Change in Autoignition Temperature with Pressure 3

[zdas04]

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.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

 

[unclesyd]

Can't help with any specifics, but you may find some information on chemical processes using air as oxidizer. I know there has been a tremendous amount of work on the explosive limits of air and butane in the manufacture of Maleic and Terapthalic Anhydrides.
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.

 

[Guidoo]

I have a book called "Physical Properties of Natural Gases", written by T.M. Geerssen and published by the Dutch company "NV Nederlandse Gasunie" (1988). It contains a section called "Effect of pressure and temperature on flammability limits". It contains two figures that shows UFL, LFL and AIT as function of pressure and temperature. One figure is for Groningen natural gas and the other for Ekofisk natural gas. According to the text "Gasunie commissioned DSM in 1984 to develop formulas which would make it possible to calculate the flammability limits and autoignition temperatures of any given natural gas from composition, pressure and temperature". and furtheron "For further information contact NV Nederlandse Gasunie". My experience is that they are quite willing to share this kind of information, so maybe you could contact them (

http://www.gasunie.nl).

They also made a computer program called Fyscal that contained this information.

 

[zdas04]

Unclesyd,
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

 

[Guidoo]

David,

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

 

[Guidoo]

I just checked the computer program Fyscal, that was made by Gasunie in 1995. It is MS-DOS based (so it doesn't run on more modern Operating Sytems like Windows 2000, but on Windows 95 you can still run it...) and is in the Dutch language. It contains the possibility to input your own gas mixture and then to calculate LFL and UFL at various temperatures and pressures. These calculations are based on the results of tests done by DSM as described above. Maybe Gasunie has a more modern version of the program available now?

 

[25362]

I recall this subject has been dealt with in previous threads. It has been published that pressure has little effect on the LFL except at very low pressures (<50 mm Hg absolute), where flames do not propagate. On the other hand, the UFL increases significantly as the pressure is increased, broadening the flammability range. An empirical expression for the UFL for vapors as a function of pressure is:

UFL_p=UFL+20.6(logP + 1)​

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:

UFL_p=15 + 20.6(log 2.101 + 1)=42% volume of fuel in air​

 

[zdas04]

25362,
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

http://www.muleshoe-eng.com

 

[25362]

To zdas04, regretfully I don't have. The AIT is a function of many variables: the volume of the flammable vapour, concentration of the vapour, pressure of the system, presence of catalytic material, and flow conditions.

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.