Page 24 - 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

Absorption (Chemical Engineering) 21

residence times and because of extensive gas backmixing. fine liquid mist from a gas steam. They are often also used
Their application is limited to easy absorption duties (one for simultaneously absorbing certain components from the
or two theoretical stages), usually in systems where the gas stream, but because of their poor mass transfer are ef-
controlling resistance to mass transfer is in the gas phase. fective only when these components are highly soluble in
Column capacity is usually limited by liquid droplet en- the liquid. Common applications are scrubbing incinerator
trainment from the top. fumes and sulfuric and phosphoric acid mists.
Spray absorbers are advantageous where low pressure
drops are critical and where the gas may contain some Wet scrubbers. These are devices in which a liquid
solids, such as in the absorption of SO 2 from coal-fired spray contacts a gas stream, primarily for the purpose of
boiler exhaust gases. removing fine solid particles or liquid mists from the gas.
In this process, the liquid spray simultaneously absorbs
Falling-film absorbers. These are usually vertical solublecomponentsfromthegas.Thespraysaregenerated
heat exchangers with the cooling medium in the shell and by a variety of mechanical devices.
the absorption taking place in the tubes. The solvent flows
downward, while the gas may enter either at the bottom
C. Hydraulic Design
(countercurrent flow) or at the top (cocurrent flow).
Mass transfer in falling-film absorbers is strongly de- This design phase determines the types, dimensions, loca-
pendent on the gas velocity in the tubes, the liquid and gas tion, and orientation of the multitude of internals used in
distribution, and the tube surface conditions. The max- absorption columns. It usually leads to refinements to the
imum capacity of falling-film absorbers is normally re- column design and sizing and, most important, is critical
stricted either by flooding or by pressure drop. Another for ensuring trouble-free operation.
important limit in these absorbers is film breakup. If heat
flux is excessive, dry areas may form at the tube wall and 1. Packed Columns
reduce mass transfer.
Falling-film absorbers make continuous heat removal The most important aspects of packed-column internals
possible and are therefore extensively used in applica- and their design are outlined in the following paragraphs.
tions where the heat released during absorption is high, Packed-tower efficiency and turndown are strongly de-
such as in the absorption of hydrogen chloride to form pendentonthequalityofinitialliquiddistribution.Uneven
hydrochloric acid. distribution may cause local variations in the liquid/gas ra-
tio, localized pinch conditions, and reduced vapor–liquid
Stirred tanks. These are mechanically stirred ves- contact. Figure 14 shows two common liquid distributor
sels, which are advantageous when absorption is accom- types, the ladder type (shown as the top distributor) and
panied by a slow liquid-phase chemical reaction. As dis- the orifice type (shown as the redistributor). The ladder
cussed earlier (Section II), this application is considered type is a horizontal header of pipes, which are perforated
a chemical reactor rather than an absorber. Stirred tanks on the underside. The orifice type is a flat perforated plate
provide high liquid residence times but are limited to low equipped with round or rectangular risers for gas passage.
gas flow rates. Other common types of distributors are a header equipped
with spray nozzles (spray distributor) and a header of hor-
Bubble columns. These are columns full of liquid izontal channels, with V notches cut in the vertical walls
into which gas is introduced by a perforated pipe or a of the channels (notched-trough distributor).
sparger. Bubble columns are used for applications similar Ladder and spray distributors rely on pressure for their
to stirred tanks, but their contact efficiency is lower. action. They provide a large gas flow area but a some-
what limited liquid flow area; they are light and cheap but
Venturi scrubbers. In a venturi scrubber, a liquid jet are sensitive to corrosion, erosion, and to a certain extent
issues from a nozzle. The jet induces cocurrent gas flow plugging. They are most suitable for high gas/liquid ratio
into the throat of the jet. Mass transfer takes place be- applications.
tween the gas and the atomized liquid downstream of the Orifice and notched-trough distributors rely on gravity
nozzle. Mass transfer is usually poor and depends on the for their action. They provide a large liquid flow area; the
throat velocity or pressure drop, the liquid/gas ratio, and notched-trough distributor also provides a large gas flow
the liquid atomization pattern. Because of the cocurrent area. They are more robust and expensive than pressure
nature of contacting, the maximum solute removal does distributors and are sensitive to levelness. The orifice dis-
not exceed a single theoretical stage. Venturi scrubbers are tributor is most sensitive to plugging, while the notched-
used primarily for separation of fine particulate matter or trough is the least sensitive to plugging, corrosion,