Page 25 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN001H-01 May 7, 2001 16:18

22 Absorption (Chemical Engineering)

or erosion. The orifice distributor has the potential to gen- Gas inlets. Gas must enter above the bottom liquid
erate a distribution pattern superior to most others, but level or, if bubbled through the liquid, through a well-
its application is often restricted to clean fluids where the designed sparger. Commonly, no sparger is used; in such
gas/liquid ratio is not high. The notched-trough distributor cases, the feed nozzle should be located at least 12 in.
is often considered the most reliable distributor, although (0.3 m) above the liquid level. Impingement on the liquid
the quality of distribution may be somewhat inferior than level, seal pan overflow, and instrument nozzles must be
that of the orifice or ladder distributors. avoided. Failure to follow these guidelines may result in
Liquid redistributors are installed at frequent intervals premature flooding, excessive entrainment, and in some
in a packed column to remix the liquid, thus counteracting cases mechanical damage to the trays.
the propagation of maldistribution effects and the natural
tendency of liquid to migrate toward the wall. A common Bottom liquid outlets. Sufficient residence time
design practice is to redistribute the liquid every 20 ft must be provided in the bottom of the column to separate
(6–7 m). any entrained gas from the leaving liquid. Gas in the bot-
Redistributor design is similar to gravity distributor de- tom outlet may also result from vortexing or from forthing
sign. The orifice type is most popular (Fig. 14). A notched- caused by liquid dropping from the bottom tray (a “wa-
through type requires a liquid collection device above it terfall pool” effect). Vortex breakers are commonly used,
to feed the liquid onto the distributor. Often, the gas risers and liquid-drop height is often restricted. Inadequate gas
are equipped with caps to prevent liquid from dropping separation may lead to bottom pump cavitation or vapor
through the gas spaces. choking the outlet line.
Liquid collectors are installed when liquid must be col-
lected for redistribution or drawoff (e.g., for external cool- Intermediate liquid outlets. Liquid may be with-
ing). The common device used is a chimney tray, which drawn using a chimney tray or from a downcomer. A chim-
is similar to an orifice redistributor, but without perfora- ney tray is a flat, unperforated plate with vapor risers. It
tions. Another common device is the Chevron-type col- permits total withdrawal of liquid; a downcomer drawoff
lector, which is a series of Chevron blades, with liquid permitsonly partial withdrawalbecause some weeping oc-
being collected at the bottom of the blades. curs through the tray. A downcomer drawoff may contain
Packing supports have to support the packed bed phys- some entrained gas, which must be separated downstream
ically, while incorporating a large free area to permit free or allowed for in downstream equipment design.
passage of gas and liquid. Grid supports are common,
especially in nonmetallic applications. Gas injection sup- Gas outlets. Sufficient liquid disentrainment from
ports(Fig.14)areusuallypreferred;theseprovideseparate the overhead gas is usually required. This may be achieved
passages for the gas and liquid and a large free area. by providing sufficient vertical height above the top tray,
Holddown plates and bed limiters are grids or wire installation of mist eliminators, or providing external
screens with openings small enough to prevent migra- knockout facilities downstream of the column.
tion of packing particles. They prevent bed fluidization,
which may cause breakage of ceramic and carbon pack- Tray layout. The preliminary tray and downcomer
ings or entrainment of metal or plastic packings with the layout is prepared in the column-sizing phase and refined
gas. during the hydraulic design phase. In addition to the pa-
rameters previously set, such parameters as hole diameter
or the type of valve unit are determined.
2. Tray Columns
Smaller hole diameters usually enhance efficiency and
The most important features of tray column internals and capacity but are also more sensitive to corrosion and plug-
their designs are outlined in the following paragraphs. ging. Holes smaller than 3 in. (5 mm) are uncommon be-
16
cause they require an expensive manufacturing technique.
Liquid inlets. Liquid enters the top tray via a hole in Half-inch (13-mm) holes are common when corrosion or
the column shell, often discharging against a vertical baffle plugging is expected.
or weir, or via a short, down-bending pipe (Fig. 17), or via The best type of valve unit depends on the corrosive
a distributor. Restriction, excessive liquid velocities, and and fouling tendencies of the service, as some valve units
interference with tray action must be avoided, as these may tend to pop out of their seats in corrosive services, while
lead to excessive entrainment, premature flooding, and others tend to stick to their seats in fouling services.
even structural damage. Disperser units (e.g., perforations, Other parameters such as level tolerance, tray supports,
values) must be absent in the liquid entrance area (Fig. 17) drainage, weir shape and type are also determined in this
or excessive weeping may result. phase.