Gas Phase Methane Oxidative Coupling Studied by Spatial Reactor Profiles and Microkinetic Numerical Simulations

Gas Phase Methane Oxidative Coupling Studied by Spatial Reactor Profiles and Microkinetic Numerical Simulations

 

 

Oxidative coupling of methane to ethylene (OCM) is a promising one-step reaction pathway to transform methane into ethylene, which is a valuable intermediate for the chemical industry. Unfortunately all research efforts have failed so far to design this high-temperature reaction (T_r  ≈ 800 °C) reaction into a competitive technical OCM process due to insufficient ethylene yields. Interestingly there seems to be a

virtual upper bound of about 25 - 30 % per reactor pass [1] with respect to combined C2 yield (C₂H₄ + C₂H₆). The kinetic reason for this upper bound is unknown. Since the pioneering work of Keller and Bhasin [2] more than 2700 research articles and reviews and about 140 patents have been published on OCM, demonstrating that hundreds of chemically different materials catalyze OCM, yet there is no convincing explanation as to why all catalytic data fall roughly on or below a conversion-selectivity trajectory given by X + S ≤ 100 [3]. The most likely explanation for this indifference of OCM towards the nature of the catalytic material is that at a certain temperature methane oxidation in the gas phase dominates over catalytic oxidation steps, so that product selectivities and yields are not longer determined by the nature of the catalyst.

In the present contribution we use a dedicated high temperature, high pressure spatial profile reactor [4] to measure spatial reactor profiles of species and temperature for methane oxidative coupling in the gas phase. Measurements up to 20 bar pressure and 850 °C were conducted. These profiles provide mechanistic information on how methane and oxygen are transformed into C₂H₆ and C₂H₄ and the unwanted by-products CO and CO₂ and are ideal datasets for validation of detailed microkinetic reaction models. Eight different microkinetic methane gas phase oxidation models were used in a boundary layer simulation and compared to the experimentally measured profiles. To illustrate the concept Figure 1 shows species and temperature profiles measured in the empty quartz tube of the profile reactor described in [4]. This experiment was performed at 8 bar reactor pressure, a typical OCM gas stoichiometry of C/O=4 and a total flow rate of 4000 mln · min^-1 (CH₄ = 3200 mln · min^-1, O₂ = 400 mln · min^-1, Ar = 400 mln · min^-1). The temperature profile of the split furnace surrounding the reactor tube was used as boundary condition to solve the energy balance in the boundary layer simulation. The dashed lines in Figure 1 represent predictions by a dedicated OCM gas phase kinetic model, developed by Zanthoff and Baerns for gas phase OCM at elevated pressures [5] comprising 33 species and 192 elementary reactions.

 

Figure 1: Experimental reactor profiles (scatter) and boundary layer numerical simulations (dashed lines) of gas phase methane oxidative coupling. Reactor pressure p = 8 bar, reactant stoichiometry C/O =4.0, CH4 = 3200 mln · min-1, O2 = 400 mln · min-1, Ar = 400 mln · min-1, total= 4000 mln · min-1.

From the experimental profiles it can be seen that CO is the major carbon containing product in gas phase methane oxidative coupling. C₂H₆ and C₂H₄ are formed in much smaller amounts, and it can be clearly discerned that C₂H₆ is the primary and C₂H₄ the secondary product. Interesting to note, in particular in view of the predictions by the microkinetic model, is the crossing of the C₂H₄ and C₂H₆ profile (here at 35 mm) which is reproducibly observed also for other experimental conditions. The negative flow rate of the ethylene trace between 16 mm and 26 mm is an experimental artifact from the mass spectrometric species analysis, which is due to an isobaric interference on m/z = 30 amu by C₂H₆, CH₃OH and CH₂O. As 30 amu is used to correct the ethylene peak at 27 amu for ethane fragmentation at this mass, negative peak areas are always obtained when the signal at 30 amu is caused by CH₃OH and CH₂O, which are formed in trace amounts at low temperatures. There is basically no CO₂ formation by gas phase methane oxidation in contrast to catalytic OCM where CO₂ is the dominant product (not shown). It can be further seen that there is an ignition-delay zone (0-16 mm) at the beginning of the free gas phase where no noticeable chemistry occurs. This ignition delay is a combined effect of the increasing temperature and the building up of a radical pool. Experimental reactant conversion and product formation begin at about 16 mm axial position and continue until the end of the free gas phase. C₂H₄ reaches maximum concentration at around 57 mm and is then consumed by steam reforming. The kinetic model [5] captures the final gas composition at the reactor outlet sufficiently well, but it does not reproduce the species development inside the reactor. Sufficient reproduction of reactor exit concentrations has been observed not only for the kinetic model used in Figure 1 but also for other models tested, probably because these models have been optimized to fit OCM reactor outlet data well. The species development in Figure 10 as predicted by the model is basically confined to a narrow region between 20 and 30 mm whereas the experimental profiles develop over a much longer length between 16 and 81 mm. Consequently the numerically predicted gradients are too steep and not in quantitative agreement with the experimental data. Also important qualitative features of the experimental profiles such as the crossing of the C₂H₆ and the C₂H₄ profiles are not reproduced by the model indicating severe deficiencies in terms of the included reaction steps and/or the kinetic parameters.

 

References

 

[1]   E. V. Kondratenko, M. Baerns, Handbook of Heterogeneous Catalysis, 2nd Edition, WILEY-VCH, Ch. 13.17 2008, 6, 3010-3023

[2]   G. E. Keller, M. M. Bhasin, J. Catal. 1982, 73, 9-19

[3]   A. M. Maitra, Appl. Catal. A General 1993, 104, 11-59

[4]   R. Horn, O. Korup, M. Geske, U. Zavyalova, I. Oprea, R. Schlögl, Rev. Sci. Inst. 2010, 81, 064102

[5]   H. Zanthoff, M. Baerns, Ind. Eng. Chem. Res. 1990, 29, 2-10