A new reactor concept for combining oxidative coupling and steam re-forming of methane: modeling and analysis

A direct method of converting natural gas into ethylene is the heterogeneously catalyzed oxidative coupling of methane (OCM), however, only with hydrocarbon yields limited to 30-35% despite enormous efforts to optimize the catalysts. By combining the exothermic OCM with a secondary process, namely steam reforming of methane (SRM), the methane conversion can be increased significantly while improving temperature control and simultaneously producing valuable synthesis gas. In this thesis, two different reactor concepts were developed to integrate the OCM and SRM reactions in an overall autothermal process, so that the OCM process is effectively cooled and the generated reaction energy is efficiently used to produce synthesis gas. The integration is most optimally achieved on the catalyst particle scale, which would eliminate the need for external heat exchange and opens up the possibility to use distributive oxygen dosing with which much higher product yields can be achieved. It is proposed to use a dual function catalyst particle in which the two chemical processes are physically separated by an inert, porous layer, such that additional diffusional resistances are intentionally created to control the reaction rates. This concept was studied with numerical simulations on the scale of a single catalyst particle and on reactor scale. It was found that the SRM and OCM reaction rates could be effectively tuned to achieve autothermal operation at the reactor scale, while the methane conversion was enhanced from 44% to 55%. An alternative integrated process can be achieved by combining OCM and SRM in a heat exchange reactor comprising of two separate reaction chambers which are thermally coupled. The OCM is carried out in packed bed reverse flow membrane reactor tubes submerged into a fluidized bed where the unconverted methane and byproducts from OCM are reformed, thus producing synthesis gas and consuming the reaction heat liberated by OCM. The feasibility of this concept is supported by experiments of OCM on a Mn/Na2WO4/SiO2 catalyst in a packed bed (porous Al2O3) membrane reactor. The results demonstrated that a C2 yield of 25-30 % can be achieved and that distributed feed of oxygen is optimal for the combined OCM/SRM reactor concept.

Oxidative coupling of methane has been studied over (Mn+A+W)/SiO2 catalysts in a continuous–flow micro reactor, where A represents an alkali ion of Li, Na, K or Cs having different weight percents. The main aim of this study is to find the role of alkali ions in interaction between Mn and W species with SiO2 to make a proper structure for catalyzing oxidative coupling of methane (OCM) reaction. The catalysts were characterized by XRD, SEM, FTIR, TPR and also the electrical conductivity was measured in air and under OCM reaction. It was found that for the formation of crystallized catalyst, the amount of alkali ion should be such that the catalyst containing tungsten transforms into A2WO4. Using a smaller amount of alkali ions does not result in crystalline catalyst by calcination under the same condition of temperature and atmosphere. However, under the OCM reaction condition the catalyst gradually turns into a crystalline structure and its catalytic performance, i.e. conversion and selectivity, for the OCM reaction is almost similar to the (Mn+A2WO4)/SiO2 catalyst. The transformation of the catalyst containing alkali ions from amorphous to crystalline one indicates a kind of structural flexibility of the catalyst under OCM atmosphere. The structural flexibility of the catalyst under the OCM reaction is considered to be the main chemical role of the alkali ions.

The catalytic oxidative coupling of methane (OCM), which can be used to obtain ethylene, is a major challenge in heterogeneous catalysis. This chapter mainly discusses the active sites of OCM catalysts, their reaction mechanisms, and their catalytic performance under various oxidative reaction conditions, including the OCM reaction network. In the OCM reaction, CH4 is oxidatively converted to C2H6 and then C2H4. After activation of CH4 on catalysts such as metal oxides, the formation of C2H6 proceeds in a homogenous gas phase via a free-radical mechanism. Thus, C2H6 is produced mainly by the coupling of the surface-generated •CH3 radical (methyl radical) in the gas phase. The C2H4/C2H6 yields are limited by the secondary reaction of •CH3 radicals with the catalyst and reactor surfaces and the further oxidation of C2H4 on the catalyst surface and in the gas phase. The nature of the active sites and the reaction mechanism have been investigated. Reactive oxygen ions, such as O− or O22−, are required for the activation of methane on catalysts. However, no feasible processes have resulted, despite a reasonable understanding of the elementary reactions in the OCM reaction. The non-oxidative coupling of methane (dehydrogenative coupling of methane) gives C2H4 and aromatic hydrocarbons at ~1000 K. Although the dehydrogenative coupling of methane is thermodynamically disadvantageous due to the large positive change in free energy, over-oxidation does not occur, and CO and CO2 are not formed. The catalytic performance of supported Fe catalysts, such as SiO2-supported Fe, are discussed, along with their catalytic properties.

