US 3537974 A
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U.S. Cl. 208-47 15 Claims ABSTRACT OF THE DISCLOSURE Corrosion of metals by aqueous acidic solutions in a non-oxidizing atmosphere is markedly inhibited by the presence of an alkoxy-substituted benzaldehyde. A particularly effective inhibitor of this type is p-anisic aldehyde. These inhibitors are useful in minimizing corrosion of chemical and petroleum process equipment handling hydrocarbon streams containing acidic gases and water vapor.
BACKGROUND OF THE INVENTION This invention relates to the prevention of corrosion of metals by aqueous acidic solutions, and more particularly to the prevention of corrosion of chemical and petroleum process equipment which is subjected to corrosive attack by aqueous acidic solutions in a non-oxidizing atmosphere as a result of condensation of water containing dissolved acidic substances.
Various acidic substances which are present in petroleum refining operations cause corrosion of metals With which they come in contact. Examples of destructive inorganic compounds include hydrochloric acid, sulfuric acid, sulfur dioxide, and hydrogen sulfide. Organic compounds causing corrosion include acetic acid, phenolic compounds, naphthenic acids, and aliphatic and naphthenic organic chlorides. Corrosion-causing acids enter the hydrocarbon process streams in petroleum refineries in various ways. For example, crude oils generally contain naphthenic acids. The organic chlorides do not usually occur naturally in crude oil, but are sometimes added by producers for removal of paraffin deposits in producing wells and pipelines. These tend to hydrolyze in the presence of water to produce hydrochloric acid. Hydrogen sulfide is formed in catalytic desulfurization processes using hydrogen, in which various hydrocarbon feedstocks including virgin and cracked naphthas, as well as gas oils, containing such impurities as mercaptans, disulfides, and thiophenes, are catalytically reacted with hydrogen in order to reduce their sulfur content. Sulfuric acid and sulfur dioxide are both processing reagents, the former being used as an alkylation catalyst and the latter as an extractant for the removal of aromatics from hydrocarbon feedstocks. Hydrochloric acid and hydrogen chloride may result from several sources, including the hydrolysis of organic chlorides, hydrolysis of salt which is mixed with crude oil as a result of the use of brine in oil production operations, and as a result of hydrolysis of chlorine gas which is used in the regeneration of platinum reforming catalysts.
Acidic substances such as the foregoing will cause severe corrosion of the metals from which conventional petroleum refining equipment is constructed. Carbon steels, such as 1020 carbon steel containing 0.2% carbon, are used predominantly as materials of construction. While it would be possible to fabricate refinery equipment from steels which are less prone to corrosive attack, such as stainless steel and special alloy steels, the cost of such equipment would be inordinately high and would make nited States Patent "ice any process being conducted with such equipment uneconomical.
Corrosion in petroleum process streams is particularly troublesome in equipment, such as condensers and heat exchangers, where condensation of water takes place. Water vapor is invariably present; both in hydrocarbon process streams and in regenerator gas streams, as is well known. When this water condenses, acidic gas present in the process stream, such as hydrogen chloride, hydrogen sulfide, sulfur dioxide, and carbon dioxide, dissolve in the condensate and attack metal equipment. Such attack occurs in hydrocarbon process streams containing only trace amounts of oxygen or none at all, since the metal is oxidized by the hydrogen ions of the acid.
The overhead stream from an atmospheric pipestill is one example of a petroleum process stream containing acidic gases. Such a stream generally contains hydrogen chloride as well as organic chlorides. Upon condensation of this overhead stream, hydrogen chloride is dissolved in the water condensate and the quantity of hydrogen chloride is augmented by hydrolysis of a portion of the organic chlorides present. This condensate attacks the condenser surfaces.
Another location Where corrosion may occur is on the downstream side of a hydrotreating unit. The efi'luent from a hydrotreater may contain Water vapor, hydrogen chloride, and hydrogen sulfide. In typical operations this efiluent is condensed and the hydrogen sulfide removed. Corrosion is prone to take place in the condenser and the hydrogen sulfide stripper.
