# Patent

## Title
Composition and method for removal of carbonyl sulfide from acid gas containing same

## Abstract

Novel solvent composition for selective removal of COS from a gas stream containing same, said composition comprising

    
    
        a) at least one polyalkylene glycol alkyl ether of the formula

R1O-(Alk-O)n—R2  (I)
        
            or
            
                1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone
            
            
            or
            
                a mixture of N-formylmorpholine and N-acetylmorpholine
            
            
        
        
        wherein R1 is an alkyl group having from 1 to 6 carbon atoms; R2 is hydrogen or an alkyl group having from 1 to 4 carbon atoms; Alk is an alkylene group, branched or unbranched, having from 2 to 4 carbon atoms, and n is from 1 to 10; and
        b) at least one alkanolamine compound of the formula

R3NHR4OR6  (II)
        
            or
            at least one piperazine compound of formula
        
        
    
    






    
    
        
        
            wherein R3 is hydrogen, an alkyl group having from 1 to 6 carbon atoms, or the R4OH group; R4 is a branched or unbranched alkylene group having from 1 to 6 carbon atoms; R5, independently in each occurrence, is hydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms; and R6 is hydrogen, an alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl group having from 1 to 4 carbon atoms.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This Application is a Divisional of U.S. Ser. No. 10/549,743 filed Sep. 9, 2005, which is a 371 of PCT/US2004/007453 filed Mar. 11, 2004, which claims the benefit of U.S. Provisional Application Ser. No. 60/456,354, filed Mar. 21, 2003.


