GB1597038A

GB1597038A - Quaternary pyridinium salt inhibitor system for gas conditioning solutions

Info

Publication number: GB1597038A
Application number: GB12129/78A
Authority: GB
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1977-03-28
Filing date: 1978-03-28
Publication date: 1981-09-03
Also published as: AU519454B2; JPS53119739A; CA1084687A; NO780989L; GR64223B; NL7803292A; AU3451378A; DE2813126A1; FR2385812A1; ES468254A1; FR2385812B1; MY8200232A

Description

(54) QUATERNARY PYRIDINIUM SALT INHIBITOR SYSTEM FOR GAS CONDITIONING SOLUTIONS (71) We, THE DOW CHEMICAL COMPANY, a Corporation organised and existing under the laws of the State of Delaware, United States of America, of Midland, County of Midland, State of Michigan, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a new inhibitor composition useful for preventing corrosion by solvents used in treating sour gas streams and to the inhibited solvent.

The conditioning of naturally occurring and synthetic gases by absorbing acidic gases such as CO2, H2S, COS, and HCN in an absorbent solution has been practiced commercially for many years. Gases such as feed gas for an ammonia plant, natural gas, and flue gas are examples. Aqueous solutions of various compounds such as alkanolamines, sulfolane (tetrahydrothiophene - 1,1 - dioxide), potassium carbonate, and mixtures of two or more of these have been used for the purpose. The water may be replaced partly or entirely by a glycol. All of these systems are plagued by corrosion of metal equipment which can be caused by products of degradation of the absorbent, by acidic components, or by products of reaction of these acidic components with the absorbent. For example, although aqueous alkanolamine itself is not particularly corrosive toward iron and steel equipment, it becomes highly corrosive when there are dissolved CO2 and minor amounts of H2S present, particularly when it is hot. To combat this problem, various metal compounds have been used alone or in combination 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 disadvantages of low solubility in most gas conditioning solutions and of relatively high toxicity. The latter property is particularly undesirable since it affects both the handling of the solvent and the disposal of waste material. They are also ineffective in the presence of H2S.

The problems of toxicity and corrosion described above have been substantially overcome by the present invention, which is a composition for use in a sour gas conditioning solution comprising (a) a quaternary pyridinium salt and (b) either (i) a thio compound which is a water-soluble thiocyanate, a water-soluble sulfide, or an organic thioamide, or (ii) a lower alkylenepolyamine, a corresponding polyalkylene-polyamine, or a mixture thereof wherein the alkylene units contain 2-3 carbon atoms. The composition of the invention is generally used in the form of a sour gas conditioning solution, in which solution the total amount of (a) and (b) is sufficient to inhibit the corrosion of iron and steel by carbon dioxide and hydrogen sulfide, when present, in the solution.

Essentially any pyridinium salt which is stable in the gas conditioning solution is operable in the invention. Preferably, this salt has the formula:

where R is an alkyl radical of 1--20 carbon atoms, a benzyl radical, or an alkylated benzyl radical wherein the aromatic ring has one or more alkyl substituents totaling 1--20 carbon atoms, each R' is a hydrogen atom or an alkyl radical of 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 and most preferably bromine. Best results are obtained when at least one R' represents an alkyl radical and particularly good inhibition has been found when the pyridine ring has multiple alkyl substituents. Preferably, R is a higher alkyl radical of about 1018 carbon atoms.

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

wherein A is a hydrocarbon radical of 1--12 carbon atoms or a pyridyl radical and each R" is a hydrogen atom or an alkyl radical of 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 representative of this class. Watersoluble sulfides such as ammonium sulfide, an alkali metal sulfide, or corresponding hydrosulfide including H2S are other operable thio compounds.

While any significant quantity of the combination of the pyridinium salt and the thio compound will provide some degree of inhibition of corrosion, at least about 100 parts per million concentration of the combination in the gas conditioning solution is usually required to provide practical protection. More than about 2,000 ppm of the inhibitor combination usually provides little or no added protection. Either the thio compound or the pyridinium salt alone will provide no inhibition or only partial inhibition. It appears that very little of the thio compound is usually needed in the presence of the pyridinium salt, however, concentrations as low as one part per million of thio compound in the presence of 50-100 parts per million of pyridinium salt having been found to give effective inhibition in some cases. About the maximum degree of inhibition obtainable with a particular combination is usually found when the concentration of the thio compound reaches a concentration of 10100 parts per million. Higher concentrations of this component appear to offer little or no added benefit under most conditions but may help when the quaternary salt concentration is at a disproportionately higher level.

