Abstract:
A composition and method of inhibiting corrosion caused by naphthenic acid in fluids includes using a combination of a thiophosphorus compound and a hydrogen sulfide scavenging compound. The hydrogen sulfide scavenging compound significantly enhances the performance of the thiophosphorus compound and may be included at relatively low levels in the composition. The treatment may be applied to fluids including crude oils and lighter fractions thereof.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/741,430, filed Nov. 30, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to corrosion inhibitors. The present invention particularly relates to corrosion inhibitors useful for applications in which naphthenic acids are present.  
         [0004]     2. Background Art  
         [0005]     It is widely known in the art that the processing of crude oil in its various fractions may lead to damage of iron-containing metal surfaces of the processing equipment. This corrosion is frequently associated with, in particular, the presence and activity of naphthenic acid. The corrosion occurs when the amount of naphthenic acids in the hydrocarbon reaches some critical value indicated by total acid number (“TAN”), expressed as milligrams of potassium hydroxide required to neutralize the acids in a one-gram sample. Older literature uses a rule of thumb that a TAN greater than about 0.5 is required for a crude oil to cause naphthenic acid corrosion, but more recent experience indicates that the critical value can vary considerably from this value. When elevated temperatures are applied to the crude, such as the 175° C. (−347° F.) to about 400° C. (−752° F.) temperatures customarily used to refine and distill the oil, the corrosion problem is typically further exacerbated.  
         [0006]     While various corrosion inhibitors are known in the art, the efficacy of any particular corrosion inhibitor is generally known to be dependent upon the circumstances under which it is used. As a result, a variety of corrosion inhibitors have been developed and targeted for use for treating particular crudes, for protecting particular metals, for inhibiting specific types of corrosion, and/or for use under particular conditions of temperature, environment, and the like. For example, U.S. Pat. No. 3,909,447 describes certain corrosion inhibitors as useful against corrosion in relatively low temperature oxygenated aqueous systems, such as water floods, cooling towers, drilling muds, air drilling and auto radiator systems.  
         [0007]     As commonly used, naphthenic acid is a collective term for certain organic acids present in various crude oils. Although minor amounts of other organic acids may also be present, it is understood that the majority of the acids in a naphthenic acid based crude are naphthenic in character, i.e., with a saturated ring structure that conforms to a formula such as one of the following:  
                         
 
 In the above formulas, m is typically 1-2, and n varies. It is basically any carboxylic acid group with at least one saturated 5 or 6 membered ring attached. One simple example is cyclopentanoic acid. 
 
