Hydrogen Production by Homogeneous Partial Oxidation of Propane

Article pubs.acs.org/EF

Hydrogen Production by Homogeneous Partial Oxidation of Propane Céline Hognon, Yves Simon, and Paul-Marie Marquaire* Laboratoire Réactions et Génie des Procédés, CNRS, Nancy Université, ENSIC, 1 Rue Grandville, BP 20451 F-54001 Nancy Cedex, France ABSTRACT: A kinetic study of the partial oxidation of propane was performed in a jet stirred reactor. The main goal of this study was to establish a detailed homogeneous mechanism of this reaction and explore the capability to produce H2 and olefins. Various operating parameters are modified: temperature, residence time, and O2/C3H8 ratio. The mechanism was validated by comparing experimental and simulated results. A different behavior was observed depending on the temperature. At 600 °C, the mechanism contains both an oxidation and a pyrolytic path. The main products are H2, C2H4, CH4, CO, and O2. At 800 °C, the mechanism mostly contains a pyrolytic path. The main products are H2 and CO.

1. INTRODUCTION The field of energy is confronted to the problem of a globally rising demand of energy and a rarefaction of certain primary energies, resulting in an increase of their cost. The growth of concentrations of greenhouse gases in the atmosphere also imposes new constraints. The use of hydrogen as an energy vector seems to be a good idea for the future. Indeed, its energetic use by oxidation in fuel cells produces only water vapor. It is also used for the production of ammonia and methanol in chemical industry and for hydrogenation in oil refining. Because hydrogen does not exist in nature, it has to be produced either from water (electrolysis), from biomass (gasification), or from fossil fuels.1 Currently, it is mainly produced by hydrocarbons steam reforming, which produces syngas (a mixture of hydrogen and carbon monoxide). However, the high endothermicity of this reaction makes it expensive and therefore requires an important energy input. Partial oxidation of hydrocarbons also produces syngas and could be used for hydrogen generation.2 Partial oxidation of various products (natural gas, heavy residues, coal, and biomass) are used. Usually this process takes place at high temperature (900−1500 °C) and high pressure (20−60 bar) in presence of oxygen and without catalyst. Two gas phase oxidation technologies are industrialized: Shell and Texaco processes. However, this reaction can take place in gas phase or in presence of a catalyst. Catalytic oxidation of hydrocarbons is more favorable than homogeneous oxidation because it can occur at lower temperatures. In our laboratory, we have already studied catalytic partial oxidation of methane.3,4 For some applications, it is interesting to produce hydrogen from hydrocarbons that are heavier than methane. Indeed, propane becomes liquid at approximately 9 bar, then it is easily stored and distributed. For example, propane could be stored in liquid phase in tanks to power fuel cells5 to supply with electricity or to cogenerated energy plants or habitats that cannot be connected to utilities. © 2012 American Chemical Society

This research concerns the study of partial oxidation of propane: 3 C3H8 + O2 → 3CO + 4H2 2 ° = −229 kJ ·mol−1 ΔH298 The final goal of our research is the study of the catalytic partial oxidation of propane and the determination of a mechanism to model this complex reaction. Due to the high temperature, the reaction could be composed of both homogeneous reactions in gas phase and heterogeneous reactions on a catalyst surface. This reaction is initiated at the catalyst surface and produces radicals. These free radicals can propagate in the gas phase. The catalytic partial oxidation of methane (e.g.,6−11) and the oxidative coupling of methane (e.g.,12−22) are reactions of this type and have been studied by many authors. To determine a complete mechanism for catalytic partial oxidation of propane, it is primordial to study initially this reaction without catalyst. Data on the total oxidation and combustion of propane (eg23−27) can be found in the literature. However, few kinetic models exist for partial oxidation. In 1992, Dagaut et al.26 studied oxidation of propane in jet stirred reactor at temperatures between 527 and 927 °C. Propane reacts initially by thermal decomposition and by reaction with oxygen. The importance of these paths depends on temperature. Propane’s oxidation mostly produces CO, CH4, C2H6, C2H4, and C3H6. Satterfield and Wilson27 studied partial oxidation of propane at lower temperatures between 350 and 475 °C. At these temperatures, one important step implies the formation of an alkyl hydro peroxide or an alkyl peroxy radical, which forms aldehyde, methanol, and carbon monoxide. In this work, an experimental study of gas phase partial oxidation of propane was carried out in a perfectly stirred reactor. This reactor is a novel reactor for the study of reactions occurring both in gas phase and at a catalyst surface. The effect Received: December 9, 2011 Revised: February 8, 2012 Published: February 9, 2012 1496

