Method and composition for inhibiting the corrosion of ferrous metals

BACKGROUND OF INVENTION
The conditioning (sweetening) of gases, natural and synthetic, i.e., acid gas stripping, the removal of acidic gases, e.g., CO.sub.2, H.sub.2 S and COS, by absorption of the acidic gases in a liquid absorbent medium, has been practiced commercially for many years. Various absorbents, such as the alkanolamines, "sulfolane" (tetrahydrothiophene-1,1-dioxide), "sulfinol" (tetrahydrothiophene-1,1-dioxide plus diisopropanolamine), potassium carbonate and the like, have been used commercially. Each of these systems is plagued by corrosion problems, some of which result from decomposition of the absorbent, some from reaction between the acidic components of the gases treated and the absorbent, and all, to more or less the same degree, from attack by the acidic components of the gases treated upon the metals of construction of the equipment. Generally, the corrosion occurs in the regenerator, heat exchangers, pumps and piping associated with these elements of the overall equipment. Numerous compounds have been suggested as additives to the absorbents to prevent the corrosion and/or the formation of corrosive elements. For example, copper sulfate was used for three years in a 15 percent monoethanolamine gas processing plant for removing 10 percent CO.sub.2 from the gas. Corrosion was observed as a decrease in reboiler and heat exchange tube life and on analysis of the amine solution, only a few parts per million copper was found thus indicating excessive copper deposition in the unit. (Gas Conditioning Fact Book, The Dow Chemical Company, Midland, Mich. 1962, pp. 157-158, Case Number 8). In another situation, described in U.S. Pat. No. 2,377,966, copper sulfate or copper carbonate in an aqueous alkanolamine solution was used as the absorbent to remove CO.sub.2 from a gas stream in which no H.sub.2 S was present. The copper compound in the absence of any H.sub.2 S was used to reduce the iron content of the solution (iron content being a measure of the oxidation of the solution and metal) thus indicating a reduction in the corrosivity of the solution to iron. The alkanolamine with the absence of H.sub.2 S reacted in the presence of the copper to yield oxalic acid after going through aminoacetic acid, which apparently is a product of the oxidation of the amine in the presence of iron. In still a further literature, reference, U.S. Pat. No. 2,771,405, copper corrosion by sulfur compounds and/or free-sulfur is reduced by solvent extracting the petroleum distillates with aqueous alkanolamine to remove the sulfur. Thus, in practice, copper has not been satisfactory as an inhibitor in acid-gas stripping plants.
U.S. Pat. No. 1,989,004, Fife, teaches using an alkanolamine or mixture of mono- , di- and trialkanolamines in a concentration of 15 to 30 percent in water and less than 1 percent of a soluble metal salt, e.g., copper or nickel sulfate or oxide.
This does not teach maintaining sulfur, that is, elemental or the necessary sulfur compounds and oxidizing agents to produce sulfur atoms.
Fife absorbs the H.sub.2 S and organic sulfur compounds from the gas using the complexing activity of each of the components, viz., the amine, the amine-metal complex and the metal. These complexes are nonthermally treated in two steps to release the amine for reuse and convert the sulfur to a solid for removal from the system.
Fife treats the spent solution (aqueous alkanolamine containing the amine-sulfur compound complex and the amine-metal sulfur compound complex) not with heat to regenerate but with air (page 1, column 2, lines 20-38) to oxidize the sulfur compounds (thiosulfates) and sulfates, then the oxidized solution is treated with lime to precipitate the sulfur as calcium sulfate and thiosulfates. It is well known that the alkanolamine also undergoes oxidation and that aeration would increase the loss of alkanolamine as is evidenced by the fact that the 15 to 30 percent amine solution used in Example II (page 2, column 1, lines 49-56) after regeneration constituted only a 10 to 13 percent amine solution, a loss of about 30 percent of the original amine.
U.S. Pat. No. 3,372,981, Ravner, discloses the use of copper to stabilize monoethanolamine solution against degradation during use as an absorbent for CO.sub.2 only from air (no H.sub.2 S present). Ravner uses iron, copper and a chromium salt but uses no sulfur.
U.S. Pat. No. 2,559,580, Alexander, discloses that iodine must be present in an alkanolamine solution in order to prevent degradation of the amine in which copper is present. This reference teaches that copper without iodine degrades amines, column 5, lines 19-59.
U.S. Pat. No. 2,405,672, Reitmeier, teaches the use of dichromates and a metal salt, e.g., zinc, copper, cadmium, mercury, arsenic, etc., in amounts of 2 to 3 percent. The metal salt combines with the H.sub.2 S to form an insoluble sulfide which precipitates and is thus available for oxidation by the dichromate. The pH of the reference solution must be controlled to be sure the CuS precipitates so that it is available for reaction with the dichromate to form the sulfur (column 3, line 38 to column 4, line 13). This reference requires no amine.
In recent years, both natural and synthetic gases are being produced which contain high concentrations of CO.sub.2 and/or H.sub.2 S. As the demand for neutral gases increases, the size of the units for treating these gases increases, thus, economically an increase in the concentration of absorbent in the system and/or an increase in the loading on the systems seems desirable. Both of the latter, although most desirable, increase the corrosion in the unit resulting in more frequent unscheduled downtimes for repair of and replacement of major elements.
It is therefore an object of the present invention to provide an inhibitor system for use in equipment in contact with acid-gas environments which reduces the corrosion of the metals of construction of equipment used under the conditions.
It is a further object to provide a process which will reduce the economic loss of plant elements and plant operating down-times between normal annual plant turn-around.
A still further object of the present invention is to provide an inhibitor system which will be effective in gas-sweetening processes at loadings in excess of 0.5 mole of acid-gas per mole of absorbent and/or concentrations of absorbents in the circulating solution in excess of 40 percent, thereby reducing plant size, increasing flexibility to operate with and from various gas sources and reduce environmental impact on the surrounding.
These and other objects will become apparent from the description and claims hereinafter set forth.