On the other hand, it has been found that at least about 50 parts per million and preferably 1001000 parts of the pyridinium salt is required to obtain optimum results. For each combination, a maximum degree of inhibition seems to occur at a particular level within the preferred ranges described above and higher concentrations of either component or of the combined components provide slight added protection, if any. In many cases, higher concentrations seem to cause a slight decline in the degree of inhibition after a maximum has been reached.

The polyamine component includes ethylenediamine, propylene diamine, the various polymeric forms of these such as tetraethylenepentamine.

hexaethyleneheptamine tripropylenetetramine, dipropylenetriamine, the higher molecular weight compounds of the same type and also the closely related polymers of ethylenimine and propylenimine as well as mixtures of any of these including polyalkylene polyamines containing mixed ethylene and propylene groups. These straight chain and branched chain polyamines can have molecular weights running as high as several hundred thousand. The term polyalkylenepolyamine is used herein to mean all of these polymeric forms and mixtures thereof. Polyethylenepolyamines are preferred, particularly those having an average molecular weight of about 100--1000.

While any significant quantity of the combination of the pyridinium salt and the polyamine will provide some degree of inhibition of corrosion, at least about 100 parts per million concentration of the combination in the gas conditioning solution is usually required to provide practical protection. Either the polyamine or the pyridinium salt alone will provide no inhibition or only partial inhibition. It appears that relatively little of the polyamine is usually needed in the presence of the pyridinium salt, however, concentrations as low as 50 parts per million of polyamine in the presence of 50--100 parts per million of pyridinium salt having been found to give effective inhibition in some cases. About the maximum degree of inhibition obtainable with a particular combination is usually found when the concentration of the polyamine reaches a concentration of 50500 parts per million. Higher concentrations of this component appear to offer little or no added benefit.

On the other hand, it has been found that at least about 50 parts per million and preferably 1001000 parts of the pyridinium salt is required to obtain optimum results.

The present invention affords effective inhibition of iron and steel corrosion by sour gas conditioning solutions containing dissolved CO2 and H2S using relatively low concentrations of an inhibitor combination which is easily handled and convenient to use. A concentration of the combined compounds when the thio compound is a thioamide or a sulfide can be made up in aqueous alkanolamine, aqueous glycol, or lower alkanol and this concentrate can be added to the gas treating solvent as required to make up or to maintain a desired concentration.

Since thiocyanates tend to react on standing with the quaternary salt to form a difficulty soluble, less active product, these thio compounds are best added separately to the gas-treating solution, thereby forming the combination in situ at higher dilution.

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

This inhibitor combination is particularly useful in aqueous lower alkanolamine solutions known as sour gas scrubbing solvents. Preferred lower alkanolamines can be defined as those having the formula:

wherein R' and R" independently represent hydrogen or -CR2CR2-OH and wherein each R may be hydrogen or an alkyl radical of 1-2 carbon atoms.

Representative alkanolamines are ethanolamine, diethanolamine, triethanolamine, isopropanolamine, diisopropanolamine, and N-methyldiethanolamine. Related alkanolamines which are useful acidic gas absorbents are Methicol (3 dimethylamino - 1,2 - propanediol) and diglycolamine (2 - (2 aminoethoxy)ethanol). Other gas-treating absorbents in which this inhibitor combination is effective include sulfolane (tetrahydrothiophene - 1,1 - dioxide) and aqueous potassium carbonate. These absorbents can be employed alone or in combinations of two or more, usually in aqueous solution although the water may be replaced partly or entirely by a glycol.

The inhibitor combination of this invention is also effective to inhibit corrosion of iron and steel by a gas-treating solution containing both CO2 and H2S when the H2S is present in the solution at limited concentration, up to about 500 ppm, for example, and preferably not more than about 150 ppm. The inhibitor combination is thus of wider applicability than many known inhibitors which are not effective in the presence of dissolved H2S.

