I am working on the design of a SO2 scrubber using NaOH to form Na2SO3 and I was wondering between fluidized bed, packed bed, spray tower and Venturi, which one would be the most efficient and cost-friendly for a inlet exhaust gas with a flow rate of around 3000kg/hr at a temperature of around 50 oC entering at atmospheric pressure? The stream needs to be treated from 0.05mg/m3 to 0.005mg/m3 and I was also wondering if anyone has previous experience working on it, what kind of packing material should be used in case of fluidized bed and packed bed? Thank you in advance!
As sulphur dioxide is a major atmospheric pollutant generating acid deposits, its abatement in flue gases is of significant industrial concern. Combustion of fossil fuels in power plants, incinerators, boilers, roasting of sulphide ore in metallurgy and sulphuric acid industry are the major sources of sulphur oxides. Large part of sulfur oxides is also added in air due to volcanic eruption. Cause of global environmental problems such as air pollution and acid rain.
Due to presence of moisture in atmospheric air these oxides react with moisture and forms sulforic acid the oxides react in the atmosphere forming sulphuric acid, which leads to acid rain. Because of the lowering of the permissible emission limits, there has been a rising interest in new treatment processes for absorbing gaseous SO2, using various reactants
The removal of SO2 from gas mixture with air and SO2 by ammonium bicarbonate aqueous solution , Calcium Oxide & calcium Sulphate as absorbent are used in a bubble column reactor. The effects of the concentration of ammonium bicarbonate, the SO2 inlet concentration of gas phase and the gas flow rate on the removal rate of SO2. The results showed that the higher the SO2 inlet concentration and the gas flow rate, the shorter the lasting time of SO2 completely removed in gas outlet and then the faster the decrease in the removal rate of SO2.
Article Mass transfer of SO2 absorption with an instantaneous chemic...
Usually a packed bed made of fiberglass rated for 5% NaOH, H2SO3, and Na2SO3 is used, but in your case of fairly high flow rate, a spray tower followed by a centrifugal separator may be more economical since you are only looking for 90% removal, and the stream appears to be mostly noncondensable.
Replacement of scrubbing fluid is the main operating cost, and the main quality control method. Temperature control is also important.
For your case, I feel, two stage packed bed scrubber will work. It will help in optimizing caustic consumption with maintaining environmental requirements. If you have spent caustic then the same can be used.
Are you sure that SO2 concentrations in raw flue gases are about 0.05 mg/m3? Such low SO2 concentrations may appear in more polluted ambient air as 1-hour or even 24-hour averages. Maybe it could be 0.05 g/m3 (50 mg/m3)?
90% removal efficiency of SO2 can be achieved in a well-designed one or two-stage packed column (filled eg. Pall rings) supplied with 30% NaOH aqueous solution, which is used in certain hazardous waste incinerators for the simultaneous removal of various acidic gases (HCl, HF, HCN etc.).
Scrubbers most often fail from excess recirculation of the scrubbing fluid of increasing concentration and failure to control the temperature. Too hot or too cold doesn't work well. For your flow rates the diameter of a packed tower will be large but not impossible. For packing the designer will have to trade off a larger size Pall ring against a smaller liquid rate.
You didn't mention dew point or water content. If the dew point is near the operating temperature, and the water vapor content is sufficiently high, you could economize with a barometric condenser by contacting the gas with substantially cooler water and caustic solution to effect a condensation of the SO2 with part of the water vapor.
Notice that noncondensable components greatly decrease scrubbing efficiency and lead to larger scrubbers.
The inlet concentration and outlet concentration are very low, so you must have a very good contact between gas and liquid. So you must exclude spray towers and ejectors.
Packed bed are good, but it should be very high. I suggest a packed bed with solution recycling and on the top some chimney trays with the concentrated solution.
NaOH , as said above by Mr. Decker, should be 5% in the fluidized bed, because if you have CO2 inside the gas you will produce solid carbonates. If you don't have CO2 you ca increase the concentration
I don't advice fluidized bed for low concentrations.
A spray tower would be the best for your application. It should be packed with telleretes for best effect but I think saddle type packing material would also be adequate. You may want to rethink your neutralizing agent though, NaOH will get you very good neutralization rates but I might kill your operating cost. Ca(OH)2 and Mg(OH)2 would also work at lower operating cost if you can keep their slurries suspended.
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