Note: Descriptions are shown in the official language in which they were submitted.
4~2
- 1 -
The present invention re1ates to water treatment, and in particular
to the treatment of scale forming or corrosive water systems, such as the
threshold treatment of evaporators, cooling systems or oil wells, in order
to inhibit scale formation and/or corrosion.
The evaporation of sea water and other brackish or non-potable waters
in order to provide drinking water entails problems of scale deposition on
the heat exchange surfaces of the evaporator. It is theoretically possible
to prevent scale formation by use of sequestering agents such as phosphates
or citrates, capable of forming soluble complexes with the metal ions of
the scale forming salts (usually calcium and magnesium ions). However,
sequestration of the ions would require the presence of a substantially
stoichiometric amount of complexant based on the scale forming ions, which
would be prohibitively expensive.
However, there exists a small class of compounds which, when dissol-
ved in water in proportions as low as 0.1 to 100 ppm have the power to
inhibit the fouling of heat exchange surfaces by scale. Such concentrations
are of a ver~ much smaller order of magnitude than those which would be
required to sequester the ions (of the order of several percent by weight).
The mechanism of this phenomenon, which is called the threshold effect, is
obscure, but it clearly differs from sequestration both because of the very
much smaller concentrations of agent required and because many common seq-
uestrants such as citrates do not exhibit threshold properties. Unlike
sequestrants, threshold agen~s may be effective in very small concentrations,
even when they do not prevent deposition of solids, by inhibiting the
tendency of the deposited solids to form a tenaceous scale on the heat
exchange surfaces.
Another scaling problem can arise when water is injected in bulk
through boreholes into partially exhausted oil bearing strata in order
to facilita~e the enhanced recovery of residual mineral oil. The water
may tend to deposit barium sulphate, barium carbonate, calcium sulphate
and/or calcium carbonate scale causing blockage of the boreholes.
'
~.~z~
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Another problem in connection with water treatment is corrosion
oF metal equipment exposed to water in, for example, cooling water
systems. It is known ~hat certain compounds usual1y in concentrations
s1ight1y greater ~han those required for thresho1d treatment ~e.g. 10
to 500 typically 50 to 200 ppm) tend to inhibit corrosion, especial1y
of iron, steel, copper and their alloys.
It is known that l-hydroxyethane-l, l-diphosphonic acid, herein-
after referred to as "HEDP" and its salts exhibit a useful threshold
effect, and they have been used successfully to treat evaporators. They
are also effectjve corrosion inhibitors. As used herein, the term HEDP
includes homo10gs of HEDP containing up to 20 carbons e.g. l-hydroxy-
butane-l,l-diphosphonic acid or l-hydroxy-3-phenylpropane-1,1diphosphonic
acid.
We have now discovered that certain dimeric condensates o~ HEDP
exhibit powerful thresho1d and corrosion inhibiting properties, and are
effective at 1cwer dosage rates thatn HEDP itself and retain effective-
ness for longer periods, in certain kinds of water system. They are
particularly useful in the treatment of oi1 wells or water syste~s which
contain traces of strong oxidising agents such as chromate and bromine,
which are sometimes present in water systems.
The 1itera~ure concerning the chemistry of the dimeric condensates
of the diphosphonic acids is contradictory. The ear1y work of Prentice,
Quimby, Grabenstetter and Nicholson reported in JACS August 1972, pp 6119
to 6124, which formed the basis of a series of Patent App1ications,
granted or assigned, to Procter and Gamb1e, indicated that the reaction
of acetic anhydride with phosphorous acid in a non-aqueous solvent cou1d
be made to yie1d a cyc1ic dimer joined by a P-O-P and a C-0-C bond. The
aforesaid workers denied the existence of a dimer containing a C-0-P
C-0-P ring. However, subsequent work by Co11ins, Frazer Perkin and Russe1
reported in JACS 1974 c1ear1y indicated that the a11eged P-O-P, C-O-C cyc1ic
dimer was identica1 with a product obtained by heating HEDP under vacuum.
~;Z84¢~2
-- 3 ~
They presented strong evidence to show that this product is in fact a
condensate of the for~ula:
O 0
P03H2 11/
\C/ \O
R ~ ¦ / R
~ I / \ PO H
FOR~
, where R is methyl.
The products which we have now found to be of particular value as
threshold agents and corrosion inhibitors are dimeric condensates of HEDP
wh;ch were obtained by heating anhydrous HEDP at 170 - 180C under vacuum
to remove 1 mole of water per mole of HEDP The products have been identi-
fied as having the formula 1 above where each R is an alkyl, aryl, aralkyl
or aliphatic ether group having 1 to 20 carbon atoms and will be referred
to hereinafter as HEDP dimer". We believe that the same products may be
obtained from the react;on of for example acetic anhydrides with phosp-
horous acid as described, for example, in B.P. 1,079,340, and is also
formed when a tetra metal salt o~ HEDP jS heated as described in B.P.
1,345,518. The term HEDP dimer" as used herein is not therefore to be
construed as limited to HEDP dimer produced by any particular process.
The preferred HEDP dimer is the dimer of l-hydroxyethane-l,l-diphosphonic
acid itself. It will be apparent that the water soluble and sparingly
atersoluble salts and esters of HEDP dimer are equivalent to the dimer
forthe purposes of this invention. For convenience, the term HEDP dimer"
will be used herein, where the context so permits, to include such salts
and esters.
',
. ' .
~ ~z~z
Our invention accordingly provitles a method for the inhibition of
scale formation or of corrosion, in aqueous systems which have a tendency
or potential to deposit scale or to corrode meta1 surfaces, which method
comprises adding thereto, respectively, a threshold or corrosion inhibiting
amount of HEDP dimRr as herein defined.
Preferably HEDP dimer is used according to the invention as its
alkali metal (e.g. sodium or potassium) or ammonium salts, e.g. its di,
tri, or tetra sodium salts or hexammonium salt. Other water soluble salts,
including salts of organic bases such as methylamine or trimethylamine
are also operative. It is also possible to employ water soluble esters,
such as polyoxyethylene esters, or partial esters or their salts, such as
the sodium salt of the dimethyl ester. Sparingly soluble derivatives are
also operative to give a controlled solubility additive.
The HEDP dimer is preferably used in the proportions conventional in
threshold treatment or corrosion inhibition i.e. at concentrations of 0~1
to 100 ppm, preferably 1 to SO ppm e.g. 2 to 20 ppm for threshold treatment
or ~ to 500 ppm, preferably 10 to 200 ppm for corrosion inhibition.
The invention is particulary applicable to the treatment of the sea
water which is fed to the evaporators used for making drinking water in
ships and arid coastal or island localities, especially chlorinated sea
water and most particularly sea water which is subject to continous chlorin-
ation. The HEDP dimer is also of particular use ;n the treatment of water
injected into o;l wells during secondary recovery of oil, in order to ;nhibit
the formation of barium or calcium conta;n;ng scales. The HEDP d;mer ;s
most effective in neutral or alkal;ne solut;ons.
We have discovered HEDP d;mer may be used, according to a further
aspect of the present ;nvent;on ;n conjunction with other threshold or
corrosion ;nhib;t;ng agents or ingredients of commercial threshold active
compositions or corrosion inhibitors such as condensed phosphates e.g.
threshold agents such as sodium tr;polyphosphate,sodium or potass;um hexa-
metaphos?hate, H~DP itself,aminomethylene phosphonic acids e.g.~amino tris
(methylene phosphon;c) acid or ethylenediamine tetra (methylenephosphon;c)
acid; threshold synergists such as d;carboxylic or hydroxycarboxyl;c acids,
~Z8~2
5 .
eOg. malonic, maleic, malic, tartaric, lactic, citric, adipic,
succinic, pimelic, sebacic or suberic acids; dispersants, e.gO l;gnin
sulphonates, tannin or ~ethylene b;s naphthalene sulphonates,
polyelectrolytes such as polymaleic acid, polyacrylic acid and
polymethacrylic acid; corrosion inhibitors such as water soluble
zinc salts, molybdates, nitrites, chromates, silicates, triazoles,
substituted thiazoles orthophosphates, polyol esters; biocides, and
antifoams~ References in the foregoing to acids are to be construed
as importing references to the corresponding water soluble or
sparingly water soluble salts and esters which are equivalent to the
acids for the purposes of this invention. It is within the scope of
the present invention that HEDP dim~r may also be used in aqueous
systems which contain various other inorganic and/or organic materials,
particularly all ingredients or substances used by the water treating
industry with the proviso that such materials do not render HEDP dimer
substantially ineffective for the desired purpose of scale or corrosion
inhibition.
Accord;ng to a further aspect, therefore, our invention provides
compositions for threshold treatment which contain from 5 to 95% by
weight of the total active material, of HEDP dimer, the balance of the
active material consisting essentially of at least one other threshold
agent or anti-corrosive agent, and/or at least one dispersent,
polyelectrolyte and/or threshold synergist.
By "active material" is meant those ingredients which have
threshold~anti-corrosive, dispersive, synergistic, or surface activity,
and excluding any inert diluent, or any solvent, such as water, which
may, optionally be present in the composition.
Threshold synergists are substances, such as maleic or adipic
acid which are not in themselves threshold agents, but which enhance
the threshold activity of HEDP di~er. Preferably the HEDP dimer is
present in a proportion of from 10 to 80% by weight of the total active
material, e.g. 30 to 70% most preferably 40 to 60%.
-- 6
EXA~IPLE 1 - TIIRESHOLD ACTIVITY OF ~IEDP CYCLIC DIMER
The ability of sub-stoichiome-tric amounts of HEDP dimer to inhibit
the precipitation of calcium carbonate has been demonstrated in a labora-
tory simulation of an evaporative cooling water system. In this, a natur-
al1y hard water from the town supply was recirculated around a system where
the flow was alternately heated to about 70C by a nickel sheathed immersion
heater and then cooled ko about 40C in a forced draught cooling column.
Scale, formed by the thermal breakdown of calcium bicarbonate, was deposited
on the heater sheath and the quantity determined by dissolution methods. A
typical analysis of the water used in these experiments was as follows:-
Total Hardness = 149 ppm CaC03Temporary Hardness = 78 ppm CaC03
Chloride = 40 ppm Cl
pH = 6.9
The effectiveness of HEDP Dimer as a threshold agent for calcium carbonate
is sho~m by the following results (Table 1):-
T A B L E
.. . . _ _
__ Scale deposited % Inhibition
Inhibitor Concn ppm expressed as mg w.r.t. the
- CaC03 b~ank
. . ... _ ... _ .... . . . _ .. _
Blank 0 403 0
HEDP Cyclic Dimer 2 83 79.4
. . . _ . _
The structure of the dimer was confimed by the following tests:-
112B~Z
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A. ELEMENTAL ANALYSIS
1.1 ~ Total P.
Breakdown Procedure % P Found
(a) Wet oxidation + persulphat:e 28.63; 28.65
(b) Acid persulphate oxidation28.66; 28.76
(c) Peroxide fusion - Parr bomb28.75; 28.92
Mean ~ Total P = 28.73
= ~ , = . _ .
1.2 Ortho P,_C, H; Moisture
Ortho P 50 ppm p
% C 11. 1%
% H 4.1%
% Moisture 12.6%
(1 hour at 120C) . .
B. MOLECULAR FORMULA
Molecular Formula Atom Ratio
% Found Theory Found Theory
P 28.7 28.84 4.00 ~ 4
C 11.1 11.2 4.00 4
H 4.1 4.19 17.7 18
(diff) O 55.3 55.8 14.9 15
H20 12.6 12.56
On the results obtained the molecular formula is:-
4 12 4 12 3H2; Mol. Wt. 430.
C. STRUCTURAL FORMULA
(i) X-ray
The X-ray powder photograph of the sample was substantially
.
.
1~284~;2
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i
identical with that of the hydrated cyclic dimer prepared
by Collins, Frazer, Perkins and Russell(JCS-Dalton, 1974,
960) as:-
OCH O
Ill3 ll
OH ~ P C - P OH
OH
O O OH
FORMU~A 2 HO - P - C - P - OH
(ii) N.M.R.
lH NMR
The spectrum consists of an 'OH' and a ' - P - C - P -' triplet
absorption. The chemical shift of the 'CH3' is 2ppmorandis consistent
with this 'CH3l being in a more strained environment than the corres-
ponding 'CH3' in HEDP monomer (chemical shift 1.8 ppmor).
31p NMR
The spectrum consists of two complex absorptions of equal intensity,
one at -5.2 ppm and the other at -16.0 ppm (from external 85% H3P04 refer-
ence), i.e. two magnetically different p nuclei are present in equal
abundance.
3y preparing the methyl ester~ other works (JACS 1972, 94, 6119)
have shown that the -16.0 ppm absorption is due to the 'P03 units attached
to the ring.
8~2
The NMR data given above cou1d be accounted for by either of the
following structures:-
O CH O O CH O
~ ~3 ~ 3
HO - P C P - OH HO - P C - P OH
1H I I bH
o o o ~
fH ~ OH
HO P C P - OH HO - P - C - P OH
Il I ~ 11 I' 11
O CH3 0 CH3 0
(A) (B)
FOR~IULA 3
-
El;minating the proton coupling gave a 31p spectrum consisting of
two triplets centred at -5.2 ppm and -16 ppm. This result favours struc-
ture B, since structure A should give rise to a more complex spectrum
showing both P-C-P and P-O-P coupling, whilst s~ructure B would give P-C-P
only.
D. POTENTIOMETRIC TITRATION
_ . _ _ _ . . . .. ...
Using 0.5 - 1.09 sample in 100 ml water a potentiometric titration
with l.ON HaOH shows two end-points. The 1st end-point at approxi-
mately pH 5.5 corresponds to 4 replaceable H and the second end-point
at pH 10.7 to a total of 6 replaceable H.
~10-
EXAMPLE 2
The high stability of the dimer compared with -the
monomer in the presence of strong oxidising agents, was
demonstrated as follows:
A. STABILITY TO OXIDATION
(i) Br2/NaHC03
HEDP monomer was added at a level of 10 ppm to a solution
containing 100 ppm NaHCO3 and 10 ppm Br2. The resulting solu-
tion (pH 7.8) was maintained lat 30C. and 20 ml aliquots
removed for analysis. The results were as follows:
TABLE_2
Time PO4 Found Per Cent
(Secs? (Mlcrograms) Decomposition
O lo9
9.4 4.1
15235 14.8 7.0
357 19.9 9.8
533 25.6 12.8
827 31.2 15.8
12~0 39.7 20.S
2~ 1470 46.0 23.9
1785 S4.2 28.3
The experiment was repeated using 10 ppm HEDP dimer. The
results were as follows:
TABLE 3
25 Time PO4 Found Per Cent
(Secs) (Micrograms)Decomposition
_ . _ . .... . _
O 1.1
320 1.3 0.10
910 1.7 0.30
1845 2.1 0.49
~Z84`~2
This example clearly shows that HEDP dimer is s-table under the
test conditions, whereas HEDP monomer is attacked wnder these
conditions.
(ii) Sea Water containing bromine - pH 8.3
The cyclic dimer was added to artificial bromine-
containing sea water at a level of 10 ppm. The mixture was
split into two portions, one of which was allowed to stand
at room temperature and the other was hea-ted on a steam
bath. The analytical results were as follows:
TABLE 4
ppm Br2 Present Overnight at room Steam bath for 1
temperature hour ppm PO4
ppm PO4 found found
0 0.31 0.29
2.9 0.32 0.30
156.1 0.35 - ~
0 30 0.28
11.0 0.50 0-34
15.1 0.30 0.20
The artificial sea water used in this experiment
had the composition.
NaCl KCl M~C126H20 CaC126H20 NaBr NaHC03 MgSO43H2O Town Water
265g 7.3g 6.3g 21.7g 2.9g 2.0g 46.9g to 10 litres
pH adjusted to 8.5
Compared to results obtained for HEDP monomer in
25 bromine containing water at 30C (See Example 2A(i)), the
sample of dimer tested shows no evidence for oxidative
breakdown in the presence of bromine.
~28~V2
-12-
B. ACID AND ALKALINE_~IYDROLYSIS
(i) 0.5g samples of the dimer hydrate were heated under reflux
4N.H2S04 for 1 hour, (B) 100 ml 4N.H2S04 for ~ hours and (C)
100 ml N.NaOH for 2 hours.
After hydrolysis, each solution was neutralised to pH 8.5
and made up to 200 ml with distilled water.
A 20 ml aliquot was then taken for the determination of HEDP
monomer. The results are given in Table 5.
T A B L E S
Hydrolysis¦ Dimer Hydrolysed
Expt. P w/w
BLANK _
(A) - acid 1 hour15 + 1 -
(B) - acid 4 hour49 + 1
(C) - alkali 2 hour _ _
*under the conditions used this result is at the limit of
reproducibility of the method.
The results obtained show that the dimer is hydrolysed to
monomer in strongly acid solution but is unaffected by N. sodium
hydroxide over a similar time scale.
- 20 (ii) The possibility of orthophosphate formation by hydrolysis was
checked by refluxing 50 mg samples of the dimer for 20 hours
with N.NaOH and 9N.H2S04. Each solution was neutralised and
made up to 250 ml with distilled water. No orthophosphate
( O.S ppm ortho P) was found in either of these solutions.
i3 -
i
The following compositions were prepared and tested by the method of Example 1. All percentages are weight.
EXAMPLE 3
6% HEDP d;mer potassium salt
14% Potassium polyacrylate
Balance Water
EXAMPLE 4
. .
50% HEDP dimer
50% Adipic acid
EXAMPLE 5
50~ HEDP dimer
50% Sodium lignin sulphonate
EXAMPLE 6
50% HEDP dimer
50X Sodium tripolyphosphate
EXAMPLE 7
80% HEDP dimer
20% HEDP
Z~
EXAMPLE 8
40% HEDP dimer
60% Zinc sulphate
EXAMPLE 9
75% HEDP dimer
25% sod;um chromate
EXAMPLE 10
60% HEDP dimer
40% sodium molybdate
/
EXAMPLE 11
70% HEDP dimer
30% sodium nitrite
In each case the composition of the example provided satisfactory
inhibition of scale formation.
