Influence of Pressure on H2s Oxidation: Experiments and Kinetic Modeling

ABSTRACT

The oxidation of H₂S at different manometric pressures (0.6-40 bar), in the temperature range of 500-1000 K and under slightly oxidizing conditions (λ=2), has been studied. Experiments have been performed in a quartz tubular flow reactor. The results have shown that H₂S conversion is shifted to lower temperatures as the pressure increases. The kinetic model used in this work is based on a previous one proposed by the authors to describe H₂S oxidation at atmospheric pressure, which was updated with a H₂/O₂ reaction subset for high pressures. Model results match fairly well the experimental ones both from the present work and from the literature. The reaction pathways of H₂S oxidation analyzed are similar to the ones at atmospheric pressure. The differences are found in the radicals that are involved in the oxidation process at the different pressures.

Keywords: H₂S, oxidation, high pressure, sour gas, kinetic modeling

1. INTRODUCTION

As the energy demand increases worldwide, an efficient utilization of available natural resources is needed. Natural gas production is expected to peak near 2035, taking into consideration a scenario of cumulative production, plus remaining reserves, plus undiscovered resources [1]. The abundance of natural gas reserves can facilitate the transition from fossil derived to fully renewable fuels and chemical generation [2, 3]. The total long-term recoverable conventional gas resource base is more than 400 trillion cubic metres (tcm), and another 400 tcm are estimated for unconventional sources (sour gas) [4]. The increasing importance of unconventional fuel sources, such as sour and shale gas (natural gas with significant amounts of H₂S and CO₂, up to 30% content in volume each one [5]), brings interest to the direct use of these fuels, developing technologies and combustion processes focused on their knowledge and understanding under high pressure conditions [6]. The technology most used for the treatment of H₂S is the Claus process, by producing sulfur from it [7, 8]. However, due to the large offer of sulfur worldwide [9] and the variable feedstock of H₂S in natural gas reserves, different processes for treating sour gas and H₂S streams are emerging [2, 10-13]. Thus, Langè et al. [14] proposed a detailed description of the phase behavior of the CH₄+H₂S system, in a wide range of temperatures and pressures, with the goal of performing the correct process design of new gas purification technologies. Another interesting process for exploitation of sour gas reserves is the direct gas combustion, for example, through oxy-combustion [15], which has been studied at high pressures to increase efficiency in power plants [16, 17].

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Some studies about mixtures of sulfur species and carbon species oxidation have been performed, with the final goal of knowing more about sour gas conversion. For example, Gersen et al. [18] experimentally studied the autoignition and oxidation of CH₄/H₂S mixtures in a rapid compression machine (RCM) and a flow reactor at high pressure. They showed prediction results with their model that agree well with the measured autoignition delay times. On the other hand, they also indicated that the H₂S oxidation chemistry and the interaction of CH₄ and H₂S at high pressure are not well understood. Zeng et al. [19], in their work on the co-oxidation of CH₄ and CS₂ (a known impurity of the Claus process) in a flow reactor, experimentally observed an inhibiting effect by CS₂ in the oxidation of methane at atmospheric pressure. They became aware of the complexity of the C-H-O-S combustion chemistry and claimed that their current model could not reflect all potentially significant reactions. Other recent studies have been devoted to validate their kinetic mechanisms on sulfur compounds oxidation with experiments from the literature. Bongartz et al. [15] and Bongartz and Ghoniem [20, 21] developed an optimized mechanism to make qualitative and even quantitative predictions on the combustion behavior of sour gas under oxy-fuel conditions. Salisu and Abhijeet [22] carried out kinetic simulations of acid gas (H₂S and CO₂) pyrolysis and oxidation for simultaneous syngas (H₂+CO) and sulfur recovery, which results will assist in the design and optimization of acid gas conversion reactors. Despite these efforts, there is still a need for more accurate direct determination of several important rate constants as well as more validation data in order to improve modeling predictions of sour gas conversion [15].

The reaction steps in the H₂S oxidation process remain unknown in many aspects and the available experimental data are limited. Some works have been carried out in the last years trying to understand H₂S kinetic behavior under combustion conditions. The experimental and theoretical work of Zhou et al. [23] about H₂S oxidation at atmospheric pressure established several kinetic parameters for different reactions involved in the process. Later, the exploratory work from Song et al. [24] on H₂S oxidation under high pressures showed that, under oxidizing and stoichiometric conditions, the H₂S oxidation depends strongly on the stoichiometry and pressure, claiming that experimental results for H₂S oxidation at elevated pressures are scarce.

