GB2130569A - Vanadium-cobalt corrosion inhibitor system for sour gas conditioning solutions - Google Patents

Abstract

The corrosion of ferrous metal surfaces by an aqueous alkanolamine solution employed in acid gas removal service can be inhibited by adding to said aqueous alkanolamine solution an anion containing vanadium in the +4 or +5 valence state, and a cation containing cobalt in the +2 valence state.

Description

SPECIFICATION Vanadium-cobalt corrosion inhibitor system for sour gas conditioning solutions The present invention relates to inhibitor compositions useful for preventing corrosion of ferrous metal surfaces by alkanolamine solvents used in treating sour gas streams and more particularly relates to such inhibitor compositions which contain vanadium and cobalt.

It has been a long standing commercial practice to use aqueous alkanolamine solutions (e.g. a monoethanolamine solution) to absorb acidic gases such as CO2, H2S, COS and HCN, to condition naturally occurring and synthetic gases. These treated gases may include feed synthesis gases, natural gas and flue gas. Frequently, the conditioning process is practiced by passing a 5 per cent to 30 per cent alkanolamine solution countercurrent to a gas stream in an absorption column to remove the acid gas. The absorbed acid gases may be later forced out of the conditioning solution at higher temperatures and the alkanolamine solution recycled for more absorbing.

Aqueous alkanolamine solutions are not themselves very corrosive toward ferrous metal equipment, however, they become highly corrosive when acid gases are dissolved therein, particularly when the solution is hot. It has been found that both general and local corrosive attack can occur. This is a particular problem in reboilers and heat exchangers where the steel is exposed to a hot, protonated alkanolamine solution. A heat transferring metal surface appears to be especially vulnerable. Previous investigation by others have revealed that under conditions corrosive products such as aminoacetic, glycolic, oxalic and formic acids were formed. The monoethanolamine sa Its of these acids present the possibility of increased attack upon ferrous metals.

One of the most economical and efficient methods of treating this corrosion problem is by including small quantities of corrosion inhibitors. Various metal compounds have been used by others, alone or in combination with co-inhibitors, for example, compounds of arsenic, antimony and vanadium. These metal compounds seem to be much more effective against CO2-promoted corrosion than they are when H2S has been absorbed in the conditioning solution.

A number of U. S. patents have been granted relating to the use of corrosion inhibitor additives. For example, the use of antimony was described in U. S. Patent 2,715,605. A number of amine compounds were found to be useful in preventing corrosion by addition to petroliferous oil well fluids containing carbon dioxide or hydrogen sulfide brines, as disclosed in U. S. Patents 3,038,856; 3,269,999 and 3,280,097. U. S.

Patent 3,808,140 relates to a combination inhibitor system using vanadium and antimony. Nitro-substituted aromatic acids and acid salts, stannous salts, organo-tin compounds, benzotriazole, vanadium and antimony were used in various combinations as inhibitor systems for conditioning solutions as described in U. S.

Patents 3,896,044 and 3,959,170. The use of vanadium compounds as corrosion inhibitors for aqueous amine gas sweetening reagents is well known; for example, see H. Ratchen and C. Kozarev, Proceedings of the International Congress on Metallic Corrosion, 5th, 1972 and E. Williams and H. P. Lackie, Material Protection, July 1968 p.21.

Pyridinium salts were found to be useful corrosion inhibitors when used together with lower alkylpolyamines, thioamides, thiocy?llates, sulfides and cobalt as noted in U.S. Patents 4,100,099; 4,100,100 and 4,102,804; as well as U.S. 4,096,085 and 4,143,119. Still another U.S. Patent, 2,826,516, uses soluble silicates as effective corrosion inhibitors. However, many of these corrosion inhibitor systems have not found industry acceptance because of factors such as cost and toxicity.

Other cases related to monoethanolamine gas scrubbing operations are U.S. Patent 4,184,855 which uses inter-coolers and flash heat exchangers to increase the energy efficiency of the method and U.S. Patent 4,183,903 which describes using cyclic ureas as anti-foaming agents in the alkaline absorption solution.

It is an object of this invention to provide an aqueous alkanolamine conditioning solution inhibitor system using components which are nontoxic relative to some of the prior art systems and which permit relatively higher amine concentrations and thus higher carbon dioxide loading making for a more efficient process.