Testing Procedure The corrosion of mild steel by aqueous alkanolamine solutions saturated with CO2 for 7 hours at 10--200C was measured at elevated temperatures and moderate pressure. Loosely capped bottles each containing 120 ml of test solution and a totally immersed lx2.5x0.0625 inch coupon (2.54 cmx6.35 cmx0.16 cm) of mild steel were put in a modified pressure filter for a period of 16-18 hours, at 1250C and 40 psig (2.8 kg/cm2) unless otherwise specified. The test solvent was 300/O by weight aqueous ethanolamine unless otherwise specified. The steel coupons were previously cleaned with 5 N HCI by immersion for 30 minutes at room temperature, followed by a soap and water wash, a water rinse, then an acetone rinse and air drying. At least two bottles of each trial solution were tested in each experiment along with three bottles of solution containing no inhibitor which served as controls. After testing, the same cleaning procedure was used except that the HCI treatment was 15 minutes with 5 N HCI inhibited with a commercial HCI inhibitor in order to remove any corrosion deposits. The corrosion rate and efficiency of inhibition were calculated according to the following formulas using the average weight loss of the test coupons: (0.0254)x534xmgs weight loss of coupon Rate in mils/yr (cm/yr)= (coupon density, g/cc) (coupon surfaces, sq. in.) (6.45 chain2) (hrs) Corrosion rate of blanks-rate of test coueons % Inhibition~ rate of blanks-rate of test coupons x100 corrosion rate of blanks Preparation of Quaternary Salts The quaternary pyridinium salts used in the inhibitor compositions were made by heating a mixture of the pyridine compound with excess alkyl halide or benzyl halide at 900C for two hours. The reaction mixture was then cooled and the quaternary salt was recovered as a solid or viscous liquid precipitate.

The inhibitor compositions 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 pyridinium quaternary salt used in these tests was the reaction product of tetradecyl bromide and high boiling alkylpyridine still bottoms (HAP). These still bottoms were from processes for making various lower alkyl substituted pyridines wherein most of the components were pyridines having multiple lower alkyl substituents, particularly methyl and ethyl groups. This mixed quaternary salt was tested in combination with NH4SCN, thioacetamide, thiourea, thionicotinamide, and thioisonicotinamide at various concentrations as noted.

Concentration, ppm by wt.

Thio Compound Quat. Salt Thio Compound % Inhibition NH4SCN 100 10 82.5 100 25 86.8 500 25 91.6 500 50 93.9 Thioacetamide 100 25 88.3 100 50 83.2 500 50 89.5 Thiourea 100 50 72.5 500 50 77.6 Thionicotinamide 100 25 92.2 100 50 92.2 Thioisonicotinamide 100 25 92.2 100 50 92.2 Example 2 Combinations of thiourea with benzyl pyridinium chloride and with dodecylbenzyl alkylpyridinium chloride (made from the alkylpyridine still bottoms described in Example 1) were tested for inhibition as described above. A combination of dodecylbenzyl alkylpyridinium chloride with thioacetamide was also tested.

Concentration, ppm by wt.

Pyridinium Concentration, ppm by wt.

Chloride Quat. salt Thiourea %Inhibition Benzyl 1000 none 11.1 1000 25 30.2 Dodecylbenzyl 1000 none 66.6 1000 1 89.7 1000 5 90.3 Dodecylbenzyl 1000 1* 91.5 1000 5* 90.6 1000 25* 90.6 *Thio compound was thioacetamide.

Example 3 Quaternary salts made from various higher alkyl bromides and alkylpyridine still bottoms were tested as inhibitors with and without NH4SCN as in the foregoing examples.

Concentration, ppm by wt.

Pyridinium Concentration, ppm by wt.

Bromide Quat. Salt NH4SCN % Inhibition Dodecyl 100 none 7.2 100 50 64.4 500 100 73.1 Cetyl 100 none -34.4 100 50 59.7 500 100 62.3 Octadecyl 100 none -14.3 100 50 43.8 500 100 53.9 Example 4 Quaternary salts made by reacting tetradecyl bromide with different alkylpyridines were tested as inhibitors in combination with NH4SCN by the procedure previously described.

