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1. Originally Posted by gainer
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.

2. Originally Posted by jgjbowen
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.)

3. 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.

4. 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!

5. Kirk & Ole,

Thanks

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