Chemical Treatment For Cooling Water

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-UTL-901

Chemical Treatment
For Cooling Water

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Process Engineering Guide:

Chemical Treatment
For Cooling Water

CONTENTS
0

INTRODUCTION/PURPOSE

2

1

SCOPE

2

2

FIELD OF APPLICATION

2

3

DEFINITIONS

2

4

PARTICULATE FOULING

2

5

CRYSTALLIZATION SCALING

2

6

CORROSION

4

6.1 Anodic Inhibitors
6.2 Cathodic Inhibitors
6.3 Combined Inhibitors
6.4 Yellow Metal Inhibitors

4
5
5
5

7

CHOICE OF COOLING WATER TREATMENT

5

8

MICROBIOLOGICAL FOULING

8

8.1 Oxidizing Biocides
8.2 Non oxidizing Biocides

9

8
8

ENVIRONMENTAL CONSIDERATIONS

9


FIGURES
1 SCHEMATIC DIAGRAM FOR CHOICE OF COOLING
WATER TREATMENT

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

7

10


0 INTRODUCTION/PURPOSE
Chemical treatment is an essential component in the avoidance of fouling in
cooling water systems. However, chemicals alone cannot correct design or
construction faults in cooling systems, nor will they work if the cooling system is
not controlled correctly or if the dosing system is incapable of delivering the
correct chemical dosage at all times. Both overdosing and underdosing of
chemicals can exacerbate fouling problems in cooling water.
1

SCOPE

This Guide covers the chemical treatments available for use in cooling systems
and the problems associated with their use. It is not a selection guide for every
cooling system since there are many factors to be taken into account.
2

FIELD OF APPLICATION

This Guide applies to process engineers and water technologists in GBH
Enterprises world-wide.
3

DEFINITIONS
No special definitions apply to this Guide.

4

PARTICULATE FOULING
For a chemical dispersant to be effective, there needs to be some
turbulence in the system. Ideally cooling water velocities of 1 to 2 m/s will
be achieved, but this may not be possible in the design of certain heat
exchangers particularly on the shell side of the exchanger. Dispersants
will only work on fine particles; particles larger than 20 om should have
been removed by filtration.
The basic mechanism of dispersion is that of charge reinforcement of the
particles to stop them coagulating in suspension and then settling out in
low flow areas.
The use of lignins or sulfonated lignins and tannins has been common for
many years for the dispersion of hydrated oxides. More recently
polyacrylates have been used to disperse silt, sand and iron oxides.
Polymaleic acid and polyacrylates have also been shown to prevent the
settling of calcium carbonate. Proprietary polymers are now available for
the dispersion of calcium and iron phosphates.


The choice of dispersant is determined by an examination of the solids
present and then by trial on line.
5

CRYSTALLIZATION SCALING
The simplest and cheapest method to prevent crystallization of calcium
carbonate is by reducing the pH with acid addition. This method also
serves to reduce calcium phosphate scaling, but can increase the
likelihood of calcium sulfate scaling if sulfuric acid is used and the
'makeup' water is high in alkalinity, since calcium sulfate scaling is not
very pH dependent.
This can be resolved by the use of hydrochloric acid. Reducing the scaling
tendency by acid addition can increase the corrosiveness of the
recirculating water, particularly if the pH is <7.0 and hence good pH
control and the use of a good corrosion inhibitor are essential if this is not
to lead to corrosion fouling. In the past the use of synergized chromate
treatments has allowed operation under these conditions, but increasing
environmental and toxicological concerns are likely to lead to a ban on the
use of chromate for cooling duties in the near future. Scale control
treatments are available which modify or inhibit the crystal growth of the
scale forming materials by adsorption on specific sites in the crystal lattice.
In this way, supersaturation of the salt is relieved by the nucleation of very
fine particles which can be easily dispersed. Such materials frequently
work at concentrations far below those expected if considering the
chemical reactions occurring, since they act on!y on specific sites on the
crystal lattice. For this reason they are known as "threshold agents".
Examples of such materials are:
(a)

Polyphosphates
Polyphosphate is a scale inhibitor at threshold levels as low as 1-5
mg/l. However it hydrolyses naturally in the recirculating water to
orthophosphate, which may then react with calcium and/or iron ions
in solution to form phosphate deposits. Thus it is only used in
systems with a relatively short half-life (<48 h) so that the degree of
reversion is small. They are now most commonly used in
conjunction with other scale inhibitors.


