Ind. Eng. Chem. Res. 2001, 40, 497-508
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APPLIED CHEMISTRY Mechanisms of CO and COS Formation in the Claus Furnace Peter D. Clark,*,† Norman I. Dowling,† M. Huang,† William Y. Svrcek,‡ and Wayne D. Monnery‡ Alberta Sulphur Research Ltd., c/o Department of Chemistry, and Department of Chemical and Petroleum Engineering, The University of Calgary, 2500 University Drive N.W., Calgary, Alberta, Canada T2N 1N4
The aim of this study was to determine the major pathways leading to COS and CO formation and consumption during the processing of H2S and CO2 in the partially oxidizing conditions of the Claus furnace. Both species were found to be produced by a multitude of pathways, which include the direct reaction of H2S with CO2 to form COS and H2O and the reaction of CO2 with S2, one of the major primary products in a Claus furnace. This last reaction produced SO2 and CO as the major products, with COS being formed in lesser quantities. The dissociation of H2S to H2 and S2 at high temperatures (>1000 °C) was shown to promote a further cascade of reactions stemming from the reduction of COS and CO2, both of which lead to CO. Because of the known formation of CS2 from hydrocarbon carry-over into the furnace, the reactions of CS2 with CO2, H2O, and SO2 were also studied as potential CO- and COS-forming reactions. Reaction with CO2 was slow at 2000 °C) will occur via radical intermediates and involve S1 as the major elemental sulfur species. Reactions in the anoxic zone (900-1200 °C), particularly those involving CO2 and CO, will involve molecular species only. The dominant sulfur species in the 900-1200 °C range is S2, although it should be appreciated that S3 (equilibrium concentration is 2.3 mol % at 1000 °C) and larger sulfur species (equilibrium concentration of S4 at 1000 °C is 0.14 mol %) must be involved in the chemistry to a small extent. For the sake of brevity and to limit speculation as to individual radical pathways, we have written the reactions as simple molecular transformations utilizing S2 as the reacting sulfur species. Also, it should be noted
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Figure 1. Schematic of experimental apparatus. Table 1. Equilibrium Constants for Possible Overall Reactions within the Anoxic Region of a Claus Furnace Kp reaction
900 °C
1000 °C
1100 °C
1200 °C
2H2S H 2H2 + S2 (1) H2S + CO2 H H2O + COS (2) H2 + COS H CO + H2S (3) 2COS H 2CO + S2 (4) CO + H2O H CO2 + H2 (5) 2CO2 + S2 H COS + CO + SO2 (6) CS2 + H2O H COS + H2S (7) CS2 + CO2 H 2COS (8) 2CS2 + SO2 H 2COS + 3/2 S2 (9 + 10) CS2 + 4H2 H CH4 + 2H2S (11) COS + H2O H CO2 + H2S (12) 2COS + SO2 H 2CO2 + 3/2 S2 (13 + 14) 2CO + SO2 H 2CO2 + 1/2 S2 (15 + 16 + 17)
1.25 × 10-3 4.59 × 10-2 30.1 1.14 0.72 1.57 × 10-4 109 4.98 1.68 × 105 2.70 21.8 6.78 × 103 5.96 × 103
5.36 × 10-3 6.18 × 10-2 29.6 4.69 0.55 3.95 × 10-4 83.3 5.15 1.45 × 105 0.34 16.2 5.48 × 103 1.17 × 103
1.85 × 10-2 7.98 × 10-2 29.0 15.5 0.43 8.84 × 10-4 66.8 5.33 1.26 × 105 5.74 × 10-2 12.5 4.45 × 103 2.87 × 102
5.33 × 10-2 9.98 × 10-2 28.4 43.1 0.35 1.81 × 10-3 55.4 5.53 1.11 × 105 1.28 × 10-2 10.0 3.63 × 103 84.2
that the cooler anoxic zone of the furnace makes up the largest region in the furnace. Thus, molecular reactions might have a dominating effect in establishing the final product composition, as radical intermediates transform to molecular species on departure from the flame zone and react to establish the equilibrium composition at the cooler conditions of the anoxic zone. Because a large number of reactions can potentially lead to or assist in the formation/destruction of COS and CO within the Claus furnace, equilibrium constants for these reactions were calculated in order to determine the most likely pathways. These values are given in Table 1 for reactions 1-17, in some cases as overall reactions. Although this type of analysis can be used to examine the relative likelihood of different reactions to act as a given pathway, it should be remembered