Environmental Engineering Reference
In-Depth Information
2. Method
Chlorine is used as a biocide to control the growth of microorganism in cooling
water. When Cl
2
is added to cooling tower, following reactions can occur in water:
Cl
2
+ H
2
O ↔ HOCl + H
+
+ Cl
−
(1)
HOCl ↔ H
+
+ OCl
−
(2)
NH
3
+ H
2
O → NH
4
+
+ OH
−
(3)
HOCl + NH
3
→ NH
2
Cl + H
2
O
(4)
H
2
O → H
+
+ OH
−
(5)
where, NH
3
is ammonia, H
2
O is water, and NH
2
Cl is monochloramine (H
+
, Cl
−
,
NH
4
+
, and OCl
−
are ions).
Concentrations of these species are dependent on pH
and temperature. Other competing reactions such as formation of other chloramines
and reactions with metals, hydrogen sulfide were not accounted for in this study.
In order to calculate the concentration of various chlorine species in water, mass
balance based on the equilibrium constants was solved. Calculated distribution of
chlorine compounds in water is shown in
Table 1
. Cooling towers are operated
at a pH of 7.0 and greater. At such conditions, the amount of Cl
2
in water is
negligible. The fraction of HOCl depends on pH. Thus, chlorine emissions from
cooling tower can be emitted in the form of HOCl, not as Cl
2
and only a fraction
of chlorine that is added to water is available for striping into the atmosphere.
Table 1.
Percent distribution of chlorine compounds (as Cl) in cooling water
Cl
−
OCl
−
pH
Cl
2
NH
2
Cl
HOCl
7.0
50.0
Negligible
0.1
38.7
11.2
7.5
50.0
Negligible
0.5
25.9
23.7
8.0
50.0
Negligible
1.4
12.5
36.2
Holzworth et al. (1984a) conducted experiments at a refinery cooling tower
with a circulation rate of 60,000 gal/min that used 184,000 gm of Cl
2
per day.
HOCl emissions for this cooling tower are estimated as follows:
HOCl emissions (gm/day) = 1.48 * TCl
2
* P
HOCl
* F/100
The factor of 1.48 is the ratio of the molecular weight of HOCl to Cl, TCl
2
is
the total Cl
2
added to the cooling tower (gm/day), P
HOCl
is the percent of total
chlorine present as HOCl in water (%), and F is the flash-off factor for HOCl (-).
While the average pH of water at this cooling tower was 8.2, other cooling
towers can be operated at lower or higher pH. For this study, a pH of 7.5 is used to
estimate emissions. At this pH, only 25.9% of the total chlorine is present as
HOCl in water. Flash-off factor determines the fraction of chlorine that can be
stripped from the cooling tower into the atmosphere. Holzwarth et al. (1984b)
determined flash-off factors for HOCl in laboratory. The highest flash-off factor
was 0.1 which is used in this study to estimate the maximum possible emissions.
Using these values, estimated HOCl emissions from this cooling tower are about
