Page 238 - Gas Purification 5E

224 Gas PuriJication

do not occur. Instead, an oxidation passivator converts the iron on the surface to the trivalent
state, i.e., to a form of ferric oxide that is very &rent and protective. This oxide is called the
passive film. Such inhibitom work well in units removing carbon dioxide only, reducing the cor-
rosion rate practically to d, and they are the only effective inhibitom for MEA in the absence of
hydrogen sulfide (API, 1990). Conversely, they are destroyed by hydrogen sulfide.
While oxidizing passivators work extremely well when they are properly maintained, they
have several drawbacks. First, regular solution analyses are required. Second, the passivator
concentration must be maintained within specified limits. Third, they must be protected
against impurities which destroy them, including hydrogen sulfide and large amounts of iron
corrosion products, both soluble and insoluble; and fourth, when they fail, they often permit
local attack, or even aggravate it. Finally, many of them contain toxic heavy metals, which
makes disposal difficult and expensive.
Chloride Attack of Stainless Steel in Amine Service

Impurities such as chloride gradually build up in amine systems until a steady-state con-
centration is reached. Since most amine systems contain some stainless steel, it is of interest
to know what chloride levels can cause pitting of stainless steels in an amine environment.
Limited information is available in the literature. Experiments reported by Seubert and Wal-
lace (1985) indicate little or no pitting tendencies with 304 SS exposed to DGA solutions
containing up to 4,000 ppm chloride. Based on these experiments, the maximum acceptable
chloride level for DGA plants containing type 304 SS was set at 1,000 ppm.
Halides have also been found to contribute to crevice corrosion and stress corrosion crack-
ing in stainless steel heat exchanger plates in ldrich amine plant plate exchangers. The
corrosion occurred under ethylenepropylene-diene monomer (EPDM) rubber gaskets that
contained significant concentrations of chlorine and bromine left over from the curing
process. The problem was resolved by replacing the heat exchanger packs with 316 SS plates
and peroxide-cured EPDM gaskets with a maximum total halogen concentration of 200 ppm
(Hay et al., 1996).

Foaming of alkanolamine solutions is probably the most common operating problem in
amine treating units. It is most frequently encountered in the contactor, but may also occur in
the stripping column. Foaming may result in excessive amine losses, off-specification prod-
uct gas, and reduced operating rates, and can also be responsible for the production of off-
specification, dark sulfur if foam is carried over into the Claus sulfur plant. Lieberman
(1980), Smith (1979A, B), Bacon (1987), Ballard and von Phul (1991), Manning and
Thompson (1991), McCullough and Nielsen (1996), Thomason (1985), and BaUard (1966,
1986A, B) review contaminants that can cause amine solution foaming, and summarize plant
operating practices and troubleshooting techniques that minimize amine plant foaming prob-
lems. A summary of amine plant foaming causes, symptoms, and remedies follows.

Causes of Foaming

Foaming in an amine unit is caused by solution contaminants since uncontaminated alka-
nolamine solutions will not form a stable foam. Common solution contaminants known to
cause foaming are condensed hydrocarbons and acidic amine degradation products formed in