I need to install a filter in the vent connection of two atmospheric (API) tanks used one for doinized water and the other one for demineralized water.

The intention is to avoid CO2 present in the air entering the tank.

Need to know how to design or specify it, absorbent material and name of possible suppliers.

i do not have a straight answer to your problem, but just a couple of hints. Why do you want to block CO2 only? Moisture in air will be even more aggresive and you should consider getting rid of it too. The way we protect Demin Water Tanks in our plant is by light purging of an inert (N2), and when this is not applicable purging of dry air (with a dew point of less than -50 C).
This protection has proved very effective .

The purge is just a hose in the tank that provides a light N2 flow within the tank, flow that is vented from the tanks' vent.
there is no pressure control in the tank, the flow is adjusted by checking if there is a light flow coming out from the vent.
The only drawback of protecting tanks in such manner is that N2 is costly and should not be wasted...
Hope that the information is valuable enough.

Here's a few extra tips for venting

1) Make the vent fairly large (4" exit 6" pipe). Dont use any block valves or equivalent. You could specify a spectable blind in the flange if somebody insists on a way to close of the vessel - but if you install a block valve then you will have to install a burting disc or RV and then it gets a lot more complivated
2) Make flowing velocity in the area around 0.1 m/s
3) Use a neddle valve and a rotary type flow meter (install the meter downstream of the valve) to control flow.
4) Use area of vent exit (4") to calculate required vol flow of N2.
5) If N2 flow is critical the you could also install a flow switch on the N2 line.

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