SO2 Wreaks Havoc in Amine Units

Lili Lyddon

Mar 21, 2013

If you are modeling an amine unit with SO2 in the feed, possibly from flue gas which contained sulfur compounds before incineration, you might notice that the Recycle block fails to converge. This is due to the build-up of non-regenerable salts in the amine loop. Once in the system the SO2 forms heat-stable salts with the amine and has no way to escape. The amine will eventually be rendered ineffective by being tied up as a salt.

From Kohl and Nielsen’s Gas Purification, 5th ed.:

“Sulfur dioxide is a relatively strong acid and reacts rapidly with amines to form sulfite. However, the sulfite does not generally remain in solution as a heat-stable salt because of its high reactivity. For example, SO2 in solution reacts rapidly with H2S by the Claus reaction (which is catalyzed by liquid water) to form elemental sulfur and polysulfides. Zero-valent sulfur can, in turn, disproportionate to form thiosulfate, which is a heat-stable anion. The sulfite may also be oxidized to sulfate which forms a heat-stable salt.”

In an amine unit simulation, you must remove the SO2 from the feed gas upstream of the absorber using some sort of pre-treating or a divider. If not, the SO2 will build up in the system and the recycle will never converge. In the actual plant you will want to pre-treat to avoid the byproducts discussed above. A caustic wash, salt water wash, or plain water wash are all possible pretreatment options. Although pretreatment is preferable to use of a reclaimer, this is another possible option for dealing with the SO2. You could model the “reclaimer” as a divider which removes the SO2 and twice that number of moles of amine to approximate the amount of amine which would be lost in the system by reaction with the SO2. Obviously this is not a very efficient solution since relatively high amine makeup would be required, not to mention disposal of the reclaimer residue.

The salt formation reactions are similar for all the amines. Physical solvents such as DEPG do not react with SO2 at all, however. The SO2 is much more soluble in DEPG than H2S or CO2, and the DEPG can be thermally regenerated.

A few ppm of SO2 in the feed doesn’t seem like much, but it doesn’t take much to cause problems in an amine unit