The present invention relates to an improved composition for selective removal of carbonyl sulfide (COS) with minimal absorption of carbon dioxide (CO2) from an acid gas containing COS and CO2, and also to a method for selective removal of COS from an acid gas containing COS and CO2 using this improved composition.
Natural and synthesis gas streams, derived from natural gas reservoirs, petroleum or coal, often contain a significant amount of carbonyl sulfide (COS) in addition to other impurities such as carbon dioxide (CO2), hydrogen sulfide (H2S), sulfur dioxide (SO2), carbon disulfide (CS2), and mercaptans. Various compositions and processes for removal of acid gases, including COS, from a gas containing same are known and described in the literature.
Certain physical solvents have been widely used for the selective removal of H2S from gas streams containing H2S, CO2 and optionally, other components. For example, dialkyl ethers of polyalkylene glycols are commonly specified for this purpose. Typical of the numerous disclosures of such solvents in the art is European Patent Application No. EP 0770420 A2 and U.S. Pat. Nos. 3,737,392; 3,824,766; 3,837,143; 4,044,100; 4,336,233; 4,581,154; 4,741,745; and 4,946,620, among others. According to such references, it is well known that the solubility of H2S in these glycols is much higher than the solubility of CO2. This difference in solubility forms the basis for the selective removal of H2S by glycol ethers and other physical solvents. Unfortunately, the solubility of COS in most physical solvents is not very different from the solubility of CO2. Hence, selective removal of COS or COS and H2S from gas streams containing CO2 and optionally other components, has proved difficult.
Certain tertiary alkanolamines (methyldiethanolamine and triethanolamine for example) have been widely used for the selective removal of H2S from gas streams which also contain CO2. Users of these amines exploit the rapid reaction with H2S and the slow reaction with CO2 to allow selective removal of H2S. Unfortunately, the reaction of these amines with COS is about 100 times slower than the reaction with CO2. Hence, tertiary amines are relatively ineffective at separating COS or COS and H2S from CO2 containing streams.
Certain primary and secondary amines (monoethanolamine, diethanolamine, methylethanolamine, aminoethoxy ethanol, piperazine for example) have been widely used for the simultaneous removal of H2S and CO2. These amines react rapidly with both H2S and CO2 and are well suited for simultaneous removal of H2S and CO2. Although the reaction of COS with these amines is also about 100 times slower than the reaction with CO2, appreciable amounts of CO2 and COS can be removed. Unfortunately, since COS reacts much more slowly with these amines than does CO2, selective removal of COS or COS and H2S with primary or secondary amines has proved difficult.
The selective removal of COS or COS and H2S from gas streams containing CO2 and optionally other components has proved difficult with both physical and chemical solvents. However, in a number of conventional hydrocarbon processing techniques, it is desirable or necessary to remove COS down to few parts per million (ppmv) for several reasons, such as catalyst sensitivity to COS in subsequent operations and statutory or contract requirements regarding sulfur content in product or waste gas streams. The presence of COS has also been identified as the cause of degradation reactions in several commercial processes for acid gas removal. However, in many cases it is neither necessary nor desirable to remove CO2 from gaseous mixtures.
Various solvent compositions and methods are known for selective removal of COS and H2S from gaseous mixtures containing same and other acidic gases such as CO2.
U.S. Pat. No. 3,989,811 (Hill) discloses a multi-step process for removal of acid gases, that is, H2S, CO2, and sulfur containing compounds including CS2, COS, and various mercaptans from sour gases. In this multi-step process, H2S, CO2 and COS are absorbed in a nonselective alkanolamine. The regenerated acid gases are then contacted with a selective amine to produce an H2S rich stripper off gas and a low pressure CO2 rich stream. Suitable alkanolamines include methyldiethanolamine, triethanolamine, or one or more di-propanolamines, such as di-n-propanolamine or diisopropanolamine. The preferred absorbent for treatment of COS containing gas streams also contains a significant amount of a tetramethylene sulfone (the unsubstituted compound is known as sulfolane). In principle, selective removal of COS and H2S from the original sour gas stream could be achieved if the CO2 rich product stream were recompressed and combined with the main treated gas stream. However, the capital and operating costs associated with requiring two absorbers, two strippers and compression of the CO2 stream would weigh against this option, assuming a less costly alternative can be found.
U.S. Pat. No. 4,482,529 (Chen et al.) discloses a single step process for the selective removal of COS from a gas stream containing CO2. Small amounts of bicyclo tertiary amine are added to a physical solvent already known to be selective for H2S in the presence of CO2. According to the invention, addition of the bicyclo tertiary amine encourages hydrolysis of the COS to H2S and CO2. The net effect is to improve the absorption of COS without substantially increasing the absorption of CO2. Applicable physical solvents include SELEXOLTM solvent (a blend of polyethylene glycol dimethyl ethers sold by Union Carbide Corporation under the trademark SELEXOL), sulfolane, methanol and others.
U.S. Pat. No. 4,524,050 (Chen et al.) discloses a process for hydrolyzing COS in gas streams to H2S and CO2 using solid supported bicyclo tertiary amine catalyst.
U.S. Pat. No. 4,504,449 (Doerges et al.) discloses a process for selective removal of H2S and possibly COS from a CO2 containing gas stream using highly volatile secondary amines in an organic solvent. A complex regeneration scheme is required. Applicable secondary amines are diethyl amine, diisopropyl amine, methyl isopropyl amine, ethyl isopropyl amine, dipropyl amine, methyl n-butylamine, methyl isobutyl amine and methyl sec-butyl amine. Applicable organic solvents include alkylated polyethylene glycol ethers, tetrahydrothiophene dioxide etc.
U.S. Pat. No. 4,532,116 (Doerges et al.) discloses a process for the removal of H2S and/or COS from synthesis gas using at least one secondary amine and an organic solvent. Although the process is selective for H2S and COS in the presence of CO2, the process is complex. Two scrubbing zones are required—a preliminary desulfurizing zone using a conventional circulated and regenerated solution and a fine desulfurizing or scrubbing zone using unregenerated solvent. The fine desulfurizing zone contains a heat exchanger to condense the unregenerated solvent. Applicable organic solvents are methanol, ethanol and isopropanol. Applicable secondary amines are N-ethylethaneamine, N-(1-methylethyl)propaneamine, N-methyl-2-propanamine, N-ethyl-2-propanamine, N-propyl-1-propanamine, N-methyl-1-butanamine, N-2-dimethyl-1-propanamine, N-methyl-2-butanamine.
U.S. Pat. No. 4,749,555 (Bush) discloses a process for selective removal of H2S and COS without absorbing CO2 from a gas stream having a relatively large concentration of CO2 using bridgehead amines (bicyclotertiary amine or bicyclo amidine), tertiary amine, physical solvent and water. Applicable physical solvents include sulfolane, polyethylene glycol dimethyl ethers and others.
U.S. Pat. No. 4,980,140 (Souby) discloses a process for selective removal of COS from H2S using a solvent comprising a tertiary amine, a physical co-solvent and a minor amount of water. The useful physical co-solvent includes sulfones, sulfoxides, glycols and the mono and diethers thereof, 1,3-dioxo heterocyclic compounds (dioxane and dioxolane), aromatic ethers, aromatic hydrocarbons and N-alkylated lactams of gamma or omega amino acids.
U.S. Pat. No. 5,413,627 (Landeck et al.) discloses the selective removal of H2S and COS from CO2 using a physical scrubbing agent comprising a heterocycle having five or more ring atoms, which contains two heteroatoms, one of which is nitrogen and the other of which is oxygen or nitrogen. The nitrogen atom present in the ring is/are either double bonded or single bonded but, if single bonded, the nitrogen is organo-substituted. A wide variety of scrubbing agents are disclosed, including 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMTP).
U.S. Pat. No. 5,589,149 (Garland et al.) discloses absorption solvents for removing mercaptans from gas streams without the use of iodine. The solvent comprises alkyl ether of a polyalkylene glycol and a secondary monoalkanolamine and, optionally, other amines, such as dialkanolamines.
U.S. Pat. No. 6,277,345 B1 (Stankowiak et al.) discloses the use of absorption liquid comprising at least one dialkanolamine, at least one polyalkylene glycol alkyl ether, and water for nonselective removal of CO2, H2S, COS and other acidic gases from a gaseous stream.
U.S. Pat. No. 6,102,987 (Gross et al.) discloses a process for removal of CO2 and sulfur compounds from natural gas and raw synthesis gas with a mixture of N-formylmorpholine and N-acetylmorpholine at temperatures between −20° C. and +40° C. at pressure of 10 to 150 bar in absorbing operation.
There is still a need in the industry for an improved composition for selective removal of COS from gaseous streams containing COS and CO2. It has now been surprisingly and unforeseeably found out that the addition of a primary alkanolamine, secondary alkanolamine or piperazine compound to a polyalkylene glycol alkyl ether or 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPT) results in selective removal of COS from the gaseous streams containing same with minimal removal of CO2.
One aspect of the present invention concerns a solvent composition for removal of COS from a gas stream containing same, said composition comprising

    
    
        a) at least one polyalkylene glycol alkyl ether of the formula

R1O-(Alk-O)n—R2  (I)

        wherein R1 is an alkyl group having from 1 to 6 carbon atoms; R2 is hydrogen or an alkyl group having from 1 to 4 carbon atoms; Alk is an alkylene group, branched or unbranched, having from 2 to 4 carbon atoms, and n is from 1 to 10; and
        b) at least one alkanolamine compound of the formula

R3NHR4OR6  (II)
        
            or
            at least one piperazine compound of formula
        
        
    
    






    
    
        wherein R3 is hydrogen, an alkyl group having from 1 to 6 carbon atoms, or the R4OH group; R4 is a branched or unbranched alkylene group having from 1 to 6 carbon atoms; R5, independently in each occurrence, is hydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms; and R6 is hydrogen, an alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl group having from 1 to 4 carbon atoms.
    
    


Another aspect the present invention concerns a solvent composition for removal of COS from a gas stream containing same, said composition comprising

    
    
        a) 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone; and
        b) at least one alkanolamine compound of the formula

R3NHR4OR6  (II)
        
            or
            at least one piperazine compound of formula
        
        
    
    






    
    
        wherein R3 is hydrogen, an alkyl group having from 1 to 6 carbon atoms, or the R4OH group; R4 is a branched or unbranched alkylene group having from 1 to 6 carbon atoms; R5, independently in each occurrence, is hydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms; and R6 is hydrogen, an alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl group having from 1 to 4 carbon atoms.
    
    


In another aspect, the present invention concerns a process for selective removal of COS from a gas stream containing same, said process comprising treating the gas stream with a solvent composition comprising

    
    
        a) at least one polyalkylene glycol alkyl ether of the formula

R1O-(Alk-O)n—R2  (I)

        wherein R1 is an alkyl group having from 1 to 6 carbon atoms; R2 is hydrogen or an alkyl group having from 1 to 4 carbon atoms; Alk is an alkylene group, branched or unbranched, having from 2 to 4 carbon atoms; and n is from 1 to 10; and
        b) at least one alkanolamine compound of the formula

R3NHR4OR6  (II)
        
            or
            at least one piperazine compound of formula
        
        
    
    






    
    
        wherein R3 is hydrogen, an alkyl group having from 1 to 6 carbon atoms, or the R4OH group; R4 is a branched or unbranched alkylene group having from 1 to 6 carbon atoms; R5, independently in each occurrence, is hydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms; and R6 is hydrogen, an alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl group having from 1 to 4 carbon atoms.
    
    


Still in another aspect, the present invention concerns a process for selective removal of COS from a gas stream containing same, said process comprising treating the gas stream with a solvent composition comprising

    
    
        a) 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone; and
        b) at least one alkanolamine compound of the formula

R3NHR4OR6  (II)
        
            or
            at least one piperazine compound of formula
        
        
    
    






    
    
        wherein R3 is hydrogen, an alkyl group having from 1 to 6 carbon atoms, or the R4OH group; R4 is a branched or unbranched alkylene group having from 1 to 6 carbon atoms; R5, independently in each occurrence, is hydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms; and R6 is hydrogen, an alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl group having from 1 to 4 carbon atoms.
    
    


Still in another aspect, the present invention concerns a solvent composition for removal of COS from a gas stream containing same, said composition comprising

    
    
        a) a mixture of N-formylmorpholine and N-acetylmorpholine; and
        b) at least one alkanolamine compound of the formula

R3NHR4OR6  (II)
        
            or
            at least one piperazine compound of formula
        
        
    
    






    
    
        wherein R3 is hydrogen, an alkyl group having from 1 to 6 carbon atoms, or the R4OH group; R4 is a branched or unbranched alkylene group having from 1 to 6 carbon atoms; R5, independently in each occurrence, is hydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms; and R6 is hydrogen, an alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl group having from 1 to 4 carbon atoms.
    
    


Still in another aspect, the present invention concerns a process for selective removal of COS from a gas stream containing same, said process comprising treating the gas stream with a solvent composition comprising

    
    
        a) a mixture of N-formylmorpholine and N-acetylmorpholine; and
        b) at least one alkanolamine compound of the formula

R3NHR4OR6  (II)
        
            or
            at least one piperazine compound of formula
        
        
    
    






    
    
        wherein R3 is hydrogen, an alkyl group having from 1 to 6 carbon atoms, or the R4OH group; R4 is a branched or unbranched alkylene group having from 1 to 6 carbon atoms; R5, independently in each occurrence, is hydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms; and R6 is hydrogen, an alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl group having from 1 to 4 carbon atoms.
    
