EP0807696A1 - Alkalizing agent for water conditioning - Google Patents

Abstract

An alkalisation agent especially for use in conditioning boiler feed water comprises three different amines: (A) an alkanolamine; (B) an optionally substituted cyclohexylamine; and (C) a long-chain aliphatic polyamine.

Description

The present invention relates to an alkalizing agent for aqueous media in the form of a mixture of amines. The alkalizing agent is particularly intended for use in water-steam cycles of power plants which have a once-through boiler as the steam generator.

It is known that steam generating plants must be protected against corrosion in order to ensure safe and trouble-free continuous operation. The feed water for once-through boilers, the feed and boiler water of circulation boilers and the injection water for steam temperature control must meet certain quality requirements so that both the steam generator can be operated safely and economically and the steam cleanliness required for superheaters and downstream turbines can be achieved. If the equipment is circulated, water and steam quality must not be considered with regard to individual system parts, but always only in connection with the entire water-steam cycle.

A very widely used possibility for conditioning is the alkalization of the water-steam cycle, which is based on the fact that the dissolution of iron from the apparatus parts, which is observed at high temperatures, is reduced by raising the pH. An overview of the conditioning of feed water with alkalizing agents can be found in the VGB guideline for boiler feed water, boiler water and Steam from steam generators over 68 bar permissible operating pressure (VGB-R 450 L), published by the VGB technical association of large power plant operators eV, Essen 1988.

Forced-flow boiler systems are found particularly in power plants of the electricity supply companies and in large industrial plants because they allow in large units, even under supercritical conditions, i.e. to generate steam at very high pressures and high temperatures. It is known that the thermal efficiency of power plants improves with increasing operating temperature.

Because of the design of such steam generators and because of the requirements that the downstream turbines have to meet, it is necessary to use salt-free water for the boiler feed, which is obtained by the complete desalination of natural fresh water. All conditioning agents that are added to the water-steam cycle to inhibit corrosion must be completely steam-volatile in the case of once-through boilers, i.e. conditioning is carried out with volatile alkalizing agents.

Ammonia has been widely used as a volatile alkalizing agent. However, it has turned out to be disadvantageous that, due to its high volatility and its unfavorable distribution between water and steam, it is not possible to achieve a uniform alkalization of all parts of the circuit. One has therefore searched for less volatile bases and has already used mixtures of differently volatile amines in order to use the special conditions of Phase transition during evaporation, condensation and in the two-phase area of wet steam to inhibit the respective forms of corrosion.

For example, European Patent Application No. 0'134'365 discloses a means for corrosion inhibition in steam generators or for conditioning boiler feed water for power plants with a circulation boiler and operating conditions of 35 bar / 320 ° C in the boiler. The agent consists of a "film-forming" amine and at least one alkalizing amine.

The use of the so-called "film-forming" amines, which show special properties of corrosion inhibition in the "cold" part of the circuit even in the presence of oxygen, has not been proposed and has not been disclosed under the extremely strict requirements for once-through boilers in the high-pressure and high-temperature range. Thus, the means proposed in the above patent application do not achieve the maximum conductivity of 0.2 µScm ^-1 in the condensate, which is required by the VGB guidelines (see below).

As for the hot parts of the circuit, in particular the boiler with its boiler tubes and the superheater, it has been years of operating experience that the steel surfaces in question spontaneously develop during operation due to the spontaneous magnetite formation corresponding to the Schikorr reaction at temperatures above 200 ° C Ultrapure water are protected.

• 1) Alkalisierung des gesamten Kreislaufs, besonders bei Phasenübergängen und im Zweiphasengebiet, um die Korrosionsanfälligkeit von Stahloberflächen auf ein Minimum zu bringen;
• 2) Bildung einer korrosionsinhibierenden Membran bei Temperaturen unter 200 °C;
• 3) Ausreichende Temperaturbeständigkeit bis 560 °C, um in der Turbine und im Kondensat nach Passieren des Überhitzers wirksam zu sein;
• 4) Maximale Leitfähigkeit des aus der Turbine abströmenden Kondensats gemäss VGB-Richtlinie von 0,2 µS·cm^-1;
• 5) Ökotoxikologische Unbedenklichkeit bei bestimmungsgemässem Einsatz; und
• 6) Annehmbarer Preis bzw. optimales Kosten-Nutzen-Verhältnis.

