Page 226 - Gas Purification 5E

21 2 Gas Pur8cation

Heat Stable Salt Neutralization. Soda ash (or caustic soda) is often added to DEA and
MDEA solutions to neutralize heat-stable salts, and there is considerable plant evidence that
this is an effective means of reducing corrosion (Smith and Younger, 1972; Butwell et al.,
1982; Liu and Gregory, 1994; Bums and Gregory, 1995; Liu et al., 1995; Rooney et al.,
1996). Adding soda ash reduces amine solution corrosiveness by raising the solution pH.
Soda ash addition may also reduce corrosion by preventing the release of weaker acids such
as formic acid during amine regeneration (McCullough and Nielsen, 1996). Although soda
ash addition can reduce corrosion, the amount that can be added is limited because solids
will eventually be precipitated, plugging equipment and piping. However, solids precipita-
tion and equipment plugging can be avoided if soda ash addition is combined with amine
solution reclaiming using either batch distillation, ion exchange, or electrodialysis. Soda ash
addition is particularly attractive for secondary and tertiary amines like DEA and MDEA
since these amines cannot be reclaimed during normal operation. Therefore, for these
amines, soda ash addition can be used to control corrosion until a contract reclaimer arrives
at the plant site.
According to Scheinnan (1973A, B), soda ash should first be added to DEA solutions
when the heat stable salt concentration reaches 0.5 wt%. Nearly 20 wt% sodium salts can be
tolerated before any solids precipitate. Potassium carbonate can also be used to neutralize
heat stable salts and has the advantage of being about 25% more soluble by weight than sodi-
um compounds (Scheirman, 1973A, B).
For MDEA solutions, Liu and Gregory (1994) recommend that soda ash should be added
to keep the amine heat stable salts concentration below 2 wt%. The MDEA solution should
be reclaimed when the total heat stable salt anion content reaches 4 wt%. Rooney et al.
(1996) have also investigated caustic soda neutralization of MDEA solutions containing heat
stable amine salts. They recommend that soda ash addition be used to keep the heat stable
amine salts level below 0.5 wt%. In addition, they recommend that individual heat stable
amine salt anions be kept below the following maximum levels:

Heat-Stable Amine Salt Anion Maximum ppmw

oxalate 250
formate, glycolate, malonate, sulfite, or sulfate 500
acetate or succinate 1,000
thiosulfate 10.000

According to Rooney et al., the amount of soda ash that can be added is limited to a maxi-
mum of about 10% of the total MDEA concentration before solution viscosity and solids
precipitation problems occur.
Effect of Amine vpe

It is well known that the choice of amine affects corrosion (Dupart et al., 1993A, B). Pri-
mary amines like MEA and DGA are more corrosive than secondary amines like DEA and
DIPA. In turn, DIPA and DEA ace more corrosive than tertiary amines like MDEA. As noted
by DuPart et al. (1993A, B) several investigators have shown that all amines are equally
non-corrosive when no acid gas is present (MacNab and Treseder, 1971; Lang and Mason,
1958; Froning and Jones, 1958; Blanc et al., 1982A, B). Therefore, differences in corrosion