Note: Descriptions are shown in the official language in which they were submitted.
209~
INTRODU(~TlOly
Efficient operation of boilers and other steam-run equipment requires chemical
treatment of feedwater to control corrosion. Corrosion in such systems generally arises
as a result of oxygen attack of steel in water supply equipment, pre-boiler systems,
boilers, and condensate return lines. Unfortunately, oxygen attack of steel is accelerated
by the unavoidable high temperatures found in boiler equipment. Since acid pH's also
accelerate corrosion, most boiler systems are run at alkaline pH's.
In most modern boiler systems, dissolved oxygen is handled by first mechanically
removing most of the dissolved oxygen and then chemically scavenging the remainder.
The mechanical degasification is typically carried out with vacuum degasifiers which will
reduce oxygen levels to less than 0.5-1.0 mg/L or with deaerating heaters, which will
reduce oxygen concentration to the range of 0.005-0.010 mg/L.
Chemical scavenging of the remaining dissolved oxygen is widely accomplished by
treating the water with hydrazine. See, for example, the Kirk-Othmer Encyclopedia of
Chemical Technology, Second Edition, Interscience, Publishers, Volume II, page 187. As
explained in Kirk-Othmer, hydrazine efficiently eliminates the residual oxygen by
reacting with the oxygen to give water and gaseous nitrogen. In addition, hydrazine is a
good metal passiv~tor since it forrns and maintains an adherent protective layer of
magnetite over iron surfaces.
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Unfortunately, however, it has become widely recognized that hydrazine is an
extremely toxic chemical. As a result, it is likely that worker exposure to this compound
~vill be subjected to extrernely severe tolerances by government occupational health
agencies. It is therefore desirable to provide alternate boiler water treatment chemicals
which are generally free of the dangers inherent in the use of hydrazine, but which
effectively scavenge oxygen and passivate steel surfaces under typical boiler conditions.
An improved oxygen scavenger composition which overcomes many of the
problems described above is the subject matter of U.S. 4,269,717, the disclosure of which
is incorporated herein by reference. This patent teaches that carbohydræide (CHz) is a
superior boiler water oxygen scavenger. Recently, Cosper and Kowalski (I&EC
Research, 1990, 29, 1130) reported that the scavenging of oxygen by CHz is most
effectively catalyzed by cupric ions.
THE DRAWINGS
Fig. 1 compares the passivation effectiveness of acetaldehyde carbohydrazone
versus hydrazine and carbohydrazide.
Fig. 2 illustrates the decomposition of acetaldehyde carbohydræone to hydrazine.
THE INVENTION
The invention is directed generally to controlling corrosion in boiler systems and
more particularly to treating boiler water to remove dissolved oxygen and to passivate
metal surfaces.
2 0 ~ 4 ~ 7 ~ 66530-534
One aspect of the invention provides a method for
removing dissolved oxygen from boiler water having an alkaline
pH. The method comprises adding to said boiler an oxygen
scavenging amount of at least one carbohydraæone compound
selected from the group of carbohydrazone compounds of the
formula:
1 R3
1) \C=NHM-C-NHN=C
R2/ \ R4
NH-NH
2) O=C \ > 5 6
NH-NH
NH-NH
3) O=C
\NH-N=CR7R8
1' R2, R3, R4, R5, R6, R7 and R8 are the same or
different and are selected from hydrogen, alkyl groups,
substituted alkyl groups, aryl groups, and substituted aryl
groups. The broad chain lengths for the alkyl groups range from
1 to 16 carbon atoms with a preferred range of 2 to 8 carbon
atoms. The aryl groups are six-membered rings. Substituen-ts
of the alkyl and aryl groups include methyl or hydroxymethyl as
the Rl group with a hydrogen at the R2 group or substituting an
additional methyl or hydroxymethyl for the hydrogen as the R2
group. A preferred substituent is a methyl in the Rl position
209~7~ 66530-534
with a hydrogen at the R2 position. Compounds of formulae 2)
and 3) above, for removing dissolved oxygen from boiler water
having an alkaline pH are another aspect of the present invention.
