Hydrocarbon Processing - March 2022 - 46

Process Optimization
Case Study results. With increases in catalysts activity, the
overall conversion of reaction increases, which utilizes more
H2
to convert sulfur to H2
S, thus reducing diesel outlet sulfur
concentration (TABLE 1). Commercially available hydrodesulfurization
catalysts are designed to achieve ULS diesel product.
However, catalyst activity must be monitored and assessed for a
cost-effective process and operation.
Relationship between H2
centration. H2
is added and recycled-with makeup H2
in downstream units, thus producing ULS diesel. If H2
purity and sulfur outlet con-as
one
of the feed streams to react with diesel in the reactor to convert
free sulfur present in diesel to H2
cycled, the process economics of the unit becomes infeasible.
H2
S, which will be separated
is not repartial
pressure in the reactor is directly related to the purity
of the H2 fed to the reactor as a feed stream (fresh H2
The lower partial pressure of H2
+ recycled).
leads to a decrease in reaction
purity in the feed stream
conversion and higher sulfur concentration in the product. Therefore,
it is important to ensure the high H2
and the removal of non-H2
components in the recycle stream.
fect of H2
centration in the DHDS product. With the change in H2
A case study was performed to determine and evaluate the efpurity
on the conversion of reaction and the sulfur conpurity,
other
factors also change and have a cumulative impact on sulfur
outlet concentration.
Case Study results. The outcomes of the case study indicate
the correlation between increasing sulfur outlet concentraTABLE
1. As catalyst activity increases, the overall conversion
of reaction increases, leading to a lower outlet sulfur
concentration in diesel
No.
1
2
3
Catalyst
activity
0.7
0.8
0.89
consumption
with respect
to feed, ft3
/bbl
364.1
403.5
436.5
H2
Overall
conversion,
wt%
9.9
11.3
12.7
TABLE 2. The case study results showed a correlation
between increasing sulfur outlet concentration in diesel
with a decrease in H2
purity
H2
No.
1
2
3
purity,
%
83.5
85
87.5
Average reactor
temperature,
°F
653
655
657.8
Purge gas
with respect
to feed,
ft3
/bbl
4.2
26.2
73.1
Overall
conversion,
wt%
11.1
11.6
12.53
Sulfur outlet
concentration
in diesel, ppm
25
17
9
ASAD ASHFAQ LODHI is a Senior Process Engineer with ZEL (an engineering
consultancy). With 10 yr of professional experience in process engineering, Mr. Lodhi
holds vast experience in FEED, detailed engineering and feasibility studies on various
oil and gas processing plants and refineries. He earned a chemical engineering
degree and MS degree in project management and is Nebosh IGC certified.
MOHIB SHAMSHAD is a Senior Process Engineer. He has 8 yr of experience in
process design engineering. He earned a BE degree in chemical engineering from
the University of Karachi, Pakistan, and an ME in chemicals from NED University of
Engineering and Technology, Karachi.
TABLE 3. Increasing the reaction temperature reduces sulfur content in product diesel
No.
1
2
3
Change in reactor inlet
temperature, °F
585
592
598
46 MARCH 2022 | HydrocarbonProcessing.com
Average temperature
of the reactor, °F
631.1
642.9
653.1
Deactivation rate,
°F/bbl/lb
0.9384
1.245
1.61
Catalyst life, d
184.6
116.4
74.75
Cumulative impact on sulfur outlet
concentration in diesel, ppm
98
30
9
Outlet sulfur
concentration
in diesel, ppm
95
30
9
tion in diesel with a decrease in H2 purity (TABLE 2). These results
confirm that high H2
maintained to achieve desired low-sulfur concentration in the
DHDS product.
Relationship between reactor temperature and sulfur
outlet concentration. Sulfur impurities present in diesel are
typically in the form of mercaptans, sulfides, disulfides, cyclosulfides
and thiophenes. In refinery operations, operators may
encounter significant variations in the types of sulfur impurities
present in diesel. Since high temperatures drive higher conversion
in the DHDS reactor, high-temperature operations are employed
to ensure on-spec diesel production even with varying
sulfur content in the feed stream.
Activation energy escalates with an increase in reaction temperature
due to an upsurge in the number of molecules involved
in the hydrogenation reaction. This phenomenon also involves
changes in the physical properties of diesel (i.e., an increase
of diffusivity and a decrease in viscosity and surface tension),
which ultimately promotes the H2
absorption rate in diesel and
catalyst pores to reach the active sites where the reaction occurs,
thus converting sulfur to H2
S.
A case study was performed to analyze the effect of reaction
temperature on catalysts (life and deactivation rate) and performance
of the DHDS unit with respect to the sulfur outlet concentration
of diesel.
Case Study results. The results indicated that increasing the reaction
temperature reduces sulfur content in product diesel; however,
optimum temperature selection is important to maintain an
economical balance between on-spec diesel production and catalyst
life and product yield (TABLE 3). Also, high-temperature operation
in the DHDS reaction should be monitored and controlled
closely, as high reaction temperatures can result in over-cracking
or catalyst coking, which reduces catalyst life and yield.
Takeaway. Important operating parameters such as reactor
temperature, catalyst activity and H2
purity have a significant impact
on outlet sulfur concentration. Understanding the impact of
these parameters is essential for selecting optimal parameters to
ensure desired product specifications and economic operation
of DHDS processes.
partial pressure and purity must be
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