Methane Dry Reforming at High Temperature and Elevated Pressure

Article pubs.acs.org/IECR

Methane Dry Reforming at High Temperature and Elevated Pressure: Impact of Gas-Phase Reactions Lea C. S. Kahle,† Thomas Roussière,†,§ Lubow Maier,‡ Karla Herrera Delgado,† Guido Wasserschaff,§ Stephan A. Schunk,§ and Olaf Deutschmann*,†,‡ †

Institute for Chemical Technology and Polymer Chemistry, and ‡Institute for Catalysis Research and Technology, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany § hte Aktiengesellschaft, Kurpfalzring 104, D-69123 Heidelberg, Germany S Supporting Information *

ABSTRACT: Catalytic dry reforming over a platinum-based catalyst is described experimentally and numerically in a laboratory pilot-plant flow reactor. The results reveal that coking in the upper part of the catalyst bed and at the entrance of the reactor occurs, depending on the composition of the reaction mixture and the respective temperature. To a significant extent, gas-phase reactions play a role as being the cause for the observed coking behavior in the reforming of methane in the presence of carbon dioxide at high temperatures of 1123−1273 K and at 20 bar. Hydrogen addition can inhibit coke formation better than water addition. The reactor is modeled by a one-dimensional description of the reacting field using elementary-step reaction mechanisms of up to 4238 gas-phase reactions among 1034 species and 58 heterogeneous reactions among 8 gas-phase species and 14 surface-adsorbed species. The study leads an optimized positioning of the catalyst in a technical reformer tube.

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methane is still the major limitation, with regard to their use.7,9,10 Noble metals are known to be less prone to coke under reforming conditions. Ross et al.11 investigated Pt/ZrO2 and Rh/ ZrO2 as the most active and stable catalysts of the Group VIII metals. Bitter et al.12 identified Pt/ZrO2 as a catalyst with satisfactory activity and stability for dry reforming of methane at high temperatures. Also, Bradford and Vannice13 studied a variety of supported Pt catalysts and examined their coking resistance. Solymosi et al.14 were the first to report the presence of coke precursors in the gaseous phase; they detected the formation of ethane and traces of ethylene under dry reforming conditions over Pt/Al2O3. At high temperatures and elevated pressure, noncatalytic reactions in the gas phase play an essential role in the formation of higher hydrocarbons. Methane can be converted directly to hydrocarbons by thermally induced coupling reactions at high temperatures.2,15−19 The stepwise dehydrogenation of methane can be explained by free-radical mechanisms.15 Zanthoff20 and Chen21 simulated homogeneous gas-phase reactions between methane and oxygen and achieved satisfactory agreement with the experimental data on the products ethane and ethylene above 1000 K. Kaltschmitt et al.22 showed that significant amounts of C3−C4 olefins are formed in the gas phase at the reactor entrance, resulting from the thermal cracking of isooctane. From these and other literature results, it can be concluded that hydrocarbon molecules may undergo either pure or oxygen-supported pyrolysis with the formation of small hydrocarbon radicals. The principal reaction pathway of the carbon deposition from

INTRODUCTION Coke formation on catalysts and walls of the reactor pipes are serious problems in many industrial reactors used, for instance, for reforming, methanation, and cracking of hydrocarbons. Coke can be a source for catalyst deactivation and, in severe cases, leads to blocking of reactor tubes, as well as physical disintegration of the catalyst support structure.1−4 Reforming of methane receives great interest both in research and technology. In order to produce synthesis gas, a mixture of CO and H2, conventional steam reforming, partial oxidation, and autothermal reforming are currently used on an industrial basis.5,6 Dry reforming, at least at elevated pressure levels beyond 10 bar, has not made it into commercialization. Still, among the different ways of syngas production, methane dry reforming has the advantage of a high potential carbon dioxide input. Another interesting feature of dry reforming is that the achievable lower H2/CO ratios can address downstream processes, which typically are run in the H2/CO ratio range of 1−2. Such CO-rich syngas can be a feedstock for the iron-based Fischer−Tropsch to olefins synthesis, hydroformylation, and carbonylation, similar to acetic acid synthesis.6 One of the main challenges in dry reforming of methane, especially at elevated pressure, is the prevention of coke formation. Coke can of course be formed by a variety of reactions. Apart from the Boudouard reaction (carbon monoxide disproportionation), another thermodynamically favored reaction that results in coke buildup is the reaction of methane decarbonation: CH4 ⇌ C + 2H 2

(pyrolysis of methane)

Therefore, dry reforming is carried out in the presence of catalysts that also can help to inhibit carbon deposition. In particular, Group VIII metal catalysts are of interest.1,6−8 Commercially, Ni catalysts are preferred, because of inherently low cost, but the proneness to coking during dry reforming of © 2013 American Chemical Society

