Ind. Eng. Chem. Res. 2003, 42, 3943-3951
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Kinetic Study of the Pyrolysis of H2S Manuel Binoist,* Bernard Labe´ gorre, and Franck Monnet Air Liquide Claude-Delorme Research Center, 1, Chemin de la porte des Loges, 78354 Jouy en Josas Cedex, France
Peter D. Clark, Norman I. Dowling, and M. Huang Alberta Sulphur Research Ltd., Department of Chemistry, University of Calgary, 2500 University Drive N.W., Calgary, Alberta, Canada T2N 1N4
Damien Archambault,† Edouard Plasari,‡ and Paul-Marie Marquaire*,† De´ partement de Chimie-Physique des Re´ actions, UMR 7630 CNRS, and Laboratoire des Sciences du Ge´ nie Chimique, UMR 6811 CNRS, ENSIC - INPL, 1 Rue Grandville, BP451, 54001 Nancy Cedex, France
The pyrolysis of hydrogen sulfide has been studied at residence times between 0.4 and 1.6 s and in the temperature range of 800-1100 °C. A continuous perfectly mixed quartz reactor was used to acquire kinetic data on the thermal dissociation of hydrogen sulfide and elemental sulfur mixtures diluted in argon (95 vol %). The kinetic auto-acceleration effect of sulfur is demonstrated. A detailed radical mechanism is written to account for the experimental results, in particular for the auto-acceleration effect, and validated against experimental results. This pyrolysis kinetic scheme is the first step and core of a complete detailed mechanism capable of modeling the various oxidation reactions encountered in an industrial Claus furnace. Introduction The thermal stage of the Claus process promotes the conversion of H2S into elemental sulfur by a controlled oxidation at high temperature (800-1500 °C). This process includes several simultaneous and coupled reactions such as oxidation, H2S + O2 f SO2 + H2O; a redox reaction, H2S + SO2 f S2 + H2O; and pyrolysis, H2S f S2 + H2. The overall global reaction (2H2S + O2 f S2 + 2H2O) is, in reality, a complex radical mechanism with many elementary processes where various reactive systems are coupled by common intermediate radicals. To limit the complexity of the reaction schemes, the various reactive systems are usually studied separately. The pyrolysis of H2S, which is part of the core of the general Claus process mechanism, produces H2 and S2. This reaction is equilibrium-limited (42% at 1000 °C for a dilute 5% H2S stream), but the equilibrium position is reached in a few seconds above 1000 °C. Several global rate expressions have been proposed to account for H2S pyrolysis that are compatible with the mass action law and with the orders of reaction observed for the direct and reverse reactions (H2 + S2 f H2S). However, these more or less complex rate expressions do not take into account all of the data available in the literature and, in particular, the autoacceleration of H2S pyrolysis that has been observed experimentally (S-curve). The effects of the different parameters remain poorly understood and have not been quantified. * To whom correspondence should be addressed. E-mail:
[email protected] (M.B.), Paul-Marie.Marquaire@ ensic.inpl-nancy.fr (P.-M. M.). † De ´ partement de Chimie-Physique des Re´actions, UMR 7630 CNRS. ‡ Laboratoire des Sciences du Ge ´ nie Chimique, UMR 6811 CNRS.
The aim of the study presented here was to investigate H2S pyrolysis in detail and to establish a detailed radical mechanism of the reaction. Using the knowledge of the impact of each elemental process, the mechanism could then be deduced and efficiently coupled into a fluid mechanism package to rigorously model H2S dissociation in the Claus furnace. Literature Review Experimental Studies. High-temperature gas-phase pyrolysis of H2S has been studied extensively in continuous flow “isothermal” tubular reactors1-5 and in shock tubes.6-9 The overall kinetics of these systems often yield conflicting results even for the form of the rate expressions. Adesina et al.3 and Tesner et al.2 have suggested a first-order process for the H2S decomposition reaction. Kaloidas and Papayannakos1 have also suggested first order for H2S decomposition, whereas for the rate of reverse reaction, they have proposed first order for H2 and one-half order for S2. The reverse reaction involving H2 and S2 has also been studied by Dowling et al.10 over the temperature range from 600 to 1300 °C. These workers observed experimental orders of 1 for both H2 and S2. In 1999, Dowling and Clark11 proposed a new reversible kinetic model with the expressions -rH2S ) kd[H2S][S2]1/2 and -rH2 ) kr[H2][S2] that is consistent with the equilibrium law. The presence of the term [S2] in the decomposition rate expression is extremely important because it leads to the acceleration of the reaction by S2. However, in the absence of S2 in the initial gas mixture, another mechanism is necessary to explain the initial formation of S2 during the induction stage of H2S pyrolysis. Modeling Studies. No specific studies on the modeling by detailed radical mechanisms of the H2S pyrolysis are reported in the literature. Furthermore, the oxidation of H2S, which is more complex than H2S pyrolysis,
10.1021/ie021012r CCC: $25.00 © 2003 American Chemical Society Published on Web 07/22/2003
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Figure 1. Expanded view of quartz CSTR-furnace assembly.
can be successfully modeled only by detailed radical mechanisms. Several detailed radical mechanisms are given in the literature, and a discussion of the key reactions of the oxidation mechanism was presented by Tsuchiya et al.12 The kinetics and mechanisms of the oxidation of gaseous sulfur compounds were recently reviewed by Hynes and Wine.13 The contribution of the pyrolysis reaction to the H2S oxidation mechanism can be extracted from the global reaction of pyrolysis, H2S ) H2 + S2, but it is usually neglected in an overall oxidation mechanism. Thus, without a systematic review of all of the elementary reactions, some important pyrolysis reactions might be missing in models for H2S oxidation. The H2S oxidation model of Frenklach et al.14 includes 57 elementary reactions among 17 species, and the pyrolysis processes are reduced to 5 reactions. The model proposed by Basevich et al.15 appears to be more complete. It includes 146 elementary reactions (104 direct and 42 with reverse) and 22 species; the pyrolysis part comprises 13 reactions, with 8 of them being reversible. We will test these two pyrolysis mechanisms against our experimental data. Experimental Section Reactor and Hydrodynamics. As mentioned above, until now, all studies concerning the pyrolysis of H2S have been carried out using tubular reactors and shock tubes. These reactors were considered to be plug-flow reactors. The use of small tubular reactors for kinetic studies in the gas phase has some inconveniences. First, the hydrodynamic regime is usually laminar, even for very low residence time values, so the assumption of plug flow in most cases is not fulfilled. Second, the temperature profile is usually not constant. As kinetics are extremely sensitive to the temperature, small fluctuations along the reactor lead to inaccurate kinetic parameters. Third, outlet concentrations of the reagents, products, and intermediates result from the integration of the gradients of concentrations along the reactors under different chemical compositions. For a very complex reaction scheme, the use of tubular reactors involves a high degree of uncertainty because different chemical pathways could correspond to the same outlet concentration results for a given residence time. In this
study, a continuous flow, jet-mixed reactor as proposed by Matras and Villermaux16 and David and Matras17 has been used (Figure 1). According to Chambon et al.18 and Marquaire et al.,19 a continuous perfectly mixed spherical reactor can successfully be used for kinetic studies of many complex reactions. In this type of reactor, the composition of the outlet flow corresponds to the gas-phase composition in the whole reactor volume. As previously validated,16,17 homogeneity within a spherical continuous perfectly mixed reactor can be realized by injecting the initial gaseous reagents in the form of free jets for mixing in the interior volume of the vessel. The reactor designers have also given the relations determining the zone of perfectly mixed conditions as a function of geometrical and operating parameters such as the gas-phase density and viscosity, the diameters of the reactor and of the inlet tubes forming gaseous jets, and the mean residence time. The reactor was constructed from quartz to avoid any catalytic effects. In fact, Adesina et al.3 used a quartz reactor near 1050 °C to study H2S pyrolysis and concluded that quartz surfaces are noncatalytic. In contrast, Harvey et al.4 used an alumina reactor at about 1200 °C and concluded that the dissociation of H2S took place predominantly on the alumina surface. Quench Section. The study of the high-temperature pyrolysis of hydrogen sulfide needs a very rapid (about 3 ms) preheating of initial reagents at the inlet and a very rapid cooling (quenching) of gaseous products at the outlet of the reactor to avoid back reaction between H2 and sulfur vapor.5 On the basis of the work of Houzelot and Villermaux,20 the quench section used in this study was a cylindrical annular heat exchanger maintained at 125°C by means of a circulating constanttemperature bath using silicone oil (rated to 200 °C) as the heat-transfer medium. The efficiency of the quench operation was estimated in the most disadvantageous case (mean residence time of 400 ms) where the outlet gas temperature was decreased from 1200 to 300 °C. The calculations gave a quench time of 4.9 ms, which represents approximately 1% of the mean residence time. Also, a simple calculation showed that, even for a reaction with a low activation energy (e.g., 10 kcal mol-1 in the worst case),
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Figure 2. Schematic of the high-temperature laboratory reactor system.
decreasing the temperature from 1200 to 300 °C led to decrease the reaction rate by a factor of 215. This demonstrates the high efficiency of the quench cooling system. Overall Setup and Operating Conditions. The high-temperature laboratory reactor system is shown schematically in Figure 2. The unit was housed in a high-temperature single-zone laboratory furnace (Lindberg 1500 °C model) with an operating range up to 1500 °C and an overall hot-zone length of 305 mm. The perfectly mixed reactor bulb was positioned at the far side of the hot zone across from the inlet, allowing the feed gases to undergo preheating to the reactor temperature on transiting the chamber. The time spent by the gases within this annular preheat section corresponded to only a small fraction (∼10%) of the time in the bulb. The reactor temperature was set to the required value (800-1100 ( 2 °C) using a K-type thermocouple inserted to the midpoint of the bulb. In this study, H2S pyrolysis was studied under isothermal conditions within the small reactor bulb at a high dilution ratio in argon (molar dilution ratio H2S/ Ar ) 1:20) to avoid transfer limitations and for residence times from 0.4 to 1.6 s. Metered flows of pure H2S and Ar were blended so as to provide the required stream of 5% H2S, 95% Ar with an accuracy of (0.1% ((1% in the metered flows) and were fed directly to the inlet of the reactor. When sulfur vapor was added to the feed, a portion of the argon flow was passed through a sulfur vapor saturator, maintained at a temperature between 300-350 °C, and reblended with the remaining part of the feed in a heated inlet section. The concentration of sulfur (mole percent as S2) within the feed was determined by gravimetric means from separate experiments with known molar flow rate of argon, time of flow, and weight of collected sulfur. These experiments were also performed with the same overall dilution ratio using a feed of 3.34% H2S, 1.67% S2, and 95% Ar (molar dilution ratio S2/H2S/Ar ) 1:2:20). Gas Analysis. Samples of the product gases were collected on a sulfur-free basis from the exit of the quench section and were analyzed by gas chromatography to determine the conversion of H2S. This analysis was performed simultaneously for H2 and residual H2S using a fixed-volume dual-loop injector and dual chromatographs equipped with thermal conductivity detectors. H2 was analyzed on a 5A molecular sieve column using argon as the carrier gas, and H2S was analyzed on a Chromosorb 108 column using helium. Experimental Results. Data from the H2S dissociation experiments run under dilute conditions with a 5% H2S, 95% Ar stream are presented in Tables 1 and 2. These results are tabulated relative to the nominal residence time for a temperature of 900 °C for concise-
Table 1. Experimental Results for the H2S Dissociation Reaction (5% H2S, 95% Ar): H2S Conversion (%) temp (°C)
residence timea (ms) 450
600
900
1500
R
0.4 (1640) 2.6 (1567) 4.4 (1500) 13.1 (1451) 24.9 (1416) 40.7 (1382) 50.4 (1330)
18.6 23.5 29.3 34.3 38.1 41.9 48.1
1200
800 0.2 (492) 0.0 (984) 850 0.8 (470) 1.2 (627) 1.8 (940) 2.0 (1253) 900 1.0 (450) 1.9 (600) 2.1 (900) 2.4 (1200) 940 5.0 (435) 5.7 (580) 9.1 (870) 10.7 (1160) 970 11.2 (425) 13.0 (566) 16.7 (849) 20.4 (1132) 1000 22.4 (415) 24.8 (553) 30.0 (829) 36.7 (1106) 1050 45.6 (399) 47.7 (532) 49.2 (798) 50.7 (1064)
a Nominal residence time based on a temperature of 900°C. Actual residence times are given in parentheses.
