Using a membrane reactor for the oxidative coupling of methane: simulation and optimization

Oxidative coupling of methane (OCM) is a promising route for the production of ethylene by fully utilizing the abundance of methane feedstock. However, this process suffers from the relatively low selectivity and yield of ethylene at a higher methane conversion due to the complete oxidation of methane and ethylene products. To overcome this limitation, the application of a membrane reactor in which oxygen selective membrane is used to prevent the deep oxidation of a desirable ethylene product is a potential alternative. In this study, a multi-stage dense tubular membrane reactor is proposed to improve the performance of the oxidative coupling of methane. Mathematic model of the membrane reactor based on conservative equations and detailed OCM kinetic model is employed to analyze the effect of key operating parameters such as temperature, methane-to-oxygen feed ratio and methane flow rate, on the efficiency of the OCM process in terms of CH4 conversion, C2 selectivity and C2 yield. Adjustment of feed distributions at each membrane stage under isothermal and non-isothermal conditions is also studied. The performance of the multi-stage membrane reactor is compared with a single stage membrane reactor. The result shows that the distributed feeding policy improves the performance of the OCM process. A surface response technique is further employed to determine the optimal operating condition of the OCM process with the aim to maximize the C2 products.

The performance of the Oxidative Coupling of Methane (OCM) reactions in a porous ceramic packed bed membrane reactor was experimentally investigated using an Mn–Na2WO4/SiO2 catalyst. A novel practical method was applied to modify the available commercial α-alumina membrane and shape it to the form of an inert fine oxygen distributor in an OCM membrane reactor. The characteristics of such modified membrane and the performance of the resultant OCM membrane reactor are reviewed in this paper. It was observed that establishing a 2 bar pressure gradient across the modified membrane ensures a safe and efficient oxygen-dosing along the OCM membrane reactor. Moreover, using a modified membrane with a descending permeation profile instead of a uniformed permeation profile improved the observed C2 selectivity (ethylene and ethane) by 10% in average. The efficient design of the membrane reactor setup and the stability of the prepared catalyst provided a robust operation and replicable results. In this experimental analysis, a very promising 25.5% C2 yield and 20.3% ethylene yield with 66% C2 selectivity were achieved under very low (20%) diluted reaction atmosphere for the methane-to-oxygen ratio 2. By proper exploiting the carbon dioxide instead of nitrogen dilution, the C2 yield was improved by 1–2% in average.

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.

For more than three decades, Oxidative Coupling of Methane (OCM) process has been comprehensively investigated as an attractive alternative for the commercially available ethylene production technologies such as ethane and naphtha cracking. Developing a suitable catalyst and proper reactor feeding policy, reviewing and deploying the efficient methods in the separation and purification of the undesired and desired products, possible energy saving and process intensification in different sections of the OCM process, each has been the subject of many researches in the past. In this paper, the interconnections of these aspects will be addressed by reviewing the performance of different alternative structures and unit operations in different sections of the OCM process. As a systematic approach in this analysis, a concurrent engineering approach supported by the experimental data which was extracted from an OCM miniplant scale facility was applied. Using an efficient porous packed bed membrane reactor and a proper set of operating conditions, up to 25% C2-yield, 20% C2H4-yield, and 52% C2H4-selectivity and the highest observed fluidized bed reactor C2-yield was achieved in this OCM miniplant. This experimental analysis was performed in the chair of process dynamics and operation at Berlin Institute of Technology under the main framework of Unifying Concepts in Catalysis (UniCat) project. The economic analysis of the industrial-scale operation showed promising potentials and also advantages of the final proposed OCM process-structure in this research.

Techno-economic analysis of integrating the methane oxidative coupling and methane reforming processes

Oxidative coupling of methane (OCM) conversion into C2 products through a CO2/O2 co-transport membrane reactor

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.

Oxidative coupling of methane (OCM) on a conventional fixed-bed reactor (FBR) and a ceramic dense membrane reactor (DMR) packed with Li/MgO catalyst is analyzed using plug-flow reactor models. The validity of OCM kinetic equations employed in the modeling is confirmed by excellent agreement between the simulation and experimental data for OCM on FBR. For FBR, a high methane to oxygen feed ratio favors the OCM reaction, with a low C2 yield because of insufficient oxygen supply. The highest C2 yield achieved with a feed mixture consisting of 70% methane and 30% oxygen is 20.7% at a selectivity of 53% and operating temperature of 750 °C. The C2 yield and selectivity increase slightly at a higher operating temperature. The optimal feed ratio does not change with temperature. DMR is made of a mixed-conducting ceramic membrane tube packed with an OCM catalyst. The membrane tube separates the methane and oxygen feed. The oxygen concentration in the DMR is much lower and more uniform than that in the FBR because ...

