Mass emission rate from a tank release 1

[Dave2614]

I have been given the task of calculating how many pounds of H2S was released through a pressure relief valve on a slop tank that was constantly fed a supply of gas for a 24 hr period. I wish I was a ChemE, but I'm not, so I would appreciate some help from someone who is.

The slop tank holds 400 barrels and is ~20' high and ~10' in diameter. For this exercise, let's assume that it is 1/2 full of liquid. It has 2 PRV's one is set at 8 psi and the other, a trap door, is set at 16 psi. The 8 psi valve was opened and flowed to atmosphere through a 3" ID vent pipe. The pipe is ~3 ft long. The trap door was not opened. The gas that is being released contains 6.5% H2S.

For this calculation, I will assume that the gas supply was steady and kept the tank pressurized at 8 psi.

I am attempting to calculate how many pounds of H2S are being released per hour. Thank you in advance for your assistance.

 

[jwysmooth]

This should be a fairly straight forward calculation. You can calculate the mass of H2S from the concentration you stated if you can determine the capacity of the 8 psi PRV. Usually, the PRV manufacturer will provide a flow curve with which you can calculate that flow rate. The additional information you need is.
1) Is the 6.5% a volume percent or mass %
2) The composition or MW of the vent gas
3) The temperature of the gas.
4) Flow curve of the PRV.

If the pressure from of the 3" dicharge pipe is low enough for the flow calculated, the pressure drop of the 3' riser pipe can ignored.

If you get that information, I can work with you to calculate the mass emissions.
jwysmooth

 

[Dave2614]

Thank you very much for the reply and your assistance. First, the composition of the gas contains 6.5% total volume of H2S. I am trying to obtain the composition of the gas and the temperature; as soon as I get those, I will post them. Regarding the flow curve of the PRV, I don't know if this will help, but I did learn that the air release coefficient of the PRV is 0.95.

I can't thank you enough for helping me with this...if I could give you ten stars, I would.

Once I gather all of the information, I might be able to calculate it myself if you could give me the formula, or the steps in which to take. Again, thanks a ton.

Dave

 

[jwysmooth]

No problem.
I'm not familiar with the term "air release coefficient", but I suspect it is the same value that the GPSA handbook calls K = coefficient of discharge. If you have contact with the manufacturer, you may confirm that. If it is, I think that K value in addition to the discharge area (A) of the PRV will allow us to calculate the flow thru the valve. That is, with the K and A of the PRV we do not need Item #4 from my previous list.

If you have or have access to a GPSA handbook the formula that we will use to calculate the flow is Eq 5-3 using the K and A values.
Cheers.
jwy

 

[IFRs]

The EPA might have methodologies that they like you to use. Hvae you reviewed the pertinent rules?

 

[Dave2614]

I have spent the last few days trying to gather information, specifically from the EPA. Do you know of the specific EPA regulation that address these types of calculations? Thank you.

 

[IFRs]

AP-42?

 

[jwysmooth]

The EPA usually provides guidelines to measure emission or estimate emissions. AP-42 are methods to estimate emissions from sources were they are unknown. I have never heard of an instance where the EPA has dictated to a manufacturer how they are to determine the capacity of their equipment. Since you already have the concentration of H2S, all that is needed now is to determine the flow rate from the PSV to estimate the mass of H2S emitted.
jwy

 

[Dave2614]

As soon as I receive the model # of the valve, I will then be able to provide the flow curve. Hopefully, that will be soon. Thanks for the help.

 

[Dave2614]

Hello, I just learned that the valve is a Varec 2"x3" model 2020B. After speaking with Tyco, I learned that the max pressure flow capacity for this valve is 8098 SCFH and the max vacuum flow capacity is 1562 SCFH. If some one could help me calculate the amount of H2S released, it would be greatly appreciated. Dave

 

[jwysmooth]

OK, the flow rates they gave you are probably for air at standard temperature of 60 deg. F. Depending on how accruate you want your answer, you should confirm that their values are for air and its temperature. The gas that you are relieving from the tank is probably not air, so ultimately, you will want to correct the 8098 SCFH flowrate for your gas. Thats why we need the MW of your particular relief gas.

For now we will assume that your gas in the tank is mostly air at 60 F

Calculation:
8098 (scf/hr)*(0.065) = 526.4 scfh of H2S

526.4 (scf/hr)*(1 lb-mole/379 scf)*(34.08 lb/1 lb-mole)= 47.3 lb/hr.

The two conversions in this equation are from the ideal gas law. 34.08 is the MW of H2S and there are approximately 379 scf in a pound-mole of an ideal gas at 14.7 psia and 60 F. Remember, this assumes that the carrier gas is air at 60F. The acutal value will be dependent on the actual MW of the gas and its temperature. When you get that info, I'll show you how to correct for that.
Cheers,
jwy

 

[Dave2614]

Thanks a ton, jwy! I also used that same approach, and came up with basically the same answer (47.9 lb/hr). The only difference was that I used .0911 lb/cu.ft. H2S

Would it make a big difference if we adjusted for the MW and temp? The temp of the gas is actually 60 F and it's specific gravity is 0.773.

Thanks again for helping me out.

Dave

 

[jwysmooth]

You are welcome.
The relationship between air and other gases is:
scfh Air = scfh gas*(MW gas/29)^0.5*(14.7/P)^0.5*(T/520)^0.5

You can use the spec. gravity instead of MW so...

8098 scfh = x scfh * (0.773/1)^0.5 or
x = 9211 scfh of the gas. So your H2S mass emissions becomes 53.8#/hr.

Glad I could help.
jwy