Hydrocarbon Processing - April 2022 - 18

Maintenance and Reliability
to 0.65. Downcomer flood ranged from
42%-64% for the same range of system
factors. The trays were not believed to
be flooding. Dry tray pressure drop was
greater than 1 in. H2
O, which is above
typical weeping limits.
The stripping trays have positive downcomer
clearance and were running slightly
above the weir loading where unsealing
has been observed. The trays were not believed
to be operating in the spray regime.
Activated carbon beds and iron sulfide
removal. Amine samples taken during
a foaming event were sent to the Phillips
66 lab in Bartlesville, Oklahoma. The
samples were dark green to blue in color
(FIG. 6), and the refinery wanted to determine
whether the foaming was related to
either: 1) reformer hydrogen used for the
tail gas catalyst that could have aromatics
in it, or 2) the presence of polysulfides
that amine vendors have stated could be
formed from the reaction of sulfur dioxide
(SO2
) and tail-gas amine. The amine
turned this color again, even though its
color had previously been markedly improved
by using the unit's rich amine particulate
filters. The Phillips 66 Bartlesville
lab showed conclusively that the emerald
green color was due to the presence
of extremely small colloidal iron sulfide
particles. After acidifying the amine, the
iron sulfide dissolved, and the color disappeared
from the solution (FIG. 6).
The Bartlesville lab passed the amine
solution through small 0.45-micron filters,
a smaller-than-practical field filter
element. The 0.45-micron filters were ineffective
at removing the color-producing
particles. The lab then tested activated
carbon as a filter media, and it was found
to be effective (FIG. 7). The lab ran foam
tests, using air as the process gas. The refinery
amine showed a higher foam column
before passing through activated carbon,
rather than after passing through it
(FIG. 8). Both color and foaming propensity
concerns were mitigated after passing
the amine through activated carbon. The
authors believe that the foaming propensity
was reduced because the activated carbon
was removing organic compounds,
such as surfactants that contribute to
foaming. Further, the authors believe that
the activated carbon removed the colloidal
iron sulfide particles, thereby reducing
the stability of any foam that was formed.
To make optimum use of the carbon
bed as an iron sulfide filtration device,
the carbon vendor was asked if there were
any limits on amine circulation through
the carbon bed. The recommended practice
per the vendor was 2 gpm/ft2
gpm/ft2
to 4
for amine flow through the carbon
bed, with a minimum contact time
of 15 min. Based on that feedback, amine
flow through the carbon bed was limited
to about 28 gpm, which corresponded
to a contact time of about 24 min. Flowrates
higher than 28 gpm would reduce
the efficiency of the carbon bed for iron
sulfide removal.
Root cause of
foaming. A graphical
FIG. 6. Samples of amine taken during a foaming event. The samples were dark green to blue in
color (left). After acidifying the amine, the iron sulfide dissolved, and the color disappeared from
the solution (right).
representation of the results of a comprehensive
two-dimensional gas chromatography-mass
spectrometry analysis of the
refinery tail gas amine is shown in FIG. 9.
The results indicated the presence of thianes,
which are sometimes called " organic
polysulfides, " and the presences of fatty
acids, including palmitic and stearic acids.
The fatty acids are foam-producing agents
commonly used in commercial soaps. Fatty
acids react with a base, such as caustic or
an amine forming a fatty acid salt, which,
by definition, is a soap or surfactant.
Therefore, the presence of fatty acids in an
amine system is expected to cause foaming.
Most likely, these fatty acids came
from machining or lubricating oils used in
metal-fabrication processes. The authors
believe that these fatty acids are the root
cause of the foaming and that thiane compounds
are not surface-active agents and
are not responsible for the foaming. No
inorganic polysulfides were found.
Phillips 66 checked with the vendor
that preactivated the catalyst with a proprietary
processa
. This process uses H2
S, hydrogen
and heat in a vapor phase to sulfide
the catalyst. It does not have any polysulfides
in the process, per vendor feedback.
At this time, the authors do not know
where the thianes originated. A hypothesis
is that the amine samples were shipped
to the Bartlesville lab without a nitrogen
blanket. Oxygen in the air mixed with H2
S
in the amine due to high lean loading may
have resulted in thiane production; however,
this hypothesis has not been verified.
FIG. 7. Removal of amine color with activated carbon.
18 APRIL 2022 | HydrocarbonProcessing.com
Antifoam. Based on laboratory tests,
a vendor-supplied polyglycol antifoam

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Hydrocarbon Processing - April 2022

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