Note: Descriptions are shown in the official language in which they were submitted.
1233722
"METHOD OF CONTROLLING SCALE IN PRESSURlSED BOILERS"
~ This invention relates to the treatment of a~ueous
: systems used in pressurised boilers.
The water used in steam generating boilers, cooling
:~ ~ towers, desalination units and other industrial aqueous
! 5 systems contains various impurities. The impurities
typically include alkaline earth cations, principally
; ~ calcium and:magnesium, and several anions including
: ~ bicarbonate, carbonate, sulphater oxalate, phosphate,
silicate and~fluoride~ The most common impurities in
~ 10 industrial water supplies are the water hardening calcium,
: : magnesium and carbonate ions although sulphate is usually
~ ~ also present. The salts of these metal ions, especislly
,
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the carbonates, tend to precipitate forming solid
accumulations on the surfaces of the system and these
accumulations can give rise to the formation of scale on
hot surfaces. The water may also contain various solids
such as mud, clay, iron oxides, silt, sand and other
mineral matter as well as microbiological debris which
accumulate as sludge in the system. Naturally sludge and
scale deposits greatly reduce heat transfer efficiency by
settling at low flow points in the system and thus
limiting the circulation of the water and insulating it
from the heat transfer surfaces. In addition, corrosion
of metal surfaces under the deposits is facilitated since
corrosion control agents are unable to contact the
surfaces effectively. Again, the deposits harbour
bacteria. Removal of the deposits can cause expensive
delays and shutdown of the system.
Since prior treatments such as softening, coagulation
and filtration do not adequately remove solids and solid
forming substances various chemicals have been used to
counteract the adverse effects of scale and sludge in
aqueous systems. In cooling water systems and
desalination plants the chemicals are commonly added in
order to increase the threshold at which precipitation
occurs; it has also been thought that the chemical forms a
25 film on the hot surfaces where scale formation is likely
....
~L;233~
to occ~r thereby preventing scale forming material from
adhering to the hot surfaces in question. A variety of
different chemicals have been used for this purpose
including pGlycarboxylates and other soluble, polar
polymers such as acrylate and methacrylate polymers. The
presence of small quantities of these polymers can have a
marked effect on the system.
Although these various chemicals are quite effective
in industrial cooling towers and desalination plants and
the like quite different problems arise in water which is
used in pressurised boilers due to the much higher
temperatures involved; the boiler point of water at the
lowest pressure normally used, 80 psig, is already 324F
(about 162~C)~
Because of the greater problems involved, the water
: used for such pressurised boilers is first subjected to a
deioniser or to base exchange, the former being the more
effective~ Even so these treatments are not always f~lly
effective with the result that some calcium and magnesium
ions, in particular, remain in the boiler feedwater along
with their associated anions.
In view of the higher temperatures involved, it is not
practical to try and keep the calcium in solution as is
: the case with cooling water; an exception to this is the
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use of chelants but these present some other problems of
application in particular correct dosing. Likewise, it is
much more difficult to prevent the solid deposits from
contacting the very hot surfaces thereby giving rise to
scaling.
As a result, different techniques have to be adopted
for the prevention of scale in pressurised boiler systems.
For boilers which are subjected only to low (up to,
say, 150 psig) pressures it is not unusual to add a water
soluble carbonate to the feed water in order to cause all
the calcium present to precipitate as carbonate and to add
also a dispersant to prevent the precipitated material
from settling on the hot surfaces. This is, of course, in
complete contrast with the situation with cooling water
where the aim is to keep the carbonate concentration to a
minimum and to keep that concentration in solution.
Calcium sulphate is very much more soluble than calcium
carbonate so that it does not present a significant
problem in cooling water and desalination systems. In
pressurised boilers, however, scaling due to calcium
sulphate can occur so that it is desirable to ensure that
all the calcium is precipitated.