    


As used herein, the terms “gas”, “acid gas”, and “gaseous stream” are intended to refer to natural gas, hydrocarbon gas, synthesis gas, steam reformer-type gases, and any other gas containing COS, CO2 and other gaseous components such as hydrogen sulfide, methane, ethane, propane, hydrogen, carbon monoxide, mercaptans etc. The expression “minimal additional removal of CO2” means additional CO2 absorption due to the presence of the amine additive is less than one third of the CO2 present in the feed gas, all other factors being the same.
In accordance with the present invention, it has been surprisingly discovered that the solvent compositions of the present invention have excellent selectivity for the removal of COS from a gaseous stream containing same with minimal additional removal of CO2.
It was completely surprising and unexpected to find out addition of an alkanolamine compound of formula II above or a piperazine compound of formula III above to least one polyalkylene glycol alkyl ether of the formula I above, and in particular to a mixture of polyalkylene glycol ethers of formula I above, such as the SELEXOLTM solvent, removes substantially more of the COS present in the gaseous streams with minimal additional removal of CO2.
It was also equally surprising and unexpected to find out that the addition of an alkanolamine compound of formula II above or a piperazine compound of formula III above to 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone removes substantially more of COS present in the gaseous streams with minimal additional removal of CO2 
Polyalkylene alkyl ethers of formula I suitable in the practice of the present invention are well known and include, without any limitation, diethylene glycol dimethyl ether, diethylene glycol diisopropyl ether, triethylene glycol dimethyl ether, triethylene glycol diisopropyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diisopropyl ether, pentaethylene glycol dimethyl ether, pentaethylene glycol diisopropyl ether, hexaethylene glycol dimethyl ether, hexaethylene glycol diisopropyl ether, heptaethylene glycol dimethyl ether, octaethylene glycol dimethyl ether, nonaethylene glycol dimethyl ether, decaethylene glycol dimethyl ether, and any mixture thereof.
Preferred polyethylene glycol alkyl ether is a mixture consisting of a dimethyl ethers of polyethylene glycols of formula CH3O(C2H4O)nCH3 wherein n is from 2 to 10. Particularly preferred polyethylene glycol alkyl ether is the mixture of dimethyl ethers of polyethylene glycols sold under the trademark SELEXOL by Union Carbide Corporation. SELEXOLTM solvent is a mixture of dimethyl ethers of polyethylene glycols comprising from 0 to 0.5 wt percent of diethylene glycol dimethyl ether, from 5 to 7 wt percent of triethylene glycol dimethyl ether, from 16 to 18 wt percent tetraethylene glycol dimethyl ether, from 23 to 25 wt percent of pentethylene glycol dimethyl ether, from 22 to 24 wt percent of hexaethylene glycol dimethyl ether, from 15 to 17 wt percent of heptaethylene glycol dimethyl ether, from 8 to 10 wt percent of octaethylene glycol dimethyl ether, from 3 to 5 wt percent of nonaethylene glycol dimethyl ether, and from 1 to 2 wt percent of decaethylene glycol dimethyl ether. SELEXOLTM solvent is widely used in gas treating applications for the removal of acidic gases. However, it is known that the removal of COS by the SELEXOLTM solvent is poor due to solvent factors (low solubility) and applications specific factors (low COS partial pressure). Certain amine additives such as bicyclic tertiary amines and tertiary amines have been used, with some success, to improve removal of COS by SELEXOLTM solvent. It has now been discovered that the SELEXOLTM solvent containing alkanolamines of formula II hereinbefore or a piperazine compound of formula III hereinabove is surprisingly selective in removing COS in the presence of CO2 from gases.
Alkanolamine compounds of formula II usefull in the practice of the present invention are well known compounds and include both primary and secondary alkanolamines. Preferred alkanolamine compounds of formula II are primary alkanolamines. Non-limiting examples of alkanolamine compounds are monoethanolamine (MEA), diethanolamine (DEA), methylethanolamine (NMEA), diisopropanolamine (DIPA), and 2-(2-aminoethoxy)ethanol (AEE) also known as diethylene glycolamine (available from Huntsman Corporation under the trademark DGA).
The present invention is particularly useful in the desulfurization portion of an Integrated Combined Cycle Gasification (IGCC) process using an IGCC system consisting of a conventional SELEXOLTM solvent acid gas removal unit, a fixed bed catalytic converter for reducing the gas phase concentration of carbonyl sulfide (COS), and heat exchanger equipment upstream of the SELEXOLTM solvent unit. It has been surprisingly found out that the addition of the alkanolamine compound of formula II above to the SELEXOLTM solvent results in selective removal of COS from the gas feed in the presence of CO2 with minimal removal of CO2. The costs associated with the fixed bed catalytic converter are also significantly reduced.
Since the process of the present invention for selective removal of COS from gaseous streams is conducted in the desulfurization portion of an Integrated Combined Cycle Gasification (IGCC) process, the operating conditions of the process are those of the IGCC process. These operating conditions are well known to a person of an ordinary skill in the art.
All parts, percentages and ratios herein are by weight unless otherwise indicated.
The invention will be further clarified by a consideration of the following examples that are intended to be purely exemplary of the present invention and not limiting in any way.
The following components are employed in the examples.

    
    
        MEA is monoethanolamine;
        DEA is diethanolamine;
        TEA is triethanolamine;
        NMEA is methylethanoalmine;
        DIPA is diisopropanolamine;
        HEP is hydroxyethylpiperazine;
        MDEA is methyldiethanolamine;
        DMEA is dimethyethanolamine;
        DBU is 1,8-diazabicyclo[5.4.0]undec-7-ene;
        DBN is 1,5-diazabicyclo[4.3.0]non-5-ene;
        DABCO is 1,4-diazabicyclo[2.2.2]octane, sold by Air Products and Chemicals, Inc. under the trademark DABCO;
        DMTP is 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone;
        Quinuclidine is 1,4-ethanolpiperidine; and
        AEE is 2-(2-aminoethoxy)ethanol.
    