Received: Revised: Accepted: Published: 11920

April 2, 2013 July 16, 2013 July 29, 2013 July 29, 2013 dx.doi.org/10.1021/ie401048w | Ind. Eng. Chem. Res. 2013, 52, 11920−11930

Industrial & Engineering Chemistry Research

Article

methane was proposed by Becker and Hüttinger23,24 and is shown in Figure 1.

heterogeneous and homogeneous reactions, which will also be relevant in reformer units of industrial scale. Homogenous conversion of the feed gas and the formation of coke precursors are investigated for different C/S ratios. In order to gain deeper understanding, experiments are performed at a pressure of 20 bar and within a temperature range of 1123−1273 K. Using elementary-step gas-phase reaction mechanisms, species profiles and the product compositions are numerically predicted at different temperatures and compared to the experimental data.

Figure 1. Model of pyrocarbon deposition from methane.49

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Under fuel-rich conditions, the small radicals react, thus leading to the formation of light hydrocarbons, particularly acetylene (C2H2). Subsequently, large hydrocarbon molecules containing a sufficiently large number of carbon atoms, e.g., aromatics (such as benzene (C6H6)) and polycyclic aromatic hydrocarbons (PAHs) (such as naphthalene, anthracene, and pyrene) are formed in a homogeneous phase.25,26 Carbon can be deposited by means of surface reactions of these coke precursors on a carbon surface by condensation of small hydrocarbons to larger entities and assemblies (macro molecules) in the gas phase.27 Various sets of elementary reactions are available in the literature, which are suitable for modeling studies for homogeneous gas-phase reactions for oxidation and pyrolysis of hydrocarbons, which originate mainly from datasets available from combustion kinetics. 28−35 Optimization of reactor conditions and design can be facilitated by the use of computational models that incorporate elementary reaction mechanisms.36 In this study, the catalytic dry reforming over a Pt-based catalyst was studied experimentally in a test unit consisting of a 6fold reactor with dimensions similar to industrially used reformer tubes. The coke formation at the entrance of the catalytic bed was observed during the experiments at different carbon-to-steam (C/S) ratios and temperatures. Figure 2 shows the comparison

EXPERIMENTAL SETUP AND REACTOR DESIGN In the laboratory pilot plant, the inside vertical flow reactor consists of a ceramic tube, 12 mm in inner diameter and 1400 mm in length. Operating temperatures up to 1273 K and pressures up to 40 bar can be applied. The reactor is surrounded by a furnace 800 mm long to reach isothermal behavior in the midsection of the reactor over a length of 300 mm, where the catalyst is placed. The experimentally obtained temperature profiles along the reactor at three different operating temperatures of the quasi-isothermal zone are shown in Figure 3.

Figure 3. Experimentally obtained temperature profiles along the reactor at three temperatures of the quasi-isothermal zone: 1123 K, 1223 K, and 1273 K. These profiles are used for the simulations.

The ceramic tube is studied without (noncatalytic case) and with a Pt-supported catalyst (catalytic case). For the catalytic tests, a 40-mL catalytic bed of Pt-containing pellets is placed in a primary ceramic tube and maintained in the isothermal zone of the furnace. Downstream of the catalyst zone, the catalyst bed is supported on a ceramic frit. The porosity of the bed is ε = 0.35 and the catalytic area-to-volume ratio is 2800 m−1. The catalytic active surface was determined by CO-TPDs. The ratio between the catalytically active and the geometrical surface area is calculated to be 20, corresponding to a platinum dispersion of 8%; this value serves as a model parameter. Upstream of the catalyst, a bed of corundum pellets is arranged in order to reduce the residence time in the hot temperature zone. The different experimental setups are shown in Figure 4. The product stream is analyzed by gas chromatography (GC) and Fourier transform infrared (FTIR) spectroscopy. The FTIR analysis was used to quantify the amount of water in the gas phase. The accuracy of the analyzed components (CH4, CO2, CO, H2O, and H2) is ∼25 ppm on an absolute basis. Traces of ethane, ethylene, propane, propylene, butane, and butylene can be quantified on the flame ionization detector (FID) in the GC system.

