Page 239 - Gas Purification 5E

Mechanical Design and Operation of Ahnolamine Plants 225

the amine circuit. Feed gas sources that can contribute to or cause foaming include coalesced
lubricating oil aerosols and surface active agents (e.g., water-soluble pipeline corrosion
inhibitors and many well treatment compounds). Materials introduced into the amine circuit
during operation and maintenance (such as excessive antifoam agents, phosphoric acid from
acid-washed activated carbon, chemicals and contaminants in make-up water, and chemicals
used in the manufacture of cotton filter elements) can also cause foaming (Pauley et al., 1988,
1989A, B; Pauley, 1991; Bacon, 1987; Ball and Veldman, 1991). Suspended finely-divided
solids, e.g., iron sulfide or pipeline mill scale, do not cause foaming, but rather tend to stabilize
foam and further aggravate any tendency to foam (Pauley, 1991). Pauley et al. (1988, 1989A,
1989B) present experimental data showing that liquid hydrocarbons, low and high molecular
weight organic acids, and acidic amine degradation products increase both the amine solution
foaming tendency and foam stability. Specific causes of foaming include the following:

1. Water-soluble surfactants in the feed gas (e.g., well treating compounds, pipeline corro-
sion inhibitors) which lower the amine solution surface tension. Excessive antifoam can
also cause foaming.
2. Liquid hydrocarbons (e.g., entrained compressor lubricating oil in the feed gas or hydro-
carbon condensation within the amine absorber).
3. Particulate contaminants (e.g., mill scale, FeS corrosion products, rust) contained in the
feed gas or produced within the amine treating unit. Solids such as FeS do not cause foam-
ing, but concentrate at the liquidgas interface and stabilize the foam by increasing the sur-
face viscosity and thereby retarding film drainage (Pauley, 1991).
4. Oxygen contamination of the feed gas or amine unit (usually at the amine sump or amine
storage tank) and reaction of the amine with oxygen to form carboxylic acids and amine
heat stable salts. Dissolved iron can catalyze the reaction of amine with oxygen to form
carboxylic acids (Pauley, 1991).
5. Feed gas contaminants, such as carboxylic acids, which react with the amine to form heat
stable salts.
6. Activated carbon that has been washed with phosphoric acid or that naturally contains
phosphorus in the form of leachable phosphates (Pauley, 1991; Bourke and Mazzoni,
1989).
7. Contamination of the amine unit with greases and oils during a turnaround.
8. Amine filter elements that have been washed with surfactants or contaminated with oils
during manufacture (e.g., string wound cartridge filters made with virgin cotton, but con-
taining cottonseed fragments).
9. Contaminants in the amine plant make-up water such as boiler feed water treating chemi-
cals and corrosion inhibitors (Smith, 1979A, B; Ballard, 1986A, B).

Symptoms and Characteristics of Foaming Systems

An amine plant subject to foaming will exhibit various characteristics including the fol-
lowing:

1. High amine losses and amine carry-over into downstream units
2. Reduced acid gas removal efficiency and failure. to meet product gas specifications
3. High or erratic differential pressure measurements across the absorber andor the stripper
4. An amine solution that is opaque and contaminated with suspended solids
5. More than 10% of the amine in the form of heat-stable salts