Heating chemicals.

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gainer

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Man, are we in deep do do. How did I ever fill my tank without blowing up the station! The flashpoint of gasoline is -40 F. If I have to wait till the temperature goes below -40 to gas up, I'll have to go to Antarctica, I suppose. Why don't you and Kirk go study up on a few things you obviously know nothing about. Partial pressure, solutions of things in air, convection currents, flashpoint, autoignition temperature, etc.
 

DWThomas

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Besides smoking up the kitchen , my biggest concern might be damaging the microwave. Most that I've seen warn you not to run them empty, as it can damage the magnetron. It also seems possible (speculating here) that some organic compounds might be very low conductivity and could behave more like an empty oven -- my understanding is that the process is very dependent on the attributes of water molecules.

DaveT
 

richard ide

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I really find this discussion hard to believe that it is taking place. Cooking gunpowder on the stove is also OK until something goes wrong. Fortunately it was only about a quarter pound.

Since then I have chosen to do everything involving chemicals in the safest way possible. I would never introduce an unknown quantity. Extra short hair was enough to make me err on the side of caution for the rest of my life.

If you want to play with flashpoint; try diethyl ether. That can give you some nasty surprises.
 

Kirk Keyes

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A clarification first: I misused the term "flammable liquid" when discussing propylene glycol. With a flashpoint above 200F, it is properly called a "combustible liquid".

gainer said:
What, for instance, is the flashpoint of diesel fuel?
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Both the MSDS from Phillips and Cenex list the flashpoint of Diesel #2 as <125F. The autoignition temp is <500F.

gainer said:
Is that the temperature required to ignite a fuel-air mixture in the cylinder by the heat of compression?
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The flashpoint of #2 Diesel is below the temp required for combustion in a Diesel cycle engine. And a Diesel engine has nothing to do with the safety concerns that are being voiced against your experiment here. Diesels operate at high compression ratios (16 to 25 or so) which cause high temperatures in the air/fuel mixture (about 700C or higher) and result in the self-ignition of the diesel fuel. We are not talking about Diesel engine here. We are talking about heating combustible liquids in a microwave oven and the potential hazards that arise from doing so.

gainer said:
If there is no external ignition source, it will not ignite until it reaches that temperature.
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I completely agree. Can you guarantee that every person that reads your advise and trys this at home has a microwave that will not generate a spark? Or that the person using this technique is not smoking? A lot of people still smoke out there.
 

Kirk Keyes

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gainer said:
Furthermore, there IS such a thing as partial pressure of glycol vapor in the atmosphere.
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I know that very well. Here's a nice graph of the vapour pressure of progylene glycol: Dead Link Removed

As you can see, the bottom line labeled 100% PG agrees with the MSDS listed value of 0.129 mm Hg at 77F. As we heat the propylene glycol up, by the time it reaches 100F, we are at about 0.3 mm Hg, at 150F we get 3 mm Hg, at about 185F it's 10 mm Hg, 210F, the flashpoint of propylene glycol, the vapour pressure is only about 21 mm Hg. Since you have a good understanding of vapour pressure, you most certainly know that this is not a very high vapor pressure. And note, we are no where near atmospheric pressure (about 760 mm Hg).

gainer said:
The gradient is diferent in the closed cup test than in the open cup test. The situation in the microwave is that of the open cup tester.
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A closed microwave is a rather large closed cup tester.

gainer said:
If there is a spark somewhere in the microwave, what conditions will allow it to ignite the surface of the container of glycol?
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The vapor goes to the spark so the spark does not have to go to the liquid.

gainer said:
The partial pressure of glycol vapor will have to be the same as it is at the surface of the glycol at its flashpoint.
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No, here's where you are missing the point. The vapor in the microwave just needs to build up as time goes by with the hot glycol sitting in it. if you have the liquid hotter than the flashpoint, it will be evaporating at sufficient rate to fill the container with a flammable mixture given enough time. As long as there is enough vapor buildup,you risk a fire.

gainer said:
In order for that to happen, the glycol will have to be at a much higher temperature than the flashpoint.
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No, it just needs to be at the flashpoint temp.

Also, keep in mind that propylene glycol is a much more viscous liquid than water. It will not react to convection currents during heating as quickly as water and it will not relieve internal hotspots as easily.

Another consideration is that microwaves are tuned to emit radiation that absorbed my the O-H bond in water. That absorbed microwave energy is turned into vibrational energy that causes friction of the water molecules against the surrounding food matter that causes it to heat up you food. While glycols do have some O-H bonds, they will not "heat" as efficiently as water does, and I believe it will require you to use longer heating times which may cause the glycol to risk superheating.
 