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Inevitably~ however, dosing with carbonate is not
fully effective with the result that some scale does still
form. Over a period, scale can accumulate to such an
extent that the hoiler has to be shut down and the scale
removed. This is essential since scale deposits can cause
localised overheating and even rupture in the boiler.
With moderate (e.g. 15D to 600 psig) or high (above
600 psig) pressure ~oilers, dosing the feed water with
carbonate is not satisfactory since at the water boiler
point under these pressures the carbonate will tend to
decompose giving rise to carbon dioxide; at these
temperatures carbon dioxide has a very corrosive effect on
the metal surfaces. Accordingly, for such boilers and,
indeed, for low pressure boilers as well (instead of
carbonate addition) it is usual to add a soluble
phosphate, typically sodium phosphate eOg. disodium
phosphate, or trisodium phosphate to the feed water,
although potassium phosphate and other phosphates
including polyphosphates e.g. sodium hexametaphosphate and
fluorophosphates can also be used. This ensures that all
the calcium present precipitates as calcium phosphate
which is then dispersed with a dispersant as before. This
material can be removed periodically, as in the other
systems, with the water drained from the boiler by
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~'33~22
blowdown where the sludge containing boiler water is
removed through a valve by rapidly reducing the pressure
within the boiler. Nevertheless, not all the calcium
phosphate i5 removed in this way with the result that a
scale of calcium phosphate forms which, in due course, has
to be removed after shutting down the boiler.
In practice, one adds more than the stoichiometric
amount of phosphate needed to reach with the calcium in
the water. The aim is to add sufficient phosphate to give
an exces~ at all times, e.g. 10 to 20 ppm in the boiler;
the excess required can be established by consulting
authoritative guidelines such as British Standard 2486.
Such standards also specify the degree of alkalinity
needed, generally a pH of 9.5 to 12. This alkalinity is
needed for seYeral reasons. First, by keeping the pH
sufficiently high one ensures that all the calcium
phosphate precipitates as hydroxy apatite, a basic calcium
phosphate which is easy to condition and also ensures low
solubility of the calcium phosphate. Second, alkaline
conditions prevent corrosion. Third, such a pH ensures
that any magnesium ions present are precipitated as
magnesium hydroxide.
This illustrates a further difference between the way
in which one tackles scale in cooling water and
desalination systems, on the one hand, and pressurised
boiler systems, on the other. In cooling water systems,
~L233722
scaling due to magnesium is seldom significant because the
magnesium stays in solution in the water which rarely
exceeds a temperature of about 50C. In desalination
systems magnesium scaling does become a significant
problem because temperatures rise to 100C and because
salt water contains a much higher magnesium content than
does ordinary industrial water and, as a result, special
steps have to be taken~ In effect the magnesium
bicarbonate is first converted to carbon dioxide and
magnesium carbonate which is hydrolysed by the hot water
to magnesium hydroxide; due to the high magnesium content
the solubility product is exceeded with a result that it
starts to come out of solution. In contrast to the
situation with pressurised boiler feed water, salt water
is not significantly alkaline with the result that the
magnesium hydroxide has a greater tendency to stay in
solution. At a pH of 9.5 to 12, however magnesium
hydroxide will precipitate even in the cold so that at the
high temperatures involved in pressurised boilers there is
usually no chance of scaling due to magnesium hydroxide.
It is also of interest to note that in cooling water
systems it is not uncommon to add acid (rather than
alkali) in order to try and keep more calcium carbonate in
solution.
3~
It has now surprisingly been found that certain
specific sulphonate copolymers are effective for
controlling the formation of scale in pressurised
boilers~ It will be appreciated that by ~pressurised
boilers~ we mean boilers operating at a pressure of at
least 50 psig, generally at least 80 psig, typically 80 to
150 psig (low pressure), generally 150 to 600 psig
~moderate pressure) and above 600 and up to, say 2000 psig
(high pressure). In such boilers the water will be at its
boiling point which will vary from about 298F at 80 PSIG,
to about 324~F at 80 PSIG, to about 366F at 150 PSIG, to
about 489F at 600 PSIG to about 637F at 2000 PSIG.