Acidic atmospheres are also found frequently during the regeneration of catalyst for hydroforming and other catalytic hydrogenation processes. It is necessary to prevent corrosion as a result of these acid gases while at the same time avoiding any significant adverse effect on catalyst activity as a result of contact of the catalyst with a corrosion inhibitor. This is particularly important in the case of hydroforming catalysts Where the catalysts are expensive and where it is extremely crucial, from a process economy point of view, to extend the catalyst life as long as possible.
One possible technique for inhibiting corrosion by acids is to neutralize the acid with a base. However, such a solution would not be practical because there is a tremendous daily throughput of feed streams through petroleum or chemical processes which contain acidic materials, thereby requiring a correspondingly large amount of .base for neutralization. A further problem arises from the fact that the most likely bases for use in such neutralizations would be either organic nitrogen compounds or ammonia. Nitrogen, however, is a severe poison for many petroleum conversion catalysts such as reforming catalysts. Its use would, therefore, be contraindicated in any fluid stream which would eventually contact such conversion catalysts. It is thus evident that a meaningful answer to the problem facing the petroleum industry would not be based on neutralization or removal of the acidic corrosive agents in the feed stream since such techniques would either be prohibitively expensive or would result in deactivation of reforming catalysts. Instead, it is necessary to provide a corrosion inhibitor whose effectiveness does not depend on neutralization of acid present and which does not adversely affect the catalyst to any significant degree, if at all.
SUMMARY OF THE INVENTION It has been found that corrosion of metals by acids can be markedly inhibited by the presence of an aryl-substituted aliphatic aldehyde having the formula (RO) ArCHO where Ar is a monocyclic aryl radical, R is methyl or ethyl, and n is an integer having a value of 1 or 2. Where there is more than one alkoxy-substituent present, these substituents may be either the same or different. These compounds are particularly effective in inhibiting corrosion by aqueous acids in non-oxidizing atmospheres.
DETAILED DESCRIPTION OF THE INVENTION A preferred corrosion inhibitor according to this invention is p-anisic aldehyde, which has the formula I OOHs Other alkoxy-substituted derivatives of benzaldehyde can also be used effectively as corosion inhibitors. Such compounds include o-anisic aldehyde, vanillin, and vanillin ethyl ether (4-ethoxy-3-methoxybenzaldehyde). To function effectively as a corrosion inhibitor, a compound must have a water solubility sufiicient to provide an eifective corrosion-inhibitng concentration. On the other hand, compounds having a substantial water solubility are also poor inhibitors for preventing corrosion by acids. Best results are obtained with compounds which are substantially, but not completely, water insoluble. Compounds having alkoxy groups with more than 2 carbon atoms and compounds having more than 2 alkoxy groups generally do not have sufiicient water solubility to be effective corrosion inhibitors. The presence of one or more hydroxyl substituents attached to the benzene ring, as in vanillin, is not harmful and may 'be beneficial, since the hydroxyl group enhances the water solubility of the compound without rendering it too soluble for good inhibition. The presence of alkoxy radicals is beneficial as indicated by the fact that alkoxy-substituted aldehydes of this invention are better corrosion inhibitors than benzaldehyde.
The inhibitors of this invention may be used effectively in widely varying concentrations. Effective inhibition is obtained in concentrations ranging from about moles per liter up to about 0.5 mole per liter in the aqueous acidic phase. Actually, there is no upper limit on the effectiveness of the inhibitors of this invention, and the maximum concentration is limited only by the solubility of the compound. However, concentrations in excess of 0.5 M/l. do not give inhibitory action substantially greater than that obtained at concentrations under 0.5 M/l..
