Process for removing carbon dioxide from combustion gases

Details Process for removing carbon dioxide from combustion gases Process for removing carbon dioxide from combustion gases

1996-02-20

2004-02-10

Silverman, Stanley S. (Department: 1754)

Chemistry of inorganic compounds

Modifying or removing component of normally gaseous mixture

Carbon dioxide or hydrogen sulfide component

C423S228000, C423S229000

Reexamination Certificate

active

06689332

ABSTRACT:

FIELD OF THE INVENTION AND RELATED ART STATEMENT
This invention relates to a process for removing carbon dioxide (CO
2
) from combustion gases. More particularly, it relates to a process for removing CO
2
from combustion gases which contain oxygen and CO
2
at atmospheric pressure, using an aqueous solution of a specific hindered amine solution as a CO
2
absorbent solution and also using a specific material for the equipment that contacts the solution.
In recent years the greenhouse effect of CO
2
has attracted attention as a factor responsible for the global warming. Counteracting this effect is urgently needed throughout the world to protect the global environment. The source of CO
2
is omnipresent in the whole areas of human activities that involve the combustion of fossil fuels, and the tendency is toward stricter emission control than heretofore. In view of these, energetic studies are under way on the removal and recovery of CO
2
from combustion gases, especially from those emitted by power-generating installations such as steam power plants that burn enormous quantities of fossil fuels, and on the storage of the recovered CO
2
without release to the atmosphere. For example, methods of contacting combustion gases from boilers with alkanolamine solution etc. have been studied.
Examples of the alkanolamine are monoethanolamine, diethanolamine, triethanolamine, methyldiethanolamine, diisopropanolamine, and diglycolamine. Usually, monoethanolamine (MEA) is used by preference.
A variety of techniques are known for separating acidic gases from various mixed gases by the use of amine compounds.
Japanese Patent Application Disclosure No. SHO 53-100180 describes a process for acid gas removal which comprises bringing a normally gaseous mixture into contact with an amine-solvent liquid absorbent composed of (1) an amine mixture consisting of at least 50 mol % of a sterically hindered amine containing at least either one secondary amino group which forms a part of a ring and is bound to a secondary carbon atom or a tertiary carbon atom or one primary amino group bound to a tertiary carbon atom and at least about 10 mol % of a tertiary amino-alcohol and (2) a solvent for the above amine mixture which is a physical absorbent for acid gases.
Examples of the sterically hindered amine cited are 2-piperidine ethanol and the like, those of the tertiary amino-alcohol are 3-dimethylamino-1-propanol and the like, and those of the solvent are sulfoxide compounds which may contain up to 25 wt % of water. As an example of the gas to be treated, the printed publication, page 11, upper left column refers to “a normally gaseous mixture containing high concentrations of carbon dioxide and hydrogen sulfide, e.g., 35% CO
2
and 10-12% H
2
S”. Working examples of the invention used CO
2
itself.
Patent Application Disclosure No. SHO 61-71819 introduces an acid-gas scrubbing composition containing a sterically hindered amine and a nonaqueous solvent such as sulfolane. As a sterically hindered primary monoamino-alcohol, 2-amino-2-methyl-1-propanol (AMP) and the like are given as examples and employed. Working examples of the invention use CO
2
-nitrogen and CO
2
-helium combinations as gases to be treated. Aqueous solutions of amine and potassium carbonate and the like are used as absorbents. The printed publication mentions the use of water too. It further explains the benefit of the sterically hindered amine by means of a reaction formula.
Chemical Engineering Science
, vol. 41, No. 4, pp. 997-1003, discloses the behavior of the aqueous solution of AMP as a hindered amine in the absorption of carbonic acid gas. The gases used for the absorption experiment were CO
2
and a CO
2
-nitrogen mixture at the atmospheric pressure.
Chemical Engineering Science
, vol. 41, No. 2, pp. 405-408, reports the CO
