Page 637 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

592 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

example, turbines and brush rotors. The latter are used in each. Within each reactor category, a variety of proprietary
shallow ditches <2m (6 ft) depth with a race-track shape. equipment is available, for example, bubble columns may be
Submerged aerators include diffusers of various materials configured with coarse bubble or fine bubble diffusers; sur-
and shapes, jet aerators (pressurized air introduced into a face aerators include turbines, brush rotors, etc.; stripping
water jet), static mixers, and a combination coarse bubble towers may be packed with any of a range of shapes and
diffuser combined with a submerged turbine to shear and materials (Lee and Tsui, 1999, p. 25).
disperse the rising bubbles (Huang, 1975). Air stripping
equipment is also varied, with packed towers being most 18.3.2.2 Turbine Aerators
common. Turbine aerators have been used since the 1960s when they
seemed to find favor over diffused bubble aeration. About
18.3.2.1 Reactor Types 1980, fine bubble aeration technology had improved and
Table 18.6 shows a sampling of reactor types used in gas evaluations established its economy as compared with surface
transfer, along with K L a values and notes on characteristics of aeration. Turbine aeration is reviewed here partly because it is

TABLE 18.6
Types of Reactors for Gas Transfer
1
Type K L a (s ) Notes
Bubble columns 0.005–0.01 Low to moderate mixing intensity
Mixing is by drag of gas bubbles
High u(water)
Gas bubbles move as plug flow
Simple construction, low capital cost
No moving parts, low maintenance
Gas bubbles may coalesce for height=diameter > 12
Turbine mixed tanks 0.02–0.2 Impeller type, size, speed give control of mixing intensity
Large range for u(water)
Back-mixing
Solids remain in suspension
Packed columns 0.005–0.02 Low Dp(gas)
Packing is basis for interfacial area
0.4–0.6 fraction of column is packing
Not suitable for suspensions
Flooding criterion must not be exceeded
Plate=tray columns Higher u(water) than packed columns
Large number of transfer units possible
Complete mixing for each tray; that is, similar to CFSTR’s in series
Can tolerate suspensions
Higher capital costs
Spray tower 0.0007–0.015 Water is the dispersed phase; gas is the continuous phase
Suitable for uptake of highly soluble gases
High Dp for atomizing water
Rapid coalescence of water droplets away from nozzle
Jet-loop 0.01–2.2 Jet of water or gas=water mixtures are injected into a column
Jet causes mixing
Higher K L a than bubble column alone
Variable degrees of gas induction
High power required
Venturi ejector 0.1–3 Unit is a Venturi section in a pipe
Gas injection is in the high velocity (i.e., low pressure) section
Very high mixing intensity
High K L a
Low u(water); therefore, V(reactor) is low

Source: Adapted from Lee, S.Y. and Tsui, Y.P., Chem. Eng. Prog., 95(7), 24, July 1999.
Note: CSTR, continuous flow stirred tank reactor.