    




EXAMPLES 1 TO 10 AND COMPARATIVE EXAMPLES 1 TO 12
The performance of SELEXOLTM solvent alone, and the mixture of SELEXOLTM solvent and prior art additives were evaluated in a bench scale glass absorber-stripper apparatus with the following characteristics:

    
    
        a) standard gas feed conditions (10 mole percent CO2, 1.6 mole percent COS, and the balance nitrogen, about 3 L/min, 80° F.),
        b) standard liquid feed conditions (about 11 cc/min, 80° F.),
        c) standard amount of absorber and stripper staging (small trays, 25 absorber, 20 stripper),
        d) approximately standard amount of stripping heat duty applied,
        e) standard amount of the additive used about 3 percent by weight),
        f) gas phase analysis by gas chromatography, liquid phase water content by Karl Fisher method,
        g) careful attention to ensuring steady state operation,
        h) liquid phase COS content was not measured, and
        i) stripper overhead not sampled for COS or H2S.
    
    


The results obtained in the performance evaluation of different gas treating solvent are provided in Tables below.







TABLE 1







Evaluation of Prior Art and Various Additives for Removal of COS by


SELEXOL TM solvent










COS
CO2



Absorption
Absorption



(percent of
(percent of


Solvent
feed)
feed)












SELEXOL TM solvent*
0.7
0.7


SELEXOL TM solvent*
1.2
0.2


S + 3 wt percent 1,4 diazabicyclo[2.2.2]octane (DABCO)*
2.6
0.0


S + 3 wt percent quinuclidine*
5.1
−0.2


S + 3 wt percent 1,5-diazabicyclo[4.3.0]non-5-ene (DBN)*
3.1
−0.6


S + 3 wt percent 1,8-diazabicyclo[5.4.0]undec-7-ene
2.5
0.5


(DBU)*


S + 3 wt percent triethanolamine (TEA)*
2.0
0.5


S + 3 wt percent triethanolamine (TEA)*
−0.7
2.7


S + 3 wt percent methyldiethanolamine (MDEA)*
1.3
1.3


S + 3 wt percent dimethylethanolamine (MDEA)*
2.0
0.0


S + 3 wt percent hydroxyethylpiperazine (HEP)
17.1
0.7


S + 3 wt percent diisopropanolamine (DIPA)
18.9
1.0


S + 3 wt percent diethanolamine (DEA)
18.9
2.8


S + 3 wt percent diethanolamine (DEA)
28.3
0.1


S + 3 wt percent monoethanolamine (MEA)
28.3
−0.7


S + 3 wt percent piperazine
29.1
0.8


S + 3 wt percent methylethanolamine (NMEA)**
6.6
−0.6


S + 3 wt percent methylethanolamine (NMEA)
52.1
0.9


S + 3 wt percent methylethanolamine (NMEA)
50.3
1.1


DMTP*
5.8
−1.0


DMTP*
3.6
−1.1


DMTP + 3 wt percent diethanolamine
40.1
0.6





*not an Example of the present invention


**spurious experimental outcome


S = SELEXOL TM solvent



















TABLE 2







Comparative


Comparative
Comparative



Example 1
Example 1
Example 2
Example 2
Example 3





















Base Solvent
SELEXOL TM
SELEXOL TM
SELEXOL TM
DMTP
SELEXOL TM



Solvent
Solvent
solvent

solvent


Additive
0
Piperazine
NMEA
none
DBU


Additive Concentration (wt
25
3
3
0
3


percent)


# Absorber Trays
20
25
25
25
25


# Stripper Trays

20
20
20
20


Total Gas Feed Rate (cc/min)
2997
2996
2995
3026
3008


Feed CO2 (mole percent)
10.09
10.18
10.06
9.64
11.06


Feed COS (mole percent)
1.48
1.51
1.51
1.21
1.63


Sweet CO2 (mole percent)
10.02
10.1
10.12
9.74
11.01


Sweet COS (mole percent)
1.47
1.07
1.41
1.14
1.59


Absorber:


Lean H2O content (wt percent)
3.2
4
4.6
7.4
4.3


Lean Solvent Flow (cc/min)
11
11
11
11
11


Lean T (F.)
80.7
78.8
79.8
79.8
80.5


Feed Gas T (F.)
80.8
79.1
79.5
79.3
80.3


Sweet Gas T (F.)
81.6
79.5
80.6
79.8
80.8


Rich T (F.)
79.3
77.6
78.1
77.9
77.1


Absorber P (in H2O)
28
28
28
28
28


Stripper:


Hot Rich Inlet T (F.)
239.1
220.3
232.5
229.1
231.5


Overhead Vap T (C.)
103
100
103
100
100


Reboiler T (F.)
264.7
258.3
254.6
270.5
259.5


COS Absorption (percent of feed)
0.7
29.1
6.6
5.8
2.5


CO2 Absorption (percent of feed)
0.7
0.8
−0.6
−1.0
0.5




















TABLE 3







Comparative
Comparative




Example 4
Example 5
Example 3



















Base Solvent
SELEXOL TM
SELEXOL TM
SELEXOL TM



Solvent
Solvent
solvent


Additive
DBN
DABCO
DEA


Additive Concentration (wt
3
3
3


percent)


