Heating chemicals.

[Photo Engineer]

BTW Patrick. I hope that your bread turned out ok, coated as it were with PG vapours and other misc. chemical residues. I'm sure it sure spiced things up. We had curry tonight. I wonder if it added a curry like flavor to your bread?

PE

 

[Kirk Keyes]

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?

Absolutely no one has said the flashpoint is the same as the autoignition temp. You need to read better.

 

[Kirk Keyes]

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 live in Oregon where we can't pump our own gas (as it is obviously too dangerous for us to do). When this happens, it's the fault of the high-school dropout that works at the gas station. I then negotiate the bill with him.

 

[Photo Engineer]

And, one of my colleagues saw an elderly woman drive away from a gas station with the hose still inserted in her gas tank. The hose, pump and rear of her car burst into flame! My colleague was late for work that morning due to the fire at his gas station as he was waiting to get gas. It tied up traffic quite a bit I guess.

I did not see this event personally, but we talked about it for several days.

PE

 

[Kirk Keyes]

So PAt, why don't you tell me at which vapour pressure a combustible liquid becomes a hazard?

 

[removed account4]

hehe..a peep show!

They slowly expand to about 10 times their normal size and then...........

hmmm i guess i can't do a whole package at once ..
our microwave is too small it seems.

will they fill the kitchen with yellowbath6 fumes ?
better get my vapor mask out, and evacuate the house
and then do it with a remote through the kitchen window ...

i am getting worried ...

 

[Jordan]

I have worked with microwaves as part of academic research in chemistry.

The primary mechanisms of microwave heating of liquids are (1) induction of molecular rotation due to the rapidly reversing electric field component of the microwave radiation, and (2) "convection" of charged substances in the same field. In both cases, it is the dissipation of energy in the frustrated motion (rotation or translation) of molecules that leads to heating. Substances with higher dielectric constants and charged substances will, in general, heat more quickly in a microwave of equivalent strength, but viscosity (which affects conduction) and heat capacity will have a big effect. You can prove the bit about charged substances by heating equal volumes of salt water and distilled water in identical vessels in a microwave at the same time. The salt water should get hotter much more quickly (be careful -- just zap it for a few seconds, not to boiling).

Certain organic solvents heat very quickly in a microwave oven. DMSO (dimethyl sulfoxide) is one of them. Toluene, which is very flammable and very non-polar, heats only very slowly.

Microwave heating has the potential to be much more even than conventional heating due to the larger penetration distance of microwave radiation, but this advantage can usually only be fully realized in a "mono-mode" reactor where the microwave field is designed to be perfectly even and no part of the sample is more than a couple of centimetres from the walls of the reactor. This is the type of setup that is usually used in chemistry labs. Samples are heated in a closed tube (with pressure and temperature monitoring) and power output is variable -- it's a much safer environment than a kitchen microwave, where vessels are open, irradiation power is high (microwave heating in a lab reactor is typically below 100W, but kitchen ovens can be 10x that IIRC) and there are waves bouncing all over the place, hot spots, dead spots, etc.

Despite the inhomogeneity of heating in a kitchen microwave, superheating does tend to happen, which IMO is the biggest safety risk in this kind of experiment.

If you need to heat propylene glycol in a hurry and only have a microwave to do it with, the safest solution is to heat a microwave-safe container full of water to the appropriate temperature and then use that as a hot-water bath (outside the microwave) to get the glycol to the right temperature.

More than you all ever wanted to know about microwaves, I think

 

[pesphoto]

hmmm i guess i can't do a whole package at once ..
our microwave is too small it seems.

will they fill the kitchen with yellowbath6 fumes ?
better get my vapor mask out, and evacuate the house
and then do it with a remote through the kitchen window ...

i am getting worried ...

yikes, your gonna do it?
I hope you arent too attached to your microwave.
I wanna see the carnage.

 

[gainer]

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

I knew it would happen.