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gainer

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So, what about the gasoline? Should it not ignite when it touches anything with a temperature above -40? Celsius and Fahrenheit are the same there. The lowest I've seen it here is -25 F. I've been known to fill my tank in the summer at close to 100 F without starting a fire. I still say you'd better study up.

What makes you think that a container of glycol maintained at 210 F will eventually fill the room or the microwave with ignitable glycol vapor? How do you know the equilibrium vapor pressure where heat loss = heat gained? How do you know the temperature gradient above the glycol surface?

The major reason for heating the glycol is to reduce its viscosity. Also, I think the fact that it does not ionize the way water does means that the molecular motion at any given temperature is less.
 

Kirk Keyes

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gainer said:
How did I ever fill my tank without blowing up the station!
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PAt - Gas stations and automobiles have been designed with many safeguards to prevent exactly this sort of situation from happening.

First off, you're out in the open with lots of ventilation around and not in a closed box. And your fuel filler is up off the ground to help prevent a buildup of vapors. And you do turn your car off when refueling, don't you? You put your cigarette out when you pump, right?

Don't you ever see the reports of vehicles catching fire while filling up? Are you telling me those reports are urban legends? A lot of those accidents are cause by small sparks. Just because you or I have not blown a station up does not mean there is no risk.
 

Ole

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The flashpoint is the lowest temperature at which the vapour in equilibrium with the liquid reaches an ignitable mixture with air.

So if the liquid is at a temperature above the flashpoint inside a microwave oven (or any other enclosure), you have an enclosure filled with a flammable fuel/air mixture. As PE said, it can ignite.
By definition, it will. If the container is to be maintained at 210F the rest of the enclosure will also eventually be at 210F, giving an equilibrium vapour pressure equal to the lowest ignitable concentration.

But since cold glycol is rather viscous, convection is slow. Heat capacity is also lower than water. Both these factors contribute to increase dramatically the risk of non-equilibrium situations where parts of the liquid is much hotter than others. This can lead to flash boiling, spurting very hot glycol all over the inside of the enclosure.

Here's a snip from the MSDS for propylene glycol:

I have both a chemistry degree and a demolition certificate, but this is one experiment that I do not wish to do.
 

Kirk Keyes

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gainer said:
What makes you think that a container of glycol maintained at 210 F will eventually fill the room or the microwave with ignitable glycol vapor? .
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You're the one that keeps bringing up vapour pressure. Well, if you thought about it you would know that that IS the reason that a conatiner of glycol maintained at 210F "will eventually fill the room or the microwave with ignitable glycol vapor."

It's because the glycol is pushing vapour out of the container with its vapour pressure of about 21 mm Hg. It is evaporating.

gainer said:
How do you know the equilibrium vapor pressure where heat loss = heat gained?
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What in the hell does that sentence mean??!! Heat loss = heat gained? Please clarify what you mean in regards to our discussion.

gainer said:
How do you know the temperature gradient above the glycol surface?
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We don't need to know the temperature gradient above the glycol surface. We know that we have a vapour pressure and we know that the liquid is evaporating. The hotter it is, the more it evaporates. The air above can be room temp, or it can be warmer. In fact, if it is warmer, it can hold more vapour.

gainer said:
Also, I think the fact that it does not ionize the way water does means that the molecular motion at any given temperature is less.
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Ionization is not a function of heating in microwave ovens. It is purely a molecular vibration that is being produced. As others here have touched on already, it is a rotation about certain molecular bonds that is being produced. That makes friction. And that causes heat.

Here's an independant source: "A microwave oven works by passing non-ionizing microwave radiation, usually at a frequency of 2.45 GHz (a wavelength of 12.24 cm), through the food." http://en.wikipedia.org/wiki/Microwave_oven

Again, ionization has nothing to do with microwave ovens. Time to read up on "dipole moment."
 

Kirk Keyes

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Ole said:
If the container is to be maintained at 210F the rest of the enclosure will also eventually be at 210F, giving an equilibrium vapour pressure equal to the lowest ignitable concentration.
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Ole, I think you'll agree that in the short term, the container does not have to reach equilibrium for ignition, we just need to get enough of the vapour into the air to create a fire risk. That's why the MSDS points out that "May travel long distances along the ground before igniting and
flashing back to vapor source." The air can be room temp and cause a fire risk if there is sufficient vapour in it and an ignition source.
 
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Kirk Keyes

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Pat - can you guarantee that no one will be smoking when they heat their glycol in their microwave? Can you be certain they have a microwave that has excellent ventillation and is free from ignition sources? Can you guarantee that people will only be using microwaves that have turntables to help minimize superheating of their glycol? And please remind people that is it very bad technique to be using the family microwave for heating their photographic chemicals in them.
 