         [0008]     The molecular weight of naphthenic acid can extend over a large range. However, the majority of the naphthenic acid in crude oils is found, after distillation, in the lighter fractions, including, for example, gas oil. When hydrocarbons containing such naphthenic acid contact iron-containing metals, especially at elevated temperatures, severe corrosion problems arise.  
         [0009]     Various approaches to controlling naphthenic acid induced corrosion have included neutralizing and/or removing the naphthenic acids from the crude being processed; blending low acid number oils with more corrosive high acid number oils to reduce the overall neutralization number; and using relatively expensive corrosion-resistant alloys in the construction of the crude&#39;s processing apparatus. These attempts are generally disadvantageous in that they require additional processing and/or add substantial cost to treatment of the crude oil. Alternatively, U.S. Pat. No. 4,443,609 discloses certain tetrahydrothiazole phosphonic acids and esters as being useful additives for inhibiting acid corrosion. Such inhibitors can be prepared by reacting certain 2,5-dihydrothiazoles with a dialkyl phosphite. While these tetrahydrothiazoles phosphonic acids or esters offer good corrosion inhibition, they tend to break down under high temperature conditions.  
         [0010]     Another disadvantage to using phosphorus-based compounds as corrosion inhibitors is that the phosphorus has been alleged to impair the function of various catalysts used to treat crude oil, such as in fixed-bed hydrotreaters and hydrocracking units. Thus, crude oil processors are often faced with a dilemma, since corrosion itself, if not inhibited, may result in accumulation in the hydrocarbon fluid of a catalyst-impairing amount of iron, as high as 10 to 20 ppm in some cases. Unfortunately, while there are a number of commercially available non-phosphorus-based inhibitors, they are known to be generally somewhat less effective than the phosphorus-based compounds.  
         [0011]     A significant advance in phosphorus-based naphthenic acid induced corrosion inhibitors is reported in U.S. Pat. No. 4,941,994. Therein it is disclosed that metal corrosion in hot acidic liquid hydrocarbons in inhibited by the presence of a corrosion inhibiting amount of a dialkyl and/or trialkyl phosphite with an optional thiazoline. Another patent, U.S. Pat. No. 5,863,415, discloses that thiophosphorus compounds of a specific formula are particularly useful for corrosion inhibition in hot liquid hydrocarbons and may be used at concentrations that add to the fluid less of the catalyst-impairing phosphorus than some of the previous phosphorus-based corrosion inhibitors. These thiophosphorus compounds also offer the advantage of being able to be prepared from relatively low cost starting materials.  
         [0012]     In view of the above, it would be desirable in the art to find additional method and compositions for inhibiting or controlling naphthenic acid induced corrosion in crude oils that offer advantages over the prior art.  
       SUMMARY OF THE INVENTION  
       [0013]     In one aspect, the present invention is a method for inhibiting naphthenic acid corrosion in a fluid containing a corrosive amount of naphthenic acid, the method comprising adding, to the fluid or to a feed therefor, a thiophosphorus compound and a hydrogen sulfide scavenging compound, wherein the thiophosphorus compound and the hydrogen scavenging compound are in amounts effective to inhibit corrosion.  
         [0014]     In another aspect, the invention is a composition useful for inhibiting naphthenic acid corrosion in a fluid containing a corrosive amount of naphthenic acid comprising an admixture of a thiophosphorus compound and a hydrogen sulfide scavenging compound. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     In one aspect, the present invention is a method for inhibiting naphthenic acid corrosion in a fluid containing a corrosive amount of naphthenic acid, the method comprising adding to the fluid or to a feed therefor, in amounts sufficient to effect corrosion inhibition in the fluid, a thiophosphorus compound and a hydrogen sulfide scavenging compound. The fluid may be, for example, a hydrocarbon such as a crude oil or a lighter fraction thereof. The thiophosphorus compounds useful with the present invention include those that are disclosed in U.S. Pat. No.5,863,415 to Zetlmeisl, the content of which is incorporated herein by reference in its entirety. For example, thiophosphorus compounds useful in practicing the present invention include the salts of the thiophosphorus compounds, alkyl and aryl esters of the thiophosphorus compounds, and isomers of the thiophosphorus compounds. Any thiophosphorus compounds that effect inhibition of naphthenic acid induced corrosion in fluids can be used in the present invention.  
         [0016]     Useful with the invention in one non-limiting embodiment are thiophosphorus compounds of the general formula:  
                         
 
 wherein R 1  is R 3 (OCH 2  CH 2 ) n —, or R 3 (OCH 2  CH 2 ) n O—, R 2  is the same as R 1  or is —XH, each X is independently sulfur or oxygen, provided however that at least one X is sulfur, R 3  is an alkyl group of about 6 to about 18 carbon atoms, and n is an integer from 0 to about 12. 
 
         [0017]     The hydrogen sulfide scavenging compounds useful with the invention include those disclosed in U.S. Pat. No. 5,169,411 to Weers and U.S. Pat. No. 6,663,841 to Salma, et al., the contents of which are incorporated herein by reference in their entireties. Included in the compounds are the non-acidic imine compound which is the condensation product of an amine or polyamine and an aldehyde, dialdehyde or ketone. For example, they include, in non-limiting embodiments, those compounds having the general formula: 
 
R 1 (N═R 2 ) x ,   FORMULA 2 
 
 wherein x is an integer of from about 1 to about 10; R 1  is independently selected from the group consisting of  
                         
 
 cycloalkyl having about 4 to about 7 carbon atoms, phenyl, benzyl, alkyl having 1 to about 20 carbon atoms, and alkenyl having 1 to about 20 carbon atoms; R 3  is hydrogen, alkyl having from 1 to about 20 carbon atoms, alkenyl having 1 to about 20 carbon atoms, or aryl; n is an integer of 1 to 6; R 4 , R 5 , and R 6  are each independently selected from the group consisting of alkyl containing 1 to about 20 carbon atoms and  
                         