(b)

Phosphonates
These have the group C-PO3H2. The most commonly used
materials are hydroxyethylidene diphosphonic acid,
CH3COH(PO3H2)2, known as HEDP and aminomethylene
phosphonic acid, or AMP, (H2O3P)CH2N(CH2PO3H2)2. These
materials will not hydrolyze as easily as the inorganic
polyphosphates and can complex metal ions by sequestration.
Most phosphonates are, however, subject to some degree, to
degradation by the presence of strong oxidizing biocides, such as
chlorine. At the same time, these materials also provide some
nutrients for microbiological growth, so well controlled biocide
application is essential.

(c)

Polyolesters
These contain the group C-O-PO3H2 and can be a range of
different compounds depending on the nature of the polyol used
and how they are made. They are threshold agents.

(d)

Acrylates
These compounds contain carboxylic groups on an aliphatic carbon
chain. The most common materials in use are polyacrylates,
polymethacrylates and the polymaleics. They retard the
precipitation of calcium carbonate by distorting the crystal habit so
that the crystals do not stick together.

(e)

Phosphonocarboxylic acids
These compounds contain both C-PO3H2 and COOH groups and
act in a similar way to the phosphonates.

The selection of a scale inhibitor depends on the water quality in the
system and which species is likely to form.


6

CORROSION
An atomic study of the metal surface would reveal that it is far from the
uniform surface expected. Manufacturing defects and interfaces between
the crystalline phases of the alloy result in polarization of the atoms, with
some sites being positively charged (anodes) compared with others
(cathodes). The presence of a conducting solution results in the electrical
connection of these sites allowing corrosion to occur, with loss of metal
into solution from the anode and deposition of metallic oxides and
hydroxides at the cathodes. The rate and severity of the attack will depend
on the nature of the metal alloy, the conductivity and the pH of the
solution, the temperature, the presence of oxygen and several other
factors.
Corrosion can also be prevented by the use of specific inhibitors which
work by several distinct mechanisms such as oxidation, deposition and
filming. Corrosion inhibitors can be divided into two basic types, anodic
and cathodic. In some circumstances the two types are used in
combination to produce a synergistic effect.

6.1

Anodic Inhibitors
These work by oxidation of the anodic sites on the metal surface, thus
preventing interaction through the water and dissolution of iron ions. In
extreme conditions, anodic attack can result in severe pitting of the metal
surface, in particular if there is poor control of addition of the anodic
corrosion inhibitor. Typical examples of anodic inhibitors are:
(a)

Chromate
Used at 300 to 500 mg/l as CrO4. Generally regarded as the best
performers, but now being phased out for environmental and
toxicological reasons.

(b)

Phosphates
Used at 6 to 30 mg/l as PO4. Good performers if well controlled,
but with the risk of heavy scale formation if not. Basis of the new
generation of inhibitors to replace chromates, but even now under
environmental scrutiny.


(c)

Molybdate
Used at 8 to 12 mg/l as Mo. Not very effective by themselves, but
enhance performance of other inhibitors.

(d)

Nitrite
Used at 800 to 1000 mg/l as NO2, higher in closed loop systems, at
pH values >8.5. Excellent performance, even at temperatures up to
100°C, but subject to microbiological degradation, so rarely used in
open systems.

(d)

Silicate
Rarely used alone. Regarded as outdated, but still the basis of
many formulations.

(e)

Phosphonate
Used at 10 to 15 mg/l as active ingredient. A moderate corrosion
inhibitor, in addition to its scale inhibiting properties. The basis for
the new "all organic" treatments.

