Civil Engineering Reference
In-Depth Information
In-Line Blenders
In-line blenders were developed to approach instantaneous mixing of chemicals, and
it is suitable for the rapid adsorption-destabilization reactions. The
G
-value suggested
for in-line blenders is in the range of 3,000 to 5,000 sec
1
. Hudson recommended
using in-line blenders with a residence time of 0.5 sec and a water hp of 0.5 hp / mgd
(0.99 kW / ML / d) of flow.
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Kawamura prefers the use of in-line blenders and gives the following reasons:
43,44
In-line blenders provide virtually instantaneous mixing with a minimum of short-
circuiting.
•
There is no need to consider headlosses.
•
In-line systems are less expensive than more conventional rapid-mix units.
•
Types of in-line mixers include:
Custom-designed units
•
Proprietary mechanical devices, such as the Water Champ manufactured by
USFilter Stranco and Gas Master manufactured by the Master Company
•
Static mixers manufactured by several companies, including Koch Engineering
and Komax systems
•
In-line mixers may not be less expensive than other methods in all cases. A detailed
cost-benefit evaluation should be conducted to determine the best engineering solution.
Jet Injection Blending
A study by Vrale and Jorden showed that a jet injection device, which introduced the
coagulant through six holes [0.028 inch (0.71 mm) diameter], was superior to a typical
backmix reactor.
40
The jet injection system required a
G
-value of 1,000 sec
1
to
achieve the maximum particle aggregation rate. Other mixing units tested in the same
study required
G
-values of 6,000 to 9,000 sec
1
to achieve the same aggregation rate.
Chao and Stone developed a typical design for a jet injection system.
45
The result was
a
G
-value
1,000 sec
1
and a detention time of 0.55 sec.
A unit used for full-scale applications is shown in Figure 10-8. The utilization of
this type of unit is limited in practice, although it has been shown to have potential
advantages. Two disadvantages were that the orifices plugged and mixing intensity
could not be varied.
The unit shown in Figure 10-8 has design criteria of:
44
G
750 to 1,000 sec
1
•
Dilution ratio at maximum alum dose
100:1
•
Flow velocity at injection nozzle
20 to 25 ft / sec (6.1 to 7.6 m / s)
•
Mixing time
1 sec
•
The power input,
P
(ft-lb / sec or watts), for this type of flash mixer can be computed
from:
