New catalysts based on silicon carbide support for improvements in the sulfur recovery. Silicon carbide as support for the selective H2S oxidation

Abstract

As fases ativas NiS 2 e Fe 2 O 3 suportadas em β-carbeto de silício (SiC) com média área específica mostraram alta atividade, seletividade e estabilidade na oxidação direta do H 2 S a enxofre elementar. Os catalisadores foram testados a temperaturas que variaram da temperatura ambiente, no caso do Ni em reator de leito gotejante, até temperaturas superiores à do ponto de orvalho do enxofre, no caso do Fe em reator de leito fixo. Para ambos os casos, foi proposta a formação de uma fase bastante ativa de oxisulfeto de Ni ou de Fe, formada pela oxidação do NiS 2 e pela sulfuração do Fe 2 O 3 . A ausência de microporosidade no suporte contribuiu à alta seletividade do catalisador. A grande estabilidade ao carregamento de enxofre sólido, apresentada pelos catalisadores suportados em SiC em temperaturas inferiores a 100 °C, foi explicada pela maneira especial da deposição do enxofre, a qual depende do papel da água presente na reação e do caráter heterogêneo (hidrofílico e hidrofóbico) da superfície do suporte.The NiS 2 and Fe 2 O 3 active phases supported on medium surface area β-silicon carbide (SiC) showed high activity, selectivity and stability in the direct oxidation of H 2 S into elemental sulfur. The catalysts were tested under a large range of temperatures, from room temperature using Ni (tricklebed reactor) to over-dewpoint temperatures using Fe (fixed bed reactor). For both cases, the formation on stream of a Ni and Fe oxysulfide high active phase was proposed by oxidation of NiS 2 and sulfidation of Fe 2 O 3 , respectively. The absence of microporosity in the support contributed to the high selectivity into sulfur. At low temperatures (below 100 °C), the high stability of β-SiC supported catalysts towards the solid sulfur loading was explained by a specific mode of sulfur deposition, involving the role of water in the feed and the heterogeneous nature of the SiC surface, being partly hydrophilic and hydrophobic.Keywords: silicon carbide, catalyst support, sulfur recovery, H 2 S oxidation, hydrophilicity, hydrophobicity IntroductionThe removal of hydrogen sulfide (H 2 S) from the acid gases generated by oil refineries or natural gas plants is a crucial aspect of air pollution control, due to ever increasing standards of efficiency required by the environmental protection pressure. The general trend is to selectively transform H 2 S into elemental sulfur and steam, by the wellknown modified Claus process.1 (equation 1) 2 H 2 S + SO 2 ↔ 3/n S n + 2 H 2 OAccording to the different running processes, typical sulfur efficiencies are only 90-96 % for a two stage reactor unit and 95-98 % for a three stage process, due to the thermodynamic limitations of the Claus reaction.2 This means that for a conventional sulfur plant, the SO 2 emissions may amount to many thousand tons per year released into the atmosphere, and a large amount of sulfur compounds remains in the tail gas, i.e., SO 2 (≈ 6000 ppm), H 2 S (≈ 12000 ppm), COS and CS 2 (≈ 700 ppm). New catalysts and processes for the tail-gas treatment of the industri...

New catalysts based on silicon carbide support for improvements in the sulfur recovery: new silicon carbide nanotubes as catalyst support for the trickle-bed H2S oxidation

“…The NiS 2 nature of the supported phase after sulfidation has been determined by XRD. 3 The loading of Ni was set at 5 wt. % and confirmed by the atomic absorption spectroscopy technique, performed at the Service Central d'Analyse of the CNRS (Vernaison, France).…”

“…3 The weight hourly space velocity (WHSV) was defined as the ratio of the inlet H 2 S weight per hour to the weight of catalyst used. Structural characterization of the samples was done by powder XRD measurements, carried out with a Siemens Diffractometer Model D-5000, using a Cu K α radiation and operating at 40 kV and 20 mA.…”

“…3,6 Such a high performance for the selective H 2 S oxidation was notably due to the mechanical removal of the sulfur formed from the active sites by a water film. The role of condensed water, acting as a conveyor belt on the catalyst surface when using SiC grains has already been reported and discussed in details.…”

“…The role of condensed water, acting as a conveyor belt on the catalyst surface when using SiC grains has already been reported and discussed in details. 3,13,17 A liquid water film allowed the sulfur formed to be removed from the NiS 2 active sites before the subsequent storage on hydrophobic SiC surfaces, outside the pores, which are free of active phase.…”

“…1 Industrial processes and recent developments have been reported in Part I of the present paper and summarized in a series of reviews published in the literature. 2,3 Previous works showed the very high performances obtained on silicon carbide grains or extrudates (SiC) supported catalysts as compared to the literature and industrial catalysts, especially at the low temperature of 60 °C. [4][5][6][7] The H 2 S oxidation at low temperature is performed in a trickle-bed mode, the water of the inlet feed condensing at the head of the catalyst bed and trickling down through the catalyst.…”