Number of Absorber Trays
25
25
25


Number of Stripper Trays
20
20
20


Total Gas Feed Rate (cc/min)
3000
2999
3004


Feed CO2 (mole percent)
10.9
10.26
10.3


Feed COS (mole percent)
1.6
1.51
1.48


Sweet CO2 (mole percent)
10.97
10.26
10.01


Sweet COS (mole percent)
1.55
1.47
1.2


Absorber:


Lean H2O content (wt percent)
4
3.6
4.1


Lean Solvent Flow (cc/min)
11
11
11


Lean T (F.)
80.7
82.3
81.1


Feed Gas T (F.)
80.3
81.8
81


Sweet Gas T (F.)
80.8
82.3
81.6


Rich T (F.)
79.4
79.5
79.8


Absorber P (in H2O)
28
28
28


Stripper:


Hot Rich Inlet T (F.)
227.6
246.5
226.7


Overhead Vap T (C.)
100
102
102


Reboiler T (F.)
259.9
260.6
256.1


COS Absorption (percent of
3.1
2.6
18.9


feed)


CO2 Absorption (percent of
−0.6
0.0
2.8


feed)






















TABLE 4








Comparative
Comparative





Example 4
Example 6
Example 7
Example 5
Example 6





















Base Solvent
SELEXOL TM
SELEXOL TM
SELEXOL TM
SELEXOL TM
SELEXOL TM



Solvent
solvent
solvent
solvent
solvent


Additive
MEA
Quinuclidine
none
DEA
NMEA


Additive Concentration (wt
3
3
0
3
3


percent)


Number of Absorber Trays
25
25
25
25
25


Number of Stripper Trays
20
20
20
20
20


Total Gas Feed Rate (cc/min)
2999
2978
2985
2990
3012


Feed CO2 (mole percent)
10.09
10.06
10.13
10.03
10.11


Feed COS (mole percent)
1.8
1.76
1.72
1.7
1.67


Sweet CO2 (mole percent)
10.16
10.08
10.11
10.02
10.02


Sweet COS (mole percent)
1.29
1.67
1.7
1.43
0.8


Absorber:


Lean H2O content (wt percent)
5.4
4.5
3.7
3.6
4


Lean Solvent Flow (cc/min)
11
11
11
11
11


Lean T (F.)
79.7
80.7
82
83.2
80.9


Feed Gas T (F.)
80.6
79.9
80.5
81.8
83.2


Sweet Gas T (F.)
81.1
81
81.4
83
83.5


Rich T (F.)
78.6
76.7
77.8
80.9
82.5


Absorber P (in H2O)
28
28
28
28
28


Stripper:


Hot Rich Inlet T (F.)
222.1
213.8
209.7
207.6
201.2


Overhead Vap T (C.)
109
103
105
101
100


Reboiler T (F.)
246.6
250.2
257.5
256.9
255.1


COS Absorption (percent of feed)
28.3
5.1
1.2
15.9
52.1


CO2 Absorption (percent of feed)
−0.7
−0.2
0.2
0.1
0.9






















TABLE 5









Comparative

Comparative



Example 7
Example 8
Example 89
Example 98
Example 9





















Base Solvent
SELEXOL TM
SELEXOL TM
DMTP
SELEXOL TM
SELEXOL TM



Solvent
Solvent

Solvent
solvent


Additive
NMEA
DIPA
none
HEP
TEA


Additive Concentration (wt
3
3
0
3
3


percent)


Number of Absorber Trays
25
25
25
25
25


Number of Stripper Trays
20
20
20
20
20


Total Gas Feed Rate (cc/min)
3023
3013
3004
2990
3000


Feed CO2 (mole percent)
10.08
10.08
10.02
10.06
10.01


Feed COS (mole percent)
1.61
1.64
1.67
1.58
1.52


Sweet CO2 (mole percent)
9.97
9.98
10.13
9.99
9.96


Sweet COS (mole percent)
0.8
1.33
1.61
1.31
1.49


Absorber:


Lean H2O content (wt percent)
3.7
4
8.2
3.2
3.6


Lean Solvent Flow (cc/min)
11
11
11
11
11


Lean T (F.)
79.4
82.9
82.4
84.2
81


Feed Gas T (F.)
78.4
81.3
81.1
82.4
79.3


Sweet Gas T (F.)
79.4
82.6
82
83.7
80.3


Rich T (F.)
78.3
80.1
79.6
80.2
77.5


Absorber P (in H2O)
28
28
28
28
28


Stripper:


Hot Rich Inlet T (F.)
200.2
206.7
198.9
197.5
180.5


Overhead Vap T (C.)
101
100
104
100
101


Reboiler T (F.)
255.6
255.6
271.3
258.4
255.7


COS Absorption (percent of feed)
50.3
18.9
3.6
17.1
2.0


CO2 Absorption (percent of feed)
1.1
1.0
−1.1
0.7
0.5





















TABLE 6







Comparative
Comparative

Comparative



Example 10
Example 11
Example 10
Example 12




















Base Solvent
SELEXOL TM
SELEXOL TM
DMTP
SELEXOL TM



Solvent
Solvent

solvent


Additive
TEA
MDEA
DEA
DMEA


Additive Concentration (wt
3
3
3
3


percent)