dx.doi.org/10.1021/ef201938t | Energy Fuels 2012, 26, 1496−1508

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of temperature, residence time, and O2/C3H8 ratio was investigated experimentally. A mechanism generated by a software (EXGAS)28 was chosen for the simulation in this study. Experimental points were compared with those computed by simulation and reaction paths were discussed. The main purpose of this paper is to obtain a mechanism for the homogeneous partial oxidation of propane validated in a wide range of temperatures, residence times, and under our experimental conditions. This gas phase mechanism is necessary before the study of the partial oxidation of propane in the presence of a catalyst. This last point will be the subject of a study in progress.

mixture arrives by an annular part where it is preheated. Next, it is injected in the reactor which is composed of a hemispherical part containing a cross-shaped injector. The latter has four nozzles at its branch extremities. The inside diameter of the nozzles is 0.3 mm. The energy of the inlet gas allows good mixing in the gas phase volume. The homogeneous reactions occur in this hemispherical part, which has a volume of 110 cm3 and a radius of 2.5 cm. The other part is a removable cylindrical support on which we will lay a variable number of catalyst’s pellets for the second part of this study. These parts fit together. The reactor is heated by four Thermocoax resistor wires that are in contact with the wall:

2. EXPERIMENTAL SECTION

temperature of the first preheating part was 100 °C lower than the reactor temperature, whereas the temperature of the second preheating part was equal to that in the reactor. We consider that the volume of the second preheating is less than 1% of reactor’s volume and the conversion during preheating is then negligible. • One coil is shaped shell around the hemispherical part. • One coil is in spiral shape inside the piston to heat the surface leading catalyst pellets. Thermocouples are inserted under the coils to check the temperature. The output of gases is released at constant and slight overpressure (PR = 1.053 atm). The outlet gases are analyzed by a microchromatograph (Agilent 3000A) used in derivation. The latter is composed of three modules; each one contains an electronic pressure regulator, a pneumatic valve assembly, an injection valve, a capillary column, and a micro-thermal conductivity detector (TCD). Permanent gases (O2, H2, CO, and CH4) are analyzed by a molecular sieve column (molsieve 5A) using argon as a carrier gas, light hydrocarbons (C2H2, C2H4, and C2H6) and CO2 are analyzed on a Poraplot-U column using helium as a carrier gas, and heavier hydrocarbons are analyzed on an Al2O3-plot (C3H8, C3H6, n-C4H10, i-C4H10, and C4H8), also using helium as a carrier gas. Products are analyzed by an online micro-GC that was externally calibrated by several standard bottles containing a gas mixture at different concentrations. Thanks to this original chromatograph technology, we are able to detect and quantify thirteen products in less than 3 min. Formaldehyde can be detected on a Poraplot-U column, but we are not able to quantify it. This is why the measurement of formaldehyde is performed through a reaction between gas stream leaving the reactor and a solution of dinitrophenylhydrazine (2,4DNPH).33−35 Formaldehyde in the gas stream reacts with DNPH to form the corresponding derivative of the very stable 2,4-dinitrophenylhydrazone:

• Two coils are on the top part for gas preheating. The

2.1. Experimental Conditions. The mechanism has been validated through an experimental study in a perfectly stirred reactor operating in its steady state conditions. The experimental conditions used for this study are

• temperatures between 500 and 800 °C • residence times (τ = (gas phase volume (cm3)/flow rates

(cm3·s−1)) between 1 and 6 s (flow rates are given in reactor conditions (TR, PR)) • outlet pressure fixed at 1.053 atm • composition of the gas inlet with C3H8/O2/Ar = 2/3/95 for most experiments. This ratio (2/3) is the stoichiometric ratio C3H8/O2 of the partial oxidation of propane. Note that this ratio C3H8/O2 was already chosen by Barison et al.29 and by Corbo and Migliardini.30 The reactants are highly diluted in argon to better control the reaction temperature and to avoid hot spots. 2.2. Setup. The experimental setup used is shown in Figure 1.

