US2715605A - Prevention of corrosion of ferrous metals by alkanolamines - Google Patents

Description

United States Patent Oflfice 2,715,605 Patented Aug. 16, 1955 PREVENTION OF C(BRRGSION OF FERROUS METALS BY ALKAYOLAMINES Joseph K. Goerner, Austin, Tera, assignor to Jeiierson Chemical Company, Inc., New York, N. Y., a corporation of Delaware No Drawing. Appiication July 2, 1952, Serial No. 295,961

9 Claims. (Cl. 202-57) This invention relates to the prevention of corrosion of ferrous metals by alkanolamines and to alkanolamine compositions which are substantially non-corrosive to ferrous metals. More particularly, this invention relates to minimizing the corrosion of ferrous equipment in which alkanolamines are distilled or stored or which equipment is employed for carrying out processes using alkanolamines. By ferrous metals is meant structural steel including steels containing chromium, and/ or nickel, stainless steel, cast iron, Wrought iron and other forms of steel and iron employed in the production of tanks, stills, pipe lines and other equipment for handling, storing or transporting alkanolamines.

Alkanolamines, such as monoethanolamine, diethanolamine, triethanolamine, isopropanolamine, diisopropanolamine, triisopropanolamine, etc., are commonly prepared by allowing aqueous ammonia solutions to react with ethylene oxide, propylene oxide, etc. N-dimethyl ethanolarnine may be made by allowing ethylene oxide to react with aqueous dimethylamine, and N-methyl monoethanol amine and N-methyl diethanolamine are prepared by reaction of ethylene oxide with aqueous methyl amine. By employing various different amines and alkylene oxides a great variety of different alkanolamines may be obtained; preferred alkanolamines contain from 2 to 18 carbon atoms.

The reaction of an alkylene oxide with ammonia inevitably results in a mixture of alkanolamines. Thus, for example, the reaction of ethylene oxide with ammonia yields a mixture of mono-, di-, and tri-ethanolamines. This mixture must be resolved to obtain commercially acceptable products. This is usually accomplished by distilling the crude alkanolamines, preferably under vacuum. When carrying out this distillation in ferrous equipment considerable iron dissolves in the hot liquid alkanolamines. Such corrosion of ferrous equipment is aggravated by the presence of moisture or elemental oxygen in the alkanolamine solution.

In carrying out certain porcedures involving the use of alkanolamines, for example, the scrubbing of gases such as petroleum refinery gases, coke oven gas and other combustible gases in ferrous equipment to remove carbon dioxide and hydrogen sulfide from the gases, considerable corrosion of the equipment takes place, particularly during the regeneration of the scrubbing medium by heating it usually at temperatures above C., usually above to C., to expel the dissolved gas. At these temperatures, particularly at about C., the corrosion is appreciable, and at C. and higher presents a serious problem. In an endeavor to minimize such corrosion in the case of gas purification processes involving the use of ethanolamines as the scrubbing medium it has been proposed to add to the scrubbing solution of ethanolamines a small amount of arsenic compounds, such as arsenous oxide and sodium arsenite, vanadium oxide, copper salts or iodine compounds, as corrosion inhibitors. Of these corrosion inhibitors the arsenic compounds appear to be most eifective to inhibit corrosion during distillation of alkanolarnines. The use of arsenous oxide or sodium arsenite as corrosion inhibitors in such process, however, has the serious disadvantage that some of the arsenic compound volatilizes with the alkanolamines and appears in the distillate contaminating the same.

It is among the objects of this invention to provide a process for minimizing the corrosion of ferrous metals by alkanolamines, which process is simple to carry out and does not result in the contamination of the alkanolamine distillate when the process is employed to minimize corrosion of ferrous equipment in which alkanolamines are distilled.

Another object is to provide an alkanolamine suitable for scrubbing gases to remove carbon dioxide and/or hydrogen sulfide and for other uses and which is substantially non-corrosive to ferrous metals.

Other objects and advantages of this invention Will be apparent from the following detailed description thereof.