Number of Absorber Trays
25
25
25
25


Number of Stripper Trays
20
20
20
20


Total Gas Feed Rate (cc/min)
3005
2995
2981
2994


Feed CO2 (mole percent)
10.17
10.01
10.09
9.95


Feed COS (mole percent)
1.48
1.52
1.52
1.47


Sweet CO2 (mole percent)
9.9
9.88
10.03
9.95


Sweet COS (mole percent)
1.49
1.5
0.91
1.44


Absorber:


Lean H2O content (wt percent)
4.2
3.8
8.6
4


Lean Solvent Flow (cc/min)
11
11
11
11


Lean T (F.)
82.6
82.6
80
80.3


Feed Gas T (F.)
80.4
80.4
79
79.7


Sweet Gas T (F.)
81.9
81.8
79.6
81


Rich T (F.)
77.8
78.5
79.4
76.9


Absorber P (in H2O)
28
28
28
28


Stripper:


Hot Rich Inlet T (F.)
185.2
189.5
182.2
198.3


Overhead Vap T (C.)
102
108
105
108


Reboiler T (F.)
256.6
257.6
264.3
253.7


COS Absorption (percent of feed)
−0.7
1.3
40.1
2.0


CO2 Absorption (percent of feed)
2.7
1.3
0.6
0.0









As can be seen from the Tables 1-6 hereinabove, certain amine additives of the prior art improve removal of COS without substantially increasing the removal of CO2. For example, the addition of DABCO, quinuclidine, DBN and DBU to the SELEXOLTM solvent increases the COS removal of to the SELEXOLTM solvent from 1 percent to 2-5 percent.
EXAMPLES 11 TO EXAMPLE 15 AND COMPARATIVE EXAMPLES 13 TO 14
The performance of a physical solvent alone, and the mixture of the physical solvent and prior art additives of the present invention were evaluated in a bench scale glass absorber-stripper apparatus with the following characteristics:

    
    
        j) standard gas feed conditions (8.7-10 mole percent CO2, 1.1-1.5 mole percent COS, and the balance nitrogen, about 3 L/min),
        k) standard liquid feed conditions (about 11 or 30 cc/min),
        l) standard amount of absorber and stripper staging (small trays, 25 absorber, 20 stripper),
        m) approximately standard amount of stripping heat duty applied,
        n) standard amount of the additive used about 3 percent by weight),
        o) gas phase analysis by gas chromatography, liquid phase water content by Karl Fisher method,
        p) careful attention to ensuring steady state operation,
        q) liquid phase COS content was not measured, and
        r) stripper overhead not sampled for COS or H2S.
    
    


The results obtained in the performance evaluation of different gas treating solvent are provided in Tables 7 and 8 below.












TABLE 7







Comparative

Example12




Example 13
Example 11
003
Example 13




















Base Solvent
SELEXOL TM
SELEXOL TM
SELEXOL TM
SELEXOL TM


Additive
None
HEP
HEP
AEE


Additive Concentration (wt percent)
0
3
3
3


# Absorber Trays
25
25
25
25


# Stripper Trays
20
20
20
20


Total Gas Feed Rate (cc/min)
2985
2985
2985
2985


Feed CO2 (mole percent)
8.65
9.34
9.76
9.75


Feed COS (mole percent)
1.48
1.47
1.44
1.43


Sweet CO2 (mole percent)
8.65
8.14
8.61
8.36


Sweet COS (mole percent)
1.33
0.52
0.58
0.41


Absorber:


Lean H2O content (wt percent)
7.2
3.9
6.8
3.9


Lean Solvent Flow (cc/min)
30
30
30
30


Lean T (F.)
81
85
84
85


Feed Gas T (F.)1
89
89
89
89


Sweet Gas T (F.)
66
72
72
70


Rich T (F.)2
83
88
88
87


Absorber P (in H2O)
28
28
28
28


Stripper:


Hot Rich Inlet T (F.)
209
203
194
204


Overhead Vapor T (F.)
215
216
215
214


Reboiler T (C.)
115
122
118
132


COS Absorption (percent of feed)
10
65
60
71


CO2 Absorption (percent of feed)
0
13
12
14





1measured at outlet of feed gas heater


2measured in pot beneath lowest absorber tray






As can be seen from the Table 7 hereinabove, the SELEXOLTM solvent alone removes about 10 percent of the COS from the feed gas while the addition of about 3 wt percent of HEP or AEE to the SELEXOLTM solvent improves COS removal to 60-70 percent. Removal of CO2 increases from 0 to 12-14 percent.











