GB2156327A - Alkanolamine process for removal of carbon dioxide from industrial gases using copper and an additional inhibitor - Google Patents

Abstract

The use of corrosion inhibiting compositions in aqueous absorbent gas-liquid contacting processes for recovering carbon dioxide (CO2) from industrial gas and oil combustion and partial combustion process flue gases, employing Cu<2+> in combination with one or more of dihydroxyethylglycine, alkali metal or ammonium permanganate, alkali metal or ammonium thiocyanate, nickel or bismuth oxides and alkali metal carbonate. The inhibitors are effective in reducing corrosion of metals in contact with the aqueous absorbent in the absorbent regeneration section of the plant as well as reduce the thermal degradation of the absorbent when high oxygen content combustion gases are treated to recover the CO2.

Description

SPECIFICATION Alkanolamine process for removal of carbon dioxide from industrial gases using copper and an additional inhibitor Processes for the removal of carbon dioxide (CO2) from industrial gases (e.g. natural gas, refinery gas, and certain synthetic gases) and the like are well known to the commercial petroleum and chemical industries.

Likewise, numerous absorbents, generally aqueous based, have been employed in the gas-liquid processes of industry. Among the more widely employed solvents are the alkanolamines (e.g., monoethanolamine (MEA), diethanolamine (DEA), methyl diethanolamine (MDEA)), sodium carbonate, sulfolane and sulfolanediisopropanaolamine (Shell's Sulfinol process), all of which are corrosive to metals, particularly at temperatures of regeneration used in most aqueous absorbent gas purification and recovery processes.

These absorbents are also subject to thermal degradation particularly in the presence of metals. The degradation products of both metal corrosion and degradation, as well as the acid gases absorbed, accelerate both corrosion and absorbent degradation.

A less well known, yet commercially practiced application of gas treating (absorbing), is the removal of CO2 from gas streams, e.g. flue gas, containing oxygen from a few parts per million to about two percent.

The degradation occurs as described in U.S. Patent 3,137,654 which reports even small amounts of oxygen cause deterioration of ethanolamine and diethanolamine. Corrosion likewise accompanies these applications due in part to the types of metals used in commercial plants.

A commercial scale operation, as carried out today, usually employs absorbents such as aqueous alkanolamine solutions having amine concentrations from 10 to about 20 percent. These processes are commonly sized such that the solvent circulates at rates to absorb (load) acid gases into the solution at about 20 to 40 percent of its theoretical capacity when the oxygen content of the gas being treated is only a few parts per million.

Lesser known commercial applications, which remove CO2 from gas streams containing a few parts to about two percent oxygen, or air, as in U.S. Patent 3,137,654, utilize alkanolamine concentrations ranging from 7 to 12 percent, and in some exceptional cases as high as 24 percent (the concentration of a 4 N amine solution reported in U.S. Patent 3,137,654).

While many of these commercial processes use various additives to abate both corrosion and degradation (most inhibitor formulations are based on patent and published literature technology, which allude to and even claim utility in the presence of oxygen) the experience in the field has been conflicting at best.

Commercial processes audited and/or reported in the literature routinely limit the amount of oxygen (in the presence of CO2) to only a few percent. Generally the plants are designed for maximum loadings of less than 50 percent of theoretical solvent capacity while employing 10 to 20 percent amine concentrations of the aqueous absorbent solutions to control corrosion and solvent degradation. However, considerable degradation as well as poor corrosion inhibition profiles are still obtained.

In some instances activated carbon absorbers are used in an effort to remove the degradation products and corrosion products. However, in the instance of C02 removal, U.S. Patent 3,137,654 teaches that activated carbon filters in the amine circuit enhance the degradation rate of MEA rather than reduce the effect.

In an industry faced with ever increasing fuel and construction costs, the desire to employ higher loadings and stronger absorbent concentrations is natural. However, many plants are not converting because of the increased corrosion and degradation problems encountered with higher loadings and/or higher absorbent concentrations. Several mild exercusions above conventional concentrations and/or loadings on a commercial scale have been made with very poor results.

It would, therefore, be advantageous to find an inhibitor or inhibitor mixture capable of maintaining or reducing corrosion and/or degradation rates while employing loadings in excess of 50% of the theoretical and absorbent concentrations in the 30 to 40 percent range.

