Method and composition for oxygen scavenging

Abstract: A method of treating an aqueous system is described which comprises incorporating in the system a hydroxylamine derivative of the general formula: wherein each of R₁, R₂ and R₃ which may be the same or different represents hydrogen, alkyl of 1 to 8 carbon atoms or aryl or a water soluble salt thereof or a precursor thereof, and a tannin in a weight ratio of at least 2:1. This is particularly useful for boiler water systems.

Description: This invention relates to a method of retarding or controlling corrosion in aqueous systems, especially boiler water systems, due to dissolved oxygen. Dissolved oxygen is objectionable in water used for industrial purposes because of the corrosive effect on ferrous and yellow metals with which the water comes into contact. In cold and hot water lines, failure may occur in the piping and the lines may become blocked with the products of corrosion. "Red water" and iron stains may result caused by iron brought into solution by the corrosive attack of dissolved oxygen. It is well known that increased temperatures and low pH values accelerate oxygen attack. In boiler systems dissolved oxygen is a principal factor influencing corrosion of metals which make up feed lines, heaters, economisers, boilers, and steam and return lines. It is known to utilise oxygen scavengers for the purpose of eliminating the dissolved oxygen. These include sodium sulphite, hydrazine as well as hydroxylamine and derivatives. It will be appreciated that there are advantages in using a volatile oxygen scavenger i.e. one which does not result in any solid residues. While sodium sulphite has been used for many years in low pressure boilers it cannot be used satisfactorily in high pressure boilers because it tends to decompose giving rise to acidic gases, principally sulphur dioxide and hydrogen sulphide which can give rise to corrosion. While hydrazine overcomes this difficulty it is a toxic chemical so that special precautions have to be taken over ics use. More recently, therefore, use has been made of certain hydroxylamine derivatives, in particular N,N-diethylhydroxylamine (see, for example, US-A-4,067,790). However, the use of such derivatives is not entirely satisfactory under all conditions because at relatively low temperatures its reaction with dissolved oxygen is rather slow. This becomes a particular problem when the derivative is added, as is normal, to the feed line in order to prevent corrosion in the boiler. Addition of the derivative to the steam and return lines is generally less of a problem because the temperatures are higher. FR-A-2560888 discloses a method of treating an aqueous system, in particular a recirculating water system, which comprises added thereto a tannin, a water soluble salt of a metal of Group II of the Periodic Table and a hydroxylamine or a neutralising amine; the amounts of tannin and hydroxylamine derivative are said to be roughly equal. EP-A-283243 discloses the use of a class of nitrones to inhibit corrosion in aqueous systems. JP-A-57204288 discloses a method of removing oxygen from water using a hydroxylamine as oxygen scavenger and tannic acid. The present invention is based on the surprising finding that the oxygen scavenging activity of certain mixtures of hydroxylamine derivatives can be catalysed by the presence of small amounts of a tannin. Accordingly the present invention provides a method scavenging oxygen from a closed aqueous system which comprises incorporating in the system a mixture of hydroxylamine derivatives of the general formula: wherein R 1 represents alkyl of 1 to 8 carbon atoms or aryl, R 3 represents hydrogen, alkyl of 1 to 8 carbon atoms or aryl and in one said derivative R 2 is hydrogen and in another said derivative R 2 is alkyl of 1 to 8 carbon atoms or aryl, each said derivative being incorporated as such or as a water soluble salt thereof such as a chloride, sulphate, acid sulphate, phosphate or sulphite, or a precursor thereof and a tannin, the weight ratio of the mixture of hydroxylamine derivatives to tannin being of at least 2:1. Typical values of R 1 , R 2 and R 3 include (where appropriate) hydrogen, ethyl, isopropyl, phenyl, benzyl and tolyl. Preferred hydroxylamine derivatives include those in which at least one or R 2 and R 3 represents hydrogen, ethyl or isopropyl. Specific preferred compounds include hydroxylamine, N,N-diethylhydroxylamine (DEHA) and N-isopropyl hydroxylamine (NIPHA). These compounds have the ability to passivate a boiler metal; it is believed that they do so by generating magnetite. A mixture of DEHA and NIPHA is preferred since they complement each other in their activity. Thus the NIPHA or other N-hydrocarbylhydroxylamines scavenge oxygen faster than the DEHA or other N,N-di-hydrocarbylhydroxylamines. As a result it is possible to use less of the tannin activator and the fee line can be scavenged of oxygen by the NIPHA (under relatively mild conditions), leaving most of the DEHA to volatilise in the boiler and pass through into the steam lines where it can scavenge oxygen. In general the weight ratio of the two types of derivative will be from 7:1 to 1:7, especially 3:1 to 1:3. By a "precursor" we mean a compound which converts to a hydroxylamine derivative in the system. Typical examples of such precursors include oximes which are converted in the boiler to hydroxylamine. These will generally have the formula: where R 1 and R 2 are as defined above; specific examples include methylethylketoxime, acetaldoxime, butyraldoxime and propionaldoxime. Other precursors include the nitrones which generally have the formula: in which R 1 and