BRIEF DESCRIPTION OF INVENTION
In accordance with the present invention, the corrosion of metal, particularly ferrous metal and its alloys, in contact with absorbent solutions of acid-gas stripping (gas-sweetening) plants, is materially reduced or essentially eliminated by maintaining dissolved in the absorbent copper ions in the presence of sulfur atoms, particularly polysulfide moieties, in an environmental state such as to prevent bright copper plating. The quantity of copper ions necessary to reduce corrosion will vary with each plant and may vary from day to day in a particular plant. Therefore, the practical manner for determining the proper quantity of copper ions is to place in the equipment at the points where the most severe corrosion occurs, generally in the reboiler and cross-exchangers to the reboiler, a metal coupon which can be examined periodically, even daily, for signs of bright copper plate and corrosion. Such a control design will be explained and described in detail in the examples. Further, when a plant is first placed on the inhibitor composition of the present invention, be it new or an older plant, a larger quantity of copper must be supplied since much is lost to removing the corrosive elements and corrosion products at the sites corrosion has or is likely to develop. Once a degree of passivation to corrosion has been attained, it has been found that as little as about 15 parts of inhibitor per million parts of absorbent solution will maintain less than 1 mpy corrosion rate. It is to be understood that in most instances where initially high concentrations of inhibitor are required it is common for the inhibitor to eventually passivate the unit by removing the innocuous components and thus enable reduction of inhibitor to more conventional quantities, i.e., in some instances even as low as 1 to 10 ppm. However, even in those instances where corrosion is only slightly reduced, downtimes due to corrosion are minimized and even eliminated between normal plant turn-arounds or shutdowns.
Copper (metal, oxide, hydroxide or salt) of a nature to provide, in situ under the conditions found in a gas treating or conditioning process using thermal regeneration, copper ions and sulfur or a sulfur yielding compound which, under the conditions of operation, viz., particularly the thermal regeneration, will produce sulfur atoms and preferably polysulfide moieties. The copper ions form a copper polysulfide with the copper atoms and polysulfide moieties:
Cu.sup.++ + S.sub.n.sup.-y-2 .fwdarw. CuS.sub.n.sup.-y
where y is an integer from 2 to 6 and n is an integer from 2 to about 50 which copper polysulfide is available to react from solution with the Fe.sup.++ in solution to form at the surface of the containers a chalcopyrite, i.e., CuFeS.sub.2. The Fe.sup.++ is the primary product of the cathodic corrosion reaction. Further, in some instances, the copper will be reduced to copper atoms which alloy with the ferrous metal below the chalcopyrite surface layer and form a copper-ferrous alloy. These phenomena provide an effective physical and electrical barrier to the corrosive environment (the alkanolamine, alkanolamine decomposition products, H.sub.2 S, CO.sub.2, Cu.sup.+2, S.degree., Fe.sup.+3 and O.sub.2); that is, the chalcopyrite acts as a kinetic barrier to the corrosion reactions by hindering the electron and mass transport required to support the anodic and cathodic-corrosion half reactions.
The phenomenon is, of course, based on theoretical concepts supported by the results of surface analysis of the ferrous surfaces of both passivated and corroded surfaces using depth profiling, Auger spectroscopy and the electron microprobe.
The copper ions may conveniently be introduced into an absorbent medium both during and after the initial period as a solution of or by dissolution in said absorbent of sufficient of one of the following to maintain the equivalent of 1 to 15 or more parts of copper per million parts of absorbent medium dissolved in the circulating medium:
I copper metal when accompanied by sulfur, or an oxidizing agent which will produce sulfur from the H.sub.2 S dissolved in said absorbent;
Ii copper sulfide (either cupric or cuprous);
Iii a copper salt (cupric or cuprous) in combination with an oxidizing agent and if little or no H.sub.2 S or COS is present in the acid gas, then sulfur or a sulfur generating compound. The copper salts may be but not limited to cupric or cuprous carbonate, benzoate, stearate, acetate, acetylacetonate, chloride, oxalate, oxide, molybdate, chromate, perchlorate, sulfate or tetrafluoroborate.
It is, of course, understood that the quantity of inhibitor (i.e., the concentration of the components of the inhibitor composition) in the circulating system may be, and oftentimes is, greater than that dissolved. Such a condition will occur during the initial stages of introduction of the inhibitor to a plant or at any upset of the plant or on start-up after any scheduled or unscheduled down-time or turn-around. During these periods, the inhibitor is reacting with the metal of the plant, passivating the sites of corrosion, and reacting with corrosion products to passivate these products; thus, the inhibitor is being consumed. Therefore, the inhibitor components must be present to replace those consumed. And where the solubility of one or more of the components is limited, excess of that component or components must be present to enable reaction and/or dissolution in order to effectively inhibit corrosion. It has been found that initially as much as 500 to 2000 ppm inhibitor must be present to passivate a plant on start-up in order to obtain satisfactory readings on the monitoring coupons. This condition can exist for from several hours to several days depending upon the condition of the plant. Similarly, during operation of a plant which has undergone an upset, it may be necessary to increase the amount of components circulating, dissolved or undissolved, until the monitoring unit indicates a non-corroding condition.
The oxidizing agents which have been found to give satisfactory results when used in combination with one or more of the above sources of copper are: sulfur, potassium permanganate, calcium permanganate, sodium permanganate, potassium persulfate, potassium iodate, calcium iodate, sodium bromate, sodium persulfate, potassium meta periodate, strontium permanganate, sodium perborate, zinc permanganate, hydrogen peroxide, sodium dichromate, potassium dichromate, sodium perborate, sodium peroxide, oxygen.
