Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 66 (2015) 61 – 64
The 12th International Conference on Combustion & Energy Utilisation – 12ICCEU
Reaction mechanism for the oxidation of aromatic contaminants present in feed gas to Claus process Abhijeet Raj* and Sourab Sinha Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, UAE
Abstract Claus process, consisting of a furnace and catalytic reactors, is used to recover sulfur from H2S found in raw natural gas and gases from refineries. H2S is accompanied by contaminants such as benzene, toluene, xylenes (collectively called BTX), other hydrocarbons, NH3, CO2, N2 and sulfur compounds. Among these, BTX have attracted several research activities as they form soot and sulfur-hydrocarbons in catalytic units that clog and deactivate the catalysts. This work focusses on BTX oxidation by SO2 as a potential solution that can be carried out in a BTX destruction unit placed between Claus furnace and catalytic units. To determine the extent of BTX destruction by SO 2, reaction mechanisms are developed using density function theory. The rates of elementary reactions are evaluated using transition state theory. The pathways leading to the formation of CO and SO are obtained. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of 12ICCEU Peer-review under responsibility of the Engineering Department, Lancaster University
Keywords: Benzene, Toulene, Xylene, SO2, Claus process, DFT
1. Introduction Claus process is widely used in oil and gas industries to recover sulfur from H 2S, an acid gas present in natural gas and in by-product gas streams [1-3]. Claus process has two major sections: (a) Claus furnace, where non-catalytic partial oxidation of H2S in air through the reaction, H2S + 1.5O2 ė SO2 + H2O, takes place. Some sulfur is also produced in this section. (b) Catalytic units, where unburnt H2S and SO2 from the furnace form sulfur catalytically (2H2S + SO2 ė 3S + 2H2O) [4]. During the separation of H2S from raw natural gas in amine sweetening units, several contaminants from the raw gases such as benzene, toluene, xylenes (collectively known as BTX), heavy hydrocarbons, NH3, CO2, N2, CS2 and COS accompany the H2S gas stream. Out of these, BTX are known to form soot or carbon-sulfur compounds in catalytic units that clog and deactivate the catalysts, and reduce sulfur quality [5]. Due to these reasons, BTX have been a major concern for sulfur recover unit operators. The
* Corresponding author. Tel.: +971-26075738 E-mail address:
[email protected]
1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Engineering Department, Lancaster University doi:10.1016/j.egypro.2015.02.032
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increase in the temperature in Claus furnace by enriching air with oxygen, by co-firing natural gas with feed, or by preheating the feed gas may help in oxidizing BTX [6]. However, these methods involve high capital and operating costs. Another solution could be the adsorption of BTX from feed gas on activated carbons, but high energy usage makes this solution economically unviable. All the BTX removal techniques have concentrated on feed pre-treatment or on the destruction of BTX in Claus furnace. However, the presence of BTX does not have detrimental effect on the efficiency of Claus furnace. The effect is primarily seen in the catalytic section. Thus, a BTX destruction unit between Claus furnace and the catalytic units is required, where BTX can be oxidized. Since SO 2 is present in the exhaust gas from furnace in high concentration, it is possible to oxidize BTX by SO 2 [7]. Our present investigation focuses on the development of reaction mechanisms for the oxidation by SO2 of phenyl, o-methylphenyl and 2,3-dimethylphenyl radicals that can facilitate process simulations to determine optimum operating condition for BTX destruction. 2. Calculation details The ground state molecular structures of the stable chemical species and the transition states were found using Density Functional Theory with B3LYP functional and 6-311++G(d,p) basis set. The molecular structures were optimized with different spin multiplicities to identify the multiplicity with a minimum energy, reasonable geometry and low spin contamination. All the calculations were performed using Gaussian 09 [8]. The reaction rate constants were evaluated using transition state theory. 