready to trip out right?

Take care

Hello "babybear & RHABE",

I have followed with some interest the responses that have been forth coming regarding the original post. While I do not pretend to comprehend all of it I do understand the basics regarding fluids, gases and super fluids. I also understand that extreme pressures are need to convert gases to liquids so on.

I am not the original poster and won't even hazard a guess as to just what he was shooting for as a realistic answer or why he posted the question at all, as he hasn't demonstrated the courtesy of further responses.

I know that micro waves generated in a "home makers" oven increase from 110/120 volts through the strength of the step up transformer through the capacitor and raytheon tube and that such micro wave ovens are rated in the amount of watts needed to reach maximum power, such a 500/600 and even 900 watts. Through experience we know that we can boil water, destroy food, and quickly ruin coffee. However, I cannot reasonably even guess how much energy would be needed to cause water to reach the condition one regards as super fluid. I do not expect either of you to do the math, but could you offer a suggestion as the amount of power needed to cause water to "reach the stage that we consider super fluid".

I ask this for the benefit of those of us who don't pretend to be something that we are not, but follow these posts and responses with respect and interest.

TMF

Hi TMF,

Really appreciate your reply post, despite the fact that, so far, the OP has not had the respec' to write in again!

I think you may have missed something, the amount of pressure needed.

It would indeed be more than interesting to hear from a 'Rocket Scientist'? LOL!

The way I see it, there would not need to be hardly any 'extra' power or heat to reach 640 odd - °Kelvin, ....... 374 °C, as apposed to the normal 1Atm, Boiling Point 212 °F (100 °C). (I realise that power and heat are not the same).

The important thing here is the extra, almost 218 atmospheres of pressure.

374 degrees °C (674 °k) a pressure of almost 22 mega-pascals (218 atmospheres). That pressure, is enough to almost make the water reach critical temperature, from fright! And, as I say I do not see much more heat is needed, other than that necessary to heat the container.

The critical state is so ->>>''out there''<<<- and almost unattainable, (as something to aim for) if not so, in real life, it is not surprising there is little evidence of the power or heat necessary to raise water to its critical state. I cannot recall how to reach it myself from written info', sorry. It also has to do with the starting temperature of the liquid and its contained pressure vessel. To a scientist this may be obvious, to me it takes a lot of thought to see 'what happens if'--------?-?-?'. I suppose you may reach the answer through Hydrogen Bonding, but I am still checking that out, if I can figure out how it works?.................. We must be nuts, doing this for our 'entertainment'?

If the water is an ice state, and the pressure vessel has to be several cm thick to contain a (potential bomb) and, this is also very cold, (the power necessary to raise the 'ice' to the critical point and the container to at least a temperature from which the out-landish critical point is attainable), will be more.

The obvious water temperature to start with may be 4 °C, its most dense. But I still have no reference to the heat or power necessary. Please note: My words, not copied, so you can't look for them directly.

Please see the site below, it really only confirms how the critical point or state if you like, is arrived at, and explains that a fraction of a change either way and it goes heywire,........... Into gas or back to water liquid........

http://www.answers.com/topic/critical-point-thermodynamics

I am still looking, even though.

Take care.

Hi babybear,

interesting to hear from a 'Rocket Scientist'?

This was a reality 15 years ago when sounding rockets were tried with supercritical water as propellant. A spherical tank, a heater, a valve (may be a bursting diaphragma) and a nozzle. Simple but not really safe, solid rocket fuel seems to be better.

http://www1.lsbu.ac.uk/water/explan4.html#Cp

At this website (cited above) you will find a lot about the odd physical data of water.

So a plot of the specific heat (at constant pressure) remarks abour specific heat (at constant volume) and a plot of the heat (energy) needed to bring from liquid to steam. Near room temperature the specific heat is near constant but nearer to the critical point the specific heat is rising considerably - may be in relation to the vanishing heat of vaporisation.

So you can make a very rough estimate up to the region where the specific heat starts to rise: specific heat times mass times temperature plus vaporisation energy.

(I will try this evening with a little bit of free time available).

This is high energy physics and technology. Guard yourself first.

RHABE

Hi RHABE,

thanks so much for getting back to me.

Yes I have always thought 'water' to be very special, but until I read more several years ago, and yet more in detail over the last week, I knew nothing of the more odd and unique abilities. Pretty amazing stuff.

I have yet to find the details of how much Energy it takes to raise water to its critical state though.

I know there is some physicists and others on this site, who may know or be able to extrapolate the answer? Though as I have said in a previous post, this state is so , well, impossible to get to, there may be available info' on machines which can do this, and pretty good details on these machines, but, I have seen a couple and they are really meant for experiment only?

Thank you once again for the post and keep in touch.

Take care.

Remeber equation

PV = nRT

If you assume volume 1000 ml and temperature 400C then you can calcualte pressure needed which is

P = nRT/V --> n R 698/1000

and you see pressure is directly related proportional to temperature and inversely proportional to volume of water

n and R are the constant

Sorry but at these pressures you must use Van der Waals equation. The universal gas law will be inacurate.

Hi,

as the density of water near or at boiling temperature is going down with rising pressure and temperature and the density of vapor is going up with the same conditions and these densities meet at the critical point there is no possibility to distinguish between liquid and gaseous above the critical point. Fluid?

Heating a pressure container: why not. Couple the waveguide with a quarter wave (in thickness) ceramic plate to the pressure container, matching impedance will work at any frequency, you certainly know the principle from the anti-reflective coating of lenses.

This ceramic "window" should be designed to withstand the pressure, quartz-glass will dissolve at these temperatures as any other oxides too so not usable, but I suppose that nitrides will work.

Maybe you have to go up to above typical 3 GHz as the minimum cross-section of a waveguide will go down, so the minimum window diameter.

We used diamond windows inside 220°C concentrated sulfuric for acid for optical sensing as no other material seemed to be stable.

RHABE

Hi Guest,

In part answer to your question..................

Every substance has a critical temperature. Some examples are shown below.

As you can see the critical point is 374 °C --- 217.7 Atm --- 3200.19 psi.

Tubes containing water at several temperatures. Note that at or above 374 oC (the critical temperature for water), only water vapor exists in the tube.

Critical Pressure The critical pressure of a substance is the pressure required to liquefy a gas at its critical temperature. Some examples are shown below.

The Critical Point of water..................

1. Video results for critical point of water

   Critical point of water with Sunn O))) 6 min www.youtube.com

   Hottest water on Earth discovered 1 min 11 sec www.youtube.com

2. Critical Temperature and PressureTubes containing water at several temperatures. Note that at or above 374oC (the critical temperature for water), only water vapor exists in the tube. ...
   www.chem.purdue.edu/gchelp/liquids/critical.html - Cached - Similar -