BOILER DOUBLE BUFFERS

SOLUTION TAMPON DOUBLE POUR EMPECHER LA CORROSION DES CHAUDIERES

English Abstract

ABSTRACT OF THE DISCLOSURE
A method of inhibiting corrosion of metallic surfaces in a
steam generating system is disclosed which comprises the step of
adding to the aqueous phase of the system a non-oxygen containing
alkylated diamine having at least two carbon atoms between the
diamine nitrogen atoms.

Claims

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THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of inhibiting corrosion of metallic
surfaces in both the liquid and vapor phases of a steam generating
system which comprises adding to the aqueous phase of a steam
generating system an amount sufficient for the purpose of a
non-oxygen containing alkylated diamine having at least two carbon
atoms between the diamine nitrogen atoms.
2. The method of claim 1 wherein said non-oxygen
containing alkylated diamine has the general formula
<IMG>
wherein R and R1 are H or alkyl, R and R1 are not both H, and
x is two or more.
3. The method of claim 1 wherein said non-oxygen
containing alkylated diamine is selected from the group consisting
of N,N,N', N'-tetramethylethylenediamine, N,N'-dimethyl-
ethylenediamine, and N,N,N', N'-tetramethylpropylenediamine.

-12-
4. A method of maintaining the pH of the liquid and
vapor phases of a steam generating system within the range of 8.5
to 9.5 which comprises adding a sufficient amount for the purpose
of maintaining pH of a non-oxygen containing alkylated diamine
having at least two carbon atoms between the diamine nitrogen
atoms.
5. The method of claim 4 wherein said non-oxygen
containing alkylated diamine has the general formula:
<IMG>
wherein R and R1 are H or alkyl, R and R1 are not both H, and
x as two or more.
6. The method of claim 4 wherein said oxygen
containing alkylated diamine is selected from the group consisting
of N,N,N', N'-tetramethylethylenediamine, N,N'-dimethyl-
ethylenediamine, and N,N,N', N'-tetramethylpropylenediamine.

Description

2~9~ ~97
R-750
BOILER DOUBLE BUFFERS
FIELD OF THE INVENTION
The present invention relates to methods for controlling
metal loss in boiler/condensate steam systems. More particularly,
the present invention relates to methods of controlling pH by
feeding a single control agent to inhibit corrosion in both the
liquid and steam phases of a boiler system.
BACKGROUND OF THE INVENTION
Corrosion in steam generating systems, especially of iron
and copper, results in damage to piping and equipment as well as
the loss of high quality ~ater and energy. The corrosion products
and process chemicals if returned to the boiler can contribute to
the formation of damaging boiler deposits thereby reducing the
overall system reliability and increasing operating and
maintenance costs.

2~9~97
As water temperatures rise, ferrous hydroxide is
converted to magnetite (Fe304) in the absence of oxygen to
form a protective film barrier. In actual boiler systems the
presence of contaminants such as dissolved oxygen and carbon
dioxide promote corrosion reactions.
In addition to iron corrosion in water which is augmented
by the presence of oxygen, corrosion of copper by oxygen may also
occur. If copper complexing agents such as ammonia are present,
a protective copper oxide film cannot become permanently estab-
lished. High concentrations of carbon dioxide in the condensatesystem, at lower pH values (less than 8), have an effect similar
to ammonia in dissolving a copper oxide film.
Due to the aqueous solubility of carbon dioxide when water
is heated in steam generating systems, the concentration of carbon
dioxide in the water decreases and the gas enters the produced
steam. ~pon condensation, carbon dioxide again dissolves in the
water to form carbonic acid. Since the condensate contains rela-
tively few dissolved solids and thus little buffering capacity,
the carbonic acid can drastically lower the condensate pH. In
turn, when acidic condensate mixes with makeup water, the steam
generator feedwater pH can also decrease.
Waters containing carbonic acid cause acidic or general
corrosion of the iron and copper metallurgies found in condensate
and feedwater systems. This type of corrosion is evidenced by a

