Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Carbon dioxide or hydrogen sulfide component
Reexamination Certificate
1998-09-30
2001-03-27
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Carbon dioxide or hydrogen sulfide component
C423S226000, C423S229000, C564S475000, C564S477000, C564S503000, C564S511000, C252S189000, C252S190000
Reexamination Certificate
active
06207121
ABSTRACT:
This invention relates to a composition and method for removing acid gases such as, for example, H
2
S, CO
2
and COS from a fluid stream containing same.
BACKGROUND OF THE INVENTION
Purification of fluids involves removal of impurities from fluid steams. Various fluid purification methods are known and practiced. These fluid purification methods generally fall in one of the following categories: absorption into a liquid, adsorption on a solid, permeation through a membrane, chemical conversion to another compound, and condensation. The absorption purification method involves the transfer of a component of a fluid to a liquid absorbent in which said component is soluble. If desired, the liquid containing the transferred component is subsequently stripped to regenerate the liquid. See, for example, A. Kohl and R. Nielsen, “Gas Purification, 5
th
edition, Gulf Publishing, 1997; A. Kohl and F. C. Riesenfeld “Gas Purification, 4
th
edition, Gulf Publishing, 1985; A. Kohl and F. C. Riesenfeld “Gas Purification, 3rd edition, Gulf Publishing, 1979; and “The Gas Conditioning Fact Book” published by The Dow Chemical of Canada, Limited, 1962; all incorporated herein by reference.
Aqueous solutions of various primary, secondary and tertiary alkanolamines, such as, for example, monoethanolamine(MEA), diethanolamine (DEA), diglycolamine (DGA), diisopropanolamine (DIPA), methyldiethanolamine (MDEA) and triethanolamine (TEA), have been used as absorbent liquids to remove acid gases from liquid and gas streams. In a regeneration method, the aqueous alkanolamine solution containing acid gas is then subjected to heat to regenerate the aqueous alkanolamine solution.
Primary alkanolamines such as MEA and DGA, or secondary alkanolamines such as DEA or DIPA are generally suitable for highly exhaustive removal of CO
2
, however they have disadvantage of requiring large expenditure of energy for regeneration.
Tertiary alkanolamines, especially MDEA and TEA, require less energy consumption for regeneration, but since they do not react directly with CO
2
, they do not remove CO
2
completely from the fluid stream. Tertiary alkanolamines are, however, suitable for selective removal of H
2
S from a fluid containing both H
2
S and CO
2
, since the absorption rate for H
2
S is about the same for all alkanolamines.
The chemistry of acid gas reactions with aqueous alkanolamine treating solutions is well known and is described in many publications such as, for example, the aforementioned publications and references cited therein, and publications described below and references cited therein.
Canadian Patent No. 1,091,429 (G. Sartori et al) describes the use of aqueous solutions containing water-soluble primary monoamines having a secondary carbon atom attached to the amino group in gas purification applications. Primary monoamines having a secondary carbon atom attached to the amino group specifically mentioned in this reference as being suitable are 2-amino-1-propanol, 2-amino-1-butanol, 2-amino-3-methyl-1-butanol, 2-amino-1-pentanol, 2-amino-1-hexanol and 2-aminocycloxexanol. It is notable that this reference is completely silent as to degradability and corrosivity of these primary monoamines which have a secondary carbon atom attached to the amino group.
Chem. Eng. Comm., 1996, Vol. 144, pp. 103-112, “Effects of Composition on the Performance of Alkanolamine Blends for Gas Sweetening”, describes the influence of blend composition and components on some of the parameters which can be used to monitor the performance of amine blends for aqueous blends of MDEA and MEA, MDEA and DEA, and MDEA and DIPA.
48
th
Annual Laurance Reid Gas Conditioning Conference, Mar. 1-4, 1998, pp. 146-160, “Amine Degradation Chemistry in CO
2
Service”, describes the degradation chemistry of various ethanolamines in CO
2
service. The paper promotes gas treating solvents which are not formulated with primary or secondary ethanolamines as a solution for the loss rates associated with the use of various ethanolamines such as MDEA, MMEA and DEA.
