6 replies to this topic

[cholman99]

Hello all,

I'm looking for some feedback on the following calculations, I'm not real confident that I'm not over simplifying the issue.

I have a storage tank containing styrene monomer, when the tank is opened (manual level check, etc.) there is the potential for pressure to be released from the tank (vent set point down to atmospheric). I'm trying to determine how many lbs of styrene vapor could be lost during this event. I have calculated this value using the ideal gas law to calculate the number of moles of vapor in the head space at the relief pressure set point and then at atmospheric pressure. Then the change in moles multiplied by the percent styrene in the vapor space (vapor pressure styrene / total pressure) would give me the amount of styrene lost. Am I missing anything in this thought process? Calcs below:

Vapor space volume = 500,000 L
P(relief) = 3.5" WC = 14.822 psia = 1.008577277 atm
P(atmospheric) = 1 atm

Vapor pressure styrene @ 72°F= 0.101 psia
MW styrene = 104.1491 g/mol
T = 72°F = 295.37 K

% styrene in vapor space = 0.101 / 14.822 = 0.00681, [0.681 %]

PV = nRT => n = PV / RT

P1 = 1.008577277 atm
V1 = 500,000 L
R = 0.0821 L atm / K mol
T1 = 295.37 K

n1 = 20795.51 mol


P2 = 1.0 atm
V2 = 500,000 L
R = 0.0821 L atm / K mol
T2 = 295.37 K

n2 = 20618.66 mol

delta n = n1 - n2 = 176.85 mol

176.85 mol * 0.00681 % styrene * 104.1491 g/mol / 453.59 g/lbs = 0.277 lbs styrene lost

Any thoughts?

Thanks,
Chad

[TS1979]

If this is an atmospheric storage tank, there should be a breathing device installed at the tank ruff. In such arrangement, the pressure inside the tank should be equal to the atmospheric pressure. When you open the vent, the loss of styrene is by diffusion instead of venting.

If this is a closed tank without breathing device, you need to figure out what the pressure is before you open the vent. Your assumption that the inside pressure is atmopheric pressure plus the vapor pressure may not be current. The rest of the calculation seems OK

[Robert Montoya]

Hello Cholman99; your calculations is OK. But take into account that the temperature in the vapor space is less of the liquid, usually is 5°F less.
I have a question, How many components addition to the styrene is in the tank? The styrene is 100% pure?

Robert Montoya

[cholman99]

TS1979 -
I would say it is a closed tank with breathing device set at 3.5" WC. In this scenario I'm not worried about breathing losses. I'm just concerned about the net losses when we go from max pressure (breathing device set point) to atmospheric pressure (when tank is opened for maintenance, etc).

Robert -
We have chillers for the liquid that we run during the summer, which is how I come to the max (normal) operating temperature of 72°F for the liquid. Interesting comment about vapor space temp being different than liquid, I had not considered that. My initial thoughts are that the liquid temp would drive how much styrene goes into the vapor space - so the styrene vapor pressure value still applies. During the summer, would we not expect the vapor space to be warmer than the liquid - considering we cool the liquid? In this case, I would expect the head space pressure to increase due to the higher temperature. Since the vent set point of 3.5" WC, limits the max pressure, we would loose vapor (vented due to excess pressure) resulting in a lower mass of styrene vapor in the head space.

I re-ran the original calcs with a new Temp of 77°F and keeping the pressures as is. The result is I went from 0.277 lbs to 0.275 lbs styrene lost.

The styrene does have a stabalizer: P-Tertiary Butyl Catechol.

Thanks,
Chad

Edited by cholman99, 05 June 2012 - 08:09 AM.

[Robert Montoya]

Dear Cholman99; I am so confused with your question. First you say that the pressure inside the tank is down with respect the atmospheric pressure? Is this true? If is this case; I am absolutely sure that the direction of flow going in the direction of higher pressure to lower pressure until the pressures are equal. When the pressure is the same starts diffusion in this case was calculated differently using Fick's law is not not the ideal gas law.

[cholman99]

Robert,

I'm not sure where the confusion comes? Are you refering to the vent set point of 3.5" WC? This setting is psig, so it's equivelant to 14.822 psia or 1.008577277 atm.

Thanks,
Chad

[process-man]

Dear cholman99
In order for us to give good comments to your case, could you please attach the sketch of your tank , I mean type of tank, nozzles, etc..?
Process man

Edited by process-man, 23 June 2012 - 05:39 AM.