Oxidative Decomposition of H2S over Alumina-Based Catalyst

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Oxidative Decomposition of H2S over Alumina-based Catalyst Vincenzo Palma, Vincenzo Vaiano, DANIELA BARBA, Michele Colozzi, Emma Palo, Lucia Barbato, and Simona Cortese Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01960 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 24, 2017

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Industrial & Engineering Chemistry Research

Oxidative Decomposition of H2S over Alumina-based Catalyst V. Palmaa*, V. Vaianoa, D. Barba*a, M. Colozzib, E. Palob, L. Barbatob, S. Corteseb a

Department of Industrial Engineering of the University of Salerno, Via Giovanni Paolo II, 132, 84084, Fisciano (SA), Italy b

KT kinetics Technology,

Viale Castello Della Magliana 75, 00148 Rome, Italy

*Corresponding author: e-mail: [email protected], Phone: +39 089969275, Fax: +39 089964057.

Abstract Al2O3-based catalyst was employed for the first time in the H2S oxidative decomposition in order to obtain simultaneously sulphur and hydrogen. The influence of the reaction temperature (in the range 700-1100 °C) and the contact time (in the range 17-33 ms) were investigated in terms of H2S conversion, H2 yield and SO2 selectivity. Good catalytic performance were obtained at 1000 and 1100 °C with experimental values very close to those ones expected from the thermodynamic equilibrium. At temperature of 1000°C, the H2S conversion and H2 yield were been respectively about 50% and 17%; in particular, the SO2 selectivity is decreased of a magnitude order (~0.5%) respect to the value observed in homogeneous case (4%). A predictive mathematical model of H2S oxidative decomposition in presence of catalyst was developed through the identification of the main reactions occurring in the system. The results obtained from the kinetic investigations evidenced that the catalyst, in addition to the H2S decomposition reaction and the partial oxidation reaction to sulphur, was able also to promote the SO2 conversion by the Claus reaction allowing to avoid the presence of SO2 at the reactor outlet.

Figure 6 b

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Keywords H2S oxidative decomposition, Al2O3- based catalyst, H2 production, SO2 selectivity.

1. Introduction The industrial treatments of fossil fuels, such as hydrodesulphurization (HDS), produce large amount of hydrogen sulphide1. H2S is considered a strong pollutant, being a weak acid very poisonous, corrosive, flammable, and therefore the industrial applications of hydrogen sulphide are very limited. Actually, the industrial process that uses H2S as feeding is the Claus process in which the main reaction is the partial oxidation of H2S to sulphur and water (H2S +½ O2 → 1/x Sx + H2O)2. This process is not economic because the price of sulphur (the primary product) is depressed and the hydrogen contained in the H2S molecule is lost as low grade steam. However, the Claus process is nowadays intensively practiced because it allows to dispose the H2S in an environmentally acceptable manner. An attractive alternative could be to produce simultaneously sulphur and H2 by the thermal decomposition of H2S in according to this reaction: H2S ↔ H2 + ½S2. In the last years, sulfides3,4 and transition metals oxides5-7 has been studied in heterogeneous hightemperature decomposition of hydrogen sulfide. Unfortunately, this reaction is very endothermic and it is not thermodynamically favored, if not for extremely high temperatures, thus requiring large amounts of energy and subsequent separation stages with high fixed and operating costs. A possible solution to these drawbacks is to couple the decomposition reaction with an exothermic reaction, making the system autothermal and more favored both from a kinetic and from the thermodynamic point of view. In particular, an appropriate amount of oxygen could be added in the reacting system in order to use the heat produced by the oxidation of a fraction of H2S (H2S +½O2 → ½S2 + H2O) to obviate to the endothermic character of the H2S decomposition reaction, obtaining simultaneously sulphur, H2O, and H28. Due to the presence of oxygen, a possible formation of SO2