The most important requirements for a good conditioning agent for a water-steam circuit in once-through boilers with a downstream turbine and return of the condensate are the following:

• 1) Alkalization of the entire circuit, especially during phase transitions and in the two-phase area, in order to minimize the susceptibility to corrosion of steel surfaces;
• 2) formation of a corrosion-inhibiting membrane at temperatures below 200 ° C;
• 3) Sufficient temperature resistance up to 560 ° C to be effective in the turbine and in the condensate after passing through the superheater;
• 4) Maximum conductivity of the condensate flowing out of the turbine according to the VGB guideline of 0.2 µS · cm ^-1 ;
• 5) Ecotoxicological harmlessness when used as intended; and
• 6) Acceptable price or optimal cost-benefit ratio.

The object of the invention was to provide an alkalizing agent that meets the requirements specified above and is particularly suitable for use as a water conditioning agent in high-performance steam boiler systems for corrosion protection. On the one hand, the corrosion protection effect in the steam boiler should be achieved, and on the other hand, all downstream components, in particular the turbine, should be protected against corrosion.

It has now been found that a mixture of certain amines fulfills the object of the invention above and also has further advantageous properties. According to the invention, the agent is composed of at least three different types of amines, namely (a) a monoalkanolamine, (b) optionally substituted cyclohexylamine and (c) at least one film-forming long-chain aliphatic polyamine, also known as fatty polyamine.

The role of the first component, namely the monoalkanolamine, is seen in the fact that it primarily serves to permanently alkalize the aqueous phase including the condensate.

The amino alcohol used is in particular one which has a boiling point between 150 and 190 ° C., for example monoethanolamine (1-aminoethanol) or isopropanolamine (2-amino-2-methyl-1-propanol). The use of monoethanolamine is currently preferred. However, other amino alcohols can also be used, provided that they have the properties described above.

The second component of the alkalizing agent according to the invention is optionally substituted cyclohexylamine. It is an amine volatile with water vapor, which forms an amine-containing, ie basic, protective layer and stabilizes the magnetite protective layer. It was found that the layer mentioned consists predominantly of dicyclohexylamine, which, under the conditions of use in a steam boiler, was formed from two molecules of cyclohexylamine according to the following scheme: $${\text{2 C}}_{\text{6}}{\text{H}}_{\text{11}}{\text{NH}}_{\text{2nd}}{\text{⇒ (C}}_{\text{6}}{\text{H}}_{\text{11}}{\text{)}}_{\text{2nd}}{\text{NH + NH}}_{\text{3rd}}$$

Normally, the present invention proposes to use unsubstituted cyclohexylamine; but there are also inert substituted cyclohexylamines, such as. B. dicyclohexylamine, methylcyclohexylamine, ethylcyclohexylamine, dimethylcyclohexylamine and others. Such compounds are by hydrogenation of the corresponding aromatic amines or nitro compounds easily accessible.

Furthermore, the optionally substituted cyclohexylamine acts as a hydrotrope in the preparation of the composition according to the invention and also has the property of increasing the volatility of the third component described below and of bringing it to the turbine in larger amounts than hitherto.

• R einen aliphatischen Kohlenwasserstoffrest mit 12 bis 24 Kohlenstoffatomen,
• p eine ganze Zahl von 1 bis 6, vorzugsweise 3, und
• n eine ganze Zahl von 1 bis 6, vorzugsweise 1.

