NO840312L - CORROSION-INHIBITED GAS TREATMENT SOLUTION AND USE THEREOF - Google Patents

Description

This invention relates to corrosion inhibiting compositions

for use in acid gas removing apparatus and methods for using the same.

It is well known from the state of the art that acid gases such as carbon dioxide, hydrogen sulphide and carbonyl sulphide can be removed from gaseous output streams such as natural gas and synthetic

gas using dilute aqueous solutions of "sweeteners" such as potassium carbonate, alkanolamines such as monoethanolamine, diethanolamine and methyldiethanolamine, and other weak bases. The aqueous method is to use a contacting device in which the absorbent solution is brought into contact with the output stream, and to use a regenerator with an evaporator in which the enriched absorbent containing the sour gas components is regenerated back to the free absorbent. The solution is then recycled to the contact device for re-use.

Great efforts have been made with a view to

solve the problem of metallic corrosion in equipment used in the above process. This problem is particularly acute and/or chronic when mild steel is used in the equipment to limit investment costs, rather than using more exotic and expensive metal alloys, such as stainless steel. It is of course common practice to use stainless steel and nickel alloys in sensitive areas, such as for heat exchange equipment.

US Patent 3,087,778 (1975) describes the inhibition of potassium carbonate solutions using 1,000-5,000 ppm (parts per million) trivalent oxides of arsenic, antimony or bismuth.

US Patent 3,808,140 (1974) describes the inhibition of alkanolamine dissolution using small amounts of vanadium and antimony compounds.

US Patent 3,896,044 (1975) describes the inhibition of alkanolamine solutions using small amounts of nitro-substituted aromatic acids or salts thereof.

US Patent 3,959,170 (1976) describes the inhibition of alkanolamine solutions using a small amount of a stannous salt.

US Patent 4,071,470 (1978) describes inhibition of alkanolamine solutions using a small amount of reaction

the product of copper, sulfur and an alkanolamine.

US Patent 4,096,085 (1978) describes the inhibition of alkanolamine solutions using small amounts of a polyamine, with or without copper and sulfur.

US Patent 4,100,099 (1978) describes the inhibition of sour gas conditioning fluids using small amounts of quaternary pyridinium salts and alkylene polyamines.

US Patent 4,100,100 (1978) describes the inhibition of sour gas conditioning fluids using small amounts of quaternary pyridinium salts, thiocyanate compounds or thioamide compounds and compounds of divalent cobalt.

US Patent 4,102,804 (1978) describes the inhibition of acid gas conditioning solutions using small amounts of a quaternary pyridinium salt and a thiocyanate compound, a sulfide compound, or a thioamide compound.

US Patent 4,116,629 (1978) describes corrosion inhibition of stainless steel (types 410 and 430) in contact with carbonate solutions using nickel salts.

US Patent 4,143,119 (1979) describes the inhibition of acid-gas conditioning solutions using small amounts of copper and a polysulfide formed in situ.

While the above-mentioned compositions are effective, each of them has various defects that limit their applications. For example, compounds of arsenic, antimony and vanadium are known to be toxic, and their use causes problems with waste disposal. Use of quaternary pyridinium compounds is known to cause foaming problems in certain cases.

It has now been discovered that the corrosion of iron and steel in gas removal equipment can be effectively reduced by the use of a gas conditioning solution such as aqueous alkanolamines inhibited by effective amounts of a soluble thionitrogen compound.

It is believed that the inhibition is a result of the interaction between the thionitrogen compound and the metals that are naturally present in the gas removal solution used.

In the event that the metals are lacking, the gas conditioning solution can be modified by adding suitable amounts of one or more soluble nickel, cobalt, calcium, copper, chromium, zinc, tin, aluminum or magnesium compounds.

The inhibitors are advantageously constantly supplemented or maintained in the gas conditioning solution so that effective passivation is achieved.

The combination of inhibitors disclosed herein is unique in that it reduces corrosion on all ferrous surfaces, i.e. not only mild steel surfaces, but also stainless steels, except for 400 series stainless steels.

The inhibitors which are used according to the invention are particularly effective in aqueous solutions of alkanolamines such as monoethanolamine, diethanolamine, methyldiethanolamine and related alkanolamines which are usually used for removing acid gases from gas streams.

