Kinetic Simulation of Acid Gas - American Chemical Society

Jun 6, 2016 - Salisu Ibrahim and Abhijeet Raj*. Department of Chemical Engineering, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab ...
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Kinetic Simulation of Acid Gas (H2S and CO2) Destruction for Simultaneous Syngas and Sulfur Recovery Salisu Ibrahim and Abhijeet Raj* Department of Chemical Engineering, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates ABSTRACT: With tight regulations on the permissible sulfur content in transportation fuels, the oil and gas industry is mandated to adopt reliable and cost-effective desulfurization and acid gas (H2S and CO2) treatment techniques. At present, the Claus process is used to treat acid gas to recover sulfur, but the hydrogen content in H2S is wasted as low grade steam. A viable method of acid gas treatment is the simultaneous production of valuable syngas (H2 and CO) and sulfur through the thermal destruction of H2S and CO2. However, the optimum process conditions and blends of H2S and CO2 to increase syngas production are not known. In this paper, a detailed reaction mechanism is developed for the simultaneous decomposition of H2S and CO2 and is validated using different sets of experimental data. The effects of the relative composition of H2S and CO2 in the acid gas and the process conditions on the production of syngas and sulfur during thermolysis are reported. A synergistic effect is observed in the simultaneous decomposition of H2S and CO2, i.e., CO2 decomposition to CO is enhanced in the presence of H2S, and H2S conversion is improved in the presence of CO2. The temporal evolution of unwanted sulfurous compounds (SO2 and COS) are also studied at different residence times and temperatures. The major reactions leading to syngas formation during acid gas decomposition are identified. The developed kinetic model can facilitate the design and optimization of the syngas and sulfur recovery process from acid gas.

1. INTRODUCTION The oil and gas industry is facing increased dependence on sour feedstocks (sulfur-bearing fuels) and tightening regulations on the maximum allowable levels of sulfur content in transportation fuels.1 Hydrogen sulfide (H2S) and carbon dioxide (CO2), also known as acid gas, are generated in large volumes as byproducts during the desulfurization of crude oil and natural gas. Acid gas also contains other impurities such as N2, NH3, CS2, COS, and hydrocarbons in small quantities. It poses a significant threat to human health, process equipment, and the environment.1,2 Consequently, its emission into the atmosphere is strictly regulated worldwide. To treat the acid gas streams, the Claus process is used, where sulfur is recovered.3 However, this process suffers from low thermal stage efficiency due to variation in acid gas composition from associated impurities.4,5 Although significant efforts have been made to improve the Claus process efficiency, it is also necessary to develop alternative and cost-effective means of utilizing acid gas.3−7 This has triggered significant interest in improving our understanding of sulfur and H2S chemistry through modeling and experimental efforts. The simultaneous production of H2 and sulfur from H2S pyrolysis has been examined in several studies to improve H2 yield and to provide better understanding of the reaction kinetics involved in this process.8−16 In ref 15, Binoist and coworkers reported a reaction mechanism and experimental data on the pyrolysis of H2S in diluted Ar at residence times of 0.4− 1.6 s and at temperatures of 1073−1375 K. Manenti and coworkers16 later revised this mechanism to improve the match between the modeling results and the experimental data. Since acid gas often contains a large volume of CO2, a more practical alternative would be to utilize both H2S and CO2 © XXXX American Chemical Society

simultaneously to recover syngas (H2 and CO) and sulfur. Syngas is a valuable commodity that can be used as a fuel in gas engines or as a reactant to produce valuable chemicals such as ammonia and liquid fuels.17,18 This is also well suited for the treatment of lean acid (with high CO2 content) gas that is known to cause serious technical problems in Claus process plants.19 The production of syngas from acid gas has been discussed in the literature both theoretically and experimentally.17,18,20 El-Melih et al.20 showed experimentally that pyrolysis of acid gas is well suited for the treatment of lean acid gas, which is difficult to treat in the Claus process. They used a gas chromatograph (GC) to quantify the syngas evolved at the exit of a laboratory-scale plug flow reactor over a temperature range of 1250−1475 K and a high residence time of 2 s in the reactor. The results revealed the importance of reactor temperature and acid gas composition on the conversion of H2S and CO2 to produce syngas with a wide range of H2 to CO ratio. The presence of CO2 in acid gas enhanced H2S conversion and CO production while reducing H2 production. However, this study mainly focused on the effect of reactor temperature and inlet CO2 concentration on the amounts of syngas produced at the reactor exit. The temporal evolution of syngas along the reactor length (residence time) was not examined. Such information can provide valuable insight into the detailed chemistry that occurs during the acid gas conversion process to facilitate reactor design and optimization. Received: March 27, 2016 Revised: May 31, 2016 Accepted: June 6, 2016