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may be expected. In this regard, the use of a catalyst could play a key role to improve the selectivity of the process to sulphur and hydrogen, depressing the SO2 formation. Al2O3-based catalysts were widely used because enhance the H2 production and reduce the SO2 formation by promoting the Claus reaction: 2H2S + SO2 = 3/2 S2 + 2H2O9-10. In a previous work, we studied the thermal H2S decomposition reaction in presence of oxygen in homogeneous phase11-12 showing an approach of the H2S conversion and the H2 yield to the equilibrium values only at high temperature (1000-1100 °C), but with SO2 selectivity higher than that one expected from the equilibrium calculations13. To our knowledge, no papers regarding the use of a catalyst in the oxidative H2S decomposition for the simultaneous production of hydrogen and sulphur have been published. Therefore, the aim of this work is to study the oxidative decomposition of H2S over an aluminabased catalyst under different operating conditions (reaction temperature, contact time) in order to minimize the SO2 production and to develop a preliminary kinetic model in order to define the possible reactions that describe in a simplified manner the behavior of the experimental results.

2. Experimental

2.1 Catalyst preparation and characterization The Al2O3 catalyst was synthetized by thermal treatment of pseudo-boehmite samples (provided by Sasol) at 900 °C for 12 hours in static air in order to obtain the stabilization of the alumina phase. Subsequently, the obtained sample was heated in a quartz tubular reactor up to 1100 °C in 20 vol% of H2S in nitrogen flow (300 Ncm3/min) with a heating rate of 10 °C/min and kept isothermal for 1 h in order to stabilize the catalyst in presence of a H2S gaseous stream. Before the catalytic test, the catalyst was characterized by X-Ray Diffraction and Adsorption of Nitrogen at -196 °C.



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X-ray diffraction (XRD) was performed using a Brucker D8 diffractometer with Ni filter and graphite monochromator using CuKa radiation (λ = 1.5401 A°) operating at 40 kV and 40 mA from 20° to 80° with a scanning speed of 3°min-1. The specific surface area was evaluated with a Costech Sorptometer 1040 by using N2 and He, respectively adsorptive and carrier gas. The BET method multipoint analysis based on N2 adsorption/desorption isotherms at -196°C was used to evaluate the specific surface area (SSA) after the pretreatment of the sample at 150 °C for 1 h in He flow (99.9990%).

2.2 Catalytic activity tests The catalytic activity tests were carried out in the laboratory plant shown in Figure 1 and it was described in detail in our previous work11.

Figure 1

Briefly, a system of three way valves allows feeding the feed stream (H2S, O2, N2) to the reaction section and the products to the analysis section (sampling line 2). Otherwise in by-pass position the reactants go directly to the analysis section to verify the composition of the feeding gas (sampling line 1). Experiments were carried out in a fixed bed quartz tubular reactor specifically designed and realized consisting of a tube with 300 mm length and internal diameter of 12 mm. The measurement of the temperature on the catalytic bed was realized by means of a thermocouple placed in a quartz sheath, concentric to the reaction zone. Sulphur produced by the reaction were trapped by using a quartz-wool filter, placed at the end of the reactor in the quenching zone. In order to avoid the SO2 absorption in the water produced from the reaction, a cold trap, working at 0 °C, was placed after the quenching zone allowing to remove selectively sulphur and water without ACS Paragon Plus Environment

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SO2 absorption11. The exhaust stream was analyzed by a quadrupole filter mass spectrometer (Hiden HPR 20). The operating conditions used for the evaluation of the catalytic performances are the following: • Temperature: 700-1100 °C • H2S concentration: 10 vol. % • Feeding O2/H2S molar ratio: 0.2-0.35 • Contact Time: 17 – 33 ms H2S conversion, SO2 selectivity and H2 yield were calculated by using the following relationships: x H2S (%) = ((zH2SIN-zH2SOUT)/zH2SIN)·100 s SO2 (%) = (zSO2OUT/(zH2SIN-zH2SOUT))·100 y H2 (%)= (zH2OUT/ zH2SIN)·100

where: zH2SIN : Inlet H2S volumetric fraction [-] zH2SOUT : Outlet H2S volumetric fraction [-] zSO2OUT : Outlet SO2volumetric fraction [-] zH2OUT: Outlet H2 volumetric fraction [-]

