Vapor recovery unit (VRU) design and methodology

[RistEcho]

I've been asked to review some VRU designs for a tank terminal containing HC (activated carbon beds with PSA followed by a scrubber), but admittedly I do not have hands-on experience with VRUs so would like to seek everyone's experience. Design is for 1000 NCMH.

Couple initial questions:
1. Tank products are mainly gasoline but do include heavier HC. Most VRU designs call for adsorption with activated carbon bed, followed by desorption/regeneration at 25-50 mmHg for 10-15 minutes cycles. Are these vacuum pressure and duration settings enough for full regeneration? I'm a bit incredulous because the heavier HC will be harder to desorb, and I worry that they will accumulate over time to kill the effectiveness of the VRU.
2. From what I understand, adsorption of lighter HC (e.g. ethanes) by activated carbon is poor. Does the adsorption truly improve with specialty activated carbon pellets (I saw a quote for a specialty 4mm cylindrical activated carbon product that was multiples of the price of a "generic" brand)?
2. Vacuum pump - From what I've read, it seems that the top reason for VRU failure is the vacuum pump. Do people have a favorite type (or brand/model?) for this application? I understand that oil seal vacuum pump seems to be most popular, but I would prefer to rely on advice from folks with real VRU experiences.

Thank you!

 

[georgeverghese]

Agreed, low temps will be required for adsorption of ethane on to activated carbon, but then there is next to no ethane in gasoline.

 

[RistEcho]

@georgeverghese - What have been your experience with why VRUs fail or do not meet their emission targets?

 

[pierreick]

Hi,
Check with those guys.

Vapor Recovery Units | John Zink

John Zink's Vapor Recovery Units safely prevent harmful VOCs from reaching the atmosphere and are highly adaptable for a variety of applications.

www.johnzink.com

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A google search keys words Ethane vapor recovery unit is another option.

Notes: Three concerns about carbon bed, the regeneration process and how to dispose the material at the end of its shelf life,
the risk of self-ignition of the bed when saturated with HC.

To me you should consider working with people in the field, define your needs (specification), ask for a quote and guaranties.
In some countries it is very difficult and expensive to dispose AC (China is an example).

Good luck,

Pierre

 

[georgeverghese]

I dont have direct experience working with or in the design of activated carbon adsorption units in gasoline tank VRU service. There doesnt seem to be a grade of mole sieves that would enable this adsorption of heavy hydrocarbons ( paraffins + naphthenes + some aromatics) from an air stream at ambient temps, and the closest appears to mole sieves 5A - talk to Dow Chemicals to confirm and explore. Otherwise, see if this document helps:

https://24198570.fs1.hubspotusercontent-na1.net/hubfs/24198570/TSM_feb-mar-2017-Activated-Carbon-3.pdf