In accordance with the present invention it has been found advantageous to add at least above 50 parts by weight of Cu+2 as a salt thereof, preferably copper carbonate, per million parts of aqueous absorbent solution used in gas-liquid contactors to remove and recover CO2 from a gas stream containing both CO2 and oxygen. In addition to the copper salts eg, copper carbonate, one or more of alkali metal permanganates, ammonium permanganate, dihydroxyethylglycine, an alkali metal thiocyanate, ammonium thiocyanate, an alkali metal carbonate, a nickel oxide, or a bismuth oxide are added to provided an additional inhibitor in an amount of from about 50 parts of one or more of these inhibitors per million parts by weight of absorbent solution.

The solution used in the method is a novel carbon dioxide absorbent solution and accordingly the scope of the invention includes an aqueous carbon dioxide absorbent solution comprising an alkanolamine absorbent-reactant, at least 50 parts per million parts be weight of solution of copper 2 and as an additional additive one or more of dihydroxyethylglyine, an alkali metal carbonate, an alkali metal permanganate, ammonium permangenate, a nickel oxide, a bismuth oxide, an alkali metal thiocyanate or ammonium thiocyanate in an aggregate amount of at least 50 parts per million parts by weight of solution.

The invention further includes carbon dioxide whenever recovered by the process of the invention as well as the use of such carbon dioxide in a carbon dioxide-consuming process, for example a conventional industrial process which employs carbon dioxide.

These additives have been found to be particularly effective to inhibit both metal corrosion as well as absorbent degradation. Thus when, in accordance with the present invention, they are employed, aqueous-absorbent solutions can exceed thirty percent and CO2 loadings can be as high as or above about 80 percent of theoretical for the absorbent even in the presence of oxygen as high as 4 to 7 percent, or even higher.

Absorbents found to be effectively inhibited are monethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), methyl diethanolamine (MDEA), sulfolane, diisopropylamine (DIPA) and the like.

While the inhibitor composition of the present invention is useful in absorbent solutions having less than thirty percent absorbent and loadings of less than fifty percent, they are most effective at the higher concentrations and loadings where the prior components have shown poor or ineffective protection.

It has now been found that when aqueous absorbents for the recovery of Cm, from industrial gases, which contain appreciable oxygen, are employed in accordance with present day liquid-gas contact/regeneration processes parameters, coupled with high loadings of the CO2 and 30 to 40 percent absorbent concentrations, the normal corrosion rate of metals in contact with such solutions and the thermal degradation of the absorbent (e.g. amine) can be markedly reduced by adding to the solution at least 50 parts, and preferably from 50 to 2000 parts by weight of Cu +2 per copper carbonate (CuCO2);; additionally with at least 50, and preferably 50 to 2000 parts of one or more of the compounds, dihydroxyethylglycine (DEG), an alkali metal permangante, an alklaki metal thiocyanate, ammonium thiocyanate, an alkali metal carbonate, nickel oxide, or bismuth oxide per million parts of absorbent solution.

It is particularly preferred for both the Cut2 and the additional inhibitor compounds each to be present in an amount of from 50 to 1000ppm, based on the weight of the total solution.

When the invention here set forth is employed, it has been found that the use of activated carbon filters materially reduces the degradation of the amine solution and indirectly effects the corrosion rate.

It is, of course, to be understood that while concentrations above the preferred upper limit of 2000 parts per million parts of absorbent solution may be employed with slight possible improvement due to the increased concentrations, such is not normally desirable due to cost. Therefore, higher concentrations are deemed to be within the scope of the invention here described.

A series of experiments were run to demonstrate the efficacy of the compositions of the present invention to effectively control corrosion in oxygen and CO2 containing industrial gas absorption processes.

