Abstract:
The corrosion of iron and steel surfaces by an aqueous alkanolamine conditioning solution used to remove CO 2  from a gas stream is effectively inhibited by a combination of a vanadium containing ion and a soluble cobalt salt. This system allows the use of higher amine concentrations which in turn allows a higher carbon dioxide loading with low corrosion thereby improving the energy efficiency of the gas sweetening process.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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. 
     2. Prior Art 
     It has been a long standing commercial practice to use aqueous alkanolamine solutions (e.g. a monoethanolamine solution) to absorb acidic acids such as CO 2 , H 2  S, 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 percent to 30 percent alkanolamine solution countercurrent to a gas stream in an absorption column to remove the acid gases. 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 salts 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 coinhibitors, for example, such as compounds of arsenic, antimony and vanadium. These metal compounds seem to be much more effective against CO 2  -promoted corrosion than they are when H 2  S 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. Pat. No. 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. Pat. Nos. 3,038,856, 3,269,999 and 3,280,097. U.S. Pat. No. 3,808,140 relates to a combination inhibitor system using vanadium and antimony. Nitrosubstituted 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. Pat. Nos. 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, thiocyanates, sulfides and cobalt as noted in U.S. Pat. Nos. 4,100,099; 4,100,100 and 4,102,804; as well as U.S. Pat. Nos. 4,096,085 and 4,143,119. Still another U.S. Pat. No. 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. Pat. No. 4,184,855 which uses inter-coolers and flash heat exchangers to increase the energy efficiency of the method and U.S. Pat. No. 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. 
     SUMMARY OF THE INVENTION 
     The invention is a corrosion inhibited composition consisting essentially of an aqueous alkanolamine solution employed in acid gas removal service and an inhibiting amount of a combination of an anion containing vanadium in the plus 4 or 5 valence state and an anion containing cobalt in the plus 2 valence state. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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 valance state for protective film formation. 
     The choice of vanadium compounds is not critical since it is the vanadium-containing anion, particularly vanadium in the plus 4 or 5 valence state, which provides this unusual corrosion inhibiting property in combination with the amines. Thus, for example, one can employ vanadium oxide such as VO, V 2  O 3 , VO 2 , V 2  O 5  and the like; vanadium cyanides such as K 4  V(CN) 6 .3H 2  O, K 3  V(CN) 6 , 2KSCN. VO(SCN) 2 .5H 2  O and the like; vanadium halides, such as fluorides, including VF 3 , VF 3 .3H 2  O, VF 4 , VOF 2 , VF 5 , chlorides including VCl 2 , VCl 3 , VCl 3 .6H 2  O, VOCl, VOCl 2 , VOCl 3 , V 2  O 2  Cl, V 2  O 3  Cl 2 .4H 2  O or VO 2  Cl 2  2.8H 2  O, and bromides including VBr 3 , VBr 3 .6H 2  O, VOBr, VOBr 2  or VOBr 3  ; vanadium sulfates including VSO 4 .7H 2  O, V 2  (SO 4 ) 3  , VOSO 4  or (VO) 2  (SO 4 ) 3  ; vanadates including orthovanadates, represented by the generic formula: M 3  VO 4 , pyrovanadates, represented by the general formula M 4  V 2  O 7  and metavanadates, represented by the general formula MVO 3  and the like where M represents a cation. The condensed vanadate ions which form in aqueous solutions, such as V 6  O 17   4  arealso useful in this invention. 
     For convenience in introducing vanadate ions into the inhibiting systems of this invention the alkali metals, ammonium 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: MVO 4 . 
     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. Salts such as CoCl 2 , CoBr 2 , CoSO 4 , Co(NO 3 ) 2 , 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: ##STR1## wherein R&#39; and R&#34; independently represent hydrogen or --CR 2  CR 2  --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)] 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 I 
     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 Teflon® 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 percent 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 1.48&#34;×0.41&#34;×0.12&#34; 1020 mild steel coupon with a 0.25&#34; 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 Teflon 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 an 115±1° C. 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 I which showed that of the transition metals tested, cobalt noticeably reduced corrosion of the mild steel coupon. 
     