Figure 2. Pictures taken of the removal of spent catalyst from the reactor.

of the fresh and aged Pt-catalyst. After several hours of experimental use, the coke deposition changes the appearance of the light-gray powder to black. Particularly, the upper region of the catalyst bed is covered by carbon deposits. This observation was our motivation to examine the gas-phase reactions in methane reforming to gain an understanding of coke formation on catalysts and reactor tube walls initiated by homogeneous reactions. In this study, first, a catalytic reference case with a Ptsupported catalyst is studied experimentally and numerically. Then, focus is placed upon upstream gas-phase reactions, to investigate coke deposition in the flow reactor. The experimental and numerical setups are treated as a laboratory pilot plant for catalytic dry reforming processes to study the potential

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KINETIC TESTS Catalytic Case. CH4 and CO2 conversions were measured under reforming conditions for a molar CH4/CO2 ratio of unity under dry conditions (absence of water in feed gas) and including 11921

dx.doi.org/10.1021/ie401048w | Ind. Eng. Chem. Res. 2013, 52, 11920−11930


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MODELING APPROACH The blank as well as the filled reactor tube is modeled using a one-dimensional (1-D) description of the reactive flow. Either a plug flow or a 1-D packed-bed model is applied. For the simulations, tools of the DETCHEM software package37 DETCHEMPLUG and DETCHEMPACKEDBEDare used. Elementary-step reaction mechanisms were coupled with the flowfield models, as described below. The model assumes that (a) there is no variation in the transverse direction, and (b) axial diffusion of any quantity is negligible, compared to the corresponding convective term. Then, both plug flow and packed bed are defined by the following equations: Continuity equation: Ng

d(ρu) = av ∑ si̇Mi dz i=1

(1)

Species conservation: Figure 4. Schematic diagrams of the noncatalytic blank reactor (left) and the reactor filled with a bed of Pt-based catalyst (right) (z is the axial position along the reactor). All schematic diagrams are used for a plugflow simulation (values given in meters (m)).

ρu

d(Yi ) + Ya i v ∑ si̇ Mi = Mi(av si̇ + ωi̇ ε) dz

(2)

Conservation of energy: ρuAc

10% H2O (Table 1) under isothermal conditions. An inlet total volume flow of 1.86 × 10−4 m3/s is used, including 5% argon as an internal standard, with a pressure of 20 bar. Noncatalytic Case. Experiments without catalyst material are conducted using different inlet gas mixtures that consist of methane and carbon dioxide with variations of the partial pressure of hydrogen and water. Three furnace temperatures (1123, 1173, and 1223 K) are applied (see Table 1). For each measurement, the same order of partial pressure settings is carried out regarding the compositional variation of the gas mixture entering the reactor. First, the composition of CH4/CO2 = 1.0, including 40% of water, is applied. When steady state is reached, the amount of steam is decreased in steps of 10% down to dry conditions. The same procedure of partial pressure adjustments is repeated for hydrogen as the co-feed, instead of water. The measurements are performed in a blank reactor with 20 mL of corundum and with a corundum-filled upper section of the reactor. An inlet total volume flow of 1.86 × 10−4 m3/s is used inn reference to standard conditions (298.15 K, 1013.25 mbar). All measurements in this study are carried out at a pressure of 20 bar.

=

d(cpT ) dz

Ng

+

Ng + Ns

∑ ωi̇ hiMiε + ∑ i=1

si̇hiMiav

i=1

4 U (Tw − Tz) dh

(3)

Equation of state: pM = ρRT

(4)

In the above equations, ρ is the density, u the velocity, av the catalytic area-to-volume ratio, ε the porosity, Ac the area of cross section of the channel, Ng the number of gas-phase species, Ns the number of surface species, ṡi the molar rate of production of species i by surface reaction, ω̇ i the molar rate of production of species i by the gas-phase reaction, Mi the molecular mass of species i, Yi the mass fraction of species i, cp the specific heat capacity of species i, hi the specific enthalpy of species i, U the overall heat-transfer coefficient, Tw the wall temperature, Tz the gas temperature, p the pressure, and M the average molecular weight. In case of the plug-flow model, the porosity is ε = 1. The areato-volume ratio (av) defines the circumference to cross section of

Table 1. Inlet Compositions Given in Mole Fractionsa 1 2 3 4 5 6 7 8 9 10 a

X (CH4)

X (CO2)

0.475 0.425 0.425 0.375 0.325 0.275 0.425 0.375 0.325 0.275

0.475 0.425 0.425 0.375 0.325 0.275 0.425 0.375 0.325 0.275

X (H2O)

X (H2)

0.1 0.1 0.2 0.3 0.4 0.1 0.2 0.3 0.4

temperature [K]

pressure [bar]

volume flow [L/h]

catalytic

1123 1123, 1173, 1223 1123, 1173, 1223 1123, 1173, 1223 1123, 1173, 1223 1123, 1173, 1223 1123, 1173, 1223 1123, 1173, 1223 1123, 1173, 1223 1123, 1173, 1223

20 20 20 20 20 20 20 20 20 20

67 67 67 67 67 67 67 67 67 67

+ + − − − − − − − −

5% argon is used as dilution gas. 11922



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a plug-flow reactor. For circular channels this parameter is 2/r, where r is the radius of the channel. The main focus of the modeling is on the chemical source terms ω̇ i and ṡi. In both cases, homogeneous and heterogeneous reactions, DETCHEM can handle elementary-step reaction mechanisms. The gas-phase reaction source term ω̇ i is written as follows: Ng