2~91~9~
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general wastage or by grooving of the metal surface. If untreated,
- corrosion can cause failure of condensate return lines, feedwater
piping, and other equipment (condensate receivers, pumps, heaters,
etc.) associated with steam generating and hot water heating
systems.
Several methods have been devised to control acid-induced
corrosion in these systems. Materials can be added that adsorb to
the metal surface to form a thin barrier between the metal and the
acidic solution. Examples of effective barrier forming materials
that are routinely used are long chain amines, such as
octadecylamine, and azoles such as tolyltriazole.
A second, more often utilized method of controlling
carbonate caused corrosion is the addition of volatile amines to
neutralize the carbonate and thereby increase the aqueous pH.
Many different volatile amines are utilized, but some commonly
used materials include cyclohexylamine, morpholine, and methoxy-
propylamine. On an equal weight basis, the most effective amines
are those that possess high basicity and low molecular weight.
The high basicity allows attainment of high pH after acid
neutralization, and low molecular weight allows a greater molar
concentration (and thus more neutralization). The addition of
volatile amines neutralizes the acid (H+) generated by the
solution of carbon dioxide in condensate. The amines hydrolyze in
water to generate the hydroxide ions required for neutralization.

2~91097
By regulating the neutralizing amine feedrate, the condensate pH
can be elevated to within a desired range (e.g. 8.5 to 9.5).
Numerous amines can be used for condensate pH neutralization and
elevation. The selection of the appropriate amine is currently
controlled by the basicity, stability and distribution ratio
characteristics of the particular amine.
Steam generating systems are operated under several dif-
ferent control programs, but the general purpose of all boiler
control programs is to minimize corrosion of the metal surfaces.
This is accomplished by maintaining the pH within a specific
window of operation, a window determined experimentally to fit the
particular metallurgy, chemistry, and pressure of the system.
In high pressure systems, the feedwater is very pure with little
hardness and iron, and little to no organic contaminants. The
boiler control methodology is designed to maintain a passive
magnetite layer on the iron metallurgy and prevent adverse
corrosion reactions at metal surfaces.
The solubility of magnetite is very pH dependent, with
the minimum in a specific pH range. Boiler corrosion control is
designed to operate in this pH window of magnetite solubility and
to prevent excessive deposition while maintaining a thin magnetite
layer for passivity. This control scheme is designed to maximize
the efficiency of heat transfer across the boiler and condensate
surfaces. Maintaining a passive magnetite is critical on both the
liquid and steam side of steam generating systems.

209~097
Con~rol agents, either a single component or a mixture of addi-
tives, must be able to maintain the pH of both aqueous phases
near the magnetite pH minimum. High pressure ~above 1500 psig)
steam generating equipment control agents can be categorized
into two groups; inorganic and organic. Inorganic agents are
typically phosphate or borate-based, while the organic systems
are usually amine-based with varying physical properties.
Inorganic control agents preserve the boiler water
chemistry in a non-corrosive state and are very effective at
preventing problems on the water side of a steam generating
system. However, they are of limited effectiveness on the
steam side. Typical inorganic control agents must be used in
combination with a volatile additive in order to provide pH
control in the steam phase. Also, inorganic control agents
often form insoluble salts with hardness and iron contaminants
resulting in increased deposition. Thus, inorganic control
agents are not typically employed on the steam side because of
solubility and/or deposition problems.
Typical organic control agents can usually be classified
as highly volatile or less volatile. The highly volatile agents
have very short residence times in steam generators and will tend
to maintain the pH of the steam phase only. The less volatile
amines tend to maintain the p~ of the liquid phase with limited
activity in the steam phase. In addition, organic additives tend
to thermally decompose and form potentially corrosive species

2 ~ 7
such as ammonia and or low molecular weight`organic acids. Such
decomposition can be caused by high temperatures.
In the current use of organic boiler control agents,
amine blends are employed to minimize these problems by providing
a formulation which provides both high and low volat;lities.
Such mixtures, while effective control agents may have undesir-
able properties such as low flash points, incompatibility with
- other additives, etc.
SUMMARY OF THE INVENTION
The present invention relates to methods of controlling
the pH of a steam generating system in which a single amine pH
control treatment agent is employed. The amine control agents of
the present invention are "double buffers" which help to maintain
the pH of both the liquid and steam phases within the system in a
preferred range, typically ~.5 to 9.5. The control agents of the
present invention also exhibit a strong buffering capacity and do
not significantly break down into ammonia within a steam
generating system.
The control agents of the present invention maintain
both liquid and steam pH's in a steam generating system by the
addition of a single "double buffer" amine. The control agents
of the present invention are non-oxygen containing alkylated