The impure gaseous/liquid exit streams that can be treated with the inhibited gas conditioning solutions of the invention to remove carbon dioxide may generally contain a few (1-5) ppm H_S and/or carbonyl sulfide and 500 ppm or less of oxygen. A few m 3 of an exit gas containing (300 ppm or less) I 2 S can be treated with the inhibited gas conditioning solutions of the invention by a simple test for selection of the suitable metal synergist. For example, metals that form insoluble sulphides will generally not provide corrosion protection.

Examples of suitable soluble nickel compounds are sulphate, nitrate, acetate, tartrate and citrate of divalent nickel.

Examples of suitable soluble cobalt salts are cobalt (II) halides such as the chloride, fluoride and bromide, and also sulphate, nitrate, acetate and benzoate of divalent cobalt.

The other metals which are used according to the invention are used in the form of their related soluble salts.

Examples of suitable soluble thionitrogen compounds are alkali metal thiocyanates, such as potassium thiocyanate and sodium thiocyanate, and metal thiocyanates such as copper thiocyanate and nickel thiocyanate. Ammonium thiocyanate can also be used and is preferred.

Other examples of soluble thionitrogen compounds are thioamides with the formula

A-C(S)-N(R) 2

where A is a hydrocarbon radical with 1-6 carbon atoms or

a pyridyl radical,

R is a hydrogen atom or an alkyl group with 1-4 carbon atoms.

Special examples of the thioamides are thioacetamide, N,N'-diethyl-thioacetamide, thiobenzamide, N,N'-dimethyl-thioacetamide, thiocaproic acid amide, N,N'-diethyl-thiocaproic acid amide and thio-nicotinic acid amide.

It should be noted that the term "soluble compound"

in the present means that the compound is sufficiently soluble in the aqueous gas conditioning solution, i.e. aqueous alkanolamine, to be able to be used for the purposes of the invention.

It has been found that thionitrogen inhibitors must be present in the gas conditioning solutions in amounts from 50 and preferably 100 or more parts per million. Since these compounds are consumed during the process, large amounts, such as 500 ppm or more, can be used at start-up, and periodic additions can be made thereafter to maintain the required amounts in the solution. A range of 50 to 1000 ppm has been found to be an effective amount, and a range of 100 to 300 ppm is the preferred range.

It has been found that when a fresh charge of the gas conditioning solution containing thionitrogen is used in a gas conditioning plant containing different metal alloys, there is a period of approx. 2-4 days in which the solution must be circulated before passivation takes place. It is believed that the gas conditioning solution dissolves sufficient trace metals to act as a synergist with the thionitrogen compounds.

In those applications of this invention in which the gas-conditioning equipment is constructed exclusively of mild steel, one or more of the above-mentioned metal salts must usually be added in the manner indicated above.

The following examples will further illustrate the invention.

Example 1

In a gas conditioning plant in which a hydrogen gas flow of 1.4 x 10<6>m<3>per. day at a pressure of 2.07 MPa and containing 18% CO 2 , was contacted with an aqueous solution of 18 wt% monoethanolamine, the enriched amine solution was regenerated in a stripping column with a reflux reboiler, and the depleted solution was pumped back to the contactor . Suitable cross-heat exchangers were used.

Stainless steel (304) and monel components were used in the heat exchangers.

Corrosion probes and metal coupons of the same metals used in the plant were placed in the cooled enriched amine solution, the hot enriched solution and the hot depleted solutions. The probes were adjusted to show the corrosion rate in mm per year.

The basic corrosion rate without inhibitor on carbon steel in the circuit for the hot enriched amine solution was 18.8 mm per year.

The basic corrosion rate without inhibitor on 304 stainless steel was 0.30 mm per year.

The basic corrosion rate without inhibitor on Monell 400 was less than 0.051 mm per year.

After the addition of 200 ppm ammonium thiocyanate, the corrosion rate of carbon steel dropped to less than 0.051 mm per year and the corrosion rate for stainless steel and monel dropped to less than 0.0254 mm per year.

The amine concentration was then increased from 18% to 25%,

and corrosion rates remained the same.