that it does not provide any information about the relative rates of these processes. Nonetheless, the range of Kp values listed, for the reactions as written, are sufficiently large that all of the listed reactions should be considered. The large values of the equilibrium constants for the reactions of CS2, COS, and CO with SO2, in particular, suggest that these reactions might represent major pathways for conversion of these species in the Claus furnace, barring any kinetic limitations. The smaller Kp values for reactions 1, 2, and 6 also do not exclude these reactions from the anoxic chemistry, as these processes represent the starting points or primary reactions for this chemistry. Clearly, the number of reactions that can lead to COS and CO could be quite large and, for CO, would include
partial oxidation and steam reforming of methane or other hydrocarbons entering the furnace. The objective of this work was to determine the dominant mechanistic pathways to COS and CO under Claus furnace conditions to allow for a logical approach to the kinetic modeling of these species. Experimental Section Gases and Materials. All gases were obtained from standard suppliers and had >99.9% purity according to the gas chromatographic procedures described below. Water was distilled before use. Experimental System. A schematic of the system used in the experiments is given in Figure 1. Two types of reactor were used: one constructed of fully dense alumina (supplier, Vesuvius McDanel) with dimensions of 19 mm o.d × 12.7 mm i.d. × 74 cm length and the other fabricated from quartz with the same dimensions. The purpose of using two reactor types was to establish the catalytic nature of the surface on some of the reactions suggested as possible COS formation pathways. Temperature profiles for these externally heated reactors were measured under a nonreactive flow of argon using an alumina sheathed S-type thermocouple. These profiles presented an initial ramp-up section, (∼400 °C to reaction temperature) corresponding to 20% of the length of the reactor, followed by an isothermal hot-zone section, and finally, a cool-down section. Noted residence times for the experiments given in the data tables refer to the hot-zone section of the furnace and
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Table 2. Effect of Reactor Type on Product Yield for the Reaction of a 50% H2S/50% CO2 Feed Mixture equilibrium, quartz, and R- alumina product yields(mol of product per 100 mol of reactant) T °C
tres (ms)
600 700 800 900 1000 1100 1200 1300 1400
3200 2870 2600 2400 2230 2070 1930 1800 1700
H2 0.6 1.3 2.4 4.0 5.8 7.5 9.3 10.9 12.2
0.18 0.22 2.4 5.9 7.6 6.7 7.4 7.9 8.2
CO 0.14 1.1 3.3 4.3 5.5 6.7 6.74 6.0 6.3
1.8 4.2 7.9 12.3 16.9 21.0 24.9 28.1 30.8
0.11 0.12 0.2 1.3 3.7 14.7 20.5 23.1 25.6
COS 0.63 1.5 6.0 8.6 11.5 14.8 16.9 17.9 20.2
4.1 4.2 3.8 3.1 2.5 1.9 1.4 1.1 0.9
can therefore be used for relative comparisons between different reactions. Although some reaction is unavoidable within the initial ramp-up zone, use of a reactor design to implement a separate reactant entry and preheat was considered but not implemented. While this approach appears to have merit, it is limited by the ability of several of the reactants (notably H2S and COS) to undergo unimolecular decomposition. Subsequent reaction within the cool-down section of the reactor was minimized by employing a quench system with a quench time on the order of 20-100 ms from reaction temperature to ca. 400 °C. Quenching of the gases was achieved by withdrawal of gas samples through a quartz capillary connected to a pumping system. Reactants were metered using mass flow controllers with reactant compositions being checked by gas chromatography (see below). In experiments using sulfur as a reactant, the reactant gas mixture was passed through liquid sulfur at 330 °C with the quantity being taken up ascertained by cooling the hot gas to 20 °C to condense and trap the sulfur carried by the gas. Experimental Procedures. A typical experiment consisted of stabilizing the reactor system at the desired temperature, passing the feed mixture through the reactor, and analyzing the dried (P2O5) product gas. Several samples were collected for each condition, with average data being presented in the tables (absolute error is estimated to be 1000 °C. The CO/SO2 Reaction. Although this reaction has been examined at low temperatures (1 s, similar to those found in commercial furnaces. The major products of the reaction were CO2 and sulfur, with only around 25%