Design of a microchannel-based reactor module for thermally coupled reactions: Oxidative coupling and steam reforming of methane

The effect of heat transfer on products of a thermally coupled shell and tube reactor consisting of two processes: Steam reforming of methane and oxidative coupling of methane

Influence of nanocatalyst on oxidative coupling, steam and dry reforming of methane: A short review

Conceptual Process Design and Economic Analysis of Oxidative Coupling of Methane

Atomically dispersed SrOx species on exposed {2 2 2} facets of pyrochlore La2Zr2O7 nanocrystals for boosting low-temperature oxidative coupling of methane

Integrated autothermal oxidative coupling and steam reforming of methane. Part 1: Design of a dual-function catalyst particle

Molecular structure and catalytic promotional effect of Mn on supported Na2WO4/SiO2 catalysts for oxidative coupling of methane (OCM) reaction

Oxidative Coupling of Methane (OCM) processes have been investigated as an alternative promising approach for ethylene production for the last three decades. Having considered the performance of the state-of-the-art OCM catalysts and the OCM reaction mechanism, improving the performance of the OCM membrane reactor could be considered as an important contribution to address such a complicated reactor engineering task. In this context, a systematic methodology implementing inorganic membranes, properly modified via silica-based materials, and the thereby achieved outstanding OCM membrane reactor performances are reported here. Moreover, the most important aspects of the performance analysis of OCM membrane reactors, especially in the context of the thermal-engineering characteristics of these systems, are discussed. Such analysis, for the most part, can be applied similarly to analyze other highly exothermic reaction systems in membrane reactors. Interactions between the membrane and the benchmark Mn–Na2WO4/SiO2 catalyst are also discussed. Furthermore, along with reviewing the general aspects of the model-based analysis of OCM membrane reactors, the potential of integrated OCM membrane reactors, such as dual-membrane reactors, is also highlighted. The special characteristics of modeling such non-isothermal reaction systems with significant mass and heat integration in both radial and axial dimensions are also reviewed.

Natural gas (NG), mainly composed of methane, is extremely resistant to autoignition. High efficiency premixed compression ignition (CI) combustion modes are therefore difficult to achieve in NG engines. One potential method for expanding the range of compression ignition operation in NG engines is to pretreat the fuel such that it becomes more reactive. This paper presents results from one-dimensional simulations of a medium duty multi-cylinder natural gas engine partially fueled by products from oxidative coupling of methane (OCM), a catalytic process by which methane is converted to C2 hydrocarbons like ethane and ethylene. First, the results of OCM reactor three-dimensional CFD simulations are presented to define the resulting product species distribution over a range of residence time. Using the reactor results, a one-dimensional model of an engine is implemented with and without OCM products to illustrate efficiency gains possible by operating in a compression ignition mode. Results of the modeling show that brake thermal efficiency improves by 1 to 7% with OCM-enabled CI combustion compared to spark ignition combustion of natural gas alone. The results also show that since the OCM reactor is exothermic, part of the incoming natural gas heating value is converted to sensible enthalpy. The OCM products therefore must be used to preheat the intake charge, thus avoiding an overall brake thermal efficiency (BTE) penalty. Overall, this study finds that OCM products have the potential to increase BTE of NG engines by allowing them to operate in premixed CI modes. Future work must look to improve OCM catalyst durability and reactor turndown ratio to provide a reliable high reactivity fuel stream for NG engines.

Rare earth oxides (REOs), particularly the sesquioxides, such as Sm2O3 and La2O3, have been investigated as promising catalysts in the oxidative coupling of methane (OCM). Much less attention has been paid to the reducible REOs because they are expected to give oxidation products, such as CO and CO2 (COx), rather than the desirable ethane and ethylene (C2+). Because Li addition can improve the performance of Sm2O3 in the OCM reaction and Li/MgO is commonly used as a reference OCM catalyst, the effects of lithium addition to a reducible oxide, TbOx, were investigated in detail in this study and compared with a Sm2O3 catalyst, which is the best single component OCM catalyst. Because of the well-documented volatility of lithium under OCM conditions, particularly for the Li/MgO system, the stability of lithium-doped samaria and terbia catalysts was examined as a function of preparation methods in this study. As expected, terbia supported on nanoparticle magnesia (n-MgO) is not a very active or selective OCM c...

Production of ethylene by oxidative coupling of methane (OCM) is a promising direct path from methane to ethylene. Li-W-Mn-O-SiO2 composite materials prepared by various methods—solid-phase synthesis, silica impregnation, sol-gel synthesis—are used as OCM catalysts. The phase states of these composite materials prepared by the different methods has been studied before and after use in the OCM reaction; the study has unexpectedly revealed the effect of the preparation technique on the phase composition of the material and the specific features of its catalytic behavior in OCM. Optimum catalysts provide an ethylene yield of 15% in terms of passed methane.

The enhancement effects of BaX2 (X = F, Cl, Br) on SnO2-based catalysts for the oxidative coupling of methane (OCM)