The third component of the alkalizing agent according to the invention is a long-chain aliphatic polyamine. Suitable polyamines are known in large numbers and are commercially available. The fatty polyamines which can be used in the context of the invention correspond to the general formula

R― [NH― (CH ₂ ) _p -] _n ―NH ₂

in which mean:

• R is an aliphatic hydrocarbon radical having 12 to 24 carbon atoms,
• p is an integer from 1 to 6, preferably 3, and
• n is an integer from 1 to 6, preferably 1.

The effect of these long-chain fatty polyamines is based on the formation of a hydrophobic, firmly adhering protective membrane on the base, which is normally a very thin layer of magnetite. This mode of action is already known from the literature. In addition, however, another property has been found: the fatty polyamine complexes iron (II) ions emerging from the metal, and this complex promotes formation when it is oxidized from Magnetit. Any oxygen present is bound in this way.

The alkalizing agent according to the invention accordingly not only acts to set the desired pH value in the feed water and in the steam boiler, but also forms corrosion protection layers in all parts of the system.

Examples of such polyamines are Dodecylaminomethylenamin, Dodecylaminodimethylenamin, Dodecylaminotrimethylenamin, the corresponding tetradecyl, hexadecyl and octadecylamine, Octadecenylaminotrimethylenamin, Octadecenylaminodi- (trimethylamino) -trimethylenamin, Palmitylaminotrimethylenamin, Hexadecenylaminotrimethylenamin, Dodecenylaminotrimethylenamin, etc. At present, the N-oleyl-1,3-propanediamine (ie octadecen (9) ylaminotrimethyleneamine) is preferred.

The proportions of the three components of the alkalizing agent according to the invention described above can vary within wide limits depending on the conditions of use and the individual substances chosen in each case. If, for example, component (a) monoethanolamine, component (b) cyclohexylamine and component (c) oleylaminopropyleneamine are chosen, the weight fractions of components (a), (b) and (c) are pure, i.e. 100% related connections, such as (40-70) :( 30-50) :( 4-10), the sum of the three components adding up to 100.

The alkalizing agent according to the invention can also contain other constituents, generally in very small amounts, so as not to impair the effect. For example, you can stain the labeling agent.

The alkalizing agent according to the invention is preferably formulated as an aqueous, relatively concentrated solution which can be easily metered into the system to be conditioned. An aqueous mixture containing about 4 to 25% by weight of the alkalizing agent is a clear, stable solution, although it contains up to about 4% by weight of fatty alkyl polyamine, which is only slightly water-soluble per se. As already mentioned above, it has surprisingly been found that the cyclohexylamine is an effective solubilizer, that is to say a hydrotropic substance, for the fatty polyamine. To prepare the aqueous solution of the agent, the procedure is advantageously firstly to prepare a relatively concentrated aqueous solution of monoethanolamine, which is freely miscible with water, and then to stir in a previously prepared mixture of the fatty aminoalkylene amine and the cyclohexylamine. A relatively concentrated aqueous solution is obtained in this way, which can then be diluted without restriction with water, preferably pure, demineralized and non-carbonated water, preferably to an approximately 15% solution. The latter has a pH of about 12 to 12.5.

When using the alkalizing agent according to the invention in high-performance steam generators and power plants, that is to say with an operating pressure of well over 68 bar, for example over 150 bar, and at operating temperatures above 500 ° C., it has surprisingly been found that the requirement specified in the above-mentioned VGB guideline Limit value of the conductivity in the steam and condensate, namely 0.2 µS · cm ^-1 , which is determined after a strongly acidic ion exchanger, could be reached and fallen below in continuous operation. This is the first time that an alkalizing agent is available that meets the requirements listed above and the requirements and after of the VGB guideline. It was also found that the use of particularly pure or purified starting products is very cheap and leads to even lower conductivity values. The reason for this is seen in the fact that impurities in the conditioning agent decompose or are oxidized to form acidic substances during operation in high-performance power plants, which increase the conductivity of the water and also render some of the alkalizing agent ineffective. For this reason, when producing the amine mixture, it is recommended to use raw materials that are as pure as possible and to work under a protective gas that keeps the carbon dioxide away.