Example 1 An 80 percent monoethanolamine aqueous solution (in order to obtain accelerated corrosion rates at elevated temperatures without use of pressure) was saturated with a CO2 stream containing four percent (4%) oxygen and the saturated solution subjected to reflux at 255 F (1 24"C) for 44 hours. Mild steel coupons were immersed in the boiling liquid and theirweight loss measured in mils penetration peryearto determine the effectiveness of various inhibitor combinations. The results of such tests are set forth in Table TABLE I Additive (p pm) Metal Loss DEGt CUC03 SCN KMnO4 mpy2 Fm/year -- -- -- -- 1.6 40.6 400 500 -- -- 0.3 7.6 400 500 -- 500 0.3 7.6 400 500 400 -- 0.3 7.6 400 500 -- 500 0.3 7.6 400 800 -- 500 0.3 7.6 1 DEG = dihydroxyethylglycine 2mpy = mils penetration per year.

Example 2 In a similar manner as described in Example 1 employing 80 percent MEA, results were obtained as shown in Table II: TABLE II Additive (ppm) MS. t Liq. Phase DEG CuCO3 SCN Ni Bi mpy ijm/year % Protection -- -- -- -- -- 138 3505 Baseline 400 - - - - - - - - 142 3606 0 400 -- 400 -- -- 97.4 2473.9 29 400 -- 400 50 5 0.3 7.6 99.8 400 -- 400 50 5 39 991 71.7 400 -- 400 50 -- 34.5 876.3 75.0 400 -- -- 50 5 95.5 2425.7 30.8 400 500 400 50 5 1.2 30.4 99.1 400 500 400 50 5 0.9 22.8 99.3 400 500 -- -- -- 0.8 20.3 99.4 1 M.S. = mild steel Example 3 Several accelerated oxidative condition runs were made while sparging CO2 and O2 to provide a pad of 30 Ibs. (13.6 kg) CO2 and 15 Ibs. (6.8 kg) O2 over a refluxing (130 C) solution of 30 percent MEA. The pad maintained the solution saturated. The results of such tests over a 24 hour period for each test, run in triplicate, are set forth in Table III: TABLE III SPARKLER FILTER TEST 30% MEA-CO2 Saturated 130"C 24 Hr. Test 1020 MS 30 Ibs.CO2 & 15 Ibs. 02 Pad CORROSION RATE INHIBITOR mpy mml year None 40.7 1033.8 52.1 1323.3 43.7 1101.0 200 ppm DEG 52.1 1323.3 65.6 1666.2 40.4 1026.2 100 ppm DEG 23.9 607.1 44.5 1130.3 51.9 1318.3 200 ppm CuC03-Cu(OH)2-H20 1.2 30.5 (eq. 56% CuCO3) .9 22.9 1.2 30.5 200 ppm Cu+2asCuCO3.Cu(OH)2.H2O .6 15.2 1.6 40.6 .9 22.9 100 ppm DEG + 200 ppm CuCO3,Cu(OH)2,H20 1.2 30.5 .9 22.9 1.3 33.0 200 DEG + 200 ppm CuCO3 ~ Cu(OH)2,H2O .9 22.9 .8 20.3 1.2 30.5 80 ppm DEG + 200 ppm Cu-2 as CuCO3#Cu(OH)2.H2O .5 12.7 .9 22.9 1.4 35.6 200 ppm DEG + 200 ppm Cu+2 as CuCO3,Cu(OH)2,H2O .9 22.9 .9 22.9 .7 17.8 Example 4 500 Milliliters of a 30 percent aqueous monoethanolamine solution were placed in a flask which having a single neck through which was attached a condenser and a side arm neck for coupling.The flask was heated on an electric heating mantel to effect boiling (104 C) of the MEA solution at a rate to match the condenser capacity. A mixture of C02 and 02 was sparged through the MEA solution at a rate of 125cm3 per minute of CO2, and 80 cm3 per minute of 02 (supplied as air). The alkalinity of the solution was measured each day after adjusting the volume to 500 ml with fresh water if necessary. The results of such experiments, the solution and additives of each flask were recorded and are set forth in Table IV.

TABLE IV 389 500 Alkalinity Flask % ppm ppm Day No. MEA CuCO2 DEG* 1 2 3 4 1 30 0 0 29.94 28.10 29.00 27.44 2 30 x 0 30.11 27.47 27.27 27.44 3 30 x x 30.30 28.24 29.23 28.75 *dihydroxyethylglycine TABLE IV (Cont'd) Alkalinity Flask Day Decline in No. 5 6 7 8 Alkalinity 1 26.20 24.12 22.80 22.09 26.2 2 27.13 27.14 26.71 25.61 14.9 3 28.86 28.56 28.38 27.46 9.4 That these tests were conducted under conditions found in actual plant operations, illustrates the effectiveness of the combination of additives.These tests also illustrate that without the DEG, alkalinity (the measure of ability of the MEA to absorb CO2) declines and corrosion is not controlled but merely slowed down, and that unless both components are present, the potential for high corrosion and loss of absorption is unpredictable and not much better than the reference teachings which all are less than satisfactory from a commercial standpoint.