Ole

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Absolutely Kirk, as I pointed out in the next paragraph. The bit you quoted was a follow-up to Gainer's "will eventually" - yes, it will eventually reach equilibrium, and at flash point that is by definition a flammable mixture.

I also listed some of the reasons why a non-equilibrium state is not only very probable, but also potentially much more dangerous. After all, the equilibrium state of an open tray with gasoline is "empty"...
 
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gainer

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OK. But if as you say, the flashpoint is the same as the autoignition temperature, which is what you say when you maintain that contact with anything at a temperature => flashpoint will cause ignition, then why does the gasoline not ignite when it touches my automobile? It certainly has air enough to form a combustible mixture, and the car's fuel tank is often 100 or more degrees hotter than the flashpoint of gasoline. If you want to preach to me, preach truth, not fiction. If I leave an open container of propylene glycol in my microwave all day or for a week after heating it above the flashpoint but below the boiling point, will I get a fire or not the next time I turn on the oven?

In spite of the "safety" devices at the service station, I accidentally ran some gasoline onto the pavement one time. The flow rate was too low to activate the overflow shut off. I reported it to the proprietor, and he said "You have to pay for it anyway" as if that was my only concern. So, I got in the car and with one motion started it and got the hell out. No fire. Why not? My hot exhaust passed right over the spill.

I have a 5 gallon gas container for my lawnmower. It has no safety device. Why do I not get a fire every time I use it? I have seen a person, admittedly an idiot, put out a cigarette in a puddle of gasoline. If you do it quickly enough, you can live to tell about it. He thought it was great because it was aviation fuel, and he thought high octane meant low flashpoint.

Can you guarantee that a person using a water bath to heat glycol will not be smoking? What's the difference how the glycol gets to the flashpoint? It is slightly below the boiling point of water at 760 mm Hg.
 

richard ide

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Please take 2 tablespoons of borax and call the doctor in the morning.
 

Photo Engineer

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Patrick;

The flashpoint of gasoline (depending on octane rating) is from 40 - 70 deg C. This comes from the Merck index.

Why don't you go study up on a few things you obviously know nothing about?

PE
 
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gainer

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The solubility of glycol in air, like that of water in air, depends on temperature. If the glycol in the air above its surface cools to a certain temperature, it will begin to condense. At this point, evaporation will take place at a slower rate. If the height at which this temperature is reached is below the rim of the container, what do you say will happen? Will the condensed glycol flow out of the container, or back in?

If I put a half-full container of glycol in the microwave and heat it to the flashpoint, let it stand there till it cools, should I be able to feel the coating of glycol that has condensed on the rotating tray and the outside of the container?
 
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gainer

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Photo Engineer said:
Patrick;

The flashpoint of gasoline (depending on octane rating) is from 40 - 70 deg C. This comes from the Merck index.

Why don't you go study up on a few things you obviously know nothing about?

PE
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I looked it up in several different sources. Whatever its number is, the flashpoint is not defined as the temperature at which a substance will self-ignite.
 

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gainer said:
OK. But if as you say, the flashpoint is the same as the autoignition temperature, which is what you say when you maintain that contact with anything at a temperature => flashpoint will cause ignition, ...
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Noone has said that. PE said that "If the vapour comes in contact with an object at or above its flashpoint, it can ignite" (emphasis mine).

If I put a half-full container of glycol in the microwave and heat it to the flashpoint, let it stand there till it cools, should I be able to feel the coating of glycol that has condensed on the rotating tray and the outside of the container?
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That depends on the overall temperature inside the oven, which will determine the diffusion distance of the glycol fumes; as well as the temperature of the vessel, the tray and the walls - as well as the sensitivity of your fingertips. But the short answer is "yes".
 

DarkroomExperimente

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so if we heat up glycol or gasoline in the microwave...it'll need a source of ignition to catch fire ...so most of the time it won't ignite

if I run across the street randomly with my eyes closed...I'll need a car driving by to get hit...so most of the time I won't get hit by a car
 

Photo Engineer

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gainer said:
I looked it up in several different sources. Whatever its number is, the flashpoint is not defined as the temperature at which a substance will self-ignite.
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Patrick, notwithstanding your rhetoric, the temperature you quote is over 100 deg F off from the quoted figures in my texts. I'm sorry, but the error was obvious to me as soon as I read the post above that you made. All I had to do was open some chemistry texts to verify the actual temperature range.

Please, please. I am saddened by this whole exchange. You are incorrect in your assumptions and statistically your instructions can lead to a fire or worse.

PE
 
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