The invention provides a corrosion inhibited acid gas-absorbing composition which comprises an aqueous alkanolamine solution, which additionally contains an anion containing vanadium in the +4 or +5 valence state, and a cation containing cobalt in the +2 valence state.

The invention also provides a method for inhibiting the corrosion of ferrous metal surfaces by an aqueous alkanolamine solution employed in acid gas removal service which comprises adding to said aqueous alkanolamine solution an anion containing vanadium in the +4 or +5 valence state, and a cation containing cobalt in the +2valence state.

The use of aqueous solutions of alkanolamines and particularly monoethanolamine for sour gas conditioning solutions is well known in the art. The surprising aspect of the instant invention is that vanadium-containing anions and cobalt ions together form a corrosion inhibitor system much better than the vanadium or the cobalt alone.

Vanadium-containing compounds are thought to act as oxidant-type inhibition catalysts which undergo a redox reaction at the ferrous metal surface. It is thought that the iron needs to be somewhat corroded for the vanadium to be effective. The limited corrosion would place the iron in the proper valence state for protective film formation.

The choice of vanadium compounds is not critical since it is the vanadium-containing anion, particularly vanadium in the +4 or +5 valence state, which provides this unusual corrosion inhibiting property in combination with the amines. Thus, for example, one can employ vanadates including orthovanadates, represented by the genericformula: M3VO4, pyrovanadates, represented by the general formula M4 V2 7 and metavanadates, represented by the general formula MVO3 and the like where M represents a cation. The condensed vanadate ions which form in aqueous solutions, such as V60174 are also useful in this invention.

For convenience in introducing vanadate ions into the inhibiting systems of this invention the alkali metals, ammonim and alkaline earth vanadates including orthovanadates, pyrovanadates and metavanadates are preferred. Exemplary of such vanadates are the following: sodium metavanadate, potassium metavanadate, lithium metavanadate, ammonium metavanadate, sodium pyrovanadate, potassium pyrovanadate, lithium pyrovanadate, ammonium pyrovanadate, sodium orthovanadate, potassium orthovanadate, lithium orthovanadate, calcium pyrovanadate, calcium metavanadate, magnesium orthovanadate, magnesium pyrovanadate, magnesium metavanadate, ferrous orthovanadate, ferrous pyrovanadate, ferrous metavanadate, and the like.

Other forms of vanadium that can be used in this invention include: the vanadovanadates, double vanadates, i.e., heteropoly acids containing vanadium and the peroxy vanadates having the general formula: MVO4.

Essentially any cobaltous compound which is sufficiently soluble in the aqueous alkanolamine solution to provide the desired concentration of divalent cobaltous ions can be used. Inorganic salts such as CoCI2, CoBr2, CoCO3, CoSO4, or Co(NO3)2; and organic salts such as cobaltous acetate and cobaltous benzoate are all suitable sources of cobaltous ions. Salts such as the sulfate, nitrate, carbonate, or chloride are particularly preferred.

As will be seen in the Examples, the corrosion inhibitor system is effective even if very small amounts of additives are used. For example, the vanadium-containing anion and cobalt-containing ion are seen to be effective in concentrations as low as 100 parts per million. Of course, now that this particular corrosion system has been discovered, it is merely a matter for one skilled in the art to optimize the system for a particular application. Upper limits on the inhibitor concentration might be 600 ppm each for vanadium and cobalt. The precise concentrations must be set as a balance between the needs of the conditioning solution and the economics of using relatively high inhibitor concentrations.

The 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-1,2propanediol) and DIGLYCOLAMINE [ 2-(2-aminoethoxy)ethanol)j agent, the latter being a product of Texaco Chemical Co. 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 combination of two or more, usually in aqueous solution although the water may be replaced in part or wholly by a glycol.

The following examples will illustrate the method of this invention as well as disclose the method of corrosion testing employed.

