Benzene Destruction in Claus Process by Sulfur ... - ACS Publications

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Benzene Destruction in Claus Process by Sulfur Dioxide: A Reaction Kinetics Study Sourab Sinha,† Abhijeet Raj,*,† Ahmed S. AlShoaibi,† Saeed M. Alhassan,†,‡ and Suk Ho Chung§ †

Department of Chemical Engineering and ‡Gas Processing and Materials Science Research Centre, The Petroleum Institute, Abu Dhabi, United Arab Emirates § Clean Combustion Research Centre, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia S Supporting Information *

ABSTRACT: Benzene, toluene and xylene (BTX) are present as contaminants in the H2S gas stream entering a Claus furnace. The exhaust gases from the furnace enter catalytic units, where BTX form soot particles. These particles clog and deactivate the catalysts. A solution to this problem is BTX oxidation before the gases enter catalyst beds. This work presents a theoretical investigation on benzene oxidation by SO2. Density functional theory is used to develop a detailed mechanism for phenyl radical −SO2 interactions. The mechanism begins with SO2 addition to phenyl radical after overcoming an energy barrier of 6.4 kJ/mol. This addition reaction is highly exothermic, where a reaction energy of 182 kJ/mol is released. The most favorable pathway involves O−S bond breakage, leading to the release of SO. A remarkable similarity between the pathways for phenyl radical oxidation by O2 and its oxidation by SO2 is observed. The reaction rate constants are also evaluated to facilitate process simulations.

1. INTRODUCTION The Claus process is considered to be the most significant method for recovering elemental sulfur from the gaseous hydrogen sulfide (H2S) found in raw natural gas and in byproduct gases derived from oil refineries.1−4 In the Claus furnace, partial oxidation of H2S takes place noncatalytically through the reaction H2S + 1.5O2 → SO2 + H2O. Sulfur is also produced in the furnace through an endothermic reaction.5 Thereafter, the remaining unburnt H2S and SO2 pass through catalytic beds to undergo Claus reaction (2H2S + SO2 → 3S + 2H2O) to form sulfur. Approximately 90−95% of the elemental sulfur is produced by the Claus process. H2S, recovered from gas and oil fields, is accompanied by contaminants such as benzene, toluene, xylene (collectively known as BTX), heavy hydrocarbons, ammonia, carbon dioxide, N2 and compounds of sulfur (e.g., CS2 and COS).6 These contaminants affect the efficiency of the Claus process, deactivate the catalysts, reduce sulfur quality, and trigger the formation of toxic compounds such as H2SO4, CO, COS, and CS2.7 They also lead to the corrosion of downstream equipment and an increase in the pressure drop through reactor beds due to the formation of sulfur hydrocarbons (similar to soot particles).8 Out of these contaminants, BTX have attracted several research activities due to their capability to form soot particles in the Claus catalytic reactor units.9−11 These particles plug the catalyst pores and make them inactive within hours of operation. The replacement of catalysts is then required, which significantly enhances the production cost of sulfur. These contaminants can be destroyed in a Claus furnace at high temperatures (above 1050 °C). However, the typical process conditions in the Claus furnace do not favor complete BTX © 2014 American Chemical Society

oxidation by O2 (since H2S oxidation by O2 is preferred), and thus they are able to reach the catalytic reactors.12−14 Several solutions have been proposed in the literature to remove or destroy BTX before the gas stream carrying them enters the catalytic units, as mentioned below. The increase in the temperature in a Claus furnace may help in oxidizing BTX.6,7,10 For this, three methods have been suggested: (a) enriching air with oxygen, (b) co-firing natural gas with feed, and (c) preheating the feed gas. Method (a) does not raise the furnace temperature high enough for complete BTX oxidation when the H2S concentration in the feed is low. Moreover, enriching air with oxygen is an expensive process. Method (b) requires burning natural gas along with acid gases in a furnace of high volume for BTX destruction and, thus, has high capital cost. Method (c) is able to raise the furnace temperature since the adiabatic flame temperature increases with feed preheating, but it is not high enough to completely destroy BTX, especially when the feed is lean in H2S. BTX can be removed from the H2S gas stream in a gas absorption unit using amine as a solvent.7 However, this technique has limited BTX removal efficiency and involves high capital and operating costs. The adsorption of BTX using activated carbon beds is a promising solution,7 but, in the presence of moisture in the gas stream, the adsorbed BTX on carbon beds are replaced with water. To avoid this problem, feed preheating is required. In general, four to five beds of activated carbon in series are Received: Revised: Accepted: Published: 10608