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Table 6. Reaction Products from the Reaction of CS2 with CO2 T °C
tr (s)
feed mole products ratio (mol per 100 mol of reactant) % CS2 CO2 CS2 CO2 CO COS CS2 S2 conversion
1055 2.0 1155 2.0 1255 2.0
5.75 1.9 5.75 1.9 5.75 1.9
effect of temperaturea 5.63 0.47 0.04 1.37 0.5 5.33 0.90 0.07 1.23 0.6 4.36 2.14 0.13 0.73 1.1
27.9 31.5 61.3
1055 0.5 1055 1.0 1055 1.5
effect of residence time 6.22 1.6 6.06 0.09 0.13 1.48 0.1 6.78 1.9 6.50 0.39 0.25 1.58 0.3 5.76 2 5.57 0.62 0.13 1.55 0.4
7.5 18.1 21.3
effect of CO2/CS2 feed ratio 1.9 19.6 0.91 0.05 1.33 0.5 2 10.7 0.80 0.07 1.52 0.5
29.8 24.6
1055 2.0 20 1055 2.0 11.1
a Equilibrium calculations predict quantitative conversion of CS2 under all conditions investigated.
of the COS appearing as CO in the experiments showing complete COS conversion. This suggests that reaction of COS with SO2 is relatively facile and is in reasonable agreement with equilibrium predictions for this system. When the data with variable residence time obtained at 1055 °C (Table 5) are compared to the model for COS kinetics published by Behie et al.,11 it is clear that reaction of COS with sulfur dioxide is considerably faster than COS decomposition. This conclusion is reinforced by the data shown in Table 10 for COS decomposition in the reactor system of this study. Thus, overall, the following reaction sequence, which is reactions 13 and 14 given in the Introduction, can be written to describe the major pathway in the SO2/COS system:
COS + SO2 f CO2 + SO + 1/2S2
(13)
COS + SO f CO2 + S2
(14)
In addition to these processes, direct decomposition of COS (eq 4) seems to account for approximately 25% of the total COS conversion. In summary, it can be concluded that the CO2/S2 reaction is an important reaction in the Claus furnace, particularly above 1200 °C (i.e., in the flame zone), and that it is reversible by reaction of either CO or COS with SO2. Reactions of CS2 with H2O, SO2, and CO2. We have reported data on CS2 formation in earlier publications20,21 and shown that it arises whenever hydrocarbons are present in the feed acid gas. Our laboratory data indicated that H2S consumed much or all of the
O2 incorporated for combustion of the hydrocarbons, allowing for formation of CS2 by reaction of the uncombusted hydrocarbon with S2 or other sulfur species. As pointed out in the Introduction, the possible decomposition reactions of CS2 are of special interest with respect to COS formation in the Claus furnace and WHB, as each can lead to CO and COS production. As in some of the previous work, we have examined the reaction of CS2 with an excess of the reagent in order to approximate conditions in Claus furnaces and so, perhaps, deduce the more important mechanisms of a commercial system. Interestingly, CO2 was found to react with CS2 under Claus furnace conditions although relatively more slowly (Table 6) when compared to some of the other reactions included in this study. Thus, equilibrium calculations predict quantitative conversion of CS2 under the conditions studied, because pf the excess of CO2 in the feed. Nevertheless, over 60% CS2 conversion was observed at 1255 °C using a 2-s residence time, giving CO and sulfur as major products. Because it is difficult to envisage how CO2 might be converted directly to CO, it is probable that COS is an intermediate product of this reaction that then undergoes further decomposition to CO in a combination of reactions 8 and 4.