Primary and secondary alkanolamines can also be used as activators in combination with tertiary alkanolamines to remove CO
2
down to as low as 100 parts per million (ppm) or less requiring less regeneration energy than is required using the primary or secondary alkanolamines alone.
U.S. Pat. Nos. 5,209,914 and 5,366,709 show how activators such as ethylmonoethanolamine (EMEA) or butylmonoethanolamine (BMEA) can be used with MDEA to achieve better CO
2
removal than MDEA alone.
U.S. Pat. No. 4,336,233 discloses that the use of a combination of piperazine and MDEA results in an improved acid gas removal. However, one particular disadvantage of piperazine is that piperazine carbamate formed from the reaction of piperazine and CO
2
is not soluble in the aqueous MDEA/piperazine solution. Thus, the additive level is limited up to about only 0.8 moles/liter, which severely limits the capacity of the solvent, or requires higher circulation rates to treat the same amount of fluid than other MDEA/alkanolamine activator blends require.
The primary disadvantage of using primary and secondary alkanolamines such as MEA, DEA and DIPA is that CO
2
reacts with these alkanolamines to form degradation compounds such as oxazolidinones and ethylenediamines. C. J. Kim,
Ind. Eng. Chem. Res.
1988, 27, and references cited therein show how DEA reacts with CO
2
to form 3-(2-hydroxyethyl)-2-oxazolidi-none (HEO) and N,N,N′-tris(2-hydroxyethyl)ethylenediamine (THEED). This reference also shows how DIPA reacts to form 3-(2-hydroxypropyl)-5-methyl-2-oxazolidinone (HPMO). These degradation compounds reduce the amount of alkanolamine available for acid gas removal, increase the viscosity of the solution and potentially increase the corrosivity of the solvent.
It is evident that there is still a great need and interest in the gas purification industry for alkanolamine compounds which will be effective in the removal of acidic gases from fluid streams and will have improved degradation properties compared to alkanolamines commonly used for this purpose.
It has now been discovered that 1-amino-2-butanol and its derivatives are effective in removing acidic gases from fluid stream and that they have superior degradation properties as compared to alkanolamines conventionally used in the gas purification industry.
In the context of the present invention the term “fluid stream” encompasses both a gaseous stream and liquid stream.
SUMMARY OF THE INVENTION
In one aspect the present invention is an aqueous solution adapted for use in the removal of acidic gases from a fluid stream containing same, said aqueous solution comprising an effective amount of an alkanolamine of the formula
R—NHCH
2
CH(OH)CH
2
CH
3
(I)
or mixtures thereof wherein R is H, —CH
2
CH(OH)CH
2
CH
3
, an alkyl group having from 1 to 6 carbon atoms, an aryl group having from 6 to 12 carbon atoms, an aralkyl group having from 6 to 12 carbon atoms, or a cycloalkyl group having from 3 to 6 carbon atoms.
In another aspect the present invention is a process for removing acidic gases from a fluid stream containing same, said process comprising contacting said fluid stream containing acidic gases with an aqueous solution comprising an effective amount of an alkanolamine of the formula
R—NHCH
2
CH(OH)CH
2
CH
3
(I)
or mixtures thereof wherein R is H, —CH
2
CH(OH)CH
2
CH
3
, an alkyl group having from 1 to 6 carbon atoms, an aryl group having from 6 to 12 carbon atoms, an aralkyl group having from 6 to 12 carbon atoms, or a cycloalkyl group having from 3 to 6 carbon atoms.
DETAILED DESCRIPTION OF THE INVENTION
The alkanolamines of the formula I above are surprisingly found to be effective for removing acidic gases, particularly CO
2
, H
2
S, COS or mixtures thereof, from a fluid stream containing same and yet exhibit much improved degradation properties compared to alkanolamines conventionally used in the gas purification industry. These compounds are known and their synthesis is described in various publications such as, for example, J. Z
Griffin Steven P.
Strickland Jonas N.
The Dow Chemical Company
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