Chemical Engineering June 2023 - 39

Depending upon the process application,
a support grid(s) will be required.
The inlet gas or liquid needs
to be evenly distributed to fully utilize
the media. In pressure-swing
or thermal-swing applications, a
process-flow-outlet distributor is required
to ensure that the media are
properly regenerated. For all downflow
applications an outlet distributor
or collector should be installed,
even if a support grid is installed.
Operational and downstream equipment
" nightmares " have resulted
when an outlet distributor or collector
was not installed.
When designing internals, the
vessel piping arrangement and the
process application will influence
the type, design and material of
construction. Piping that enters or
exits from the side of the vessel versus
piping that enters or exits from
the vessel heads will limit the options.
Thermal-swing units require
support grids to allow expansion
and contraction in concert with the
vessel. Compatibility with the process
gas or liquid will determine the
material of construction choices.
Flow distribution, end-of-run pressure
drop, vessel diameter, media
particle size and total weight need
to be carefully considered.
Fabrication profiles. As shown in
Figure 1, profile wire, slotted metal,
mesh screen and bar grating are the
typical fabrication options for these
vessel internals. They can be used
individually or in combination with
each other. All internals should be
inspected prior to installation and
during media replacement. Internals
made with mesh screen can tear
or become separated so careful inspection
is required. Mesh screen
repair or replacement can impact the
planned schedule.
Distributors. An inlet-flow distributor
needs to evenly distribute the gas or
liquid before contact with the media.
The open area should be greater
than 100% of the inlet pipe diameter.
Based upon the vessel piping arrangement,
it is either welded to the
inlet pipe, welded to interior vessel
head flange, attached with bolting
or sandwich flanges, or attached to
interior vessel head bolting tabs. For
vessel diameters greater than 10 ft,
an enhanced flow distributor should
be considered over a standard design.
In down-flow processes, the
recommended distance between
the bottom of the distributor and the
media is 3 ft. If the inlet distributor is
installed in the bottom of the vessel,
it must prevent media leakage and
be designed for static and dynamic
forces. Fabrication choices are profile
wire, slotted metal with profile
wire, slotted metal or slotted metal
with mesh screen.
An outlet-flow distributor or collector
needs to retain the smallest
media particle size and prevent
media leakage, especially if installed
in the bottom vessel head. The open
area should be at least 200% of the
outlet pipe diameter. In regenerative
processes, it must evenly distribute
the heating and cooling gas prior to
contact with the media. This will ensure
that the media is completely regenerated.
In down-flow processes,
it should be designed for both the
static and dynamic forces - even
if a support grid is installed above
it. Based upon the vessel piping arrangement,
it is either welded to the
inlet pipe, welded to interior vessel
head flange, attached with bolting
or sandwich flanges, or attached
to interior vessel head bolting tabs.
Fabrication choices are profile wire,
slotted metal with profile wire, slotted
metal, or slotted metal with mesh
screen. During media replacement,
the mesh screens should be rigorously
inspected to ensure no tears
or separations have occurred.
Grid supports. Support grids require
a vessel ring, vessel beam seats and
support beam(s), because the grid
lays on top of the vessel ring. Some
types of support grids incorporate
beams into the grid which also eliminates
the need for beam seats. The
number of beams required is based
upon the vessel diameter. In thermalswing
applications, the support grid
needs to be designed for expansion
and contraction in coordination with
the vessel. The diameter of a support
grid should be less than the vessel
inside diameter. The void space calculation
is performed by support grid
suppliers to ensure that the support
grid does not fall off the vessel ring
or crush itself against the vessel wall.
Rope packing is placed in the void
space. Compatibility with both expansion
and contraction, and the process
gas or liquid will impact material-ofconstruction
options. Common materials
of construction are 300 or 400
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series stainless steel, Inconel, Monel
and carbon steel. The individual grid
panels need to be sized for easy vessel
entry and grid assembly.
A support grid must retain the
smallest media particle size. The slot
size should be one half the size of
the smallest media particle diameter.
In down-flow processes, the grid
needs to be designed for the static
and dynamic force, media weight
plus end-of-run pressure drop.
Fabrication
choices
are
profile
wire, slotted sheet panels, or multiple
mesh screens over bar grating.
Multiple (two or three) mesh screens
of different slot sizes are required to
ensure particle retention and screen
strength. During media replacement,
the mesh screens should be rigorously
inspected to ensure no tears
or separations have occurred.
Vessel out of roundness, especially
on vessels greater than 15 ft
in diameter can present installation
and operational headaches. Ideally,
the assembly fabrication of the individual
grid panels should begin after
the " as built " inside diameter and
the vessel ring width are measured
for each vessel.
Enhanced support-grid designs
offer increase operational and reliability
benefits. These designs minimize
the impact of vessel out of roundness,
allow for increased vessel volume
of media, and energy savings.
One comment on support grids:
the term " Johnson screens " has
been, and still is, used interchangeably
for a support grid, no matter if
it is fabricated from profile wire, slotted
metal, or mesh-over-bar grating.
Johnson Screens was the company
name and is a product brand name
for its profile wire product (Johnson
Screens Vee-wire).
n
Edited by Gerald Ondrey
Author
Eugene A. Kuchta is an independent
consultant at Kuchta Consulting
Co. (Houston; Email: eugene@
kuchtaconsulting.com).
Prior to
this, he worked for 12 years for
Johnson Screens, where he focused
on vessel internals for the
natural
petrochemical and industrial gas
industries. Before this, he worked
for Union Carbide/UOP for 31 years, primarily focused
on molecular sieve adsorbents, PSA Hydrogen, and
PolySep membranes in a technical sales function. He
holds B.S.Ch.E. degree from the New Jersey Institute of
Technology (1978) and has been a member of AIChE
since his undergraduate years.
39
gas, petroleum refining,
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Chemical Engineering June 2023

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