NO780989L - ANTI-CORROSION AGENT FOR IRON AND STEEL - Google Patents

Description

Anti-corrosion agent

for iron and steel

The present invention relates to a new inhibitor

which is suitable for preventing corrosion of solvents used in the treatment of sour gas streams, as well as the inhibited solvent.

Conditioning of naturally occurring and synthetic gases by absorption of acidic gases such as CC>2, H2^' COS 0<^ HCN

in an absorbent solution has been used in industry for many years. Gases such as the starting gas for the production of ammonia, natural gas and pipe gases are examples. Aqueous solutions of various compounds such as alkanolamines, sulfolane (tetrahydrothiophene-1,1-dioxygen), potassium carbonate and mixtures of two or more of these have been used for this purpose. The water can be completely or partially replaced with a glycol. All these systems are exposed to corrosion on metallic parts of the apparatus, which corrosion can be caused by products formed by the breakdown of the absorbent, by acidic components or by reaction products of these acidic components and the absorbent. For example, it is mentioned that although alkanolamine in and of itself is not very strongly corrosive to iron and steel, it becomes very strongly corrosive in the presence of dissolved CO2 and smaller amounts of H2S, especially after heating. For

to combat this problem, various metal compounds have been used, alone or together with other compounds, as corrosion inhibitors, for example compounds of arsenic, antimony and vanadium. While such metal compounds are effective corrosion inhibitors, they have the disadvantage that they are poorly soluble

in most gas conditioning solutions, and that they are relatively highly toxic. The latter property is particularly undesirable, as it entails disadvantages both in the treatment of the solvent and the disposal of waste material. They are also ineffective in the presence of H2S.

The problems with the toxicity and corrosion described above have essentially been overcome by means of the present invention. This relates to an agent for inhibiting corrosion of iron and steel by carbon dioxide and possibly smaller amounts of hydrogen sulphide in gas conditioning solutions, characterized by the wood inhibiting concentration in said solution of a combination of one part by weight of a quaternary pyridinium salt and (1) 0.001-10 parts of a thio compound which is a water-soluble thiocyanate, a water-soluble sulfide, or an organic thioamide, or (2) 0.01-10 parts of a lower alkylene pblyamine, a corresponding polyalkylene polyamine, or a mixture thereof, wherein the alkylene units contain 2-3 carbon atoms.

In principle, any pyridinium salt that is stable can

in the gas conditioning solution, is used according to the invention. Preferably, this salt has the formula:

where R is an alkyl radical with 1-20 carbon atoms, a benzyl radical or an alkylated benzyl radical, where the aromatic ring has one or more alkyl substituents with a total of 1-20 carbon atoms, R<1> is a hydrogen atom or an alkyl radical with 1-6 carbon atoms, and X is any convenient anionic radical such as halide, sulfate, acetate, or nitrate. In the above general formula, X is preferably a bromine or chlorine atom, of which bromine is preferred.

The best results are obtained when at least one of the substituents

R' represents an alkyl radical, and particularly good inhibition has been found when the pyridine ring has several alkyl substituents. Preferably, R is a higher alkyl radical with approx. 10-18 carbon atoms.

The thio compound in the inhibitor combination is preferably a water-soluble thiocyanate, such as a. alkali metal thiocyanate or - most preferably - ammonium thiocyanate. It can also be an organic thioamide, and in principle any such compound can be used. This class of compounds includes a thiourea, a polythiourea, a hydrocarbon-substituted derivative thereof or a thioamide of the formula:

where A is a hydrocarbon radical with 1-12 carbon atoms or a pyridyl radical, and R" is a hydrogen atom or an alkyl radical with 1-8 carbon atoms. Thioamides such as thiourea, 1,2-diethylthiourea, propylthiourea, 1,1-diphenylthiourea, thiocarbanilide, 1 ,2-dibutylthiourea, dithiobiurea, thioacetamide, thionicotinamide, and thiobenzamide are typical representatives of this class. Water-soluble sulfides such as ammonium sulfide, an alkali metal sulfide, or similar hydrosulfide, including ^S, are other thio compounds that may be used.

