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-4lkanolaniines for Hydrogen Surfide and Carbon Dioxide Removal 173

(equivalent to 2,500 gaY(day)(ft2) used to set the settler dimensions. Separators downstream
of contactors, using either random packing or sieve trays, can be smaller than those follow-
ing mixers or eductors because the degree of agitation and, therefore, the formation of fine,
entrainable droplets is less in the packed or tray columns. Stewart and Laming (1991,
1994A, Bj recommend LPG residence times of 15 minutes for a gravity settler downstream
of a pzcked bed LPG treater.

Amine Filters
Figure 2-96 shows an optional amine filter on the lean amine solution flowing to the
LPG/amine contactor. Although many LPG maters do not have dedicated mechanical or
activated carbon filters, their use should be considered. Bacon (1972) recommends full flow
mechanical and activated carbon lean amine filters for LPG treaters. Russell (1980) reports
particulate plugging of the random packing beds used in LPG treaters and recommends full
flow amine filtration to minimize plugging. DuPart and Marchant (1989) also recommend
both full flow mechanical and activated carbon filtration of the amine flowing to LPG
treaters. As noted by both DuPart and Marchant (1989) and Strigle (1994), failure to remove
impuritie.s can reduce the capacity of the LPG treating system.
Removal of COS from LPG by Amines

A common specification for LPG is the Copper Strip corrosion test. This test indicates the
level of elemental sulfur and H2S by the degree of darkening of a strip of pure copper. A
#1A Copper Strip test result corresponds to 1-2 ppmw or less H2S in the LPG. COS in dry
LPG is not detectable by the Copper Strip test (Bacon, 1972; Perry, 1977A, Bj. However, if
the LPG comes in contact with water, e.g., during shipment in a tank car containing water,
the COS hydrolyzes to H2S and C02 and the LPG can fail the Copper Strip test upon retest-
ing (Holmes et al., 1984: Perry, 1977A, B). The COS hydrolysis reaction is
COS + H20 = H2S + CO? (2-49)

Because of this possible reaction, COS removal during amine LPG treating is desired.
DGA, MEA, and possibly DIPA can react with COS during the amine treating operation so
that a #lA Copper Strip test result can be achieved even if the LPG is wet. DGA is reported
to reduce COS levels from 4,500 ppmw to less than 2 ppmw (McClure and Morrow., 1979A1
B, Cj. The DGA reacts with COS to form a degradation product that can be converted back
to DGA by thermal reclaiming. MEA reacts hversibly with COS and, when MEA is used
for LPG treating, the treated LPG can pass the #1A Copper Strip test. However, MEA is not
preferred for simultaneous C02, H2S, and COS removal from LPG because of the amine loss
caused by the irreversible reaction with COS (McClure and Morrow, 1979C). DIPA is also
reported to be capable of removing COS to several ppmw (S.I.P.M., 1979). Mick (1976)
reports that DEA can only remove COS to 90-135 ppmw. Bacon and Pearce (1985) report
that MDEA can remove only 33%. of the COS in LPG and cannot remove COS to levels low
enough to pass the #1A Copper Strip test.
Although neither MDEA nor DEA can remove COS to levels low enough to pass the Cop-
per Strip test, use of a formulated caustic wash downstream of the amine/LPG treater is
reported to remove COS to levels significantly less than 1 ppmw (Bacon and Pearce, 19853.
The composition of the formulated caustic may be based on technology outlined by Johnson