2
and H
2
S absorption rates of aqueous solutions of a hindered amine such as AMP and a straight-chain amine such as MEA at about the ordinary temperature. According to the report, the two aqueous solutions do not differ appreciably in the absorbent concentration range of 0.1-0.3 M when the partial pressure of CO
2
is 1 atm. However, when aqueous solutions at a concentration of 0.1 M was used and the CO
2
partial pressure was reduced from 1 atm. to 0.5 and 0.05 atm., the absorption rate of AMP decreased substantially below that of MEA at 0.05 atm.
U.S. Pat. No. 3,622,267 teaches a technique of purifying a synthetic gas containing CO
2
at a high partial pressure, e.g., 30% CO
2
at 40 atm., such as the gas synthesized by partial oxidation of crude oil, using an aqueous mixture containing methyl diethanolamine and monoethyl monoethanolamine.
German Patent Disclosure No. 1,542,415 reveals a technique of adding monoalkylalkanolamines and the like to physical or chemical absorbents so as to enhance the rates of absorption of CO
2
, H
2
S, and COS. Likewise, German Patent Disclosure No. 1,904,428 teaches the addition of monomethylethanolamine for the improvement of the absorption rate of methyldiethanolamine.
U.S. Pat. No. 4,336,233 discloses a technique by which an aqueous solution of piperazine at a concentration of 0.81-1.3 mol/l or an aqueous solution of piperazine and a solvent such as methyldiethanolamine, triethanolamine, diethanolamine, or monomethylethanolamine is used as a scrubbing solution for the purification of natural, synthetic, or gasified coal gas.
Similarly, Japanese Patent Application Disclosure No. SHO 52-63171 sets forth CO
2
absorbents prepared by adding piperazine or a piperazine derivative such as hydroxyethylpiperazine as an accelerator to a tertiary alkanolamine, monoalkylalkanolamine or the like.
The aqueous MEA solution, used in continuous removal of CO
2
by absorption through gas-liquid contact with high-temperature combustion gas that contains oxygen and CO
2
, would cause a corrosion problem. The CO
2
absorption column in which the combustion gas containing CO
2
and oxygen comes in contact with the absorbent solution, the regeneration column that heats the absorbent solution to liberate CO
2
and thereby regenerate the solution, and pipings, heat exchangers, pumps, and all other metallic components located between and around those two columns can be corrosively attacked. Equipment designs using the conventional materials for chemical plants might be implemented in laboratories but would never be feasible for industrial-scale processes because of a too short service life.
A proposal has been made in U.S. Pat. No. 4,440,731 to overcome the corrosion of the equipment where CO
2
is taken up from combustion gases containing oxygen and CO
2
by the use of a CO
2
absorbent solution which consists of an aqueous solution of MEA or other similar compound.
According to the proposal, at least 50 ppm of bivalent copper ion is added to such an absorbent solution, with or without the further addition of dihydroxyethylglycine, a carbonate of an alkali metal, a permanganate of an alkali metal or ammonium, a thiocyanate of an alkali metal carbonate or ammonium, an oxide of nickel or bismuth, or the like. The method is claimed to inhibit the decomposition of the absorbent MEA or the like even during the treatment of combustion gases containing a high concentration of oxygen.
Working examples of this U.S. patent actually describe tests with only an aqueous solution of MEA as a sole amine compound. To be more exact, 30 lbs. of CO
2
and 15 lbs. of oxygen were supplied to an aqueous solution of 30% MEA being refluxed, and corrosion promotion tests were conducted with mild steel coupons in the presence of various corrosion inhibitors at 130° C. It is reported that the addition of 200 ppm cupric carbonate [CuCO
3
.Cu(OH)
2
.H
2
O, CuCO
3
accounting for 56% of the total amount] inhibited the corrosion to 0.9-1.2 mil/y (mpy) whereas the corrosion in the absence of such an inhibitor was 40-52 mpy.
OBJECT AND SUMMARY OF THE INVENTION
In resisting the corrosive attack of an absorbent solution, stainless steel is naturally superior to carbon steel. This apparently favors the use of sta

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