~1097
diamines with at least two-carbons between the diamine nitrogens
of the general formula:
R R
N - [ CH2 ] - N
/ x
Rl Rl
wherein R is H or alkyl, Rl is H or alkyl, R and Rl are not
both H and x equals 2 or more and preferably 2 to 6. The control
agents of the present invention exhibit very little decomposition
to ammonia in either the steam or liquid phases while maintaining
both steam and liquid phase pH above a threshold level. The
control agents are effective when added at relatively low
concentrations.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The inventors discovered a specific class of amines which
act as "double buffers" in the treatment of a boiler system. By
"double buffer" is meant that the control agents will effectively
control pH in both the liquid and steam phases. The control agents
were found to provide ef~ective pH buffering to maintain a desir-
able magnetite layer throughout a boiler system at relatively low
treatment concentrations. The control agents maintain a desirable
magnetite layer by controlling pH to the range of about 8.5 to
9.S. The buffers of the present invention show a resistance to
degradation into ammonia.

2~910~7
The buffers of the present invention are non-oxygen.
containing alkylated diamines having at least two carbons between
the diamine nitrogens of the general formula:
R \ R
N [ CH2 ] N
x
Rl Rl
wherein R and Rl are hydrogen or alkyl (preferrably Cl - C6
alkyl), R and Rl are not both hydrogen and x is 2 or more
preferrably 2 to 6. Exemplary diamines include:
N,N,N',N'-tetramethylethylenediamine, N, N'-dimethylethylene-
diamine, and N,N',N'-tetramethylpropylenediamine.
The effectiveness of representative non-oxygen containing
alkylated diamines of the present invention was evaluated in
testing perfor~ed in research scale boilers. The research boilers
employed in the following example are D-shaped electrically heated
stainless steel assembl;es with 4000 watt electrical immersion
heaters that produce 19.8 pounds per hour of steam at a maximum
operating pressure of 1,500 psig. Operation o~ the research
boilers is described in U.S. Patent No. 4,288,327 incorporated
herein by reference.

9 7
The tests were run at 1,45~ psig at 50 cycles for 44 hours
with no condensate return. Daily inalyses were made of the steam
and blowdown (liquid) products. The feedwater for the boilers was
supplied by a demineralizer train and treated with the control
agent alkylated diamine. Hydroquinone at a treatment rate of 0.3
ppm was added to each feedwater tank as an oxygen scavenger.
Hydroquinone was selected as the oxygen scavenger due to the need
for quantitative testing for ammonia. The control agent being
tested was fed at a concentration of 10 ppm in the feedwater tank.
The boilers were run for 44 hours to establish equilibrium between
the test chemistry and the research boiler. After 44 hours
samples were analyzed for ammonia and pH.
A control agent was deemed successful if it produced the
following results for this test methodology:
(1) steam pH above 8.5
(2) blowdown pH above 8.5
(3) difference between steam and blowdown pH not
greater than 1.5 pH units
(4) ammonia levels in the steam and blowdown not
to exceed 0.4 ppm
Table 1 summarizes the results of such testing on a
variety of amines and shows the unique effectiveness of the class
of amines of the present invention.

~91~97
.
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TABLE 1
- Test Run Results
All concentrations in ppm
Control Agent
5 (10 ppm in DH Ammonia
Feedwater~ _ BD Stm BD Stmlest Results
TMEDA 9.66 9.10 < 0.05 0.09 Pos
DMEDA 9.40 9.25 0.21 0.26 Pos
TMPDA 9.26 8.66 0.27 0.22 Pos
10 Morpholine 8.73 8.30 0.12 0.16 Neg
DMAPA 9.48 9.02 0.33 0.75 Neg
DMA 7.32 9.76 < 0.3 0.3 Neg
. NOTES: BD = Blowdown
Stm = Steam
TMEDA = N,N,N'-tetramethylethylenediamine
DMEDA = N,N'-dimethylethylenediamine
TMPDA = N,N,N',N'-tetramethylpropylenediamine
DMAPA = Dimethyleaminopropylamine
DMA = Dimethylamine
While this invention has been described with respect to
particular embodiments thereof, it is apparent that numerous other
forms and modifications of this invention will be obvious to those
skilled in the art. The claims and this invention generally
should be construed to cover all such obvious forms and modifi-
cations which are within the true spirit and scope of the present
invention.