April 27, 2014 June 8, 2014 June 11, 2014 June 11, 2014 dx.doi.org/10.1021/ie501732a | Ind. Eng. Chem. Res. 2014, 53, 10608−10617

Industrial & Engineering Chemistry Research

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Figure 1. Potential energy diagram at 0 K for H-abstraction from benzene to form phenyl radical by the chemical species, H, OH, and HS, present in the Claus furnace in appreciable concentrations.

shown to take place at temperatures above 540 °C. Since the exhaust gas from the Claus furnace is well above this temperature, the reactions between SO2 and BTX can take place. The high temperature studies of SO2 addition to hydrocarbon flames16−19 also suggest that SO2 reduces the concentrations of aromatic hydrocarbons and soot through their oxidation. In order to determine the optimum operating conditions under which BTX could be oxidized by SO2 through simulations, a detailed reaction mechanism is required. However, so far, there has not been any attempt in the literature to study the detailed chemistry behind the oxidation by SO2 of benzene, toluene or xylene. Our present investigation focuses on the development of a reaction mechanism for H atom abstraction from benzene to form phenyl radical (C6H5•) and the oxidation by SO2 of phenyl radical. The mechanisms for the oxidation of toluene and xylene will be addressed in forthcoming studies. The energetics and molecular parameters for various species involved in their reactions have been determined through quantum chemical calculations. The rates of the elementary reactions have been evaluated using transition state theory. The possible reaction pathways leading to the formation of some

required for effective removal of BTX. To regenerate the beds, steam is used to desorb BTX. This is followed by drying of beds. As evident, the high energy usage makes this solution expensive. With the brief description of some existing solutions for BTX removal presented above, it is clear that they either require huge capital costs or involve high energy consumption, but they still do not provide complete removal of BTX. Moreover, all the BTX removal techniques have concentrated on the pretreatment of feed or on the destruction of BTX in a Claus furnace. However, the presence of BTX in an acid gas stream does not have any noticeable detrimental effect on the Claus furnace. The effect is primarily seen in the catalytic section of the Claus process, as explained before. Thus, a BTX destruction unit between the Claus furnace and the catalytic units is required, where BTX can be oxidized. Since SO2 is already present in the hot exhaust gas from the furnace in high concentration, it is possible to oxidize BTX by SO2 if favorable process conditions, such as temperature and residence time, are maintained in the BTX destruction unit. An experimental study was conducted by Levy et al.,15 where the oxidation of benzene by SO2 was carried out in sealed Pyrex bulbs at temperatures in the range of 400−540 °C. The reaction between them was 10609

dx.doi.org/10.1021/ie501732a | Ind. Eng. Chem. Res. 2014, 53, 10608−10617


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Figure 2. Reaction mechanism for the destruction of phenyl radical (C6H5•) by SO2. The numbers given below the chemical species will be used for their identification in the paper.

chemical species, such as CO, SO, CO2, S, and COS, detected in the experimental studies, have been identified.

identify the multiplicity with a minimum energy, reasonable geometry and low spin contamination. All the calculations were performed using Gaussian 09 software.28 Spin contamination in open shell systems can significantly affect the energy, geometry and calculated spin density of the molecules.29 The expectation value of the spin operator Ŝ2, ⟨S2⟩, gives a measure of the amount of spin contamination introduced by a given level of theory. By observing ⟨S2⟩ for a number of stable species and transition states, it is shown in refs 21 and 22 and in this work that DFT introduces a negligible amount of spin contamination. The values of ⟨S2⟩ for all the chemical species and transition states involved in the proposed processes are presented in the Supporting Information.

