Page 21 - Gas Purification 5E

Introduction 11

As compared to tray columns, packed columns are generally preferred for small installa-
tions, corrosive service, liquids with a tendency to foam, very high liquidgas ratios and
applications in which a low pressure drop is desired. Their use in larger sizes appears to be
increasing, and there is also a growing use of packing to replace trays where an improvement
in column performance is required.
Typical random packing elements are illustrated in Figure 1-3 and one example of an
ordered packing is shown in Figure 1-4.
Spray Contactors

Spray contactors are important primarily when pressure drop is a major consideration and
when solid particles are present in the gas, for example, atmospheric pressure exhaust gas
streams. They are not effective for operations requiring more than one theoretical contact
stage or a close approach to equilibrium. Since the surface area for mass transfer in a spray
chamber is directly proportional to the liquid flow rate, it is common practice to recycle the
absorbent to increase absorption efficiency.
Types of equipment classified as spray contactors include countercurrent spray columns,
venturi scrubbers, ejectors, cyclone scrubbers, and spray dryers. The use of spray dryers as
absorbers is of particular interest in the removal of sulfur dioxide from hot flue gas (see
Chapter 7).

Figure 1-3. Examples of random packing elements. Courtesy of Koch Engineering
Company, Inc,