Wet scrubber - what happens if you continue to re-circulate a scrubbing liquid?

[RistEcho]

I have a general question about wet bed scrubber (packed bed scrubber).

If I re-circulate a scrubbing liquid (e.g. water) to capture a pollutant (e.g. a chemical compound) from a gas stream, I understand that the concentration of that chemical will accumulate in the scrubbing liquid.

At what point will the re-circulated scrubbing liquid stop absorbing the chemical pollutant, assuming no blowdown and no fresh water make-up?

If the chemical has a very low Henry constant and is completely miscible in water, then wouldn’t I be able to form a high (say, >80% or >90%) concentration solution of the chemical via this method?

Thank you for entertaining my thought experiment.

 

[pierreick]

Hi,
Probably good for you to review this document.
If you don't add make up water, the level of water in the sump will decrease because the gas leaving the tower is saturated. You will damage the pump soon or later.
Other consequences, viscosity, charge of pollutant, superficial tension will increase, and Foam may appear thus will affect the mass transfer.
By the way, check the validity of your model using Henry's law.

Good luck.
Pierre

 

[shvet]

water will be completely lost with a gas or will be substituted by components of a gas having lower vapor pressure than water has

 

[georgeverghese]

Agreed, it will stop absorbing when the recirc. absorbent liquid concentration reaches the Henry's Law limit at this operating temp

 

[RistEcho]

I appreciate everyone’s input.

To summarize, provided that the solubility limit has not been reached, the ratio of the molar fraction of the chemical in vapor and recycling liquid will simply equal its Henry constant. Chemical concentration in vapor needs to be low (say, <3%) for Henry's Law to apply.

Other factors to consider:
- Need a minimum liquid level in sump to protect pump
- Consider changes in liquid properties, such as viscosity, surface tension, and foam, that can affect mass transfer

Is this correct?

 

[shvet]

Incorrect.

Mass transfer is impossible once the equilibrium concentration is reached. This is why you make up fresh solvent in a circulation loop—to keep the concentration of a pollutant in the circulating solvent below the equilibrium one. The further the concentration in the circulation loop is from the equilibrium one, the less packing height is required in the scrubber.

This entire "thought experiment" of yours is meaningless; it doesn't work as you believe.

Note that you're ignoring that solvent vapors (e.g., water) are continuously lost with the gas. Also, you're overlooking that Henry's Law assumes a gas component being absorbed is above its critical temperature and therefore insoluble in any solvent.

You should try dynamic modeling of this scrubber, as it's a relatively simple activity.

 

[georgeverghese]

Mass transfer is also affected when you have maldistribution of liquid over the cross section of the absorber column. The configuration of the first distributor, redistributors every several ft of length also are key factors, especially for packed bed units. Keep vapor velocity within permissible upper limit approach to floodpoint. Also liquid flow rate must be above the minimum wetting rate.

What exactly are your concerns here ?

 

[RistEcho]

Let me try to illustrate better with an example.

Suppose I have a scrubber for DMF vapor at 40C at 10000ppm. I want the outlet gas concentration to be 2500ppm or less.

DMF is completely miscible so solubility limit is not an issue. DMF’s Henry’s law constant is also very low (7.47 × 10−5 at 25C).

My absorbent (water) starts at 0% DMF. I use a level control in the scrubber sump to add just enough fresh water to keep the minimum level needed to keep the scrubber running, but I am mainly recycling the absorbent.

At 40C and 1 atm, 50% DMF solution (% wt) would have a vapor pressure of ~2000ppm at equilibrium. Suppose I want to create this 50% DMF solution while not exceeding my goal outlet gas concentration (2500ppm). Could I run this system until my absorbent becomes 50% DMF solution? I can then pump this solution to a separate tank, and repeat this process again.

(I know 40C is hot for a wet scrubber, but that’s the VLE data that I have on hand.)

 

[pierreick]

Hi,
Did you hear about Google?

https://www.kianresin.com/userfiles/files/eastman_Dimethylformamide%20(DMF).pdf

many more data are available!
Do you know that Henry's coefficient has a unit?
Did you check the MSDS of your product and acceptable threshold for releasing this material to atmosphere?
Your objectives should be to release DMF at the low level possible to atmosphere, not to produce a solution of concentrated DMF.

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my view only,
Pierre

 

[shvet]

college coursework is not welcomed in case those are misposted

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