While any significant amount of the combination of the pyridinium salt and the thio compound will provide some degree of corrosion inhibition, to obtain practically satisfactory protection it will usually be necessary to use at least 100 parts per million (ppm) of the combination in the gas conditioning solution. More than approx. 2,000 ppm of the inhibitor combination will usually provide little or no additional protection. The thio compound alone or the pyridinium salt alone gives no inhibition or only partial inhibition. However, it has been found that very small amounts of the thio compound in the presence of the pyridinium salt are usually required, with concentrations as low as 1 ppm of the thio compound in the presence of 50-100 ppm of the pyridinium salt being found to provide effective inhibition in some instances. Near the maximum degree of inhibition that can be achieved with a given combination will usually be achieved when the concentration of the thio compound reaches a concentration of 10-100 ppm. Higher concentrations of this component seem to provide little or no additional benefit under most conditions, but may help when

the concentration of the quaternary salt is disproportionately high.

On the other hand, it has been found that at least approx. 50 ppm and preferably 100-1000 ppm of the pyridinium salt is required to achieve optimal results. For each combination, a maximum of inhibition appears to occur at a particular level within the preferred ranges described above,

and higher concentrations of one or the other of the components or both provide little or no additional protection. In many cases, higher concentrations seem to cause an insignificant reduction in the degree of inhibition after a maximum has been reached.

The polyamine component includes ethylenediamine, propylenediamine, the various polymeric forms thereof, such as tetraethylenepentamine, hexaethyleneheptamine, tripropylenetetramine, dipropylenetriamine, the higher molecular weight compounds of the same type, as well as the closely related polymers of ethyleneimine and propyleneimine, as well as mixtures of any of these, including polyalkylene polyamines containing mixed ethylene and propylene groups. These polyamines with branched or unbranched chains can have molecular weights of up to several hundred thousand. The term polyalkylene polyamine shall in the present context be understood to include all these polymeric forms and mixtures thereof. Polyethylene polyamines are preferred, especially those having an average molecular weight of about 100-1000.

While any significant amount of the combination of the pyridinium salt and the polyamine will provide some degree of corrosion inhibition, to obtain practically satisfactory protection it is usually necessary to use a concentration of at least about 100 ppm of the combination in the gas conditioning solution. The polyamine alone or the pyridinium salt alone gives no inhibition or only partial inhibition. However, it has been found that relatively small amounts of the polyamine are usually required in the presence of the pyridinium salt. concentrations as low as 50 ppm of the polyamine in the presence of 50-100 ppm of the pyridinium salt have been found to provide effective inhibition in some cases. Near the maximum degree of inhibition that can be achieved with a given combination is usually achieved when the concentration of the polyamine reaches a concentration of 50-500 ppm. Higher concentrations of this component appear to provide little or no additional benefit.

On the other hand, it has been found that at least approx. 50 ppm and preferably 100-1000 ppm of the pyridinium salt is required to achieve optimal results.

The present invention provides effective inhibition against corrosion of iron and steel under the influence of acid gas conditioning solutions containing dissolved CO 2 and H 2 S using relatively low concentrations of an inhibitor combination which is easy to process and convenient to use. When the thio compound is a thioamide or a sulfide, a concentrate of the compounds can be prepared in aqueous alkanolamine, aqueous glycol, or lower alkanol, and this concentrate can be added to the gas treatment solvent as needed to provide or maintain a desired concentration. As thiocyanates tend to react with the quaternary salt on standing to form a difficult-to-dissolve, less active product, these thio-compounds are best added separately to the gas treatment solution, whereby the combination is formed in situ at higher dilution.

When the co-inhibitor is a polyamine, the concentrate should contain approx. 0.01-10 parts polyamine per part pyridinium salt, and a concentrate containing 0.1-1 part by weight polyamine per salt by weight is most preferred.

This inhibitor combination is particularly well suited in aqueous solutions of lower alkanolamine, which solutions are known in the USA as "sour gas scrubbing solvents". Preferred lower alkanolamines can be defined as compounds of the formula:

where R' and R" independently represent hydrogen or

-CR2CR2-OH, and where each R can be hydrogen or an alkyl radical with 1-2 carbon atoms. Typical alkanolamines are ethanolamine, diethanolamine, triethanolamine, isopropanolamine, diisopropanolamine and N-methyldiethanolamine. Related alkanolamines which are suitable absorbents for acid gases are "Methicol" (3-dimethylamino-1,2-propanediol) and diglycolamine (2-(2-aminoethoxy)ethanol). Second

gas treatment absorbents in which this inhibitor combination is effective include sulfolane (tetrahydrothiophene-1,1-dioxide) and aqueous potassium carbonate. These absorbents can

are used alone or in combinations of two or more, usually in aqueous solution, although the water can be wholly or partially replaced with a glycol.

