Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to the treatment of aqueous
systems and, more particularly, to the inhibition and
removal of solid deposits in industrial heating and
cooling systems.
The water used in indust~ial aqueous systems such as
steam generating boilers, hot water heaters, heat
exchangers, cooling towers, desalination systems, cleaning
systems, pipe lines, gas scrubber systems, and associated
equipment contains various impurities. The impurities
typically include alkaline earth cations such dS calcium,
barium, and magnesium and several anions such as
bicarbonate, carbonate, sulfate, oxalate, phosphate,
silicate, and fluoride. These anions and cations combine
and form preclpitates due to the pH, pressure, or
tempeeature in the system or the presence of additional
ions with which they form insoluble products. The most
common impurities in industrial water supplies are the
water hardening ions such as the calcium, magnesium and
carbonate ions. In addition to precipitating as
carbonates, calcium and magnesium as well as any iron or
copper present can also react with p`nosphate, sulfate, and
silicate ions and form the respective complex insoluble
salts. These solid reaction products accumulate on
surfaces of the system and form scale. The water may also
contain various solids such as mud, clay, iron oxides,
silt, sand, and other mineral matter and microbiological
debris that accumulate as sludge deposits in the system.
Iron oxides may be present in the feedwater and may be
produced by corrosion of metal surfaces in contact with
the water. The sludge may become incorporated in the
scale deposits and the precipitates tend to cement the
sludge particles and form a strongly adherent scale.
Sludge and scale deposits greatly reduce heat transfer
eEficiency by settling at low flow points in the system
and limiting the circulation of the water and insula-ting
it from the heat surfaces. In addition to interfering
with heat transfer and fluid flow, corrosion of metal
surfaces underneath the deposits is facilitated since
corrosion control ~gents are unable to contact the
surfaces effectively. The deposits also harbor bacteria.
Removal of the deposits can cause expensive delays and
shutdown of the system. Water at the relatively high
temperatures in steam generating boilers and hard waters
are e~pecially susceptible to scale formation. Extremely
severe scale deposits can cause localized overheating and
rupture in boilers.
Since external treatments of the feedwater to
industrial systems such as softening, coagulation, and
filtration do not adequately remove solids and
solid-forming substances, various internal chemical
treatments have been used to preven-t and remove scale and
sludge in aqueous systems. The chemical treatment
generally involves the combined use of a precipitating
agent and a solid conditioner to maintain the solids in
the boiler water in a suspended state for effective
removal. The precipitating chemicals commonly employed
for calcium salts are soda ash and sodium phosphates.
Magnesium is precipita~ed by the alkalinity of boiler
water as magnesium hydroxide.
A variety of polycarboxylate and other water soluble,
polar polymers such as acrylate polymers have been used as
solids conditioners in industrial water systems. The
presence of small quantities of these polymers improves
the flui~ity of the precipitated sludge and results in t~e
formation of amorphous, frangible and se~rat~d
precipitates instead of hard, dense, crystals that form
scale on surfaces. The finely dispersed solid p~rticles
remain suspended and are carried out of the system by the
flow of water or by blowdown.
The precipitation of scale forming compounds can be
prevented by inactivating their cations with chelating or
sequestering agents so that the solubility of their
reaction products is not exceeded. Various nitrogen
containing compounds such as ethylenediamine tetraacetic
acid and nitrilotriacetic acid have be~n used as chelants
in water treatment.
Phosphonates are used extensively in water treatment
as precipitation inhibitors and are effective in threshold
amounts that are markedly lower than the stoichiometric
amount required for chelating or sequestering the scale
forming cation.
U.S. Patents 3,666,664 and 3,804,770 of Lorenc et al.
disclose scale inhibitors containing nitrilotriacetic acid
or ethylenediamine tetraacetic acid and an organic amino
methylene phosphonate. Preferably, the composition also
includes a polymer such as a water soluble s~lfoxy-free
polar addition polymer. The preferred water soluble
anionic polymers are the maleic anhydride and
non-sulfonated styrene copolymers of U.S. Patent 2,723,956
of Johnson and U.S. 3,549,538 of Jacklin in which the
copolymers are employed with a nitrilo compound,
especially a nitri]o tricarboxylic acid salt, as a scale
inhibitor.
