H2S emission rate 1

[Dave2614]

I am attempting to calculate the emission rate of H2S from an oil/gas production slop tank. The pressure relief valve that was venting to atmosphere released at 8 psi through a 3" pipe. The volume of the tank is 16,800 gallons and there was a continuous supply of gas flowing into the tank.

The temperature was 294 degrees K.
VP of H2S is 17.6 atm
MW is 34.1

I am not an engineer and need some help in providing a relatively accurate answer. Thank you very much to the kind sole who takes the time to help me.

 

[stevethermo]

This is not a straight forward calculation. Even to get a rough number will take some engineering judgement. You will need to have information on the H2S composition in the gas. If that is available, the next issue is the flow rate from the tank. The flow rate leaving the tank is largely dependent on the restrictions in the relief valve itself, and will require documents with information on the flow characteristics on the valve, or some kind of test.
Hopefully the vaporization of H2S from the liquid to the gas can be neglected, but will depend on the liquid and gas compositions.
Post more info if you can.
Regards,
Steve

 

[Dave2614]

Hello Steve, thanks for your reply. I do not have any other information on the gas/liquid charateristics, but I do know that the air release coefficient of the PRV is 0.95. Using your engineering judgement, can you provide me with an approximate amount of the H2S release rate? Thank you very much for your time and assistance.

Dave

 

[maddocks]

Hi Dave:

You really need to get us a gas composition. We need to come up with a specific gravity to determine flowing properties. As well, we need some info on the PSV/PRV. Can you provide a model # and some details? Anything?

jim

 

[Dave2614]

Hello guys, I have some additional information and hopefully this can help solve the problem. The gas components are approximately:
Methane = 75-78%
H2S = 6-13.5% (normally, 6.5-8%)
Propane = 3-4%
Butane = 2%
CO2 = less than 1%

I know it doesn't add up to 100%, but that's as close as I can get.

The temp of the incoming gas is 60-70 degrees F

The PSV is a 2.5" Varec, I do not know the model #, but it releases at 8 oz. of pressure.

The tank is empty and it has a vapor pressure of 14 lbs. The tank is designed to be kept under atmosheric pressure.

That's all I have. Thank you all who are helping me with this.

Dave

 

[Dave2614]

Hello, I have learned that the PRV is a Varec model #2020B, 2"x3". After speaking with the manufacturer, I learned that this valve has a maximum pressure flow capacity of 8098 scfh and a max vacuum flow capacity of 1562 scfh. Given the gas composition and this flow information, would someone be able to assist me in calculating (and understanding the process) the amount of H2S being released from this tank. I am willing to pay for your assistance.

 

[pichpich]

Hello, I have alsmost the same problem.
the composition of gas is ch4, h2s (250ppm),c2h6,...
My concern is the H2S which is present with a high quantity. my client is asking to prove that the vents are designed considering the high content of H2S.

Please help me to find a code treating thisissue.

many thnaks

tschüss

 

[mbeychok]

Dave2164:

I am going to make a number of assumptions because you have been giving us contradictory information and using terms that you obviously do not understand.

Assumptions

(1) The temperature in the tank at the time of release is 294 kelvins which is equal to 70 degrees F.

(2) The gauge pressure in the tank at the time of release is 8 oz as given you by the Varec manufacturer (rather than the "8 psi" that you first gave us). A gauge pressure of 8 oz is 0.5 psig and that is equivalent to an absolute pressure of 14.696 + 0.5 = 15.196 psia.

(3) The manufacturer's release flow rate of 8,098 scfh is in terms of standard cubic feet at 60 degrees F and 14.696 psia (there are many different temperatures and pressures used to define scfh).

(4) The H2S content of the gas that you gave us as a range of 6 to 13.5% is volume percent (rather than weight percent). I will use the average of those 2 values, which is essentially 10 volume %.

Calculations

(a) A released gas flow of 8,098 scfh (at 60 deg F and 14.696 psia) is the same as 7,986 cubic feet at 15.196 psia and 70 deg. F

(b) The amount of H2S released is 10 percent of 7,986 cubic feet = 799 cubic feet which is approximately 2.11 pound-moles or approximately 72 pounds.

Some of the info you gave is is completely irrelevant and/or in error:

(1) "vapor pressure of H2S is 17 atmospheres" is the wrong terminology. You probably meant the saturation vapor pressure of H2S at 294 kelvins ... which, in this case, is completely irrelevant.

(2) You gave us a release pressure 8 psi in your original posting, but then told us that the Varec valve opens at 8 oz. of pressure

(3) You must always state whether pressures are gauge pressures or absolute pressures.

(4) You must always state whether percentages are volume percent or weight percent.

(5) You must always define the reference temperature and pressure for defining a "standard" gas flow. There are many different reference teperatures and pressures.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[owg]

Mr. Beychok has brought some welcome rigour to the question and the answer. However I find it unlikely that the amount of gas coming into the vessel happens to exactly match the capacity of the relief valve. It is more likely that the relief valve is relieving intermittently since the pressure is not reported to be building in the tank. Perhaps Dave2614 could observe the frequency of release, without getting too close to this toxic gas, and then multiply the frequency fraction by the calculated spot release rate of 799 SCFH.

HAZOP at

http://www.curryhydrocarbons.ca