TABLE 8







Comparative
Example
Example



Example 14
14
15



















Base Solvent
DMTP
DMTP
DMTP


Additive
none
HEP
NMEA


Additive Concentration
0
3
3


(wt percent)


# Absorber Trays
25
25
25


# Stripper Trays
20
20
20


Total Gas Feed Rate (cc/min)
2985
2985
2985


Feed CO2 (mole percent)
9.75
9.73
9.66


Feed COS (mole percent)
1.43
1.43
1.42


Sweet CO2 (mole percent)
9.47
9.14
8.69


Sweet COS (mole percent)
1.31
0.45
0.0


Absorber:


Lean H2O content (wt percent)
9.1
4.9
3.9


Lean Solvent Flow (cc/min)
11
11
11


Lean T (F.)
76
79
82


Feed Gas T (F.)1
85
84
77


Sweet Gas T (F.)
71
72
68


Rich T (F.)2
82
83
82


Absorber P (in H2O)
28
28
28


Stripper:


Hot Rich Inlet T (F.)
238
261
244


Overhead Vapor T (F.)
216
212
211


Reboiler T (C.)
129
149
150


COS Absorption (percent of feed)
8
69
100


CO2 Absorption (percent of feed)
3
6
10





1measured at outlet of feed gas heater


2measured in pot beneath lowest absorber tray






As can be seen from the Table 8 hereinabove, DMTP solvent alone removes about 8 percent of the COS from the feed gas while the addition of about 3 wt percent of HEP or NMEA to the DMTP solvent improves COS removal to about 69-100 percent. Removal of CO2 increases from 3 to 6-10 percent.
As can also be seen from the Tables 1-8 hereinabove, all the solvent compositions of the present invention showed a marked improvement in COS removal with minimal additional CO2 removal. Overall, the solvent compositions of the present invention comprising the SELEXOLTM solvent and the alkanolamine of formula II or piparazine compound of formula III removed COS in an amount of from 17 percent to 52 percent as compared to the SELEXOLTM solvent alone which removed COS in an amount of only about 1 percent. Similarly, the solvent composition of the present invention comprising DMTP and the alkanolamine of formula II removed COS in an amount of 40-100 percent as compared to DMTP alone which removed COS in an amount of from 3-8 percent. Such magnitude of improved removal of COS by the solvent compositions of the present invention is completely unexpected and unpredictable.
Other embodiments of the invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.

## Claims

1. A solvent composition for selective removal of COS relative to CO2 from a gas stream containing COS and CO2, said composition comprising
a) a mixture of N-formylmorpholine and N-acetylmorpholine; and
b) at least one alkanolamine compound of the formula


R3NHR4OR6  (II)



or
at least one piperazine compound of formula




wherein R3 is hydrogen, an alkyl group having from 1 to 6 carbon atoms, or the R4OH group; R4 is a branched or unbranched alkylene group having from 1 to 6 carbon atoms; R5, independently in each occurrence, is hydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms; and R6 is hydrogen, an alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl group having from 1 to 4 carbon atoms, and
c) 9 percent by weight water or less.

2. The solvent composition according to claim 1 wherein the component b) is an alkanolamine of formula II in which substituent R3 is hydrogen.

3. The solvent composition according to of claim 1 wherein the component b) is monoethanolamine.

4. The solvent composition according to claim 1 wherein the component b) is an alkanolamine of formula II in which substituent R3 is an alkyl group having from 1 to 6 carbon atoms or the R4OH group.

5. The solvent composition according to claim 4 wherein the alkanolamine of formula II is selected from the group consisting of diethanolamine, methylethanolamine and diisopropanolamine.

6. The solvent composition according to claim 1 wherein the component b) is piperazine.

7. The solvent composition according to claim 1 wherein the component b) is hydroxyethylpiperazine.

8. A process for selective removal of COS relative to CO2 from a gas stream containing COS and CO2 with minimal removal of CO2 from the gas stream, said process comprising treating the gas steam with a solvent composition comprising
a) a mixture of N-formylmorpholine and N-acetylmorpholine; and
b) at least one alkanolamine compound of the formula


R3NHR4OR6  (II)



or
at least one piperazine compound of formula




wherein R3 is hydrogen, an alkyl group having from 1 to 6 carbon atoms, or the R4OH group; R4 is a branched or unbranched alkylene group having from 1 to 6 carbon atoms; R5, independently in each occurrence, is hydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms; and R6 is hydrogen, an alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl group having from 1 to 4 carbon atoms, wherein removal of CO2 from the gas stream due to the presence of component b) is less than 1/3 of the CO2 present in the gas stream.

9. The process according to claim 8 for selective removal of COS from a gas stream, wherein R3 is hydrogen.

10. The process according to claim 8 for selective removal of COS from a gas stream wherein the component b) is monoethanolamine.

11. The process according to claim 8 for selective removal of COS from a gas stream, wherein the component b) is an alkanolamine of formula II in which substituent R3 is an alkyl group having from 1 to 6 carbon atoms or the R4OH group.

12. The process according to claim 11 for selective removal of COS from a gas stream wherein the alkanolamine of formula II is selected from the group consisting of diethanolamine, methylethanolamine, and diisopropanolamine.

13. The process according to claim 8 for selective removal of COS from a gas stream, wherein the component b) is piperazine.

14. The process according to claim 8 for selective removal of COS from a gas stream, wherein the component b) is hydroxyethylpiperazine.

15. A solvent composition for selective removal of COS relative to CO2 from a gas stream containing COS and CO2, said composition comprising
a) a mixture of N-formylmorpholine and N-acetylmorpholine; and
b) at least one alkanolamine compound of the formula


R3NHR4OR6  (II)



or
at least one piperazine compound of formula




wherein R3 is hydrogen, an alkyl group having from 1 to 6 carbon atoms, or the R4OH group; R4 is a branched or unbranched alkylene group having from 1 to 6 carbon atoms; R5, independently in each occurrence, is hydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms; and R6 is hydrogen, an alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl group having from 1 to 4 carbon atoms.