Derivated hydrazone is analyzed by high pressure liquid chromatography (HPLC) (LC-10AS Shimadzu) with UV detection. The calibration of the HPLC instrument was performed under analysis conditions identical to those of the samples, by using a commercial standard solution of HCHO−DNPH derivative (Sigma-Aldrich). All data were collected at a steady state regime and were verified by five reproducible sets of GC data. In figures, each point corresponds to a set. The quantified products are H2, O2, CO, CO2, CH4, C2H4, C2H6, C2H2, C3H8, and C3H6. Formations of formaldehyde and acetaldehyde are quantified for only one condition (τ = 3 s and T = 600 °C).

Figure 1. Schematic representation of the experimental setup. The reactant mixture is introduced into the reactor at controlled flow rates (2% C3H8, 3% O2, and 95% Ar), regulated by mass flow rate controllers (RDM 280 Air liquide). The jet stirred reactor is considered as a continuous flow stirred tank reactor.31,32 It is made in quartz because of its inert property, thus no catalytic reactions occurred on the wall of the reactor. The reactant 1497


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Figure 2. Carbon balance versus temperature for Ar/C3H8/O2 = 95/2/3 and τ = 3 s.

nondetected species predicted by the mechanism at T = 650 °C and τ = 3 s. These products are in low concentration (10−250 ppm) and the carbon balance with these species is equal to 98%. 3.2. Influence of Temperature. We studied the partial oxidation of propane by varying temperature from 525 to 800 °C. All experiments were performed for a constant residence time of 3 s. We have chosen this time value because it is the middle of our range of residence times. In our conditions of high dilution, the conversion of oxygen and propane are estimated as follows:

3. RESULTS AND DISCUSSION 3.1. Reaction Products and Carbon Balance. Thirteen species have been measured in line by GC at the outlet of the reactor. Major products are hydrogen, carbon monoxide, carbon dioxide, methane, ethane, and propene. Minor products are ethylene, acetylene, i-butane, n-butane, and butene. For some experiments, traces of formaldehyde and acetaldehyde are quantified. The carbon balance (CB) is given by the following expression, in which the xin represents inlet mole fractions and xout is exit mole fractions of the constituent: CB = [3(xC3H8)out + (xCO)out + (xCH4)out

XO2 =

+ (xCO2)out + 2(xC2H4)out + 2(xC2H6)out + 2(xC2H2)out + 3(xC3H6)out + 4(xiC4H10)out

XC3H8 =

+ 4(xnC4H10)out + 4(xC4H8)out ]/3(xC3H8)in

Table 1. Molar Fractions of the Main Nondetected Species Predicted by the Mechanism at T = 650°C, τ = 3 s, and Ar/ C3H8/O2 = 95/2/3

CH2CO C4H6 C5H10 C7H12 cycle C3H6O

126 241 7.45 4.21 215

molar fractions (ppm) CH3CHO C2H3CHO C6H10 cycle C2H4O cycle C3H6O

(xO2)in (xC3H8)in − (xC3H8)out (xC3H8)in

where xin is the inlet mole fraction and xout is the exit mole fraction. Conversions of propane and oxygen, versus temperature, are given in Figure 3. In our operating conditions, conversions become significant at 525 °C. The conversion of propane increases from 2% at 525 °C to 98% at 800 °C. It grows faster than the conversion of oxygen when temperature increases. The conversion of oxygen is limited because propane is mostly consumed. Molar fractions of hydrogen and carbon monoxide, normalized by initial fraction of propane, are shown in Figure 4 as a function of temperature and for a constant residence time of 3 s. The amounts of hydrogen and carbon monoxide increase with temperature. Figure 5 shows molar fractions of main products (CH4, CO2, C2H6, C2H4, and C3H6) (normalized by the initial fraction of propane) versus temperature and for a constant residence time of 3 s. The concentrations of methane and carbon dioxide increase with temperature while those of ethane, ethylene, and propene increase, then reach a maximum and finally decrease. This shows that these molecules are decomposed by secondary reactions of oxidation or pyrolysis when conversion increases.

The carbon balance is shown in Figure 2, and without HCHO, it is verified at least at 90% and at 95% when HCHO is quantified. This slight difference can be explained by an analytical error and/or because some carbon products (present in a very low concentration) were not measured. Simulation and experiment with HCHO and CH3CHO measurements confirm this explanation. Our analytical methods do not allow us to determine the low concentration of some products such as hydrocarbons with a number of carbons atom greater than 5, or aromatic compounds. Table 1 presents the main

molar fractions (ppm)

(xO2)in − (xO2)out

115 73.5 58.1 43.3 11.5 1498


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Figure 3. Conversion versus temperature for Ar/C3H8/O2 = 95/2/3 and τ = 3 s.

