How much CO2 is required to neutralize caustic? 1

[nicholascz]

I'm looking at designing a CO2 effluent treatment system for a microbrewery, and I need to make some preliminary calculations to see if the system is feasible.

This may seem a silly question but:

How much CO2 is required to neutralize say, a pound of caustic? (CO2 becomes carbonic acid, which is a polyprotic acid, so there are multiple dissociation constants) I am trying to get a ballpark number without doing a titration.

What about alkalinities? Will that affect my calculations?

CO2 is in the gas phase (supplied from a tank), so how do I take into account the gas liquid efficiencies of the reaction?

Any help is appreciated. Thanks!

 

[kenvlach]

Hi,
The dissolved CO₂ species is a function of pH. Geochemistry, groundwater and pollution by Appelo and Postma (1996), p.94, shows that HCO₃^- is the predominant species for the pH range 6.5-10, so I suggest that the neutralization reaction is:
Na⁺ + OH^- + HCO₃^- + H⁺ = NaHCO₃ + H₂O

Hence, 44 grams of CO₂ will neutralize 40 grams of NaOH.

 

[nicholascz]

Thanks! That was very helpful!

 

[saxon]

nicholascz, Although you can calculate the quantity of CO2 needed to neutralize a given amount of NAOH, in pure solution, most waste streams are "brews" containing chemical compounds that function as buffers to the reaction usually requiring more reagent than the calculation indicates. To real world the actual CO2 requirement, take a sample of the waste stream to the local lab and run a pH titration on the waste stream and develop its titration curve. This approach will supply you with the necessary info to design the appropriate pH control system.

Hope this helps.
saxon