benzene recovery 4

[poli60]

Dear Experts,
I need to define the basis of design for a vapor recovery unit that shall treat the vapors coming from an internal floating roof tank for the storage of benzene.
My assumptions are the following:
1) flowrates from the tank can be calculated following API 2000 rules.
2) yearly emissions of VOC (benzene) can be estimated with AP-42 methods.
My questions as follows:
a) are the assumptions right?
b) how the figures from 1) and 2) have to be combined to obtain a design flowrate to the VRU and, especially, the VOC content in air?
Thank you for any help you can provide.

 

[poli60]

Just a little clarification.... in this case the "main gas" is nitrogen, not air.

 

[TD2K]

Just to be sure I understand you, you need/want to process and recover the benzene that gets past the vapor seals of the internal floating roof into the vapor space of the tank?

 

[poli60]

TD2K,
as you say benzene goes into the vapor space and then to the atmosphere due the volume variations in the the tank (outbreathing).
The VRU will have to purify nitrogen from benzene.
Thanks.

 

[RaRo]

unless you are storing the benzene at high temperature, I don't think a recovery system would be financially beneficial.
Consider that.
You may have to switch to simple emission control by ad- or absorption, or incineration.

 

[poli60]

RaRo,
actually the VRU will have an ads/abs scheme.
My question is "how can I size the VRU? For which quantity of benzene?"
Thanks

 

[TD2K]

http://www.epa.gov/ttnchie1/ap42/ch07/bgdocs/b07s01.pdf

That should give you an idea of the losses from a floating roof.

I wouldn't use API 2000. While API 2000 is used to size vent and vacuum valves it's intended to handle all tanks and I suspect is going to give you higher volumes than in actual fact because you don't want your vacuum or vent valves to be undersized and vent and vapor valves are relatively inexpensive, trying to optimize a 6" vent valve versus a 4" just isn't much money.

 

[BigInch]

RaRo, Financial reward is not the motivation for everything.

What would you be doing, if you knew that you could not fail?

 

[RaRo]

I agree BigInch. But a recovery system is different from a treatment system...

The AP42 material linked by TD2K seems quite substantial. Thank you!
Any addition of benzene to the tank will generate an emission of nitrogen+VOC load to your VRU. Your mass balance for the unit should be enough to give you the yearly amount of N2+vapor volume to be treated. Your instantaneous flowrate will be given by your benzene loading pump.
The ambiant thermal effect will also contribute, but presumably in a far lesser order of magnitude. You can judge that from you location meteorological characteristics.
Can you however consider the average VOC load to be your design load? I don't think so. Depending on the time your benzene remains in the tank without level change, you should probably consider the VRU design load equal to the nitrogen top saturation in benzene (at storage temperature and benzene vapor pressure).

Hope this helps.

 

[poli60]

Thanks to TD2K and RaRo for the discussion.
It would be great to have other contributions from members with specific experiences (design and/or operation) on ads/abs VRUs installed in benzene/aromatics storage and delivery (trucks, ships, trains) systems. For instance:
- liquids used for the vacuum pumps and the absorber: do you use the same (benzene) for both?
- do you recycle the cleaned nitrogen from the absorber either to the tank or to the adsorbers?
- which discharge pressure do you obtain from the vacuum pump (ie absorber operating pressure)?
- other items
Thank you in advance.

 

[shokridesign]

you can adsorb benzene vapor on activated carbon in a column filter and for carbon and benzene recovery i must know your system diagram and amount of benzene vapor.