Example 5 A series of corrosion tests were run using a pyrex tube having a flat flange at its bottom. Fastened to form a liquid tight seal by the use of "0" rings and a flat flange blank, was a mild still coupon of a size to extend beyond the interior diameter of the tube, but less than the flange diameter, so as to permit the flange blank to be drawn tight against the "0" rings to seal the coupon to the bottom of the tube. The upper end of the tube was fitted with a water cooled atmospheric condenser. A variable electrical current was impressed across the coupon. The coupon thus became the heater element which simulates the skin temperature of a reboiler in a commercial plant.The current was adjusted to maintain the liquid surrounding the coupon heating element boiling (1 04#C-106#C). The coupon actually recorded 300oF (1 48#C). The results of operating the tube containing the same compositions as Flasks 1, 2 and 3 of Example 4 with respect to coupon weight lost as a measure of corrosion are set forth in Table V.

TABLE V 389 500 ppm ppm Coupon Weight Loss Flask No. MEA Cut02 DEG* mpy** CLmlyear 1 30 0 0 268 6807 2 30 x x 1.5 38.1 3 30 x x 1.4 35.6 * dihydroxyethylglycine ** mpy, mils penetration per year This data confirms the data set forth in Example 2, second entry of Table II, that dihydroxyethylglycine does not contribute to corrosion inhibition resulting from the MEA solution containing CO2 and 02 but does contribute to the reduction in degradation of the amine solution and thereby contributes to the effectiveness of the Cut2 as a corrosion inhibitor.

Claims (10)

1. A process for removing carbon dioxide from a gas containing C01 and 02 comprising: contacting the gas, in a gas-liquid contactor, with an aqueous solution which contains an alkanolamine absorbent-reactant and, as additive to the solution to reduce degradation of the absorbent under the conditions of use and corrosion of the metals in contact with said solution during use, at least 50 parts per million parts by weight of solution of copper~7 and as an additional inhibitor one or more of dihydroxyethylglycine, an alkali metal carbonate, an alkali metal permanganate, ammonium permanganate, a nickel oxide, a bismuth oxide, an alkali metal thiocyanate, or ammonium thiocyanate in an aggregate amount of at least 50 parts per million parts by weight of solution; and circulating the solution to a regeneration step wherein the CO2 is released from the absorbent and the essentially CO2-free absorbent recycled to the contactor.

2. A process as claimed in claim 1 in which the copper 2 is as copper carbonate.

3. A process as claimed in claim 1 or 2 in which the copper 2 and the additional inhibitor are each present at 50 to 1000 ppm, based on the weight of the total solution.

4. A process as claimed in claim 1 for removing carbon dioxide from a gas substantially as hereinbefore described in any one of the Examples.

5. Carbon dioxide whenever recovered by a process as claimed in any one of the preceding claims.

6. The use of carbon dioxide as claimed in claim 5 in a carbon dioxide-consuming process.

7. An aqueous carbon dioxide absorbent solution comprising an alkanolamine absorbent reactant at least 50 parts per million parts by weight of solution of copper 2 and as an additional additive one or more of dihydroxyethylglyine, an alkali metal carbonate, an alkali metal permanganate, ammonium permanganate, a nickel oxide, a bismuth oxide, an alkali metal thiocyanate or ammonium thiocyanate in an aggregate amount of at least 50 parts per million parts by weight of solution.

8. An absorbent solution as claimed in claim 7 wherein the copper 2 is as copper carbonate.

9. An absorbent solution as claimed in claim 7 or claim 8 in which the copper 2 and the additional inhibitor are each present at 50 to 1000ppm, based on the weight of the total solution.

10. An absorbent solution claimed in claim 7 substantially as hereibefore described in any one of the Examples.