The absorbents for acid-gas stripping which have been found to be susceptible to use in accordance with the present invention are the alkanolamines having the formula ##STR1## wherein each R.sub.1 and R.sub.2 represent independently hydrogen, or --C(R.sub.3).sub.2 C(R.sub.3).sub.2 --OH and where R.sub.3 represents an independently selected radical of hydrogen or a C.sub.1-3 alkyl radical. This absorbent may be used alone or in combination with each other or one of the following, also useful, gas-stripping absorbents: sulfolane (tetrahydrothiophene-1,1-dioxide), potassium carbonate or diglycolamines. The presence of hydrogen sulfide (H.sub.2 S) is required if the inhibitor is to be effective in absorbent systems of potassium carbonate and the dialkanolamines. The inhibitor has limited effectiveness in the presence of CO.sub.2 alone in these absorbents. The preferred absorbents are monoethanolamine, diisopropanolamine-sulfolane, diethanolamine, diglycolamine(2-(2-aminoethoxy)ethanol), 3-dimethylamino-1,2-propanediol (Methicol) all as aqueous solutions. It is to be understood that instead of water glycols may be employed.

DETAILED DESCRIPTION OF THE INVENTION
GENERAL PROCEDURE
Various aqueous solutions of monoalkanolamine with or without inhibitors were prepared and the solutions saturated with H.sub.2 S at 25.degree. C. Mild steel coupons, pickled in HCl (15 percent) for 30 minutes, rinsed with water, scrubbed with pumice soap and a toothbrush, rinsed first with water, and then with acetone and air dried, were weighed and immersed in the test solutions with or without inhibitor. Then the solution jars were sealed and placed in a pressure vessel and maintained at 120.degree. C. under a pressure which just exceeds the vapor pressure of the stripping solution (absorbent) at the 120.degree. C. temperature used for stripping the acid gases from the absorbent for 15 hours. The metal coupons were removed, pickled for 10 minutes in 10 percent HCl which was inhibited with a commercial acid corrosion inhibitor, scrubbed, rinsed and dried as before. The so-prepared coupons were weighed and the difference in weight converted to corrosion rate in mils penetration per year (mpy). Such a procedure determines the corrosion rate of the solutions on AISI 1010 or 1020 Steel. This test determines whether a compound or mixture of compounds has potential as an inhibitor, e.g. it is a screening test. The coupons are similar to the metal in a freshly cleaned, new plant and thus simulate start-up in such a plant. However, in an operating plant, there is an additional problem of corrosion products which act as a copper sink (absorb copper by chemical-combination), e.g., FeS will rapidly absorb copper ions from solution to form chalcopyrite, CuFeS.sub.2, a poor inhibitor.
Solutions, prepared containing various compounds and mixtures of compounds, were tested in accordance with the above General Procedure to determine the effectiveness of the compounds as inhibitors. The results are set forth below:
TABLE I__________________________________________________________________________ % Monoethanol Amine % RateInhibitor in Water H.sub.2 S MPY % Inhibition.sup.1__________________________________________________________________________Control None 80 Sat at 25.degree. C. 74 --1 1000 ppm CuCO.sub.3 80 Sat at 25.degree. C. 14 81 1000 ppm KMnO.sub.4Control None 40 Sparged 16 14.4 -- hours w/H.sub.2 S2 0.3 gm. (1000 ppm) CuCO.sub.3 40 Same 1.1 92 0.3 gm. (1000 ppm) KMNO.sub.4__________________________________________________________________________ ##STR2##
In a similar manner, the following compounds were tested yielding the results set forth in the tables. The numbers under "Percent Inhibition" represent the average results of replicates. The data hereafter presented establishes the utility of many of the copper salts listed as well as the effectiveness of the inhibitors of the present invention to reduce corrosion in various gas treating solutions used commercially. It is to be understood that these data do not define the lower limits of the compositions of the present invention, only the indication that these compositions have utility as a corrosion inhibitor. The lower limits are determined by the test methods later set forth in this application. It will be readily observed, for example, that CuS and S added to maintain about 90 ppm and 45 ppm, respectively, will hold a badly corroded commercial plant whereas most of the data from the laboratory hereafter set forth indicates 100 ppm of any combination usually gives less than 50 percent protection.