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DOI: 10.1021/acs.iecr.6b01176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. H2S Pyrolysis Reactionsa

The residence time and temperature in the reactor are critical parameters that can affect optimal reactor dimensions for high syngas yield.17,20,21 The production of unwanted sulfurous compounds (such as SO2, S2O, and COS) is another important issue to consider during the design and optimization of acid gas treatment units, as their formation can reduce the acid gas conversion efficiency and increase the emission of sulfurous byproducts to the environment.22−24 The theoretical characterization of the acid gas conversion process using a detailed and validated reaction mechanism can improve our understanding of the chemistry involved in syngas production and can assist in reactor design, scale-up, and optimization. This paper presents a kinetic modeling study of the simultaneous decomposition of H2S and CO2 for syngas production, for which a detailed reaction mechanism containing pyrolysis, oxidation, and organo-sulfur reactions has been developed. The developed mechanism has been validated by comparing modeling results with different sets of experimental data to ensure that the chemistry of H 2 S and CO 2 decomposition and COS and SO2 formation is effectively captured in the mechanism. Simulations are further conducted to obtain insight into the temporal evolution of syngas and to predict the concentrations of unwanted sulfur compounds (SO2, S2O, CS2, and COS) at different residence times and temperatures.

2. DEVELOPMENT OF REACTION MECHANISM The detailed mechanism consisting of elementary reactions for H2S, CO2, CS2, and COS combustion was developed by adopting reactions from several studies.15,16,21−29 The pyrolysis reactions of H2S were obtained from the works of Binoist et al.,15 Manenti et al.,16 Zhou et al.,21 and Cong et al.22 The oxidation reactions of H2S were taken from the Leeds mechanism23 and the mechanisms presented in Cerru et al.24 and Shabin et al.25 The chemistry sets on the formation of COS and CS2 were adopted from systematic studies reported in refs 25−28 for the Claus furnace. The thermodynamic properties of the species involved in the elementary reactions were obtained from the studies reported in refs 23 and 29. The resulting list of elementary reactions in the developed mechanism for H2S pyrolysis and oxidation, CO2 decomposition, and COS/CS2 production is presented in Tables 1−3.

1.76 2.00 1.08 8.30 6.02 2.00 1.87 3.20 4.58 6.20 2.00 1.20 4.80 1.15 5.16 1.50 1.28 1.00 1.40 1.10 4.19 1.50 1.00 1.20 2.00

× × × × × × × × × × × × × × × × × × × × × × × × ×

1015 1014 1011 1013 1012 1013 1018 1015 1019 1016 1014 1017 1013 1025 1014 1013 1014 1014 1015 1013 1008 1008 1016 1007 1013

n

E

0.0 0.0 0.0 0.0 0.0 0.0 −1.0 0.0 −1.4 −0.6 0.0 −1.0 0.0 −2.8 0.0 0.0 0.0 0.0 1.0 0.4 1.6 1.6 0.0 2.1 0.0

64000.0 66000.0 2969.7 2052.7 4968.0 7400.1 0.0 0.0 104380.0 0.0 76600.0 0.0 77103.9 1665.0 21000.0 0.0 0.0 430.0 57030.0 210.0 472.0 2149.6 0.0 715.4 7400.0

a The rate constants are expressed as k = ATn exp (−E/RT), and the units of the kinetic parameters A, n, and E are in mol, cal, K, cm, and s.

