Chemical Engineering August 2014 - 58

Cooling tower
CyCles of ConCentration (CoC)
Environmental Manager
changing NPDES regulations, consider
the changes required to meet
the new guidelines at a power plant
operated in the Southern U.S. Prior
to 2013, the plant's NPDES permit
primarily focused on pH and residual
oxidanta. However, the new
permit now imposes an average
monthly limit of 1,200 mg/L TDS.
Given that the TDS concentration
of the makeup water sometimes
reaches 400 mg/L, the tower cycles
of concentration (COC) may be limited
to three under the new regulations,
whereas previously the tower
was allowed to operate at a significantly
higher COC (See the sidebar,
right) for a brief discussion of the
COC concept).
Another impurity that now receives
more scrutiny is sulfate
(SO4). Managing sulfate can be particularly
problematic with regard
to the overall process chemistry
of properly managed cooling towers,
as sulfuric acid is commonly
added to cooling-tower makeup to
remove bicarbonate alkalinity and
thus minimize calcium carbonate
(CaCO3) scale formation in the
condenser and cooling system. The
common treatment step follows this
reaction pathway:
H2SO4 + Ca(HCO3)2 → CaSO4 +
2H2O + 2CO2↑
However, tighter regulations on
sulfate in the discharge stream
may curtail or eliminate this common
and straightforward method of
scale control at some plants.
On a related note, phosphorus is
also being banned in many waste
streams [1]. Phosphorus serves as
a nutrient that encourages plant
growth. When released to open bodies
of water, excess levels of phosphorus
can initiate and propagate
toxic algae blooms. The challenge for
tower owners is that organic and inorganic
phosphates are widely used
for corrosion and scale control in
cooling-water systems. To meet this
challenge, a variety of all-polymer
programs have emerged for corrosion
and scale control, to minimize
phosphate use.
As has been noted, some heavy
metals are also on EPA's proposed
n a cooling tower, warm water from condensers or other heat exchangers is sprayed
or is allowed to fall through uprising ambient air. Typically 65-80% of cooling is accomplished
by evaporation of perhaps 2-3% of the circulating water into the air. At
atmospheric conditions, the latent heat of evaporation is roughly 1,000 Btu/lb - so
much heat is transferred by evaporation. As the water evaporates, minerals are left
behind. Thus, the dissolved-solids concentration of the circulating water continually increases
during tower operation. The cycles of concentration (COC) is simply the ratio of
the dissolved solids concentration in the circulating water compared to the concentration
in the makeup water. For example, if the circulating water has a chloride concentration
of 250 ppm, and the makeup has a chloride concentration of 50 ppm, the COC is 5.0.
Unlimited COC is not possible, as eventually, the concentrating effects of evaporation
I
will lead to scale formation by some of the minerals. So, periodically a portion of the
circulating water is " blown down " to purge the system of dissolved solids and replenish
the system with fresh makeup water. Very common is automatic blowdown based on
continuous measurement of an easily analyzed property, such as specific conductivity.
The relationship between blowdown volume and COC is outlined in standard cooling
tower texts. It is represented by the fundamenal equation:
BD = E/(COC-1)
Where:
BD = Blowdown rate, gal/min
E = Evaporation rate, gal/min
Thus, higher COC equates to lower blowdown rate. This can be very important, particularly
if the blowdown must be minimized for discharge purposes or to conserve water.
At low COC, any increase in the cycles of concentration greatly reduces the blowdown
volume. This effect diminishes at higher COC values.
❏
upgraded NPDES list, with primary
examples being zinc and chromium.
State regulations may impose other
limits. For the plant mentioned
above, the expectations are that
copper discharge will, by 2015, be
limited to less than 30 parts-perbillion
(ppb). According to Ref. 2,
copper limits are as low as 12 ppb in
some parts of the U.S. At these very
low limits, copper discharge can
potentially be a problem for units
equipped with copper-alloy condenser
tubes. However, additional
sources of copper, which often affect
older wooden cooling towers, are the
copper compounds that are used as
wood preservatives. One possible
solution is to replace older cooling
towers with modern, fiberglass towers.
Another possibility is to install
a wastewater-treatment plant that
includes a precipitation step to remove
heavy metals.
These examples underscore the
fact that at existing plants, the costs
to comply with new liquid discharge
guidelines may be significant. For
instance, a switch to all-polymer
chemistry in a large cooling tower
to avoid strict limits on COC, or
to eliminate phosphate in the discharge
may result in annual cost
increases in the six figures. The
capital cost to install a treatment
system to remove newly regulated
58 ChemiCal engineering www.Che.Com august 2014
impurities from the discharge can
easily reach or exceed $1 million.
Plus, the addition of waste-treatment
systems adds complexity and
operational costs to the plant over
the lifecycle of the facility.
Moving to ZLD
In addition to the impurities mentioned
above, there is always the
possibility that additional wastewater
contaminants could be regulated
in the future. For this reason, some
experts recommend that plants
consider a zero-liquid discharge
(ZLD) process at the beginning of
the project. However, ZLD is often
rather complex. Perhaps the most
" straightforward " ZLD disposal
technique - albeit with a large
caveat - is deep-well injection.
The wells would have to be several
thousand feet deep to avoid the possibility
of the discharge stream ending
up in the shallow groundwater
sources that are used for residential
purposes. While this concept sounds
simple, experience has shown that
some wastewater streams can generate
scale within the well shaft,
particularly as the water temperature
rises further underground.
High-pressure is generally required
for this process, and if scale formation
occurs, then capacity may decrease
as the piping becomes more

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Chemical Engineering August 2014

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