How do you calculate the autoignition temperature of a gas mixture? 2

[Axf]

Is it possible to calculate the autoignition temperature of a gas mixture? I know the autoignition temperatures for each component in the mixture.

I need this information to determine which temperature class the gas mixture is in to select equipment for a hazardous area. (IEC 79, I'm not very familiar with this as I'm a process engineer)

My gas mixture is:

Methane 86.98% AIT. 595 dec
Ethane 9.35% AIT. 515 deg C
Propane 2.33% AIT. 470 deg C
Butane 0.63% AIT. 365 deg C
Nitrogen 0.71%

 

[owg]

This book may provide a method. Check out FT Bodurtha, Industrial Explosion Prevention and Protection, McGraw-Hill. I will check my copy next time I am in the office.

HAZOP at

http://www.curryhydrocarbons.ca

 

[Axf]

Thanks OWG but I haven't got a copy of the book you recommended. I'd appreciate some more help.

Thanks

 

[Guidoo]

Literature values of the AIT for natural gases vary widely. I found values ranging up to 670 °C. However, they never were below 450 °C. According to IEC 60079-0, section 5.3.2.2, table 2, least severe temperature class is T1, with a maximum surface temperature of 450 °C.

So my conclusion is: select temperature class T1, and just fill-in AIT > 450°C. There is no need to do a detailed calculation of the AIT, since it will result in same temperature class... Who cares whether it is 537 or 616 °C?

 

[25362]

Little that I can add consists of the following:

Minimum ignition energy, about 0.25-0.29 mJ.

The AIT of a vapor, aka spontaneous ignition temperature (SIT), is a function of the concentration of the vapour, volume of the vapor, pressure of the system, presence of catalytic material, and flow conditions. Experts recommend to experimentally determine the AITs at conditions as close as possible to process conditions.

Composition affects the AIT; rich or lean mixtures have higher AITs; larger system volumes decrease the AITs; an increase in pressure decreases AITs; increases in oxygen concentration decreases the AITs.
This strong dependence on conditions illustrates the importance for exercising caution when using AIT data. For example, my sources call for an AIT of 1000^oF for methane, 103^oF lower than the value quoted by Axf.

The fire triangle teaches us that fires and explosions are preventable by eliminating ignition sources. This should be given greatest attention. Combination of sources should be investigated. The goal being to eliminate or minimize ignition sources, since the probability of a fire or explosion increases significantly as the size of the plant increases. These potential ignition sources may be in the thousands.

The NEC classifies process areas according to process hazards. An example of the rating method:

Class I: Locations where flammable gases or vapours are present.
Group D: ethane, butane, ethyl alcohol.
Division 2: Flammables are normally contained in closed containers or systems. Hazardous only under abnormal conditions. Flammable concentrations are not normally present.

 

[owg]

It turns out that Bodurtha is silent on the AIT of mixtures. He quotes somewhat different values. Here they are for comparison. They are in air at 101.325 kPa
Methane 537 dec C
Ethane 515
n-butane 405
isobuane 462

HAZOP at

http://www.curryhydrocarbons.ca

 

[Guidoo]

Anyway, as I wrote before, AIT > 450°C and Temperature Class is T1.

 

[owg]

Guidoo's approach makes sense to me.

HAZOP at

http://www.curryhydrocarbons.ca

 

[jcaiken]

There is a lot of research going on into AITs which suggests that in practise applying the results of the standard tests are too conservative and theoetical. They do not reflect the external effects of an open site on hot surfaces.

One standard (can't remember which) identifies that the impact of gas components less than 10% by vol can be discounted but the overall AIT should then be based on the lowest component AIT.