SODIUM BASED SULFUR DIOXIDE REMOVAL
PROCESS DESCRIPTION: SULFUR DIOXIDE ABSORPTION VIA CAUSTIC USAGE
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- Reaction #2: SO2 + Na2SO3 +H2O → 2NaHSO3 (Bisulfite)
The most efficient method of removing sulfur dioxide gas from a main body such as flue gas is by direct contact of the gas with a liquid stream containing an alkali. This contact is achieved over some type of surface such as baffles, trays, spray droplets or media known as “packing”. Furthermore, “countercurrent” contact is the most efficient of the many types of contacts available. This technique is achieved by a rising gas meeting a falling liquid. Contact where a horizontally moving gas meets a falling liquid is not true countercurrent contact and is not as efficient for a given volume of contact.
The equipment used for this is classified as “absorbers”, “absorber towers”, or “scrubbers”. The overall reaction of Sulfur Dioxide with Caustic Soda is
- Reaction #1: SO2 + 2NaOH → Na2SO3 (Sulfite) + H2O
This cannot be done solely via this reaction, rather this reaction must be split in two as:
and
- Reaction #3: NaHSO3 + NaOH → Na2SO3 (Sulfite) + H2O
These two reactions, if combined, will yield reaction #1.
In order to carry out high SO2 removals, reaction #2 is the only reaction that can occur in the absorber. This occurs at much more moderate alkalinity levels than the overall reaction and this is necessary or else the caustic will preferentially react with carbon dioxide in the gas. This reaction will diminish the removal of sulfur dioxide.
The reaction #3 is a regeneration reaction and usually takes place in a tank away from contact with the gas. That is, the bisulfite is regenerated back to sulfite which serves both as the byproduct and the absorbent.
A typical absorber system consists of both the gas/liquid contact and the regeneration tank. The alkali is added to the tank on e.g. automatic pH control such that appropriate alkalinity is maintained in this vessel. The resultant sulfite solution is pumped from this tank back to the absorber stage where it reacts with more sulfur dioxide to produce bisulfite. A small predetermined blowdown stream is taken away from the recycle line so that sulfite is removed. This stream can be removed automatically also by controlling its density.
In cases where very high initial sulfur dioxide concentrations are present, it may become necessary to have more than one stage of absorption/regeneration. These are usually employed in a countercurrent method where the caustic is added to the last stage the gas sees so that the strongest alkali contacts the weakest SO2 loadings. Blowdown solutions from intermediary stages are not removed from the system, but rather are fed to the immediate upstream stage in a cascading technique. This is achieved by pH control. Only the blowdown from the initial stage is eliminated from the system. Although, in theory, there can be a multiple of cascading stages, the inaccuracy of pH control makes this possibility unjustified.