Page 211 - Gas Purification 5E

Mechanical Design and Operation of Alkanolamine Plants 197

Continuous Rejlux Putge Ammonia and cyanide build-up in the amine regenerator over-
head system can be reduced and corrosion controlled with a continuous purge of amine regerr
erator reflux water. See Figure 3-1. A reflux purge is effective if the cyanide and ammonia
concentrations in the sour gas are low (Neumaier and Schillmoller, 1955). If the cyanide con-
tent of the reflux water or upstream wash water is less than 100 ppmw, cyanideinduced blis-
tering (HIC) of carbon steel will be minimal (Neumaier and Schillmoller, 1955). To minimize
wet HzS corrosion problems (SSC, HIC, SOHIC), the cyanide content of the wash water
should be below 20 ppm (NACE, 1994B). On the basis of guidelines developed for hydropro-
cessing unit air coolers, carbon steel can be used in the regenerator overhead condenser and
downstream piping if the ammonium bisulfide concentration in the reflux water is less than
2-3 wt% and if exchanger tube and piping velocities are kept below 20 feet per second (Piehl,
1975). Inhibitor injection (ammonium polysulfide) may be required for the amine regenerator
overhead system if the cyanide concentration is too high and, in refineries, a standby inhibitor
injection system is often provided (Neumaier and Schillmoller, 1955). If a reflux purge is
used to control the NH3 and cyanide content of the reflux water, it may be desirable to avoid
the use of return bends in either air- or water-cooled overhead condensers (F’iehl, 1975).
The disadvantage of an amine regenerator reflux purge is that there may be very costly
amine losses because amine concentrations in the reflux water can be between 0.5 and 2.0
wt8 depending on amine entrainment, the type of amine, and the number of water wash
trays in the amine regenerator (Bucklin and Mackey, 1983). Also, the purge rate required to
reduce cyanide and ammonia concentrations below corrosive levels is difficult to determine
and control. Although amine losses can usually be reduced by installing a demister pad in the
amine regenerator, some losses will occur, and corrosive conditions may exist even with a
continuous reflux purge. For these reasons, when both ammonia and cyanide are present,
amine units are typically designed with corrosion resistant alloys as shown in Figure 3-1.
The amine regenerator is usually all 304L SS or is lined with type 304L integral cladding or
weld overlay from two trays below the feed tray up to and including the upper head. The
overhead piping is usually 304L SS, and titanium tubes are frequently used in the overhead
condenser. Ehmke (1981A, B) notes that aluminum has been used successfully for the over-
head condenser if pH, chloride ion content, impingement, and velocity are controlled. Car-
bon steel is frequently utilized for the reflux accumulator; however, a conservative corrosion
allowance is necessary and HIC-resistant material is sometimes used. The reflux pump cas-
ing and impeller should be 316 SS. All of the reflux piping is typically 304L SS.

Upstream Water or Caustic Wash. Caustic washing was recommended by Polderman
and Steele (1956) and used by Noms and Clegg (1947) for removal of formic and acetic
acids from gas streams entering amine treating units. Since both formic and acetic acids are
stronger acids than either H2S or C02, they will replace the sulfide (or carbonate) salt. How-
ever, as noted by Polderman and Steele (1956) and Ehmke (1981A, B), caustic washing can-
not remove HCN from gas streams because HCN is a weaker acid than H2S or C02 and will
be displaced from the caustic solution by either of these two acid gases.
Since HCN is more soluble in water than either H2S or C02, water wash has been used to
remove hydrogen cyanide from gas streams (Kelley and Poll, 1953; Neumaier and Schill-
moller, 1955; Polderman and Steele, 1956). Table 3-1 indicates that a single stage water wash
alone is relatively ineffective in removing ammonia and cyanide (Neumaier and Schillmoller,
1955; Ehmke, 1981A, B). This means that additional protective measures are required, includ-
ing a back-up inhibitor injection system (ammonium polysulfide) for the amine regenerator
overhead system and an amine regenerator reflux purge (Neumaier and Schillmoller, 1955).