The aldehydes of this invention function most effectively in a non-oxidizing atmosphere. The atmosphere may be either inert, e.g., a nitrogen atmosphere, or reducing, e.g., a hydrocarbon gas atmosphere. These aldehydes offer more effective protection against corrosion by non-oxidizing acids than against corrosion by oxidizing acids. Oxidation of the aldehyde to the corresponding acid may occur in the presence of an oxidizing acid or an oxidizing atmosphere.
Any metals which are subject to acid attack in a nonoxidizing atmosphere can be protected with the inhibitors of this invention. These inhibitors are particularly useful for protection of ferrous metals, and especially low carbon steel, such as 1020 carbon steel (containing 0.2% carbon). Low carbon steels are ideal for construction of petroleum processing equipment from the standpoint of cost and other significant qualities such as strength and their ability to withstand the process stream temperatures. The principal drawback to low carbon steel is its susceptibility to acid corrosion, and problems arising from this are substantially obviated by the use of the inhibitors of this invention.
Non-oxidative corrosion by acids is ordinarily a problem where the pH of the acidic solution is about 4 or lower. The aldehyde inhibitors of this invention offer excellent protection even in solutions which are decidedly on the acid side, e.g., those having a pH of l or lower.
A few types of apparatus used in the petroleum processing industry will be cited as' examples of apparatus which may be protected against corrosion according to this invention. One such type of apparatus is the regeneration circuit used in hydroforming. It is necessary in hydroforming to use a catalyst having a small chloride content. During regeneration coke is burned from the catalyst, producing an efiiuent which has a fair concentration of CO and small quantities of S0 and S0 During this step, the chlorides to be found in the gas vapor will increase due to an increase in water content of the gas which serves to strip chlorine off the catalyst. The second step is to remove any water left on the catalyst. This means thorough drying of the flue gas which is a mixture of nitrogen, CO CO, S0 S0 and HCl. After most of the water has been removed, chlorination is started in a manner such that chlorine will be progressively absorbed by the catalyst. During the subsequent rejuvenation of the catalyst to rearrange the crystal structure, some chlorine will still be carried over with the flue gas. The last step in the regeneration operation is purging the stream with nitrogen, an inert gas, to remove air and finally pressure up with hydrogen. The inhibitor of the instant invention is injected into the hydroformer regenerating gas stream either continuously throughout the regeneration cycle or by intermittent high rate injection of inhibitor at the same total amount per regeneration cycle. The presence of the inhibitor serves to reduce or minimize corrosion in heat exchange equipment and transfer lines where water condensate, containing the acidic components mentioned preciously, accumulates. The inhibitor compound is adsorbed in the metal surfaces and minimizes corrosion by markedly lowering the rate of corrosion reactions. As indicated earlier, the presence of alkoxy-substituted benzaldehydes, preferably in the absence of oxygen, serves to inhibit the corrosion effects of various acid base corrosion causing substances. An inert gas, e.g., nitrogen, is present and, in addition, corrosive substances such as hydrogen sulfide, hydrochloric acid, and sulfuric acid are present. The addition of an alkoxysubstituted derivative of benzaldehyde serves to minimize corrosion with no adverse effect on the platinum or palladium catalyst.
Another area where corrosion in an inert atmosphere is widespread and has a deleterious effect is hydrotreating. Substantial quantities of hydrogen sulfide are produced in the hydrotreater by reduction of sulfur compound such as mercaptans, disulfides, and thiophene. This causes corrosion in the presence of water condensate. Also present is hydrogen chloride, which may result from the decomposition of organic chlorides such as carbon tetrachloride and trichloroethylene in the process stream, or from the hydroformer treat gas which contains HCl from decomposition of the chlorine treated catalyst base. In any event, the hydrotreater effluent condenser and other overhead equipment has been plagued with problems instigated by the presence of hydrogen chloride. Again, corrosion is greatly reduced by the injection of an arali phatic aldehyde into the process stream.
An advantage of the aldehydes of this invention is that they do not poision platinum catalysts, as do inhibitors containing nitrogen or sulfur. The hydrotreater efiluent is frequently passed through a platinum catalyst bed in a hydroformer, and in those cases it is imperative to avoid corrosion inhibitors which could poison the platinum catalyst.