TABLE II__________________________________________________________________________80% Monoethanolamine in WaterSaturated with H.sub.2 S at R.T.Copper Oxidizer % Inhibition.sup.(1)__________________________________________________________________________Cu.sub.2 S 1000 KMnO.sub.4 1000 84,28CuS 100 K iodate 100 corrosion.sup.(2)CuS 100 K persulfate 100 corrosionCuS 100 KMnO.sub.4 100 corrosionCuS 500 KMnO.sub.4 500 90CuS 1000 KMnO.sub.4 1000 96CuS 500 K iodate 500 91CuS 1000 K iodate 1000 90CuS 500 K persulfate 500 90,86CuS 500 K persulfate 1000 92CuS 500 K persulfate 100 92CuS 1000 Na bromate 1000 93CuS 500 Na bromate 100 93CuS 500 Na chlorate 1000 93CuS 500 Na chlorate 100 93CuS 1000 H.sub.2 O.sub.2 (30%) 1000 60Cu oxalate 1000 KMnO.sub.4 1000 77,35,95,84Cu oxalate 2000 KMnO.sub.4 1000 95,93Cu oxalate 5000 KMnO.sub.4 1000 93,94Cu oxalate 1000 K persulfate 2000 corrosionCu oxalate 2000 K persulfate 5000 94Cu oxalate 1000 H.sub.2 O.sub.2 (30%) 5000 88,53Cu oxalate 2000 H.sub.2 O.sub.2 (30%) 1000 85Cu oxalate 5000 H.sub.2 O.sub.2 (30%) 1000 85Cu (acetate).sub.2 1000 KMnO.sub.4 1000 91Cu (acetate) 1000 KMnO.sub.4 1000 84Cu (benzoate).sub.2 2000 KMnO.sub.4 1000 80CuB.sub.4 O.sub.7 1000 KMnO.sub.4 1000 85CuI(P.phi..sub.3).sub.4 2000 KMnO.sub.4 1000 80CuI(P.phi..sub.3).sub.4 1000 KMnO.sub.4 1000 35Cu(CF.sub.3 CO.sub.2).sub.2 1000 KMnO.sub.4 1000 77CuSO.sub.4 1000 K persulfate 1000 87CuSO.sub.4 1000 Na bromate 1000 3,67,53,28CuSO.sub.4 1000 KMnO.sub.4 1000 66Cupric chromate 1000 KMnO.sub.4 1000 77Cu stearate 1000 KMnO.sub.4 1000 62,20Cu stearate 2000 KMnO.sub.4 1000 81Cu stearate 2000 KMnO.sub.4 2000 85Cu perchlorate 1000 KMnO.sub.4 1000 64Cu(BF.sub.4).sub.2 1000 KMnO.sub.4 1000 84Cupric per- 1000 KMnO.sub.4 1000 49,75chlorateCupric per- 2000 KMnO.sub.4 1000 74,92chlorateCupric per- 5000 KMnO.sub.4 1000 94,95chlorateCupric 1000 KMnO.sub.4 1000 58,44acetylacetonateCuprous 1000 KMnO.sub.4 1000 84acetateCu.sub.2 O 1000 Sulfur 1000 49Cu.sub.2 O 2000 Sulfur 1000 82Cu.sub.2 O 1000 Sulfur 2000 70Cu.sub.2 O 2000 Sulfur 2000 75CuCO.sub.3 1000 Sulfur 1000 30,43CuCO.sub.3 2000 Sulfur 1000 77CuCO.sub.3 1000 Sulfur 2000 72CuCO.sub.3 2000 Sulfur 2000 83Cu(NO.sub.3).sub.2 1000 Sulfur 1000 67,52,62Cu(NO.sub.3).sub.2 1000 Sulfur 5000 46CuCO.sub.3 1000 KMnO.sub.4 1000 94,81,90,93,92 94,87,87,90,89 93,92,78,52,92 92,76,80,76,98 93,94,93,68CuCO.sub.3 1000 KMnO.sub.4 2000 80CuCO.sub.3 1000 KMnO.sub.4 5000 75CuCO.sub.3 1000 KMnO.sub.4 500 corrosionCuCO.sub.3 1000 KMnO.sub.4 100 corrosionCuCO.sub.3 1000 KMnO.sub.4 50 29CuCO.sub.3 2000 KMnO.sub.4 1000 91CuCO.sub.3 2000 KMnO.sub.4 500 92CuCO.sub.3 2000 KMnO.sub.4 100 87CuCO.sub.3 2000 KMnO.sub.4 50 86CuCO.sub.3 5000 KMnO.sub.4 1000 91CuCO.sub.3 5000 KMnO.sub.4 500 90CuCO.sub.3 5000 KMnO.sub.4 100 88CuCO.sub.3 5000 KMnO.sub.4 50 86CuCO.sub.3 10000 KMnO.sub.4 1000 90CuCO.sub.3 10000 KMnO.sub.4 100 89CuCO.sub.3 1000 K persulfate 1000 corrosion, 67CuCO.sub.3 1000 K persulfate 2000 80CuCO.sub.3 2000 K persulfate 1000 81CuCO.sub.3 2000 K persulfate 2000 85CuCO.sub.3 1000 K iodate 1000 79CuCO.sub.3 1000 Na bromate 1000 85CuCO.sub.3 500 Na persulfate 500 corrosionCuCO.sub.3 1000 Na persulfate 500 corrosionCuCO.sub.3 1000 Na persulfate 1000 corrosionCuCO.sub.3 2000 Na persulfate 1000 86,83CuCO.sub.3 1000 Na persulfate 2000 84,81CuCO.sub.3 2000 Na persulfate 2000 85,86CuCO.sub.3 1000 Na persulfate 5000 82,80CuCO.sub.3 1000 K meta- 1000 71 periodateCuCO.sub.3 1000 Na permanga- 1000 26,74 nateCuCO.sub.3 1000 Na permanga- 2000 33,64 nateCuCO.sub.3 2000 Na permanga- 1000 86,75 nateCuCO.sub.3 2000 Na permanga- 2000 81 nateCuCO.sub.3 1000 Sr permanga- 1000 66 nateCuCO.sub.3 1000 Sr permanga- 2000 82,58 nateCuCO.sub.3 1000 Sr permanga- 5000 94 nateCuCO.sub.3 2000 Sr permanga- 2000 96 nateCuCO.sub.3 5000 Sr permanga- 2000 94 nateCuCO.sub.3 1000 Na perborate 5000 68CuCO.sub.3 1000 Zn permanga- 1000 72 nateCuCO.sub.3 2000 Zn permanga- 1000 80 nateCuCO.sub.3 1000 Zn permanga- 2000 59 nateCuCO.sub.3 1000 Ca iodate 1000 75CuCO.sub.3 1000 NaHSO.sub.3 1000 corrosionCuCO.sub.3 1000 Na chlorate 1000 46, corrosionCuCO.sub.3 1000 Na chlorate 2000 corrosionCuCO.sub.3 1000 KIO.sub.3 1000 26CuCO.sub.3 1000 KI 1000 63,77 KMnO.sub.4 1000CuCO.sub.3 1000 H.sub.2 O.sub.2 (30%) 5000 63Cu.sub.2 O 1000 K persulfate 1000 18,89 corrosionCu.sub.2 O 1000 K persulfate 2000 90Cu.sub.2 O 1000 K persulfate 5000 93Cu.sub.2 O 2000 K persulfate 1000 corrosionCu.sub.2 O 2000 K persulfate 2000 89Cu.sub.2 O 2000 K persulfate 5000 93Cu.sub.2 O 1000 KMnO.sub.4 1000 corrosion 92,93,86Cu.sub.2 O 2000 KMnO.sub.4 1000 44Cu.sub.2 O 2000 KMnO.sub.4 500 corrosionCu.sub.2 O 2000 KMnO.sub.4 100 38Cu.sub.2 O 1000 Ca iodate 1000 87CuO 1000 KMnO.sub.4 1000 85,95,92CuO 1000 Ca iodate 1000 91CuO 1000 K iodate 1000 95CuO 1000 Na permanga- 1000 59 nateCu(NO.sub.3).sub.2 