 
 R 7  is hydrogen or alkyl having 1 to about 20 carbon atoms, and is the same as R 2  with the proviso that only one of R 4 , R 5  and R 6  may be  
                         
 
 and R 2  is independently selected from the group consisting of ═CH 2 , cyclohexyl,  
                         
 
 alkyl containing 1 to about 20 carbon atoms, and alkenyl containing 1 to about 20 carbon atoms. In FORMULAS 5 and 6, “S” denotes the presence of at least one sulfur atom in the ring, without being specific as to its location or locations. 
 
         [0018]     The inhibitors of the present invention have been found to be both effective and to have very low levels of hydrogen sulfide evolution during storage and use. The amount of the thiophosphorus compound is desirably, in certain non-limiting embodiments, from about 40 to about 70 percent by weight of a total additive comprising both the thiophosphorus compound and the hydrogen sulfide scavenging compound. In other embodiments the hydrogen sulfide scavenging compound may be from 1 percent to 10 percent by weight, and preferably from about 4 percent to about 6 percent by weight. The remainder of the additive may be an inert material that is compatible with the fluid, such as, for example, mineral oil. In other non-limiting embodiments the thiophosphorus compound may represent from about 90 percent to about 99 percent by weight of the total additive, and in still other non-limiting embodiments, it may represent from about 94 to about 96 percent by weight.  
         [0019]     The additive combination of the thiophosphorus compound and the hydrogen sulfide scavenging compound may be employed in the fluid, containing the corrosive naphthenic-acid, in any amount that is effect to inhibit the corrosion effected by the fluid/naphthenic acid. Such may be relatively low, from about 100 ppm to about 5,000 ppm, but greater or lesser amounts may be employed if such are effective. In many desirable embodiments it has been found that a level of from about 500 ppm to about 2,000 ppm is effective, and in other desirable embodiments such may range from about 750 ppm to about 1250 ppm.  
       EXAMPLES  
       [0020]     The following examples are provided to illustrate the present invention. The examples are not intended to limit the scope of the present invention and they should not be so interpreted. Amounts are in weight parts or weight percentages unless otherwise indicated.  
       Example 1  
       [0021]     A test of the invention is performed by first determining the level of corrosion for a blank.  
         [0022]     The procedure includes the step of adding 282 grams of mineral oil to a reaction flask. A test cell is sparged with nitrogen at a rate of from 10-20 mL/min for a period of 30 to 60 minutes with continuous stirring. The heaters are set at 288° C. and the test cell is sparged with 1 mol-percent H 2 S in nitrogen for the remainder of the test. The test procedure is run with a total acid number of 10 mg KOH, a temperature of 288° C., a run time of 24 hours, and 1 mol-percent of hydrogen sulfide in nitrogen, using C1018 carbon steel coupons. A “blank” is first tested, without corrosion inhibitor, to determine the amount of corrosion that occurs. About 20 g of commercial naphthenic acids having a total acid number of 151 are injected 30 minutes after the coupon is inserted into the test cell. Then a first test of the invention is done using another coupon and the same procedure, except that the corrosion inhibitor of the invention is included. The corrosion inhibitor is a mixture of a thiophosphorus compound and a hydrogen sulfide scavenger compound at a level of 1000 ppm by volume where the thiophosphorus compound is present at 55 weight percent and the scavenger is present at 5 weight percent, with the remainder of the admixture being a mineral oil. The coupons are in all cases removed after 24 hours in the test cell for evaluation. The procedure to test the invention is also completed with another coupon in a second test.  
         [0023]     In the first test, and when compared to the “blank,” the level of protection afforded by the invention is determined to be 94.2 percent. In the second test of the invention, the level of protection is determined to be 99.7 percent.  
       Comparative Example 2  
       [0024]     Example 1 is repeated substantially identically except that no hydrogen sulfide scavenger is used, but the same amount of the thiophosphorus compound is used. In a first test, the level of protection is determined to be 80.1 percent. In a second test, the level of protection is determined to be 89.2 percent.