6.2

Cathodic Inhibitors
Cathodic inhibitors work by coating the cathodic sites on the metal surface
with salts which then prevent interaction with the anodic sites. Deposition
of the salts occurs naturally at the high pH conditions at the cathodes
which are the result of the formation of hydroxide ions during the first
stages of corrosion. Cathodic inhibitors are not as good as anodic
inhibitors for carbon steel and are generally used synergistically. The best
examples of cathodic inhibitors are:
(a)

Zinc
Used at 1 to 5 mg/l as Zn, but above pH 8 can only be used in
conjunction with solubilizing polymer. At pH values <7.5, the high
levels of zinc required make the treatment expensive and
environmentally unacceptable.


(b)

Polyphosphates
Used at 10 to 20 mg/l as PO4. Rarely used alone due to
degradation to orthophosphates which can result in fouling of the
metal surface.

(c)

Calcium carbonate
Naturally occurs as scale in hard waters. This is the basis of the
Langelier Index control (see GBHE-PEG-UTL-900).

6.3

Combined Inhibitors
Generally used to produce a better performance than either of the
component parts but, depending on the synergy, may bring further
constraints of pH control, etc. For example, zinc/chromate inhibitors permit
operation at chromate levels of 15 mg/l (c.f. 300 to 500 mg/l if
used alone) and zinc levels of 2 to 4 mg/l, but require pH values of 6.5 to
7.0. Various combinations are available, for example zinc/phosphate,
zinc/phosphonate, molybdate/ phosphonate, molybdate/nitrite,
orthophosphate/polyphosphate etc., but most are proprietary
blends with closely guarded formulations.
Corrosion inhibitors can also be added in combination with scale inhibitors
when the cooling water is on the border line of scale forming and
corrosive. Examples are zinc/carboxylic acids, zinc/polyolesters and
phosphonate/phosphonate.

6.4

Yellow Metal Inhibitors
The above treatments are all for carbon steel. In most cases stainless
steels will not be susceptible to corrosion except where deposits have
already formed and there are high metal temperatures in the presence of,
for example, high concentrations of chloride ions. With many treatments,
however, it is common to find a copper corrosion inhibitor which acts by
forming a film on the surface of the metal in copper, brass or cupronickel
forms, thus preventing attack, particularly in low pH conditions. Examples
of such inhibitors are mercaptobenzothiazole, benzotriazole and
tolyltriazole.


7

CHOICE OF COOLING WATER TREATMENT
Figure 1 illustrates the steps involved in deciding the correct cooling water
treatment for a given application.
In most cases there is no choice of water available for cooling. It is,
therefore necessary to be able to use the water given without any further
external treatment and modify the internal treatment to give acceptable
cooling performance. The choice of treatment is dictated by:
(a)

The raw water quality
Whether it is potable or non-potable, whether it is hard or soft, etc.

(b)

The size of the system
Whether it is possible to justify expenditure on control equipment
for acid addition or whether the system half life is short or long.

(c)

The cooling duties
For example, water flow, metal skin temperatures and water
temperature.

There are two basic strategies, as indicated in Figure 1:
(1) Adjust the pH to make the recirculating water corrosive and non-scaling in
nature and then treat it with a strong corrosion inhibitor (see Example 1).
This is used for large systems where pH control can be afforded and
where high water or skin temperatures are involved, since these can lead
to local scaling problems. It is also used where the incoming raw water
can vary in quality.
(2) Allow the water to concentrate to make it scaling, but non-corrosive, and
then treat it with a good scale inhibitor (see Example 2). This is used for
small systems where there are no high temperatures, but pH control is
considered to be too expensive, and for harder make up waters.
A third strategy is to make the recirculating water mildly corrosive and nonscaling in nature by a combination of allowing it to concentrate and/or adjusting
the pH and then treating it with a mild corrosion inhibitor plus a low concentration
of scale inhibitor. This is used for softer make-up waters where scaling is not a
major risk and for systems where the materials of construction present no major
risks of corrosion (e.g. stainless steel ).

