Optimum pH for NaOH based Wet SO2 Scrubber
Optimum pH for NaOH based Wet SO2 Scrubber
(OP)
I am looking for assistance with the best pH to operate a small NaOH based scrubber for controlling low concentrations (75 ppm ) of SO2 from a CHP exhaust, with a high CO2 content. I wish to minimise NaOH losses to carbonate formation while still removing SO2. Technical references as well as field experience most welcome.
Thanks in advance,
TISM
Thanks in advance,
TISM





RE: Optimum pH for NaOH based Wet SO2 Scrubber
RE: Optimum pH for NaOH based Wet SO2 Scrubber
RE: Optimum pH for NaOH based Wet SO2 Scrubber
Thank you anyway
RE: Optimum pH for NaOH based Wet SO2 Scrubber
Technical references may easily be found in the internet.
Since the scrubber is small, have you considered using just water? If you simply dissolve the gases in water you'd probably absorb much more CO2 than SO2 because of their relative proportions in the gas (divided by their respective Henry Law constants). For example, if the relative volume fraction of CO2 /SO2 in the gas is, say, 190:1 the mol fractions absorbed in water @ 30oC, would be: (190)(50)/(1900) = 5. Five times more CO2 than SO2.
The reactions with water:
CO2(gas) <-> CO2(dissolved); +H2O<->H2CO3<->H+ +HCO3-
SO2(gas) <-> SO2(dissolved); +H2O<->H2SO3<->H+ +HSO3-.
The right concentration of H+ in solution that drives the first reaction to the left while driving the second to the right happens at 10-4 to 10-6 mols per liter (pH= 4-6). This pH is not achievable with NaOH solutions. NaOH solutions (pH>7) would absorb more CO2 than SO2 at 30oC.
To remove SO2 without absorbing CO2, the scrubbing solution should be a weak acid. That's the reason I suggested you visit the other thread.
Industrial-size solutions consider using soda ash (sodium carbonate) or bicarbonate which are cheaper and can be regenerated. A typical reaction would then be:
Na2CO3 + SO2 -> Na2SO3 + CO2 for the scrubber
And in the holding tank one may regenerate the sodium carbonate with calcium salts in a reaction such as:
Na2SO3 + CaCO3 + 0.5 O2 -> CaSO4(Gypsum) + Na2CO3
then to a thickener for precipitation and separation of calcium sulphate (with a flocculating agent), while the supernatant liquid would be reused in the scrubber. Some carbonate is lost in the cake, and some calcium would be retained in solution but in sufficiently low concentrations not to form solids in the scrubber.
Yours is a apparently a small outfit so my suggestion is you heed PeterAB's advice.
RE: Optimum pH for NaOH based Wet SO2 Scrubber
Note also that whereas the absorption of SO2 into aqueous solution is a virtually instantaneous process limited only by the diffusion rate of the SO2 molecules to the gas-liquid interface, the absorption of CO2 is a much slower process controlled by resistance to mass transfer in the liquid phase. Quoting Perry's Chemical Engineer's Handbook (p. 18-44 in 5th ed.):
"Sherwood and Pigford ("Absorption and Extraction," 2nd ed. McGrawHill, NY 1952) indicate that the controlling step is a slow pseudo-first-order irreversible reaction between dissolved CO2 and OH-,..."
Hence in addition to minimizing pH, the absorption of SO2 over CO2 may be favored by minimizing the reaction time available for absorption. For this purpose you may wish to consider a finely atomized spray scrubber, which due to its large gas-liquid interfacial area completes the absorption in about one second. see www.turbosonic.com
RE: Optimum pH for NaOH based Wet SO2 Scrubber