Methane catalytic decomposition over Cu-Ni-Al catalyst: reaction rate and catalyst regeneration
Advancing the chemical engineering fundamentals
Catalysis - III (T2-13c)
Keywords: hydrogen, Cu-Ni-Al co-precipitated catalyst , regeneration
METHANE CATALYTIC DECOMPOSITION OVER Cu-Ni-Al CATALYST: REACTION RATE AND CATALYST REGENERATION
Taís E. Machado, Marla A. Lansarin and Oscar W. Perez-Lopez
aDepartamento de Engenharia Química – Universidade Federal do Rio Grande do Sul, Rua Eng. Luiz Englert s/n. CEP 90040-040, Porto Alegre, RS, Brasil.
The hydrogen has been considered as ideal energy source because its combustion generates just water. Among other applications, it has been used in Fuel Cell. Methane is the most used hydrogen source due to its high hydrogen to carbon ratio and its abundant storage. The natural gas steam reforming is currently the largest and most economic way to produce hydrogen. Others pathways are the partial oxidation, the autothermal reforming and the dry reforming.
These processes supply hydrogen in mixture with carbon monoxide, which is a strong poison for Proton Exchange Membrane Fuel Cell, the most potential fuel cell to propel vehicles. This way, an interesting alternative process for hydrogen production is the methane catalytic decomposition, or methane decarbonization:
CH4 → C + H2 (∆H298K = 74,54 kJ/mol) (1)
The hydrogen produced by methane decomposition is of high purity, and can be utilized directly as the fuel of H2–O2 fuel cell. Besides, in specific conditions the carbon is produced in the nanotubes form. The carbon nanotubes have been turned an active field due to their exceptional properties, that make them suitable for many applications such as polymer reinforcements for composites or breakthrough materials for energy storage, electronics and catalysis.
The objective of the present work is the kinetic study and determination of the methane catalytic decomposition reaction rate over Cu-Ni-Al catalyst in order to produce high purity hydrogen. The regeneration on the catalyst was also studied by oxidation of the produced carbon.
The catalyst was prepared by continuous coprecipitation method, starting from solution metals nitrate and sodium carbonate as precipitating. After the precipitate will be crystallized, filtrate and dried, samples were calcined at 600°C, using synthetic air. The activation was carried out with a H2/N2mixture. Than the methane decomposition was carried out with a CH4/N2 mixture at 600°C, during two hours. The catalytic tests were carried out in a thermobalance SDT Q600 model of the TA instruments.
The influences of diffusion in porous catalyst and of the operation conditions in the reaction rate were appraised. The catalyst was separate in four particle size ranges in order to determine the influence of the internal diffusion in the reaction rate.
Plots of deposited carbon weight versus time were built and the curves showed similar behavior indicating that, in the studied conditions, the diffusion effect is not limiting. So, it was established adequate diameter to the objectives of this work: 0,248 <dp <0,3551mm.
The methane decomposition was carried out under different temperatures and methane concentrations for reaction rate determination. Methane concentrations between 0.5 and 1.25 gmolm-3 were used. The Cu-Ni-Al co-precipitated catalyst stability was shown for temperatures above 500°C, and it began to lose activity at 700°C. The temperature range chosen for tests were 500 to 600°C.
The reaction rate grows quickly in the initial minutes and than, after 140 min time-on-stream, reaches a constant value. The catalyst was shown quite stable up to nine hours of reaction, when the test was interrupted due to pan capacity limitation.
The kinetic parameters were obtained and it was observed that the reaction is first order; the pre-exponential factor and the activation energy were 2 and 50655 Jmol-1, respectively.
Since the reaction forms carbon which is deposited on the catalyst surface causing its deactivation, regeneration studies were carried out. The regeneration was done by oxidation with air and its effects on the catalyst activity were also investigated. The catalyst was submitted to reaction-regeneration cycles, being the reaction carried out in larger methane concentration (1.25gmolm-3). After two hours of reaction (600°C), the methane was removed of the line by purge with nitrogen and air was added. Firstly, a 30 minutes long regeneration was carried out in temperatures between 500 and 600°C. After, 20, 30 and 45 min. long regenerations were carried out at 560°C. It was done at least three reaction-regeneration cycles for each case. The best regeneration condition, that is to say, that allows a larger number of cycles with low activity loss, was done at 560°C during 30 min. In this condition the catalyst could be regenerated three times, with loss of 20% of the activity.
During the regeneration, besides the carbon oxidation, metallic Ni and Cu can be partially converted to the oxide form, what requests a new catalyst activation stage after each regeneration. In the reaction-regeneration cycles mentioned previously, the methane acting as gas reducer during the reaction stage beginner. The activation was evidenced by a weight loss in the first minutes of the reaction. A test using H2/N2 reduction stage after each regeneration, was carried out (560°C during 30 min). This test result was better, being the catalyst after the first regeneration more active than the fresh catalyst. Four cycles reduction-reaction-regeneration was carried out and the catalyst lost 15% of its activity after the third regeneration.
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Presented Thursday 20, 15:40 to 16:00, in session Catalysis - III (T2-13c).
