Mass emission rate from a tank release
Mass emission rate from a tank release
(OP)
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.
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.





RE: Mass emission rate from a tank release
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
RE: Mass emission rate from a tank release
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
RE: Mass emission rate from a tank release
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
RE: Mass emission rate from a tank release
RE: Mass emission rate from a tank release
RE: Mass emission rate from a tank release
RE: Mass emission rate from a tank release
jwy
RE: Mass emission rate from a tank release
RE: Mass emission rate from a tank release
RE: Mass emission rate from a tank release
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
RE: Mass emission rate from a tank release
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
RE: Mass emission rate from a tank release
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