A significant advantage of the use of corrosion inhibitors is that it is possible to use inexpensive construction materials such as low carbon steel, instead of costly corrosion resistant alloy steels which would render the cost of the process prohibitive.
In both of the above illustrations, the oxygen content of the process stream is substantially nil. Best results are obtained with the corrosion inhibitors herein described when little or no oxygen is present.
While ferrous metals have been cited as an illustrative example of metals which can be protected according to this invention, it should be understood that other metals and alloys such as nickel, zinc, brass, and copper, may also be protected. While copper is more resistant to acid attack in a non-oxidizing atmosphere than the other metals and alloys mentioned, nevertheless it may be prone to slight attack by strong acids, and such attack is mitigated by aliphatic aldehyde corrosion inhibitors.
The problem of corrosion attack is most severe in those units, such as condensers, heat exchangers, and transfer lines, where water condenses. The acid gases present in the process stream are dissolved in the condensate, and attack the metal process equipment. It has been found that the corrosion inhibitors herein are eifective under the entire temperature range in which water is present in the liquid phase. Since some processes are run at high pressure, the actual temperature may be considerably above the atmospheric boiling point of water; nevertheless, the inhibitors do not lose their eifectiveness at such temperatures. Likewise, they remain effective at low temperatures down to 32 F.
The inhibitor is preferably injected into the process stream just a short distance upstream for best results. This mitigates loss of the inhibitor, and also protects the inhibitor from decomposition from high temperatures which prevail in some units of process streams.
While the mechanism for the inhibiting action of aldehydes of this invention is not completely understood, the following explanation is offered for the purpose of illustration and as an aid in understanding the invention, and should not be taken as limiting the scope of the invention in any manner. The corrosion additive is believed to be adsorbed on the metal surface in the form of a continuous or nearly continuous thin film. This film would serve to inhibit any chemical or electro-chemical interaction between the acidic corrosive material in solution and the metal surface. The very small quantities of inhibitor that are utilized to form this thin film are not believed to undergo any significant chemical reaction with the acidic corrosive material. Thus, only small amounts of additional inhibitor would be necessary to maintain long term protection on metal surfaces, these additions being possibly necessitated by attrition losses due to physical interactions of the flowing stream with the film.
As previously noted, the corrosion inhibitor should not be markedly water soluble, nor should it be substantially completely insoluble. In short, the water solubility must be enough to establish an effective concentration, which as earlier noted is generally at least M/l. in the aqueous acidic phase.
The present invention will be more fully understood with reference to the following specific examples. It is understood that these examples are illustrations of specific embodiments of this invention and are not to be taken as limitations.
EXAMPLE 1 This example illustrates the efficacy of p-anisic aldehyde as an inhibitor of acid induced corrosion of 1020 carbon steel exposed to 0.1 N hydrochloric acid, which has a pH of 1. Corrosion rates were measured by weight loss of carbon steel specimens having a size of approximately 1" x 4 x s, and a surface area of approximately 58 square centimeters. The specimens were abraded through 4-0 emery paper, degreased in benzene, and washed in distilled water. Immediately after drying, the specimens were weighed and placed in a corrosion cell and immersed in the corrosive solution. Each of the corrosive solutions, except those used for control purposes, contained a predetermined concentration of p-anisic aldehyde. The amount of corroded metal was determined by weight loss. The corrosion cell was basically a 2000 ml. Erlenmeyer flask with a special top to permit entrance and exit of nitrogen for deaeration and to prevent air contamination. The cell had a removable chimney with Pyrex hooks from which the metal specimens were suspended. The corrosive solution was deaerated with nitrogen before a run. Nitrogen was also bubbled through the solution continuously during a run to prevent contamination with air. A constant temperature was achieved by the use of constant temperature oil bath. All runs were gzggrigl out for two days at a constant temperature of The results of representative experiments utilizing the above procedure are summarized below in Table I. In this table, corrosion rate in milligrams per square deci meter per day (mg./dm. /day or mdd.) and percentage inhibitor efficiency (which equals is the corrosion rate with inhibitor) are given for various concentrations of inhibitor in moles per liter (M/L.).