                       TABLE I______________________________________CORROSION INHIBITOR SCREENING TESTSMEA.sup.a, CO.sub.2.sup.b,           Inhibitor      Vanadium.sup.d,                                  Corrosion%     mole/mole A.sup.c, ppm   ppm     Rate.sup.e, mpy______________________________________50.0  0.39      A═ Ni                    100   0       17050.0  0.39      A═ Ni                    100   100     2650.0  0.39      A═ Cu                    100   0       3950.0  0.39      A═ Cu                    100   100     4750.0  0.39      A═ Co                    100   0       750.0  0.39      A═ Co                    100   100     1150.0  0.39      A═ Zn                    100   0       6550.0  0.39      A═ Zn                    100   100     2750.0  0.39      --             --      24______________________________________ .sup.a Monoethanolamine, low iron grade, &lt;10 ppm Fe; made by Texaco Chemical Co. .sup.b Mole CO.sub.2 per mole of MEA. .sup.c Nickel was introduced as nickel nitrate, copper was introduced as cupric nitrate, cobalt was introduced as cobalt nitrate, and zinc was introduced as zinc nitrate. .sup.d Introduced as ammonium metavanadate, used in all examples. .sup.e The corrosion rate is a measurement of linear penetration in thousandths of an inch per year as computed by the following formula: ##STR2## 
    
     EXAMPLE II 
     The effect of soluble iron on an ammonium metavandate 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 I. 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 THEVANADIUM INHIBITED SYSTEM              Post-test     Additives              Analysis.sup.c,MEA,  CO.sub.2, Fe.sup.a,                   V.sup.b,                        Fe,  V,   Corrosion Rate%     mole/mole ppm     ppm  ppm  ppm  mpy______________________________________30.0  0.30      100     100  22   87   &lt;130.0  0.30      200     100  .sup.d                             19   1230.0  0.30      300     100  .sup.d                             11   730.0  0.30      400     100  .sup.d                             9    2830.0  0.30      500     100  .sup.d                             8    830.0  0.30       50     200  3    234  &lt;130.0  0.30      100     200  8    197  &lt;130.0  0.30      150     200  3    156  &lt;130.0  0.30      200     200  5    142  &lt;130.0  0.30      250     200  3    110  &lt;1______________________________________ .sup.a Iron was introduced as freshly prepared aqueous solution of ferrou ammonium sulfate. .sup.b Vanadium was introduced as ammonium metavanadate. .sup.C By atomic absorption analysis. .sup.d Not analyzed. 
    
     EXAMPLE III 
     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 III______________________________________EVALUATION OF COBALT--VANADIUMINHIBITOR SYSTEMCO.sub.2  Additives.sup.a                  Post-test Analysis.sup.b                               Cor-MEA,  mole/   CO,    V,   Fe,  Co,  V,   Fe,  rosion%     mole    ppm    ppm  ppm  ppm  ppm  ppm  Rate______________________________________50.0  0.39    --     100  --   --   116  1134 5750.0  0.39    --     200  --   --   211  578  2250.0  0.39    --     300  --   --   296  506  1850.0  0.39    100    --   --    82  --   1061 3950.0  0.39    200    --   --   214  --   813  5150.0  0.39    300    --   --   226  --   570  2650.0  0.39    100    100  --    82  103  393  1950.0  0.39    200    100  --   130   88  92   1250.0  0.39    300    100  --   223  100  100  650.0  0.39    100    200  --    66  197  198  1250.0  0.39    200    200  --   139  199  275  1250.0  0.39    300    200  --   245  200  7    &lt;150.0  0.39    300    200  40   261  163  44   &lt;150.0  0.39    300    200  80   267  170  72   &lt;150.0  0.39    300    200  120  265  173  109  &lt;150.0  0.39    300    300  40   266  278  42   &lt;150.0  0.39    300    300  80   266  269  77   &lt;150.0  0.39    300    300  120  264  274  108  &lt;150.0  0.39    300    400  40   269  388  44   &lt;150.0  0.39    300    400  80   268  387  72   &lt;150.0  0.39    300    400  120  272  391  108  &lt;1______________________________________ .sup.a Cobalt was introduced as cobalt nitrate, vanadium was introduced a ammonium metavanadate, and iron was introduced as aqueous solution of ferrous ammonium sulfate. .sup.b By 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 ten unit range or plus or minus five mils/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 III, the monoethanolamine concentration was 50 weight percent which is much higher than the 5 to 30 percent used in the prior art methods. As a result, the sour gas conditioning solution can be more concentrated and more effective in removing CO 2  than current solutions and provide corrosion protection in addition. 
     It is anticipated that many modifications may be made in the method of this invention without departing from the scope of this invention which is defined only by the appended claims.