Change in Autoignition Temperature with Pressure
Change in Autoignition Temperature with Pressure
(OP)
In doing research for a class on purging air from hydrocarbon gas lines, I've come across many references that said that as pressure increases Autoignition Temperature goes down, Lower Explosive Limit (LEL) goes down, and Upper Explosive Limit (UEL) goes up. I have been unable (after several hundred hours of searching) to find any trail towards a way to quantify these changes. I've occasionally found someone who claimed that at x pressure, the UEL went up to y%, or Autoignition temp went down to z, but never any references to actual arithmetic.
Does anyone have a feel for the magnitude of these changes or a place to look for the references? I've been doing this class for 10 years, and usually get by with an (unsatisfactory) "the values change with pressure" and rarely get the question "how much?". When I do get the question I shrug and try to change the subject.
Does anyone have a feel for the magnitude of these changes or a place to look for the references? I've been doing this class for 10 years, and usually get by with an (unsatisfactory) "the values change with pressure" and rarely get the question "how much?". When I do get the question I shrug and try to change the subject.
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com





RE: Change in Autoignition Temperature with Pressure
We manufactured Maleic Anhydride and exceeded these limits several times, an "excursion". Every time we pushed the envelope we made the rupture disk salesman happy.
I'll try to get some leads on the studies.
RE: Change in Autoignition Temperature with Pressure
RE: Change in Autoignition Temperature with Pressure
I've found several references over the years wrt specific components, but I've never been able to assemble enough of them to be able to make up a typical natural gas stream. Since the reaction of any given component is so different from other components, I've been unsuccesssful in extrapolating the missing components.
Guidoo,
Thank you very much for the lead, I've e-mailed them and hope springs eternal. Do you know if this book was published in English or just in Dutch?
David
RE: Change in Autoignition Temperature with Pressure
Book was published in English in 1988, while earlier editions were in Dutch. They used to send it for free if you asked for it, but it may be out of stock now.
Regards,
Guido
RE: Change in Autoignition Temperature with Pressure
RE: Change in Autoignition Temperature with Pressure
where P is pressure in megapascals absolute, UFL is the upper flammable limit (vol% of fuel plus air) at 1 atm.
For example, if the UFL for a substance is 15% by volume at 1 atm abs, the UFL at 2 MPa ga. would be:
RE: Change in Autoignition Temperature with Pressure
Thanks, I just did a search within eng-tips and found the thread you were talking about. You're right it covers the topic pretty well, but not as throughly as Guidoo's reference. Maybe it is worthwhile to resurect some of these topics from time to time.
That relationship matches Guidoo's plots very closely (for high heat-content gas it matches at 130C and low heat-content it matches at 140C).
The work from Holland also supports the idea that the change in LFL with pressure is small and probably insignificant.
Do you happen to have a similar empirical relationship for Autoignition temp?
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
RE: Change in Autoignition Temperature with Pressure
Rich or lean mixtures have higher AITs; large system volumes decrease AITs; increases in pressure decrease AITs; and increases in oxygen concentration decrease the AITs.
Therefore, it is essential to determine AITs experimentally at conditions closest to prevailing process conditions, and I believe it is important to exercise caution when using AIT published data.