1000 Zn permanga- 1000 63 nateCuO 1000 K persulfate 1000 94CuO 1000 Na bromate 1000 92Cu(NO.sub.3).sub.2 1000 Na dichromate 1000 corrosion 40,58Cu(NO.sub.3).sub.2 1000 Na dichromate 5000 49Cu(NO.sub.3).sub.2 1000 Br.sub.2 1000 corrosionCu(NO.sub.3).sub.2 1000 I.sub.2 1000 49,15 corrosionCu(NO.sub.3).sub.2 1000 I.sub.2 2000 6Cu(NO.sub.3).sub.2 1000 I.sub.2 5000 corrosionCu(NO.sub.3).sub.2 1000 K persulfate 5000 corrosionCu(NO.sub.3).sub.2 1000 K persulfate 1000 corrosionCu(NO.sub.3).sub.2 1000 Na persulfate 1000 42Cu(NO.sub.3).sub.2 1000 Na persulfate 5000 50Cu(NO.sub.3).sub.2 1000 Na chlorate 500 corrosionCu(NO.sub.3).sub.2 1000 Na chlorate 100 corrosionCu(NO.sub.3).sub.2 1000 Na chlorate 1000 corrosionCu(NO.sub.3).sub.2 1000 Na chlorate 5000 corrosionCu(NO.sub.3).sub.2 5000 Na chlorate 5000 88Cu(NO.sub.3).sub.2 1000 La perchlorate 1000 corrosionCu(NO.sub.3).sub.2 1000 Na perborate 1000 41,14Cu(NO.sub.3).sub.2 1000 Na perborate 5000 15,55Cu(NO.sub.3).sub.2 1000 Sr permanganate 1000 63Cu(NO.sub.3).sub.2 2000 Sr permanganate 2000 91Cu(NO.sub.3).sub.2 5000 Sr permanganate 2000 95Cu(NO.sub.3).sub.2 1000 Sr permanganate 2000 55,79Cu(NO.sub.3).sub.2 1000 Sr permanganate 5000 79Cu(NO.sub.3).sub.2 1000 Na peroxide 1000 corrosionCu(NO.sub.3).sub.2 1000 Periodic acid 1000 20Cu(NO.sub.3).sub.2 1000 NH.sub.4 persulfate 1000 16,24,62Cu(NO.sub.3).sub.2 1000 NH.sub.4 persulfate 2000 corrosionCu(NO.sub.3).sub.2 1000 NH.sub.4 persulfate 5000 28Cu(NO.sub.3).sub.2 1000 K persulfate 5000 corrosionCu(NO.sub.3).sub.2 1000 K persulfate 5000 61Cu(NO.sub.3).sub.2 1000 KMnO.sub.4 1000 60,59,81,90Cu(NO.sub.3).sub.2 1000 Ca permanganate 1000 91,68Cu(NO.sub.3).sub.2 1000 Ca iodate 1000 13Cu(NO.sub.3).sub.2 1000 K-meta 1000 93,63 periodateCu(NO.sub.3).sub.2 1000 H.sub.2 O.sub.2 (80%) 5000 37,53Cu(NO.sub.3).sub.2 1000 H.sub.2 O.sub.2 (30%) 1000 68Cu(NO.sub.3).sub.2 1000 H.sub.2 O.sub.2 (30%) 5000 37Cu(NO.sub.3).sub.2 1000 Cu selenate 1000 corrosionCu(NO.sub.3).sub.2 1000 Na chlorate 1000 93Cupric 1000 KMnO.sub.4 1000 77chromateCupric 2000 KMnO.sub.4 1000 70molybdateCu(BF.sub.4).sub.2 1000 KMnO.sub.4 1000 84CuI(P.phi..sub.3).sub.4 1000 KMnO.sub.4 1000 35Copper 5000 KMnO.sub.4 1000 41tungstateCopper 5000 KMnO.sub.4 1000 28phosphideCuCl.sub.2 1000 KMnO.sub.4 500 92CuCl.sub.2 1000 KMnO.sub.4 1000 84CuCl.sub.2 1000 K persulfate 500 92CuCl.sub.2 1000 K iodate 500 92CuI.sub.2 1000 KMnO.sub.4 1000 71CuBr.sub.2 1000 KMnO.sub.4 1000 77__________________________________________________________________________ .sup.(1) See Table I .sup.(2) Corrosion means no inhibition, i.e. same or greater corrosion rate as that observed with no inhibitor
EFFECT OF PRESENCE OF OXYGEN
To demonstrate the beneficial effect of the presence of oxygen in the system the following tests were run. Oxygen was bubbled through 120 milliliters of an 80% by volume monoethanolamine solution, containing various inhibitors, which was previously saturated at room temperature with H.sub.2 S for 1/2 hour. A coupon prepared in the manner under General Description was immersed in the solution during the heating part of the test in the usual manner. The corrosion rate (loss of metal per unit time) for each run was compared with the corrosion rate for the solution containing no inhibitor. The results are recorded below as % inhibition:
TABLE III______________________________________ % InhibitionCopper ppm w/oxy w/o oxy______________________________________Cu(NO.sub.3).sub.2 1000 52 corrosiveCu(NO.sub.3).sub.2 2000 89 --Cu(NO.sub.3).sub.2 5000 -- corrosiveCuS 1000 87 91CuS 2000 23 --Cu.sub.2 O 1000 69, 65 corrosiveControl None 0 0CuCO.sub.3 1000 92 84, 0, corrosive______________________________________
To demonstrate the effectiveness of the corrosion inhibitor compositions of the present invention in other commercial solvents used in gas-treating plants the following data was collected:
COPPER SALT (CuCO.sub.3 or CuS) PLUS OXIDIZER, (S.degree. OR KMnO.sub.4) IN VARIOUS SOLVENTS USED TO REMOVE ACID GASES FROM GAS STREAMS
PROCEDURE:
The conditions for testing the various solvents, including gas ratios, as volume ratios, are described in the following tables. The saturated solutions, at whatever concentration used, were poured into 4 oz. bottles which had the inhibitor candidates and the 1020 mild steel coupons (prepared as in the general procedure) already weighed and placed in them. The bottles were capped with loose fitting caps and placed in a pressure vessel which was then held at 125.degree. C. with a total pressure of 40 psig of either H.sub.2 S, or CO.sub.2 or both, for 16 hrs. All inhibitor components are reported in parts inhibitor component per million parts of absorbent medium.