Figure 4. Amount of hydrogen and carbon monoxide versus temperature for Ar/C3H8/O2 = 95/2/3 and τ = 3 s.

Figure 5. Amounts of main products versus temperature for Ar/C3H8/O2 = 95/2/3 and τ = 3 s. 1499


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cracking.36−39 For our experimental study, the olefins yield reached 34% at τ = 3 s and T= 625 °C. 3.3. Influence of Residence Time. We studied the partial oxidation of propane by varying the residence time from 1 to 6 s. All experiments were carried out at a constant temperature of 600 °C. This temperature corresponds to the average temperature that we fixed for the study of the catalytic reaction. The conversions of propane and oxygen as a function of residence time are given in Figure 7. The conversion of propane increases from 18% at τ = 1 s to 68% at τ = 6 s, whereas that of oxygen increases from 1% at τ = 1 s to 64% at τ = 6 s. Molar fractions of hydrogen and carbon monoxide (normalized by the initial fraction of propane) are shown in Figure 8 as a function of residence time and for a temperature of 600 °C. The amount of carbon monoxide increases with residence time. CO selectivity increases from 2.45% at 1 s to 22% at 6 s. Hydrogen selectivity increases from 2.5% at 1 s to 4% at 6 s. Figure 9 shows molar fractions of main products (CH4, CO2, C2H6, C2H4, and C3H6) (normalized by the initial fraction of propane) as a function of residence time and for a constant temperature of 600 °C. The concentration of ethylene increases with residence time. The selectivity of C2H4 increases from 20.2% at 1 s to 31.4% at 6 s. We observe the same behavior for methane. The concentration of propene increases up to 3 s and then slightly decreases. The concentrations of carbon dioxide and ethane are constant with residence time. 3.4. Influence of O2/C3H8 Ratio. We studied the partial oxidation of propane by varying the O2/C3H8 ratio. All experiments were carried at constant temperature of 600 °C and constant residence time of 3 s. Molar fraction of propane is constant (2%) and molar fraction of oxygen varies according to the ratio. Conversions of propane and oxygen versus ratio are given in Figure 10. The conversion of propane increases with the ratio. It increases from 44% at O2/C3H8 = 1 to 62% at O2/C3H8 = 2. C3H8/O2 ratio is therefore a very sensitive parameter. The molar fractions of hydrogen and carbon monoxide (normalized by the initial fraction of propane) are shown in Figure 11 as a function of O2/C3H8 ratio and for a temperature of 600 °C and a residence time of 3 s. The amount of carbon monoxide increases with this ratio. Carbon monoxide selectivity increases from 6.8% at O2/C3H8 = 1 to 14.7% at O2/C3H8 = 2. The H2 selectivity seems to be constant versus ratio.

We define the selectivity of carbon products and hydrogen by SH2 = SCi =

(x H2)out 4XC3H8(xC3H8)in ϑi(xCi)out 3XC3H8(xC3H8)in

where XC3H8 is the propane conversion, (xi)out is the exit mole fraction, (xi)in is the inlet mole fraction, and νi the number of carbon atoms. Hydrogen selectivity increases from 1.4% at 550 °C to 19% at 800 °C. Carbon monoxide selectivity increases from 5% at 575 °C to 38% at 800 °C.

Figure 6. Olefin yield versus temperature for Ar/C3H8/O2 = 95/2/3 and τ = 3 s.

Figure 6 shows the yield of olefin versus temperature and for a residence time of 3 s. The yield of olefin is defined by YCi =

ϑi(xCi)out 3(xC3H8)in

where (xCi)out stands for the exit mole fraction, (xC3H8)in for inlet mole fraction, and νi for the number of carbon atoms. As the yield of olefins (C2H4 + C3H6) was high, the reaction of partial oxidation of propane could be an alternative to steam

Figure 7. Conversion versus residence time for Ar/C3H8/O2 = 95/2/3 and T = 600 °C. 1500


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Figure 8. Amounts of hydrogen and carbon monoxide versus residence time for Ar/C3H8/O2 = 95/2/3 and T = 600 °C.

Figure 9. Amounts of main products versus residence time for Ar/C3H8/O2 = 95/2/3 and T = 600 °C.

Figure 10. Conversion versus O2/C3H8 ratio for τ = 3 s and T = 600 °C. 1501


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Figure 11. Amounts of hydrogen and carbon monoxide versus O2/C3H8 ratio for τ = 3 s and T = 600 °C.