Conceptual Process Design and Economic Analysis of Oxidative Coupling of Methane

Simultaneous Synthesis of the Downstream Process and the Reactor Concept for the Oxidative Coupling of Methane (OCM)

Oxidative coupling of methane (OCM) is a process to directly convert methane into ethylene. However, its ethylene yield is limited in conventional reactors by the nature of the reaction system. In this work, the integration of different membranes to increase the overall performance of the large-scale oxidative coupling of methane process has been investigated from a techno-economic point of view. A 1D membrane reactor model has been developed, and the results show that the OCM reactor yield is significantly improved when integrating either porous or dense membranes in packed bed reactors. These higher yields have a positive impact on the economics and performance of the downstream separation, resulting in a cost of ethylene production of 595–625 €/tonC2H4 depending on the type of membranes employed, 25–30% lower than the benchmark technology based on oil as feedstock (naphtha steam cracking). Despite the use of a cryogenic separation unit, the porous membranes configuration shows generally better results than dense ones because of the much larger membrane area required in the dense membranes case. In addition, the CO2 emissions of the OCM studied processes are also much lower than the benchmark technology (total CO2 emissions are reduced by 96% in the dense membranes case and by 88% in the porous membranes case, with respect to naphtha steam cracking), where the high direct CO2 emissions have a major impact on the process. However, the scalability and the issues associated with it seem to be the main constraints to the industrial application of the process, since experimental studies of these membrane reactor technologies have been carried out just on a very small scale.

The oxidative coupling of methane (OCM) in a fixed bed reactor (FBR) and a membrane reactor (MR) were studied by two-dimensional numerical simulations. In FBR, a suitable catalyst was selected by comparing between Li/MgO, La2O3/CaO and Na-W-Mn/SiO2 catalysts. The simulation results indicated that Na-W-Mn/SiO2 catalyst offers the best performances. Different operating conditions, such as temperature, CH4/O2 ratio and GHSV were studied. Increasing operating temperature resulted in increasing of CH4 conversion but decreasing C2 selectivity. However, the effects of CH4/O2 ratio and GHSV showed the contrary results. In MR, the suitable membranes were selected by comparing between porous Membranox, a dense BSCFO and LSGFO membrane. Simulation results indicated that BSCFO membrane offers the best performances. Various operating conditions, such as methane flow rate, air flow rate and temperature have influences on performance of OCM reaction. Increasing operating temperature resulted in increasing of CH4 conversion and decreasing of C2 selectivity. Moreover, increasing of methane feed flow rate resulted in lower CH4 conversion but increased C2 selectivity. The effect of air flow rate showed the contrary results. When comparing the performance between FBR and MR, it was found that the yield of MR was higher than FBR. The temperature profiles of FBR and MR revealed that significant hot spot temperature was observed for the FBR unlike that of the MR. Optimum dimension of MR was 0.018 m diameter and 0.2 m length. The best performance was found at GHSV of 38904.54 h-1 and temperature of 1073 K, offering CH4 conversion of 43.713 %, C2 selectivity of 61.352 % and C2 yield of 26.82 %.

On the design, development and operation of an energy efficient CO2 removal for the oxidative coupling of methane in a miniplant scale

Oxidative coupling of methane in a modified γ-alumina membrane reactor

Oxidative coupling of methane (OCM) to higher hydrocarbons over Sr-promoted La2O3 supported on commercial low surface area porous catalyst carriers (containing mainly alumina and silica) at 800 and 850 °C and a space velocity of 102 000 cm3·g-1·h-1 has been thoroughly investigated. Effects of support, catalyst particle size, linear gas velocity (at the same space velocity), Sr/La ratio, CH4/O2 ratio in the feed, and catalyst dilution by inert solid particles on the conversion, yield, or selectivity and product ratios (C2H4/C2H6 and CO/CO2) in the OCM process have been studied. The catalysts have been characterized for their basicity, acidity, and oxygen chemisorption by the TPD of CO2, ammonia, and oxygen, respectively, from 50 to 950 °C and also characterized for their surface area. The supported catalysts showed better performance than the unsupported one. The best OCM results (obtained over Sr-La2O3/SA-5205 with a Sr/La ratio of 0.3 at a space velocity of 102 000 cm3·g-1·h-1) are 30.1% CH4 conversion with 65.6% selectivity for C2+ (or 19.7% C2+-yield) at 800 °C (CH4/O2 = 4.0) and 12.8% CH4 conversion with 85.1% selectivity for C2+ (or 10.9% C2+-yield) at 850 °C (CH4/O2 = 16.0). The basicity is strongly influenced by the Sr/La ratio; the supported catalysts showed the best performance for their Sr/La ratio of about 0.3. The methane/O2 ratio also showed a strong influence on the OCM process. However, the influence of linear gas velocity and particle size is found to be small; it results mainly from the temperature gradient in the catalyst. The catalyst dilution has little or no effect on the conversion and selectivity. However, it has beneficial effects for achieving a higher C2H4/C2H6 ratio and also for reducing the hazardous nature of the OCM process because of the coupling of the exothermic oxidative conversion reactions and the endothermic thermal cracking reactions and also due to the increased heat transfer area.