Furthermore, it has been found that these particular
copolymers provide the very real practical advantage that
they will, in factl when present in the boiler water,
actually remove scale which is already presentO In other
words, these specific copolymers have an "in service"
cleaning effect. This cleaning effect i5 not specific to
the particular scale which has been deposited from the
feed water currently in use; in other words the copolymers
will also remove scale which may have formed from previous
operation of the boiler using a different feed water.
Thus if a boiler has been operated incorrectly by allowing
the composition of the feed water to vary without adequate
controls and, as a result calcium phosphate scale allowed
33~22
to form, that is there is old scale present, regardless of
the additive which may have been used, then by dosing the
feed water with the specific copolymers used in the
present invention the scale can be removed while the
boiler is operating under load.
It should be added that this surprising in service
cleaning effect is only observed when using the high
temperatures involved in pressurised boilers. Thus, these
same copolymers are not effective for removing scale from
c~oling systems even though they may be effective in
~ inhibiting the formation of fresh scale.
According to ~he present invention there is provided a
method of controlling scale in a pressurised boiler which
comprises adding to the boiler at least a scale
controlling amount of a copolymer which possesses
recurring units of the formula
- CH - CH-
COOH COOH
; : and the formula
~ -CH2- CH -
z
: 20 where Z represents -SO3H or -CH2SO3H, or a water
soluble salt thereof, typically an alkali metal salt such
as the sodium salt, although potassium, ammonium, zinc and
lower amine salts and other water soluble salts may be
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used. The free acids may also be used and all of the acid
hydrogens need not be replaced nor need the cation be the same
for those replaced. Thus, the cation may be any one of or a
mixture of NH~, E, Na, K. etc. In another aspect the present
invention provides a method of removing scale from a scaled
pressurised boiler which comprises adding to the aqueous
system of said boiler a scale removing amount of the
specified copolymer. The invention further provides a
composition suitable for addition to a pressurised boiler
water system which comprises a copolymer which possesses
xecurring units of the formula
- C~ - CH -
COOH COOH
and of the formula
- CH2 - CH -
where Z represents ~SO3H or -CH2S03H, or a water soluble
salt thereof, and a water soluble calcium precipitating agent.
The copolymers are conveniently prepared by polymerising
maleic acid or anhydride or fumaric acid with the vinyl or
allyl sulphonic acid or an alkali metal salt thereof using
conventional procedures. Thus con~entional addition
polymerisation methods using light or free radical initiators
may be employed. Generally, the copolymerisation may be
effected at from about 30 to about 120C using a peroxide
catalyst such as hydrogen peroxide or ben20yl peroxide in an
inert medium. The copolymer may be derived, for example,
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by solution polymerisation of maleic acid and sodium allyl
sulphonate in the presence of hydrogen peroxide. The salts
can of course be obtained using conventional methods.
The relative proportions of sulphonate and carboxylate
components depend on some extent upon the degree of
scaling to be treated. The copolymer generally contains
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from about 10 to about 80 mol per cent of sulphonate
moieties and correspondingly rom 90 ~o abou~ 20 mol
percent of the carboxylate moieties. Preferably, the
sulphonate moieties comprise about 25 ~o about 75 mol per
cent of the copolymer and the carboxylate moieties
comprise from about 75 to about 25 mol per cent. For the
vinyl sulphonate copolymer, the sulphonate moieties
especially comprise about 50 to about 75 mol per cent of
the copolymer and the carboxylate moieties from about 50
to about 25 mol per cent.
The average molecular weight of the copolymer is not
critical 50 long as the polymer is water soluble.