3. Results and Discussion The oxidation mechanisms of BTX radicals (phenyl, o-methylphenyl and 2,3-dimethylphenyl radicals) by SO2 are presented. Due to space constraint, other toluene and xylene radicals are not discussed. Phenyl radical – SO2 reactions: Figure 1 presents the potential energy diagram for phenyl radical oxidation by SO2 with the energies of the chemical species and the transition states relative to the energy of the reactants, phenyl radical and SO2. The reaction begins with the attack of O-atom of SO2 on the radical site due to higher electronegativity of O atoms than the S atom. This addition reaction forms a stable chemical species, CS1 after overcoming a small energy barrier of 6.5 kJ/mol, and with the release of 182 kJ/mol of reaction energy. Thereafter, the removal of SO from CS1 forms a phenoxy radical, CS2, which is also formed during phenyl oxidation by O2 through the breakage of O-O bond [9]. The reactions thereafter become identical to some reactions in phenyl oxidation mechanism by O2 [9]. The species CS2 forms a fused bicyclic intermediate, CS3 having high energy and low stability. A C-C bond in CS3 dissociates to form CS4, which, in turn, forms cyclopentadienyl radical (CS5) after CO elimination. O-methylphenyl radical – SO2 reactions: Figure 2 presents the reaction mechanism for the oxidation of o-methylphenyl by SO2. Similar to phenyl radical, reaction initiates with the addition of SO2 to the radical site of o-methylphenyl through one of the O atoms, and this requires overcoming a small activation energy barrier of 4.8 kJ/mol to form CS1. This reaction is highly exothermic with a reaction energy of 181.6 kJ/mol. Thereafter, the elimination of the SO group from CS1 leads to the formation of CS2 which lies 87.7 kJ/mol above CS1. The next step involves the conversion of a six-member ring in CS2 to a five-member and a three-member ring in CS3. This requires crossing an activation energy barrier of 224.7 kJ/mol. The species CS3, thus formed, is highly unstable, and it can barrierlessly form CS4 and CS5 through the breakage of the three-member ring, with CS4 being more stable than CS5 by 14.1 kJ/mol. This is followed by the loss of CO from CS4 and CS5 after overcoming energy barriers of 25.3 and 10.2 kJ/mol, respectively, to form 2-methylcyclopentadienyl radical (CS6). 2,3-dimethylphenyl – SO2 reactions: Figure 3 presents the reaction mechanism for the oxidation of 2,3-dimethylphenyl radical by SO2. The addition of SO2 on the free radical site to form CS1 requires a very small activation energy barrier of 4.1 kJ/mol, and results in the release of a reaction energy of 178.3 kJ/mol. Thereafter, the elimination of the SO group from CS1 leads to the formation of
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CS2 which lies 86.6 kJ/mol above CS1. The next step involves the conversion of a six-member ring in CS2 to a five-member and a three-member ring in CS3, requiring an activation energy of 216.0 kJ/mol. The species CS3, thus formed, is highly unstable, and it can barrierlessly form CS4 and CS5 through the breakage of the three-member ring, which lies 53.4 and 41.5 kJ/mol respectively below CS3. This is followed by the loss of CO from CS4 and CS5 after overcoming energy barriers of 25.1 and 12.6 kJ/mol, respectively, to form 2,3-dimethylcyclopentadienyl radical (CS6). 200
TS3 135.2
TS4 154.8
CS3 122.6
+ SO '+, KJ/mol
TS5 96.6
CS4 72.7
O
100
C O
TS1 6.5
+ SO
0
CS5 41.7 + SO + CO
Reaction Progress
+ SO2
CS2 -85.9
TS2 -94.7
O
-100
+ SO CS1 -182.3 -200
O
0.0
S O
0.2
0.4
0.6
0.8
1.0
Figure 1. A potential energy diagram at 0 K for the oxidation of phenyl radical by SO2.
'+, kJ/mol
200
TS3 130.8
100
0
H3C .
TS1 4.8
+ SO
CH3
+ SO2
CS1 -181.6
-200
CH3
0
CH3
TS5 91.8
TS6 90.8
O
CS5 H C 80.6 3
. O C
+ SO
CS4 66.5
CS6 29.2
CH3 . CO
+ SO
CH3
+ SO + CO
Reaction Progress
CS2 -93.9
.
-100
CS3 121.6
O.
