6 replies to this topic

[abhivmairal]

I was reviewing a design of a vapor recovery unit (VRU) for recovery of gasoline vapors (VOC's) by absorbing in gasoline for one terminal. In the design, a heavy gasoline with IBP 140 C and FBP 210C has been considered. Normally gasoline available in terminal also has light ends as per the BS specs. My understanding is that quantity (flowrate) of heavy gasoline required will be less to absorb the VOC's in absorption column as compared to standard gasoline which has light HC's in it. Can anybody confirm regarding this? Hence, according to me the unit is not correctly designed. Also request to please share any supporting study/literature data regarding the same. 

 

The HC concentration considered in feed vapors to VRU is 60%(along with air)for the design of Vapor recovery unit. Seems on higher side to me. Can anybody confirm regarding this also ?

[PingPong]

 

 Hence, according to me the unit is not correctly designed.

How do you come to that conclusion?

 

 

The HC concentration considered in feed vapors to VRU is 60% (along with air) for the design of Vapor recovery unit. Seems on higher side to me.

What concentration would you then design the unit for?

 

If the vapor comes from gasoline storage with gasoline that has an RVP of 9 to 10 psi then the partial pressure of light hydrocarbons in the air at 38 ^oC (100 ^oF) will be about that number, so concentration in the air would be 60 to 70 % in case of 38 ^oC gasoline storage temperature.

Exact number depends om the storage temperature at your location.

[Pilesar]

The lighter gasoline should have a higher absorption rate which would allow less flow. 

[PingPong]

In any absorber the lean oil should not contain any of the components that are to be absorbed from the vapor.

 

It is not possible to absorb light gasoline components from air using a light or full range gasoline as lean oil.

Hence the use of heavy gasoline as a lean oil in this case.

[abhivmairal]

Dear Mr. Pilesar and Pingpong,

 

Thanks for your quick response.

 

Mr. Pilesar, 

 

What is the basis that light gasoline should have higher absorbtion rate and shall require lower flowrate statement ? Request to please elaborate. Please note that in terminal storage tanks all the gasoline are mixed. As such there no heavy or light gasoline readily available for absorption.

 

Mr. Pingpong,

 

Thanks for your reply. I agree that lean gasoline should only be used for absorption of light ends and shall require lowest flow for absorption. As mentioned, in terminal, we dont have separate light or heavy gasoline available since everything in added to storage tank. Hence, my understanding was that heavy gasoline should not be considered for design. Normal gasoline (with light ends) shall require more flow. Please confirm my understanding.

[Pilesar]

Efficiency of solvents vary. Engineering using good vapor/liquid equilibrium data is required to find the best design. I don't know whether your solvent is suitable or what amount of removal is required. The solvent used should have a high solubility for the pollutant vapor and low viscosity and low vapor pressure. If the material you have available will work, then it is probably the best choice. If it will not work, then you will need to find another way to control VOC.

[PingPong]

Mr. Pingpong,

 

Thanks for your reply. I agree that lean gasoline should only be used for absorption of light ends and shall require lowest flow for absorption. As mentioned, in terminal, we dont have separate light or heavy gasoline available since everything in added to storage tank. Hence, my understanding was that heavy gasoline should not be considered for design. Normal gasoline (with light ends) shall require more flow. Please confirm my understanding.

Once again: you cannot absorb light gasoline components from air using a full range gasoline that contains those light components. It won't work no matter how huge you would make that gasoline flow. The gasoline vapors were produced in equilibrium with the full range gasoline due to its rather high vapor pressure (RVP 9 tom 10 psi). Recontacting those vapors with the same gasoline from which they originated will not reabsorb any of them.

 

You need a lean hydrocarbon that does not contain the components to be absorbed. If that is not available then you need to consider another type of VRU using refrigeration, or membranes, or ........