Canadian Centre for Occupational Health and Safety says -40 for gasoline.
MSDS Hyperglossary says -40.
Pasadena Fire Department says -40
ContactPakistan.com says -45
Engineering Toolbox says -45
Highest I saw by reputable source was 30 F for high octane aviation fuel. You can't use that in your auto.
I saw values as low as -49
Are you sure Merck didn't lose a - somewhere?

You know that in Rochester you would not be able to start your car in winter if the flashpoint of gasoline were +40 F.

Another important characteristic of liquids is flammability range. For gasoline, the range is from 1.4g/100 g air to 7.6. Below or above that range it will not sustain combustion. I do not know the numbers for propylene glycol.

You are using ridicule again instead of valid arguments.

If you did not mean that the flashpoint and the autoignition point were the same, why did you say that a fire could be started if propylene glycol vapor at the flashpoint came in contact with an object at that temperature? The proper statement would be that a fire could be started iff the vapor was of a concentration higher than the minimum of the flammability range and came into contact with an object that was at least at the autoignition temperature. The spark or other ignitind device used in the flashpoint tester meets the ignition temperature requirement, and the vapor pressure in the tester must meet the minimum required at the fire point, which is usually somewhat higher than the flame point if a sustained flame is to be formed. I didn't make these things up.

 

[PhotoJim]

IYou know that in Rochester you would not be able to start your car in winter if the flashpoint of gasoline were +40 F.

I am no chemist, but I've started a car in temperatures as cold as -45 C (-49 F). They drive in the far north of Canada too and the temperatures can occasionally get down into the mid minus-60s Celsius (-80s or so Fahrenheit) and cars still work, although they complain violently.

I can't tell you the physics of how it works, but it does work.

 

[Ole]

...Highest I saw by reputable source was 30 F for high octane aviation fuel. You can't use that in your auto.
...
Are you sure Merck didn't lose a - somewhere?

Well - yes, you could. car engines aren't made to run on it, but something basically simple like a US-made V8 engine should run very well on 100% octane.

Maybe Merck did lose a "-". Your numbers are in agreement with most MSDS'es.

You know that in Rochester you would not be able to start your car in winter if the flashpoint of gasoline were +40 F.

That's totally wrong. You can start a car at -50 F - and I have done so. Flashpoint has no bearing on how gasoline works in an internal combustion engine (well - a little, but it's a lot more complex. There doesn't need to be gasoline vapour for ignition to take place, for example. But it helps a lot).

Another important characteristic of liquids is flammability range. For gasoline, the range is from 1.4g/100 g air to 7.6. Below or above that range it will not sustain combustion. I do not know the numbers for propylene glycol.

I know, it's in the MSDS: 1.78% to 17.4% (slight differences between different MSDS'es). Note that the range is much greater than for gasoline...

 

[Photo Engineer]

Well, Patrick, you are correct. The 6th edition of Merck says -40 but the 12th edition says +40, so they lost a minus sign somehow which could lead to big problems, but that is for them to worry about.

The point being that almost everyone here has pointed out a number of your errors and when I make one I stand up and say as I just did, I made an error. I didn't look far enough! Sorry! Can you do the same in the face of others who have pointed out problems to you?

Now, as for sarcasm, I lifted one comment out of one of your own posts and put it in mine, so the sarcasm was yours to begin with.

No, you made nothing up, you just are misusing tools as Jordan inferred above. If we wish to have a litany of sarcasm and your errors, hark back to a comment about borax on your bread. I didn't make that one, but I think it relevant here that we examine and are questioning the fact that your proposals will work. They may or may not.

I worked with ethyl ether for years (Flashpoint is -40). I got away with it 98% of the time, but the two fires in 4 years that I did have were doozies. So, it can happen and that is what we are telling you about.

I'm sorry, but I cannot convince you of the potential danger of using a microwave to heat chemicals. Jordan tried, Kirk tried and I've tried. Others have joined in as well.