Solvent used: N-Methyldiethanolamine 45% + water 55%
Gas Loading ratio: 1/5 CO.sub.2 /H.sub.2 S
______________________________________CuCO.sub.3 KMnO.sub.4 MPY % Inhibiton______________________________________-- -- 38.5 0.01000 1000 11.2 71.01000 1000 13 66.21000 -- 13.4 65.21000 -- 12 68.8-- 1000 61 -58.5______________________________________
Solvent used: Diisopropanolamine 45%, Sulfolane 35%, Water 20%
Gas Loading ratio: Saturated with H.sub.2 S only
______________________________________CuCO.sub.3 KMnO.sub.4 MPY % Inhibition______________________________________-- -- 18.4 0.050 50 9.85 46.450 500 3.1 83.150 2500 9.0 51500 50 5.85 68.2500 500 5.56 70500 2500 6.14 66.62500 50 8 56.52500 500 7.65 58.42500 2500 5.74 68.825 25 11.1 39.75000 5000 7.46 59.4______________________________________
Solvent used: Diglycolamine: (2-(2-aminoethoxy)ethanol) 60% + water 40%
Gas loading ratio: Saturated with H.sub.2 S only
______________________________________CuCO.sub.3 KMnO.sub.4 MPY % Inhibition______________________________________-- -- 20.4 0.050 2500 6.21 69.5500 50 2.8 86.3500 2500 3.91 80.82500 50 3.19 84.32500 500 3.27 842500 2500 3.1 84.85000 5000 4 80.4500 500 3.22 83.750 50 37.5 -8450 500 28.3 -39______________________________________
Solvent used: METHICOL: (3-dimethylamino-1,2-propanediol) 50% + water 50%
Gas loading ratio: CO.sub.2 sparged 30 min., H.sub.2 S sparged the 20 min. External H.sub.2 S gas pressure was applied to the sparkler filter to get its pressure to 40 psig.
______________________________________CuCO.sub.3 KMnO.sub.4 MPY % Inhibition______________________________________-- -- 25.55 0.01000 1000 9.44 63.0 12.0 53.0 11.11 56.5______________________________________
Solvent used: Catacarb: (potassium carbonate) from Cooperative Farm Chemicals 72-FS-224 as delivered
Gas loading ratio: 50/50 CO.sub.2 /H.sub.2 S saturated
______________________________________CuCO.sub.3 KMnO.sub.4 MPY % Inhibition______________________________________-- -- 27.63 0.01000 2000 12.4 55.01000 2000 17.8 35.61000 100 20.0 27.61000 1000 20.2 27.01000 1000 16.9 38.8______________________________________
The following two tables illustrate that a high CO.sub.2 content, i.e., CO.sub.2 alone or ratios greater than 1 to 1, CO.sub.2 to H.sub.2 S in potassium carbonate or diethanolamine systems are not effectively inhibited. However, when the CO.sub.2 to H.sub.2 S ratio is 1 to 1 or less than 1 to 1, e.g., 1/4.5 or H.sub.2 S alone, inhibition is effected.
Solvent used: Exxon catacarb (Potassium Carbonate) their sample solution #1 containing no inhibitor
Gas ratio: CO.sub.2 only
______________________________________ %CuS S.degree. MPY Inhibition______________________________________-- -- 42 03000 -- 72 -671000 -- 58 -36500 -- 56 -30100 -- 39 +83000 3000 19 +543000 1000 105 -1443000 100 63 -461000 3000 54 -281000 1000 87 -1041000 100 52 -22500 1000 69 -58100 500 40.5 +5.7100 100 60.2 -40-- 1000 42 2.2-- 500 80.2 -86.6-- 3000 29 32.5-- 100 46 -7______________________________________
Gas ratio: 9/1 CO.sub.2 /H.sub.2 S
______________________________________CuCO.sub.3 S.degree. MPY % Inhibition______________________________________-- -- 7 03000 -- 14.7 -114, -72, 11.76 -102 13.851000 -- 9.7, 10, -41, -46, 15.1 -120500 -- 9.5, 9.9, -38.6, -44, 10.8 -58100 -- 10.06, 10.8, -47, -57.6, 9.81 -433000 3000 13.87 -102 15.83 -1303000 1000 15.1 -1203000 500 14.13 -106 14.86 -1173000 100 14.9 -117 13.03 -901000 3000 16.33 -138 20.3 -1961000 1000 17.3 -152 12.8 -871000 500 15.04 -1191000 100 12.9 -88 12.6 -84500 3000 16.07 -134500 1000 12.77 -86 14.38 -110500 100 10.8 -57100 3000 17.01 -148 17.82 -160100 1000 14.6 -113 14.75 -115100 500 16.1 -135100 100 13.06 -90.6______________________________________
Solvent used: Diethanolamine: (DEA) 30% + 70% water
Gas ratio: 9/1 CO.sub.2 /H.sub.2 S
______________________________________CuCO.sub.3 S.degree. MPY % Inhibition______________________________________-- -- 9.3 0 9.09 0 9.13 05000 -- 17.04 -86 18.5 -1011000 -- 13.4 -46 12.63 -38500 -- 12.95 -41 11.04 -20100 -- 9.44 -3 9.43 -2.85000 5000 33.02 -260 33.4 -2645000 1000 25.3 -176 17.2 -87.65000 100 18.7 -104 19.5 -1131000 5000 26.2 -185 32 -2491000 1000 19.71 -115 23.3 -1541000 500 23.2 -154 19.78 -1161000 100 13.35 -46 13.85 -51500 5000 27.8 -205 26.3 -187500 1000 21.4 -133 19.82 -116500 500 21.8 -138 20.5 -124500 100 15.7 -71 14.9 -62-- 5000 36.14 -294-- 1000 22.4 -144-- 500 19.9 -117-- 100 15.11 -65100 5000 22.76 -148100 500 26.2 -186______________________________________
Solvent used: Diethanolamine: (DEA) 40% + 60% water
Gas loading ratio: CO.sub.2 /H.sub.2 S 1/4.5