The CSTR simulations were carried out using the perfectly stirred reactor (PSR) model present in the Chemkin Pro software and the developed reaction mechanism. With a reactor volume of 33.3 cm3, the mass flow rates ranging from 0.00834 to 0.0302 g/s provided the residence times of 0.4−1.45 s. The governing equations for CSTR that are solved in Chemkin Pro can be found in refs 30 and 31. Figure 1 presents a comparison between the experimental data and the simulation results using the developed mechanism. A good agreement between them for the profiles of H2S conversion and H2 and S2 production was found at different temperatures and residence times. The complex chemistry of H2S conversion at high temperatures (above 1273 K) could also be captured using the developed mechanism, where the other kinetic models for H2S pyrolysis in the literature are less accurate.15,16 Acid Gas Pyrolysis. In a previous study,20 El-Melih et al. presented an experimental study on syngas production from the thermal decomposition of H2S and CO2 at different temperatures in a plug flow reactor at a high residence time of 2 s. Figure 2 shows the comparison of simulation results alongside the experimental data from ref 20. Clearly, the kinetic model satisfactorily predicts the experimentally observed profiles of syngas, H2, and CO during the simultaneous decomposition of the two reactants, though a small difference between the experimental and the simulated concentrations was observed at a low reactor temperature of 1373 K. The uncertainty in experimental data was not available. SO2 and COS Formation. As mentioned before, during the conversion of acid gas to syngas and sulfur, SO2 and COS are formed as byproducts from H2S oxidation and CO−sulfur interactions. The mechanism is expected to predict their concentrations reliably since their formation affect sulfur and

3. RESULTS AND DISCUSSION 3.1. Validation of Reaction Mechanism. To ensure that the developed reaction mechanism captures the complex chemistry of H2S and CO2 decomposition as well as production of syngas and sulfur reliably, a varied set of experimental data from the literature related to H2 S, CO 2 , CO, and H 2 combustion and/or pyrolysis were selected for model validation. All the kinetic simulations and reaction pathway analysis were conducted using Chemkin Pro software. H2S Pyrolysis. Binoist et al.15 reported experimental data on the pyrolysis of H2S in a continuously stirred tank reactor (CSTR) at different temperatures and residence times. The H2S conversion, i.e., the amount of H2S consumed in the reactor, is defined on mole basis as H 2S conversion (%) =

A

reactions H2S + M = SH + H + M H2S + M = H2 + S + M H2S + H = SH + H2 H2S + S → SH+SH H2S + S = H2 + S2 H2S + S = HS2 + H H + H + M = H2 + M H + H + H = H2 + H H2 + M = H + H + M S + H + M = SH + M S + H2 → SH + H S + S + M → S2 + M S2 + M → 2S + M S2 + H + M = HSS + M SH + H → H2 + S SH + S = H + S2 SH + SH = S2 + H2 SH + SH → H2S + S HSSH + M → SH + SH + M HSS + H = SH + SH HSS + H = H2 + S2 HSS + H = H2S + S HS2 + H + M = H2S2 + M H2S2 + H = HS2 + H2 H2S2 + S = HS2 + SH

(moles of H 2S at inlet − moles of H 2S at outlet) × 100 moles of H 2S at inlet B

DOI: 10.1021/acs.iecr.6b01176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 2. H2S Oxidation Reactionsa A

reactions H2S + O2 = HSO + OH H2S + OH = SH + H2O H2S + HSO = SH + HSOH H2S + HOS = SH + HSOH H2S + SO = HSO + SH SH + OH = HOS + H SH + HSO = S + HSOH H2 + S2O = SH + HOS SH + SO = HSO + S HSSO2 + M = SH + SO2 + M HSO + S2 = HSS + SO HSO + H = SH + OH HSO + OH = H2O + SO HOS + O = H + SO2 HSSO + O = SH + SO2 HSSO + H = S2O + H2 HSSO + OH = S2O + H2O HSSO + S = HSS + SO HSSO + HSS = S2O + HSSH HSSO + HO2 = S2O + H2O2 S2O + HSO2 = HSSO + SO2 S2O + H + M = HSSO + M SO + H + M = HSO + M SO* + M =SO + M SO* + O2 = SO2 + O SO + O(+M) = SO2(+M) SO + HO2 = SO2 + OH SO + SO + M = OSSO + M S + SO2 → 2SO HSO2 + O = SO2 + OH HSO2 + OH = SO2 +H2O HSO2 + SH = SO2 + H2S HSO2 + S = SO2 + SH SO + SH = S2 + OH S2O + O = 2SO S2O + S = SO + S2 S2O + SH = HSO + S2 HSS + O = SH + SO HSSH + O = HSO + SH HSSH + OH = HSS + H2O HSOH + H = HSO + H2

