Materials of Construction - Quench Towers
June 8, 2003
Introduction
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Tower Shell
The tower shell is generally carbon steel which has been lined with a corrosion
resistant lining. Once the gas has been quenched and cooled, the materials
of construction can be changed to a plastic material or FRP.
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Lining
The lining will
be dependent in the gas and circulating liquid composition as well as the
temperature of the gas.
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Rubber
A rubber lining is used to prevent the weak acid from attacking the carbon
steel shell . The type of rubber used will vary from application to
application. Natural and chlorobutyl rubber are two types of
rubber that have been specified for the service. The selection of the
type of rubber will depend on the availability, cost, installation
requirements, etc. If the ambient temperature of the site is low,
chlorobutyl rubber should be specified due to its better low temperature
flexibility over natural rubber. The minimum thickness of rubber is
generally 5 mm.
The maximum operating temperature is 60°C (140°F). Brick thicknesses
should be specified to achieve this temperature at the rubber lining.
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Acid Brick
At least one layer of acid resistant brick is required to protect the rubber
lining from the high gas temperatures especially in the region of the gas
inlet. Multiple layers of brick are preferred as this provides an
added degree of safety if a flaw or crack should develop in the outer layer.
Each additional layer of brick is laid so as to stagger the joints. This
eliminates any direct paths to the rubber lining should a crack develop in the
mortar.
The mortar used to lay the acid brick should be a furan mortar. The
mortar is applied with a trowel to the sides of the brick before laying it in
the course. Either a red shale or fireclay brick is suitable
for the service. If there is no requirement for an outer layer of carbon
brick, a fireclay brick is recommended due to its greater resistance to
thermal shock. If a layer of carbon brick will be laid on top of the
acid brick, the less expansive red shale brick can be used.
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Carbon Brick
Carbon brick is required when there are fluorides present in the gas and
circulating liquid. Fluorides will attack the silica in the acid brick
resulting in failure of the acid brick lining. A single layer of carbon
brick laid on top of the layer(s) of acid brick will prevent fluoride attack.
A thin layer of carbon filled furan mortar (3 mm) is trowelled on top of the
acid brick prior to laying the carbon brick.
The mortar use to lay carbon brick should be furan mortar. As with the
acid brick, the carbon brick is laid by applying mortar to the sides of the
brick before laying it in the course.
Carbon can be oxidized if the brick is exposed for prolonged periods to the
hot gas with no liquid flowing over the brick surface. Risk of
oxidization occurs when the gas temperature exceeds 350°C . The design
of the tower should ensure that the carbon brick is not exposed to the hot gas
without liquid flowing on the brick surface.
Specification: Low-Ash Chemical Carbon Brick
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Spray Nozzles and Feed Pipes
Many materials have been specified for quench tower service. The selection
of a suitable material will be dependent on the gas and liquid compositions, gas
and liquid temperature, mechanical strength, resistance to abrasion and wear,
etc.
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PTFE
PTFE has excellent resistance to chemical attack and is suitable for all gas
and liquid compositions. However, PTFE lacks mechanical strength at high
temperatures. PTFE nozzles are best suited for counter-current tower
designs where they are not exposed to hot gases.
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Silicon Iron
Silicon iron is resistant to most gas and liquid compositions that are found
in typical quench tower applications. The material has excellent
resistance to abrasion and wear due to solids in the circulating liquid.
However, silicon iron is brittle and can break easily if there is excessive
mechanical forces exerted on the nozzle or feed pipe.
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Metals
There are some metals that maybe suitable for spray nozzles and feed pipes
depending on the gas and liquid compositions. Hastelloy C-276, G, X,
alloy 20, stainless steel maybe specified for the service. Careful study
of the suitability of the metal must be done before specifying it.
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