Pure VOC recovery from solution in product water tank

[B_DeW]

Dear all,
My first post so please let me know if anything is wrong, and thank you in advance for your help!
Without going into detail about the process our company designed (we're a scale-up), I'll try to give relevant details.
We have designed a process that produces nearly pure water (think RO quality). The problem is that we have a small amount of a VOC dissolved in the product water due to our process. Since it is an expensive and flammable gas we're trying to recover it.
The idea is to design a closed product water tank where the VOC can come out of solution which takes about 2 hours (99.7% removal at atmospheric pressure). The first design was a product tank filled to the top with water (after purging the air out) and then letting bubbles of our VOC form within this tank, while continuously draining and filling the liquid with a residence time of around 2 hours. The VOC will be pumped from the top of the tank.
The questions are: will this work? Are there better ways of recovering a fairly pure gas from a binary mixture of water with a gas?
Thanks in advance!
-B

 

[SuperSalad]

If you cool the solution, that may aid your process. It should decrease the evaporation of the water. So that may get you a purer stream as you pull the VOC gas out. If you were to do that though, I would think pulling vacuum would also be advisable.

Is the goal here purely recovery of the stream to reduce loss? Or do you need to recover it to prevent environmental release?

Andrew H.

http://www.MotoTribology.com

 

[EdStainless]

Though if you warm the water you may reduce the solubility of the gas allowing it to out-gas faster.
Either way I would think that a slight vacuum would move the process along faster.
Another option is to bubble very coarse bubbles of nitrogen through the water to assist in removing the VOC.
There are ways to design flow-thru multi-chambered tanks for this kind of separation.


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P.E. Metallurgy

 

[Compositepro]

You can use a steam stripping process, which works very rapidly. In a small vessel you can heat the liquid to boiling, using a heating element. The steam will almost instantly remove all volatiles from the liquid. This steam gets condensed (refluxed) in a vent pipe at the top of the vessel, leaving the volatiles behind to vent off as they accumulate. The water leaving the stripper goes through a counter-flow heat exchanger where most of the heat is recovered by pre-heating the feed water to the stripper. The amount of moisture in the volatile gas depends on the condenser temperature.

 

[B_DeW]

Thanks all for the tips!
Andrew: We're removing it because of the cost and fire risk (we're bound to our industrial partner's ATEX zones and HSE management...).
We'll look into warming or cooling the water, and test if any of those will make a significant difference.
I'm afraid we don't have the energy availabilty or budget to implement steam stripping but we will keep this in mind for a scaled up pilot, thank you @Compositepro!
We're now looking at pumping the VOC from the tank based on the internal pressure (if it's rises, we pump), to ensure that we maintain the level.

 

[LittleInch]

With a residence time of two hours you can't have much of an inflow.

Thought about a vacuum de-aeration tower? I think that would strip out all your VOC a lot faster.

Just letting it slowly gas off seems really rather inefficient and slow, but depends on your process and space.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[B_DeW]

Thank you LittleInch,
We do have a very low flow rate, approx 200L/minute of product with at most 5kg/hr inflow of the VOC (it's a relatively large tank).
We did think about a tower-like structure but not a vacuum deaeration tower, will have a look if that suits our flow rate. Thank you!
-b

 

[LittleInch]

That's a pretty low rate so a full vacuum tower is probably rather OTT, but you could try a mini one.

But in a big tank I think you'll find it difficult to guarantee a VOC limit.

Some level of agitation or cascade flow should be better.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.