4a
2094~i76
According to a preferred embodiment, the carbohydrazone is added at a level of at
least 0.5 moles of the carbohydrazorle per mole of dissolved oxygen. Preferably, the
boiler water is subjected to deaeration to reduce the level of dissolved oxygen and the
carbohydrazone is added to the bciler water after deaeration to remove remaining
dissolved oxygen. The carbohydrazone may also be used in conjunction with an
oxidation-reduction catalyst. Preferred carbohydrazone compounds are acetaldehyde
carbohydrazone (ACHz), dihydroxyacetone carbohydrazone (DHACHz), methyl
tetrazone (MTz), dimethyl tetrazone (DMTz), hydroxymethyl tetrazone (HMTz), and
dihydroxymethyl tetrazone (DHMTz).
A further aspect of the invention provides a method of removing dissolved oxygen
from boiler water having alkaline pH and passivating boiler surfaces comprising adding
to the boiler water at least 0.5 moles of the carbohydrazone per mole of dissolved
oxygen along with from 0.2 up to about 20% by weight based on the carbohydrazone of a
catalyst capable of undergoing oxidation-reduction reactions. Preferred catalysts of the
invention are copper, hydroquinone, cobalt, and diethylhydroxylarnine.
The carbohydrazones described above are readily prepared by reacting,
carbohydrazide (CHz) with either aldehydes or ketones having the appropriate number
of carbon atorns in an allyl or substituted alkyl group defined by the formula previously
shown.
Typical of the aldehydes that may be reacted with carbohydrazide are
acetaldehyde, butyraldehyde, propionaldehyde, hydroxyacetaldehyde and the like.
Starting ketones are acetone, butanone, hydroxyethyl ketone dihydroxy acetone and the
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like. In certain instances, it is possible to perform the reaction using two or more
different aldehydes or ketones.
The reaction of any molar amount of an ald~hyde or ketone from 0.1 to 2 with
carbohydrazide yields a product which is more active and stable than carbohydrazide
itself. The aldehydes or the ketones reacted with carbohydrazide to yield the
corresponding carbohydrazone. In the case of (ACHz) this compound has been
characterized by l3C-NMR, ~ l lR and elemental analysis. All of these compounds are
stable at room temperature.
Products formed in a typical carbohydrazone reaction are illustrated by the AC~Iz
products shown below:
~H-NH ~NH-NH2 0
O = C CH(CH3) O = C (CH3) HC = NHN-C-NHN = CH (CH3)
NH-NH H-N = CHCH3ACHz
MTz mono-ACHz
Although ACHz or MTz may be added to the boiler system at any point, it is most
efficient to treat the boiler feedwater, preferably as it comes from the degasifier.
Residence times prior to steam formation should be maximized to obtain maximum
corrosion protection. W~ile ACHz or M7`z will control corrosion even if the residence
times are as low as 2-3 minutes, residence times of 15-20 minutes or more are preferred.
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The amount of ACHz or MTz required tO effectively scavenge oxygen from the
boiler water is dependent upon the amount of oxygen actually present therein. It is
generally desirable that at least 0.5 moles of ACHz or MTz be used per mole of oxygen.
These rninimum levels of ACHz or MTz will have the added benefit of effectively
passivating metal surfaces. Of course, levels of ACHz or MTz considerably in excess of
0.5 moles per mole of oxygen may be required, particularly for treating boiler feedwater
under static storage conditions. Under such static conditions, for example, treatment
levels of 160 moles or more of ACHz or MTz per mole of oxygen have proven effective
in controlling corrosion.
ACHz and MTz are effective oxygen scavengers and metal passivators over the
entire range of temperatures to which boiler feedwater is generally subjected. Typically,
these temperatures will be in the range of 120-350 F.
While it is well known that each molecule of ACHz or MTz is capable of being
hydrolyzed to 2 molecules of hydrazine, the extent of hydrolysis under typical boiler
conditions is very minor. This fact has been illustrated in the examples below which
further demonstrates that ACHz and MTz are effective oxygen scavengers and metal
passivators in their own right.
In one important embodiment, the present invention provides a method of
removing dissolved oxygen from boiler water by adding to the water an oxygen
scavenging amolmt of either acetaldehyde carbohydrazone (ACHz) or methyl tetrazone
(MTz) which are soluble in water. ACHz or MTz may be used either as a dry powder or
as a solution.
2094~
While ACHz or MTz may be used alone in the present application, it is preferred
that they be catalyzed. For this purpose, it is desirable to use catalysts which undergo
oxidation-reduction reactions. For example, hydroquinone, other quinones and
diethylhydroxylamine (DEHA) can be used to catalyze the ACHz or MTz since they are
capable of undergoing oxidation-reduction reactions. When a quinone or DEHA catalyst
is used, the amount of quinone added in relation to the carbohydrazone should be in the
range of 0.2 to 20% by weight of the carbohydrazone.
