Optimum pH for NaOH based Wet SO2 Scrubber 2

[TISM]

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

 

[PeterAB]

Since SO2 is much easier absorbed than CO2, the presence of CO2 should not pose much of a problem. Very little CO2 is absorbed below a pH of 9. By contrast, the blowdown pH from an SO2 absorber is typically around 6. Pick a blowdown pH of 7 if you must have a neutral discharge, or if you plan the neutralize the discharge with caustic anyways. On the other hand, if you can handle an acid discharge, then to minimize the caustic consumption let the pH drop as low as possible whilst maintaining the outlet SO2 concentration required.

 

[25362]

I suggest you pay a visit to thread127-56418.

 

[TISM]

I appreciate the reference but that thread relates to lime (CaO) based wet scrubbing systems where I am interested in an NaOH only system.
Thank you anyway

 

[25362]

OK.
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 @ 30^oC, 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 30^oC.

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.

 

[PeterAB]

pH control is really a function of how much caustic or lime you add, not a function of which alkali you choose. Keeping a low pH is more critical for lime than for caustic based scrubbers, because the slightest CO2 absorption can cause scaling (CaCO3 formation) in lime-based scrubbers. Since sodium carbonate/bicarbonate is more soluble, scaling is typically less of a problem with caustic-based scrubbers.

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.):
&quot;Sherwood and Pigford (&quot;Absorption and Extraction,&quot; 2nd ed. McGrawHill, NY 1952) indicate that the controlling step is a slow pseudo-first-order irreversible reaction between dissolved CO2 and OH-,...&quot;
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

http://www.turbosonic.com

 

[25362]

Even for fine atomization the absorption of SO₂ in one-stage contact spraying may be only around 70% efficient, when the molar ratios of alkali to SO₂ are up to 20% above stoichiometric, for flue gas entering at about 400^oF.