U.S. Patent 3,959,167 of Hwa et al. discloses a
composition for inhibiting or preventing accumulation of
scale or the like on heating surfaces in an aqueous
system. The composition comprises an acrylic polymer, a
water soluble chelant which may be nitrilotriacetic acid
or ethylenediamine tetraacetic acid, and an
organophosphonic acid which may be
aminotri(methylenephosphonic acid) or a hydroxyalkylidene
diphosphonic acid such as hydroxyethylidene diphosphonic
acid.
U.S. Patents 4,255,259 and 4,306,991 of Hwa and Cuisia
disclose a composition for inhibiting scale in aqueous
systems which comprises a copolymer of styrene sulfonic
acid and maleic anhydride or maleic acid and a water
soluble phosphonic acid or salts thereof. Various
phosphonic acids including hydroxyethylidene disphosphonic
acid, nitrilo tri(methylene phosphonic acid), and other
amino methylene phosphonic acids may be used.
V.S. Patent 3,~30,~37 of Baum et al. discloses a
composition for boiler water treatment which contains a
sulfonated polystyrene as a dispersant and sludge
conditioner and an optional chelating agent such as
nitrilotriacetic acid, ethylenediamine tetraacetic acid,
or their sodium salts.
The composition for inhibiting formation of scale in
an aqueous system of the present invention comprises a) a
copolymer of ma]eic acid or anhydride and styrene sulfonic
acid or a water soluble salt thereof; b) an organic
phosphonate of the general formula:
~ ~ (C 2 H2)
wherein R is
-CH2 ~ OH,
OH
n is to 0 to 6, and x is 1 to 6, or of the general formula:
R ~
~O-P~¢-I-OH
HO X OH
wherein X is -OH or -NH2 and R is an alkyl group of from
1 to 5 carbon atoms, or a water soluble salt thereof; and
c) an a~inocarboxylate chelating compound of the general
formula:
RN[-(CH2)x-Z]2
wherein x is 1 or 2, R represents -(C~2)x-Z or
2 2 2)x Z]2 and each Z individually
represents a -COOH group, or a water soluble salt
thereof. The method of inhibiting the formation of scale
in an aqueous system of the present invention comprises
adding to the system a scale inhibiting amount of the
composition.
The present invention provides unexpectedly superior
inhibition of deposition and formation of scale,
particularly those containing calcium and magnesium
phosphates and silicates and iron oxide, on the metallic
structures of industrial water systems. The composition
and method are effective when used in water at high
temperatures and pressures in steam generating boilers and
the copolymer remains soluble in water of high hardness
and alkalinity. The invention exhibits the threshold
effect of the inhibition of formation of metallic salt
~5 crystals and the prevention of their adherence to heat
transfer surfaces at low treatment levels.
The chelating compounds used in the present invention
are water soluble aminocarboxylates. The preferred
aminocarboxylate chelating compounds are ethylenediamine
9~
tetracetic acid and nitrilotriacetic acid. In other
words, in these preferred compounds, x is 1 and all the
three Z radicals are the same. Nitrilotriacetic acid is
an especially preferred chelating compound.
The present invention employs water soluble amino
alkylene phosphonic acids, hydroxy or amino alkylidene
phosphonic acids, or water soluble salts thereof. The
preferred compounds are aminotri(methylene phosphonic
acid) 7 hydroxyethylidene-l,l-diphosphonic acid and water
soluble salts thereof. ~ydroxyethylidene-l,-l-diphosphonic
acid is especially preferred. Other suitable phosphonic
acids having these formulas include ethylenediamine
tetra(methylene phosphonic acid), diethylenetriamine penta
(methylene phosphonic acid), triethylenetetraamine hexa
(methylene phosphonic acid), hexamethylenediamine
tetra(methylene phosphonic acid), aminoethylidene
diphosphonic acid, aminopropylidene diphosphonic acid,
hydroxypropylidene diphosphonic acid, hy~roxybutylidene
diphosphonic acid, and hydroxyhexylidene diphosphonic acid.
The composition of the present invention further
comprises a water soluble copolymer of maleic acid oe
anhydride and styrene sulfonic acid or water soluble salts
thereof. The polymer may be prepared by copolymerizing
maleic acid or anhydride with styrene sulfonic acid or an
alkali metal salt thereof. Conventional addition
polymermization methods in the presence of light or free
radical initiators may be employed. Another method of
producing the copolymers is to copolymerize the maleic and
styrene monomers and sulfonate the copolymer in accordance
with conventional methods such as with a sulfur
trioxide-organic phosphorus compound as des_ribed in U.S.
Patent 3,072,618. The degree of sulfonation can vary but
substantially complete sulfonation is preferred.
The rel3tive proportions of styrene sulfonate and
maleic anhydride depend upon the degree of scale
inhibition needed. The copolymer generally contains from
about 10 to about 90 mole percent of the sulfonate.
Preferably, the mole ratio of styrene sulfonate moieties
to maleic acid or anhydride derived moieties is from about
1:1 to about 4:1 and especially is from about 1:1 to about
3:1.
The average molecular weight of the copolymer is not
critical so long as the polymer is water soluble.
General]y, the molecular weight is preferably from about
1,000 to about 25,000 and especially is from about 6,000
to about 10,000.
The amino^arboxylates, phosphonates and copolymers a~e
generally used in the forln of an alkali metal salt and
usually as the sodium salt. Other suitable water soluhle
salts include potassium, ammonium, zinc, and lower amine
s~lts. The free acids may also be used and all of the
acidic 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 NH4, H, Na, K, etc. The
copolymer is converted into the water soluble salts by
conventional methods.
Whil_ it is possible to add each of the components
separately to an aqueous system, it is generally more
convenient to add them together in the form of a
composition. The composition of the present invention
generally comprises from about 0.1 to about 100,
preferably about 2 to about 6, parts by weight of the
copolymer; from about 0.1 to about 100, preferably about
0.5 to about 5, parts by weight of the phosphonate, and
from about 0.1 to about 100, preferably about 0.5 to about
5, parts by weight of the aminocarboxylate. The polymer
and phosphonate are used in weight ratios genPral]y of
~2~
from abou' 10:1 to about 1:10, preferably of from about
4:1 to about 1:4, and especially of about 1:1. In
general, the aminocarboxylate and the copoly~er are used
in the weight ratios of from about 50:1 to about 1:10,
preferably of from about 30:l to about 10:1, and
especially of about 18:1. The ratio of aminocarboxylate
to phosphonate is generally from about 50:1 to about 5:1,
preferably from about 20:1 to about 5:1, and especially of
f rOQ about lO:l to about 8:1.
The compositions may be added as dry powders and
permitted to dissolve during use but normally are used in
the form of aqueous solutions. The solutions generally
contain from about 0.1 to about 70 weight percent of the
composition and preferably contain from about 1 to about
40 weight percent. The solutions can be made by adding
the ingredients to water in any order.
The amount of the composition added to the water is a
substoichiometric amount that i6 effective to inhibit
scale and sludge and depends on the nature of the aqueous
system to be treated. The phosphonate and
aminocarboxylate dosage depends to some extent on the
amounts of hardness causing and scale forming compounds
present in the system. The copolymer dosage depends to
some extent on the concentration of suspended solids and
existing ]evels of solids bui]dup in the system. The
composition generally is added to the aqueous system in an
amount of from about 0.01 to about 500 parts per million
(ppm) and preferably of from about 0.1 to about 50 parts
per million of system water.
The compositions of this invention may include or be
added to water containing other ingredients customarily
employed in water treatment such as alkalies, lignin
derivatives, other polymers, tannins, other phosphonates,
biocides, and corrosion inhibitors. The composition may
~2~
be introduced at any location where it will be q~ickly and
efficiently mixed with the water of the system. The
treatment chemicals are customarily added to the makeup or
feed water lines through which water enters t~e system.
Typically, an injector calibrated to deliver a
predetermined amount periodically or continuously to the
makeup water is employed.
The present invention is especially useful in the
treatment of alkaline boiler water such as the feed or
makeup water in a steam generating boiler. Such boiler
systems are generally operated at a temperature of from
about 298 to about 637F. and a pressure of from about 50
to about 2,000 psig.
The composi~ion and method for its use of this
invention are illustrated by the following examples in
which all parts are by weight unless otherwise indicated.
E~AMPLES 1 and 2
Aqueous solutions of a composition containing Gne part
of hydroxyethylidene diphosphonic acid, one part of
nitrilotriacetic acid, and three or six parts of a
copolymer of sodium styrene sulfonate and maleic anhydride
were prepared. The treatment solutions also contain
sodium phosphate, sodium sulfate, sodium sulfite, sodium
hydroxide, and sodium chlorlde in amounts sufficient to
provide the boiler water composition shown below in Table
I. Solutions containing the same amounts of the treatment
chemicals and the same parts of each component of the
composition were also prepared.
The sludge conditioning and scale inhibiting
properties of these solutions we~e evaluated in a small
laboratory boiler which had three removable tubes as
described in the Proceedings o~ the Fifteenth Annual Water
Conference, Engineers Society of Western Pennsylvania, pp.
87-102 (1954). The feedwater for the laboratory boiler
was prepared by diluting Lake Zurich, Illinois tap water
~i~h distilled water to 40 ppm total hardnesss as CaCO3
and adding calcium chloride to provide a 6 to 1 elemental
calcium to ~agnesium ratio. The feedwater and chemical
treatment solutions were fed to the boiler in a ratio of 3
volumes of feedwater to 1 volu.~e of solution giving a
feedwater total hardness of 30 ppm of CaCO3. The
scaling tests for all the treatment solutions were
conducted by adjusting boiler blowdown to 10 pe~cent of
the boiler feedwater giving approximately 10
co~centrations of the boiler water salines and adjusting
the composition of the treatment solution to give a boiler
water after the 10 concentrations having the composition
shown in Table I.
TABLE :[
Sodium Hydroxide a~ NaOH 258 ppm
Sodium Carbonate as Na2CO3 120 ppm
Sodium Chloride as NaCl 681 ppm
Sodium Sulfite as Na2SO3 50 ppm
Sodium Sulfate as Na2SO4 8l.9 ppm
Silica as SiO2less than 1 ppm
Iron as Feless than 1 ppm
Phosphate as PO410-20 ppm
The scaling tests were run for 45 hours each at a
boi].er pressure of 400 psig. Upon the completion of a
test, the boiler tubes were individually removed from the
boiler and the scale or deposit present on 6 inches of the
central length of each tube was removed by scraping,
collected in a tared vial, and weighed. The results of
the tests are shown in Table II.
-- 10 --
TAsLE II
Additive Scale
Dosage in Reduction
Run No. Additive the Feed (~)
(ppm)
1 Styrene 0.5 66.1
sulfonate and
maleic anhydride
copolymer (I)
2 Hydroxyethyli- 0.5 20.0
dene diphospho-
nic acid (II)
4 Nitrilotriacetic 1.0 600
acid (III)
I + II + III 0.5 95.5
(3:1:1 active)
6 I + II + III 0.5 96.0
(6:1:1 active)
The comparative results on scale formation shown in
Table II demonstrate that the composition and method of
the present invention provide scale inhibition that is
very considerably superior to that of the components added
separately.
EXAMPLE 3
The same laboratory boiler was used to study the
efficiency of the composition of this invention and ea_h
of its components as additives in preventing the formation
of new scale or removal of existing scale in an already
scaled boiler. The boiler was first operated to form
scale on the tubes and the boiler water surfaces. The
amount of calcium phosphate (hydroxyapatite) scale was
established by conducting several runs.
After the prescaling, the boiler was shut down to
remove one tube specimen and determine the initial amount
of scale on the tubes. The operation was continued for
another 45 hours using feedwater containing 30 ppm (as
CaCO3) total hardness and the treatment additive. Other
boiler water chemicals such as those described in Examples
1 and 2 were also used. The boiler water pressure was 400
psig and the boiler water concentration was ten times.
The scale deposited on the testing tubes was 5.98
grams (average) during the first stage (prescaling) and an
additional 8.99 grams during the second stage where no
additive treatment (blank) was addedO The results of the
tests are shown in Table III.
~ABLE III
Additive Scaling Scale
Dosage in Rate Reduction
Run No. Additive the Feed (g/ft2) (%)
(ppm)
1 Blank (No - 8.99
~dditive)
2 Styrene 2 (0.07) 100.8
Sulfonic Acid and
maleic anhydride (I)
3 Hydroxyethylidene 3 1.52 83.1
diphosphonic acid
(II)
4 Nitrilotriacetic 3 3.64 59.5
acid (III)
I + II ~ III (3:1:1 2 (0.65) 107.2
active)
The results demonstrate the unexpectedly superior
effectiveness of the composition and method of this
invention in removing existing scale .
- 12 -
