Page 227 - Gas Purification 5E

Mechanical Design and Operation of Alkanolamine Plants 21 3

cannot be due to the amine alone. Several explanations have been offered to account for the
effect of amine type on corrosion. For example, MDEA differs from MEA, DEA, DJPA, and
DGA in that it does not form amine-C02 degradation products. However, investigations by
Polderman et al. (1955A, B) for MEA and by Chakma and Meisen (1986) for DEA suggest
that although amine-C02 degradation products contribute to corrosion, they are not the pri-
mary cause. As noted by DuPart et al. (1993% B), it is possible that the ability of primary
and secondary amines to form carbamates according to reaction 3-18 may account for the
differences in corrosion. Perhaps the more basic amines such as MEA are more corrosive
due to the presence of undecomposed salts such as allcanolammonium carbamate and the
resultant concentration of allcanolammonium ions in the hottest sections of the amine regen-
erator (see equation 3-19). Also, less basic amines such as DEA are easier to strip than
MEA, while MDEA is very easily stripped. Consequently, DEA and MDEA alkanolammo-
nium ion concentrations are low in the bottom section of the amine regenerator where the
amine solution is the hottest. Therefore, these less basic amines are less corrosive than pri-
mary amines such as MEA and DGA.
Erosion-Corrosion

Erosion-corrosion is caused by high amine solution velocities, solution turbulence, and
impingement of gas and amine on metal surfaces. Erosion-corrosion removes the protective
iron sulfide, carbonate, or oxide film protecting the piping and equipment from corrosion.
Areas that are subject to erosion-corrosion include the piping from the amine contactor pres-
sure let-down valve to the rich amine flash drum, the piping from the rich amine flash drum
level control valve to the amine regenerator, and the lean amine pump (see Figure 3-1).
Other areas that are affected include heat exchanger tubes near the inlet nozzle, the amine
contactor near the sour gas inlet, and the amine regenerator near the rich amine inlet. Ero-
sion-corrosion is aggravated by dirty amine solutions containing suspended solids. Erosion-
corrosion can be reduced by choosing the correct materials of construction and mechanical
design details which minimize impingement, reduce turbulence, and lower amine solution
velocities. Amine solution mechanical filtration to remove suspended solids also reduces
erosion-corrosion. Several authors recommend that suspended solids levels be kept below
0.01 wt% (Liebennan, 1980; Hall and Polderman, 1960). Mechanical filtration of amine
solutions is reviewed in more detail later in this chapter.
Erosion-Corrosion of Piping

API 945 recommends designing both lean and rich amine carbon steel piping for veloci-
ties less than 1.8 dsec (6 Wsec) (MI, 1990). Table 3-5 summarizes piping velocity recom-
mendations from several sources. As noted, recommended velocities range from 0.9 to 1.8
dsec (3 to 6 ft per sec). Although there is no published research to support these recommen-
dations, it is thought that API 945 represents good practice because it is an industry consen-
sus document. Sheilan and Smith (1984) and Dingman et al. (1966) recommend the use of
seamless pipe and long-radius elbows to reduce amine piping erosion-corrosion. Sheilan and
Smith also suggest that threaded connections or socket weld fittings be avoided.
Erosion-Corrosion of Heat Exchangers and Reboilers

Ballard (1966) and Dingman et al. (1966) recommend the use of multiple inlets and out-
lets to reduce corrosion of kettle and horizontal thermosyphon reboilers. See Figure 3-12.