Figure 12. Amounts of main products versus O2/C3H8 ratio for τ = 3 s and T = 600 °C.

• A detailed submechanism C0−C2 composed of 444 reversible or directs elementary reactions where only species containing less than 3 atoms are considered. This submechanism is not generated but used by EXGAS; it was developed by Barbe et al.42 4.2. Validation of the Mechanism. To improve the agreement between our experimental results and those simulated, we must adjust kinetic constants. To determine the most sensitive reactions involved in the homogeneous mechanism, a sensitivity analysis was performed at 600 °C and τ = 3 s. The analysis was performed for major products. The first order sensitivity coefficient for species n and reaction i is defined according to:

Figure 12 shows the molar fractions of the main products (CH4, CO2, C2H4, and C3H6) (normalized by the initial fraction of propane) versus O2/C3H8 ratio for constant temperature of 600 °C and a residence time of 3 s. O2/C3H8 ratio did not influence the concentrations of propene and methane, whereas ethylene concentration increases slightly from 25% at O2/C3H8 = 1 to 28% at O2/C3H8 = 2. Oxygen excess is mainly an effect on COx concentrations.

4. MODELING The reactor was modeled as a perfectly stirred reactor (PSR).40,41 The kinetic simulation was carried out using Chemkin II software. 4.1. Description of the Mechanism. The simulation mechanism used was generated by means of the EXGAS28 software, a program developed by our laboratory. This mechanism is composed of 134 gaseous species (molecules and radicals) and 937 reactions. It includes the following: • A detailed primary mechanism: primary reactions are those implying starting reagents and radicals derived from them. • A secondary mechanism: secondary reactions are those implying primary products. In this part, reactions are usually globalized and then reactions are not elementary steps.

Si , n =

k i dx n dk i x n

where ki is the kinetic constant of reaction i and xn is the molar fraction of species n. The higher the Si,n coefficient is, the more sensible against species n the reaction i is. Moreover, a positive sensitivity coefficient Si,n means that an increase of the kinetic constant ki leads to an increase of the concentration of species n. Sensitivity coefficients for reactions whose kinetic constants were modified are represented in Figure 13. Some reactions have sensitivity coefficients greater than |0.05| but their kinetic 1502


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Figure 13. Sensitivity analysis of the homogeneous mechanism at 600 °C, τ = 3 s for major products.

Figure 14. Comparison between experimental (symbols) and computed (solid and dashed lines) mole fraction profiles of CO2 obtained for τ = 3 s and O2/C3H8 = 3/2.

Table 2. Reactions Whose Kinetic Constants Were Adjusted initial reaction

C3H8 + CH3· → CH 4 + i‐C3H7· C3H6 + H· → n‐C3H7·

−1 −1

(1)

factor

E = 9600

A = 2.2 × 1011

1.10

A = 1.3 × 10

E = 3260

12

A = 7.3 × 10

0.56

A = 1.5 × 1014

E = 23.6 × 103

A = 1.5 × 1013

0.10

12

A = 1.4 × 10

E = −1040

11

A = 1.4 × 10

0.10

A = 1.4 × 1012

E = −1040

A = 1.4 × 1011

0.10

A = 2 × 10

11 13

(2)

CO + HO2 · → CO2 + HO·

(3)

C3H6 + HO· → C2H5· + HCHO C3H6 + HO· → CH3· + CH3CHO

(4) (5)

modified cm3·mol−1·s−1

cm ·mol ·s 3

cal·mol

−1

reactions 4 and 5 can be found because they are not elementary processes. As a result of these changes, the values of the concentrations of O2, C3H8, H2, CO, CO2, C2H4, and C2H6 from the modeling are in good agreement with those obtained experimentally (Figures 3−12) in a wide range of temperatures, residence times and ratios. However, we note that the concentration of methane is slightly overestimated in the mechanism, whereas that of propene is underestimated. The concentration of propene is underestimated probably because of the globalization of reactions in the secondary mechanism. As there is a globally good agreement between the experimental results and the modeling throughout the experimental

constants can not be modified because they are well-known in the literature. Figure 14 compares the computed and experimental mole fractions profiles of carbon dioxide for a residence time of 3 s and a O2/C3H8 ratio of 3/2. The solid dashed curve corresponds to mole fractions after adjustment and the dashed line before modification of five kinetic constants. Table 2 summarizes the reactions whose kinetic constants were modified. We only adjusted the pre-exponential factor. In this table, A is given in units of cm3·mol−1·s−1 and E in cal·mol−1. The new values of kinetic constants are in agreement with the literature, as shown in Figures 15 and 16. Reaction 1 is only slightly modified. In the literature, no kinetic constants for 1503