R 2 each independently represent hydrogen, alkyl of 1 to 8 carbon atoms or aryl and R 3 represents a hydrocarbon group of 1 to 10 carbon atoms such as a saturated or unsaturated aliphatic acyclic or cyclic group, an aryl group or an aralkyl group, or a water soluble salt thereof. Specific examples include formaldehyde isopropylnitrone, formaldehyde ethylnitrone, acetaldehyde isopropylnitrone, acetaldehyde ethylnitrone, acetone isopropylnitrone and acetone ethylnitrone. Further details of the use of such nitrones can be found in EP-A-283243, to which reference should be made for further details. References hereafter to hydroxylamine derivatives should therefore be interpreted as including the precursors as well. Tannins are a large group of water-soluble, complex organic compounds which can be obtained from the various wood and vegetation materials found throughout the world. Almost every tree or shrub that grows contains some tannins in the leaves, twigs, barks, wood or fruit. Examples of barks are wattle (or mimosa), mangrove, oak, eucalyptus, hemlock, pine, larch and willow. Examples of woods are quebracho, chestnut, oak and urunday. Examples of fruits are myrobalans, valonia, divi-divi, tara and algarrobilla. Examples of leaves are sumac and gambier and examples of roots are canaigre and palmetto. Among the preferred materials is the quebracho wood. A spray-dried quebracho powder is commercially available. These natural tannins can be categorised into the traditional "hydrolyzable" tannins and "condensed tannins" as disclosed by A. Pizzi in "Condensed Tannins for Adhesives", Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 359-369. Condensed tannin extracts include those manufactured from the bark of the black wattle tree (or mimosa tannin of commerce), from the wood of the quebracho tree (Spanish: Quebra haca, axe-breaker,) from the bark of the hemlock tree, and from the bark of several commonly used pine species. The preparation of wattle (mimosa) and quebracho extracts is a well established industrial practice and such extracts are freely available in considerable amounts, as are extracts from chestnut. While hydrolyzable tannins are generally mixtures of simple phenols and esters of a sugar, condensed tannin extracts, such as wattle and quebracho, are composed of approximately 70% polyphenolic tannins, 20% to 25% non-tannins, mainly simple sugars and polymeric carbohydrates (hydrocolloid gums), the latter of which usually constitute 3% to 6% of the extract and heavily contribute to extract viscosity, while the balance is accounted for by a low percentage of moisture. Although the exact structure is not known, it is believed that the main polyphenolic pattern in quebracho tannins is represented by flavenoid analogues based on resorcinol A and pyrogallol B rings as shown in formula 1 below: Preferred tannins for use in the present invention are those derived from mimosa, chestnut and quebracho with tannins derived from mimosa being most preferred. It will be understood that the natural tannin may be replaced in whole or in part by a synthetic tannin or tannin derivative, in particular synthetic tannins whose structure contains functionalities of phloroglucinol, resorcinol, catechol and pyrogallol, for example Resorcinex 9901L obtainable from Advance Resin Systems. Sulphonated synthetic tannins have been found to be particularly effective. Sulphonated Resorcinex 9901 L is believed to have the structure: While it is known that tannins have some ability to prevent corrosion of yellow metal such as copper (various vegetable tannins can absorb oxygen and react with iron to form a protective iron tannate film), it is surprising that they have a catalytic effect on the action of the mixture of hydroxylamines to eliminate oxygen. Thus the use of the combination of the mixture of hydroxylamine derivatives and tannin results in faster scavenging and is effective at signficantly lower temperatures than the use of hydroxylamine derivatives alone. While it is possible to incorporate the derivatives separately it is generally more convenient to add them together. Accordingly, the present invention also provides a composition suitable for addition to a closed aqueous system which comprises a mixture of hydroxylamine derivative as defined above or a water soluble salt thereof or a precursor thereof and a tannin in a weight ratio of at least 2:1. The amounts of hydroxylamine derivatives used in the present invention will depend on the amount of dissolved oxygen in the system. In general 1 to 70 parts of the mixture of hydroxylamine derivatives are used per part of oxygen, more particularly to 2 to 20 parts. Preferred amounts are about 4 parts of DEHA and 7 parts of NIPHA. The weight ratio of the mixture of hydroxylamine derivatives to tannin will in general be from 2:1 to 200:1, preferably 2:1 to 40:1 and especially 3:1 to 20:1. The preferred weight ratio is about 4:1. In the formulation the amount of the mixture of hydroxylamine derivatives will generally be from 5 to 50% by weight, especially 10 to 30% by weight and more especially 15 to 25% by weight, with corresponding concentrations of the tannin. With the preferred ratio the most preferred composition contains about 20% by weight of hydroxylamine derivative and about 5 weight % of tannin. It will be appreciated that the compositions will normally take the form of aqueous solutions. The method of the present invention is particularly useful for dosing feed lines where the temperatures are relatively low, typically from 5 to 120°C, generally from 15 to 80°C, and especially from 20 to 75°C. The products can, though, also be added directly to