2. COMPUTATIONAL DETAILS The ground state molecular structures of the stable chemical species as well as the transition states were found using density functional theory (DFT) with Becke three-parameter exchange and the Lee, Yang and Parr correlation (B3LYP) functional with the 6-311++G(d,p) basis set. For large molecules such as aromatic hydrocarbons, DFT is considered to be a standard choice, as it is computationally less expensive than higher levels of theory and has already been validated and used for aromatic hydrocarbons in previous studies. 20−27 The molecular structures were optimized with different spin multiplicities to 10610



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transition state for H-abstraction by HS was not found. Moreover, this reaction had a high reaction energy of +103 kJ/ mol, indicating that H-abstraction from benzene by HS is less likely. The rest of the paper presents a detailed mechanism for the reaction between phenyl radical (CS1) and SO2 (CS2), as shown in Figure 2. This reaction can proceed via two routes: (a) the addition of SO2 on the radical site of CS1 through one of the O atoms, and (b) the addition of SO2 on the radical site of CS1 through the S atom. The O-addition route was found to be more feasible than the S-addition one. There are two reasons for this. (a) Higher electronegativity of O than S: The electrophilicity of O atom is much higher than that of S atom. Thus, the electron-rich O atom in SO2 is more likely to attack the free radical site of CS1 than the electron-deficient S atom of SO2. (b) Higher stability of the chemical species formed when SO2 is added on CS1 through the O atom than that of the species formed when SO2 is added on CS1 through S atom. The energy difference between the two resultant chemical species was found to be about 60 kJ/mol. In view of these arguments, the progress of the mechanism only through the Oaddition route has been considered in this work. Some of the important functional groups that were found on the chemical species present in the reaction mechanism are shown in Figure 3. The entire mechanism, presented in Figure 2, has been divided into three pathways, as discussed below.

The rate constants (k) for the reactions involved in the proposed processes were evaluated with transition state theory using k(T ) =

⎛ −ΔEa ⎞ kBT Q ≠ exp⎜ ⎟ h Q AQ B ⎝ kBT ⎠

for bimolecular reaction, and k(T ) =

⎛ −ΔEa ⎞ kBT Q ≠ exp⎜ ⎟ h QA ⎝ kBT ⎠

for unimolecular reactions. In the above expressions, Q‡ is the total partition function of the transition state, QA and QB are the partition functions of the reactants A and B, ΔEa is the activation energy of the reaction, T is the temperature, h is the Planck’s constant, and kB is the Boltzmann’s constant. The total partition function is temperature dependent and is calculated as the product of translational, rotational, vibrational and electronic partition functions. The partition functions for the transition state and reactants were calculated at a range of temperatures (3003000 K) using the vibrational frequencies, moments of inertia, mass and electronic multiplicity, all of which are given by the quantum chemical calculations. A linear least-squares fitting algorithm was used to fit the modified Arrhenius expression (k = ATne(−E/RT)) to the data points of the rate constants in order to obtain the kinetic parameters of the frequency factor A, the temperature exponent n and the activation energy E. To assess the role of quantum tunnelling, in the literature, there are four efficient methods to evaluate the tunnelling correction factor (or, transmission constant): Wigner correction, Eckart correction, zero-curvature tunnelling correction, and small curvature tunnelling correction. At temperatures above 500 K, the correction factors from all the methods converge to similar values.30,31 In this paper, the Wigner method was employed to obtain the tunnelling correction factors for all the elementary reactions. The correction factor, CW(T), is expressed as

Figure 3. Some oxygen- and sulfur-containing functional groups formed during the progress of the reaction between phenyl radical and SO2.

2 1 ⎛ h ϑ≠ ⎞ C W (T ) = 1 − ⎜ ⎟ 24 ⎝ kBT ⎠

3.1. Pathway 1. Figure 4 presents the potential energy diagram for pathway 1 with the energies of the chemical species and the transition states relative to the energy of the reactants, C6H5• and SO2 (i.e., CS1 and CS2). This addition reaction forms a highly stable chemical species, CS3 after overcoming a small energy barrier of 6.4 kJ/mol. It is an exothermic reaction where an energy of 182 kJ/mol is released. Since SO2 addition on CS1 is very similar to the initiation reaction during CS1 oxidation by O2,32−34 a comparison between their mechanisms, wherever possible, is presented in this paper. Upon the barrierless addition of O2 to CS1, an energy of 177 kJ/mol is released,34 which is comparable to the reaction energy of 182 kJ/mol for CS1 + SO2 → CS3. For further reactions, CS3 can follow three possible channels: (a) the elimination of sulfur monoxide (SO) molecule, (b) the attack on a nearby C atom by the dangling O atom in CS3, and (c) the attack on a nearby C atom by the S atom in CS3. Possibility (a) is discussed here, while the discussions on possibilities (b) and (c) will be presented in the forthcoming sections. The removal of SO from CS3 leads to the formation of a phenoxy radical, CS4. The SO molecule, thus produced, is highly reactive, exists in a triplet state similar to O2, and plays a vital role in the combustion of sulfur compounds in a Claus

where ϑ‡ is the imaginary frequency of the transition state. The rate constant is then calculated as a product of CW(T) and k(T).