Table 3. CO2, CS2, and COS Reactionsa

1.00 8.70 1.00 1.00 5.38 1.00 1.00 1.00 1.00 1.00 1.00 4.90 1.70 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 6.42 5.00 1.00 1.00 3.20 3.70 3.23 5.88 1.00 1.00 1.00 1.00 1.00 9.27 1.00 1.00 1.00 1.00 1.00 1.00

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

1011 1013 1013 1013 1003 1013 1011 1013 1013 1017 1012 1019 1009 1014 1013 1013 1013 1013 1013 1013 1013 1022 1015 1013 1013 1013 1003 1032 1012 1013 1013 1013 1013 1012 1011 1013 1012 1014 1014 1014 1014

n

E

reactions

0.0 −0.7 0.0 0.0 3.2 0.0 0.0 0.0 0.0 0.0 0.0 −1.9 1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 −2.6 0.0 0.0 0.0 0.0 2.4 −5.8 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

49100.0 0.0 17300.0 12500.0 26824.0 7400.0 11000.0 46000.0 25000.0 3000.0 3000.0 1560.0 470.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 32000.0 286.6 0.0 0.0 0.0 0.0 7660.0 3044.2 9034.0 0.0 0.0 0.0 0.0 4320.0 0.0 0.0 5000.0 0.0 0.0 0.0 0.0

CO2 + M = CO + O + M CO2 + O = CO + O2 COS + O = CO + SO CO + S2 = COS + S S + COS = S2 + CO O + COS = CO2 + S HCO + M = H + CO + M HCO + H = CO + H2 HCO + O = CO + OH HCO + O = CO2 + H HCO + OH = CO + H2O HCO + HCO = CH2O + CO HCO + H2O = CO + H + H2O CH2O + M = HCO + H + M CH2O + OH = HCO + H2O OH + CO = H + CO2 CO + O = CO2 H + H + CO2 = H2 + CO2 C + SO2 = CO + SO C + H2S = CH + SH O + CS = CO + S COS + M = CO + S + M CH + SO = CO + SH SO2 + CO = SO + CO2 H + HCS = CS + H2 SH + CS = H + CS2

A 6.06 1.24 2.00 4.77 2.95 5.00 5.70 7.34 3.02 3.00 1.02 1.80 2.24 3.31 7.59 6.76 5.90 5.50 4.16 1.20 1.63 1.43 1.00 2.70 1.21 3.23

× × × × × × × × × × × × × × × × × × × × × × × × × ×

1013 1010 1013 1013 1008 1008 1011 1013 1013 1013 1014 1013 1018 1016 1012 1007 1015 1020 1013 1014 1014 1015 1013 1012 1014 1010

n

E

0.0 0.0 0.0 0.0 0.0 0.0 0.7 0.0 0.0 0.0 0.0 0.0 −1.0 0.0 0.0 1.1 0.0 −2.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.5

104445.5 43830.0 7385.0 13314.3 3404.0 10990.1 14870.0 0.0 0.0 0.0 0.0 0.0 17000.0 81000.0 170.0 70.0 4100.0 0.0 0.0 8843.7 1510.3 61007.1 0.0 48289.0 0.0 495.0

a

The rate constants are expressed as k = ATnexp (−E/RT), and the units of the kinetic parameters A, n, and E are in mol, cal, K, cm, and s.

At all temperatures, the model could successfully predict the experimental observation, indicating that the mechanism contains adequate reactions for COS formation. 3.2. Acid Gas Simulations. After validating the mechanism for acid gas pyrolysis and oxidation, reactor simulations were performed to examine the effect of acid gas composition (H2S/ CO2/N2), temperature, and residence time on the production of syngas and unwanted sulfurous compounds (SO2 and COS). A temperature range of 1373−1800 K, residence times between 0 and 2 s, and a pressure of 1 atm were selected for simulations that reflect typical process conditions in a Claus furnace. Effects of Acid Gas Composition and Temperature. Figures 5 and 6 present the effect of varying H2S concentration in H2S/ CO2/N2 feed on the decomposition of CO2 to CO and on the concentrations of reaction products at different temperatures for a residence time of 2 s. For the simulation results reported in these figures, the concentration of CO2 was fixed to 5%, while the amounts of H2S and N2 in the inlet acid gas were changed. The decomposition of CO2 is an endothermic process that is only favored at high temperatures to produce CO and O radicals. As shown in Figure 5, when H2S was not present in feed, CO2 decomposition was only about 0.3% at 1800 K. By increasing the amounts of H2S in feed from 0.5% to 3%, CO2 conversion increased by 10%−40% depending upon the temperature (1400−1800 K). Clearly, CO2 decomposition is greatly enhanced in the presence of H2S in a pyrolytic environment. This is because H2S decomposition and partial oxidation generate radical species such as H, SO, and S that react with CO2 to enhance its decomposition to CO (discussed later). In line with the CO2 profiles, the amount of CO produced increases with increasing temperature as well as with

a

The rate constants are expressed as k = ATn exp (−E/RT), and the units of the kinetic parameters A, n, and E are in mol, cal, K, cm, and s.