the boiler or to the condensate and return lines although the effectiveness varies with the precise hydroxylamine derivative employed. In particular DEHA is effective in the condensate lines having volatilised in the boiler. On the other hand NIPHA will tend to decompose more readily in the boiler. It will be appreciated that the boiler water systems can also have included small amounts of additives normally used for the control of corrosion, scaling, sedimentation, and hardeners as well as pH control. In connection with the latter the boiler water will normally be kept at a pH from 8 to 13 while the pH of the feed will generally be from 7 to 10.5 and more particularly from 8.5 to 9.5. In fact it has been found that a high pH results in a faster oxygen removal rate. A pH of 8 to 12 is generally preferred in the boiler. Particular additives which may be included to advantage are neutralising amines; typical examples of these are cyclohexylamine, butanolamine, morpholine, monoethanolamine, dimethylaminothanol, methoxypropylamine and dimethylaminopropylamine. Other useful additives include the molybdates, such as sodium molybdate. The further Examples further illustrate the present invention: EXAMPLES The following tests carried out on a laboratory recirculating rig illustrate the very good oxygen scavenging, and hence oxygen corrosion inhibition, which can be obtained using a blend of hydroxylamine derivatives and tannin. Test Conditions System water Demineralised water pH 10.5 Temperature 50°C Flow rate of water 200 mls/min past probe head Stirring rate 400 revs/min Test Conditions Cont'd. Duration of test 120 secs Concentration of oxygen scavenger in reaction flask 50 ppm Active Concentration of activator in reaction flask 12.5 ppm Active (MIMOSA Tannin) Total blend concentration in reaction flask 62.5 ppm Active (DEHA and NIPHA + Mimosa Tannin) The Figure illustrates the effectiveness of Mimosa tannin in various blends of NIPHA with DEHA using 20% by weight of DEHA and 1% by weight of activator. The Figure illustrates the % oxygen remaining after 45 seconds.

Claims: A method of scavenging oxygen from a closed aqueous system which comprises incorporating in the system a mixture of hydroxylamine derivatives of the general formula:- wherein R 1 represents alkyl of 1 to 8 carbon atoms or aryl, R 3 represents hydrogen, alkyl of 1 to 8 carbon atoms or aryl and in one said derivative R 2 is hydrogen and in another said derivative R 2 is alkyl of 1 to 8 carbon atoms or aryl, each said derivative being incorporated as such or as a water soluble salt thereof or precursor thereof, and a tannin, the weight ratio of the mixture of hydroxylamine derivatives to tannin being at least 2:1. A method according to claim 1 in which at least one of R 2 and R 3 represents hydrogen, ethyl or isopropyl. A method according to claim 1 or 2 in which one of the hydroxylamine derivatives is N, N-diethylhydroxylamine or N-isopropylhydroxylamine. A method according to claim 1 in which the hydroxylamine precursor is an oxime or nitrone. A method according to any one of the preceding claims in which the tannin is derived from mimosa, chestnut or quebracho. According to any one of the preceding claims in which 1 to 70 parts of the mixture of hydroxylamine derivatives are present per part of dissolved oxygen. A method according to claim 6 in which from 2 to 20 parts of the mixture of hydroxylamine derivatives are present per part of dissolved oxygen. A method according to any one of the preceding claims in which the weight ratio of the mixture of hydroxylamine derivatives to tannin is from 2:1 to 200:1. A method according to claim 8 in which the said weight ratio is from 3:1 to 20:1. According to any one of the preceding claims in which the weight ratio of the two different hydroxylamines in said mixture is from 1:7 to 7:1. According to any one of the preceding claims for the treatment of a boiler water system. A method according to claim 11 in which the components are incorporated in the feedline at a temperature of 15 to 80°C. A composition for addition to a closed aqueous system in order to scavenge oxygen therefrom which comprises a mixture of hydroxylamine derivatives defined in any one of claims 1 to 3 each said derivative being present as such or as a water soluble salt thereof or precursor thereof and a tannin in a weight ratio of at least 2:1. A composition according to claim 13 in which the hydroxylamine derivative is present as the corresponding oxime or nitrone. A composition according to claims 13 or 14 in which the tannin is derived from mimosa, chestnut or quebracho. A composition according to any one of claims 13 to 15 in which the weight ratio of the mixture of hydroxylamine derivatives to tannin is from 2:1 to 200:1. A composition according to claim 16 in which the said weight ratio is from 3:1 to 20:1. A composition according to any one of claims 13 to 17 which comprises 5 to 50% by weight of the mixture of hydroxylamine derivatives as an aqueous solution. A composition according to any one of claims 13 to 18 in which the weight ratio of the two different hydroxylamines in said mixture is from 1:7 to 7:1. A composition according to any one of claims 13 to 19 which also comprises a neutralising amine. A method according to any one of claims 1 to 12 which comprises adding to the system a composition as claimed in any one of claims 13 to 20.

Inventors: FAN JOSEPH CHWEI-JER, GREAVES BRIAN, HWA CHIH MING, POOLE STEPHEN CLIFFORD, FAN, JOSEPH CHWEI-JER, GREAVES, BRIAN, HWA, CHIH MING, POOLE, STEPHEN CLIFFORD
Applicants: BETZDEARBORN INC
Application year: 1993
Main classification: C23F11
IPC codes: C23F11, C02F1