Page 309 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

286 10 Post-combustion Air Emission Control

In Sect. 5.2, we only analyzed counter flow packed bed scrubber. There are
actually many other types of wet scrubbers in engineering practice. Despite of their
differences in configuration, the ultimate objective is to maximize the gas-liquid
contact surface area. The larger the contact area, the greater mass transfer speed,
and the faster air cleaning process is.
In choosing a wet scrubber, we have to consider the fabrication costs, operating
costs, and environmental impact. The capital cost of a wet scrubber is lower than
that of a bag-house filter or an ESP but higher than a cyclone. Operation and
maintenance costs, however, are much higher due to consumption of liquid,
accessories required to handle high pressure drops and slurries, and problems
related to corrosion, abrasion, solids buildup, failure of rotating parts and restart
problems after a shutdown. Most importantly, wet scrubber transforms an air pol-
lution problem to a water treatment problem.
Schematic diagrams of typical wet scrubbers are shown in Fig. 10.5. Typical wet
scrubber designs include

• Bubble column
• Packed bed scrubber
• Spray tower
• Venturi scrubber.
A bubble column is an apparatus used for gas–liquid absorption. Liquid is stored
in a vertical column. Gas is introduced from the bottom of the column and dis-
tributed through a bubble generator, called a sparger. Large bubbles can be gen-
erated by passing through the holes in the sparger; millimeter range or micron-sized
bubbles can be generated by porous media. Gas-liquid mixing can be done by
buoyancy, or it can be mechanically mixed by stirring. The liquid can be static or
parallel flow or counter-current with respect to the gas phase.
A bubble column is useful in reactions where the gas-liquid reaction is slow in
relation to the absorption rate. It is useful for small quantity of gas. However, when
the gas flow rate is high, it would require extreme power to pass this large volume
of gas through a liquid tower. And, the dust accumulation may plug the column too.
Therefore, other types of wet scrubbers should be considered for large volume of air
cleaning.
In a spray tower, liquid (water) droplets are sufficiently large to overcome the
upward gas speed. The droplet sizes are controlled to optimize particle contact and
to provide easy droplet separation. The liquid to gas ratio is much smaller than that
of a bubble column.
One special type of spray tower is called cyclonic spray tower. The body of a
cyclonic spray tower is just like a cyclone. When air or gas enters the cyclone, the
centrifugal forces are created by either a tangential inlet or vane-induced inlet.
The liquid sprays are aligned in the center of the tower. Centrifugal forces increase