Table 2. Comparison of Experimental and Equilibrium Conversions for H2S Dissociation at Long Residence Times (5% H2S, 95% Ar): H2S Conversion (%) residence timea (s) temp (°C)
5.0
10.0
15.0
R
900 1000 1100
18.6 41.4 53.8
26.3 40.2 53.3
31.6 -
29.3 41.9 54.1
a Nominal residence time at a temperature of 900°C. Corresponding residence times at 1000 and 1100°C: 4.6, 4.3 and 9.2, 8.5 s, respectively.
ness. All of the experiments within each column were performed with the same overall feed flow rate, but actual residence times for each experiment, taking temperature changes into account, are provided in parentheses. Further experiments were also conducted at lower temperatures of 600 and 700 °C. They showed no conversion, indicating that essentially no dissociation of H2S occurs at temperatures up to 800 °C for residence times as long as 1.5 s. Table 1 also lists the predicted equilibrium conversion (R) at each of the temperatures under study (far right column). These values can be compared with the experimental data set, showing that, in the temperature and residence time range studied, the measured conversions at all but the highest temperature of 1050 °C are kinetically controlled. The acquired data set was therefore deemed to be an adequate basis on which to carry out the kinetic modeling described later in this paper. Experiments with longer residence times on the order of 5-15 s were also performed as a means to validate the experimental apparatus. These data are presented in Table 2 as a comparison of the experimentally measured and equilibrium calculated conversions for H2S dissociation at 900-1100 °C. The data in Table 2 show that good experimental agreement was observed with the equilibrium predictions in the appropriate temperature range, validating the design of the apparatus (reactor + quench section). No reaction between H2 and S2 takes place downstream of the reactor outlet.
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Table 3. Experimental Results for the H2S Dissociation Reaction in the Presence of S2 (3.34% H2S, 1.67% S2, 95% Ar): H2S Conversion (%) residence timea (ms)
temp (°C)
450
600
900
1200
1500
R
850 900 940 970
9.0 (462) 14.7 (443) 20.4 (428) 22.4 (418)
10.2 (617) 15.8 (590) 22.2 (570) 25.3 (557)
10.6 (925) 16.5 (885) 23.9 (856) 27.6 (835)
11.6 (1232) 16.5 (1180) 24.2 (1141) 28.3 (1113)
12.0 (1541) 16.6 (1475) 24.3 (1427) 29.0 (1393)
14.6 20.1 25.1 29.0
a Nominal residence time based on a temperature of 900°C. Actual residence times are given in parentheses.
Table 4. Results of H2S Dissociation in Equivalent Quartz and Alumina Reactorsa (5% H2S, 95% Ar) % H2S temp (°C)
tr (ms)
1100
43 128 40 119
quartz reactor 9.1 28.5 33.9 63.9
54.1 64.7 -
43 128 40 119
alumina reactor 18.9 44.6 55.6 62.2
54.1 64.7 -
1200
1100 1200 a
experimental
equilibrium
Annular plug-flow reactor design.
Data for the corresponding kinetic experiments on H2S dissociation carried out in the presence of added S2 are tabulated in Table 3. The results show that the rate of H2S pyrolysis is clearly accelerated in the presence of S2. Thus, substantially more reaction was observed at 850 °C over the entire residence time range of ∼0.5-1.5 s than without S2 in the feed, and equilibrium was further achieved at the lower temperature of 970 °C after only ∼1.4 s. This aspect of the kinetic behavior of the system is more fully explored in the discussion of the reaction mechanism and successfully incorporated into the kinetic modeling of the reaction. A study complementary to the main kinetic evaluation was carried out using equivalent reactors fabricated out of quartz and alumina. This study was used to examine any potential difference in the effects of these materials on the kinetics of the H2S pyrolysis reaction and, hence, to determine the suitability of alumina as a reactor material for higher-temperature studies. The reactor design used in this case was that of an annular plugflow type, since fabrication of the more complex continuously perfectly stirred reactor from alumina was not possible. Both reactors were similarly designed with an integrated quench section and 0.8-mm annular clearance. Fabrication of the quartz reactor presented no difficulty; however, special-order alumina tubes (Vesuvius McDanel, Beaver Falls, PA) were required for assembly of the alumina reactor. The data collected at 1100 and 1200 °C for both reactors, together with the calculated equilibrium values, are presented in Table 4. These data show that the corresponding conversion of H2S in the alumina reactor is always higher than that in the quartz system, except for the 119-ms residence time condition at 1200 °C. In this case, equilibrium is achieved in both reactors, as the conversion is similar to the predicted value of 64.7%. The consistently higher conversions of H2S in the alumina reactor operated under kinetic control suggests that the reactor walls exert a catalytic effect, making
Table 5. Detailed Mechanism of H2S Pyrolysis
H2S + M ) H + SH + M H2S + M ) H2 + S + M H + SH ) H2 + S reverse H + H2S ) H2 + SH reverse H2S + S + M ) H2S2 + M reverse SH + H2S2 ) H2S + HS2 reverse SH + HS2 ) H2S + S2 reverse SH + S + M ) HS2 + M reverse H + S2 + M ) HS2 + M reverse SH + S ) H + S2 reverse SH + SH + M ) H2S2 + M reverse SH + S2 ) S + HS2 H2S2 + M ) H + HS2 + M H2S + S ) H + HS2 SH + SH ) H2S + S reverse S + S + M ) S2 + M HS2 + HS2 ) H2S2 + S2 H2S2 + H ) H2S + SH H + HS2 ) H2S + S H + HS2 ) H2 + S2 reverse SH + M ) S + H + M reverse S + H2S2 ) SH + HS2
A (cm3‚mol-1‚s-1)
n
E (cal‚mol-1)
no.
1.76 × 1015 2.39 × 1015 1.29 × 1013 2.70 × 1014 2.90 × 1014 1.18 × 1014 3.60 × 1012 3.00 × 1020 5.20 × 1014 6.80 × 1014 2.00 × 1013 4.80 × 1013 6.00 × 1011 6.00 × 1012 1.00 × 1018 1.00 × 1017 8.97 × 1015 6.98 × 1016 3.20 × 1013 2.70 × 1022 1.40 × 1012 9.10 × 1019 1.80 × 1013 0.75 × 108 2.29 × 108 1.00 × 1015 6.00 × 1012 6.00 × 1012 6.00 × 1013 4.30 × 1013 4.72 × 1013 6.00 × 1012 3.60 × 1015 6.00 × 1012
0.0 0.0 0.0 0.0 0.0 0.0 0.0 -1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -1.0 0.0 -1.0 0.0 1.14 1.30 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
66 200 60 250 7210 21 030 13 000 27 100 0.0 52 800 6100 29 700 4700 38 400 0.0 70 900 1510 31 070 15 000 31 300 0.0 58 600 33 700 70 200 25 800 100 17 100 0.0 9100 5200 8400 1400 57 610 73 600 9300 8200
R1 R2 R3 R-3 R4 R-4 R5 R-5 R6 R-6 R7 R-7 R8 R-8 R9 R-9 R10 R-10 R11 R-11 R12 R13 R14 R15 R-15 R16 R17 R18 R19 R20 R-20 R21 R-21 R22
this material unsuitable as a substitute for quartz in higher-temperature studies. Kinetic Modeling First, the pyrolysis components of the H2S oxidation models developed by Frenklach et al.14 and Basevich et al.15 were tested. Overall, we reached the conclusion that the simulation of H2S pyrolysis using these models was not in agreement with our experimental data. Assuming that our experimental results are valid, we postulated that the discrepancy arises from missing reactions and/or incorrect kinetic or thermochemical parameters in the previous models. A new detailed mechanism has been written and is shown in Table 5. Development of the Mechanism of H2S Pyrolysis. To construct a mechanism of H2S pyrolysis in a systematic way, the first step was to determine the primary mechanism (only the formation of primary products). In the second step, the secondary mechanism, during which molecules formed by the primary mechanism react to form other molecules and new radicals, is taken into consideration. The initiation of the radical reaction is the dissociation of an HS-H bond, indexed as reaction R1 (reaction numbers correspond to the final mechanism presented in Table 5). The H and SH radicals can then react with the initial reagent (reaction R4). When all possible reactions of metathesis are exhausted, the reactions of radical decomposition by β-scission have to be considered. These reactions are not considered in the H2S pyrolysis scheme because the radicals are too small (mono- or diatomic) to undergo such reactions. Then, the radicals react with one another by the processes of recombination (reaction R11), taking into account that
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Figure 3. H2S pyrolysis: H2S conversion.
the recombination of free atoms (here H and S) is generally negligible. The primary mechanism of consumption of the reagent is written below.
Primary mechanism H2S + M f H + SH + M
(R1)
H + H2S f H2 + SH
(R4)
SH + SH + M f H2S2 + M
(R11)
To generate the secondary mechanism, it is assumed that the molecules formed by the primary mechanism react with other species following the same rules as the ones used to write the primary mechanism.
Secondary mechanism Reactions of H2S2 H2S2 + M f H + HS2 + M
(R13)
H2S2 + M f SH + SH + M
(R-11)
(reverse of reaction R11) H2S2 + SH f H2S + HS2
(R6)
HS2 + SH f H2S + S2
(R7)
HS2 + M f H + S2 + M
(R-9)
Reactions of H2 H2 + SH f H2S + H
(R-4)
The mechanism of pyrolysis thus obtained includes seven reactions with two of them being reversible. From the literature,7,8 it appears that another initiation could occur by the elimination of S: H2S + M f H2 + S + M. This reaction has also been included in the mechanism (see Table 5), and its kinetic parameters have been obtained in the high-temperature range of 18872891 K by Woiki and Roth7 from concentration measurements of H and S atoms by ARAS spectroscopy and also by Tsuchiya et al.12 This mechanism was completed by the reaction of the biradical S, which can be formed in other reactions such as H + S2 f S + SH. The complete mechanism is presented in Table 5. It includes
22 reactions of which 15 are reversible. The initial kinetic parameters are essentially from Basevitch et al.,15 Chnerysheva et al.,21 and the NIST database,22 and the thermochemical data used are from Burcat and McBride’s database.23 A comparison between the proposed mechanism and that of Basevitch et al.15 reveals the following differences: (1) Basevich et al. do not consider the decomposition reaction of the radical HS2 by β-scission (reaction R-9 of the secondary mechanism). We included this reaction in Table 5 by writing this process in a reversible way, i.e., reactions R9 and R-9. (2) Initiation by the elimination of S (H2S + M f H2 + S + M) is not included by Basevitch et al. (3) In Basevich et al., the value of the rate constant is erroneous for the reaction S + H2 ) SH + H (same kinetic parameters for the direct and reverse reactions). For this reaction, we will use the value proposed by the NIST database. Validation of the Mechanism. A simulation of our mechanism, using the software CHEMKIN II,24 was compared to the experimental results. Some minor adjustments to the rate parameters were used to improve the fit between the model and the experimental results. The values of the parameters used are given in Table 5. Good qualitative agreement (order of magnitude and shape of the curves) is obtained with the experimental results of the pyrolysis of pure H2S at five temperature values (Figures 3-5). Simulations of the pyrolysis of H2S in the presence of S2 also fit the experimental results (Figures 6-8). As shown in Figure 3, the conversion is predicted accurately by the postulated mechanism with an index of correlation of 0.97. In addition, Figures 4 and 5 show that the H2 and S2 production curves are in good agreement with the experimental data. Here, it should be noted that the experimental quantity of S2 is obtained by a material balance on S, which is less precise because it considers all possible species present as S2. In the presence of S2, the results of simulations (Figures 6-8) show that the mechanism of pyrolysis proposed in this study describes the phenomenon of rate acceleration by S2. This effect is amplified when the temperature increases. Thus, S2 is the species responsible for the auto-acceleration of H2S dissociation. This effect also explains that equilibrium is rapidly reached. The good agreement obtained between the simulations and the experiments in the whole range of the
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Figure 4. H2S pyrolysis: H2 formation.
Figure 5. H2S pyrolysis: S2 formation.
Figure 6. H2S pyrolysis with S2: H2S conversion.
operating conditions (residence time, temperature, and initial amount of S2) provides a good validation of the mechanism of H2S pyrolysis. Kinetic Analysis and Mechanism Reduction. We have also carried out an analysis of flows, rates, and
sensitivities using the developed simulations. Simulations were performed at 1000 °C for two mean residence time values, 0.1 and 1 s, because the chemistry is different at low conversion. Thus, it is possible to identify the main reaction paths for a better under-
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Figure 7. H2S pyrolysis with S2: H2 production.
Figure 8. H2S pyrolysis with S2: S2 production.
standing of the reaction with the aim of developing a reduced mechanism. Behavior at Low Mean Residence Time (on the Order of 0.1 s). Under these conditions, the process is initiated by the two reactions
H2S + M f H + SH + M
(R1)
H2S + M f H2 + S + M
(R2)
These two reactions represent 17% of the overall H2S consumption (3% for reaction R1 and 14% for reaction R2). Sulfur is accumulated as shown by reactions R10 (SH + S f H + S2) and R14 (H2S + S f H + HS2) followed by reaction R-9 (HS2 + M f H + S2 + M). These processes allow a sufficient concentration of S2 to accumulate, thus promoting the catalytic effect of S2 on H2S pyrolysis. Behavior at High Mean Residence Time (on the Order of 1 s). In this case, reactions R1 and R2, which are consumers of H2S (forward direction) at low residence time, are reversed and produce H2S. Thus, there is an inversion of the direction of reaction for these processes when the residence time increases.
Under these conditions, the initiation is now obtained by a sequence of the two reactions R-7 and R-9
H2 S + S2 f SH + HS2
(R-7)
HS2 + M f H + S2 + M
(R-9)
S2
H2S + M 98 H + SH + M
(R1)
From the total balance of these two reactions, it is seen that introduction of H2S into the reaction sequence occurs by a reaction with S2. A flow rate sensitivity analysis indicates that the rate of reaction of R-7 is identical to the rate of reaction of R-9, which means that the quantity of sulfur consumed by the first reaction is replaced by the second reaction. Overall, it is concluded that there is a catalytic effect of sulfur (S2) and that the HS2 radical is a very reactive intermediate at low concentration. From a detailed analysis of the molar fractions and sensitivity analysis, it is seen that H2S2 is a very minor product, with a concentration approximately 100 times lower than those of the radicals S and H. The reactions in which this species occurs are negligible for the production or the consumption of other species. Thus,
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Figure 9. Main reaction pathways in the pyrolysis of hydrogen sulfide. Table 6. Reduced Mechanism of the H2S Pyrolysis
H2S + M ) H + SH + M H2S + M ) H2 + S + M H + SH ) H2 + S reverse H + H2S ) H2 + SH reverse SH + HS2 ) H2S + S2 reverse H + S2 + M ) HS2 + M reverse SH + S ) H + S2 reverse H2S + S ) H + HS2 SH + SH ) H2S + S reverse
A (cm3‚mol-1‚s-1)
n
E (cal‚mol-1)
no.
1.76 × 1015 2.39 × 1015 1.29 × 1013 2.70 × 1014 2.90 × 1014 1.18 × 1014 2.00 × 1013 4.80 × 1013 1.00 × 1018 1.00 × 1017 8.97 × 1015 6.98 × 1016 1.80 × 1013 0.75 × 108 2.29 × 108
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.14 1.30
66 200 60 250 7210 21 030 13 000 27 100 4700 38 400 1510 31 070 15 000 31 300 25 800 100 17 100
R1 R2 R3 R-3 R4 R-4 R7 R-7 R9 R-9 R10 R-10 R14 R15 R-15
Table 7. Comparison of the Simulation of H2S Conversion and the Outlet Molar Concentrations of H2 and S2 Using Detailed and Reduced Mechanisms (Mean Residence Time ) 1 s, T ) 1000 °C)
detailed mechanism reduced mechanism
H2S conversion (%)
H2 (%)
S2 (%)
36.7 36.7
1.91 1.91
0.92 0.92
the species H2S2 and its corresponding reactions can be removed without significantly affecting the overall results. Thus, it is possible to reduce the reaction mechanism to seven species and nine reactions (see Table 6). The flow diagram of the reduced mechanism, giving the main reaction pathways for the dissociation of hydrogen sulfide, is represented in Figure 9. A comparison between the simulations obtained by the detailed and reduced mechanisms confirms the validity of this analysis as shown by the results obtained at a temperature of 1000 °C and a mean residence time of 1 s (see Table 7). Conclusions The pyrolysis of H2S has been studied using a continuous flow, perfectly mixed reactor, specifically designed for kinetic studies. A comparison of the new experimental data with previously published kinetic models of radical mechanisms for H2S dissociation shows significant discrepancies. A detailed mechanism has been developed to account for all of the different phenomena involved in H2S pyrolysis. This model is in good agreement with the new experimental data. Global kinetic models can represent H2S pyrolysis in a narrow range of experimental conditions, but a detailed mechanism is necessary to account for large