Concentration, ppm by wt.

Alkylpyridine Quat. Salt NH4SCN % Inhibition 2-methyl- 100 50 27.0 3-methyl- 1000 none 54.5 1000 50* 88.8 2-ethyl- 50 50 3.2 100 50 31.1 3-ethyl- 100 50 83.7 500 50 93.6 2,4-dimethyl- 100 50 83.9 500 50 83.7 3,5-dimethyl- 100 50 60.8 500 50 73.3 5-ethyl-2-methyl- 100 50 82.5 500 50 90.9 3-ethyl-4-methyl- 100 50 88.1 100 100 89.9 500 100 95.7 2i4,6-trimethyl- 100 50 73.5 500 50 84.9 *Thio compound was thioacetamide.

Example 5 The quaternary salt of Example 1 (tetradecyl alkylpyridinium bromide) was tested in combination with NH4SCN as before except for using 35% by weight aqueous ethanolamine. Blanks were also run for comparison.

Concentration, ppm by wt.

Quat. Salt NH4SCN % Inhibition 100 none -24.2 1000 none -36.9 none 100 - 8.4 none 500 -20.2 50 25 39.3 50 500 26.4 100 25 88.5 100 50 94.5 100 500 92.3 500 10 87.4 500 50 92.6 500 100 96.4 500 500 92.0 1000 25 81.0 1000 50 87.6 1000 100 89.2 1000 500 89.5 Example 6 The same quaternary salt described in Examples I and 5 was tested as before in combination with NH4SCN at various concentrations using 15% by weight aqueous ethanolamine as the test solvent.

Concentration, ppm by wt.

Quat. Salt NH4SCN % Inhibition 50 10 68.3 50 50 91.9 50 500 95.9 100 10 96.4 100 50 95.8 100 500 96.2 500 10 93.2 500 50 93.3 500 500 94.8 1000 10 89.0 1000 50 87.6 1000 500 91.7 Examples 7-10 The quaternary salt described in Examples 1 and 56 was tested in combination with NH4SCN as before using various aqueous alkanolaminecontaining solutions as test solvent.

Concentration, ppm by wt. Corrosion mils/yr.

Quat. Salt. NH4SCN Solvent (mm./yr.) %Inhibition - - 70% TEA' 10.1 - (0.26) 100 50 70% TEA1 0.8 92.6 (0.02) 500 100 70% TEA1 0.7 93.1 (0.02) - - 50% DEA2 10.4 - (0.36) 100 50 50% DEA2 0.6 93.7 (0.02) 500 100 50% DEA2 1.0 90.4 (0.03) - - 60% DEA2 27.1 - (0.69) 100 50 60% DEA2 0.6 97.6 (0.02) 500 100 60% DEA2 1.1 96.1 (0.03) - - Mixed3 19.0 (0.48) 100 - Mixed3 2.6 86.5 (0.07) 500 - Mixed3 2.0 89.4 (0.05) 100 50 Mixed3 1.6 91.8 (0.04) 500 100 Mixed3 1.5 92.1 (0.04) 'TEA=Triethanolamine, by weight. 2DEA=Diethanolamine, by weight.

3Mixed=45% diisopropanolamine, 35% sulfolane, 20% water, by weight.

Example 11 Combinations of tetradecyl alkylpyridinium bromide and NH4SCN were tested in 30% by weight aqueous ethanolamine saturated with CO2 and containing 100 ppm by weight of sulfide ion added as ammonium sulfide under test conditions otherwise as previously described.

Concentration, ppm Quat. Salt NH4SCN % Inhibition 100 - 76.5 500 - 94.1 100 50 76.6 500 50 89.2 100 100 77.1 500 100 93.3 In the above tests, the ammonium sulfide present in the alkanolamine solution to simulate the presence of H2S served as the thio compound and so the quaternary salt was 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 procedure was used, except that an 112S equivalent was added to the aqueous alkanolamine. The H2S was added to the CO2-saturated aqueous alkanolamine as an amount of aqueous (NH4)2S sufficient to supply sulfide and hydrosulfide ions in about the same concentrations as the listed H2S concentration would provide. In Examples 12 to 14 the corrosion inhibition testing was done at 1250C in 30 percent by weight aqueous ethanolamine saturated with CO2 and containing the equivalent of, by weight, 100 ppm, 300 ppm, and 500 ppm H2S as (NH4)2S, respectively.

