POWER May 2013 - 82

WATER TREATMENT
2. Bleach and bromine dissociation
are very dependent on pH.
Source: Preventive Macrofouling Strategies
for Nuclear and Fossil Fired Power Plants, Edward
W. Ekis, Jr., and Michael G. Trulear, Nalco
Chemical Co.
HOCI
80
60
40
20
4 5 6 7 8 9 10 11
pH
particularly during the hottest months of
the year.
Understanding how the bleach and bromine
chemistry control organics is important.
As bleach is fed to water, it dissociates
into hypochlorous acid and sodium hydroxHOBr
20
40
60
80
100
ide,
NaOCl + H2
O→HOCl + NaOH. The
hypochlorous acid further dissociates into
hydrogen and hypochlorite ion, HOCl ↔ H+
+OCl-
, which is highly dependent on the pH
of the water. According to the FILMTEC Reverse
Osmosis Membrane Technical Manual
published by Dow Water & Process Solutions,
" The germicidal efficiency of free
residual chlorine is directly related to the
concentration of undissociated HOCl. Hypochlorous
acid is 100 times more effective
than the hypochlorite ion OCl-
. The fraction
of undissociated HOCl increases with decreasing
pH " (Figure 2).
In practical terms, this means that as the
pH increases, the amount of free available
HOCl decreases. For example, a pH of 8.0,
which is typical for cooling tower applications,
leaves approximately 20% HOCl, with
80% as OCl-
. Therefore, it takes considerably
more bleach at elevated water pH to maintain
a specified TRC. Said another way, the
amount of bleach needed to control microbiological
activity increases as the water pH
increases.
Furthermore, when bleach reacts with organics,
considerably less-effective biocides
called chloramines are formed. Chloramines
are derivatives of ammonia by substitution of
one, two, or three hydrogen atoms with chlorine
atoms. The possible reactions proceed as
follows:
HOCl + NH3
HOCl + NH2
HOCl + NHCl2
- → NH2Cl + H2
Cl → NHCl2
→ NCl3
+ H2
O
+ H2
O
O
On the other hand, bromine chemistry is
appreciably different than bleach. Because
bromine is typically used in conjunction with
bleach, HOCl + NaBr → HOBr + NaCl.
As with hypochlorous acid, the hypobromous
acid (HOBr) will dissociate into the hypobromite
ion (OBr-
is also pH dependent: HOBr ↔ H+
.
), which, as with bleach,
+ OBr-
However, the percentage of available
HOBr is much higher at elevated pH relative
to available HOCl. Recall in the previous
example, that at a pH of 8.0, there is
approximately 20% HOCl, with 80% as the
less-biocidal OCl-
is approximately 80% HOBr, with 20% as
OBr-
. At that same pH, there
, a marked improvement.
In addition to dissociation advantages,
there are also two other notable benefits to
using bromine chemistry: The short-term
biocidal activity of OBr-
is very similar to
HOBr, and any bromamines formed have
Enhanced PDF version now available
The 2013 UDI Who's Who Directory covers more than 4,200
U.S. and Canadian generating plants. The directory provides:
* Nearly 8,500 plant management and support
contact names, titles, and primary job functions.
* Basic plant operating statistics for more than 1,500
power stations, including:
Generation (MWh)
Availability (%)
Heat rate
Capacity Factor (%)
* Power plant design characteristics
Choose which purchase option that best suits your needs:
* Hardcopy Book
* Enhanced Directory PDF (CD-ROM) - NEW FOR 2013
* Mailing List (CD-ROM), Enhanced Directory PDF & Hardcopy Book
UDI WHO'S WHO AT ELECTRIC POWER PLANTS
For more detailed information and a list of all available data, visit us online at UDIDATA.COM or contact the UDI Editorial team at UDI@Platts.com.
82
www.powermag.com
POWER | May 2013
Un-ionized acid (%)
Ionized acid (%)

http://www.UDIDATA.COM http://www.powermag.com

POWER May 2013

Table of Contents for the Digital Edition of POWER May 2013