CO2 + CS2 H [2COS] H 2CO + S2 The observation of small concentrations of COS in all experiments and more COS than CO at a 0.5-s residence time at 1055 °C supports the formation of COS as an initial product. The last set of data in Table 6 shows that the conversion of CS2 by CO2 appears to be independent of the CO2 concentration (within this limited data set), although a more extensive study of this reaction would be required to confirm this tentative conclusion. Most workers have assumed that conversion of CS2 in the Claus furnace occurs by reaction with H2O, which is present in large concentrations. The data in Table 7 bear out this supposition, as 100% conversion was observed over 2 s even at the relatively low temperature of 1055 °C. In addition, conversions >90% were obtained over 0.5 s at 1055 °C. Both CO2 and CO were formed as major products, along with lesser amounts of COS. Interestingly, CO2 was the major oxide product at 1055 °C, whereas CO was the predominant oxide at 1255 °C. These observations, together with the increased CO2/ CO ratios at higher H2O/CS2 feed ratios, suggest that COS is an intermediate product that can undergo either decomposition to CO and sulfur or hydrolysis to CO2
Table 7. Reaction Products from the Reaction of CS2 with H2O feed mole ratio H2O CS2
T °C
tr (s)
1055 1155 1255
2.0 2.0 2.0
5 5 5
1055 1055 1055
0.5 1.0 1.5
1055 1055
2.0 2.0
a
products (mol per 100 mol of reactant) H2 CO COS SO2 CS2
H2S
CO2
S2
H2O
2 2 2
1.53 1.49 1.55
1.09 0.90 0.74
effect of temperaturea 1.71 0.80 0.11 2.00 0.95 0.15 1.88 1.09 0.17
0 0 0
0 0 0
1.2 1.2 1.1
1.8 1.5 1.6
5 5 5
2 2 2
1.78 1.81 1.61
0.95 1.00 0.96
effect of residence time 1.86 0.82 0.08 1.96 0.81 0.10 1.85 0.88 0.12
0 0 0
0.15 0.10 0.05
0.9 1.0 1.1
1.4 1.3 1.5
20 10
2 2
2.29 1.90
1.49 1.34
effect of H2O/CS2 feed ratio 3.22 0.48 0.03 2.52 0.62 0.04
0.16 0.08
0 0
0.8 1.0
14.5 5.6
Equilibrium calculations predict quantitative conversion of CS2 under all conditions investigated.
% CS2 conversion 100 100 100 92.5 95.0 97.5 100 100
Ind. Eng. Chem. Res., Vol. 40, No. 2, 2001 505 Table 8. Reaction Products from the Reaction of CS2 with SO2 feed mole ratio SO2 CS2
products (mol per 100 mol of reactant) CO CO2 COS CS2
% CS2 conversion
T °C
tr (s)
1055 1155 1255
2.0 2.0 2.0
5 5 5
2 2 2
2.99 2.89 2.84
effect of temperaturea 0.32 1.60 0.44 1.54 0.65 1.50
0 0 0.05
0 0 0
3.0 3.1 3.1
100 100 100
1055 1055 1055
0.5 1.0 1.5
5 5 5
1.7 1.8 2
2.81 3.20 2.90
effect of residence time 0.38 1.26 0.40 1.49 0.44 1.56
0. 0.05 0.08
0.17 0.05 0
2.5 2.7 3.0
90 97 100
1055 1055 1055 1055 1055
2.0 2.0 2.0 2.0 2.0
20 11 5 5 5
0 0.07 0.25 0.94 1.40
0 0 0.10 0.28 3.79
2.7 3.2 5.9 7.8 8.0
100 100 97.5 95.3 62.1
a
2 2 4 6 10
SO2
effect of SO2/CS2 feed ratio 18.65 0.29 1.63 8.35 0.30 1.71 0.83 0.78 3.28 0 2.79 2.60 0 3.68 1.62
S2
Equilibrium calculations predict quantitative conversion of CS2 under all conditions, except for the 5:10 feed mole ratio of SO2/CS2.