This invention is based upon the surprising discovery that the corrosion of ferrous equipment in contact with hot alkanolamines at a temperature above 150 C. can be greatly minimized, if not substantially completely prevented, by the addition of a small amount of an antimony compound to the alkanolamines. By antimony compound is intended to include antimony as such, as well as antimony in combined form. Examples of antimony compounds which can be used are elemental antimony, antimony trioxide (SbzOs), antimony pentaoxide (SbzOs), antimony tetraoxide (Sb2O4), alkali salts including the alkali metal and alkaline earth metal salts, e. g., sodium, potassium, lithium, calcium, magnesium salts of antimonous and antimonic acids, potassium antimonyl tartrate, known as tartar emetic, other alkali metal and alkaline earth metal antimonyl tartrates, such as sodium antimonyl tartrate and other organic antimony containing compounds such as the antimony acetates. The preferred corrosion inhibitor is antimony trioxide.

When adding the antimony inhibitor to alkanolamines subjected to distillation in ferrous equipment, it is preferred to employ an inorganic antimony compound to avoid adding to the distillation system organic material which decomposes at the high temperatures employed in the distillation and thus tends to contaminate the distillate and possibly result in a product having poor color stability.

The amount of antimony compound added to the alkanolamine may be from 0.001% to 5.0% by Weight based on the weight of the alkanolamines, preferably from 0.01% to 1% by Weight. The antimony compound may be added continuously or intermittently to the alkanolamine solution to insure the presence of an amount of antimony inhibitor within the range above noted. In the case of the distillation of alkanolamines it is preferred to add the antimony inhibitor continuously, for example, by preparing a slurry of antimony trioxide in say diethanolamine containing about 10% by weight of the antimony trioxide and feeding this slurry into the distillation system at any point which permits the antimony stream to join the crude alkanolamine stream before the latter is heated to about 150 to 160 C. Thus, in the case of the distillation of ethanolamines the antimony containing slurry may be added either to the crude aqueous reaction mixture which Will contain Water, ammonia, mono-, diand tri-ethanolamines as well as high boiling residues, or it may be added to a relatively anhydrous ethanolamine mixture before the latter is subjected to distillation to resolve it into its components.

While the explanation for the behavior of antimony compounds to minimize corrosion of ferrous equipment by alkanolamines is not fully understood, it is believed that the addition of the antimony compound to the 3 alkanolamines coats or plates with a thin film of antimony the ferrous metal surfaces with which the alkanolamines would otherwise come in contact. As long as this antimony coating is present the underlying ferrous metal does not corrode to a serious extent. If no antimony compound is added after the formation of the initial coating or plating, the protection lasts for a short time, say from a few minutes to several hours,'depending on such factors as the relative amount of antimony compound added to the system, the surface to volume ratio of the system, temperature, rates of flow, amount of oxygen or moisture present, etc. After this short period of time, recominencement of corrosion takes place and is indicated by the appearance of excessive amounts of iron in the liquid being discharged from the still or other vessel in which 'the hot alkanolamines are treated or stored. Further Example I A sample of mild steel with an area of cm. was immersed in 1000 ml. of a liquid mixture containing approximately monoethanolamine, 40% amine, 19% triethanolamine and 1% water disposed in a 2000 ml. flask, to which was attached a reflux condenser. The liquid was protected from the atmosphere by a stream of dry nitrogen gas introduced into the flask below the liquid level. After 51 hours at 190 C. it was 7 r found that the steel sample showed an average penetration rate of 0.09 inch per year, and that the ethanolamines liquid had picked up 355 parts of iron per million parts of ethanolamines.

' Ina comparative test carried out in exactly identical manner, except that 0.027%' of SbzOs by weightbased on the weight of the ethanolamines was added to the ethanolamines before the start of the heating, the peneitration was 0.0019 inch per year, and the iron pick-up wasonly 29 parts per million parts of ethanolamines.

Example II .A commercial unit of mild steel for the continuous'distillation of monoethanolamine was operated for 48 hours without any corrosion inhibitor. It was found that the still bottoms liquid contained 600 parts of iron per million parts of monoethanolamine. Addition of antimony trioxideto the feed was then commenced at a rate of 0.03% by weight based on the weight of the ethanolamaine fed to the still. After a very short time the iron content of the still bottoms liquid fell to 50 parts per million parts of monoethanolamine.

Example III This example differs from Example I in that 0.031% by weight of sodium antimonate (NaSbOs) was added to the ethanolamine mixture before the start of the heating. The penetration was 0.0060 inch per year, and the iron pick-up was parts per million parts of ethanolamines.

Example IV A sample of stainless steel withan area of 20 square centimeters was immersed in 1000 ml. of a mixture of alkanolamines the same as that used in Example I. After 51 hours of heating at 185 190 C. it was found penetration was at the rate of 0.0026 inch per year and the 'iron pick-up was 44 parts per million parts of alkanolamines.