CO yields. In ref 27, Chin et al. have reported the experimentally observed species concentrations during the combustion of 1.65 mol % H2S with 1 mol % O2 in a plug flow reactor at different residence times (0−1.4 s) and temperatures. The experimental data from this study was used to predict SO2 formation from H2S oxidation at different temperatures of 1273, 1373, and 1473 K, as shown in Figure 3. As evident, both quantitative and qualitative trends, observed in experiments, were satisfactorily captured with the mechanism. Previous experimental studies have showed that CO is the precursor to COS formation.26,27 In ref 26, Karan et al. reported experimental data on the formation of COS from the reaction of CO with S2 in a plug flow reactor at different temperatures. Figure 4 compares the simulation results with the experimental data on COS concentration within a temperature range of 1173−1423 K and for reactor lengths of 6.4 and 16 m. C

DOI: 10.1021/acs.iecr.6b01176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. Comparison between simulated and experimental data, reproduced with permission from El-Melih et al.20 (2016, Elsevier) on syngas production for feed containing 3% H2S/2% CO2/95% N2 at 1 atm.

decomposition) and consumption (due to their oxidation by O atoms donated by CO2 and their recombination to form H2S) in the reactor, as shown later. Syngas (H2 + CO) production and H2/CO ratio profiles are also presented for different temperatures and H2S concentrations in Figure 5, where syngas concentration and the H2/ CO ratio are shown to be increasing with the increase in H2S concentration in feed. The utilization of syngas for different applications is highly dependent on the ratio of H2 to CO (Table 4). A ratio of 0.36−1.1 is required to power Siemens gas engines, while this ratio should range from 1 to 5 for liquid fuel production.18 For H2 and ammonia production, the complete removal of CO is necessary. The total content of inert gases such as N2 and Ar should often be as low as possible (preferably less than 2%) due to economic reasons, except in ammonia synthesis, where 25% nitrogen in feed is required.32 Thus, before utilizing syngas produced from acid gas, a purification (mainly desulphurization) process is required for the removal of impurities such as H2S, COS, SO2, and N2, and gas conditioning for the adjustment of the H2 to CO ratio depending on the desired application.32 Syngas can also be conditioned via the oxidation of CO to CO2 (a water−gas shift reaction) in a dedicated converter to alter the H2 to CO ratio or to essentially remove CO.32 In addition to the required H2 to CO ratio, syngas to be used in gas engines and turbines for power generation is required to have a minimum lower heating value of about 4−9 kJ/g and a high degree of purity due to

Figure 1. Comparison between experimental data (points) from Bionist et al.15 and model predictions (lines) on H2S conversion and H2 and S2 production for feed containing 5% H2S/95% Ar at 1 atm.

the increase in the amount of H2S in feed. While H2S conversion increased with increasing temperatures, it decreased with increasing concentration of H2S in feed, owing to its production in the reactor from the reactions of H2 and S2, when the concentrations of these products increased in the reactor. The profiles of H2 and S2 are shown in Figures 5 and 6, respectively, where at a given temperature their concentrations increased with increasing H2S concentration in feed. With up to 10% H2S in feed and for the temperature range studied in this work, the H2 profiles showed a slight reduction in its concentration with increasing temperature, but after reaching a minimum value, its concentration increased with further rise in temperature. With 40% H2S in feed, the H2 profile showed a continuous increase in its concentration with increasing temperature. While S2 concentration increased initially with increasing temperature and then decreased slightly for H2S concentrations up to 10%, S2 concentration increased continuously with increasing temperature for H2S concentration of 40%. Such trends in the profiles of H2 and S2 are a result of competing reactions for their formation (from H2S D