Generally, the weight average molecular weight ranges from
about 500 to about 100,000. The molecular weight is
preferably from about 800 to about 25,000 and especially
is from about 1,000 to about 15,000. A copolymer having a
mol ratio of maleic acid or anhydride to allyl sulphonic
acid of about 1:1 and a molecular weight of about 2,000 is
especially preferred. Other preferred copolymers include
those havin9 a mol ratio of maleic acid or anhydride to
vinyl sulphonic acid of about 1:1.5 or about 1:3, and a
molecular weight of about 7,000 to 9,000. Although the
best results are generally obtained with the 1:3 mole
ratio, in practice because of the relatively high cost of
the vinyl sulphonic acid, a mole ratio 1:1.5 is generally
preferred eYen though the results are not quite so good.
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It will be appreciated that it will sometimes be
simpler to dose the feed water simultaneously with the
specific copolymer and the water soluble carbonate,
typically sodium carbonate, or phosphate such as those
specified above or other hardness precipitating agent as
appropriate to the temperatures and pressures to be used
in the boiler; the pH will normally be adjusted if
necessary to 9.5 to 12, preferably 10 to 11. This pH can
be achieved by maintaining the recommended alkalinity
value for the particular boiler employed by adding
appropriate quantities of caustic soda. This alkalinity
value can be determined using well known methods, such as
by titration against standard acid. Accordingly, the
present invention also provides a composition suitable for
the addition to a pressurised boiler water system which
comprises a copolymer having recurring units of the
formulae set out above and a water soluble hardness
precipitating agent. Typically the copolymer is added as
an aqueous solution generally containing 0.1 to 50%,
preferably 2.5 to lD~, especially 3 to 5~ by weight
(active) of the copolymer. The amount of hardness
precipitating agent in the solution is suitable from 5 to
50% (or solubility limit) preferably from 15 to 35~,
especially 25 to 35~, by weight. Thus the relative weight
proportions of the copolymer and hardness precipitating
~;33'~
agent are suitably from 0.1:50 ~o 10:1, preferably from
1:15 to 2:3, especially from 1:11 to 1:3.
Of course other conventional additives can also be
incorporated in the water including alkalis, lignin
derivatives, other polymers, tannins, biocides and
corrosion inhibitors.
The copolymer may be introduced at any location where
it will be quickly and efficiently mixed with the water of
the system although it will generally be most convenient
to add it to the make-up or feed water lines through whish
the water enters the boiler. Typically, an injector
calibrated to deliver a predetermined amount periodically
or continuously to the make-up water is employed.
The amount of the copolymer added to the water is a
15 substoichiometric amount that is-effective to control,
i.e., inhibit and remove the scale and naturally this
depends on the nature of the aqueous system to be treated,
especially its calcium content. The amount depends to
; some extent on the concentration o~ suspended solids and
20 existing levels of solids build up in the system.
Typically, amounts from about 0.1 to about 400 ppm,
preferably from about 1 to about BO ppm and especially
from about 5 to about 50 ppm active in the boiler water
are used. In general, as the amount of precipitating
25 agent needed for the calcium increases so does the amount
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of copolymer. Typically for an especially preferred
composition the amount of composition added will be about
20 to about 2500 ppm.
The following Examples further illustrate the present
invention. In these Examples the properties of the
additives were evaluated in a small laboratory boiler
which had three removable tubes as described in the
proceedings of the 15th Annual Water Con~erence, Engineers
Society of the Western Pennsylvania, pages 87-102 (1954).
The feed water for the laboratory boiler was prepared by
diluting Lake Zurich, Illinois, tap water with distilled
water to 40 ppm total hardness as CaCO3 and adding
calcium chloride tD provide a 6:1 elemental calcium to
magnesium ratio. The feed water and chemical treatment
solutions were fed to t~e boiler in a ratio of 3 volumes
of feed water to 1 volume of solution giving a feed water
total hardness of 30 ppm CaCO3. The-scaling tests for
all the treatment solutions were conducted by adjusting
boiler blow-d~wn -to 10~ of the boiler feed water giving an
20 approximately 10 fold concentration of the boiler water
salines and adjusting the composition of the treatment
solution to give a boiler water after the 10 fold
concentration haYing the composition shown in Table I.