KG

∑ νikkfk ∏

ωi̇ =

(11)

CH4 ⇌ C + 2H 2 2CO ⇌ CO2 + C

(5)

where KG is the sum of gas-phase reactions, Ng is the number of gas-phase species, νik is the stoichiometric coefficient, and cj are the species concentrations. The surface species equation ṡi can be written in an analogous way as a sum of KR reactions among Ng gas-phase and Ns surface species: Ng + Ns

KR

si̇ =

∑ νikkfk ∏

ν′

c j jk (6)

j=1

k=1

kfk is the rate coefficient, for which a modified Arrhenius expression is applied for surface reactions: k fk = Ak T

βk

Si0 Γτ

(13)

CO2 (s) + H(s) ⇌ COOH(s) + Pt(s)

(7)

RT 2πMi

⇌ CO(s) + OH(s)

(8)

Γ is the surface site density that describes the maximum surface concentration of adsorbed species, τ the number of adsorbing sites, and S0i the sticking coefficient. The platinum surface site density, using a value of Γ = 2.72 × 10−5 mol/m2, is defined from the number of surface Pt atoms in one monolayer (1 ML = 1.67 × 1015 atoms/cm2) estimated for Pt(111) surface with a rectangular unit cell. The system of equations is solved using the differential algebraic equation solver LIMEX.37 When provided with a set of input conditions, the program solves the 1D equations and outputs the velocity, temperature, and mole fraction of each species along the centerline of the reactor. Inlet and boundary conditions of the simulation are taken from the experiment.

(15)

CO2 (s) + C(s) ⇌ 2CO(s)

(16)

C(s) + O(s) ⇌ CO(s) + Pt(s)

(17)

C(s) + OH(s) ⇌ CO(s) + H(s)

(18)

CH4 + (4 − x)OH(s) + Pt(s) ⇌ CHx(s) + CH4 + (4 − x)H2O(s)

(19)

CH4 + (4 − x)O(s) + Pt(s) ⇌ CHx(s) + CH4 + (4 − x)OH(s)

OH(s) + H(s) ⇌ H 2O(s) + Pt(s)

(20) (21)

[Note: In these equations, Pt(s) is a free platinum side; (s) denotes adsorbed species.] Coke formation is caused by methane decomposition and the Boudouard reaction (CO disproportionation). Carbon may be gasified by steam or oxygen, as well as by an excess of hydrogen. Noncatalytic Case. In order to avoid incorrect conclusions due to uncertainties of one individual reaction mechanism, three models are used to model the homogeneous chemical conversion in the gas phase. Aromatic hydrocarbons (AHs) and polycyclic aromatic hydrocarbons (PAHs) are included to accentuate effective coke precursors. No adjustments were made to the kinetic parameters of any elementary chemical reaction. Gas-Phase Reaction Mechanism 1 (M1). The mechanism is based on the well-known detailed scheme developed for isooctane combustion at Lawrence Livermore National Laboratory (LLNL),38,39 which consists of 4238 reactions among 1034 species, most of which are reversible. This mechanism was later coupled with a detailed toluene scheme by Dagaut et al.,40 which is discussed by Andrae et al.41 The toluene mechanism was validated by experiments on toluene oxidation in an atmospheric

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REACTION MECHANISM Catalytic Case. Especially for a description of the influence of water on dry reforming, a surface reaction mechanism was developed for combined dry/steam reforming over platinum. The detailed reaction mechanism makes it possible to describe the main “global” processes in the system CH4/CO2/H2O as dry and steam reforming reactions, water-gas shift reaction, carbon formation reactions, methane cracking, and gasification of carbon by steam or oxygen. ΔR H 0 = 247 kJ/mol (9)

CH4 + H 2O ⇌ CO + 3H 2

ΔR H 0 = − 172.4 kJ/mol

(12)

(14)

⎛ −Eak ⎞ Ns ⎛ εik Θi ⎞ ⎟ exp⎜ ⎟ ∏ exp⎜ ⎝ RT ⎠ RT ⎝ ⎠ i=1

CH4 + CO2 ⇌ 2CO + 2H 2

ΔR H = 74.9 kJ/mol

CH4 + (5 − x)Pt(s) ⇌ CHx(s) + CH4 + (4 − x)H(s)

Here, the parameter εik is used to define coverage-dependent activation energies. For adsorption reactions, the rate coefficient is described through sticking coefficients: k fk =

