Chemical Engineering September 2018 - 43

Low-pressure flow
Restricted flow,
increased pressure
FIGURE 5. Slime buildup in the
system can affect flow turbulence
and pressure
do more with less falls on everyone's
shoulders, so having a reliable solution
and the additional expertise available to
support process operations are more
critical than ever. Bleach is a common
microbial control chemistry and is effective
with constant and consistent delivery.
This requires a continuous supply of
bleach moving around your facility to the
appropriate application points. Bleach
has a relatively short shelf life, especially
under hot conditions, which adds to the
need for frequent deliveries. Suppliers
of bleach and chlorine gas tend to be
commodity chemical suppliers who do
not focus on unique situations or custom
applications that arise frequently in
cooling water systems.
On the other hand, chlorine dioxide
FIGURE 6. Corrosion rates and corrosion
pit depths are different with the three
chlorine-based biocides, as shown in the
bar graph
10
9
8
7
6
5
4
3
2
1
Chlorine
(CI2)
42
Hypochlorite
(NaCIO)
requires on-site generation equipment
that must be properly maintained to
consistently and reliably deliver chemistry
to water systems. Established chlorine
dioxide suppliers have the expertise
necessary to evaluate your application
in order to survey, size and select the
appropriate generation equipment for
the needs of a particular process, and
to apply the chemistry under the appropriate
dosage scheme. The onsite
generation of chlorine dioxide also enables
the optimization of production
throughput, as the system can accommodate
fluctuations in production levels
that may require variable production of
chlorine dioxide.
Sustainability. All biocidal programs
impact the sustainability of your operation.
Energy-use and chemical-use
cost savings are possible with bleach,
but are greater with chlorine gas, and
most significant with chlorine dioxide.
This is mostly due to the reduction of
biofilms, which greatly impact the efficiency
of heat exchangers. In addition,
there will be a decrease in energy
consumption by cooling water pumps,
as flow restrictions due to clogging are
removed. Water savings are greatest
for cooling water systems when the
cycles of concentration can be maximized.
Chlorine dioxide is superior for
this driver due to the limited production
of chloride ions.
System A
System B
Chlorine
(CIO2)
Specific considerations
Assembled here are five additional considerations
for use of chlorine-based
microbial-control strategies in industrial
cooling water applications.
pH. At a pH of 6.0-7.0, bleach has
similar efficacy to chlorine dioxide,
and is often used as a microbial control
chemistry. Less chlorine dioxide
is required to effectively reduce bacteria
counts when compared to other
chlorine-based programs at higher pH.
Chlorine dioxide is effective across a
broader pH range compared to chlorine
gas and bleach. Figures 2 and 3
compare the equivalent dosage of ClO2
per 100 pounds of NaOCl required to
treat water at different pH ranges with
the greatest benefit being seen at the
upper end of the pH range.
Contaminant effects. Chlorine dioxide
is less affected by pH and contaminants
that create demand, such as oil,
grease and ammonia. In addition to an
overall lower rate of use, this means
that the pH impact is minimized,
thereby maximizing the microbial impact
of the solution.
Figure 4 shows the effect of organic
demand and pH on the free oxidant available
for microbial control. The volume of
bleach needed to maintain a similar level
of free or available oxidant compared to
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
SEPTEMBER 2018
Corrosion severity
http://WWW.CHEMENGONLINE.COM
Chemical Engineering September 2018

Table of Contents for the Digital Edition of Chemical Engineering September 2018