DOI: 10.1021/acs.iecr.6b01176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. Comparison between simulated results and experimental data on COS production from Karan et al.26 using different reaction lengths for feed containing 5% CO/0.9% S2/94.1% Ar.

undesired products (SO2 and COS) through CO2 separation from acid gas using techniques discussed in the literature.33−35 Figures 7−9 show the effect of the presence of CO2 on the conversion of H2S and the concentrations of products at different reactor temperatures for a residence time of 2 s. For the simulations, feeds containing 5% H2S with varying amounts of CO2 (0.5%−80%) and N2 were used. An increase in H2S and decrease in CO2 conversion with increasing CO2 concentration in feed can be observed. For a given temperature, the increase in H2S conversion with increasing CO2 concentration was due to its partial oxidation by the oxidizers generated from CO2 decomposition to form SOx, as explained later. Increasing temperature increased H2S and CO2 conversions. With increasing CO2 concentration, while H2 concentration decreased at a given temperature due to its oxidation, CO concentration increased. The syngas concentration is shown in Figure 8 to increase with increasing CO2 addition. Thus, a high amount of usable syngas with different H2 to CO ratios can be produced depending on the composition of CO2 in acid gas. This further confirms that acid gas thermolysis offers a great alternative for the treatment of lean acid gas (with higher CO2 content than H2S) that poses technical problems such as flame extinction in sulfur recovery plants. The S2 profiles, shown in Figure 8, exhibit a complex behavior with increasing CO2 concentration and temperature. At low CO2 levels (0%−5%), an increase in CO2 concentration in feed slightly increased S2 production due to enhanced H2S

Figure 3. Comparison between simulated and experimental data, reproduced with permission from Chin et al.27 (2016, John Wiley and Sons transaction) on SO2 production at different temperatures for feed containing 1.6% H2S/1% O2/97.4% N2.

strict emission regulations, though extensive syngas gas treatment is not required in boilers used for steam cycles.18,32 Figure 6 shows the profile of the undesired products, COS and SO2, that are produced in low concentrations during acid gas pyrolysis. The decomposition of COS to CO and S is responsible for the decrease in its concentration at high temperatures. With increasing concentration of H2S in feed, SO2 concentration was found to decrease due to the preferential consumption of O atoms by H2 (produced in high amounts for high H2S concentrations in feed) that reduces S2 oxidation. The H2S content in acid gas can be increased to enhance the desired H2 and CO production while minimizing E

DOI: 10.1021/acs.iecr.6b01176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. Profiles of CO2 and H2S conversion, CO, H2, and syngas production, and H2/CO ratios at different temperatures, and H2S concentrations for feed containing H2S/5% CO2/N2.

Table 4. Syngas Ratio for Different Applications no.

syngas application

H2:CO ratio

1 2 3 4 5

methanol production Fischer−Tropsch process synthetic natural gas production oxo-alcohols production gas engines (Siemens)

1−4 0.6−2 1.5−3 1−1.5 0.36−1.1

conversion. At such CO2 levels, S2 concentration increased with increasing temperature. However, at higher CO2 levels in feed (above 5%), while the S2 concentration increased marginally with increasing CO2 concentration at low temperatures (below 1373 K), the increase in temperature reduced the amount of S2 produced due to its oxidation to form SO2 and COS (Figure 9). When CO2 concentration was increased at a given temperature, SO2 and COS concentrations increased. Besides, traces of S2O (0.14−2.5 ppm) and CS2 (0.002−0.25 ppm) were observed at high temperatures (1700−1800 K) with CO2 concentrations of 40%−80% in the acid gas. These unwanted sulfur compounds present in syngas can be effectively captured using desulfurization processes commonly found in sulfur recovery plants. Numerous proven technologies exist that can be used for the effective cleaning of syngas containing sulfur impurities, such as (a) amine extraction of H2S and CO2 using alkanol amines such as monoethanolamide (MEA), diethanolamine (DEA), and methyl-diethanolamine (MDEA), (b) wet scrubbers, (c) spray dry scrubbers, (d) sorbent injection, and (e) regenerable processes. The detailed reviews on these technologies can be found in ref 2 and 3. Effect of Residence Time at Different Temperatures. Figure 10 presents the effect of residence time on the thermal

Figure 6. Profiles of S2, COS, and SO2 at different temperatures and H2S concentration for feed containing H2S/5% CO2/N2.

