Hydrocarbon Processing - May 2021 - 23

Special Focus

Maintenance and Reliability
S. M. LEE, S. H. BAE and D. S. LEE,
SK Energy, Seoul, South Korea

Stability evaluation of hot spots in the
RDS process reactor
During the end-of-run (EOR) period of the authors' company's residue
hydrodesulfurization (RDS) process
in early February 2019, some hot spots
occurred in the catalysts of the reactors.
The normal operating temperatures of
the catalytic bed of the reactors are about
370°C-400°C, and the temperature
gradually rises as the severity of the process increases during the EOR period.
However, the temperatures at that time
reached 600°C-1,100°C. Fortunately,
the planned shutdown of the process
was in March, so personnel were able to
adjust the operational conditions for the
remaining month.
Impurities such as metal, sulfur and
asphaltene accumulate in the catalysts,
resulting in uneven feedstock flow. Areas
that are not in contact with the feedstock
undergo a rapid exothermic reaction
inside, which is called a " hot spot " phenomenon. It occurs occasionally during
the EOR period because a lot of impurities accumulate in the catalysts and the
operational severity of the process is increased to fully use the remaining catalyst life. In addition, if the catalyst loading quality is poor, it rarely appears, even
during normal operation.
When a hot spot occurs, the bed
temperature rises rapidly in a short time
due to a rapid heat reaction, and the
range is expanded and continuously deteriorated. The inside of a hot spot mass
comprises a solid chunk of coke and
is made worse by not penetrating the
feedstock coolant.
In this study, the authors modeled hot
spot phenomena experienced to analyze
the temperature effects of the catalysts,
reactor shells and internal parts. This
was completed through computational
fluid dynamics (CFD) analysis and,

with finite element analysis (FEA) techniques, these results were used to evaluate structural reliability. Also reviewed
was the corrosion mechanism of internal
structures exposed at high temperatures
during this time.
Ultimately, the purpose of this study
was to assess structural stability by estimating the exposed temperature of reactor internals and the shell in the event of
similar hot spot phenomena in the future-and to predict corrosivity and devise measures during operation. In other
words, if an abnormal temperature zone
occurs due to a hot spot, then an operational guide could be provided to determine whether the process can be operated continuously or if feedstock supply
must be halted and measures taken (such
as nitrogen cooling).

Hot spots. Hot spot cases are very di-

verse, and their characteristics may differ
depending on shape, temperature and
location of occurrence. Hot spot sizing is not easy, but it can be estimated
using thermocouples for temperature
sensing of catalysts. It could be easier if
thermocouples for multi-point sensing
were installed. The actual size can be
checked during the catalyst replacement
work during a shutdown. The hot spot
area rises rapidly in a short time and is
observed as a hard coke mass.
The analysis was done in three stages.
These include:
1. The temperature profile-
in axial and radial directions-
was analyzed through CFD after
modeling hot spots. The authors
compared several items to find

TABLE 1. Variable for CFD analysis
Condition

Description/properties

Hot spot location

Axial direction: 500 mm above the interbeam
Axial direction: 900 mm above the outlet collector
Radial direction: 50 mm away from the wall

Hot spot shape

Spherical, having diameters of 500 mm, 1,000 mm and 1,500 mm

Hot spot temperature

800°C-1,100°C

Feed phase

Single phase

Feed velocity

0.0031 m/sec

Viscosity

0.000023 Pascal second (Pa-sec)

Catalyst thermal conductivity

0.25 watts/m Kelvin (W/mk)* vs. 12 W/mK vs. 25 W/mK**

Speciﬁc heat

1,886.7 J/K/kg

Feedstock temperature

400°C

Ambient temperature

21°C

Shell outside

Adiabatic condition

*Thermal conductivity for coke in literature
**Thermal conductivity reﬂecting the temperature change (10°/cm) in the radial direction experienced by SK Energy

Hydrocarbon Processing | MAY 2021

Hydrocarbon Processing - May 2021

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