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Content archived on 2024-04-15

ETHYLENE FROM NATURAL GAS BY DIRECT CATALYTIC OXIDATION.

Objective

TO COLLECT DATA FOR THE MODELLING OF THE CHEMICAL REACTION, TO OPTIMIZE THE PHYSICAL PARAMETERS OF THE CATALYSTS AND TO DETERMINE OPTIMUM REACTOR DESIGN AND REACTION CONDITIONS.
For the chemical industry it would be extremely valuable to be able to convert natural gas into olefines such as ethylene and propylene. Research was carried out into the design of an optimal catalyst in view of the complicated kinetics and the exothermicity of the reaction. Data were collected for modelling the chemical reaction and for determining the optimal reaction conditions and reactor design and the technical and economical feasibility of a methane oxidative coupling process was investigated.

The oxidative coupling of methane was a very promising new process for converting natural gas to ethylene. It was technically feasible and economically attractive, when hydrocarbons containing chains of 2 or more carbons (C2+) gave yields of 25% (C2+ selectivity greater than 60%) were achieved.

Lithium doped magnesium oxide was a very promising catalyst system for this process with which C2+ yields of 19% were reached. Lithium is the active component in lithium magnesium oxide. Very small amounts of lithium were sufficient to create an active and selective lithium magnesium oxide catalyst. The largest disadvantage however, was its stability because rapid deactivation occured by lithium loss which was caused by volatilization of lithium hydroxide formed by reaction of lithium carbonate and water by reaction with the quartz reactor wall forming lithium silicates.

From the reactor point of view the fluidized bed reactor seemed most suitable because of the high exothermicity of reaction. The new fluidized bed catalysts developed combine a good mechanical strength, which is essential for a fluidized bed catalyst, with a good selectivity and a good stability.

Experiments in an elevated pressure reactor set up have shown that moderate pressures are feasible without oxidizing all the desired C2 products. However, the high heat production can result in hot spots or run away of the reactor.

The kinetics of the methane coupling over lithium magnesium oxide can be de scribed as an Eley-Rideal mechanism in which gas phase methane reacts with absorbed oxygen at the catalyst surface. Methyl radicals are generated which couple to ethane in the gas phase. Ethane is dehydrogenated to ethylene which in turn is oxidized into carbon monoxide and carbon dioxide. A kinetic model, which simulates the gas phase methane coupling supported this reaction mechanism and the main role of the catalyst as a methyl radical generator. However, it also made clear that the catalyst has another role: a nonselective radical scavenger, which retards all gas phase radical reactions.
THE DIRECT OXIDATION OF METHANE ON APPROPRIATE CATALYSTS CAN LEAD EITHER TO C2 HYDROCARBONS OR TO METHANOL FORMATION. DEVISING AND TESTING NEW EFFICIENT CATALYST FORMULATIONS FOR BOTH REACTIONS IS THE AIM OF THE TWO COOPERATING CONTRACTORS. TECHNISCHE UNIVERSITEIT EINDHOVEN WILL UNDERTAKE THE CHEMICAL ENGINEERING STUDIES NEEDED FOR THE PROCESS DESIGN, STARTING FROM THE PARTNERS RESULTS. THUS, KINETIC STUDIES WILL BE PERFORMED TO COLLECT DATA FOR THE MODELLING OF THE REACTIONS. RUNS WILL BE MADE ON MICRO FIXED-BED REACTORS WITH AND WITHOUT STEAM ADDITION AND OVER A RANGE OF PRESSURES, AS WELL AS ON MICRO FLUIDIZED BED REACTORS. PULSE EXPERIMENTS WILL ALSO BE CONDUCTED TO REVEAL THE CONTRIBUTION OF MOLECULAR OXYGEN AS COMPARED WITH THAT OF ADSORBED OXYGEN SPECIES IN THE REACTION. OPTIMUM CATALYST PORE DISTRIBUTION AND PARTICLE SIZE WILL BE SOUGHT, IN VIEW OF THE COMPLEXE KINETICS AND THE EXOTHERMICITY OF THE REACTIONS. FOR THIS PURPOSE, EXPERIMENTS IN MICRO FIXED-BED REACTORS WILL BE CARRIED OUT TO DETERMINE THE EFFECT OF THE INTERNAL WALL-TO-VOLUME RATIO AND OF DIFFUSION LIMITATIONS; A CAPILLARY TUBE REACTOR COATED WITH CATALYST WILL ALSO BE USED. OPTIMUM REACTOR DESIGN AND REACTION CONDITIONS ARE TO BE DEFINED THROUGH EXPERIMENTS IN A VERSATILE FLUID-BED REACTOR (7 CM IN DIAMETER) AND IN A MULTISTAGE FIXED-BED REACTOR, BOTH OPERATING AT PRESSURES UP TO 15 BAR. VARIATION OF THE OXYGEN CONCENTRATION PROFILE WILL BE POSSIBLE IN BOTH REACTORS. SWING OPERATION WILL ALSO BE POSSIBLE. A FAST QUADRUPOLE SPECTROMETER WILL ENABLE CONCENTRATION PROFILES INSIDE THE REACTOR TO BE OBTAINED. FINALLY, A FEASIBILITY STUDY FOR THE METHANE-TO-METHANOL PROCESS, INCLUDING PRODUCT SEPARATIONS AND RECYCLES, WILL BE PERFORMED, SIMILAR TO THAT FOR THE ETHYLENE PROCESS. THIS WILL ASSESS, INTER ALIA, THE SELECTIVITY CONVERSION TARGETS TO BE ACHIEVED IN ORDER TO RENDER THE PROCESS ECONOMICALLY VIABLE.

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Coordinator

EINDHOVEN UNIVERSITY OF TECHNOLOGY
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Address
Den Dolech 2
5600 MB EINDHOVEN
Netherlands

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