Ethylene (C2H4) is the building block for a vast range of chemicals from plastics to antifreeze solutions and solvents. It is among the most often produced organic compounds of the petrochemical industry. The current market demand is more than 150 million tons per year with a global growth rate of around 3.5% over the next 5 years . It is usually produced in steam-cracking units from a range of petroleum-based feedstocks, such as naphtha. Therefore its production capacity and cost are strongly depending on the availability and price of crude oil. Because of the increasing demand for ethylene besides the limitation in the oil reserves an alternative process to produce this essential compound is required. Oxidative coupling of methane (OCM) which converts methane directly into C2 products and higher hydrocarbons (1), may be the way to realize this aim. The most important advantage of this process is that it converts the less expensive methane, with its reserves exceeding those of crude oil, to highly demanded product, i.e. ethylene .
CH4 + O2 → C2H6 or C2H4 + H2O(1)
Nevertheless, the OCM process has not yet been commercialized, as a minimum yield of about 30% towards C2 products is required for making it economically feasible. The main reason for this limitation is the occurrence of partial and total oxidation reactions which are thermodynamically more favorable than the coupling reaction. Noteworthy, simultaneous presence of oxygen and the main hydrocarbons of this reaction, i.e. methane, ethane and ethylene in the reactor increase the chance of occurrence of unselective oxidation reactions. Therefore, their separation by implementing the chemical looping concept can be a good alternative to classical co-feed reactors. This reactor concept exists since about hundred years and is well-known for combustion processes, chemical looping combustion (CLC). There, it is used to separate the formed carbon dioxide from nitrogen. We employed the same concept to OCM and could successfully improve the performance of this process by suppressing the rate of unselective oxidation reactions [4,5].
To take advantage of the chemical looping reactor concept for OCM, there is a vital need to a catalyst material which can provide a sufficient storage capacity for one of the reactants, in general oxygen.
Fortunately, the Na2WO4/Mn/SiO2 which is already known as the most stable and active catalysts for this reaction [2,6], can also fulfill this requirement; therefore the studies was done over this catalyst. The reactor concept in the laboratory scale was realized by installing two pneumatic pulse valves in the inlet of a fixed bed reactor. Each of the valves was used for feeding one of the reactants, O2 or CH4, to the reactor one after the other. Figure 1 shows the operation procedure of this reactor . In the first step the catalyst material which is already heated to process temperature is oxidized with air. Afterwards, the reactor is purged with an inert gas to remove the oxygen left in the gas phase of the reactor. Then, methane is passed through the catalyst bed and forms methyl radicals which couple in the gas phase to form ethane and later ethylene. Finally, un-reacted methane and the products are removed from the reactor and the catalyst can be re-oxidized. The OCM is tested in this reactor under several different reaction conditions. The performance obtained in these experiments is compared to that reported in the literature for the reaction operated in fixed bed reactors (fig-2). The results show that this method achieves a significantly higher yield in comparison to the classical co-feed reactors. In addition, a high conversion of methane can be safely achieved since no explosion regime has to be taken into account in the chemical looping reactors.
The next step in this project is to scale up the process. This can be done in two circulating fluidized bed reactors or in a sequence of fixed bed reactors that simulate a moving bed by a switchable feed strategy. The compatibility of our catalyst with the fluidized-bed reactor under stationary condition was already demonstrated in earlier studies .
, Vinzenz Fleischer1
, Ulla Simon1
, Reinhard Schomäcker1
1 TU Berlin/School II, Department of Chemistry, Berlin, Germany
Prof. Dr. Reinhard Schomäcker
TU Berlin/School II
Department of Chemistry
 “The Ethylene Technology Report 2016,” can be found under www.researchandmarkets.com, 2016
 S. Arndt, T. Otremba, U. Simon, M. Yildiz, H. Schubert, R. Schomäcker, Appl. Catal. A Gen. 2012, 425–426, 53–61
 S. Arndt, G. Laugel, S. Levchenko, R. Horn, M. Baerns, M. Scheffler, R. Schlögl, R. Schomäcker, Catal. Rev. 2011, 53, 424–514
 V. Fleischer, P. Littlewood, S. Parishan, R. Schomäcker, Chem. Eng. J. 2016, 306, 646–654
 J. Adanez, A. Abad, F. Garcia-Labiano, P. Gayan, L. F. De Diego, Prog. Energy Combust. Sci. 2012, 38, 215–282
 M. Yildiz, U. Simon, T. Otremba, Y. Aksu, K. Kailasam, a. Thomas, R. Schomäcker, S. Arndt, Catal. Today 2014, 228, 5–14
 S. Jašo, S. Sadjadi, H. R. Godini, U. Simon, S. Arndt, O. Görke, a. Berthold, H. Arellano-Garcia, H. Schubert, R. Schomäcker, et al., J. Nat. Gas Chem. 2012, 21, 534–543
More on ethylen