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|Selection Of Reactor Type||Multi - stage Bubble Columns|
|Stirred Tank Reactor||Wash and Spray Columns|
of material between a gas and a liquid plays a very important part in many
This transfer forms the basis of the unit operations of
distillation, absorption and humidification.
In these cases there is a physical transfer of material across the
phase boundary controlled by the physical properties of the system.
In some instances, physical transfer may he followed by a second
step, chemical reaction, the unit operation then becomes gas-liquid
The rate and extent of the reaction and the choice of equipment is
influenced not only by the physical transfer of material but also by the
produces a range of equipment specially designed for gas-liquid reactions.
The range includes continuous and batch stirred tank reactors,
bubble columns with and without internal fittings, wash, spray and
The Garg units, together with other items such as heat
exchangers, pumps and valves, and our process engineering capability in
the fields of distillation, absorption, extraction, etc. enable us to
design complete systems for laboratory, pilot plant or production use.
reactions may be either spontaneous or initiated thermally,
photochemically or catalytically.
They include processes such as oxidation, halogenation, hydration,
nitration and sulphonation, and are widely used throughout the chemical
industry from petrochemicals to pharmaceuticals. The complexity of the
processes that occur in gas-liquid reactors, involving not only
mass-transfer but also chemical reaction, means that on many occasions
pilot plant studies must be carried out before the production plant design
can be finalised.
The use of glass is an asset at both the pilot plant and production
With almost all gas-liquid reactions, parallel or series side reactions take place which lead to unwanted by-products. This can frequently have a decisive effect on the process econornies and the correct choice of reactor type and informed process design is essential.
order to achieve the aim of an optimal production unit the process design
engineer needs to know
The process chemistry involved including possible side reactions.
many cases this information is not fully available and laboratory and
pilot plant tests are required using small scale continuous or batch
These tests can be carried out by our engineers, using customer
materials, in experimental units we have available at our headquarters.
Alternatively units are available to borrow in order for customers
to carry out their own experimental work.
following information gives an initial guide to the preferred reactor
The stirred tank reactor has a long retention time for the liquid phase.
addition to retention time the initial choice must also be influenced by
the following considerations.
gas-liquid ratio, which can be adjusted to a desired value by recycle if
this does not adversely affect the chemical kinetics.
The supply or removal of heat, whether an isothermal or adiabatic reaction cycle is desirable and the influence of the reactor temperature distribution on the desired product spectrum.
figure shows a stirred tank reactor which is capable of being operated in
batch liquid/continuous gas
gas may be dispersed in two ways, either using a stirrer or using a Garg gas-mixing valve which is described in detail Heat may be supplied
or removed from the system through the jacket of the reactor or in a heat
exchanger in the recycle line. To enable photochemical reactions to be
carried out submersible lamps may be put into the reactor.
bubble column is predominantly used for continuous operations, in contrast
to the stirred tank reactor it has no moving parts. The unit consists of
an unpacked column section (except that a short packed
section is provided at the top to
remove scum). Gas is
introduced at the base of the column through a gas manifold, or
alternatively using a Garg gas-mixing valve, the gas may be recycled if
required. The liquid can also
be recycled via an external heat exchanger.
Photochemical reactions can be carried out by fitting lights around
the column. Columns of this form have similar retention times to that of a
continuous stirred tank reactor. However,
the continuous stirred tank is a thoroughly mixed system whereas the
column is a dispersed plug flow system.
It is possible to improve the retention time of the liquid phase by
including short packed sections in the column, this however leads to an
increase in back mixing and a reduced gas loading.
Multi - Staged Bubble
bubble columns allow greater variation of the retention time of the liquid
phase. The simple construction of
the columns enables a cascade mode of operation to be used, with a well
defined flow path and no remixing between stages. Multi-stage bubble
columns can be supplied with constant or variable bubble formation layer
heights. The column with
constant bubble formation height is based on a filter plate column with
fixed overflow heights and external liquid transfer lines.
In the case of the column with variable bubble layer height, the
liquid outlets are completely flooded and provided with restrictions which
make it possible to adjust the heights of the bubble formation layers
independently of the gas and liquid loadings
. This enables a
mean liquid retention time to be maintained under load changes.
If required heat exchange coils can be fitted in the column and the
temperature profile controlled. Multi-stage bubble columns are an
advantage when an undesirable side reaction must be controlled and they
Wash and Spray Columns
figure shows this very simple unit.
Wash columns are commonly used for purely physical absorption
processes, but they are also suitable for gas-liquid reactions if a short
retention time is required for the liquid phase.
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