Hydrocarbon Processing - December 2021 - 25

Special Focus Catalysts
A. KITTUR, Saudi Aramco, Ras Tanura, Saudi Arabia
Establish the maximum allowable pressure drop
across the catalyst bed structure in a hydrocracker
The reaction section of a hydrocracker unit has three reactors
configured in two stages. Most of the cracking takes place
in the third reactor (of interest), which uses a high-activity hydrocracking
catalyst in its top four beds. As the feed and recycle
gas mixture enters the top of the reactor, it flows evenly over
the reactor cross-sections before flowing down to each of the
catalyst beds. The catalyst is designed to remove organic metals
from the feed and is graded to provide maximum surface area to
collect pipe scale and other solid particles with minimal impact
on reactor pressure drop, as shown in FIG. 1.
As per original equipment manufacturer (OEM) specifications,
each catalyst bed can handle an 85-psig pressure drop.
Pressure differential transmitters measure the differential pressure
over the reactor and can also be lined up to indicate the
pressure drop over the top catalyst beds.
A steady increase in pressure drop is typically expected across
the first catalyst bed structure during normal operations (FIG. 2).
In operational runs, it was observed that this pressure drop approached
the set value of 85 psig much earlier than the scheduled
end-of-run (EOR) conditions. Operational adjustments were
required to avoid this pressure drop from exceeding an 85-psig
set value before the scheduled EOR, which often compromised
the efficiency and productivity of the hydrocracker.
Problem statement. To establish the maximum allowable
pressure drop that the reactor internal structural components can
withstand, the mechanical strength of the a) catalyst bed support
structure, b) expansion skirt structure, c) shell integral support
ring, and d) expansion skirt plug weld to the shell wall, must be
established. A preliminary assessment of the reactor's mechanical
design was completed, and several important characteristics of
internal mechanical and fluid loads were identified for the existing
maximum allowable 85 psid pressure drop permitted across
the first catalyst bed and expansion skirt assembly, combined.
Basically, the dead weights for the reactor internals contributing
toward the mechanical loads remain constant through the
changing fluid loading scenarios. The correlation for pressure
drop data across the various elevations of the reactor were obtained
from the process trends of existing field instrumentation,
which established that 86% of the pressure drop was found to
occur across the catalyst beam structure of the first bed, while
the remaining 14% was found to occur across the expansion skirt
assembly of the first bed.
Among the contributions of the various fluid loads and the
weights of the reactor internals contributing toward the mechanical
loading of 398 psi on the top general surface of the
shell integral support ring, the effect of the pressure drop across
the first bed had the maximum impact of 94% when compared
with the other. The contribution of the various fluid loads and
the weights of the reactor internals toward the mechanical loading
of 655 lb/in. (117 kg/cm) on the expansion skirt plug weld
to shell is shown in FIG. 3. The effect of the pressure drop across
the first bed had the maximum impact of 87% when compared
with the other loads.
The complex geometry associated with the reactor internals,
coupled with the thermo-mechanical loading, mandated
an exhaustive and thorough stress analysis based on American
Society of Mechanical Engineers ASME Sec-VIII Div-2 methodology.1
The
accurate stress distribution was simulated using
the finite element method (FEM).
FIG. 1. Second-stage reactor in a hydrocracking unit.
FIG. 2. Pressure drop process trends.
Hydrocarbon Processing | DECEMBER 2021 25
Hydrocarbon Processing - December 2021

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