TABLE I Percent Corrosion rate, inhibitor Inhibitor concentration, M/l. mgJdmfl/day efficiency EXAMPLE 2 This example demonstrates the corrosion inhibition properties of p-anisic aldehyde to control corrosion of 1020 carbon steel in a catalytic reformer regeneration circuit condensate. The pH of this solution was 0.5 and the temperature of the test was 100 C. This represents an extremely corrosive environment for carbon steel. With the exception of solution identity and temperature, the procedure utilized was that of Example 1. The time of testing in this instance was about 2 days. The average corrosion rate of steel test specimens exposed to solutions containing 10 M/l. of p-anisic aldehyde was 1690 mg./dm. /day (mdd.), while the average corrosion rate of steel test specimens exposed to solutions containing no inhibitor was 41,114 mdd. This represents an inhibitor efiiciency of 95.7%.
While this invention has been particularly described with reference to petroleum processing streams, it will be understood that similar acid corrosion problems may also occur in chemical processing equipment and in containers for storage and shipment of acidic materials. Such equipment and containers can also be protected with the corrosion inhibitors of this invention.
1. A process for inhibiting corrosion of a metal by an aqueous acidic solution which comprises adding to said solution a corrosion inhibiting amount of a corrosion inhibitor consisting essentially of a compound having the formula (RO) A,CHO
where Ar is a monocyclic aryl radial, R is methyl or ethyl, and n is 1 or 2.
2. A process according to claim 1 in which the compound is p-anisic aldehyde.
3. A process according to claim 1 in which the metal is a ferrous metal.
4. A process according to claim 1 in which said solution and the surrounding atmosphere are non-oxiding.
5. A process according to claim 1 in which said solution has a pH not greater than about 4.
6. A process according to claim 1 in which said corrosion inhibiting compound is present in a concentration of about 10- M/l. to about 0.5 M/l. in the aqueous acidic solution.
7. A process according to claim 1 in which said acidic solution is a condensate in a hydrocarbon process stream.
7 8. A process according to claim 1 in which said corrosion inhibiting compound is added to a hydrocarbon process stream upstream of the area to be protected.
9. A process for inhibiting acid induced corrosion in a metal vessal containing hydrocarbonaceous fluids containing acidic corrosive agents, said process comprising adding a corrosion inhibitor consisting essentially of a compound having the formula (RO A CHO where Ar is a monocyclic aryl radical, R is methyl or ethyl, and n is 1 or 2.
10. A process according to claim 9 in which said compound is p-anisic aldehyde.
11. A process according to claim 9 wherein said vessel carries a hydrocarbon process stream.
12. An aqueous acidic solution inhibited against corrosive attack on meatls, said solution comprising water, an acidic substance normally tending to cause corrosion of metals, and a small but effective amount of a corrosion inhibiting compound having the formula (RO) A,.CHO
8 where Ar is a monocyclic aryl radical, R is methyl or ethyl, and n is l or 2.
13. A solution according to claim 12 having a pH not greater than about 4.
14. A solution according to claim 12 in which said compound is present in a concentration of about 10- M/l. to about 0.5 M/l.
15. A solution according to claim 12 in which said compound is p-anisic aldehyde.
References Cited UNITED STATES PATENTS 2,415,161 2/1947 Camp 208-47 2,571,739 10/1951 Marsh 2l2.5 2,965,577 12/1960 Heimann et al. 252--148 3,453,203 7/ 1969 Foroulis 208-47 2,908,640 10/1959 Dougherty 2039 X DELBERT E. GANTZ, Primary Examiner G. E. SCHMITKONS, Assistant Examiner US. Cl. X.R. 2l2.7; 252396