In a comparative test in which 0.05% by weight of diethanolrosion of ferrous metals by hot alkanolamines.

antimony trioxide was added to the amines as inhibitor on the same stainless steel it was found that penetration was at the rate of 0.0008 inch per year on the basis of 65.75 hours heating at 185 to 190 C. The iron pick-up was 12 parts per million parts of alkanolamines.

Example V Diethanolamine containing about 0.05 by weight of antimony trioxide based on the weight of the diethanolamine and diluted with water to a concentration of about 8% by weight of diethanolamine is employed as the scrubbing medium to remove carbon dioxide from boiler stack gas passing through a steel scrubbing tower with no appreciable corrosion of the steel Walls taking place. The exhausted scrubber liquid is regenerated by heating to about C. in a steel vessel without any serious corrosion of the steel walls of the regenerator.

It Will be noted this invention effectively inhibits cor- Furthermore, the use of this invention in inhibiting corrosion of ferrous metals, such as stills employed in the distillation of alkanolamines, results in the production of an alkanolamine product which isnot contaminated by the corrosion inhibitor.

It will be further noted this invention provides a new composition of matter, namely, a mixture of alkanolamines with an antimony compound, which when used as a solvent for removal of carbon dioxide, hydrogen sulfide, or for other uses is substantially non-corrosive to ferrous equipment. The preferred compositions are the ethanolamines, mono-, di-, or trior mixtures thereof containing from 0.001% to 5.0% by weight, preferably from 0.01% to 1.0% by Weight, of an antimony compound, preferably an antimony oxide, especially antimony trioxide.

It is to be understood that this invention is not restricted to the present disclosure otherwise than as defined by the appended claims.

What is claimed is:

1. A process of minimizing the corrosion of ferrous metals by hot lower alkanolamines, which comprises admixing with said lower alkanolamines from 0.001% to 5.0% by weight of an antimony compoundbased on the weight of the alkanolamines.

2. A process of minimizing the corrosion of ferrous metals by lower alkanolamines at a temperature above C., which comprises maintaining in said lower alkanolamines a concentration of an antimony compound of from 0.001% to 5.0% by weight based on the weight of the alkanolamines.

3. A process of minimizing corrosion of ferrous metals by lower alkanolamines during the distillation of said alkanolamines, which comprises admixing with said lower alkanolamines prior to being subjected to distillation from 0.001% to 5.0% by weight of an inorganic antimony compound based on,the weight of the alkanolamines.

4. The process defined in claim 3, in which from 0.01 to 1% by Weight of antimony compound is added to the lower alkanolamines based on the weight of the alkanolamines subjected to distillation.

5. A process of minimizing the corrosion of a steel still in which a mixture of lower alkanolamines is subjected to distillation, which comprises adding to the stream of said lower alkanolamines introduced into the still from 0.01%

' to 1% of an antimony oxide by weight based on the weight of the alkanolamines.

6. In a process of resolving by distillation in a ferrous metal vessel a mixture of monoethanolamine, diethanolamine and triethanolamine, the improvement which comprises adding to said mixture from 0.01% to 1% by weight of antimony oxide based on the weight of said mixture of ethanolamines to inhibit corrosion of the ferrous metal walls of the vessel by said ethanolamines.

7. As a new composition of matter a lower alkanolamine containing from 0.001% to 5.0% by weight of an 6 antimony compound based on the weight of said lower 0.01% to 1.0% by weight of antimony trioxide based on alkanolamine. the weight of the diethanolamine.

8. As a new composition of matter an ethanolamine containing from 0.001% to 5.0% by weight of an anti- References Clted m the file of thls Patent mony oxide based on the weight of the ethanolamine. 5 UNITED STATES PATENTS 9. As a new composition of matter a solvent consist- 2,031,632 Bottoms Feb. 25, 1936 ing primarily of diethanolamine and containing from 2,303,399 SChWartz Dec. 1, 1942

Claims (1)

1. 5. A PROCESS OF MINIMIZING THE CORROSION OF A STEEL STILL IN WHICH A MIXTURE OF LOWER ALKANOLAMINES IS SUBJECTED TO DISTILLATION, WHICH COMPRISES ADDING TO THE STREAM OF SAID LOWER ALKANOLAMINES INTRODUCED INTO THE STILL FROM 0.01% TO 1% OF AN ANTIMONY OXIDE BY WEIGHT BASED ON THE WEIGHT OF THE ALKANOLAMINES.