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TABLE I
Sodium Hydroxide as NaOH 258 ppm
Sodium Carbonate as Na2CO3 120 ppm
Sodium Chloride as ~aCl 681 ppm
Sodium Sulfite as Na2SO3 50 ppm
Sodium Sulfate as Na2SO4 819 ppm
Silica zs SiO2 less than 1 ppm
Iron as Fe less than 1 ppm
Phosphate as PO4 10-20 ppm
In the first series of tests, the boiler was run for
45 hours at a boiler water pressure of 400 psigO At the
completion of each test, the boiler tubes were
individually removed from the boiler and the scale or
deposit present on 6" of the central length of each tube
was removed by scraping, collected in a tared vial, and
weighed. The results obtaioed are shown in Taùle II.
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TABLE II
Run Additives Active Scaling Scale
No. Dosage Rate Reduction,
in the g/ft2/hr
Boiler Water
ppm
1 None - 0.213
2 Allyl sulphonic 5 D.063 ~0.4
acid and maleic
acid copolymer ~1 1; wt.av.M.W. = about 2,000)
3 Sodium vinyl 5 0.046 78.4
sulfonate and
maleic acid
copolymer (1.5:1; wt.av.M.W. = about 7,000 to 9,000)
4 Sodium 10 0.094 55.9
vinyl sulfonate
and fumaric acid
copolymer (1.5:1; wt.av.M.W. = about 7,000 - 9,000)
Allyl sulphonic 10 0.006 97.2
acid and maleic
acid copolymer
(as in Example 2)
6 Sodium vinyl 10 0.014 93.4
sulfonate and
maleic acid
copolymer (as in
Example 3)
These results show that the copolymers used in the present
invention are effective ~or reducing the rate of scaling in a
pressurised boiler.
In a second series of tests, the system was first run for 45
hours without any addition of polymer and then one of the three
tubes was taken out and replaced by a clean tube. The system was
then run for a further 45 hours but this time with polymer added.
After this further period of 45 hours the scale in the three tubes
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is weighed as before. Thus comparison of the results of this test
with those of an untreated blank (no polymer added during the
second 45 hour period) enables one to assess whether the polymer is
capable sf r~moving scale and also preventing the formation of
scale on a clean tube. The results obtained are shown in following
Table III.
TABLE III
Run Additives Active Dosage Scale
No. in the Boiler Reduction
Water, ppm %
7 Allyl sulphonic 30 112.0
acid and maleic acid
copolymer (as in
Example 2)
8 Sodium vinyl sulfonate 30 108.8
and maleic acid copolymer
~as in Example 3)
9 Sodium vinyl sulfonate and 30 123.9
maleic acid copolymer
(3:1; wt.av.M.W. = about 6,000)
It is clear therefore,-that the use of these polymers is
effective not only in inhibition of scale formation but also, since
the scale reduction is greater than 100~, in removing existing
scale.
In order to assess the inservice cleaning ability of the
copolymers in a coolln~ water system a clean metal heater was fixed
in a glass t~be assembly through which water heated to 60C. was
circulated by means of a pump. The assembly formed part of a
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closed system provided with an expansion tank open to the
atmosphere. The heater was removed and placed in dilute acid to
~ remove the scale thereon. The acid solution was then titrated with
; a standard EDTA solution to determine the amount of calcium
carbonate scale (as Ca ) from which the weight of calcium
carbonate scale was determined.
In the first test, a synthetic water adjusted to 400 ppm of
calcium and 400 ppm alkalinity (as bicarbonate) was circulated for
6 hours. On weighing the heater it was found that 780 milligrams
1~ Of calcium carbonate scale had formed.
The test was then repeated, after 6 hours 10 ppm of a
copolymer of maleic acid and allyl sulphonic acid (as in Example 2)
was added and circulation continued for 45 hours. ~he heater was
then removed and tested as above; it was again found that 7B0
1~ milligrams of calcium carbonate scale had formed. It is apparent,
therefore, that the copolymer did not remove any scale under these
conditions.
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