variations of experimental conditions and for the influence of co-reactants, such as S2. Only a detailed radical mechanism can be used to model coupled reactions (pyrolysis, oxidation, redox reactions of H2S) because these reactive systems are linked by common intermediate radicals and by cross reactions. This detailed mechanism is reduced in accordance with data for low and/or high mean residence time conditions. This reduced mechanism gives essentially the same results as the detailed one. The development and validation of the detailed kinetic mechanism of H2S pyrolysis is the first step in the development of a rigorous Claus furnace kinetic model. The pyrolysis model is the core of a general oxidation mechanism for hydrogen sulfide. Of course, this model must be efficiently coupled with fluid mechanics and thermal effects to correctly account for the various physical and chemical phenomena occurring in a sulfur plant Claus reaction furnace. Accurate simulations of the Claus furnace’s chemical and physical phenomena can allow one to optimize the design of new plants or to increase the capacity of existing units by taking advantage of oversized equipment or by implementing oxygen enrichment technologies. In both cases, a detailed knowledge of the composition and temperature profiles inside the combustion region is critical to guarantee the reliability of the equipment and the performance of the plant under the new operating conditions. Literature Cited (1) Kaloidas, V.; Papayannakos, N. Kinetics of thermal noncatalytic decomposition of hydrogen sulphide. Chem. Eng. Sci. 1989, 44, 2493-2500. (2) Tesner, P. A.; Neemirovskii, M. S.; Motyl, D. N. Kinetics of the thermal decomposition of hydrogen sulfide at 600-1200 °C. Kinet. Katal. 1989, 31, 1232-1235. (3) Adesina, A. A.; Meeyoo, V.; Foulds, G. Thermolysis of hydrogen sulphide in an open tubular reactor. Int. J. Hydrogen Energy 1995, 20, 777-783. (4) Harvey, W. S.; Davidson, J. H.; Fletcher, E. A. Thermolysis of Hydrogen Sulfide in the temperature range 1350-1600 K. Ind. Eng. Chem. Res. 1998, 37, 2323-2332. (5) Hawboldt, K. A.; Monnery, W. D.; Svrcek, W. Y. New experimental data and kinetic rate expression for H2S pyrolysis and re-association. Chem Eng Sci 2000, 55, 957-966. (6) Bowman, C. T.; Dodge, L. G. Kinetics on the Themal Decomposition of Hydrogen Sulfide Behind Shock Waves. Symp. Combust. [Proc.] 1976, 16, 971-982. (7) Woiki, D.; Roth, P.; Kinetics of the High-Temperature H2S Decomposition. J. Phys. Chem. 1994, 98, 12958-12963. (8) Olschewski, H. A.; Troe, J.; Wagner, H. G. UV absortion study of the thermal decomposition reaction H2S f H2 + S(3P). J. Phys. Chem. 1994, 98, 12964-12967. (9) Shiina, H.; Oya, M.; Yamashita, K.; Miyoshi, A.; Matsui, H. Kinetic studies on the pyrolysis of H2S. J. Phys. Chem. 1996, 100, 2136-2140. (10) Dowling, N. I.; Hyne, J. B.; Brown, D. M. Kinetics of the reaction between hydrogen and sulfur under high-temperature Claus furnace conditions. Ind. Eng. Chem. Res. 1990, 29, 23272332. (11) Dowling, N. I.; Clark, P. D. Kinetic modeling of the reaction between hydrogen and sulfur and opposing H2S decomposition at high temperatures. Ind. Eng. Chem. Res. 1999, 38, 1369-1375. (12) Tsuchiya, K.; Kamiya, K.; Matsui, H. Studies on the oxidation mechanism of H2S based on direct examination of the key reactions. Int. J. Chem. Kinet. 1997, 29, 57-66. (13) Hynes, A. J.; Wine, P. H. Kinetics and mechanisms of the oxidation of gaseous sulfur compounds. In Gas-Phase Combustion Chemistry; Gardiner, W. C., Ed.; Springer-Verlag: New York, 2000; pp 343-382. (14) Frenklach, M.; Lee, J. H.; White, J. N.; Gardiner, W. C., Jr. Oxidation of hydrogen sulfide. Combust. Flame 1981, 41, 1-16.
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Received for review December 11, 2002 Revised manuscript received May 29, 2003 Accepted June 3, 2003 IE021012R
