Hydrocarbons evaporation reduction

[barqueiroenmarte]

Dear all,

I have sometimes heard that increasing the storage pressure of a tank containing hydrocarbons (i.e with inert blanketing) can help to reduce emissions but I don't understand it.

As far as I know, a liquid contained in a tank will evaporate until the partial pressure of that component in the vapor space above reaches its vapor pressure. If it is so, in my understanding, the mass of hydrocarbon vapor in the vapor space doesn't depend on the storage presuure, therefore the mass of hydrocarbon vapors which are expelled to the atmosphere whenever there is an increase in liquid level is the same.

Is my understanding correct? Increasing the storage pressure of a hydrocarbon can reduce the amount of hydrocarbon vapor in the vapor space?

Thank you for your help.

 

[dcasto]

Those who told you were correct, but all you do is move some of the emmissions from one place to the next. Think about some extremes here. Lets say you put 1000 psi on the tank, then the equalibrium (partial pressures) will be such that the gas phase will be 99% N2 and the liquid phase will be 5% N2. Any venting of the tank will only be 1% HC's. When you take the liquid out, the N2 will come out of the liquid and carry some HC's with it. The N2 gas + HC's will be about 50% HC's so where ever you vent the gas of the liquid, they will have the emmission problem.

Now, without detailed compositions and pressure, there maybe a point where you can add N2 and vent it as needed and vent the dissolved N2 from the liquid later and have a net zero change to emissions. My recommendations is to recovery the HC' vapors when you vent them to lower your emissions. Charcoal, refrigerations, compressions, incineration,......

 

[barqueiroenmarte]

Hello and thank your for your answer.

There are a couple of things I am afraid I don't understand very well:

- You mention that the liqid phase will be 5% N2. Does it mean that some nitrogen can become liquid or what you mean is that it is dissolved in the HC liquid?

- In your example at 1000 psi in the equilibrium there is 1% HC's. In my understanding, if the pressure was 500psi the HCs content would be 2% so when there was an increase in liquid level the amount of HCs expelled would be the same (the HC content, 2%, would be the double than in the case of 1000 psi storage but the total amount of gases expelled would be half that in the case of 1000 psi because of being at a lower pressure, so the amount of HCs would remain the same). Is it wrong?
- Finally, could you please explain a bit your last sentence "there maybe a point where you can add N2 and vent it as needed and vent the dissolved N2 from the liquid later and have a net zero change to emissions"? Do you mean that it is possible to achieve a zero emission of HCs through the venting?

Thank you

 

[dcasto]

The nitrogen is in the liquid phase, you could say it is disolved, but it's energy is that of a liquid.

I did some quick simulations of a mixed HC stream and N2. At 1000 psig, the HC stream was 11% N2 and the vapor stream was 98% N2. At 500 psig, the HC liquid was 65 N2 and the vapor stream was 97.5% N2.

putting N2 in the N2 will stop a large amount of HC's from venting on your site. When you move the liquid HC's and their pressure is reduced, then at that time HC's will also leave with the vented N2. I do not believe that the amount of lost HC's when you put N2 into the tank will be less than if you didn't put N2 in, when you include venting at your site and the site where the HC's are used assumeing they are used at low pressure (less than 15 psig).

 

[barqueiroenmarte]

According to your simulations the HC content at vapor phase at 1000 psig would be 2% whereas at 500 psig would be 2,5%. So, when there was an increase in the liquid level of the tank of, for example, 1 m3, it would be expelled 1 m3 of the vapor. Since the vapor of the tank at 1000 psig has a density twice than the vapor at 500 psig, the mass of HC expelled at 1000 psig would be 60% higher than at 500 psig ((1000*0,02)/(500*0,025)=1,6, so an increase in the storage pressure causes an increase in HCs expelled through the venting?

Finally, could you please tell me the software or documentation you usually use for this type of calculations?

Thank you

 

[dcasto]

I use

http://www.WinSim.com.