3. Results and discussion 3.1 Catalyst Characterization The XRD patterns of the catalyst are reported in Figure 2.

Figure 2

The results evidenced the presence of mixed phases of Al2O3. In detail, the peaks at about 37, 45.5, 60 and 67 degree are ascribable to the γ-phase of alumina obtained from the thermal decomposition of pseudoboehmite (AlOOH)14, in agreement with a literature paper15 in which it was reported the



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formation of γ-Al2O3 by thermal treatment of precursors at temperatures similar to those ones used in the preparation method. The signals at about 20, 33 and 40 degree could be instead due to the presence of θ-phase of Al2O3 crystalline structure. This result is related with the structural transformation of a fraction of the γphase to the θ-phase structure16. The presence of γ- θ phase of alumina is also confirmed by the value of SSA (100 m2/g) lower than the one observed for γ- alumina17.

3.2 Catalytic activity results

3.2.1 Influence of the reaction temperature

The comparison of the experimental data obtained with and without the catalyst by increasing the temperature from 700 up to 1100 °C at fixed contact time (τ) of 33 ms is illustrated in Figure 3.

Figure 3 a-b-c

The H2S conversion and the H2 yield increased with the increase of the reaction temperature. In particular, in presence of catalyst, the experimental data were very close to the thermodynamic equilibrium in all the investigated temperature range while, in absence of catalyst, the approach to the equilibrium values was obtained only at 1100 °C. It is worthwhile to note that, in the absence of catalyst, the SO2 selectivity (Figure 3) was equal to 4 %, higher than that one expected from the thermodynamic equilibrium at 1000 °C (~0.5 %). For temperatures lower than 1100 °C, the difference between the equilibrium values and the experimental data is even more dramatic. This result can be explained considering that the total oxidation reaction of H2S to SO2 is more favored from a kinetic point of view with respect to the other reactions above reported and, as a consequence, the SO2 concentration in the gas phase was higher.


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The influence of the catalyst on SO2 selectivity with respect to the equilibrium values can be better evidenced from the Figure 4.

Figure 4

The experimental SO2 selectivity is even lower than the equilibrium data up to 900 °C, while for higher temperatures it is possible to observe an approach to the equilibrium, maybe because it begins to be significant the contribution of the homogeneous reactions.

3.2.2 Influence of the Contact time

The effect of the contact time on catalytic performances at 1000 °C is shown in Figure 5. Figure 5

As it is possible to see, no significant changes in H2S conversion, H2 yield and SO2 selectivity was obtained by increasing the contact time.

3.3 Kinetic evaluation Starting from the obtained experimental results, a mathematical model for H2S catalytic decomposition in the presence of oxygen was developed through the identification of the main reactions able to describe the reaction system. In particular, in addition to the homogeneous reactions, it was considered also the effect of the catalytic ones. Therefore, the mathematical model was developed assuming that the following five reactions occur in the reaction zone: 1 S2 ↔ H 2 S 2

1)

H2 +

2)

3 H 2 S + O2 → SO2 + H 2O 2

3)

H2S +

r1

1 3 SO2 ↔ H 2O + S 2 2 4


r2 r3


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4)

H 2 S + O2 → H 2 + SO2

r4

5)

1 1 H 2 S + O2 → H 2O + S 2 2 2

r5

It was considered that these reactions occur both in homogeneous phase and thank to the presence of catalyst. The kinetic expressions employed in the model, are the following:

(

1 2 S2

1 − PH K eq1

r 1= k 1 P H P − 2

2

S

)

1)

r2 = k 2 PO2 PH 2S

(

r 3= k 3 P H S P 2

1 2 SO2

2) 3

1 − P P4 K eq2 H O S 2

2

)

