Hydrocarbon Processing - March 2022 - 45

Process
Optimization
A. A. LODHI, Zishan Engineers Ltd.,
Karachi, Pakistan; and M. SHAMSHAD,
Energy Enterprise Associates, Karachi, Pakistan
Choosing controlled and optimal parameters
for the DHDS unit
Environmental considerations and stringent government
regulations drive incessant interest and efforts to decrease diesel
sulfur content to ultra-low levels. New deep hydrodesulfurization
technologies configured to meet diesel product sulfur specifications
of less than 10 ppm continue to attract the attention of
refiners. The medium-pressure ultra-low-sulfur (ULS) diesel hydtrotreating
unit converts sulfur to hydrogen sulfide (H2
presence of hydrogen (H2
S) in the
) and catalysts, with the H2
S removed
at a later stage. Significant effort is being made to optimize the
process and operations to comply with Euro-5 quality specifications
for diesel sulfur content and maintain an economic balance.
The improvement in the process requires an understanding
of various factors that control outlet sulfur concentration, so that
refiners and technology providers can design and operate the hydrodesulfurization
unit as a controlled operation to achieve the
desired results. This article examines the relationship of catalyst
activity and reactor operating factors with conversion rate and
sulfur concentration in outlet diesel. Process simulations were
performed to analyze how each of the considered variables impact
the outlet sulfur content. The results show that an increase
in each of the contributing factors (i.e., reaction temperature, H2
purity and catalyst activity) improved the conversion rate of reaction,
thus a decline in the outlet sulfur concentration in diesel was
achieved. However, optimal parameters must be set to prolong
catalyst life and maintain excellent and cost-effective outcomes.
Process scheme. The diesel hydrodesulfurization (DHDS)
unit is designed to hydrotreat straight-run diesel from the crude
distillation unit and diesel from bottom upgradation units such
as the thermal cracker or visbreaker to achieve a target sulfur
specification of 10 wppm.
The H2
former (if available in the refinery) and a separate H2
requirement of the DHDS unit is fulfilled from the replant.
The
feed is pumped to a required reaction pressure of 53 barg, combined
with the H2
stream, and introduced to a reactor after being
preheated by heat exchangers and furnaces to a temperature of
318°C. The reaction is facilitated using a high-activity cobaltmolybdenum
(CoMo) catalyst in a fix-bed reactor, where conversion
of sulfur to H2
S takes place. The reaction is a quenched
-rich
reaction as the reaction products pass from the heat exchanger
and cooler to a cold separator at reduced pressure. The H2
Case Study feed data. A simulation-based case study was
performed to present an analysis of sulfur outlet concentration,
with respect to different parameters and recommendations
made against each scenario to achieve optimum results. The following
feed data was considered and used in the simulation for
evaluation: a sulfur concentration in the diesel feed of 1 wt%; a
feed flowrate of 32,920 bpd; an operating pressure of 53 barg;
and the use of a CoMo catalyst.
Relationship between catalyst activity and sulfur outlet
concentration. Catalyst activity is a function of chemisorption
of reactants on the catalyst surface, which also affects the rate of
conversion of reaction. Coking or carbon deposition, sintering
and poisoning of catalysts all refer to physical or chemical deactivation
of catalyst active sites, resulting in a decrease in catalyst
activity and impacting catalyst performance. As a result, the sulfur
content in outlet diesel is high and target specifications are
not met. Sometimes, the molecules acting as a catalyst poison
get chemisorbed on catalyst active sites, resulting in irreversible
changes to the geometric structure of the catalyst's surface or
the chemical nature of active sites. As a result, catalysts must
be replaced or regenerated. Therefore, it is important to handle
catalysts as per industry standard practice and avoid/minimize
carbon deposition and catalyst poisoning.
Carbon formation and deposition on the catalyst can be
avoided by providing high partial pressure of H2
in a DHDS
reactor, whereas sintering can be avoided by selecting catalyst
constituents that have high thermal stability. To preserve catalyst
activity, the licensor or technology providers choose catalyst
formulation, design, pore size distribution and pellet size as
per diesel feed composition.
A case study was performed to analyze the direct impact of
catalyst activity on the conversion of reaction and sulfur outlet
concentration in the DHDS process.
Hydrocarbon Processing | MARCH 2022 45
vent gas from the cold separator is routed to the amine contactor.
Lean H2
gas compressor to minimize consumption of makeup H2
gas from the amine contactor is routed to the recycle
, while
the rest is vented. The liquid from the cold separator is routed
to a distillation column where desired desulfurized diesel is obtained
as a bottom product.
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