Example 1 In this Example the equipment involved a set of copper strip corrosion test bombs that met ASTM D130 specifications. The covers were modified with valves and dip tubes to allow sampling of the liquid phase when the vessel was pressurized due to autogenous pressures. A polytetrafluoroethylene (PTFE) coupon mount was attached to the dip tube and a polypropylene liner was fitted to the vessel in a manner so that the test solution was not in direct contact with the body of the vessel. In a typical experiment, 90 ml of a 50 weight per cent aqueous monoethanolamine was premixed with carbon dioxide, ammonium metavanadate and certain transition metal salts, The solution was placed in the liner of the vessel. A piece of 37.6 x 10.4 x 3.1 mm. 1020 mild steel (hereinafter referred to as "coupon") with a 6.35 mm. diameter hole for mounting was freshly polished with fine Emery cloth (#JB5R, RED-I-CUT Carborundum), followed by rinsing with water and acetone. The dried clean coupon was then weighed and attached to the PTFE mounting in a manner such that when the vessel was closed the coupon would be totally immersed in the test solution. The vessel was sealed and placed in a 115 + 10C shaker bath for a period of 96 hours. Then the coupon was recovered and cleaned by scrubbing with a bristle brush. When needed, a mild abrasive, PUMACE FFF (supplied by Central Texas Chemical Co.), was employed for post-test cleaning. After the coupon was clean and dried, weight loss was determined.A series of such experiments provided the results listed in Table which showed that of the transition metals tested, cobalt noticeably reduced corrosion of the mild steel coupon.

TABLE I Corrosion inhibitor screening tests MEA% CO2b, Vanadiumd, Corrosion Ratee, molelmole Inhibitor AC, ppm ppm mmpy 50.0 0.39 A = Ni 100 0 4.25 50.0 0.39 A = Ni 100 100 0.65 50.0 0.39 A = Cu 100 0 0.975 50.0 0.39 A = Cu 100 100 1.175 50.0 0.39 A = Co 100 0 0.175 50.0 0.39 A = Co 100 100 0.275 50.0 0.39 A = Zn 100 0 1.625 50.0 0.39 A = Zn 100 100 0.675 50.0 0.39 --- --- 0.6 aMonoethanolamine, low iron grade, < 10 ppm Fe; made by Texaco Chemical Co.

bMole CO2 per mole of MEA.

CNickel was introduced as nickel nitrate, copper was introduced as cupric nitrate, cobalt was introduced as cobalt nitrate, and zinc was introduced as zinc nitrate.

dintroduced as ammonium metavanadate, used in all examples.

eThe corrosion rate is a measurement of linear penetration in thousandths of an inch per year as computed by the following formula: Rate (mm. /year) = 8.,6 > < x weighweight loss of coupon, mgs (coupon density, g/cc) (coupon surface, sq. cm.) (hrs) Example Il The effect of soluble iron on an ammonium metavanadate inhibited system was tested in a 30% aqueous monoethanolamine loaded with 0.30 moles of carbon dioxide per mole of amine reagent according to the same procedure given in Example 1. Results given in Table II indicated that increasing soluble iron in the test solution reduced the effective soluble vanadium in the test solution.

TABLE II Effect of soluble iron on the vanadium inhibited system Post-test Additives AnalysisC MEA, co, Fea, Vb, Fe, V, Corrosion Rate molelmole ppm ppm ppm ppm mmpy 30.0 0.30 100 100 22 87 < 0.025 30.0 0.30 200 100 d 19 0.3 30.0 0.30 300 100 d 11 0.175 30.0 0.30 400 100 d 9 0.7 30.0 0.30 500 100 d 8 0.2 30.0 0.30 50 200 3 234 < 0.025 30.0 0.30 100 200 8 197 < 0.025 30.0 0.30 150 200 3 156 < 0.025 30.0 0.30 200 200 5 142 < 0.025 30.0 0.30 250 200 3 110 < 0.025 alron was introduced as freshly prepared aqueous solution of ferrous ammonium sulfate.

bVanadium was introduced as ammonium metavanadate.

CBy atomic absorption analysis.

dNot analyzed.

Example 111 The effectiveness of the cobalt-vanadium inhibitor system was further tested in a 50% aqueous monoethanolamine loaded with 0.39 moles of carbon dioxide per mole of amine reagent. To further increase the corrosiveness of the test, the bath temperature was increased to 120 C. Results of these tests indicated the combination of cobalt and vanadium provided protection to mild steel coupon while either cobalt or vanadium alone was not effective.