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Table 3. Rate of Production for Some Reactions at T = 550 °C, τ = 3 s, and C3H8 Reactant Conversion = 12% reaction

Figure 15. Rate constants of C3H6 + H· → n-C3H7 reactions according to several authors,43−45 chosen from the NIST Web site database.

domain studied, we can consider that this homogeneous mechanism is validated in our experimental conditions (1 < τ < 6 s; 500 < T < 800 °C; 1 < O2/C3H8 < 2). 4.3. Analysis of the Reaction Mechanism. A reaction scheme can be determined at low and high temperatures to represent the chemistry. We studied flow consumption analysis for each species at two different temperatures (550 and 800 °C) for a residence time of 3 s. For each temperature, we also studied the rate of production of reactions (Tables 3 and 4) that have a rate greater than 1 × 10−9 mol·cm−3·s−1 (main reactions). At 550 °C and for a residence time of 3 s, the reaction diagram is composed of an oxidative path and a pyrolytic path, as shown in Figure 17. As usual, HO· radical is the most reactive radical, so propane first reacts with this species to product n-propyl and iso-propyl by H-transfer reaction but can also react with H·, HO2·, and CH3· radicals. The rates of these reactions are high, as shown in Table 3. The two propyl radicals are produced in the same order of quantity.

absolute rate of production (mol·cm−3·s−1)

C3H8 + H· → H2 + n‐C3H7·

1.07 × 10−9

C3H8 + HO· → H2O + n‐C3H7·

5.77 × 10−9

C3H8 + CH3· → CH 4 + n‐C3H7·

1.43 × 10−9

C3H8 + HO· → H2O + i‐C3H7·

5.84 × 10−9

i‐C3H7· + O2 → C3H6 + HO2 ·

5.87 × 10−9

i‐C3H7· + O2 → C3H7O2 ·

1.52 × 10−9

n‐C3H7· → CH3 + C2H 4

6.60 × 10−9

n‐C3H7· + O2 → C3H6 + HO2 ·

3.02 × 10−9

C3H7O2 · → HO· + C3H6O

1.33 × 10−9

C3H7O2 · → ·C3H6OOH

1.49 × 10−9

·C3H6OOH → HO· + cycleC3H6O

1.33 × 10−9

C3H6 + H· → n‐C3H7·

1.30 × 10−9

·CH3O + M → HCHO + H· + M

3.88 × 10−9

O2 + ·CHO → CO + HO2 ·

2.16 × 10−9

HO2 · + CH3· → CH3O· + HO·

3.80 × 10−9

2HO2 · → H2O2 + O2

1.55 × 10−9

H2O2 + M → 2HO· + M

3.30 × 10−9

Whereas n-propyl radical decomposes mainly thermally to form ethylene and methyl radicals (at T = 550 °C and τ = 3 s; r = 6.60 × 10−9 mol·s−1·cm−3), or by reaction with oxygen to produce propene (at T = 550 °C and τ = 3 s; r = 3.02 × 10−9 mol·s−1·cm−3). n‐C3H7· → CH3· + C2H 4 n‐C3H7· + O2 → C3H6 + HO2 ·

C3H8 + HO· → i‐C3H7· + H2O

The n-propyl radical reacts more than i-propyl, as shown in Tables 3 and 5. Propene reacts mainly by addition of H· radical to form n-C3H7· and by H-transfer reaction with H·, HO·, HO2· and CH3· radicals to form C3H5·. Propene is not very reactive.

C3H8 + HO· → n‐C3H7· + H2O

Iso-propyl radical decomposes mainly by reaction with oxygen to give propene and to a lesser extent by adding O2. i‐C3H7· + O2 → C3H6 + HO2 ·

Figure 16. Rate constants of CO + HO2· → CO2 + HO reactions according to several authors,44−47 chosen from the NIST Web site database. 1504


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Table 4. Rate of Production for Some Reactions at T = 800 °C, τ = 3 s and C3H8 Reactant Conversion = 98% reaction

Table 5. Molar Fractions of Mains Radicals at T = 550 °C and T = 800 °C for τ = 3 s and O2/C3H8/Ar = 2/3/95 molar fractions ×106

absolute rate of production (mol·cm−3·s−1)