F

DOI: 10.1021/acs.iecr.6b01176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 8. Profiles of syngas, H2/CO ratios, and S2 formation at different temperatures and CO2 concentration for feed containing 5% H2S/CO2/N2.

Figure 7. Profiles of H2S and CO2 conversions and H2 and CO production at different temperatures and CO2 concentration for feed containing 5% H2S/CO2/N2.

decomposition of the reactants, H2S, and CO2 at different temperatures. At all the temperatures considered, H2S conversion reached its maximum value within a residence time of about 0.6 s and did not vary much after that. At a low temperature of 1373 K, only about 13% of CO2 decomposed into CO and O over a residence time of 2 s, and the increase in CO2 conversion with time was almost linear. As the temperature was increased, CO2 conversion increased, and the time required to reach the maximum value of conversion decreased. Figure 11 presents the temporal profiles of H2, CO, syngas, S2, and O atoms at different temperatures. While CO profiles were in line with the profiles of CO2 conversion, H2 profiles differed significantly from the H2S conversion profiles. At all the temperatures, the H2 concentration reached a maximum value at low residence time, and then it decreased with increasing time. This was primarily due to the oxidation of H2 by O atoms released during CO2 decomposition. It is shown in Figure 11 that O atoms are present at the initial stage of the acid gas conversion process (i.e., low residence times) at all the temperatures. The difference between the maximum H2 concentration and its concentration at the exit of the reactor remained within 0.9−1.4 mol % at all the temperatures

Figure 9. Production profiles of COS and SO2 at different temperatures and CO2 concentration for feed containing 5% H2S/ CO2/N2.

considered in this work. The low reactor temperatures (1323 and 1373 K) did not support H2 oxidation by O atoms, and a negligible decrease in its concentration after reaching its maximum value was observed. The O atom concentrations at low temperatures (1323 and 1373 K) increased continuously, G

DOI: 10.1021/acs.iecr.6b01176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 10. H2S and CO2 conversion profiles at different reactor temperatures and residence times for feed containing 5% H2S/3% CO2/92% N2.

while at higher temperatures, its concentration decreased to very low values due to their involvement in H2S and H2 oxidation. Figure 11 also presents the amounts of S2 produced from the given feed. The profiles at all the temperatures complement the H2S conversion profiles, where increasing the reactor temperature increased S2 concentration and decreased the residence time required to reach its maximum value. Figure 12 shows the profiles of SO2 and COS at different reactor temperatures and residence times. An increase in the residence time and temperature (above 1550 K) led to enhanced formation of SO2 when CO2 conversion was above 70%. With increasing temperature, the residence time required to reach the maximum value of SO2 concentration reduced. The COS concentration increased continuously with increasing residence time for temperatures up to 1500 K. However, with further increase in temperature, the maximum value of COS concentration and the residence time required to reach this maximum value decreased. This trend in COS profiles is attributed to the occurrence of competing formation and consumption reactions in the reactor (discussed later). Mechanistic Pathways of H2S and CO2 Conversion. The most favorable reaction pathways involved in the acid gas conversion process were identified through the reaction path analysis. A schematic of the major pathways involved in H2S decomposition (reactions R1−R12) is shown in Figure 13. The conversion of H2S occurred thermally via its collision with other molecules in the gas phase (M) to generate SH and H radicals (reaction R1). These radical species further attack the H2S molecules to produce several other radicals (see reactions R2−R9). The production of H2 and S2 was observed to occur predominantly via radical recombination reactions (reactions R8 and R10) rather than directly from H2S thermal and chemical decomposition pathways (reactions R2 and R4). Increasing the reactor temperature enhanced the reaction rates that resulted in increased radical production and, subsequently, H2 and S2 production as the reaction progressed with time. The decomposition of CO2 had a significant impact on the

Figure 11. Production profiles of H2, CO, syngas, S2, and O atoms at different reactor temperatures and residence times for feed containing 5% H2S/3% CO2/92% N2.

enhanced conversion of H2S as well as decomposition of the H2 produced. Figure 14 shows a schematic of the major reaction pathways involved in the H2S and CO2 conversion processes (R1−R21). The decomposition of CO2 occurred through the thermal and chemical pathways to release oxygenated atoms in the reactor (reactions R1−R3). The release of O and OH radicals then promoted the oxidation of H2S, H2, and other radicals formed in the reactor (reactions R7). Thus, the decrease in H2 production is due to reaction with available oxidizers (OH and O radicals) emanating from CO2 decomposition. The higher reactor temperature enhanced the conversion of CO2 and, subsequently, caused a decrease in the amounts of H2 produced with higher residence times of the reactor. The produced oxygenated sulfur radicals did not only oxidize H2 and H2S but also reacted with CO and CO2 to trigger SO2 and COS production. H