Example 12 (100 ppm H2S) Concentration, ppm by wt.

Quat.

Salt Polyamine Quat. Salt Polyamine % Inhibition TABU"' PEI-3'2' 100 - 76.5 500 - 94.2 100 64.3 500 49.1 100 100 94.3 500 100 95.5 100 500 94.2 TAPB"' E-l003 - 100 47.5 500 57.7 100 100 94.9 500 100 95.2 "'TAPB=Tetradecyl bromide salt of polyalkylpyridines in lower alkylated pyridine still bottoms (HAP). These still bottoms were from processes for making various lower alkyl substituted pyfldines wherein most of the components were pyridines having multiple lower alkyl substituents, particularly methyl and ethyl groups.

'2'PEI-3=Polyethylenimine of about 300 average molecular weight.

'3'E-100=Ethylenediamine plant still bottoms contain 8590% pentaethylenehexamine and hexaethyleneheptamine with some tetraethylenepentamine and small amounts of branched and cyclic isomers and derivatives.

Example 13 (300 ppm H2S) Concentration, ppm, by wt.

Quat. Concentration, ppm, by wt.

Salt Polyamine Quat. Salt Polyamine % Inhibition TAPB E-100 100 - 72.6 500 - 86.0 1000 - 83.7 100 - 4.6 500 10.1 100 100 90.8 500 100 88.7 100 500 90.1 Example 14 (500 ppm H2S) Concentration, ppm, by wt.

Quat.

Salt Polyamine Quat. Salt Polyamine % Inhibition TAPB E-100 100 - 57.1 500 - 84.8 100 100 89.0 500 100 88.1 500 500 91.3 Examples 15-17 are essentially a repeat of Examples 12-14 using 60 percent by weight aqueous diethanolamine as the ethanolamine solution. Equivalent amounts of aqueous (NH4)2S were added as before to the CO2-saturated alkanolamine to provide about the concentrations of sulfide and hydrosulfide ions formed by the listed amounts of H2S.

Example 15 (100 ppm H2S) Concentration, ppm, by wt.

Quat.

Salt Polyamine Quat. Salt Polyamine % Inhibition TAPB E-100 100 - 92.8 500 - 93.6 1000 - 92.3 100 25.8 500 55.8 100 100 96.2 500 100 96.2 Example 16 (300 ppm H2S) Concentration, ppm, by wt.

Quat.

Salt Polyamine Quat. Salt Polyamine % Inhibition TAPB E-100 100 - 88.6 500 - 90.3 100 49.1 500 60.2 100 100 93.2 500 100 91.8 100 500 92.6 Example 17 (500 ppm H2S) Concentration, ppm, by wt.

Quat.

Salt Polyamine Quat. Salt Polyamine % Inhibition TAPB E-100 100 - 84.0 500 - 83.2 1000 - 84.0 100 39.6 500 46.3 1000 27.6 100 100 86.6 500 100 85.5 Similar effective inhibition of corrosion is found when the quaternary salt of the above examples is replaced by the same amount of other pyridinium salts as previously defined, for example, dodecylbenzyl 3 - ethyl - 4 - methylpyridinium chloride, dodecyl alkylpyridinium bromide (made from HAP alkylpyridine still bottoms), tetradecyl 3-ethylpyridinium bromide, and tetradecyl trimethylpyridinium bromide. Similarly, closely comparable results are obtained when the polyamine component in these examples is replaced by the same concentration of polypropylenimine of 500 average molecular weight, triethylenetetramine, hexapropyleneheptamine, or other such polyamine as defined above.

In the same way, effective inhibition of ferrous metal corrosion is also obtained when these quaternary pyridinium salt-polyamine combinations are maintained at the disclosed concentration in other sour gas conditioning solutions such as previously described. For example, aqueous or glycol-containing solutions of diethanolamine, N-methyldiethanolamine, diisopropanolamine, and mixtures of these including mixtures with sulfolane and other known gas conditioning solvents, also aqueous potassium carbonate are all protected by these inhibitor combinations.