As a dosing quantity for use as a water conditioning agent in high-performance steam boilers, the alkalizing agent according to the invention is generally used in proportions which correspond to an active ingredient concentration in the water of about 0.5 to 5 ppm. Higher shares can also be provided, should this be necessary in special cases. Since the agent of the invention can be provided as an aqueous solution, it is possible to dose easily and precisely.

The alkalizing agent according to the invention was tested in a large-scale test for its effectiveness. A solution of the above-described alkalizing agent was added to the feed water in the metering station for conditioning agent of a power plant block, containing a once-through boiler, operating pressure 180 bar (18 MPa), main steam temperature 540 ° C, and a steam turbine with an output of 311 MW, such that a Concentration of 1 to 2 mg / l (1 to 2 ppm) was set and maintained. Steam samples were continuously taken from the live steam line, the samples through Cooling condenses, the condensate is passed through a strongly acidic ion exchanger and finally the conductance is measured in the outlet of the ion exchanger. The pH value of the condensate (in front of the ion exchanger) was in the range from 8.7 to 9.5 with a most common value of 9.4. The conductivity was approximately 0.15 to 0.16 µS · cm ^-1 . It must be said that the guidelines of the VGB (Technical Association of Large Power Plant Operators eV) for boiler feed water for corresponding power plants have a pH value of 9.5 ± 0.2 and for conductivity a value of <0.2 µS · cm ^-1 provide.

The use of the alkalizing agent of the invention is not limited to a once-through boiler; it is clear to the person skilled in the art that the concept of the invention can be applied to all boiler and power plant systems, e.g. also circulation boiler.

The alkalizing agent according to the invention can not only be used as a water conditioning agent for high-performance steam boilers, but it can also be used wherever a mixture of amines is to be used, for example as a neutralizing agent, for the production of soil waxes based on fatty acid amides and imides, complexing agents and the like.

Claims (12)

Alkalizing agent for aqueous media in the form of a mixture of amines, characterized in that it is composed of at least three different types of amines, namely (a) at least one alkanolamine, (b) optionally substituted cyclohexylamine and (c) at least one long-chain aliphatic polyamine. Alkalizing agent according to claim 1, characterized in that it is present as an aqueous solution. Alkalizing agent according to Claim 2, characterized in that the concentration of the amine mixture in the solution is 10 to 18% by weight, in particular approximately 15% by weight. Alkalizing agent according to claim 1, characterized in that it contains as component (a) monoethanolamine. Alkalizing agent according to Claim 4, characterized in that it contains cyclohexylamine as component (b). Alkalizing agent according to claim 1, characterized in that it is as component (c) a polyamine of the formula

R― [NH― (CH ₂ ) _p -] _n ―NH ₂

contains, in which mean: R is an aliphatic hydrocarbon radical having 12 to 24 carbon atoms, p is an integer from 1 to 6 and n is an integer from 1 to 6. Alkalizing agent according to Claims 4 and 5, characterized in that it contains N-oleyl-1,3-propanediamine as component (c). Alkalizing agent according to one of the preceding claims, characterized in that the components (a), (b) and (c), calculated as dry matter, are in a weight ratio of (40-70) :( 30-50) :( 4-25) , where the sum of the three components adds up to 100. Process for conditioning boiler feed water, in particular for high-performance boiler systems, characterized in that an alkalizing agent according to one of claims 1 to 8 is metered into the feed water. A method according to claim 9, characterized in that one uses the agent according to claim 8. A method according to claim 9 or 10, characterized in that the amount of the agent is metered so as to adjust and maintain a concentration of 0.5 to 5 ppm of the agent in the boiler. Method according to one of claims 9 to 11, characterized in that it is used as a conditioning agent for the boiler feed water of a once-through boiler with a downstream turbine, the operating pressure above 150 bar and the steam temperature above 500 ° C.