DOI: 10.1021/acs.iecr.6b01176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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production. Therefore, the rate of COS production is expected to increase with higher reactor temperatures. However, it was noted that reaction 23 had a significant impact on the decrease in COS concentration at higher residence times and temperatures. Reactions R3, R11, and R12 are competing reactions that contributed to the observed trends of COS mole fractions. The production of SO2 occurred through the reactions of oxygenated sulfur species formed in the reactor (reactions R17−R20). The formation of SO and S2O, which predominantly occurred through the oxidation of sulfur (reactions R14 and R16−R21), had significant impact on SO2 concentration in the reactor. Clearly, the low concentrations of SxOy species at low reactor temperatures (1373 K) were due to the limited availability of oxygen atoms from low conversion of CO2 (of about 10%). This implies that the concentration of oxygenated sulfur radicals can be reduced by controlling the amounts of oxygen atoms in the reactor. The mechanism presented in this study could be used to obtain an optimum blend of H2S and CO2 depending upon the desired H2/CO ratio, allowed SO2 concentrations in the product stream, and the availability of an energy source. The amount of energy required to maintain a high temperature in the pyrolytic reactor could be high. Table 5 lists the energy

Figure 12. Sulfur dioxide (SO2) and carbonyl sulfide (COS) production profiles at different reactor temperatures for feed containing 5% H2S/3% CO2/92% N2..

Table 5. Energy Requirements for Pyrolytic Reactor at Different Temperatures T, K

required energy, kJ/mol

1323 1373 1500 1550 1600 1800

36.32 38.31 43.30 45.30 51.35 55.39

required to heat feed from room temperature to a desired temperature in the pyrolytic reactor, calculated by Chemkin Pro software using the reactor inlet parameters (reactor diameter of 2 m, inlet velocity of 4 m/s, inlet flow rate of 12.6 m3/s, and feed composition of 5% H2S, 3% CO2, and 92% N2, along with the thermodynamic parameters). Clearly, the high energy requirements suggest that retrofitting the pyrolytic reactor producing syngas with a combustor would be required to obtain the desired heat energy.

Figure 13. Reaction pathways of H2S decomposition.

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CONCLUSIONS A detailed reaction mechanism for the simultaneous production of syngas and sulfur from H2S and CO2 thermolysis was developed. The mechanism was validated by comparing the simulation results with different sets of experimental data, and a satisfactory agreement between the two was found. Through reactor simulations at different temperatures, the effect of the relative composition of H2S and CO2 in the acid gas on their thermal decomposition was studied. The results revealed that CO2 is easily reduced to carbon monoxide in the presence of H2S in a pyrolytic environment, and H2S conversion is enhanced in the presence of CO2. Higher temperatures (above 1473 K) and long residence times of the reactor promoted syngas as well as unwanted byproduct (SO2 and COS) formation, but COS production was mitigated at temperatures above 1600 K. Sulfur yield had a strong dependence on CO2 concentration in feed. While low CO2 concentrations enhanced sulfur production, the higher CO2

Figure 14. Reaction pathways of H2S and CO2 decomposition.

The production of SO2 and COS occurred through the reactions of CO and CO2 with sulfur species. It was observed that the reaction of CO and sulfur (reactions R3 and R11) were the major pathways that accounted for most of the COS I

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concentrations decreased the amount of sulfur formed due to the formation of oxygenated sulfurous compounds (SO2 and COS). The high residence time and high reactor temperature can be used to minimize the production of COS, while producing sulfur and syngas with suitable H2 to CO ratio for different applications. The reaction pathway analyses revealed the reactions that were responsible for syngas production from acid gas and for the observed trends in species profiles. The developed kinetic model alongside the simulation results will assist in the design and optimization of acid gas conversion reactors, while minimizing the environmental burden.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +971-2-6075738. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by the Petroleum Institute Gas Processing and Materials Science Research Center (GRC), Abu Dhabi, UAE.

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REFERENCES

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DOI: 10.1021/acs.iecr.6b01176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX