Synthesis Gas and Refinery Hydrogen Applications Support balls or more accurately, support balls and hold-down balls will at times need to be specified to our customers. Advice may also be needed opposite the usage of meshes and hold-down screens. This brief report focuses on some key aspects to consider. 1. Support Balls 2. Hold-down Balls 3. Meshes over Exit Collector & Exit Screen 4. Hold-down Screens 5. Support and Hold-down Ball Compositions and Applications 6. High Surface Area Dust Collectors

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Usage of Support Balls, Meshes & Hold-
down Screens
Synthesis Gas and Refinery Hydrogen Applications
By: Gerard B. Hawkins
Managing Director, CEO
Support balls or more accurately, support balls and hold-down balls will at times
need to be specified to our customers. Advice may also be needed opposite the
usage of meshes and hold-down screens. This brief report focuses on some key
aspects to consider.
1. Support Balls
Balls are needed to support the catalyst bed. This primarily allows the exit
collector and screen to have sufficiently sized slots/holes to avoid adding to the
reactor pressure drop and clog-up with catalyst dust etc. Thus balls are usually
filled in graduating sizes from large at the bottom. The attached sketch shows
some typical arrangements.
These are only guidelines, but can be generally recommended:-
Support balls on top of
Support Screen
Support balls filled in base of
reactor
Thickness Size of balls Thickness Size of balls
Top Layer 100mm
(4")
6mm - 13mm
(1/4" -1/2")
100mm
(4")
1/4" - 1/2"
Bottom Layer 150mm
(6")
25mm
(1")
As
needed
25mm - 50mm
(1" - 2")
The sizes of the slots or mesh holes needs to be understood with relation to the
size of the support ball adjacent to it. There are examples of the ball size being
such that every third mesh hole would be theoretically blocked by a support ball.
Under such circumstance, a larger size ball would be needed otherwise the
ball/mesh combination would start to act like a filter (see 3).
The minimum thickness of a support ball layer is practically 100mm (4").

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2. Hold-down Balls
The application of hold-down balls is far more critical. The primary purpose is to
avoid any disturbance of the catalyst bed by the velocities of the inlet process
feed. If any doubt exits over the specified sizes for the hold-down balls, then a
check should be carried-out by the GBHE, C2
PT Process Engineering. We have
formulas which allow us to predict when a given support ball will be disturbed or
not by the process gas. Although the accuracy of the methods are questionable
and conservative, if they indicate problems, then they should be heeded. A
19mm (3/4") hold-down ball requires a "disturbance" velocity of at least 3 times
that of 3/16” sphere.
As a rough rule of thumb, the minimum ball size at the top should be no smaller
than 25mm (1"), but there are many applications with smaller sizes. Certainly
6mm (1/4") is very prone to movement and although more dense than most
catalysts and absorbents, it makes little sense to use as a hold-down as its size
is similar to that of the catalyst!
Case 1 Case 2
Thickness Size of balls Thickness Size of balls
Top Layer 10cm (4") 25mm - 50mm
(1" -2")
15cm (6") 25mm - 50 mm
1" - 2 "
Bottom Layer 10cm (4") 6mm - 13mm
(1/4" - 1/2")
- -
Apart from the size versus "disturbance" consideration, the minimum depth of the
support balls is theoretically 3 particle diameters; e.g. 114mm (4.5") for 38 mm
(1.5") balls. However, even with smaller ball sizes it is not recommended to
reduce the bed depth below 100 mm (4")
3. Meshes over Exit Collector & Exit Screen
Provided the correct sizes of balls are used, there is little need to install meshes
in the exit system. In many cases, meshes are excellent collectors of fine dust
which rapidly leads to a pressure drop build-up. This ultimately leads to poor bed
distribution and poor catalsyt/absorbent performance and perhaps premature
plant shut-down due to pressure drop limit. We therefore do not recommend
them.

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4. Hold-down Screens
Historically, these were quite popular. Here, a wire mesh screen is placed
between the bed and support balls to ensure that the balls do not sink down in to
the bed. Such meshes are however an additional complication in bed charging
and discharging and are now generally believed unnecessary. The downside is
that increased care is needed to ensure complete bed coverage to a satisfactory
depth without visual aid of the mesh.
5. Support and Hold-down Ball Compositions and Applications
There are a variety of compositions that can be offered, but generally, this is
limited to two types:-
1.SiO2/Alumina
2.Alumina
The first type of ball is the cheaper and more standard one. The key
disadvantage to this is the high silica content (typically 67 wt%) which can leach-
out under certain conditions. This means that in the presence of process stream
containing steam, the temperature should be limited to prevent the leaching-out
of SiO2. The clue to the temperature threshold is the application of HTS catalyst
where problems have been experienced with silica contamination of the catalyst.
This therefore indicates that in the presence of wet streams, the temperature of
around 300 - 350oC (572 - 662oF) might represent a limit for silica based balls.
Thus for all purification (i.e. "dry"), LTS and methanation applications, high silica
based balls are acceptable. For Pre-Reforming, secondary reforming, and HTS
applications, we would recommend the second more expensive type; alumina
balls. We are currently using high silica based balls for an isothermal shift
application because the inlet temperature is less than 300oC (572oF). This may
be reviewed once we obtain catalyst samples.

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It is worthwhile to note that some plants are constructed by contractors who pay
little attention to the types of support and hold-down balls. Purchasing of these is
usually non-technical with the price being the main incentive and in many cases
without the consultation of the catalyst supplier. It is therefore common to see
high silica balls used throughout the plant. In severe situations, there is example
of balls actually being
crushed in the applications. This may be due to SiO2 leach-out and/or poor
manufacturing quality of the ball itself
Historically, alumina lumps were used instead or with conjunction with balls, but
more recently seemed to be mostly displaced by balls.
6. High Surface Area Dust Collectors
The use of inert catalyst supports to collect solids etc. on tops of beds is
sometimes applied. The most "formal" application is seen with the Japanese
contractor MKK who uses these routinely on the top of HTS beds when certain
naphtha steam reforming catalysts are installed upstream. The reasoning is
around collection of migrated potash to avoid pressure drop increase across the
HTS bed. A high surface area shape, often an inert (un-dipped) reforming
rasching rings, may be recommended in these applications. These have a much
higher surface area than balls and also larger voidage so a greater amount of
solid material may be collected prior to an observable pressure drop increase.
However, it should be noted that inert catalyst supports are much less dense
than support balls and may be prone to movement if the velocities are high
enough. Hold-down balls are also usually still required (under inert catalyst
supports).

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Downflow Reactors
Inert Support Ball Applications
15cm (6") of
25mm - 50mm (1" - 2") balls
(May use 15cm layer of 25mm - 50mm
balls on top of 10cm layer of smaller size)
10cm (4") of
6mm - 13mm (1/4" - 1/2") balls
15cm (6") of
25mm (1") balls
Notes:-
1. Typical specification:-
66wt% SiO2, 25wt% Al2O3, Balance
Na2O/CaO/MgO/TiO3
2. Use +99wt% alumina balls (SiO 2 <0.2wt%) for
streams containing steam at temperatures greater
than 300 degC (572 degF)
3. Support grids not always recommended
Can plug with catalyst dust - High pressure drop.
4. Wire mesh around outlet collector not
recommended
Easily plugged with catalyst dust - High pressure drop
5. Floating hold-down screens not normally
recommended
Support
grid
Catalyst/Absorbent
Flat Distribution
Plate
15mm (3/4")
slots
25mm -50mm (1"- 2") balls
{
10cm (4") of
6mm - 13mm (1/4" - 1/2") balls
or
15cm (6") split layer of 6mm (1/4")
balls
on top of 13mm (1/2") balls

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Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
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Information contained in this technical document or as otherwise supplied to
Users is believed to be accurate and correct at time of going to press, and is
given in good faith, but it is for the User to satisfy itself of the suitability of the
Product for its own particular purpose. GBH Enterprises, C2
PT, Catalyst
Process Technology gives no warranty as the fitness of the Product for any
particular purpose and any implied warranty or condition (statutory or otherwise)
is excluded except to the extent that exclusion is prevented by law. GBH
Enterprises, C2
PT accepts no liability for loss or damaged resulting from
reliance on this information. Freedom, under Patent, Copyright and Designs
cannot be assumed.