3. RESULTS AND DISCUSSION In the high temperature environment of Claus furnace, H atom removal from benzene to form phenyl radical can take place through H-abstraction by gas-phase radical species such as H, OH, and HS present in the Claus furnace in appreciable concentrations. Figure 1 presents the potential energy diagrams for H-abstraction reactions for H, OH, and HS. The Habstraction reaction by OH was found to be exothermic, while it was endothermic in the cases of H and HS. The Habstraction reaction from benzene by H atom was also studied in (22 and refs therein). The barrier height for this reaction was found to be 46 kJ/mol using B3LYP/6-31G(d,p) in,22 which agrees reasonably well with the barrier height of 50.9 kJ/mol found in this work. In ref 31 the H-abstraction from benzene by OH was studied, and the energy barriers of 12 kJ/mol from experiments and 14.7 kJ/mol using CBS-QB3 were found, which are close to 11.4 kJ/mol found in this work. The 10611



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Figure 4. Pathway 1. A potential energy diagram at 0 K showing the oxidation of phenyl radical (CS1) by SO2 to form cyclopentadienyl (CS7) as the end product.

variational transition state theory23 that, at most of the temperatures in the range of 300−3000 K, this reaction had an energy barrier of 74 kJ/mol, with the transition state structure having a CO−SO bond length of 2.3 Å (this bond length in CS3 was 1.69 Å). Upon the elimination of a sulfur-containing molecular fragment, the reactions appearing thereafter become identical to some reactions present in the CS1 oxidation mechanism by O 2 , 32,33,37−41 as can be seen in Figure 4. After the decomposition of CS4, a fused bicyclic cyclopropanone intermediate, CS5, is formed having a high energy and low stability. A C−C bond of the tricyclic ring of CS5 dissociates to form CS6, which, in turn, forms cyclopentadienyl (CS7) after the elimination of a CO molecule. The energetics for this pathway were also calculated using M06-2X/6-311++G(d,p), and the results were found to be similar to those presented in Figure 4. Figure S4 in the Supporting Information presents the energy diagram with the M06-2X/6-311++G(d,p) level of theory. 3.2. Pathway 2. This section provides an alternate pathway that CS3 can follow for the subsequent reactions after the attack of a nearby C atom by its dangling O atom, as shown in Figure 6. This attack leads to the formation of a chemical species, CS8, which is a result of resonance stabilization, as shown and explained below.

furnace. The phenoxy radical (CS4) can also be formed during the oxidation of CS1 by O2 through the breakage of the O−O bond.34−36 A high activation energy of 152 kJ/mol is required for this breakage reaction.23 On the other hand, during CS1 oxidation by SO2, the formation of such a radical (i.e., CS4) through the barrierless dissociation of CS3 appears to be a favorable reaction. Figure 5 presents the potential energy surface for this barrierless reaction, which was obtained through partial geometry optimization at different CO−SO bond lengths with spin multiplicity of 2. Note that, in Figure 4, the spin multiplicities used to determine energies of CS3, CS4, and SO were 2, 2 and 3, respectively. It was found through

The breakage of one of the bonds in SO gives the terminal O atom a free radical and places a lone pair of electrons over the S atom that can be easily accommodated in the low lying vacant d-orbitals of the S atom. The free radical on the terminal O atom then attacks the C atom at the ortho position of the aromatic ring to form a fused bicyclic structure, CS8. This reaction CS3 → CS8 requires a large energy barrier of 205.6 kJ/mol to be overcome. Moreover, it is an endothermic

Figure 5. Potential energy surface at 0 K for the elimination of SO from CS3 to form CS4 through CO−SO bond dissociation. The energies presented here are relative to the energy of CS3. 10612



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Figure 6. Pathway 2. Potential energy diagram at 0 K showing the formation of a pyran radical as one of the end products during the oxidation of CS1 by SO2. Route a, for the decomposition of CS9, is shown by dotted lines, while Route b is shown by solid lines.