0

The reactions can be described by a recently developed detailed surface reaction mechanism consisting of 58 reactions among 8 gas-phase and 12 surface-adsorbed species used. The mechanism suggests that methane adsorbs dissociatively on the platinum surface. The first step involves CH4 decomposition in which the cleavage of C−H bonds at the catalytic surface leads up to adsorbed carbon and hydrogen. Further interaction of adsorbed carbon species CHx (x = 0, 1, 2, 3) with adsorbed atomic oxygen, formed from CO2 or H2O decomposition, produces CO. Adsorbed CO2 can also react catalytically by the reverse water-gas shift reaction to CO and H2O via an adsorbed COOH intermediate. The formed H2O can, in turn, react with methane and hydrocarbon radicals. The dry reforming process on the Pt catalyst surface is described as follows:

ν′ c j jk

j=1

k=1

ΔR H 0 = − 41.2 kJ/mol

CO + H 2O ⇌ CO2 + H 2

0

ΔR H = 206 kJ/mol (10) 11923



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jet-stirred reactor, by the simulation of benzene oxidation from 0.46 atm to 10 atm, the ignition of benzene−oxygen−argon mixtures, and the combustion of benzene in flames. The merged mechanism consisted of 8927 reactions among 1082 species. Gas-Phase Reaction Mechanism 2 (M2). This second mechanism from the Golovitchev group42 consists of 690 reactions among 130 species. Species from C1 through C8 are considered, and the mechanism is applicable for a wide range of conditions (640−1760 K; 1−55 bar).43,44 Gas-Phase Reaction Mechanism 3 (M3). The third mechanism was applied as well. It is derived from various literature sources based on the work by the Dean group,45,46 and eventually led to 3611 reactions among 420 species, including the reaction pathway for aromatic and some polyaromatic hydrocarbons, such as naphthalene, anthracene, and pyrene.22 For the simulation of noncatalytic reforming of methane, mechanism M2 is mainly used, because of its short processing time. More-detailed descriptions of, e.g., the formation of polyaromatic precursors or C3/C4 olefins can be achieved through the use of mechanisms M1 and M3.

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RESULTS AND DISCUSSION Catalytic Case. Methane and CO2 conversion under dry conditions and including 10% H2O is modeled at different temperatures and compared to the experimental data. Gas-phase simulations, calculated with reaction mechanism M1, are included upstream of the catalytic zone. As shown in Figures 5a and 5b, the conversions are close to the thermodynamic equilibrium. The methane conversion is lower than the CO2 conversion, although they are fed into the system with a molar ratio of 1:1. This difference indicates the occurrence of the reserve water-gas shift (rWGS) reaction, Figure 5. Catalytic conversion of (a) CH4, (b) CO2, and (c) the H2/CO molar ratio, all as a function of temperature for CH4/CO2 = 1/1 under dry conditions; 20 bar; 5% Ar as dilution. Symbols = experiment at 1123 K, lines = simulation, dashed lines = equilibrium composition at given temperature.

CO2 + H 2 → CO + H 2O

The H2/CO molar ratios that are less than unity are also explained by the rWGS (see Figure 5c). Figure 6 shows the numerical product profiles along the catalytic bed at 1123 K and under dry conditions. Catalytic conversion takes place and not only the major products H2 and CO but also H2O are formed. The rapid formation of H2O and CO and the slower formation of H2 can be related to the rWGS reaction again. Rostrup-Nielson et al. have also shown that the WGS reaction is extremely rapid under typical methane reforming conditions.47 The computed surface coverage along the reactor length of the catalyst shows a surface carbon concentration at the catalytic entrance zone that decreases with reactor length (see Figure 6, right). The experiments confirm this behavior. Under dry reforming conditions, coke is observed upstream of the catalytic zone and at the entrance of the catalyst bed, even at 1123 K (Figure 2). There is less coke formation within the catalyst section and downstream of the catalyst section, because of the lower concentration of methane in the gas phase, which is mostly converted on the catalyst surface. Since gas-phase reactions can take place in front of the catalytic zone, coke can be formed by homogeneous pyrolysis as well. It is supposed that coke is not only formed catalytically on the surface but also from the gas phase. It should be noted that coke formation and buildup is a transient process in real catalytic systems, including many carbonaceous atomic layers. The growth of a coke layer is not implemented in the present surface reaction model, i.e., the simulations can only compute a maximum of one monolayer of

carbon on the surface. Therefore, the model has shown where coking take place in the reactor instead of the amount of coke deposition. Subsequently, the catalytic effect of carbon to further coke deposition is not considered in the model and the time of the reactor breakdown due to blockages cannot be predicted. To avoid coke deposition in the experimental reactor setup, 10% steam was included to shift the equilibrium and allow for less coke formation. The experiments were carried out at temperatures higher than those under “totally dry” reforming conditions. Figure 7 shows the calculated and measured CH4 and CO2 conversions, as well as the resulting H2/CO ratios. The same trends in conversion and H2/CO ratio, as discussed with regard to Figure 5, are observed. Methane and CO2 are not fully converted at the temperatures studied. Consequently, gas-phase reactions may also occur downstream of the catalyst bed. The remaining methane may be converted by cracking and coupling reactions that produce carbon precursors downstream of the catalytic zone. However, little coke deposition was observed. The model does not predict coke formation downstream the catalyst bed in the gas phase, which can be understood being due to high hydrogen and water concentrations and a lower methane concentration in the downstream gas flow. 11924