______________________________________CuCO.sub.3 KMnO.sub.4 MPY % Inhibition______________________________________-- -- 14.35 0.025 25 13.6 5.225 50 16.8 -17.0725 100 18.96 -32.125 500 22.2 -5525 1000 20.5 -4325 2000 17.05 -18.825 5000 11.6 19.2500 25 8.05 44.0500 50 8.17 43.0500 100 8.19 43.0500 500 9.05 37.0500 1000 7.91 45.0500 2000 8.95 37.6500 5000 8.84 38.41000 50 9.27 35.41000 500 12.25 14.61000 2000 11.0 23.31000 500 14.7 -2.45000 50 10.1 29.65000 5000 13.6 5.21000 -- 9.55 33.4-- 1000 17.2 -19.8______________________________________
Solvent used: Diethanolamine: (DEA) 80% + 20% water
Gas ratio: Saturated with H.sub.2 S only
______________________________________CuCO.sub.3 KMnO.sub.4 MPY % Inhibition______________________________________-- -- 17.25 0.05000 -- 6.3 63.52000 -- 6.77 60.71500 -- 6.19 64.01000 -- 5.74 66.7 500 -- 6.75 60.9-- 5000 13.6 21.1-- 2000 14.3 17.1-- 1500 14.06 18.5-- 1000 13.64 20.9-- 500 13.64 20.95000 5000 6.7 61.22000 2000 6.69 61.21000 1000 5.66 67.21000 1000 5.9 65.85000 1000 7.06 59.075000 2000 7.02 59.31000 5000 6.00 65.22000 5000 6.4 63.002000 1000 6.85 60.01000 2000 6.5 62.32000 2000 4.8 72.21000 1000 5.2 69.81000 1000 4.52 73.8______________________________________
Solvent used: Diethanolamine (DEA) 30% + 70% water
Gas ratio: 1/9 CO.sub.2 /H.sub.2 S
______________________________________CuCO.sub.3 S.degree. MPY % Inhibition______________________________________-- -- 35.3 0 28.12 0 30.0 05000 -- 18.6 40 17 451000 -- 14.18 54 13.6 56500 -- 11 64.6 13.4 57100 -- 12.9 58.6 18.2 41.55000 5000 21.5 31 23.06 265000 1000 20.6 34 21.3 315000 500 21 31.6 21.4 31.35000 100 18.3 41.2 19.2 381000 5000 23.5 24.5 25.1 19.41000 1000 17.2 44.7 16.4 47.31000 500 16.8 46 14.2 541000 100 17.27 44 18.2 41.5500 5000 39.6 -27 58.8 -89500 1000 16.3 47.6 20 36500 500 14.8 52.5 17.7 43500 100 13.84 55.5-- 5000 105.7 -239-- 1000 72 -131-- 500 62.1 -99-- 100 54 -73______________________________________
It is apparent from the above data that the copper inhibitor composition of the present invention is not a satisfactory inhibitor for diethanolamine solutions of less than 50 percent diethanolamine when used for sweetening acid-gases high in carbon dioxide content. Similarly the inhibitor composition has limited utility in potassium carbonate solutions of greater than 50 percent CO.sub.2. However, as H.sub.2 S content increases, the inhibitory effect is apparent.
FIELD TRIAL
An off-gas stripper in a medium size refinery operating at 250.degree. F. on the reboiler and with 17 to 19% aqueous monoalkanolamine absorbent (a 6000 gallon capacity) was seriously corroded and recording 37.4 mpy corrosion rate on an electrical resistance corrosion monitoring probe at the reboiler outlet, 74.6 mpy and 95.5 mpy corrosion rate on coupons at the reboiler outlet and the reboiler cross-exchange, respectively. This rate of corrosion was experienced during a five-day period following a turn-around. On the sixth day, copper sulfide and sulfur were added to the aqueous amine solution continuously at a rate of approximately 50 pounds CuS every 6-8 days and approximately 24 pounds sulfur every day.
Beginning the sixth day of the test, the day the corrosion inhibitor was added, throughout the twelfth day the corrosion rates were 9.75 mpy, 4.77 mpy and 16.08 mpy at the probe at the reboiler outlet and the coupons at the reboiler outlet and reboiler cross-exchanger, respectively. From the twelfth day to the end of the test the corrosion rates were as follows:
______________________________________ Probe Coupons Reboiler Reboiler Cross-Day Outlet Outlet Exchanger______________________________________12-14 1.85 1.26 9.4419-26 1.85 1.25 14.627-39 1 10.5 13.739-45 1 4.7 18.1______________________________________
On several instances the corrosion rate on the probe was allowed to increase to about 24 mpy by discontinuing the addition of inhibitor, then inhibitor addition resumed. Within one hour after the addition was resumed, the probe registered 1 mpy or less corrosion rate.
In another test using a pilot plant, copper rods were placed in a filter chamber for an alkanolamine gas treating plant. Sulfur dissolved in monoethanolamine was added periodically. The full flow of alkanolamine passed through the filter containing the copper rods in the rich absorbent to the stripper line. The corrosion probes showed passivation during the test, i.e., zero corrosion rate.
TABLE IV
Copper rods in full flow filter. S.degree./MEA solution added as slugs.
______________________________________ Corrosion Rate (MPY) Hot Lean 1.sup.(1) Hot Lean 2.sup.(1)______________________________________200 mls S.degree. /MEA 52.6 12.0400 mls S.degree. /MEA 0.0 0.0Depassivate 0.0 14.95300 mls S.degree. /MEA 0.0 0.0Depassivate 0.0 37.0______________________________________
Remove copper rods -- only residual copper present in plant.
