Chemical Engineering August 2021 - 48

Solids Processing
Considerations for Solid Catalyst Loading
Special considerations must be made for loading expensive solid catalyst materials
into fixed-bed and tubular reactors
John Deajon
Maviro Inc.
M
ost units in the chemical
process industries (CPI)
use complex and expensive
heterogeneous
catalysts aimed at improving reaction
performance. Although sometimes
overlooked, the solids-handling
practices assocated with loading and
unloading solid catalyst materials into
and out of reactors can have a significant
impact on the ultimate performance
of the catalyst, the operation
of the process and the safety of the
personnel involved. Even the most
highly engineered solid catalysts can
exhibit reduced performance and
present safety hazards if they are not
correctly handled and loaded.
Efficient catalyst loading significantly
increases the process unit's
performance by improving the flow
distribution inside the reactor and
also preventing any settling of the
bed, resulting in more reliable operation.
This article discusses some
of the potential issues that can arise
from improper loading of solid catalysts
and explores best practices
and current technology for achieving
proper loading.
Catalyst materials and usage
Catalysts, inlcuding homogeneous,
heterogeneous and biological cataMaviro
FIGURE
1. Tubular reactors contain a series of
tubes that contain the solid catalyst material, as
shown here
46
lysts, are added to a chemical reaction
to increase the reaction rate.
They are manufactured globally by a
number of specialized suppliers that
design them to reduce environmental
emissions and production costs.
Many solid hetergeneous catalyst
substances have transition metals
as active sites, and the desired
reaction occurs on the surface of
the solid. The electron structure of
transition metals adsorbs reactant
molecules strongly enough to allow
the stabilization of a transition state
for the reaction, but not so strongly
as to prevent desorption of product
molecules from the catalyst surface.
Cobalt is a key constituent of a
number of catalyst systems used
worldwide for hydrotreating and hydrodesulfurization,
Fischer-Tropsch
synthesis and gasoline mercaptan
removal. Other catalytic applications
of cobalt include production
of the monomer for manufacturing
polyethylene terephthalate (PET), a
plastic often used for beverage containers.
Precious metals, such as
platinum, palladium, ruthenium and
silver, are effective catalysts in many
applications because of their extreme
stability at high temperatures
and pressures, and their inertness to
chemical reactions.
The range of different ways catalysts
are used is broad. In polymer
production, the catalyst material is
often injected into a reactor containing
the monomer and hydrogen
under pressure. The polymer particles
are formed and removed from
the reactor.
Fixed-bed reactors are the most
common type of reactor for solid
catalysts. These are typically cylindrical
vessels filled with catalyst pellets.
The gas or liquid reactants flow
through the solid catalyst bed and
are converted into products.
Tubular reactors/reformers consist
of a series of tubes containing the
catalyst (Figure 1). This type of reacMaviro
FIGURE
2. Loading of solid catalysts is often best
accomplished by specialist companies with specific
experience and equipment to carry out the
operation
tor is normally used in cases where
the process must be carried out at
high temperatures, and where hot
gases are present on the outside of
the tubes. Tubular reactors are also
used where the process is highly
exothermic and coolant needs to be
applied to the outside of the tube.
In both cases, the presence of
multiple tubes makes it possible to
reach and maintain the very high or
low temperatures required. Achieving
the temperatures needed would
be difficult if a single bed was utilized.
Fluidized-bed reactors suspend
small catalyst particles by the upward
motion of the fluid to be reacted.
The fluid is typically a gas
with a flowrate high enough to mix
the particles without carrying them
out of the reactor. Fluidized-bed reactors
are typically multiple-vessel
units whereby under normal operation,
the catalyst will migrate between
vessels.
Catalyst handling
Catalyst handling applications involve
full containment of catalysts
and require very limited exposure
to workers. Loading and unloading
of catalyst materials is often best
completed by specialist catalysthandling
companies that have the
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