Figure 7. Route c. Potential energy diagram at 0 K showing the formation of thiophene as one of the end products during the oxidation of CS1 by SO2.

reaction with reaction energy of 180.8 kJ/mol. The forward reaction for CS8 to form CS9 requires a small activation energy of 30.91 kJ/mol. The new species CS9 has higher stability than CS8. There are three possible routes (a, b and c) that CS9 can follow, as discussed below. Route a. Represented by dotted lines in Figure 6, this route leads to the loss of an SO molecule from CS9 to form CS4 after crossing an energy barrier of 30 kJ/mol. The further reactions of CS4 are already shown in Figure 4. Route b. Represented by solid lines in Figure 6, this route proceeds via the loss of S atom from CS9 through the breakage of the S−O bond, thus leaving two O atoms on the resultant species, CS10. The reaction CS9 → CS10 + S is highly endothermic with reaction energy of 239 kJ/mol and with no barrier for the reverse reaction. The released S atom, in the absence of O2, can form S2 by combining with another S atom.42 The decomposition of oxygenated species, CS10, thus formed, has already been studied by Tokmakov et al.34 Three possible products from the decomposition of CS10 were identified in ref 34. Out of the three, CS11 was the product requiring the lowest energy barrier (13 kJ/mol) through the migration of an O atom at the ortho-position in CS10 to the meta-position to form a bridged structure. The other two possible (but less likely) products (CS18 and CS25) and their possible decompositions have been shown in the Supporting

Information. The species CS11 lies 40 kJ below the energy of CS10 in the potential energy diagram. The low energy barrier for CS10 → CS11 indicates that the intermediate CS10 is a short-lived species. The dissociation of the C−C bond connecting the ortho- and meta-positions of CS11 forms 3oxypinyloxy, CS12. This molecule rearranges itself by crossing an energy barrier of 143.3 kJ/mol to form a pyran molecule with a terminal CO group in CS13. This reaction also involves the conversion of a 7-membered ring to a 6-membered ring. Thereafter, the elimination of the CO group takes place, which requires crossing an energy barrier of only 3 kJ/mol to form an energetically favored product, CS14 (a pyran radical). Figure S1 in the Supporting Information provides an alternate channel for CS11 involving the migration of the epoxide ring from the ortho-meta position to the meta−para position to form CS15 after crossing a high activation energy barrier of 181.8 kJ/mol. The C−O bond at the meta-position with respect to the carbonyl group dissociates to form CS16. This bond dissociation reaction involves a small activation energy of 30.8 kJ/mol. Thereafter, the elimination of the H atom from the para-position of CS16 after overcoming a high activation energy barrier of 93.4 kJ/mol forms a closed shell species, 1,4-benzoquinone (CS17). Compared to Route b in Figure 6 for CS11, a higher overall activation energy is involved in this channel. 10613



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Figure 8. Pathway 3. Potential energy diagram at 0 K showing the formation of cyclopentadienyl from CS3 as one of the end products during the oxidation of CS1 by SO2.

Figure 9. Potential energy diagram at 0 K showing the formation of cyclopentatriene as one of the end products during the oxidation of CS1 by SO2.

Route a. This route, shown by dotted lines in Figure 8, involves the barrierless elimination of the SO group and the formation of CS4, a chemical species that is also present in Pathway 1. The further reactions of CS4 are given in Figure 4. Route b. Represented by solid lines in Figure 8, this route involves the molecular rearrangement of CS33 to form a seven membered ring in CS34 with two exocyclic O atoms: one attached to a C atom as a carbonyl group, and the other with a S atom as a thionyl group. A high activation energy of 202 kJ/ mol is required for this reaction. The seven membered ring is converted to a six membered ring with a carbonyl group in CS35. However, a small activation energy of 17 kJ/mol for the backward reaction (CS34 → CS33) and a high activation energy of 183 kJ/mol for the forward reaction (CS34 → CS35) are required, indicating that CS35 will mainly be formed at high temperatures. Thereafter, CS35 can lose a CO molecule to form CS36. Similar to CS4, CS36 leads to the formation of a cyclopentadiene ring (CS37) with a SO group on it after crossing an energy barrier of 145 kJ/mol. The loss of the SO group from CS37 gives cyclopentadienyl (CS7). Route c. An alternate route for the progress of the mechanism beyond CS33 is presented in Figure 9. The attack of the C atom containing the carbonyl group in CS33 by the S radical forms a seven membered ring with a carbonyl and a thionyl group in CS39, but a high activation energy of 148 kJ/ mol is required. The further reactions of CS39 provide the

