H2Tech - Q1 2022 - 31

ADVANCES IN HYDROGEN TECHNOLOGY
Shift reactor. The primary function of the shift reactor is to
convert CO formed in the reformer to H2
by reacting it with
steam. The key parameters are inlet temperature, outlet temperature,
steam-to-dry gas ratio, pressure drop and temperature
profile. The KPIs include CO conversion, pressure drop
and WGS approach.
Plotting the temperature profile clarifies how the catalyst
activity declines with age. A clear trend can be obtained by
plotting the percent exotherm profile rather than the temperature
profile. The percent exotherm at any bed temperature
equals [(T - Tmin
) / (Tmax
- Tmin)] × 100. A sample illustration
of both bed temperature and exotherm percentage plots across
a high-temperature shift (HTS) reactor is shown in FIG 3. Profiles
1 and 2 are different profiles for the same feed at different
inlet temperatures. Not much can be inferred by plotting the
bed temperatures. However, when the profiles are plotted by
taking the exotherm percent at each position, it becomes clear
that, by increasing the inlet temperature, the reaction profile
becomes steeper.
Pressure drop. The shift reactor, either HTS or mediumtemperature
shift (MTS), being downstream of the WHB/
PGB, is vulnerable to fouling issues due to upstream boiler
water leaks. This causes pressure drop to increase due to the
buildup of boiler solids. The pressure drop trend should be
carefully monitored, especially after a trip incident. In addition,
any wetting incident can reduce the strength of catalyst
pellets, which could contribute to a high pressure drop. Any
inappropriate or inadequate hold-down layer on top can also
cause high pressure drop issues.
Mass balance across the shift reactor. Assume there is an
HTS reactor in the flowsheet and that mass balance will need
to be calculated.
Case 1. The inlet temperature is 320°C, and the outlet
temperature is 394°C. The outlet composition (dry basis) reported
by the laboratory was the following:
* H2
* CH4
* N2
= 74.86 %
= 4.16%
* CO = 3.38%
* CO2
= 1.23%.
TABLE 8 shows the inlet and outlet compositions after doing
C and O balances. At the end of the shift reactor, 3,038.25
kmol/hr of H2
is produced in Case 1. Using Eqs. 26 and 27,
the WGS approach can be calculated as:
Kp (WGS) = [([H2
][CO2])/([H2
Ln (Kp WGS) = 0.63508Z3
4.1778Z + 0.31688
- 0.29353Z2
= 10.2 and Teq (WGS)
= 403.2°C
O][CO])]
+
Eqs. 26 and 27 are used to calculate the following:
* Kp(WGS)
* WGS approach = Teq (WGS)
* CH4
= 71.96 %
= 2.46%
- T = 403 - 394 = 9.2°C.
Case 2: The naphtha case. In this case, the inlet temperature
is 320°C, and the outlet temperature is 396°C. The outlet composition
(dry basis) reported by the laboratory was the following:
* H2
* CO = 3.2%
* CO2
= 22.37%.
(26)
(27)
= 16.36%
SPECIAL FOCUS
TABLE 8. C and O balance for Case 1 (shift reactor)
Inlet, kmol/hr Outlet, kmol/hr Outlet wet mol fraction
H2
N2
CH4
CO2
CO
H2
O
2,672.3
49.83
168.68
298.97
502.35
1,809.71
3,038.25
49.92
168.84
663.98
137.18
1,444.85
0.552
0.009
0.031
0.121
0.025
0.263
TABLE 9. C and O balance for Case 2 (shift reactor)
Inlet, kmol/hr Outlet, kmol/hr Outlet wet mol fraction
H2
H2
O
CO2
CO
CH4
973.14
915.48
182.47
225.24
39.19
1,147.25
741.34
356.64
51.02
39.22
0.491
0.317
0.153
0.022
0.017
TABLE 9 shows the inlet and outlet compositions after doing C
and O balances. At the end of the shift reactor, 1,147.25 kmol/hr
of H2
proach are the following:
* Kp(WGS)
= 397.1°C
* WGS approach = Teq (WGS)
PSA section. Generally, H2
- T = 397.1 - 396 = 1.1°C.
recovery across the PSA is 85%-
90%. Assuming 87% H2 is recovered across the PSA, the H2
production in Case 1 would be 3,038.25 × 0.87 = 2,643.3
kmol/hr. In Case 2, it would be 1,147.25 × 0.87 = 998.1
kmol/hr. This needs to be cross-checked with the PSA purge
gas flow and the H2
content.
Takeaway. There is no denying that proper checks and balances
in the plant are crucial for ensuring maximum operational efficiency-and
that mass balance is one of those crucial checks.
The primary objective of this practice-oriented article is to ensure
that the process engineer understands the significance of
KPIs across each section of the plant and is confident in doing
calculations (e.g., approaches to equilibrium), using available
plant and lab information. By following the monitoring aspects
highlighted in this article, the process engineer will be in a better
position to make sound technical judgments when doing a
technical bid and/or routine plant evaluations.
LITERATURE CITED
1
2
Riazi, M. R., Characterization and Properties of Petroleum Fractions, ASTM
International, West Conshohocken, Pennsylvania, 2005.
Twigg, M. V., Catalyst Handbook, Second Edition, CRC Press, Boca Raton,
Florida, 1996
K. R. RAMAKUMAR is a Senior Technical Service Engineer
for Johnson Matthey Catalyst Technologies, and is based
in Dubai. He is a chartered chemical engineer with 16 yr
of experience in the downstream oil and gas industry.
Prior to joining Johnson Matthey in 2014, Mr. Ramakumar
worked in refineries in India and the UAE and was involved
in operations, technical services, and the commissioning
of hydrogen and hydroprocessing units.
H2Tech | Q1 2022 31
is produced in Case 2. The temperatures and WGS ap=
10.8 and Teq (WGS)
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