3)

r4 = k4 PO2 PH2S

4)

r5 = k5PO2 PH2S

5)

r1cat = k1cat PH 2 P

1 2

S2

− k1cat reverse PH 2 S

6)

r2 cat = k 2 cat PO2 PH 2S 1 2 SO2

r3cat = k3cat PH 2 S P

7) 3 4 H 2O S 2

− k3cat reverse P

P

8)

r4cat = k4cat PO2 PH2S

9)

r5cat = k5catPO2 PH2S

10)

where: r1: Rate of homogeneous reaction 1 r2, r4, r5: Rate of homogeneous reactions 2, 4 e 5 r3: Rate of homogeneous reaction 3 Keq1: Equilibrium constant of homogeneous reaction 1 [atm-0.5] Keq2: Equilibrium constant of homogeneous reaction 3 [atm+0.25] Pi: Partial pressure of component i-th [atm] k1, k3: Kinetic constant of homogeneous reactions 1 and 3 [min-1·atm-1.5] ACS Paragon Plus Environment

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k2, k4, k5: Kinetic constant of homogeneous reactions 2, 4 e 5 [min-1·atm-2] r1cat: rate of the catalytic reaction 1 r2cat, r4cat, r5cat: rate of the catalytic reactions 2, 4 e 5 r3cat: rate of the catalytic reaction 3 k1cat, k3cat: Kinetic constant of catalytic reactions 1 and 3 [min-1·atm-1.5·cm3/gcat] k1cat_reverse:: Kinetic constant of reverse catalytic reaction 1 [min-1·atm-1·cm3/gcat] k3cat_reverse: Kinetic constant of reverse catalytic reaction 3 [min-1·atm-1.75·cm3/gcat] k2cat, k4cat, k5cat: Kinetic constant of catalytic reactions 2, 4 e 5 [min-1·atm-2·cm3/gcat] The kinetics expressions for the homogeneous reactions 1, 2, 3, 4 and 5 were provided by the literature papers in which it was evidenced that these reactions follow first-order kinetics with respect to the H2S and O2 concentrations18-20. For the catalytic reactions, the kinetics expressions of reactions 2, 4 and 5 are the same of the homogeneous reactions while, for reactions 1 and 3, it was assumed that the catalyst is able to promote both the direct and the reverse reaction. By assuming the reactor as a plug flow reactor (PFR), the mass balances for each component can be written as: .

H2S: Q .

H2 : Q

dy SO2 dV

dy S2

.

O2: Q

= − r1 + r4 + (− r1cat + r4 cat ) ⋅ ρ cat

dV

.

.

= r1 − r2 − r3 − r4 − r5 + (r1cat − r2 cat − r3 cat − r4 cat − r5 cat ) ⋅ ρ cat

dV

dy H 2

SO2: Q S2: Q

dyH 2S

= −0.5r1 + 0.5r5 + 0.75r3 + (−0.5r1cat + 0.5r5 cat + 0.75r3 cat ) ⋅ ρ cat

dV

dy O2 dV .

H2O: Q

= r2 − 0.5r3 + r4 + ( r2 cat − 0.5r3 cat + r4 cat ) ⋅ ρ cat

= −1.5r2 − r4 − 0.5r5 + ( −1.5r2 cat − r4 cat − 0.5r5 cat ) ⋅ ρ cat

dy H 2O dV

= r2 + r3 + r5 + (r2 cat + r3 cat + r5 cat ) ⋅ ρ cat

The boundary conditions are the following: ACS Paragon Plus Environment

11)

12)

13)

14)

15)

16)


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V=0

yH2S = 0.1

V=0

yO2 = 0.1·R (where R is the O2/H2S feeding molar ratio)

V=0

yH2 = ySO2 = yS2= yH2O = 0

where: Q: Total flow rate [cm3 (stp)·min-1] yi: fraction volume of component i-th[-] V: Reaction volume [cm3] pcat :apparent density of the catalyst The differential equations system was solved by using the Euler method. The values of the apparent kinetic constant for each catalytic reaction were obtained by using the least-squares approach, based on the minimization of the sum of squared residuals between the experimental data obtained at different temperatures, and the values given by the mathematical model with a molar feed ratio (O2/H2S) equal to 0.2. The values of the kinetic constants of the reactions occurring in homogeneous phase have been set equal to the values previously estimated11. The results of the model calculations in comparison with the experimental data for the Al2O3-based catalyst are displayed in Figure 6 as a function of the contact time.