Example 2

In a plant in which CO2 was removed from a flue gas using 18-25% monoethanolamine (MEA), the off-gas contained from 0 to 500 ppm oxygen, and the MEA solution contained 1-5 ppm nickel in solution. The corrosion rate was found to be approx. 2.54 mm per years for carbon steel. A heavy metal corrosion inhibitor was first used to reduce the corrosion rate. Ammonium thiocyanate in an amount of 200 ppm was added as a substitute for the heavy metal. It was found that the corrosion rate remained in the range of 0.0254-0.0762 mm per

year while the MEA resolution varied from 18 to 25%.

Example 3

In a similar plant to that of Example 2, ammonium thiocyanate in an amount of 250 ppm was added as a heavy metal replacement, and the corrosion rate was still in the range of 0.0254-0.0762 mm per year.

Example 4

In a hydrogen gas scrubber in which 18-25% monoethanolamine solutions were used, the basic corrosion rate for carbon steel without inhibitor was 8.89 mm per years in the circuit for the hot enriched solution.

The basic corrosion rate for 304 stainless steel was 1.27 mm per years in the circles for the hot enriched and lean solution.

After the addition of 200 ppm ammonium thiocyanate, the corrosion rates fell to below 0.0508 mm per years for both metals.

Example 5

In a similar plant as in Example 4, containing less than 1 ppm nickel in the monoethanolamine solution, the basic corrosion rate without inhibitor was found to be 3.56 mm per year for carbon steel and 0.254 mm per year for 304 stainless steel.

After adding 300 ppm ammonium thiocyanate and 50 ppm cobalt sulfate to the MEA solution, the corrosion rate of carbon steel dropped to 0.381 mm per year, and the corrosion rate for stainless steel dropped to 0.203-0.254 mm per year.

Example 6

In a plant for the removal of CC^ from natural gas in which a 30% MEA solution was used, and where all equipment was of carbon steel, the basic corrosion rate was found to be 1.52 mm per years in the circuit for the hot lean solution.

After adding 200 ppm of ammonium thiocyanate, the corrosion rate dropped to 0.762-1.02 mm per year. Later, 3 ppm nickel sulphate (NiSO^) was added, and the corrosion rate dropped to below 0.0508 mm per year.

Example 7

In a natural gas purification plant as in example 6 in which

16% MEA was used, the basic corrosion rate for carbon steel was found to be 0.305 mm per year in the MEA stripper section. The basic corrosion rate for 304 stainless steel was found to be 0.305 mm per year.

After the addition of 200 ppm ammonium thiocyanate, these corrosion rates changed to 0.178 mm per second respectively. year and 0.914 mm per year. After adding 7 ppm nickel sulfate (NiSO4) both of these corrosion rates dropped to 0.00254

etc. per year.

The MEA concentration was then increased to 27%, with the above combination of ammonium thiocyanate and nickel sulfate. The corrosion rates changed to 0.00254 mm per year (carbon steel) and 0.0152 mm per year (stainless steel).

Example 8

In a hydrogen gas scrubber in which 16-27% MEA solutions were used, the basic corrosion rates for carbon steel and 30 4 stainless steel were found to be 1.78 mm per 1, respectively. year and 0.381 mm per year.

This speed was reduced to below 0.0508 mm per years for both metals using an arsenic inhibitor.

When the above inhibitor was replaced with 200 ppm ammonium thiocyanate and 5 ppm nickel sulfate, the corrosion rate for both metals was still below 0.0508 mm per year.

From the foregoing, it will be seen that the use of thiocyanate compounds greatly reduces the rate of corrosion in the case of the mild steel equipment used here, when the equipment also contains stainless steel or nickel alloys.

Examples 9-103

The effectiveness of the corrosion inhibitors which are used according to the invention was determined in a static coupon cox corrosion test. In this test, a solution of 25 and 30% by weight monoethanolamine (MEA) in deionized water was saturated with CC>2. This solution simulates an enriched amine solution commonly found in gas conditioning plants.

About. 350 ml of this inhibitor solution is then placed in a Teflon-lined steel cylinder, 50.8 mm x 254 mm, fabricated mild steel test coupons are inserted, and the cylinder is closed and bolted.

