2 replies to this topic

[B.M. Process]

Dear all, 

 

I am determining the low and upper explosive limits for a gas mixture which comprises of the given compounds in the table below.   

 

1) Methane 8.18% 2)_ Ethane 2.12% 3) Propane 1.47% 4) i-Butane 0.61% 5) n-Butane 1.22% 6) i-Pentane 0.32% 7) n-Pentane 0.42% 8) n-Hexane 0.4% 9) n-Heptane 0.2%

 

All the compositions are % by volume and all these gases are to be vented to a cold vent stack via a cold vent pit. I just need to know if there is a chance of some explosion by calculating the lower and upper limits as shown on the attached Excel sheets. Based on this attached calculation though, it does not seem likely that there will be chances of any explosion. However, I will appreciate if someone else can give  a different opinion. The explosive limit for individual compound in the mixture is included on the attached Excel sheet.   

 

Attached Files

•  Low & Upper explosive limits.xlsx   14.65KB   29 downloads

[Saml]

The possibility of an explosion is due to air entering into the stack.

And yes, your numbers show that if air is allowed into the system you may have concentration in the flammable range.

 

Now, if you are venting in a open stack without air entering into it and you are counting on atmospheric dispersion, the most you can expect is a fire on the top of the vent stack. Any source of ignition (static, lightning, etc) can produce it. An open air explosion , specially if the stack is separated from any building that can produce any sort of "enclosure", is a very unlikely event. If there is an ignition source you would probably have a fire.

 

On a side not: You have the environmental issue of venting methane and other hydrocarbons, and that is something to consider. it seems that this gas, once water is condensed, is able to sustain self combustion, so a small flare might be the best option.

[B.M. Process]

Saml,

 

Thanks for your opinion. The cold vent stack is open to atmosphere (ATM), which is where the concern is. The only thing that the designer is taking a bet on is the presence of high concentration of carbon dioxide and water vapor that account for 46.1% and 8.4% respectively. It is hoped that if these concentrations stay the same, there will not be any chances of explosion which also means that trying to install a small flare stack will be be practical as CO2 in the vent gases will not support combustion.

 

 One possible idea is to continuously introduce nitrogen into the line leading to the cold vent stack. Please find the attached PFD for the vent stack.

Attached Files

•  stack.pdf   104.42KB   14 downloads