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Figure 6. Computed product distribution in the catalyst bed and numerically predicted surface coverage of adsorbed species as a function of axial position along the catalytic bed of the reactor for dry reforming conditions at 1123 K and 20 bar; CH4/CO2/Ar = 0.475/0.475/0.05.

and that mechanism M2, in principle, is able to treat the problem. Figure 10 shows the species mole fractions along the reactor length. Unsaturated light hydrocarbons are formed using all three reaction mechanisms (M1−M3), even though there are significant quantitative differences. The formation of products in the gas phase significantly starts at a reactor length of 0.3 m, where the wall temperature is higher than 1150 K. This behavior indicates that the formation of coke precursors is mainly leveraged by the temperature. Downstream, at z = 1 m, the wall temperature decreases and the mole fractions of the byproducts remain constant or even decrease, except for ethane (C2H6). It is assumed that C2H6 increases due to ongoing coupling processes. The decrease of C2H4 and C2H2 at higher temperatures can be explained by formation of aromatics from acetylene and ethylene, which is shown in Figure 11. Significant amounts of C3−C4 olefins (1,2-propadiene, propene, propyne, n-butene (1-butene, 2-butene), isobutene, and 1,3-butadiene) are formed as a result of methane pyrolysis and coupling of hydrocarbons. Ongoing coupling processes further downstream form benzene, for instance, by dimerization of acetylene and vinylacetylene.18 Furthermore, a combination of acetylene and AHs may lead to the formation of PAHs, such as naphthalene, anthracene, and pyrene, all of which are potential precursors for carbon formation and deposition. Norinaga et al.48 showed the chemical kinetics of pyrolysis of ethylene and under conditions relevant for the chemical vapor deposition of pyrolytic carbon at 1173 K. Their kinetic model predicted the profiles of the major pyrolysis products well. Higher temperatures favor H-abstraction, thus enhancing the formation of the highly unsaturated hydrocarbons acetylene, diacetylene, and benzene. The behavior observed in our reforming reactor is in agreement with those pyrolysis studies. Figure 12 reveals the production of byproducts as a function of temperature in simulation and experiment. The higher the temperature, the higher is the mole fraction of byproducts. The simulation with mechanism M2 predicts more C2H6 and less C2H4 than the other two mechanisms. These characteristics of the mechanisms are likely to be related to the occurrence of less hydrocarbon species and missing coupling reactions in the simpler mechanism. At 1173 K, mechanisms M1 and M3 predict the experiment well, in contrast to the prediction at 1223 K, where all mechanisms predict too little ethane.

Figure 7. Catalytic conversion of CH4 and CO2 and H2/CO product ratio as a function of temperature for CH4/CO2 = 1/1 including 10% H2O; symbols = experiment, lines = simulation.

Noncatalytic Case. At high temperatures and high pressure, chemical reactions do not exclusively occur on the catalyst surface but also in the gaseous flow. Therefore, homogeneous conversion was computed for a blank reactor as a first step toward a better understanding of the role of gas-phase reactions. The experimentally obtained temperature profiles as shown in Figure 3 are used for the simulations (see Figure 8). As expected,

Figure 8. Computed mole fractions as a function of axial position along the reactor; blank reactor tube due to homogeneous conversions in the gas phase only, 10% H2, CH4/CO2 = 1; 20 bar, mechanism M2; the cross-hatched region shows the position of the demonstrated catalyst bed.

the conversion of the reactants increases with rising temperatures. Significant gas-phase reactions begin to occur in the section upstream of the catalytic bed. In experiments, severe coke formation has been observed in front of the catalyst. In Figure 9, the conversions of CH4 and CO2 are shown at the outlet of the empty reactor tube. The agreement between computed and measured conversion for all temperatures implies that only gas-phase reactions are responsible for the conversion

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VARYING THE CH4/CO2 RATIO Certainly, the formation of coke precursors and the occurrence of carbon deposition are dependent not only on the reactor temperature but also on the gas inlet composition, e.g., the CH4/ CO2 inlet ratio (see Figure 13). Increasing CH4 inlet 11925



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Figure 9. Computational conversion of CH4 and CO2 at the end of the reactor tube; blank reactor tube, 10% H2, CH4/CO2 = 1; 20 bar, mechanism M2.