Sorry for the error, but my error was believeing Merck Index, 12th Edition.

PE

 

[Photo Engineer]

Now to address another issue. An automobile works by using a very hot spark of electricity, and it works by creating an explosion, not a fire. These are totally different from the supposition that flash point is involved. The spark is far hotter than the flash point, and the mix of air to gasoline fumes is optimum to create a high powered explosion.

Of course if you want to do that in your microwave oven, then we can now tell you how to do it. We have all of the data above. This is not sarcasm, it is just fact.

PE

 

[jim appleyard]

Does anyone know the flashpoint of a Gremlin in a microwave?

 

[richard ide]

Jim,

Is that with or without the wheels??

 

[gainer]

So PAt, why don't you tell me at which vapour pressure a combustible liquid becomes a hazard?

Varies with the liquid. For gasoline it is between 1.4 and 7.6 weight percent of the air. You could convert that to partial pressure. It's been a long time since I had to do anything like that. I know percent relative humidity converts to pounds of water per pound of dry air as a function of temperature. for propylene glycol, I found 2% to 17%, but specified as volume %. Outside of those limits it is not a hazard. You know your carburetor is outside those limits when it is too lean or too rich to run.

Hold on. I just found the equation I need in my old "Elements of Chemical Engineering" text book. I should, after a little but of head scratching, be able to tell you the flammable range of partial pressures and the actual weight of glycol vapor I could allow in my microwave without danger of fire. I,m off to the head scratching room.

 

[jgjbowen]

Gentlemen,

I need to SAFELY heat PG to 150 degrees F in order to mix Pyrocat HD. What are your recommendations to achieve this. Can I just heat it in a pyrex container on my heater/stirrer? What if any precautions should I take? I've done this twice so far and haven't had a fire, but I also don't want to ever experience a fire in the darkroom.

Thanks for your advice.

John

 

[Ole]

Partial pressure = volume %, where 100% is 760 mm Hg. Since an equal volume of two gases contains the same number of molecules at the same pressure and temperature, the weight % is easily calculated from the molecular weights.

John, that is how I would do it. A stirrer helps with heat transfer, and the stirrer (mine at least) has a thermostat so it will not exceed the desired temperature (much). Heating on a hot plate withous stirring can be a problem due to the high viscosity when cold.

 

[Kirk Keyes]

Another important characteristic of liquids is flammability range. For gasoline, the range is from 1.4g/100 g air to 7.6. Below or above that range it will not sustain combustion. I do not know the numbers for propylene glycol.

Well, you must have missed my post at the end of your "Experiments with Metol..." thread. See: (there was a url link here which no longer exists)

In it I pointed out to you the your beloved MSDS sheets will tell you the flammable limits in air for propylene glycol. Here's an excerp of what I wrote for you:

Try this one: http://www.jtbaker.com/msds/englishhtml/p6928.htm Look in Section 5 - fire fighting info. You will see right after the flashpoint and the autoignition point values, it gives the info for explosive mixtures of propylene glycol in air. It says, "Flammable limits in air % by volume: lel: 2.6; uel: 12.5" That's a lower explosive limit (lel) of 2.6% propylene glycol in air, and an upper explosive limit of 12.5%. Not enough vapour in the air and there cannot be a fire or explosion, between the lel and uel and you can get a fire/explosion, and above the uel and there cannot be an fire/explosion.

 

[Kirk Keyes]

The spark or other ignitind device used in the flashpoint tester meets the ignition temperature requirement, and the vapor pressure in the tester must meet the minimum required at the fire point, which is usually somewhat higher than the flame point if a sustained flame is to be formed. I didn't make these things up.

Well, if by "flame point" you mean fire point, you did make that statement up. The fire point is always higher than the flashpoint. The liquid must be hot enough to continuously generate enough vapour to sustain a fire. With the flashpoint, it must merely be high enough to generate enough vapor allow the air/vapour mixture to burn once.

