Modelling of Carbon Tetrachloride Decomposition in Oxidative RF Thermal Plasma

Abstract

Decomposition of carbon tetrachloride in a RF thermal plasma reactor was investigated in oxygen-argon atmosphere. The net conversion of CCl 4 and the main products of decomposition were determined by GC-MS (Gas Chromatographic Mass Spectroscopy) analysis of the exhaust gas. Temperature and flow profiles had been determined in computer simulations and were used for concentration calculations. Concentration profiles of the species along the axis of the reactor were calculated using a newly developed chemical kinetic mechanism, containing 34 species and 134 irreversible reaction steps. Simulations showed that all carbon tetrachloride decomposed within a few microseconds. However, CCl 4 was partly recombined from its decomposition products. Calculations predicted 97.9% net conversion of carbon tetrachloride, which was close to the experimentally determined value of 92.5%. This means that in RF thermal plasma reactor much less CCl 4 was reconstructed in oxidative environment than using an oxygen-free mixture, where the net conversion had been determined to be 61%. The kinetic mechanism could be reduced to 55 irreversible reaction steps of 26 species, while the simulated concentrations of the important species were within 0.1% identical compared to that of the complete mechanism.

Methane Conversion: a Case Study for Simplification of Plasma Chemistry Models by the Omission of Charged Species

“…Therefore, it can be concluded that it is a good assumption that at thermal plasma conditions the ionic reactions have no importance compared to the neutral ones and can be neglected from the reaction mechanism without causing any significant error in the kinetic results and the product concentrations. For carbon tetrachloride thermal plasma the maximum temperature is 7,000 K. Considering a small ionization [5,12,13] the neutral concentration is far higher than the concentrations of the ions and electrons. Experimental results also prove that at atmospheric pressure in thermal plasmas the ion and electron containing reactions are slower compared to the thermal reactions, and they can be neglected from a reaction kinetic model [12,13].…”

“…For carbon tetrachloride thermal plasma the maximum temperature is 7,000 K. Considering a small ionization [5,12,13] the neutral concentration is far higher than the concentrations of the ions and electrons. Experimental results also prove that at atmospheric pressure in thermal plasmas the ion and electron containing reactions are slower compared to the thermal reactions, and they can be neglected from a reaction kinetic model [12,13]. Orlandini and Riedel [12] investigated the NO conversion at atmospheric pressure and they found that the simulation results were in good accordance with the previous experimental results, although the ionic species were completely ignored.…”

Estudo da eficiência de degradação de tetracloreto de carbono por plasma térmico e caracterização dos produtos formados

“…Kovács et al 3,21 propuseram um modelo teórico para a decomposição do CCl 4 por pirólise em atmosfera de argônio, considerando que em temperaturas próximas a 7000 K a etapa dominante é a decomposição térmica seguindo as reações:…”

“…A adição de gases oxidantes ao plasma, por exemplo, na degradação de hidrocarbonetos reduz a formação de carbono sólido (fuligem). 3 Os oxidantes mais comuns utilizados são oxigênio ou vapor d'água, que gaseificam o carbono sólido a dióxido de carbono ou monóxido de carbono. 3 O processo de degradação por plasma térmico segue basicamente duas etapas: na primeira, a alta temperatura em conjunto com o fluxo de elétrons gerados pela ionização do gás, formando o plasma, quebra as ligações químicas das moléculas formando radicais livres e/ou íons muito instáveis que, numa segunda etapa, no esfriamento da mistura gasosa, se recombinam espontaneamente formando novas substâncias em um processo entropicamente favorável.…”

“…3 Os oxidantes mais comuns utilizados são oxigênio ou vapor d'água, que gaseificam o carbono sólido a dióxido de carbono ou monóxido de carbono. 3 O processo de degradação por plasma térmico segue basicamente duas etapas: na primeira, a alta temperatura em conjunto com o fluxo de elétrons gerados pela ionização do gás, formando o plasma, quebra as ligações químicas das moléculas formando radicais livres e/ou íons muito instáveis que, numa segunda etapa, no esfriamento da mistura gasosa, se recombinam espontaneamente formando novas substâncias em um processo entropicamente favorável. 4 A tecnologia de plasma térmico aplicado à degradação química, apesar de muito promissora, requer ainda muito esforço para se entender a cinética e o mecanismo das reações que ocorrem no meio e, desta forma, predizer, identificar e quantificar os produtos preferencialmente formados para o total controle do processo.…”