______________________________________ Corrosion Rate (MPY) Hot Lean 1.sup.(1) Hot Lean 2.sup.(1) Hot Rich 3.sup.(2)______________________________________300 mls S.degree. /MEA 0.0 0.0 0.0Depassivate 0.0 0.0 157.0300 mls S.degree. /MEA 0.0 0.0 0.0Depassivate 0.0 0.0 181.0300 mls S.degree. /MEA 0.0 0.0 0.0Depassivate 95.0 120.0 260.0200 mls S.degree. /MEA 95.0 120.0 260.0150 mls S.degree. /MEA 100.7 33.7 100.0______________________________________
Passivation was possible for 118 hours after removing copper rods.
.sup.(1) Both probes in line from stripper bottom to reboiler.
.sup.(2) In line from absorber to top of stripper.
METHODS FOR DETERMINING THE CORROSION RATES AND INHIBITOR EFFECTIVENESS
There are two methods which can be used to measure the corrosion rate and also the effectiveness of the copper based inhibitors of the present invention at various points throughout plants which are using solvents such as monoethanolamine to remove mixtures of acid gases, such as H.sub.2 S and CO.sub.2, from liquid or gas process streams. The first is by using metal corrosion test coupons. The second is by using a metal resistance probe and an electrical resistance measuring bridge such as that manufactured by Corrosion Monitoring Systems, Inc., 33 Lincoln Rd., Springfield, N.J. 07081. A very good probe, the F. Jefferies probe is especially useful because of its all metal construction and essential freedom from temperature fluctuation sensitivity. This probe is also sold by the above named corporation. Another probe which is also useful is one made by Magna Corporation, who also manufactures a resistance bridge which will measure the corrosion experienced by their probe.
It is not unusual to have different sections of any gas treatment plant made from different metals. Thus, if the corrosion rate of a particular plant section is sought, it is necessary to measure the corrosion on a probe or coupon made of that same alloy which has been placed in the plant at that section.
A convenient coupon test method for mild steel is the one described below. Similar tests are available for other alloys.
First, a coupon of the metal of interest is machined. A convenient size is 1/2 in. .times. 2 in. .times. 3/8 in. A 3/8 in. hole is drilled near one end so that the coupon can be mounted on a coupon holder. This holder is used to place and hold the coupon, in electrical isolation from the plant's base metal, in the plant at the point of interest.
Second, the machined coupon is cleaned as follows
Place the raw metal coupon in a 15% HCl solution. The solution can be in a 2 oz, wide mouth bottle or beaker tall enough so the coupon can lean against the side of the container for support and still be completely covered by the HCl solution. Let the coupon soak in the acid for a half hour, then remove and rinse it with cold water. Next, scrub the coupon with pumice soap (such as Lava) using a toothbrush while holding the coupon in a rubber gloved hand. Use 15-20 strokes per side and edge; then, water and acetone rinse and air dry. Weight the coupon accurately on a balance to a tenth of a milligram and mount on the coupon holder and place it into the plant's solution.
Entry to the corrosive environment is achieved by one of several techniques widely known in the art. One such method is to insert the coupon and its holder through a packing gland and then through an open valve.
Third, after the coupon has been in the corrosive environment for several days, it is removed. The exact number of days is not important except that, in order to be able to place more significance in the measured corrosion rates, times of 20-30 days are desirable; but, times as short as one day can be used. Very short test periods generally result in higher apparent corrosion rates than would be expected from longer time period tests. The coupon is then recleaned according to the following procedure:
Place the corroded metal coupon in 15% HCl solution containing a good acid corrosion inhibitor. Then, follow the same instructions as listed above for the first cleaning of the coupon.
When the coupon is reweighed, subtract to determine the difference in weight from the weight of the untested coupon. The original weight, minus the final weight, will give the weight loss in grams. Convert this to milligrams (by moving the decimal to the right three places).
By using the following method, a corrosion rate for the solution-alloy system can be calculated. ##EQU1## Where: 0.183 is the conversion factor from milligrams/square decimeter/day (mdd) to mils penetration per year (mpy) for mild steel (1020). ##EQU2## Where: for coupons used 0.0176 was strip factor.
Fourth, inhibitor in the way and amount described in accordance with the present invention is added to the plant and a new corrosion rate determined by the method outlined just above.
Fifth, a percent inhibitor is calculated for the particular inhibitor concentration used as follows: ##EQU3##
Sixth, the amount of inhibitor is increased or decreased to achieve the desired corrosion protection as measured by the coupon method just outlined. Usually, 5 mpy is achievable; but corrosion has been reduced in the hot, corrosive environments from 20 mpy to values less than 1 mpy as measured by this technique which is found to correlate well with plant inspection and measurements made on specific elements by a state licensed boiler inspector.
The corrosion rate of a plant's solution can also be measured by using metal probes such as those described earlier. In this case, the probe is designed to replace the coupon as described above. The probe can be used directly as received from the manufacturer where it was sand-blasted before shipment or it can be cleaned with unihibited HCl as described in the above by the coupon cleaning method.
After the corrosion probe is cleaned, it is inserted into the plant's solution at the location in question and the temperature allowed to equilibrate. A resistance reading is taken with a resistance bridge. The bridge sold by the probe manufacture is very convenient. By drawing a graph of the resistance probe vs. time, the corrosion rate can be calculated from the slope of the line using the formula supplied by the probe manufacturer.
When the inhibited corrosion rate has been determined, inhibitor is added according to the present invention and the corrosion rate is measured and calculations made as above outlined.

Number	Name	Date
1989004	Fife	Jan 1935
2405672	Reitmeier	Aug 1946
2559580	Alexander	Jul 1951
3372981	Ravner et al.	Mar 1968
3932583	Schievelbein	Jan 1976

Method and composition for inhibiting the corrosion of ferrous metals

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Continuation in Parts (1)