Figure 13. Numerical coke precursor distribution for varying CH4/CO2 ratios at 1173 K at the end of the empty reactor tube (mechanism M2).

Figure 10. Species mole fraction in the quasi-isothermal temperature zone (1223 K) of the reactor; blank reactor tube, 10% H2, CH4/CO2 = 1; 20 bar (case 3 in Table 1).

ongoing coupling reactions of hydrocarbons may be the sensitive reactions leading to coke deposition. This indicates an influence of the partial pressure of methane on the formation of coke precursors.

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INFLUENCE OF H2 ADDITION The addition of hydrogen inhibits the pyrolysis of methane and hydrocarbons in general, at least to a certain extent, therby limiting the deposition rates of carbon from hydrocarbons.49,50 The strength of the inhibiting effect for coke precursor formation depends largely on the carbon/hydrogen ratio and the molecular structure of the hydrocarbons. Carbon deposition from highly unsaturated, linear hydrocarbons, such as acetylene, is less inhibited than that from less-unsaturated hydrocarbons, such as ethane.50 The formation and growth of hydrocarbons23 and, especially, the formation of carbon from aromatic hydrocarbons are strongly inhibited by the addition of hydrogen.50,51 The simulations of the homogeneous conversion of dry reforming of CH4 show the same trends in Figure 14. The higher the additional hydrogen partial pressure in the gas phase, the less coke precursors are formed. These experimental observations can be predicted by simulation with all three models (Figure 15).

Figure 11. Numerically predicted profiles of coke precursors as a function of axial position along the reactor; blank reactor tube, CH4/ CO2/H2 = 0.425/0.425/0.1, quasi-isothermal zone 1223 K, 20 bar, mechanism M1.

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INFLUENCE OF H2O ADDITION Water is also known to be an inhibitor of carbon deposition from hydrocarbons. However, experiments under “dry” reforming conditions with water at 1223 K could not be carried out, because of massive coking in the reactor system. Even at 1123 K, coke formation in the reactor tube was experimentally observed. The illustrated simulation data in Figure 16 show the influence of H2O on the formation of C6H6. At 1223 K, the concentration of benzene is predicted to be 3000 ppm for 40% water and 7000 ppm for 10% water addition. Even at 1173 K, a concentration of

Figure 12. Byproduct formation as a function of temperature for CH4/ CO2/H2/Ar = 0.325/0.325/0.3/0.05; blank reactor tube; symbols = experiment, lines = simulation (dotted line = mechanism M1, dashed line = mechanism M2, solid line = mechanism M3).

concentration leads to more aromatics, such as C6H6. The concentration of ethylene and acetylene is found to decrease slightly with increasing CH4:CO2 ratio. Cracking of methane and 11926



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Figure 14. Numerical ethane (left) and benzene (right) concentrations for varying H2 inlet concentrations at different temperatures at the end of the empty reactor tube, mechanism M2.

Figure 17. Numerical reactant conversion, as a function of axial position along the reactor; blank ceramic tube, CH4/CO2 = 1, 10% H2 (10% H2O), 1223 K, mechanism M2.

Figure 15. Product distribution as a function of temperature for CH4/ CO2 = 1/1 at different hydrogen concentrations of the empty flow reactor; symbols = experiment, lines = simulation.

Figure 16. Numerical C6H6 mole fraction as a function of temperature for CH4/CO2 = 1/1 at different H2O concentrations of the empty flow reactor; mechanism M2.

Figure 18. Alternated reaction pathway analysis (mechanism M2) of the conversion of methane at 1173 K; 20 bar; CH4/CO2 = 1 for totally dry reforming conditions (DR) and with 10% water and 10% hydrogen addition, respectively. The pathways lead to coke precursor formation from methane.

500−2000 ppm benzene is observed. These high amounts of benzene will surely lead to coke deposition. Comparing the influence of hydrogen addition versus H2O addition, a much higher conversion of methane is observed for water addition (see Figure 17). This behavior is explained by the fact that hydrogen inhibits the rate of methane cracking stronger than water. The reaction flow analysis in Figure 18 shows two parallel pathways to form methyl:

CH4 − H ⇌ CH3

(pathway (1))

CH4 + OH ⇌ CH3 + H 2O

(pathway (2))

This step decreases by 11% because of water addition and by 74% because of hydrogen addition. To summarize, both additives, water and hydrogen, decrease the conversion of methane, with regard to totally dry conditions. Nevertheless, hydrogen has a stronger effect than water. The conversions of CH4 and CO2 with additional 10% water at the end of the empty reactor tube in the experiment and the simulation are given in Table 2. The addition of water slightly decreases the formation of coke precursors, with regard to totally dry conditions. The simulations are in agreement with the experimentally observed methane and CO2 conversions. Figure 19 shows the mole fraction of the products along the reactor length, simulated with mechanism M2. In the case of steam addition, significant amounts of C2H4 are formed around z = 0.5 m. The isothermal zone starts at this position. Usually, the initial part of the catalyst bed would be placed here in a laboratory