By the way, have you looked at the wikipedia page for flashpoint? While talking about flashpoint and fire point right near the beginning of the page it says, "Neither of these parameters are related to the temperatures of the ignition source or of the burning liquid, which are much higher." See: http://en.wikipedia.org/wiki/Flash_point
Much of your contention seems to be involve with your belief that the vapor must be near the temp of the ignition source, and that is incorrect as this quote points out.

Also, a "spark" is never used as the ignition source in the ASTM standardized Pensky-Martens Closed Cup (PMCC) tester or in the open cup tester. The ignition source is a simple natural gas flame.

A couple interesting things I can tell you from my experience with flashpoint testing. First, as you approach the flashpoint, you can sometimes see the size of the flame that is applied to the sample will increase in size as it appears to be burning some of the vapour that is in the air above the sample. There is not enough to ignite the sample flash, but it is a noticeable increase in the size of the flame. This reminds me that if you are near the flashpoint and have a open flame, you may increase the intensite of that flame when the air comes in contact with the flame.

Second, a flashpoint is determined by heating the sample at a prescribed rate. If you heat the sample slower than the test proscribes, then you can get a flashpoint lower than if you heated it slower. This will depress by a few degrees of temperature the measured flashpoint. So you can get a flash at a few degrees lower than the published temps if you get the conditions right.

 

[Kirk Keyes]

Varies with the liquid. For gasoline it is between 1.4 and 7.6 weight percent of the air. You could convert that to partial pressure.

Pat, we are talking about propylene glycol. And I don't have to do any calculations for propylene glycol - I already told you that you can look up on the graph of glycol vapor pressure vs. temp that about 21 mm Hg is the answer. That's the vapor pressure of propylene glycol at the flashpoint of 210F.

 

[Kirk Keyes]

What are your recommendations to achieve this. Can I just heat it in a pyrex container on my heater/stirrer?

John, I agree with Ole. Pyrex on a stirring hotplate will work well. Just monitor the temp to make sure that you keep the temp of the glycol at a safe level. And of course, have a fire extinguisher in good charge nearby. (Which you hopefully have already as a regular household item.)

 

[gainer]

The necessary equation is quite simple.
Volume % = Pressure % = mol %. You will remember this as Dalton's Law of partial pressures. I had to refresh my memory. After all, it has been close to 60 years, 6 kids, an atack of encephalitis, and a blowout patch on my abdominal aorta between then and now. I kept that book mostly for the excellent air conditioning chart.

The specified volume % at the minimum of the flammable range in the MSDS is 2. The density of air (for the standard atmosphere) is 1.225 g/liter. the average molecular weight of any gas is the weight contained in 22.41 liters, or 27.45, which is a reasonable number. The partial pressure of glycol at minimum of the flammable range will be about 15 mm. Hg. The weight of glycol vapor will be close to 0.02 * 27.45 because my microwave contains about 25 liters.

If I get the glycol out of the oven before about half a gram leaves the container, it won't make any difference how hot it is. It won't burn.

 

[gainer]

I was delayed in submitting the last post. That's why it looks the way it does. I learned to drive when cars were very much different. Let me remind you of my opening statement. I did not say anyone had to heat glycol above its flashpoint. I said I found out long ago that was not necessary. But some were recommending using a hot plate and a saucepan or something similar. If in fact, the theories about how glycol vapors violate the laws of fluid dynamics are true, the vapor would be more likely to ignite upon hitting the hot plate than upon striking the very much cooler bottom of the microwave. I'm not going to recant that statement. There was no mention in any of the early objections to my statement of the fact that there is both a minimum flammable vapor pressure and a maximum flammable one. There was some scoffing at my mentioning partial pressures, that it was too complicated. There were comments that indicated that those who should know about the difference between flash point and autoignition temperature did not. Oh H-E-double hockeysticks!

 

[jgjbowen]

Kirk & Ole,

Thanks