As H₂S conversion can be affected by the operating conditions, such as temperature, pressure and combustion atmosphere, the present work addresses the oxidation of H₂S at high pressures, with the basis of the work from the authors at atmospheric pressure [25], where H₂S oxidation was studied in a flow reactor from reducing to oxidizing conditions and different temperatures (700-1400 K). In that study, H₂S conversion was reasonably predicted by the kinetic model proposed, which included the addition of HSOO⇌HSO₂ isomerization reaction, supported by recent theoretical works, as a key step in a faster reaction path of SH oxidation.

In this context, the present work studies the influence of manometric pressure (0.6-40 bar) at different temperatures (500-1000 K) and at slightly oxidizing conditions (approximately λ=2), on the conversion of hydrogen sulfide, performing flow reactor experiments, under laboratory controlled conditions. Moreover, a kinetic model capable of describing the oxidation process of H₂S at high pressures has been used to interpret the experimental results. The results obtained in this work can be also useful for different industrial processes, such as the Claus process [26] or oxy-combustion of the sour gas [15, 20, 21, 27].

2. EXPERIMENTAL METHODOLOGY

The experimental set-up used to perform the high-pressure H₂S oxidation experiments has been previously described in detail elsewhere [28]. Therefore, only a brief description of the main features is provided here. Reactants: H₂S (approximately 500 ppm), O₂ (approximately 1500 ppm) and N₂ as carrier gas, were supplied from gas cylinders through mass flow controllers with an uncertainty in the flow rate measurements of approximately 0.5%. The oxygen required to carry out each oxidation experiment is determined by the air excess ratio (λ, defined as inlet oxygen divided by stoichiometric oxygen). Slightly oxidizing conditions (λ=2) were selected to study the oxidation of hydrogen sulfide at different manometric pressures. Table 1 contains the conditions for the different experiments performed. The moderate concentration of oxygen used in this work was chosen to avoid deposition of sulfur in the experimental set-up, if reducing conditions were used, and to minimize SO₃ formation, which is enhanced at oxidizing conditions and could lead to corrosion problems. The reactant gases were premixed before entering the reactor, which consists of a quartz tube (inner diameter of 6 mm and 1500 mm in length) designed to approximate plug flow conditions [29]. The reactor is enclosed in a stainless-steel tube that acts as a pressure shell. The steel tube is placed horizontally in a tubular oven, with three individually controlled electrical heating elements that ensure an isothermal reaction zone of approximately 500 mm, with a uniform temperature profile (±5 K). Gas residence time depends on pressure and temperature and it can be expressed as t_r(s) = 232*P(bar)/T(K) in the isothermal part of the reactor. Previously to the gas analysis systems, gases pass through a filter and a condenser to ensure gas cleaning. Products are analyzed by a gas chromatograph equipped with a thermal conductivity detector (TCD) to quantify H₂S and O₂, and a continuous UV analyzer to quantify SO₂. The uncertainty of the measurements is estimated within 5%.

Table 1. Experimental conditions. N₂ as bath gas. t_r(s)=232*P(bar)/T(K).

3. KINETIC MODEL

The kinetic model used in this work has been taken from a recent work by the authors, where H₂S oxidation at atmospheric pressure was studied in a flow reactor, from reducing to oxidizing conditions [25]. The simulations have been carried out with the software Chemkin-Pro and the plug flow reactor code [30]. The oxidation mechanism proposed in [25] supported the evolution of the SH+O₂ reaction (key reaction step in H₂S oxidation) through isomerization from HSOO to HSO₂, leading to the final product SO₂, as proposed by Garrido et al. [31], in a high level ab initio study of the HSO₂ system. The main reactions for H₂S oxidation belong to the work from Zhou et al. [23] and Alzueta et al. [32]. The H₂/O₂ subset was updated for high pressures [33] and has been included in the present study with no modifications. However, no big differences have been observed in the simulations in this work using the updated H₂/O₂ subset.

The oxidation of H₂S under high pressure conditions might be a source of O₃, as mentioned by Song et al. [24], according to the SH+O₂+O₂⇌HSO+O₃ reaction, and ozone may promote H₂S conversion, as it is a much more reactive molecule than molecular oxygen. Thus, a subset for O₃ reactions has been added from their work [24], which is mainly based on the work from Atkinson et al. [34]. Reactions involving O₃ have not been found to be important under the present conditions.

Hydrogen sulfide oxidation proceeds via its reaction with the O/H radical pool to form SH, as can be seen in the reaction pathways shown in Figure 1. Under slightly oxidizing conditions, SH radicals react mainly with O₂ to form the HSOO peroxide, which isomerizes to HSO₂ and forms by decomposition the SO₂+H final products, feeding the radical pool with H radicals. The HSO₂ radicals can also react with oxygen, if it is available, or isomerize again to the radical HOSO, which will react in the same way as HSO₂, decomposing or reacting with O₂ to form SO₂. The changes in reactivity during the H₂S conversion process at different pressures are attributed to different radicals that participate depending on pressure. In the case of relatively low pressures (0.6, 5 and 10 bar manometric pressure), H₂S will react mainly with radicals H (R1).