It is essential that each case is viewed individually to ensure that the correct
balance between dosing and control costs, treatment costs and water costs have
been achieved.
FIGURE 1 SCHEMATIC DIAGRAM FOR CHOICE OF COOLING WATER
TREATMENT


8

MICROBIOLOGICAL FOULING
Cooling systems are excellent incubators for a wide variety of
microbiological organisms. Many of these are innocuous and cause few
problems directly, but can act as nutrients for other organisms. Some form
slimes which will foul the metal surface and restrict the activity of the
corrosion inhibitor. Others will cause corrosion themselves, or will produce
corrosive acids as part of their life cycle. Yet more can be harmful to
humans if allowed to thrive, although they appear not to cause fouling
problems. In short, the discovery of any significant microbiological
activity in a cooling system is an indication of poor control, although it is
impossible to operate open systems under aseptic conditions. Control can
only be achieved by the correct application of a biocide program.
Biocides used in cooling systems can be divided into two classes,
oxidizing and non-oxidizing, although non intrusive biocide treatments
such as UV light are also used.

8.1

Oxidizing Biocides
In general, oxidizing biocides are rapid acting at low concentrations, but
are not persistent. They will react with organic material in the system
indiscriminately, including some of the organic polymers added as
corrosion and/or scale inhibitors, and can be corrosive at high
concentrations. One major failing is their lack of dispersancy and their
inability to penetrate existing microbiological deposits very rapidly. They
may also be more effective in particular pH ranges.
Addition can be carried out continuously or intermittently, depending on
the choice of treatment, but the body of opinion is moving towards low
level, continuous dosing. Examples of oxidizing
biocides are:
(a) Chlorine.
(b) Bromine.
(c) Sodium hypochlorite.
(d) Chlorine dioxide.
(e) Peracetic acid.


(f) Ozone.
(g) Hydrogen peroxide.
(h) Hydantoin + chlorine or bromine.
But it is the application of biocide (frequency, duration, concentration, pH)
which is often more important than the choice of biocide.
8.2

Non-oxidizing Biocides
There is a wide range of biocides for cooling systems, each of which has
its own application requirements of pH, concentration, and time if it is to be
effective. Such biocides are frequently specific and will therefore not
provide a general kill of all microorganisms (c.f. an oxidizing biocide). They
may be rapid acting or persistent, depending on type.
Because of their nature, they are frequently used alternately, in pairs, with
additions once or twice per week, depending on system half-life, etc..
Dosing requirements are typically between 50 and 200 mg/l, which makes
treatment much more expensive than most oxidizing biocides. Their ability
to penetrate existing deposits, however, increases their value as
supplementary biocides. When nitrite is used as the corrosion inhibitor, the
use of non-oxidizing biocides is essential as it reacts with oxidizing agents
such as chlorine.
Examples of non oxidizing biocides are:
(a) Quaternary ammonium compounds.
(b) Chlorinated phenols.
(c) Methylene bis-thiocyanate.
(d) Isothiazalones.
(e) Dibromonitrilopropionamide.
(f)

Biguanides.


9

ENVIRONMENTAL CONSIDERATIONS
A critical assessment of the environmental impact of the chemical
treatment of any cooling system is needed before a decision can be made
about the treatment program. Pressure has already resulted in the ban on
the use of chromate in Europe and (shortly) in the USA. Organotins and
chlorinated phenols are now actively discouraged as biocides. Attention is
also being drawn to the discharge of zinc and phosphates, and many
biocides are, by their nature, toxic to humans and to the environment.
The inevitable result of this concern will be a move towards less toxic, but,
by their nature, less effective treatment chemicals which will require much
tighter control of dosing if they are to give comparable performance. This
will place even greater emphasis on the dosing and control
system installed and the need to keep it properly maintained.
There will also be pressure to move towards "zero discharge" of water,
bringing with it the requirement for water 're-use' and the likelihood of
worse fouling problems.
At some stage, sooner rather than later, an assessment of the "life time"
cost of fouling of cooling systems will reveal that, in many cases and for all
but the largest duties, the installation of closed, recirculating cooling
systems with indirect air or once through water cooling will be an
economic proposition.


Chemical Treatment For Cooling Water

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