Another oxidation-reduction catalyst useful with ACHz or MTz in the present
application is cobalt, preferably in a stabilized form. The amount of cobalt used in
relation to the carbohydrazone should be in the range of 0.2 to 20% by weight of the
carbohydrazone. Typical useful stabilized cobalt complexes are described in the
following U.S. Patent Nos., which are hereby incorporated by reference: 4,012,195;
4,022,711; 4,022,712; 4,026,664; and, 4,095,090. Also, as in the case of CHz, cupric ions
effectively catalyze the scavenging of oxygen by ACHz and MTz.
PREPARATIONS
The reaction of one mole of carbohydrazide (CHz = (H2NHN)2C=O) with two
moles of acetaldehyde, CH3CHO, in water yields acetaldehyde carbohydrazone
(ACHz = (CH3CH=NHN)2C-O). ACHz has been characterized by 13C NMR, FTIR
and elemental analysis and has been shown to scavenge oxygen efficiently while not
decomposing at room temperature to hydrazine.
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The reaction of one mole of CHz with one mole of CH3CHO in water yields a
solution of methyl tetrazone. The solution was actually found to contain about 9 parts
MTz to 1 part mono-ACHz, but will be referred to as a MTz solution for simplicity.
To illustrate the invention, the following examples are presented:
EXAMPLE 1
Solutions of ACHz were shown to outperform CHz in oxygen-scavenging efficiency
at 185 F. A laboratory bench-top feedwater simulator was used under air-saturated
conditions (~6 ppm 2) and the feedwater was adjusted to pH 9-10. A solution of
oxygen-scavenging chernical was fed using a syringe pump at a rate of 0.385 mL/minute.
Under these flow conditions, the retention time of the feedwater in the hot zone of this
stainless steel system (185 F) was about 12 minutes. Using a 2:1 mole ratio of
scavenger to oxygen, as shown in Table 1, uncatalyzed ACHz removed over two times as
much oxygen as uncatalyzed CHz from the feedwater. The reaction time was defined as
beginning immediately after the 12 minute retention time and ending when the reaction
was completed. Hydroxyacetaldehyde carbohydrazone (HACHz =
(HOCH2CH=NHN)2C=O) showed reactivity comparable to that of CHzl while acetone
carbohydrazone (AcCHz = ((CH3)2C=NHN)2C=O) was a less effective oxygen
scavenger. Hydraziue was mostly ineffective at this temperature. In the absence of
catalyst, no carbohydrazorle, carbohydrazide or hydrazine removed oxygen completely at
this temperature.
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abl- I. Compari~on of U~catalyzod o~ygen-8cavenger~ at 185F with
a 2:1 Molar Ratio of 8cavenger to oxyg~n.
Solution % Oxygen Removed Reaction Time/min
. . . _
ACHz 34 30
I~CHz 11 30
AcCHz 6 30
CHz 15 30
N2H4 7 30
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EXAMPLE 2
The effect of catalyst on the oxygen-scavenging ability of various compounds was
studied using the bench-top feedwater simulator under conditions identical to those in
Example 1. A 2:1 molar ratio of scavenger to oxygen was used and these results are
displayed in Table II. In the presence of copper catalyst, both ACHz and CHz
completely removed oxygen from the feedwater, but ACHz achieved this result two times
faster than CHz. Note that the reactivity of HACHz was enhanced in the presence of
catalyst, while AcCHz showed the least activity.
''3~ s7~;
blo IIo Co~p~ri~ of Catalyz~d Oxygcn~5~a~renger s a~ 185F with
a 2 :1 ~olar E~atio of Scaveugsr to O~ygç~
(catalyst = 2 ppm CuC12)
Solution% Oxygen Removed Reaction Time/min
AC~Iz 100 30
HACHz 85 30
AcCH z 3 4 7
CHz 100 70
N2H4 74 80
2 ~ 9 4 5 l ~
EXAMPLE 3
The feedwater temperature simulator measures the efficiency of oxygen-removal by
scavengers at oxygen concentrations comparable to actual boiler feedwater conditions.