+ SO
O . O S
2
4
6
8
Figure 2. A potential energy diagram at 0 K for the oxidation of o-methylphenyl radical by SO2. 4. Reaction rate constants Figure 4 compares the rate constants for two important reactions: addition of SO2 on the radical sites, and the breakage of O-S bond to release SO molecule. Due to similarity between the oxidation mechanisms of BTX radicals by O2 and by SO2, the rate constants for the addition of O2 on phenyl, and the breakage of O-O bond in phenylperoxy radical are also presented [10]. It is evident that, with the addition of subsequent methyl groups to phenyl, the rate constant for SO2 addition decreases. At temperatures above 1100 K, the rate constants for O2 addition on phenyl and SO2 addition on phenyl and o-methylphenyl radicals are within an order of magnitude, indicating that oxidation of these radicals by SO 2 should be very competitive at high temperatures. Furthermore, the rate constants for the breakage of O-S bond in phenyl, o-methylphenyl and 2,3-dimethylphenyl radicals are significantly higher than O-O bond breakage in phenylperoxy radical, which is a result of weaker O-S bond (due to higher electronegativity of O atoms than S atom) than O-O bond.
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Abhijeet Raj and Sourab Sinha / Energy Procedia 66 (2015) 61 – 64 TS3 124.3
150
.C
CH 3
100
.
50
'+, kJ/mol
CH 3
CS3 114.5
+ SO
TS1 4.1
CS5 73.0
CH 3 O
CS4 61.1
+ SO CH 3
TS6 86.2
O
TS7 85.6
CH 3
.
CH 3
C O
0
+ SO
CH 3
+ SO + CO
Reaction Progress
CH 3 CH3
.
-50
CH 3
CS6 22.8
CS2 -91.7
+ SO2 -100
CH3 CH 3 O.
CS1 -178.3
-150
+ SO
CH 3 CH 3
-200
.
O SO
0
2
4
6
8
10
Figure 3. A potential energy diagram at 0 K for the oxidation of 2,3-dimethylphenyl radical by SO2. 14
10
12
10 13
10
10
10
8
12
10
10
6
10
11
10
phenyl o-methylphenyl 2,3-dimethylphenyl phenylperoxy
4
C6H5+SO2 o-methylphenyl+SO2 2,3-dimethylphenyl+SO2 C6H5+O2
10
10
9
10 300
900
1500
2100
Temperature, K
2700 3000
10
2
10
1 300
900
1500
2100
2700 3000
Temperature, K
Figure 4. Left: Rate constants for SO2 addition to phenyl, o-methylphenyl and 2,3-dimethylphenyl radicals, and O2 addition to phenyl radical. Right: Rate constants for O-S bond scission to release SO during the oxidation by SO2 of phenyl, o-methylphenyl and 2,3-dimethylphenyl radicals, and for O-O bond scission in phenylperoxy radical.
5. Conclusion The reaction mechanisms for the oxidation by SO2 of the radicals of BTX (phenyl, o-methylphenyl and 2,3-dimethylphenyl) were developed using B3LYP/6-311++G(d,p). The rate constants for SO2 addition to the radical sites of phenyl and o-methylphenyl radicals were found to be comparable to O2 addition rate at high temperatures, while the breakage of O-S bond was faster than O-O bond. Process simulations are now required to determine the operating conditions for the high efficiency of BTX destruction unit. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Gary J.H.; Handwerk G.E. Petroleum Refining Technology and Economics; Marcel Dekker; New York, 1984. Gens T. A. Ind. Eng. Chem. Res. 1994, 33, 1654. Goar B. Energy Progress 1986, 6, 71. Jensen A.B.; Webb C. Enzyme Microb. Technol. 1995, 17, 2. Duncan C.; Diaz A.; Bagajewicz M. BTEX Removal from Natural Gas Streams, Oklahoma University, 2009. Crevier, P. P.; Al-Haji, M. N.; and Alami, I. A. Brimstone Sulphur Recovery Symposium; Colorado, 2002. Levy A.; Ambrose C. J. J. Am. Chem. Soc. 1959, 81, 249. Frisch M. J. et al. Gaussian, Inc., Wallingford:CT, 2009. Tokmakov I. V.; Kim G.-S.; Kislov V. V.; Mebel A. M.; Lin M. C. J. Phys. Chem. A 2005, 109, 6114. da Silva G.; Bozzelli J. W. J. Phys. Chem. A 2008, 112, 3566.