When a water-soluble sulfide is used as the thio compound it should not be used in the presence of cobalt since the latter is precipitated as cobalt sulfide and causes plugging in the gas treating unit.

WHAT WE CLAIM IS: 1. A sour gas conditioning solution having dissolved therein (a) a quaternary pyridinium salt and (b) either (i) a thio compound which is a water-soluble thiocyanate, a water-soluble sulfide, or an organic thioamide, or (ii) a lower alkylenepolyamine, a corresponding polyalkylenepolyamine or a mixture thereof wherein the alkylene units contain 2-3 carbon atoms, the total amount of (a) and (b) being sufficient to inhibit the corrosion of iron and steel by carbon dioxide and hydrogen sulfide, when present, in the solution.

2. A solution as claimed in Claim 1 in which (a) is present in an amount of at least 50 ppm by weight of the solution.

3. A solution as claimed in Claim 2 in which (a) is present in an amount of from 100--1000 ppm.

4. A solution as claimed in any one of the preceding claims in which (b)(i) is present in an amount of at least 1 ppm.

5. A solution as claimed in Claim 4 in which (b)(i) is present in an amount of from 10--100 ppm.

6. A solution as claimed in any one of the preceding claims in which the total amount of (a) and (b)(i) is at least 50 ppm.

7. A solution as claimed in Claim 6 in which the total amount of (a) and (b)(i) is not more than 2000 ppm.

8. A solution as claimed in any one of Claims 1 to 3 in which (b)(ii) is present in an amount of at least 50 ppm.

9. A solution as claimed in Claim 8 in which (b)(ii) is present in an amount of from 5500 ppm.

10. A solution as claimed in any one of Claims 1 to 3, 8 and 9 in which the total amount of (a) and (b)(ii) is at least 100 ppm.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

Example 17 (500 ppm H2S) Concentration, ppm, by wt.

Quat.

Salt Polyamine Quat. Salt Polyamine % Inhibition TAPB E-100 100 - 84.0

500 - 83.2

1000 - 84.0

100 39.6

500 46.3

1000 27.6

100 100 86.6

500 100 85.5 Similar effective inhibition of corrosion is found when the quaternary salt of the above examples is replaced by the same amount of other pyridinium salts as previously defined, for example, dodecylbenzyl 3 - ethyl - 4 - methylpyridinium chloride, dodecyl alkylpyridinium bromide (made from HAP alkylpyridine still bottoms), tetradecyl 3-ethylpyridinium bromide, and tetradecyl trimethylpyridinium bromide. Similarly, closely comparable results are obtained when the polyamine component in these examples is replaced by the same concentration of polypropylenimine of 500 average molecular weight, triethylenetetramine, hexapropyleneheptamine, or other such polyamine as defined above.

In the same way, effective inhibition of ferrous metal corrosion is also obtained when these quaternary pyridinium salt-polyamine combinations are maintained at the disclosed concentration in other sour gas conditioning solutions such as previously described. For example, aqueous or glycol-containing solutions of diethanolamine, N-methyldiethanolamine, diisopropanolamine, and mixtures of these including mixtures with sulfolane and other known gas conditioning solvents, also aqueous potassium carbonate are all protected by these inhibitor combinations.

When a water-soluble sulfide is used as the thio compound it should not be used in the presence of cobalt since the latter is precipitated as cobalt sulfide and causes plugging in the gas treating unit.