H₂S + H ⇌ SH + H₂   (R1)

However, as the pressure increases (20 and 40 bar), other pathways become also important, involving HO₂ and OH radicals (R2 and R3).

H₂S + HO₂ ⇌ SH + H₂O₂  (R2)

H₂S + OH ⇌ SH + H₂O  (R3)

Formation of HO₂ radicals is favored at high pressures (R4) [e.g. 18, 24, 35]. These radicals react with H₂S to form SH and H₂O₂ (R2), and promote the oxidation via the branching reaction of H₂O₂ to give OH radicals (R5), which then interact again with H₂S (R3).

O₂ + H (+M) ⇌ HO₂ (+M)  (R4)

H₂O₂ (+M) ⇌ OH + OH (+M) (R5)

Figure 1: Reaction pathways for the oxidation of H₂S.

4. RESULTS AND DISCUSSION

The experimental (symbols) and simulated (lines) results of the concentrations of H₂S, SO₂ and O₂ as a function of temperature are plotted from Figure 2 to Figure 6. All the experiments were carried out at slightly oxidizing conditions (λ=2), being SO₂ the main product from the oxidation of H₂S. The sulfur balance closes in all cases within ±5%. The conversion of H₂S is shifted to lower temperatures as the pressure increases. Thus, the onset temperature for H₂S conversion is 775 K in the case of 0.6 bar and 600 K at the highest pressure studied (40 bar). The consumption of O₂ follows the same trend as hydrogen sulfide.

The simulated results obtained with the kinetic model used in this work reproduce well the experimental data, mainly, those obtained at near atmospheric pressure (0.6 bar manometric pressure). This fact confirms that the kinetic model is capable of predicting H₂S conversion at near atmospheric pressure. For the rest of the pressures studied, the kinetic model underpredicts slightly the oxidation of H₂S at the beginning of the reaction and overpredicts to some extent the H₂S conversion at the end of the conversion. The exception would be the case at the highest pressure (40 bar), where a shift of 50 K temperature between experimental H₂S concentrations and model predictions is seen.

Figure 2: Experimental results of H₂S oxidation at 0.6 bar (Set 1 of Table 1).

Figure 3: Experimental results of H₂S oxidation at 5 bar (Set 2 of Table 1).

Figure 4: Experimental results of H₂S oxidation at 10 bar (Set 3 of Table 1).

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Figure 5: Experimental results of H₂S oxidation at 20 bar (Set 4 of Table 1).

Figure 6: Experimental results of H₂S oxidation at 40 bar (Set 5 of Table 1).

If the results near atmospheric pressure of this work (0.6 bar manometric pressure) are compared with the ones obtained by the authors in their study of H₂S oxidation at atmospheric pressure in other flow reactor [25], Figure 7, it can be observed that the results are quite similar, although a difference in the conversion onset temperature of 50 K exists, which might be due to the pressure difference (0.6 bar) or the residence time, as both experiments are carried out in different experimental installations. Previously, these set-ups operating at atmospheric pressure have already shown a similar behavior for CH₃SH oxidation under stoichiometric conditions [36].

Figure 7: Concentrations of H₂S vs. temperature at λ=2 and atmospheric pressure. Open symbols from [25] and solid symbols from the present work (Set 1 of Table 1).

It is worth to mention that the gas residence time in the reactor increases with the pressure. An additional analysis, through modeling results, of the effect of the residence time at two different pressures (1 and 21 bar absolute pressure) on H₂S conversion can be seen in Figure 8.  In this figure, at 1 bar and 21 bar, the solid lines on the right correspond, respectively, to the gas residence time of the atmospheric pressure set-up used in [25] (t_r = 194/T(K)) and the gas residence time of the present work high-pressure set-up (t_r = 4872/T(K)), which are used as the reference cases. For both pressures, their reference residence time values are subsequently increased up to being approximately doubled. . As it can be observed, at 1 bar a 50 K temperature difference in H₂S concentration profiles exists when the residence time is doubled. At high pressure (21 bar), the effect of the residence time is less important; in this case the solid line represents the residence time of set 4 in Table 1. Thus, under these conditions, the influence of gas residence time is less significant as the pressure increases. However, it is worth to highlight that the specific effect of the gas residence time depends on the given process studied. For example, Marrodán et al. [37], in their study about DME oxidation at high pressure, observed an important effect of the residence time at pressures even higher than those presented in Figure 8 on DME conversion when doubling the residence time at stoichiometric conditions.

Figure 8: Concentration of H₂S vs. temperature at different pressures and different residence times.