The addition of 10% water does not affect pathway (1) but pathway (2) decreases by 26%. Because of the addition of 10% hydrogen, pathway (1) shifts the equilibrium and passes in the direction of the methane formation. Methyl is formed totally via pathway (2). The next step to form carbon precursors is a coupling reaction of two methyl radicals to form ethane (C2H6), which further reacts to form ethylene (C2H5) via H-separation. 11927



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This study also reveals that the strength of the inhibition depends on the carbon/hydrogen ratio and the molecular structure of the hydrocarbons. Experiments including different amounts of water show coking on the reactor tube wall. Homogeneous numerical product distributions under dry reforming conditions including water predict a faster increase of the precursors at the isothermal zone as in the hydrogen case. This infers that water does not inhibit coke precursor formation in the gas phase strongly and, as a consequence, coke deposition. The inhibition effect of H2O could be clearly shown in the presence of a catalyst. The impact of gas-phase reactions on the overall performance in catalytic reforming of hydrocarbons can support the design and optimization of technical reformers. Long residence times in the noncatalytic reactor part lead to a significant formation of coke precursors and, therefore, to coke deposition in the reactor. The study gives background data that allow optimum positioning of the catalyst bed in a laboratory or industrially used reactor and give indications why certain regimes of temperature, pressure, and reactive gas composition pose high challenges for catalyst performance for dry reforming on a technical scale. Therefore, this study can aid in reactor design and the choice of reaction conditions in order to limit coke deposition.

Table 2. Comparison of Experimental and Simulated Conversions of Methane and Carbon Dioxide Including 10% H2O at Two Different Temperatures 1123 K CH4 amount [%] CO2 amount [%]

1173 K

experiment

simulation

experiment

simulation

0.5 0

1.4 0.1

10 4.3

12 5

Figure 19. Numerical product profiles as a function of axial position along the reactor; blank reactor tube, 10% H2 (H2O), CH4/CO2 = 1; 20 bar, mechanism M2.

■

and industrial reactor. In that case, a high amount of C2-species would already enter the bed and support the formation of carbonaceous layers and coke on the catalytic surface.

ASSOCIATED CONTENT

S Supporting Information *

■

The surface reaction mechanism that was used for the catalytic simulations but not shown. This information is available free of charge via the Internet at http://pubs.acs.org/.

OPTIMIZATION OF THE REACTOR SETUP The results discussed above are used for the optimization of the design of the reactor, such as position and length of catalytic bed and the operating conditions, such as the feed composition. One of the key design variables is the residence time of the gases in the noncatalytic section of the reactor, i.e., the entrance zone, and the temperature profile in that zone. Since the diameter and the length of the reactor were fixed because of design reasons, the residence time can be varied by choosing the porosity and the amount of insert material inside the reactor. Furthermore, fewer precursors will form at low temperatures and with an additional catalyst, especially in the nonisothermal front-zone of the reactor. Large homogeneous sections and long residence times at high temperatures lead to gas-phase reactions and precursor formation. Hence, a compromise must be made between saving catalyst material and increase of coke precursors and carbon formation due to gas-phase reactions.

■

AUTHOR INFORMATION

Corresponding Author

*Fax: 0049 721 608 44805. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support of the German Federal Ministry of Economics and Technology (No. FKZ 0327856A) and our project partners BASF, Linde AG, hte AG, Technische Universität München, and Universität Leipzig for fruitful discussions on the dry reforming of methane. L.K. deeply values Dr. Steffen Tischer’s input on simulations pertaining to DETCHEM.

■

CONCLUSION Catalytic dry reforming of methane at high temperatures of 1123−1223 K and a pressure of 20 bar was studied in order to derive a model for coke formation. Homogeneous reactions play a major role, being the cause of precursor-induced coke deposition in the catalyst bed and in the preheating zone upstream of the catalyst bed. Conversion of CH4 and CO2 also occurs in the gaseous phas,e which could be confirmed by pure gas-phase experiments and numerical investigations. From a mechanistic viewpoint, a significant extent of hydrocarbons is initially formed by methane dehydrogenation and further coupling reactions of hydrocarbon radicals, which could be numerically simulated using detailed gas-phase kinetic models. It is shown numerically and experimentally that CO2 and CH4 conversion increase with rising temperature; therefore, the mole fraction of the hydrocarbons rises significantly above 1223 K. In contrast, hydrogen inhibits the formation of coke precursors.

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Methane Dry Reforming at High Temperature and Elevated Pressure

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