The cylinder of contents was then heated and held at 121°C for 24 hours. The coupons were taken out, cleaned and weighed. The corrosion rate in mm per year is calculated from the following equation

In Tables I-VIII, which refer to trials over one

24 hour period, the corrosion rate is stated for mild steel (1020MS) at 121°C (250°F). The inhibitors used in these experiments were known salts such as ammonium thiocyanate, nickel sulphate, cobalt sulphate, zinc sulphate, copper carbonate and calcium sulphate. In each example, two parallel tests were usually carried out, and the corrosion rate indicated is the average of the two tests. Single trials are denoted by (SR) ("Single Run").

Examples 104-106

A pre-charge of aqueous 30% monoethanolamine (MEA)

was prepared for each series of experiments and saturated by introduction of CO2 overnight. To 400 g portions of this CC₂ saturated MEA solution was added a sufficient amount of ammonium thiocyanate to give 200 ppm of the thiocyanate ion in the final solution. Similarly, 50 ppm of Ni 2 + or C0<2+> were added in the form of NiSO -6H_0 or CoSO -7H-0 to some

4 2 4 2

of the solutions after the addition of the thiocyanate. The solution was then divided into three approximately equal portions and placed in 118 ml glass flasks containing a weighed coupon of mild steel 1020 with a surface area of 35 cm<2>.

All samples were then placed in a Sparkler Filter (a water bath apparatus with a tight lid) and pressurized to 207 kPa with CO 2 . The pressure was then raised to 310 kPa overpressure with oxygen. The steam hood in the water bath was then used to heat all the samples for 24 hours at 130°C. After cooling, the weight loss for each coupon was used to calculate the corrosion rate. The corrosion rates given in Table IX represent the average of three individual determinations.

Examples 107-109

In a similar manner as described in Examples 104-106, sufficient Ni(SCN) 2 , Cu(SCN) 2 or Co(SCN) 2•3H 2 O was added to the CO 2 -saturated MEA solution to give a final concentration of 200 ppm SCN ~. The corrosion assessment method was the same as in the previous examples. The results are given in Table X.

Claims (11)

1. Corrosion-inhibited gas conditioning solution containing a gas sweetener in an aqueous medium, characterized in that it also contains an effective amount of a thionitrogen compound, wherein the thionitrogen compound is a water-soluble thiocyanate or a water-soluble thioamide of the formula A-C(S)-N(R) where A is a hydrocarbon radical with 1-6 carbon atoms or a pyridyl radical, and the groups R independently of each other stand for a hydrogen atom or an alkyl group with 1-4 carbon atoms. 2. Conditioning solution according to claim 1, characterized in that the gas sweetener is an aqueous alkanolamine solution. 3. Conditioning solution according to claim 1, characterized in that it also contains an effective amount of one or more soluble metal salts where the metal is selected from the group of cobalt, nickel, calcium, copper, chromium, zinc, tin, aluminum and magnesium. 4. Conditioning solution according to claim 3, characterized in that the amount of soluble metal salt is from 2 to 200 parts per million. 5. Conditioning solution according to claim 3, characterized in that the metal salt is a nickel or chromium salt. 6. Conditioning solution according to claim 1, characterized in that the thionitrogen compound is a water-soluble thiocyanate. 7. Conditioning solution according to claim 6, characterized in that the thiocyanate is ammonium thiocyanate. 8. Conditioning solution according to claim 6, characterized in that the thiocyanate is an alkali metal thiocyanate. 9. Conditioning solution according to claim 1, characterized in that the thionitrogen is present in an amount of 50-1000 parts per million. 10. Conditioning solution according to claim 1, characterized in that the thionitrogen is present in an amount of 100-300 parts per million. 11. Method of inhibiting the corrosive effect of gas conditioning solutions on ferrous metals in acid gas removal equipment, wherein the solutions are contacted with a gaseous outlet stream containing CC^ and H^ S with or without oxygen, comprising recycling a gas conditioning solution containing a gas - sweetener and a corrosion inhibitor through the aforementioned equipment, characterized in that the gas conditioning solution is the solution according to claim 1.