Processes > Reactive transport modelling

Water cleaning by air stripping

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MichaelZ20:
Dear David,
Thank you for the explanation.
Actually, your code simulates the cleaning of 1 kgw by bubbling air, but in the case of the water column, "the last bubbles" before opening to the atmosphere will have a much higher benzine(g) concentration (the result of cleaning all the lower water). What do you think about the concept of the giant bubble in equilibrium with all the volume of water in the column? Though it has kinetics constraints, it seems to be closer to reality.

dlparkhurst:
First, I would say that regardless of the history of the bubble, when it leaves the system, it is assumed to be at 1.0 atmosphere pressure and in equilibrium with the solution. Those are the assumptions of the calculation, which seem reasonable. If there were more Benzene in the bubble at depth so that its partial pressure at the surface is greater than equilibrium, ideally, it should lose Benzene to re-equilibrate to the conditions at the surface of the column before it exits the system. If indeed, there were more Benzene in the bubble when it exits than expected from equilibrium at surface conditions, then that should enhance the stripping, but I don't think that is the way it would work.

In concept, if the column is well mixed, the concentration of Benzene is the same at the top and the bottom of the column. The calculated partial pressure of Benzene(g) would also be the same (the pressure dependence of the Henry's constant is minimal). Now a bubble introduced at the bottom of the column at 2 atm will double in size as it reaches the top of the column; however, equilibrium would stipulate that the partial pressure of Benzene(g) in the bubble remain the same at the bottom and the top of the column to be in equilibrium with the solution. Thus, as the bubble grows, you would need to continue to add Benzene to the bubble to maintain the same partial pressure of Benzene(g). So, it seems more likely that the bubble will be undersaturated relative to the solution when it exits the column.

If you use one large bubble, I think it requires greater than 500 L gas per L of water, which is a lot more gas, and more expensive. I don't know anything about gas exchange, but if the transfer between water and gas is slow, it may take much more than 1 L of gas per liter of water.

MichaelZ20:
Dear David,
Thank you for your comments!
Sincerely,
Michael

MichaelZ20:
Hi David!
I tried to modify the input replacing all
USE SOLUTION 0; USE GAS_PHASE 0; SAVE SOLUTION 0; END
by
RUN_CELLS; cell 0; END ,
but it does not work.
What may be a reason?

dlparkhurst:
The two calculations are different.

RUN_CELLS is equivalent to


--- Code: ---USE solution 1
USE gas_phase 1
SAVE solution 1
SAVE gas_phase 1

--- End code ---

So, in one case, a new gas phase (always of composition gas phase 1) is brought into the reaction at each step, whereas with RUN_CELLS, the gas phase evolves with each reaction step and no new gas enters the system.

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