Best method for removing water vapor from offgas?

[jforbes]

I am looking for some advice. In a pump and treat plant that my company operates, an air stripper is used to remove VOC's from the process water. The air used by the stripper is then directed to a vapor phase carbon adsorber before it is released into the environment. Currently we have a heater installed between the air stripper and carbon adsorber to heat the offgas so that the water won't condense. The offgas is 100% humid/completely saturated with water vapor and the heater heats the air 20 degs, from 60 degs F. to 80 degs F. We have been having problems with the heater and are considering replacing it with another heater or a dehumidifier.

Are there any envt. engrs. out there that operate similar sites?
How does your site handle this issue?
Any suggestions as to which choice would be best, heater or dehumidifier, or both?

It is my understanding that condensate in the cabon adsorber decreases the efficiency of the carbon. I also heard that air temperatures greater than 80 degs. F also decrease the efficiency of the carbon.

I would greatly appreciate any help you could offer. I'm a recent grad and don't know where to go for advice.

Thanks in advance!

 

[fvincent]

I don't know the flows, but some ideas follow:

1 - you could use a recuperative heating system, that is, leaving warm gas would heat incoming cold gas. After the recuperative heater you would install the conventional heater. The aim here is to save energy, but I think it is applicable only for somewhat large flows

2 - cooling and recuperative reheating is also possible with the same benefits of energy saving, provided you have large flows


3 - heating main stream by adding heated air (that is, by mixture). Your heater would not face wet gases

4 - 1 + 3

5 - The simplest idea: just dilution of wet gas with fresh air: relative humidity would decrease (avoiding in some extent condensation) and temperature would keep at 60F

Hope this helps





fvincent

 

[jforbes]

Thanks for the ideas! Overall, the flow is very small. I really liked your idea about diluting the gas with fresh air to reduce the relative humidity and I'm going to discuss it with the project manager.

Again, thanks for the help!

 

[hydrae]

jforbes

Be carful with diluting the gas with fresh air, what is the temperature and humidity of the fresh air? look up on the psychrometric chart for mixing. Two streams of air may result with a condensing mixture. (that is how clouds are formed)
The air out of the stripper will be colder than supply air due to the evaporative cooling effect.
Also your carbon adsorber is designed for a specific flow rate you may not get the residence time that is needed if you increase the flow.

You may want to contact the manufactorer or designer of the treatment system before making modifications.

Hydrae

 

[fvincent]

hydrae

You are correct: it is necessary to check the psychrometric chart.
However, if air at ambient temperature is used to strip VOC from water at probably a lower temperature (or close to wet bulb temperature of ambient air), then the saturated air stream will be at a temperature whose maximum is the dry bulb temperature of the ambient (what happens only if ambient air is already saturated).

If ambient air is not saturated, the air downstream the stripper + dilution air won't...

Am I correct?

fvincent

 

[hydrae]

fvincent
That is why I suggested to jforbes to check the psy chart. If the ambeint air is from the outdoors, its properties are variable according to weather... if from indoor, how well is RH and temp controlled?...
Hydrae