Route c. Figure 7 presents the potential energy diagram for an alternate route that the species CS9 may follow. The S atom may attack the β-carbon (with respect to the ketonic group) to form a fused bicyclic structure, CS27. The intermediate species, CS27 lies 44 kJ/mol above CS9. The breakage of the C−C bond common to the two fused rings leads to the formation of an eight-membered ring with a carbonyl group, and endocyclic oxygen and sulfur atoms after crossing an energy barrier of 133.2 kJ/mol. The CS28 formed as a result lies 38.5 kJ/mol above CS27 but can undergo molecular rearrangement to form a seven membered ring with an exocyclic CO group, CS29. This requires an activation energy of 116.1 kJ/mol to take place. The elimination of CO from CS29 results in a stable intermediate (CS30) lying 107.9 kJ/mol below it. CS30 undergoes molecular rearrangement to form a 5-membered ring with an endocyclic S atom and an exocyclic CHO group in CS31. Thereafter, the CHO group desorbs from CS31, resulting in the formation of thiophene (CS32) after overcoming an energy barrier of 94 kJ/mol. 3.3. Pathway 3. An alternate pathway for CS3 is presented in Figure 8. It involves the migration of the SO group on CS3 to the C atom at the ortho-position, resulting in the formation of CS33. A high energy barrier of 202 kJ/mol is required to be overcome for this reaction. The further decomposition of CS33 can take place through three routes, as discussed below. 10614



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possible channels for the formation of carbonyl sulfide (COS), a chemical species found readily in the Claus furnace. The first channel, shown by solid lines, involves the migration of an H atom from a nearby C atom to the O radical site of the thionyl group to form a sulphenyl radical group in CS40. Thereafter, the migration of a OH molecule to a nearby C atom forms CS41, which is a highly stable molecule, lying 307.7 kJ/mol below CS40. The molecular rearrangement in CS41 gives a fused bicyclic structure in CS42, having a five membered ring with a dangling OH group and a four membered ring with a carbonyl group. The breakage of the C−S bond in the fourmembered ring of CS42 forms a metastable intermediate, CS43, which has very low forward as well as backward activation energies. The loss of a COS molecule requires a small activation energy of 25 kJ/mol to be overcome. The resultant species, CS44 (cyclopenta-2,4-diene-1-ol), can further lose the OH group to form CS45 (cyclopenta-1,2,3-triene). Another channel for the formation of COS from CS39 has been shown by dotted lines in Figure 9. It involves the elimination of an O atom from the thionyl group to form CS46. This barrierless reaction is highly endothermic with a reaction energy of 240 kJ/mol. A molecular rearrangement in CS46 forms CS47, which has a five membered ring with an exocyclic COS group. The energy barrier for this step was found to be 175 kJ/mol. A high overall activation energy of 415 kJ/mol (corresponding to the energies of CS39 and TS48) involved in this channel makes it unfavorable. The COS group in CS47 can be eliminated by crossing a small energy barrier of 8 kJ/mol to form cyclopentadienyl (CS7). Out of the two channels presented here for CS39, the former one is more favorable for the release of COS than the latter one. 3.4. Reaction Rate Constants. Tables S2 and S3 in the Supporting Information provide the high pressure limit rate constants for the elementary reactions involved in Habstraction from benzene and in the oxidation of phenyl radical by SO2, as discussed in the previous sections. For the barrierless reactions (CS3 → CS4 + SO, CS9 → CS10 + S, CS33 → CS4 + SO, and CS37 → CS7 + SO), variational transition state theory was used to determine their rate constants, and the method is described in our previous work.21 For two such reactions (CS44 → CS45 + OH and CS39 → CS46 + O), partial optimization could not be carried out successfully, and therefore, their rate constants are not provided. Figure 10 provides the comparison between the rate constants for the addition of SO2 and O2 on CS1 since its oxidation by SO2 and O2 proceeds in a very similar fashion. The rate constant for the addition of O2 on CS1 to form phenylperoxy (C6H5O2) was obtained from ref 23. The difference between the two rate constants diminished with increasing temperature. It may appear that SO2 addition on CS1 is only competitive at temperatures above 1100 K (827 °C) where both the rates are within an order of a magnitude. However, given that SO2 is present in a high concentration in the anoxic region of the Claus furnace with sufficiently high temperature, the reactions between SO2 and benzene may be possible. This needs to be verified in the future by carrying out flame simulations. Figure 11 provides the comparison between the rate constants for three possible pathways for CS3 discussed in this paper. It is clear that the elimination of SO through the breakage of the CO−SO bond in CS3 (i.e., Pathway 1 shown in Figure 4) is more favorable than the others at all the