A sensitivity analysis of model calculations has been performed for the highest pressure studied (40 bar), at the temperature when H₂S is starting to react (675 K). In Figure 9, the sensitivity analysis for H₂S shows the isomerization reaction (R6) as the most sensitive one:

HSOO ⇌ HSO₂    (R6)

followed by the branching reaction of H₂O₂ (R5), which is comparatively a well stablished reaction. The next most important reaction is (R7):

SH + SH (+M) ⇌ H₂S₂ (+M)  (R7)

which has also been seen in the top 3 most sensitive reactions in works about H₂S oxidation [24, 35]. In this work, the observed sensitivity has been related to the capability of H₂S₂ to convert HO₂ radicals into H₂O₂, which would enhance the oxidation process and reaction (R8) is found as the fourth most sensitive reaction.

H₂S₂ + HO₂ ⇌ HS₂ + H₂O₂  (R8)

The formation of these species (H₂S₂ and HS₂) is usually more important at reducing conditions. As mentioned by Zhou et al. [23], disulfur interactions as (R9) are extremely important in the ignition and propagation reactions during H₂S oxidation even under fuel lean conditions, being (R7) chain terminating reaction, while (R9) is chain propagating reaction, due to the subsequent reaction of S with oxygen to form SO and then SO₂.

SH + SH ⇌ H₂S + S   (R9)

The same kinetic parameters for (R9) as in the work from Gao et al. [38] have been used. However, these authors mention that care should be taken in applying (R9) above 1 bar. Gao et al. also mention that H₂S₂ is more stable at high pressures and low temperatures than SH. Discrepancies in the model results compared to experimental ones might be related to the chemistry of disulfur species and claim for a better characterization of these reactions at high pressures.

Figure 9: Sensitivity analysis for H₂S conversion, at λ=2, 40 bar and 675 K.

With the purpose of evaluating the performance of the kinetic model used in the present work, it has been tested against literature ignition delay time measurements at different pressures. Some cases from the work of Mathieu et al. [35], about the effects of H₂S addition on hydrogen ignition behind reflected shock waves, have been taken, and the results are shown in Figure 10. In that study, ignition delay times were measured behind reflected shock waves for mixtures of 1% H₂/1% O₂, diluted in Ar and doped with various concentrations of H₂S (100, 400, and 1600 ppm), over large pressure (around 1.6, 13, and 33 atm) and temperature (1045–1860 K) ranges. Their results showed a significant increase in the ignition delay time due to the addition of H₂S, in some cases by a factor of 4 or more, over the baseline mixtures without H₂S. This behavior was explained because H₂S initially reacts before the H₂ fuel does, mainly through the reaction H₂S+H⇌SH+H₂ (R1), thus taking H atoms away from the main branching reaction (R10), which would produce OH radicals, and thus inhibit the ignition process.

 H + O₂ ⇌ OH + O  (R10)

 However, an increase in the reactivity was observed at the highest pressure investigated (33 atm) and at the temperature of 1100 K, using the highest H₂S concentration investigated (1600 ppm). This fact would be in accordance with the discrepancy found in this work at high pressure (40 bar), where a higher experimental reactivity is observed. The present kinetic model fits well the ignition delay time measurements under different conditions of pressure and H₂S concentration. The results from the present work and the previous study at atmospheric pressure in a flow reactor [25] suggest that the update in the kinetic model, adding the isomerization of HSOO to HSO₂ as a possible step in H₂S oxidation until the final product SO₂, should be taken into consideration for future works on the oxidation of H₂S.

Figure 10: Ignition delay time measurements vs. temperature for different experimental conditions, using a mixture of 1% H₂/1% O₂, diluted in Ar and doped with H₂S. Experimental data are taken from the work of Mathieu et al. [35].

5. CONCLUSIONS

The present work addresses the oxidation of H₂S at different manometric pressures (0.6-40 bar), in the temperature range of 500-1000 K, using a quartz tubular flow reactor. The experiments were performed at slightly oxidizing conditions (λ=2). The results show that the oxidation of H₂S is shifted to lower temperatures as the pressure increases. The onset for H₂S conversion starts at 775 K in the case of 0.6 bar and 600 K at the highest pressure studied (40 bar). The kinetic model about H₂S oxidation at atmospheric pressure developed by the group in a previous work, together with an updated H₂/O₂ subset for high pressures, seems to work fairly well at different pressures. The kinetic model matches the experimental trends, except for the experiment at 40 bar, where a gap of 50 K in temperature between experimental concentrations and model predictions is observed. The sensitivity analysis performed at 40 bar indicates that the H₂S conversion is mainly sensitive to the HSOO⇌HSO₂ isomerization reaction, and some branching reactions involving H₂S₂ and H₂O₂.

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