Chemical Engineering June 2023 - 38

Engineering Practice
The Selection and Design of Fixed-Bed,
Axial-Flow Vessel Internals
Follow these key considerations when deciding which internals to use in fixed-bed,
axial-flow vessels
Eugene A. Kuchta
Kuchta Consulting Co.
F
ixed-bed, axial-flow vessel
internals are installed in natural
gas plants, petroleum
refineries, petrochemical
plants, chemical plants and industrial
gas facilities. Thermal-swing
gas and liquid dehydrators, treaters
and purifiers, along with non-regenerative
mercury- and sulfur-removal
units are commonly installed in natural
gas plants. Fixed-bed catalyst
units are installed in refineries and
petrochemical plants, along with
thermal-swing dehydrators, treater,
purifiers, non-regenerative purifiers
and treaters, and pressure-swing
adsorption units. Chemical plants
typically have thermal-swing and
non-regenerative process units.
Industrial gas plants utilize
pressure-swing, thermalswing
and non-regenerative
processes to produce highpurity
hydrogen, oxygen, nitrogen
and helium.
Selection and design
When selecting and designing
vessel internals for
a fixed-bed, axial-flow process,
the initial capital cost
of a particular internal should
be balanced with its durability
and its impact on the installation,
maintenance and turnaround
schedules. Choosing
the lowest cost internal may
result in unexpected consequences.
If an internal component's
durability is low,
then frequent repairs or replacement
may be required,
which will result in extended
turnaround schedules. The
design needs to ensure that
an internal component allows
for an easy installation. Input
from the operations and
maintenance group should
be solicited.
38
Internal materials for pressureswing,
thermal-swing and non-regenerative
process applications are
examined in the next section; specifically
the inlet-flow distributor, the
outlet-flow distributor and the support
grid.
Pressure swing. Pressure-swing
units typically operate on very short
cycles (minutes) and have long run
times between media replacement,
usually longer than 10 years. Internals
in this process application need
to be very durable to withstand these
pressure changes and possible operational
upsets. Otherwise, unplanned
shutdowns will result in lost production
time, replacement media cost,
and the cost of repair or replacement
of the internal components.
Thermal swing. Thermal-swing units
typically operate on cycle times of
eight hours or greater; however, some
processes run on shorter cycles,
and the schedule for replacement
of media is three to five years. Internals
need to withstand temperature
swings between ambient to greater
than 600°F, be compatible with the
process gas or liquid, and withstand
possible operational upsets.
Non-regenerative. Non-regenerative
units can utilize the " cheapest
option. " However, with run times
usually less than one year before
media replacement is required, internals
can be damaged during the
media unloading, the vessel/internals
inspection or the media loading.
All vessels should have a processflow-inlet
distributor and a processflow-outlet
distributor or collector.
Shutterstock
Shutterstock
Shutterstock
Johnson Screens
FIGURE 1. Shown here are four typical fabrications options: bar grating (upper left), slotted metal grid (upper right),
mesh screen (lower left) and profile wire (lower right)
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Table of Contents for the Digital Edition of Chemical Engineering June 2023