Hydrocarbon Engineering - August 2019

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August 2019 HYDROCARBON
ENGINEERING 32
bicine, the solvent had to be regularly reclaimed with an
ion exchange reclaiming unit. While HSS and bicine are
removed by ion-exchange, degradation products such as
DEA and MMEA are not removed by this method. DEA
and MMEA are known to be non-selective amines,
absorbing both H2S and CO2. After years of reclaiming,
significant amounts of these amines (5 wt% DEA and
2 wt% MMEA) have accumulated to the point of causing
a decline in CO2 slip, resulting in an increase in energy
usage. The unit had to operate the reboiler at close to
the maximum steam rate in order to meet the treating
objectives.
A study was then undertaken to determine the impact
of switching from MDEA. FLEXSORB solution does not
degrade to form DEA and MMEA, and therefore H2S
selectivity performance would not decline over time.
Table 2 shows the results of the study and the capacity
that can be regained with a changeout. Other
improvements include reduced steam consumption,
reduced circulation rate, and lower levels of H2S in the tail
gas to the incinerator.
Case 2
In contrast to Case 1, a similar sized TGTU operated with
FLEXSORB SE for eight years. Analysis of the solvent
sample showed little heat stable salts accumulation over
time, and no accumulation of DEA and MMEA overtime.
The solvent in this TGTU was never replaced and required
no reclaiming during this eight year period. Solvent
makeup rate was low. Table 3 compares the performance
of the unit shortly after start-up in 2010 with performance
eight years later.
Case 3
A US Gulf Coast refinery TGTU had been operating with
MDEA solvent since initial start-up. The sulfur capacity was
limited by SO2 emissions from the thermal oxidiser during
the summer months. This limitation required the renting of
chillers when using MDEA to lower the level
of H 2S in the treated gas going to the
incinerator. The unit also had corrosion
problems in the reboiler shell and the
absorber tower, resulting in frequent
re-tubing of the reboiler and a replacement
of the tower shell. A study was undertaken
to quantify the incentives for a solvent
changeover with the goal of both reducing
operating costs and increasing reliability.
The study determined that a solvent
changeover would be justified based on the
increased H2S selectivity (or CO2 slip) of
SE Plus over MDEA. Improved selectivity
results in less CO2 recycled back to the front
of the Claus SRU thereby unloading the SRU,
creating additional capacity. The new solvent
would also achieve a lower H2S level in the
treated gas at the same lean amine
temperature as the MDEA. The changeover
resulted in energy savings due to the lower
circulation rate. Additional savings were
realised by eliminating the need for chillers during the
summer months, and lowered maintenance costs on heat
exchanger replacements.
Conclusions
MDEA and hindered amines such as FLEXSORB SE/SE Plus
are used in the TGTUs to improve the SRE and to lower
SO2 emissions. Case studies show that hindered amines
provide greater capacity and lower energy usage than
MDEA. Over time, MDEA degraded to form other
components that negatively affected its performance,
increasing costs as reclaiming services were required. In
contrast, hindered amine exhibited good stability over
time, which sustained its long-term performance:
„ Lower solvent circulation rate resulting in lower energy
costs.
„ Smaller unit equipment sizes.
„ ‘Drop-in’ TGTU debottlenecks possible with a solvent
changeover to hindered amines.
„ Lower life cycle costs.
„ Experience with large SRU/TGTU train sizes of over
1000 tpd.
The increased operational flexibility and reliability of
FLEXSORB SE over MDEA offsets the lower solvent
replacement costs for MDEA.
References
1. CHOW, T., D’HAENE, P., FLOWERS, J., HALON, G., KIMTANAS, C.,
NASASTO, E., SCHENDEL, R., and WONG, V., ‘Fundamentals of
Sulfur Recovery’, Laurance Reid Gas Conditioning Conference,
Norman, Oklahoma, US, 2011, (edited in 2015 by KELLER, A.,
GRIGSON, S., JENSEN, D., LARUE, K., and OYEDEJI, A.).
2. ‘Environmental, Health, and Safety Guidelines For Petroleum
Refining’, International Finance Corp. – World Bank Group,
IFC_External_Corporate_Site/Sustainability-At-IFC/Policies-
Standards/EHS-Guidelines
3. ‘Environmental, Health, and Safety Guidelines For Natural Gas
Processing’, International Finance Corp. – World Bank Group,
IFC_External_Corporate_Site/Sustainability-At-IFC/Policies-
Standards/EHS-Guidelines
Table 5. Comparison of plant performance for solvent
changeover
Solvent MDEA FLEXSORB SE Plus
Acid gas feed rate to SRU X 138% of X
Circulation rate X 60% of X
H2S leak rate (vppm) 358 19
CO2 slip (% of feed) 50 87
H2S in acid gas recycle (mol%) 23 71
Recycle gas rate to SRU X 71% of X
Lean amine temperature (˚C) 34.3 36
Table 4. Comparison of energy savings for solvent changeover
Solvent MDEA FLEXSORB SE Plus
Circulation rate X 60% of X
Chiller rental in summer Yes No
Steam rate X 79% of X
Steam costs X 79% of X