C3H8 + H· → H2 + n‐C3H7·

3.02 × 10−8

C3H8 + H· → H2 + i‐C3H7·

1.69 × 10−8

n‐C3H7· → CH3 + C2H 4

7.56 × 10−8

i‐C3H7· → H· + C3H6

2.12 × 10−8

C3H6 + H· → n‐C3H7·

1.61 × 10−8

O2 + C2H3· → HCHO + ·CHO

2.04 × 10−8

O2 + C2H5· → C2H 4 + HO2 ·

2.38 × 10−8

HO2 · + CH3· → CH3O + HO·

2.54 × 10−8

CH3O + M → HCHO + H· + M

2.31 × 10−8

HCHO + H· → ·CHO + H2

3.40 × 10−8

·CHO + M → H· + CO + M

6.02 × 10−8

O2 + ·CHO → CO + HO2 ·

2.94 × 10−8

HO· + H2 → H· + H2O

2.36 × 10−8

H· HO· HO2· i-C3H7· n-C3H7· C3H5· H· HO· HO2· i-C3H7· n-C3H7· C4H7· C3H5·

molar fractions × 106

T = 550 °C, τ = 3 s, and O2/C3H8/Ar = 2/3/95 8.49 × 10−4 C2H5· 1.24 × 10−3 5.72 × 10−4 CH3· 2.15 × 10−1 4.48 C3H7O2· 3.31 × 10−2 −3 9.01 × 10 CH3OO· 4.55 × 10−2 3.81 × 10−3 CH3O· 1.14 × 10−3 2.81 × 10−1 T = 800 °C, τ = 3 s, and O2/C3H8/Ar = 2/3/95 1.56 × 10−1 C2H5· 2.85 × 10−1 7.79 × 10−3 C2H3· 1.74 × 10−2 3.80 CH3· 2.88 2.70 × 10−3 C3H7O2· 1.06 × 10−5 −4 6.87 × 10 CH3OO· 1.67 × 10−3 1.72 × 10−1 CH3O· 1.25 × 10−3 9.22 × 10−1

The last reaction is the most important (Table 3 and Figure 17). HCHO seems to be formed mainly by dehydrogenation of methoxy CH3O· radical.

Transformation of C3H5· radical in carbon monoxide occurs through the intermediary of a hydroperoxyde.

CH3O· + M → HCHO + H· + M

C3H5· + HO2 · → C3H6O2 → C2H3CHO → CO

The transformation of formaldehyde into carbon monoxide is expected through the intermediary of ·CHO radical.

Methyl radicals react to form either ethane, either methane or either CH3O· radical.

HCHO + HO· → ·CHO + H2O

2CH3·( + M) → C2H6( + M)

·CHO + O2 → CO + HO2 ·

CH3· + RH → CH 4 + R· CH3· + HO2 · → CH3O· + HO·

The second reaction is easier, formyl radical concentration is low (Table 3).

Figure 17. Flow consumption analysis at T = 550 °C, τ = 3 s, and a C3H8 reactant conversion of 12%. 1505


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Figure 18. Flow consumption analysis at T = 800 °C, τ = 3 s and a C3H8 reactant conversion of 98%.

Finally, carbon monoxide is oxidized into carbon dioxide.

The formation of ethylene predominates, as we can see in Table 4. Ethylene is also produced by reaction between O2 and C2H5·. At this temperature, consumption of C3H5· occurs without oxygenated intermediates and gives alkenes (C5H10, C4H8, and C2H4).

CO + RO· → CO2 + R·

Where RO· is HO· or an alkoxy radical. Hydrogen mainly comes from metathesis between propane and H· radical following reactions:

C3H5· + CH3· → C4 H8

C3H8 + H· → H2 + i‐C3H7·

C4 H8 + H· → C4 H7· + H2 C4 H7 ·+CH3· → C5H10

C3H8 + H· → H2 + n‐C3H7·

Table 5 gives the concentrations of the main radicals at 550 °C where propane conversion is 12%. At 800 °C and for a residence time of 3 s, the reaction diagram is mainly composed of a pyrolytic path, as we can see in Figure 18. Propane conversion is equal to 98%, secondary reactions are more important than at 550 °C. At this temperature, the main radicals are H·, HO2·, CH3·, and C2H5·. Propane decomposes either thermally to give ethyl radical or by metathesis with H· to give the two types of propyl radicals.

