Gas separation: processes – Liquid contacting – And recycle or reuse of contact liquid for further contact
Reexamination Certificate
2000-12-29
2002-08-20
Smith, Duane S. (Department: 1724)
Gas separation: processes
Liquid contacting
And recycle or reuse of contact liquid for further contact
C095S235000, C095S236000, C423S226000
Reexamination Certificate
active
06436174
ABSTRACT:
The invention relates to a process for removing acidic gas constituents such as CO
2
and H
2
S from gases using an absorption medium, to the absorption medium itself and to its use.
It is known to remove unwanted acidic gas constituents, such as CO
2
, H
2
S or COS, from gases comprising these constituents by gas scrubbing using aqueous or nonaqueous mixtures of organic solvents as absorption medium. In this operation, both physical and chemical solvents are used. Known physical solvents are, for example, cyclotetramethylene sulfone, N-methyl-pyrrolidone and N-alkylated piperidones. Of the chemical solvents, those which have proved especially useful industrially are the aqueous solutions of primary, secondary and tertiary aliphatic amines or alkanolamines, such as monoethanolamine (MEA), diethanolamine (DEA), monomethylethanolamine (MMEA), diethylethanolamine (DEEA), triethanolamine (TEA), diisopropanolamine (DIPA) and methyldiethanolamine (MDEA). Amines act as bases, forming, in the presence of water, the corresponding ammonium carbonates or ammonium hydrogen carbonates with CO
2
and the corresponding ammonium sulfides or ammonium hydrogen sulfides with H
2
S. Primary and secondary amines can, in addition, react with CO
2
, forming carbamates.
To increase the absorption rate of the solvent mixtures for CO
2
, H
2
S and COS, said aliphatic amines and alkanolamines are used with certain saturated nitrogen heterocycles, such as piperazine or morpholine. DE-A 25 51 717 describes a process for removing CO
2
and/or H
2
S and, if appropriate, COS from gases by scrubbing with absorption media which comprise piperazine and aliphatic alkanolamines in aqueous solution. According to the information in this publication, piperazine acts as an accelerator for the absorption. Therefore, piperazine, according to this teaching, is preferably used in catalytic amounts as absorption accelerator in aqueous solution together with physical or chemical solvents known per se or their mixtures. Said publication also discloses the use of piperazine in a mixture with physical solvents, such as methanol, N-methylpyrrolidone and polyethylene glycol dimethyl ether, in which case, because of the carbamate formation by the piperazine, only substantially dilute aqueous solutions can be used. Piperazine is therefore preferably used in aqueous solution with chemical solvents, preferably with tertiary aliphatic alkanolamines.
The known solvent mixtures have the disadvantage that piperazine in these mixtures principally influences the absorption rate of CO
2
, but the solubility of piperazine in aqueous solutions of aliphatic alkanolamines, such as MDEA, is restricted.
It is an object of the present invention to provide a liquid absorption medium for removing acidic gas constituents from gases, which absorption medium, in addition to a high absorption rate, has a high capacity for acidic gas constituents.
We have found that this object is achieved by a process for removing acidic gas constituents, of the group consisting of CO
2
, H
2
S, COS, CS
2
and mercaptans, from gases, in which, in an absorption step, an untreated gas rich in acidic gas constituents is brought into contact with an absorption medium, as a result of which a clean gas low in acidic gas constituents and an absorption medium laden with acidic gas constituents are obtained, which process comprises using as absorption medium a mixture comprising
a) from 0.1 to 50% by weight of one or more mono-cyclic or bicyclic nitrogen heterocycles which are unsubstituted and/or monosubstituted or polysubstituted on the carbon by OH, C
1
-C
3
alkyl and/or C
1
-C
3
hydroxyalkyl and which have from 5 to 14 ring atoms and 1 or 2 heterocyclically bound nitrogen atoms per ring as component A,
b) from 1 to 60% by weight of a monohydric and/or polyhydric alcohol as component B,
c) from 0 to 60% by weight of an aliphatic alkanolamine as component C,
d) from 0 to 98.9% by weight of water as component D,
e) from 0 to 35% by weight of K
2
CO
3
as component E,
where the sum of components A, B, C, D and E is 100% by weight.
