OCM Reaction Mechanism and Operating Conditions . 23

Many authors have suggested that methane coupling occurs through formation of methyl radicals CH3* via hydrogen abstraction by oxygen species on active sites (Gambo et al., 2018; Karakaya et al., 2018). These methyl radicals are then coupled in gaseous phase into ethane (C₂H₆) and subsequently

dehydro-genated into the main product ethylene (C2H4). However, the educt methane, the intermediary ethane, and the main product ethylene can all suer deep ox-idation into thermodynamically stable COx species. This leads to a trade-o between conversion and selectivity, which is typical of selective oxidation pro-cesses (Vandewalle et al., 2019). The desired high methane conversion leads to undesired low product selectivity due to the oxidation side-reactions. Su, Ying, and Green, 2003 suggested an upper bound of28 %for the product yield on a conventional single-pass co-fed OCM Packed-Bed Reactor (PBR). It is generally suggested that for industrial implementation, yields around30 % are required, or a selectivity above80 %with a methane conversion of at least 20 % (Arndt et al., 2012; Kondratenko et al., 2017). However, recent patents claim a process with much lower yields, e.g., 15 % (Siluria Technologies, 2015). The reaction is typically carried out at high temperatures, i.e., >973 K, and is highly exothermic, which also poses major challenges for reaction and thermal engi-neering. Important operating conditions aecting the reaction performance in the OCM system are listed below:

CH4:O2 Ratio: The educt feed ratio is an important decision variable. A stoichiometric ratio (2:1) is desired, but high oxygen availability leads to lower selectivity and high heat release. On the other hand, higher ratios will limit methane conversion. Special attention should also be paid to avoid the forma-tion of explosive atmospheres. For industrial practice, values between 2-20 or even more can be used depending on reactor and process conguration.

Gas Dilution: A dilutant is often added for improving selectivity and heat management. In lab-scale, inert gases like N₂, He, or Ar are frequently used.

For industrial practice, N2 would be present, if air is used as oxidizing agent, although this signicantly increases equipment size and energy consumption for removing N₂ in the downstream separations. Another possibility is CO₂, which can be added to the system or present in the CH4 source, e.g., biogas. Carbon dioxide is advantageous because of its high heat capacity and because it is produced in the OCM reactor, making a CO₂-removal step a prerequisite for the downstream processing. Some authors suggest that the positive eect of CO2

goes beyond heat management and may be also related to the active catalyst phase (Shi, Yao, and Hu, 2015). Finally, steam can also be applied to manage the reactions' heat release and eventually also to prevent coke formation. Water can be easily separated by condensation in the downstream. The amount of dilutant is also an important decision variable to be considered.

1.2 Oxidative Coupling of Methane (OCM) Temperature and Heat: Most OCM catalysts require temperatures of at least 973 K, operate optimally between 1073 and 1143 K, and suer deactivation above 1173 K. An isothermal reactor would provide the highest yields, but it is impractical because of the diculties in temperature control and cost of supplying heat at such high temperatures. Hence, heat management and tem-perature control are critical aspects to design OCM reactors. Microcatalytic xed-bed reactors operated pseudo-isothermally are predominant at lab-scale, while uidized-bed and membrane reactors have reached mini-plant scale inves-tigations, but adiabatically operared xed-bed reactors currently remain the only feasible solution for industrial implementation (Aseem et al., 2018).

Pressure: Most experimental investigations on OCM are conducted near at-mospheric or at light pressures, i.e., 2 barto 3 bar. Nevertheless, higher pres-sures, e.g.,10 bar, are still relevant for industrial implementation, because they allow for a reduction in equipment size. If a high-pressure recycle stream, e.g., unconverted methane from a demethanizer column in the downstream, is fed back to the reactor, having the reactor at higher pressure can also eliminate the need for expanding and re-compressing large gas ows.

1.2.2 OCM Catalysts

Metal oxides are typically used as OCM catalysts. Common materials include Li/MgO, La₂O₃/CaO, and MnNa₂WO₄/SiO₂, which are active between 973 and 1123 K, i.e., high temperature catalysts, and generally applied in PBR (Vandewalle et al., 2019). Despite being used in several studies, Li/MgO is un-stable irrespective of the preparation procedure and suers strong deactivation, making it unsuitable for industrial application (Arndt et al., 2011).

The group of Prof. Manfred Baerns (Ruhr-Universität Bochum, Germany) made signicant progress on OCM in the 1990's and they concluded that, among the 400 tested materials, La₂O₃(27%)/CaO presented the highest catalytic per-formance and also good uidizability, which would also enable the application of Fluidized-Bed Reactor (FBR)s. A SrO(16.7%)La2O3(33%)/CaO catalyst has recently been applied to improve the heating value of biogas via OCM (Friedel, Nitzsche, and Krause, 2017), and a high-performance La₂O₃/CaO nanorod catalyst has been successfully synthesized and tested (Huang et al., 2013).

Even though the mechanistic aspetc of OCM over Mn Na₂WO₄/SiO₂ re-main largely unknown, this catalyst has been frequently cited as an excellent candidate for industrial implementation due to its performance and stability at high temperatures (Arndt et al., 2012; Shi, Yao, and Hu, 2015). Several

authors obtained 20-30 % methane conversions at 70-80 %selectivity on long-term experiments with MnNa₂WO₄/SiO₂ (Arndt et al., 2012). The cata-lyst prepration method can also have a signicant eect on its performance.

Godini et al., 2014 developed a sol-gel method for synthesizing Mn(1.9% )-Na₂WO₄(5%)/SiO₂ providing up to 15 % higher selectivity than the catalyst prepared by the more established incipient wetness impregnation method. A maximum of24.2 %yield in C2 products has been reached by applying this cat-alyst on a Packed-Bed Membrane Reactor (PBMR) under light (20 %) nitrogen dilution.

From an implementation point of view, low temperature catalysts (T <

973 K) oer signicant advantage, but these are at an earlier development stage (Vandewalle et al., 2019). Judging by Siluria's patent, it can be indeed observed that the claimed yields are not particularly high, e.g.,12 %C₂H₄ yield, but the major advantage is rather the fact that this can be achieved on an adiabatic PBR with an inlet temperature of813 K(Siluria Technologies, 2015). By using a La₂Ce₂O₇ catalyst doped with Ca²⁺ cations on its lattice, Xu et al., 2019 obtained a C2 product yield of 22.5 % at only1023 K. In this sense, La-based catalysts perform better at lower temperatures, while MnNa₂WO₄ catalysts typically require temperatures above 1073 K to eciently promote OCM (Xu et al., 2019).

Another interesting novel approach is the utilization of O² conductive ma-terials as membranes and catalysts. Igenegbai, Meyer, and Linic, 2018 applied gadolinium-doped barium created as a combined catalyst-membrane for OCM, obtaining C2 product yields of 14 % at 1023 K. Phase-decomposition, which occurred due to the presence of CO2, could be suppressed by further doping the material with zinc.

In spite of research eorts, little improvement has been achieved in terms of catalyst development after 2010, hence improvements in reactor operation and cost-ecient separation methods are required to make OCM commercially feasible (Kondratenko et al., 2017).