TABLE Ill Evaluation of cobalt-vanadium inhibitor system Additivesa Post-test Analysisb CO2 Corrosion MEA, % molelmole CO, ppm V, ppm Fe, ppm Co, ppm V, ppm Fe, ppm Rate 50.0 0.39 -- 100 -- -- 116 1134 1.425 50.0 0.39 -- 200 -- -- 211 578 0.55 50.0 0.39 -- 300 -- -- 296 506 0.45 50.0 0.39 100 -- -- 82 -- 1061 0.975 50.0 0.39 200 -- -- 214 -- 813 1.275 50.0 0.39 300 -- -- 226 -- 570 0.65 50.0 0.39 100 100 -- 82 103 393 0.475 50.0 0.39 200 100 -- 130 88 92 0.3 50.0 0.39 300 100 -- 223 100 100 0.15 50.0 0.39 100 200 -- 66 197 198 0.3 50.0 0.39 200 200 -- 139 199 275 0.3 50.0 0.39 300 200 -- 245 200 7 < 0.025 50.0 0.39 300 200 40 261 163 44 < 0.025 50.0 0.39 300 200 80 267 170 72 < 0.025 50.0 0.39 300 200 120 265 173 109 < 0.025 50.0 0.39 300 300 40 266 278 42 < 0.025 50.0 0.39 300 300 80 266 269 77 < 0.025 50.0 0.39 300 300 120 264 274 108 < 0.025 50.0 0.39 300 400 40 269 388 44 < 0.025 50.0 0.39 300 400 80 268 387 72 < 0.025 50.0 0.39 300 400 120 272 391 108 < 0.025 aCobalt was introduced as cobalt nitrate, vanadium was introduced as ammonium metavanadate, and iron was introduced as aqueous solution of ferrous ammonium sulfate.

bBy atomic absorption.

The effectiveness of the corrosion inhibitor system of this invention may be readily seen from the examples where the inhibiting effect of both co-inhibitors is greater than either inhibitor singly. The corrosion rates given are generally good over a range of + 0.1 mm/year.

It may be seen that the vanadium-cobalt inhibitor system worked well even with quantities of soluble iron present. It is also noted that in all instances of Tables I and lil, the monoethanolamine concentration was 50 weight per cent which is much higher than the 5 to 30 per cent used in the prior art methods. As a result, the sour gas conditioning solution can be more concentrated and more effective in removing CO2 than current solutions and provide corrosion protection in addition.

Claims (14)

1. A corrosion inhibited acid gas-absorbing composition which comprises an aqueous alkanolamine solution, which additionally contains an anion containing vanadium in the +4 or +5 valence state, and a cation containing cobalt in the +2 valence state.

2. A composition as claimed in Claim 1 wherein the alkanolamine is monoethanolamine.

3. A composition as claimed in Claim 1 or 2 wherein the vanadium-containing anion is derived from an orthovanadate, metavanadate, pyrovanadate, vanadium oxide or vanadium halide.

4. A composition as claimed in any preceding Claim wherein the cation containing cobalt is derived from CoC12, CoBr2, CoCO3, CoS04, Co(NO3)2, cobaltous acetate or cobaltous benzoate.

5. A composition as claimed in any preceding Claim wherein the anion containing vanadium in the +4 or +5 valence state has a concentration in the composition of at least 100 parts per million.

6. A composition as claimed in any preceding Claim wherein the anion containing cobalt in the +2 valence state has a concentration in the composition of at least 100 parts per million.

7. A composition as claimed in Claim 1 and substantially as hereinbefore described with reference to any of the Examples.

8. A method for inhibiting the corrosion of ferrous metal surfaces by an aqueous alkanolamine solution employed in acid gas removal service which comprises adding to said aqueous alkanolamine solution an anion containing vanadium in the +4 or +5 valence state, and a cation containing cobalt in the +2 valence state.

9. A method as claimed in Claim 8 wherein the alkanolamine is monoethanolamine.

10. A method as claimed in Claim 8 or 9 wherein the vanadium-containing cation is derived from an orthovanadate, metavanadate, pyrovanadate, vanadium oxide or vanadium halide.

11. A method as claimed in any of Claims 8 to 10 wherein the cation containing cobalt is derived from CoC12, CoBr2, CoC03, CoS04, Co(NO3)2, cobaltous acetate or cobaltous benzoate.

12. A method as claimed in any of Claims 8 to 11 wherein the concentration of vanadium-containing anion in the resulting composition is at least 100 parts per million.

13. A method as claimed in any of Claims 8 to 12 wherein the concentration of cobalt anion in the composition is at least 100 parts per million.

14. A method as claimed in Claim 8 and substantially as hereinbefore described with reference to any of the Examples.