Scheme 2. Conversion of CS2 by H2O at Claus Furnace Conditions
and H2S. The appearance of H2 as a significant product in this reaction at levels above the equilibrium concentration expected for just dissociation of the H2S product shows that several mechanistic pathways are operating for this “simple” bimolecular reaction (see Scheme 2). Interestingly, it is seen that high H2O/CS2 feed ratios (bottom set of data in Table 7) result in higher H2 and H2S product levels. This is most likely because of the water gas shift of CO, formed initially by COS decomposition, and consumption of sulfur species by the H2 product of that shift. Trends for CO and CO2 levels in this data set support this conclusion. The data summarized in Table 8 show that SO2 is also a very effective agent for conversion of CS2. As with H2O, SO2 promotes complete conversion at 1055 °C over 2 s and >90% over 0.5 s at the same temperature, in agreement with the theoretical equilibrium for these systems. This illustrates that reaction with SO2 could also be an important mechanism for conversion of CS2 in the Claus furnace. The major products of the reaction were CO2 and sulfur, with lesser quantities of CO and only traces of COS. Only slightly higher quantities of CO were observed on increasing the reaction temperature from 1055 to 1255 °C, and no increase in CO was measured for longer residence times at 1055 °C. Only when the SO2/CS2 feed ratios were decreased did CO exceed CO2 as the major carbon oxide product. All of these observations can be explained by reaction of CS2 with SO2 to form COS, elemental sulfur, and SO (Scheme 3), with rapid conversion of COS by either SO or another molecule of SO2. Only in experiments with small amounts of SO2 in the feed did COS appear as a significant product and have an opportunity to produce CO and sulfur by direct decomposition. Field observation shows that CS2 concentrations never exceed SO2 values in commercial furnaces; thus, any conversion of CS2 by SO2 should result in CO2 and sulfur as major products.
Scheme 3. Pathways for Reaction of CS2 with SO2
Reaction of COS with H2O and SO2 and the Direct Decomposition of COS. Because COS is implicated as an intermediate in the reaction of CS2 with CO2, H2O, and SO2, data were obtained for the reaction of COS with H2O and SO2. Clearly, the reaction of COS with CO2 leads to no net change, but the direct decomposition of COS might be implicated in the reaction of CS2 with CO2 and, hence, was also examined experimentally. The data in Table 9 show that COS reacts rapidly and to completion with H2O, forming CO2 and H2S as the major products and CO, S2, and H2 in lesser quantities. Although H2O was always used in excess in these experiments to mimic Claus furnace conditions, results at 1055 and 1255 °C show that hydrolysis dominates over direct decomposition, even at the higher temperature. Interestingly, more CO was formed by reaction of CS2 with H2O (Table 7) than by reaction of COS with H2O under the same reaction conditions and reagent molar ratios. One explanation for this observation is that conversion of CS2 results in more H2S, which can dissociate to H2 and drive the product CO2 to CO and H2O. Thus, again it is observed that the final product distribution for even relatively simple bimolecular processes is dependent on numerous, linked equilibria. As was shown earlier (Table 5), reaction of COS with SO2 is also fast, leading to CO2 and sulfur as major products with some CO formed, presumably, from direct decomposition of COS. Overall, only minor differences in CO2/CO product ratios were noted for low (1055 °C) and high (1255 °C) temperatures, but CO formation was limited to some extent when SO2 was present in large excess (see bottom data set of Table 5). The major reaction under all conditions of this study is the direct conversion of COS by SO2. Although it has been studied by numerous groups,11,31-35 the decomposition of COS in argon was
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Table 9. Reaction Products from the Reaction of COS with H2O feed mole ratio H 2O COS
T °C
tr (s)
1055 1155 1255
2.0 2.0 2.0
5 5 5
1055 1055 1055
0.5 1.0 1.5
1055 1055
2.0 2.0
a
products (mol per 100 mol of reactant) H2 CO COS SO2 CS2
H2S
CO2
2 2 2
1.28 1.05 0.80
1.65 1.60 1.53
effect of temperaturea 0.55 0.35 0 0.75 0.41 0 0.94 0.47 0