Using the feedwater ternperature simulator, solutions of ACHz were shown to
outperform CHz in oxygen-scavenging efficiency at 300 F. The feedwater was adjus~ed
to a flowrate of 70 ml/minute, pH of 9-10 and oxygen baseline of ~100 ppb. Then a 6û
mL solution of oxygen-scavenger was fed using a syringe pump at a rate of 0.028
mL/minute. Under these flow conditions, ~he retention time of the feedwater in the hot
zone of the system (300 F) was lX.9 min. The results are shown in Table III. Using a
2:1 molar ratio of scavenger to oxygen, an uncatalyzed ACHz solution removed 35% of
the oxygen in the system while CHz removed 18% of the oxygen. At a 4:1 ratio, 63% of
the initial oxygen concentration was scavenged. Therefore, as with the bench-top unit,
ACHz was shown to be twice as effective as CHz in scavenging oxygen using the
feedwater temperature simulator.
The ACHz scavenging efficiency is noteworthy since no catalyst was ennployed in
these studies and the feedwater temperature simulator is made of stainless steel. It is
likely that copper (II) contan~nation in a mild steel boiler will enhance the extent of
oxygen-scavenging.
~ i3 ~ j3
T~b ~ o~c~ ~ ly2 e~ O:~f ge~ Y~ a~ 3 0 0 F .
Soll.ltior~ % Oxygen Remov~d
_~
2: 1 ACHz 3 5
4 ~ 6 3
i3: 1 ~C1~z 81
2:1 EAa 40
2:1 C~ 18
aE.~ - erythorbic acid.
14
~9~7~
EXAMPLE 4
Corrosion studies indicated that ACHz is an e~ectiYe metal passivating agent,
under both low and high temperature conditions. In the experiment, feedwater
containing less than 2 ppb 2 was passed through a series of mild steel tubes of
increasing temperatures ~or three days. This caused the formation of an initial oxide
layer on the tube su}faces. For the next three days the feedwater was treated with
ACHz solution. Analysis of these tubes by linear polarization resulted in polarizafion
resistance values (Rp) which show a further developing passivation layer on each tube.
Observation of the tubes revealed that each was covered with an adherent black
magnetite surface. The resul~s of three runs were averaged and are shown in Figure 1,
indicating that ACHz is an efficient metal passivating agent, though not as effective as
CHz. In comparison to hydrazine, ~CHz is a much better passivating agent at low
temperatures and shows comparable passivation at elevated temperatures.
EXAMPLE 5
Stability s~udies have shown that ACHz solutions are resistant to decomposition to
hydrazine over a period of seven months at room temperature. An experimental design
was performed to determine the effects of cupric ion, ferrous ion, and temperature on
the decomposition of ACHz solutions to give hydrazine. The values of the curves in
2~9d~
Figure 2 represent the hydrazine concentration under varying conditions of temperature
~x-axis,F) and copper (y-axis, ppm Cu2+~. As shown in Figure 2, temperature was found
to have the largest effect on hydrazine accumulation, while copper concentration had a
more mild effect. The e~fect of iron was small, though measurable amounts of hydrazine
were detected in iron-containing solutions over time. Note from Fig. 2 that storage of
ACI Iz solutions at 80 F, 0 ppm Cu2+ and 2.5 ppm iron for two months resulted in a
hydrazine concentration of less than 10 ppm. In the absence of metal ions, ACHz
solutions were shown to accumulate less than 4 ppm hydrazine after seven months at
room temperature.
EXAMPLE 6
Carbohydrazide solutions (6.5 wt. ~o) were prepared and varying amounts of
acetaldehyde were added. The reaction between CHz and acetaldehyde was performed
by adding neat CH3CHO slowly to the vigorously stirring carbohydrazide solution. Thus,
solutions containing a range of molar ratios were prepared from 1:0.25 to 1:1.25
(CHz:CH3CHO). These solutions were then stored in glass jars at room temperature for
one month. At this time, the solutions were analyzed for residual hydrazine. The
resulting hydrazine concentrations (in ppm), shown in Table IV, were observed to
decrease with increasing CH3CHO. This result is consistent with the formation of MTz
and ACHz which are more stable toward decomposition to hydrazine than
carbohydrazide; thereby imparting greater stability to the solution upon storage.
16
7,,
abla IV. Elydr21zina an~lysi~ of 6 . 5 wt. % Cllz solutions
cont~inia~g various a~ounts o acetaldehy~o.
. _
Molar Ratio _ rN2E~4 1 / PDm_ _
(CHz: CH3_HO) _ Day_ 35) at Room ~em~era~ _
1:0 26
1:0.25 19
1:0.50 15
1:0.75 12
1:1 5
1:1.25 6
2 ~ 9 ~ ~' r~ ~
Having described our invention, we claim:
18
