Organic compounds -- part of the class 532-570 series – Organic compounds – Amino nitrogen containing
Reexamination Certificate
2000-04-18
2002-03-05
Richter, Johann (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Amino nitrogen containing
C564S499000
Reexamination Certificate
active
06353138
ABSTRACT:
This invention relates to a process for removing primary and secondary amine and alkanolamine impurities from aqueous tertiary amine and alkanolamine solutions used for removal of acid gases 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; incorporated herein by reference.
Aqueous solutions of various primary, secondary and tertiary alkanolamines, such as, for example, monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA) and triethanolamine (TEA), have been widely used as absorbent liquids to remove acid gases such as carbon dioxide (CO
2
), hydrogen sulfide (H
2
S), carbonyl sulfide (COS) and carbon disulfide (CS
2
) 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 or secondary alkanolamines such as DEA are known to be very reactive and thus 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, and since they do not react directly with CO
2
, they are often used for selective removal of H
2
S from a fluid stream containing both H
2
S and CO
2
.
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 publication and references cited therein.
It is known that oxygen can degrade MDEA to form DEA impurities [about 1600 parts per million (ppm) DEA]. See, Rooney at al, The Proceedings of the 48
th
Annual Laurance Reid Gas Conditioning Conference, March 1-4, 1998, p. 335-347, incorporated herein by reference.
Thermal degradation of tertiary alkanolamines has also been reported to form primary and secondary amine and alkanolamine impurities such as N,N,N-tris(2-hydroxy-ethyl)ethylenediamine (THEED), DEA and methylaminoethanol (MAE). See, for example, A. Chakma and A. Meisen, The Canadian Journal of Chemical Engineering, vol. 75, pp 861-871; and O. F. Dawodu and A. Meisen, The Canadian Journal of Chemical Engineering, vol. 74, pp 960-966, both incorporated herein by reference. In addition, it has been reported that DEA formed a secondary amine impurity identified as 4-(2-hydroxyethyl)piperazine (HEP). See, M. L. Kennard and A. Meisen, Journal of Chromatography, vol. 267, pp 373-380.
For gas treating applications where tertiary alkanolamines such as MDEA and TEA are used to selectively remove H
2
S in the presence of CO
2
, the presence of primary alkanolamines such as MEA, secondary alkanolamines such as MAE and DEA, or secondary amines such as HEP will cause the reaction of CO
2
to increase, resulting in reduced H
2
S removal. This increases costs by having to increase the amine circulation rate and/or having to lower the gas flow rate. For plants having an additional sulfur recovery unit, this increased CO
2
and decreased H
2
S reaction results in increased operational difficulties such as having to increase the oxygen content to the burner, corrosion concerns and other increased costs for the sulfur unit.
Up to now, the practice used in the industry for removing primary and secondary amine and alkanolamine impurities from solutions of MDEA and/or TEA is to use vacuum distillation. However, this process is expensive since the whole system volume of the plant must be vacuum distilled. In addition, it is extremely difficult to remove DEA from especially MDEA because the boiling point of each is somewhat similar. Also, having to remove small amounts of impurities by distillation often requires high losses of the desired tertiary alkanolamine.
A. Chakma and A. Meisen in Carbon, Volume 27, No. 4, p 573-584, (1984) have reported the use of activated carbon to remove degradation products of DEA and MDEA, however, it is shown that activated carbons has very low capacity and become saturated within short periods of time.
U.S. Pat. No. 5,292,958 (Blanc and Claud) discloses a process in which DEA impurities in TEA are removed using glyoxal. However, it is taught that greater than 1 equivalent of glyoxal is required, and when this is done that bicine is the reaction product. As has been disclosed in Rooney et. al in The Proceeding of the 1997 Laurance Reid Gas Conditioning Conference (p 12-30), bicine is a particularly corrosive agent in alkanolamine plants.
There is still a great need and interest for the removal of primary and secondary amine and alkanolamine impurities from aqueous tertiary amine and alkanolamine solutions used for removing acid gases from a fluid stream.
It has now been discovered that primary and secondary amine and alkanolamine impurities can be removed from aqueous tertiary amine and alkanolamine solutions without affecting the tertiary amine and/or alkanolamine by treating these solutions with a monoaldehyde or dialdehyde.
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 embodiment the present invention is a process for removing primary and secondary amine and alkanolamine impurities from an aqueous tertiary amine or alkanolamine solution used for removal of acid gases from a fluid stream which process comprises treating the aqueous amine or alkanolamine solution with a monoaldehyde.
In another embodiment the present invention is a process for removing primary and secondary amine and alkanolamine impurities from an aqueous tertiary amine or alkanolamine solution used for removal of acid gases from a fluid stream which process comprises treating the aqueous amine or alkanolamine solution with less than one equivalent of a dialdehyde per equivalent of the amine or alkanolamine in the solution treated.
DETAILED DESCRIPTION OF THE INVENTION
Without being bound by theory, it is believed that the aldehyde reacts with the primary and secondary alkanolamine to form a tertiary oxazolidine. This tertiary oxazolidine (a tertiary amine) would be expected to have much lower reactivity with CO
2
than the parent primary or secondary amine or alkanolamine. This tertiary oxazolidine may stay in the solution as a stable tertiary amine, part may revert back to the parent amine or alkanolamine, or, under reducing conditions in which the gas stream contains hydrogen, or optionally, if hydrogen is added to the gas stream, the oxazolidine may further react to form a stable tertiary amine or alkanolamine (see, for example, Eq. 1 showing the reaction of DEA and formaldehyde forming oxazolidine-3-ethanol, which then can react with hydrogen to form a tertiary alkanolamine, MDEA). Surprisingly, When less than 1 equivalent of a dialdehyde is used, the amount of bicine formed is kept very low while also forming a high conversion of the DEA.
Any known monoaldehyde can be used in the process of the present invention. Non-limiting examples of suitable monoaldehydes include formaldehyde, acetaldehyde, propionaldehyde and the like. Formaldehyde is the preferred monoaldehyde. The amount of monoaldehyde used is not critical but it is preferred to use from about 0.01 to about 1.5 equivalents of the monoaldehyde per one equivalent of the primary and secondary amine or alkanolamine in the soluti
Davis Brian J.
Garvey, Smith, Nehrbass & Doody LLC
Richter Johann
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