C4 H8 + H· → C4 H 9· C4 H 9· → C2H5· + C2H 4

Ethylene reacts with alkyls radicals to produce C2H3·, which also reacts with O2 to form formaldehyde. C2H 4 + R· → C2H3· + RH

C2H3·+O2 → HCHO + ·CHO

The second reaction is easier, vinyl radical concentration is low (Table 5). Hydrogen mainly comes from metathesis between propane and H· and from reactions between the formaldehyde and hydrogen atoms.

C3H8 → C2H5· + CH3·

In our experimental conditions (800 °C), the formation of n-C3H7· is twice that of i-C3H7· as observed by Dagaut et al.26 Species n-C3H7· and i-C3H7· decompose essentially by thermal path to give alkenes C2H4 and C3H6.

C3H8 + H· → H2 + i‐C3H7· C3H8 + H· → H2 + n‐C3H7·

n‐C3H7· → C2H 4 + CH3· i‐C3H7· → C3H6 + H·

HCHO + H· → · CHO + H2 1506


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A mechanism modeling the catalytic partial oxidation of propane will be validated through experimental data by varying temperature, residence time, O2/C3H8 ratio, and catalyst surface.

Table 5 gives the concentrations of the main radicals at 800 °C where propane conversion is 98%. H2 is mainly formed by pyrolytic path whereas CO is produced by a path of oxidation. The importance of the pyrolytic path increases with temperature. This is shown the comparison between the two conditions studied at 550 and 800 °C (Figures 17 and 18). Production of H2 should increase in proportion to that of CO, as validated by experimental results (Figure 4). H2 comes mainly from the metathesis of propane: hydrogen is then a primary product. On the contrary, CO is produced from the decomposition of reaction products such as C2H2 and HCHO. Production of CO becomes significant at high conversion. Figure 8 shows that the formation of CO increases faster with residence time than that of H2. Ethylene and radical methyl are principally produced from the decomposition of n-C3H7·. At high temperatures, the concentration of n-proyl is higher than at low temperatures. Moreover, the decomposition of n-C3H7· is faster at 800 °C (at T = 550 °C and τ = 3 s; r = 7.56 × 10−8 mol·s−1·cm−3) than at 550 °C (at T = 800 °C and τ = 3 s; r = 6.60 × 10−9 mol·s−1·cm−3). Methane is produced from radical methyl. These results are in agreement with Figure 5; ethylene and methane concentrations increase with temperature. Carbon monoxide concentration increases with temperature (Figure 4). Moreover CO oxidation is faster at T = 800 °C than at T = 550 °C. Figure 5 shows that carbon dioxide increases with temperature. The rate of addition H· + C3H6 → n-C3H7· increases with temperature, which explains why propylene concentration decreases with temperature.

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AUTHOR INFORMATION

Corresponding Author

*Tel.:+33 383 175 070. Fax.: +33383 378 120. E-mail: paul-marie. [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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5. CONCLUSION AND PERSPECTIVES Under our experimental conditions, high syngas concentration has been achieved by gas phase partial oxidation of propane. The maximum hydrogen selectivity of 25% is obtained at 800 °C, 6 s and O2/C3H8 = 3/2. High temperatures, high O2/C3H8 ratios, and long residence times promote propane conversion. Under our experimental conditions, high olefins selectivity has been achieved. We determine experimentally that the optimal conditions for olefins production are a low residence time, a low ratio O2/C3H8, and a low temperature. By simulation, we obtain an olefins selectivity equal to 75% for T = 575 °C, τ = 1 s, and O2/C3H8 = 1/2. The mechanism generated by Exgas software with small adjustment allows a good adequacy between simulation and experimental results in our experimental conditions (1 < τ < 6 s; 525 < T < 800 °C; 1 < O2/C3H8 < 2). Our homogeneous mechanism is validated. The analysis of the mechanism shows the consumption paths of propane in function of temperature. At low temperatures, the mechanism is composed of a path of oxidation and a pyrolytic path, whereas at high temperatures it mostly contains a pyrolytic path. Hydrogen comes mainly from the metathesis of propane with H· radical. Partial oxidation of propane without catalyst produces a syngas with a selectivity of 25% in hydrogen at 800 °C, 6 s, and O2/C3H8 = 3/2. The temperature required for this process is high. The use of catalyst can enhance the rate of reaction by decreasing the activation energy and can improve the selectivity of some products. Then catalytic oxidation of propane can occur at lower temperatures. A catalyst will be added in the reactor for our future investigation to improve hydrogen selectivity. 1507


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Hydrogen Production by Homogeneous Partial Oxidation of Propane

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