Figure 6 a-b-c

A very good agreement between the mathematical model calculations and the experimental data was achieved relatively to the H2S, H2 and SO2 concentration. The kinetic constants values (calculated from the mathematical model) were used to estimate the apparent activation energy value for each reaction through the use of the Arrhenius plot (Figure 7).

Figure 7


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It is possible to observe that the alumina based catalyst has promoted the partial oxidation of H2S to sulphur, the H2S decomposition to hydrogen and sulphur and the Claus reaction. The values of the kinetic constants of the homogeneous reactions, in comparison with those ones of the catalytic reactions calculated by the model are shown in Table 1.

Table 1

It’s possible to observe in all cases that the presence of the catalyst promoted the rate of the reactions 1, 3, and 5. The predictive capability of the developed model was verified in the temperature range between 900 and 1000 °C with a H2S inlet concentration equal to 20 vol% (Figure 8). Figure 8 a-b

Also in this case, the obtained results evidenced that the model is able to predict with a good accuracy the experimental data of H2S, H2 and SO2 concentration.

3. Conclusions The oxidative decomposition of hydrogen sulphide has been assessed for the first time using an alumina based-catalyst prepared by thermal decomposition of pseudo-boehmite. Characterization data of the obtained catalyst evidenced the presence of mixed phases of Al2O3 (γ and θ-phase). The catalytic performance has been investigated at different operating conditions (reaction temperature and contact time). In the overall range of investigated temperature (700-1100°C), it was observed that H2S conversion, H2 yield and SO2 selectivity very close to those ones expected by the thermodynamic equilibrium.



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Very different results were observed by the comparison with the reaction carried out in absence of catalyst, especially at temperature lower than 1000 °C. In particular, the SO2 selectivity was equal to 4 %, higher than that one expected from the thermodynamic equilibrium at 1100 °C (~0.5 %). No significant changes in H2S conversion, H2 yield and SO2 selectivity was obtained by increasing the contact time. A macroscopic kinetic model able to reproduce the collected experimental data was also developed. The results obtained from the kinetic investigations evidenced that the catalyst, besides the H2S decomposition reaction and the partial oxidation reaction to sulphur, is able also to promote the conversion of SO2 by the Claus reaction, decreasing the SO2 selectivity.


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FIGURE 1

Figure 1: Scheme of laboratory plant.



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FIGURE 2

Figure 2: XRD Patterns of Al2O3-based catalyst.


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FIGURE 3

Figure 3. Effect of the temperature in terms of H2S conversion (a), H2 yield (b) and SO2 selectivity (c) with and without catalyst (zH2SIN =10 vol%, O2/H2S = 0.2).



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FIGURE 4

Figure 4. Comparison of SO2 selectivity as function of the temperature between catalyst and the equilibrium value (zH2SIN =10 vol%, O2/H2S = 0.2).


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FIGURE 5

Figure 5. Effect of the contact time on H2S conversion, H2 yield, SO2 selectivity (T=1000°C, zH2SIN =10 vol%, O2/H2S = 0.2, τ=33 ms).



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FIGURE 6

Figure 6: Comparison between experimental data and mathematical model at 800°C (a), 900°C (b), 1000°C (c) as function of the contact time (zH2SIN =10 vol%, O2/H2S = 0.2).


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FIGURE 7

Figure 7: Arrhenius plot of the catalytic reactions of H2S decomposition, Claus and partial oxidation of H2S to Sulphur. (zH2SIN =10 vol%, O2/H2S = 0.2).



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FIGURE 8

Figure 8: Comparison between experimental data and mathematical model at 900°C (a), 1000°C (b) as function of the contact time for the Al2O3 (zH2SIN =20 vol%, O2/H2S = 0.2).


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TABLE 1 Table 1: Kinetic Constants of the reactions in Homogeneous Phase and in presence of the catalyst at T=1000°C.



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FOR TABLE OF CONTENTS ONLY

Al2O3-based Catalyst 1 S2 ↔ H 2 S 2 1 3 H 2 S + SO2 ↔ H 2O + S 2 2 4 H2 +

1 1 H 2 S + O2 → H 2O + S 2 2 2


Oxidative Decomposition of H2S over Alumina-Based Catalyst

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