WHAT WE CLAIM IS: 1. A sour gas conditioning solution having dissolved therein (a) a quaternary pyridinium salt and (b) either (i) a thio compound which is a water-soluble thiocyanate, a water-soluble sulfide, or an organic thioamide, or (ii) a lower alkylenepolyamine, a corresponding polyalkylenepolyamine or a mixture thereof wherein the alkylene units contain 2-3 carbon atoms, the total amount of (a) and (b) being sufficient to inhibit the corrosion of iron and steel by carbon dioxide and hydrogen sulfide, when present, in the solution.
2. A solution as claimed in Claim 1 in which (a) is present in an amount of at least 50 ppm by weight of the solution.
3. A solution as claimed in Claim 2 in which (a) is present in an amount of from 100--1000 ppm.
4. A solution as claimed in any one of the preceding claims in which (b)(i) is present in an amount of at least 1 ppm.
5. A solution as claimed in Claim 4 in which (b)(i) is present in an amount of from 10--100 ppm.
6. A solution as claimed in any one of the preceding claims in which the total amount of (a) and (b)(i) is at least 50 ppm.
7. A solution as claimed in Claim 6 in which the total amount of (a) and (b)(i) is not more than 2000 ppm.
8. A solution as claimed in any one of Claims 1 to 3 in which (b)(ii) is present in an amount of at least 50 ppm.
9. A solution as claimed in Claim 8 in which (b)(ii) is present in an amount of from 5500 ppm.
10. A solution as claimed in any one of Claims 1 to 3, 8 and 9 in which the total amount of (a) and (b)(ii) is at least 100 ppm.
11. A solution as claimed in any one of the preceding claims in which the

pyridinium salt has the formula:

wherein R is an alkyl radical of 1--20 carbon atoms, a benzyl radical, or an alkylated benzyl radical wherein the aromatic ring has one or more alkyl substituents totalling 1--20 carbon atoms, each R' is a hydrogen atom or an alkyl radical of 1-6 carbon atoms, and X is an anionic radical.
12. A solution as claimed in Claim 11 in which R is an alkyl radical of 1018 carbon atoms.
13. A solution as claimed in any one of the preceding claims in which the pyridinium salt is tetradecyl polyalkylpyridinium bromide.
14. A solution as claimed in any one of Claims 1 to 7, and 11 to 13 when dependent on any one of such claims, in which the thio compound is ammonium thiocyanate, ammonium sulfide, thiourea, a polythiourea, a hydrocarbon substituted derivative thereof, or a thioamide having the formula:

wherein A is a hydrocarbon radical of 1--12 carbon atoms or a pyridyl radical and each R" is a hydrogen atom or an alkyl radical of 1-8 carbon atoms.
15. A solution as claimed in any one of Claims 1 to 3, 8 to 10, and 11 to 13 when dependent on any one of such claims, in which the polyamine is a polyethylenepolyamine having an average molecular weight of from 3001000.
16. A solution as claimed in any one of the preceding claims in which the solvent component thereof is a lower alkanolamine, sulfolane, potassium carbonate or mixture thereof, in water, glycol, or a water-glycol mixture.
17. A solution as claimed in any one of Claims 1 to 15 in which the solvent component thereof is an aqueous solution of ethanolamine or diethanolamine.
18. A solution as claimed in Claim 1 substantially as hereinbefore described in any one of the Examples.
19. A sour gas conditioning solution inhibited against CO2 promoted corrosion of iron and steel by having dissolved therein an inhibiting concentration of a combination of one part by weight of a quaternary pyridinium salt and 0.001-10 parts of a thio compound which is a water-soluble thiocyanate, a water-soluble sulfide, or an organic thioamide.
20. A sour gas conditioning solution inhibited against CO2 and H2S promoted corrosion of iron and steel by having dissolved therein an inhibiting concentration of a combination of one part by weight of a quaternary pyridinium salt and 0.01- 10 parts of a lower alkylenepolyamine, a corresponding polyalkylenepolyamine, or a mixture thereof wherein the alkylene units contain 2-3 carbon atoms.
21. A method of conditioning a gaseous mixture which comprises contacting said gaseous mixture with a sour gas conditioning solution as claimed in any one of the preceding claims.
22. A composition for use in a sour gas conditioning solution to inhibit the corrosion of iron and steel by carbon dioxide and hydrogen sulfide, when present, in the solution, which composition comprises (a) a quaternary pyridinium salt and (b) either (i) a thio compound which is a water-soluble thiocyanate, a water-soluble sulfide, or an organic thioamide, or (ii) a lower alkylene polyamine, a corresponding polyalkylenepolyamine or a mixture thereof wherein the alkylene units contain 2-3 carbon atoms.
23. A composition as claimed in Claim 22 comprising 0.01-10 parts by weight of the polyamine per part by weight of the pyridinium salt.
24. A composition as claimed in Claim 23 comprising 0.1-I part by weight of the polyamine per part by weight of the pyridinium salt.