We have found that the object is achieved, in addition, by a liquid absorption medium of the above specified composition.
Gases which comprise said acidic gas constituents are, for example, natural gases, synthesis gases, coke furnace gases, coal gasification gases and cycle gases in the production of ethylene oxide. These gases, in addition to one or more of said acidic gas constituents, comprise other inert gas constituents which are not absorbed to a significant extent by the liquid absorption medium. Examples are highly volatile hydrocarbons, preferably C
1
-C
4
hydrocarbons and, particularly preferably, methane, and in addition nitrogen and hydrogen. The process of the invention is suitable, inter alia, for cleaning these gases by removing the acidic gas constituents. The gases to be cleaned can comprise CO
2
, preferably in amounts up to 75% by volume, and H
2
S, preferably in amounts of up to 50% by volume. Furthermore, the gases to be cleaned can comprise COS, preferably in amounts up to 5% by volume, CS
2
, preferably in amounts up to 1% by volume, and mercaptans, preferably alkyl mercaptans, in particular methyl mercaptan, preferably in amounts up to 1% by volume. The process of the invention is particularly suitable for removing CO
2
and H
2
S.
The absorption medium of the invention comprises as component A from 0.1 to 50% by weight, preferably from 5 to 30% by weight, particularly preferably from 8 to 25% by weight, of one or more monocyclic or bicyclic nitrogen heterocycles which are unsubstituted and/or monosubstituted or polysubstituted on the carbon by OH, C
1
-C
3
alkyl and/or C
1
-C
3
hydroxyalkyl and which have from 5 to 14 ring atoms and 1 or 2 heterocyclically bound nitrogen atoms per ring. Bicyclic heterocycles are those which have two anellated rings or two rings bound via a single bond. These are preferably bound via carbon atoms. The rings can have further heteroatoms, for example oxygen or sulfur. Examples are pyrrolidine, pyrazolidine, imidazolidine, piperidine, piperazine, hexahydropyrimidine, azepan, diazepan, octahydroindole, octahydrobenzimidazole, octahydropurine, decahydroquinoline, decahydroiso-quinoline, decahydroquinazoline, decahydroquinoxaline, decahydropteridine, 2-[2-pyrrolidyl]pyrrolidine, 2-[2-imidazolidyl]imidazolidine, 3-[3-pyrrolidyl]-piperidine, 2-[3-pyrrolidyl]piperazine, 3-[3-piperidyl]piperidine, 3-[2-piperazinyl]-piperidine and 2-[2-piperazinyl]piperazine. Said heterocycles can be mono-substituted or polysubstituted by OH, methyl, ethyl, propyl, hydroxymethyl, hydroxyethyl and hydroxypropyl.
Preferably, the absorption medium of the invention comprises as component A unsubstituted piperazine and/or one or more piperazines monosubstituted or poly-substituted on the carbon by OH, C
1
-C
3
alkyl and/or C
1
-C
3
hydroxyalkyl. Particularly preferably, the absorption medium of the invention comprises as component A unsubstituted piperazine.
The liquid absorption medium comprises as component B from 1 to 60% by weight, preferably from 2 to 45% by weight, particularly preferably from 3 to 35% by weight, of a monohydric or polyhydric alcohol. For the purposes of the invention, monohydric or polyhydric alcohols are only those which have no amino groups in addition to one or more alcoholic hydroxyl groups. Alkanolamines are thus not considered to be monohydric or polyhydric alcohols. Suitable monohydric alcohols are, for example, C
1
to C
5
alkanols, such as methanol, ethanol, propanols, butanols and pentanols, preferably methanol. Methanol is particularly suitable for a low-temperature process (absorption at down to −70° C.). Suitable polyhydric alcohols are, for example, C
2
-C
8
alkanediols, C
3
-C
10
alkanetriols, C
4
-C
12
alkanetetraols, C
5
-C
16
alkanepentaols, C
6
-C
20
alkanehexaols, for example ethylene glycol, propylene glycol, glycerol, butanediols, butanetriols, pentanediols, trimethylol-propane, neopentyl gl
Asprion Norbert
Grossmann Christoph
Hänzel Karl-Heinz
Kolassa Dieter
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