Figure 10. Comparison between the rate constants for the additions of O2 and SO2 on the radical site of CS1.

Figure 11. Comparison between the rate constants for the three possible destruction reactions for CS3 presented in this work.

temperatures studied in this work for the destruction of phenyl radical by SO2. At high temperatures, benzene oxidation by O2 proceeds through O−O bond scission in C6H5O2. Its oxidation by SO2 most preferably takes place through CO-SO bond scission in CS3. Figure 12 presents a comparison between the rate constants for these two bond scission reactions. Clearly, the breakage of the dangling OSO chain can take place more easily in CS3 than the breakage of the O−O bond in C6H5O2. This behavior can be attributed to the high electronegativity of the two O atoms present on either side of the S atom in CS3 that weakens the CO−SO bond. As discussed before, along with benzene, toluene and xylene are also present as contaminants in the H2S gas stream. The mechanisms for their destruction by SO2 as well as the further 10615



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and those of S−O and O−O bond breakage reactions, indicate that phenyl radical oxidation by SO2 may be competitive above 1100 K. These findings need to be verified through detailed simulations.

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ASSOCIATED CONTENT

S Supporting Information *

The energies, coordinates, vibrational frequencies, moments of inertia, and spin multiplicities for all the chemical species and transition states as well as the energy diagrams for the reactions of CS10 and CS11, the rate constants of all the elementary reactions, and the structures of transition states. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 12. Comparison between the rate constants for the breakage of the O−O bond in pnenylperoxy (C6H5O2) and the CO−SO bond in CS3.

ACKNOWLEDGMENTS This work has been financially supported by the Gas Processing and Materials Science Research Centre, The Petroleum Institute, UAE.

decomposition of 5-membered rings formed during benzene oxidation are required to be studied in the future to determine, through simulations, the optimal process conditions for BTX destruction by SO2. Moreover, along with BTX and SO2, other gas-phase species present in the exhaust gas from the Claus furnace, such as H2O, O2, N2, CO2, and sulfur, may affect BTX−SO2 interactions, and should be investigated.

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REFERENCES

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4. CONCLUSION A detailed mechanism for the reaction between SO2 and phenyl radical using the B3LYP/6-311++G(d,p) level of theory was presented to investigate the possibility of the destruction of benzene by SO2 at high temperatures. The addition of SO2 on phenyl radical was found to be highly exothermic with a reaction energy of 182 kJ/mol, and it required a very small activation energy barrier of 6.45 kJ/mol to be overcome. This indicates that their interaction is energetically favored. Three possible pathways for the further reactions were studied, and it was found that the breakage of the CO−SO bond leading to SO elimination is highly likely to take place. Thereafter, the remaining O atom can carry out the oxidation of benzene through CO removal. This was found to be very similar to the pathway for benzene oxidation by O2 at high temperatures, where the O−O bond breaks in phenylperoxy (C6H5O2) to facilitate the progress of benzene oxidation. The rate constant for CO−SO bond scission was found to be higher than that for O−O bond scission. This is possibly due to the high electronegativity of O atoms present on either side of the S atom, which weakens the CO−SO bond. Several routes for the formation of commonly observed species in the Claus furnace, such as COS, CO, CO2, OH, HCO, SO, and S, were identified. All the pathways had comparable overall activation energies, with Pathway 1 having the lowest. The energy profiles may indicate that CO and SO would be formed in higher concentrations than the other species mentioned above, since their elimination requires lower overall activation energy than that of the other products. Further, the comparisons of the rate constant profiles of SO2 and O2 additions on phenyl radical, 10616



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