The inhibitor combination according to the invention is also suitable for inhibiting corrosion of iron and steel by a gas treatment solution containing both CC^ and H^S when the latter is present in the solution in limited concentration, e.g. up to 500 ppm, preferably up to 150 ppm. The inhibitor combination is thus of wider applicability than many known inhibitors which are not effective in the presence of dissolved I^S.

Test method

The corrosion of mild steel under the influence of aqueous alkanolamine solutions saturated with CC>2 for V hours at 10-20°C was measured at elevated temperatures and moderate pressure. Flasks that were fitted with loose-fitting capsules, each containing 120

ml of test solution and a fully submerged test plate with dimensions 2.54 cm x 6.35 cm x 0.16 cm were placed in a modified pressure filter and held in this for 16-18 hours at 125°C and 2.72 ato unless otherwise stated. The test solution was 30% by weight aqueous ethanolamine unless otherwise stated. The steel plates were previously cleaned by treatment in 5N HCl for 30 minutes at room temperature, followed by washing in aqueous soap solution, washing in water and then in acetone and finally drying in air. At least two flasks with each test solution were used in each experiment, in addition to three flasks with solution containing no inhibitor, these serving as controls. After the testing, the same cleaning method was used with the exception that the HCl treatment was carried out for 15 minutes using 5N HCl inhibited with a commercial HCl inhibitor to remove any corrosion deposits. The corrosion rate and inhibition efficiency were calculated according to the formulas below using the average weight loss of the test fleets:

where the plate's density is in g/cm 3 and the plate's surface is in cm 2.

Preparation of quaternary salts

The quaternary pyridinium salts used in the inhibitors were prepared by heating a mixture of the pyridine compound with an excess of alkyl halide or benzyl halide at 90°C for two hours. The reaction mixture was then cooled, and the quaternary salt was precipitated and recovered as a solid or a viscous liquid.

The inhibitor preparations were added to the aqueous ethanolamine as a solution in a small amount of 60% by weight aqueous ethylene glycol or isopropyl alcohol.

Example 1

The quaternary pyridinium salt used in these experiments was the reaction product of tetradecyl bromide and high-boiling alkylpyridine distillation residues (HAP). These distillation residues were obtained from processes for the preparation of various lower-alkyl-substituted pyridishes, where most of the components were pyridines with several lower-alkyl substituents, especially methyl and ethyl groups. This mixed quaternary salt was tested in combination with NH₂SCN, thioacetamide, thiourea, thionicotinamide and thioisonicotinamide at various concentrations as indicated.

Example 2

Combinations of thiourea with benzylpyridinium chloridebg with dodecylbenzyl-alkylpyridinium chloride (prepared from alkyl-pyridine distillation residues as described in Example 1) were investigated with regard to inhibitory action as described above.

A combination of dodecylbenzyl-alkylpyridinium chloride with thioacetamide was also investigated.

Example 4

Quaternary salts prepared by reaction of tetradecyl bromide with various alkylpyridines were investigated as inhibitors in combination with NH^SCN by the previously described method.

Example 5

The quaternary salt in example 1 (tetradecylalkylpyridinium bromide) was examined in combination with NH₂SCN as before with the exception that 35% by weight of aqueous ethanolamide was used. Blank samples were also examined for comparison.

Example 6

The same quaternary salt as in Examples 1 and 5 was tested as before in combination with NH₂SCN at various concentrations using 15% by weight aqueous ethanolamine as the test solvent.

Examples 7-10

The quaternary salt described in Examples 1 and 5-6 was examined in combination with NH₂SCN as before using various aqueous alkanolamine-containing solutions as the test solvent.

Concentration, ppm on a weight basis Dissolution Corrosion

Example 11

Combinations of tetradecylalkylpyridinium bromide and NH^SCN were examined in 30% by weight aqueous ethanolamine saturated with C02 and containing 100 ppm sulphide ion on a weight basis added as ammonium sulphide, under otherwise the same experimental conditions as described above.

In the above experiment, the ammonium sulphide simulates the presence of H2S and serves as a thio compound in the alkanolamine solution, and the quaternary salt was thus active even in the absence of NH4SCN.

