Hydrocarbon Processing - August 2021 - GP-28

ENGINEERING AND DESIGN
Based on the actual performance
test and a desktop study of
the existing SWS train, it was
recommended that a new, smaller
SWS train should be provided with
all tie-in points to the existing
SWS. This would allow operational
flexibility during normal operations,
as well as during maintenance,
turnaround and inspection.
10,000
12,000
8,000
6,000
4,000
2,000
40
50
60
70
80
90
100
Steam injection rate
110
120
Sour water feedrate, gal/min
H2S in treated water
FIG. 5. H2S in treated water vs. steam injection rate for Case 1.
10,000
12,000
8,000
6,000
4,000
2,000
40
50
60
70
80
90
100
Steam injection rate
110
120
Sour water feedrate, gal/min
H2S in treated water
FIG. 6. H2S in treated water vs. steam injection rate for Case 2.
10,000
12,000
14,000
8,000
6,000
4,000
2,000
40
50
60
70
80
90
100
Steam injection rate
110
120
Sour water feedrate, gal/min
H2S in treated water
FIG. 7. H2S in treated water vs. steam injection rate for Case 3.
28 JULY/AUGUST 2021 | GasProcessingNews.com
130
140
150
160
0.02
0.04
0.06
0.08
0.1
0.12
Actual field test. The actual performance tests of the SWS
unit were conducted multiple times to confirm the simulation
findings.
Field Test 1 showed off-specification treated water due to a
significantly lower steam supply to the unit of less than 12 lb/hr
130
140
150
160
0.1
0.2
0.3
0.4
0.5
0.6
0.7
130
140
150
160
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
set to a temperature approach of 20°F to minimize the effect
of temperature differences across the stripping section.
TABLE 1 shows that a higher pressure has a negative effect on
water quality with respect to ammonia, and can cause a significant
increase in residual H2
under these conditions.
Simulation cases. Case 1 examined SWS column efficiency
at 100% and the steam rate as per the unit process flow diagram
(PFD). FIG. 2 showed the simplest configuration of the existing
SWS at the gas plant. The unit is equipped with a stripper
column containing 2-in. flexi rings (metal packing) with
a 10-ft-high packing bed using six trays. Sour water was fed to
the column from the top, and live steam saturated at 60 psig entered
below the bottom of the packing. The column was sized
for 85% of jet and downcomer flood in all cases.
Case 1 was performed under the assumption that the system
efficiency was at 100% (used all column trays to match
the efficiency) and that all system components (including the
vessel, pumps, filters, exchanger and column internals) were
working as per the original design with maximum efficiency.
Based on the simulation results, FIG. 5 shows the trend lines
S content in treated water, as well as the steam injection
for H2
rate to the SWS, while maintaining the column temperature
and pressure conditions as per the process flow diagram data.
The simulation results for Case 1 show that the existing SWS
unit is able to meet H2
S in the treated water specification at a
maximum efficiency of column internals.
Case 2 examined SWS column efficiency at 85% and a
steam rate as per the unit PFD. Case 2 was performed with
the assumption that the SWS column internal efficiency was
at 85% (with a smaller number of trays to match the 85% column
efficiency) due to solids buildup and fouling issues in the
system, while other unit operating conditions remained as per
the unit PFD. Under these conditions, multiple simulation
runs were carried out to check the unit performance, which
shows that the system is still able to meet the treated water
specification (FIG. 6). This indicates that the system was originally
designed with sufficient margin.
Case 3 examined SWS column efficiency at 85% and a
steam rate at 15% higher than that specified on the unit PFD.
Another sensitivity simulation was conducted at a column
efficiency of 85% and a steam/water ratio increase of 15%
over the design ratio, while the operating pressure increased
to 40 psig from 31 psig. This simulation run was conducted
to match the column actual operating conditions and cover
any uncertainties in the simulation model. With these conditions,
the column top operating temperature was increased to
275°F compared to 229°F (the PFD value), which matched
the actual field test. Note: This high operating temperature is
still below the maximum design temperature of 400°F. Under
these conditions, the system is still able to meet the treated
water specification (FIG. 7).
S content of the stripped water
Steam injection rate, lb/hr
Steam injection rate, lb/hr
Steam injection rate, lb/hr
H2S in treated water, ppmw
H2S in treated water, ppmw
H2S in treated water, ppmw
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Hydrocarbon Processing - August 2021

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