Examples 12-17

In the following examples, the quaternary salts were prepared as in examples 1-11, and the same test method was used with the exception that an H 2 S equivalent was added to the aqueous alkanolamine. This H 2 S was added to the CO 2 -saturated aqueous alkanolamine in the form of aqueous (NH 4 ) 2 S in an amount sufficient to supply sulfide and hydrosulfide ions in approximately the same concentrations as the stated H 2 S concentration would entail. In Examples 1-3, the corrosion inhibition tests were carried out at 125°C in 30% by weight aqueous ethanolamine saturated with CC^ and containing the equivalent of 100 ppm, 300 ppm and 500 ppm E^ S respectively as (NH^^ S, alt . on weight basis.

Ad example 12 (100 ppm H2 S)

Concentration, ppm by weight

TAPB = Tetradecyl bromide salt of polyalkyl pyridines in lower alkyl pyridine distillation residues (HAP). These distillation residues were obtained from processes for the production of various lower-alkyl-substituted pyridines where most of the components were pyridines with several lower alkyl substituents, especially methyl and ethyl groups.

K ( 2) PEI-3 = Polyethyleneimine with an average molecular weight of approx. 300. (<3>) E-100 = Ethylenediamine distillation residues containing 85-90% pentaethylenehexamine and hexaethyleneheptamine with some tetraethylenepentamine and small amounts of branched and cyclic isomers and derivatives.

Ad example 13 (300 ppm H^S)

Concentration, ppm by weight

Ad example 14 (500 ppm H2 S) Concentration, ppm on a weight basis

Examples 15-17 are essentially a repetition of examples 12-14 using 60% by weight aqueous diethanolamine as the ethanolamine solution. Equivalent amounts of aqueous (NH 2 ) 2 S were added as before to the CO 2 -saturated alkanolamine to provide approximately the same concentrations of sulfide and hydrosulfide ions as were formed from the indicated amounts of H 2 S.

Ad example 15 (100 ppm H2 S)

Concentration, ppm by weight

Ad Example 16 (300 ppm H2 S)

Concentration, ppm by weight

Ad example 17 (500 ppm H^S) Concentration, ppm on a weight basis

Similar effective corrosion inhibition is obtained when the quaternary salt in the above examples is replaced by the same amount of other pyridinium salts as previously indicated, e.g. dodecylbenzyl-3-ethyl-4-methylpyridinium chloride, dodecyl-alkylpyridinium bromide (prepared from HAP alkylpyridine distillation residues), tetradecyl-3-ethylpyridinium bromide and tetradecyl-trimethyl-pyridinium bromide. To a good approximation, equally good results are obtained when the polyamine component in these examples is replaced by the same concentration of polypropyleneimine with an average molecular weight of 500, triethylenetetramine, hexapropyleneheptamine or other similar polyamines as indicated above.

In the same way, effective inhibition of corrosion on iron-based metals is achieved when these combinations of quaternary pyridinium salt and polyamine are used in the indicated concentration in other sour gas conditioning solutions, as described above. E.g. aqueous or glycol-containing solutions of diethanolamine, N-methyldiethanolamine, diisopropanolamine and mixtures thereof, including mixtures with sulfolane and other known gas conditioning solvents, also aqueous potassium carbonate, will be protected by these inhibitor combinations.

Claims (4)

1. Means for inhibiting corrosion of iron and steel by carbon dioxide and possibly smaller amounts of hydrogen sulphide in gas conditioning solutions, characterized by an inhibitory concentration in the solution of a combination of one part by weight of a quaternary pyridinium salt and (1) 0.001-10 parts of a thio - compound which is a water-soluble thiocyanate, a water-soluble sulphide or an organic thioamide, or (2) 0.01-10 parts of a lower alkylene polyamine, a corresponding polyalkylene polyamine or a mixture thereof, where the alkylene units contain 2-3 carbon atoms. 2. Agent according to claim 1, characterized in that the pyridinium salt has the formula: where R is an alkyl radical with 1-20 carbon atoms, et benzyl radical or an alkylated benzyl radical, where the aromatic ring has one or more alkyl substituents with approximately 1-20 carbon atoms, R' is a hydrogen atom or an alkyl radical with 1-6 carbon atoms, and X is an anionic radical. 3. Agent according to claim 2, characterized in that the inhibitor is said pyridinium salt and said thio compound. 4. Agent according to claim 2, characterized in that the inhibitor is said pyridinium salt and said alkylene polyamine or polyalkylene polyamine.