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Düsel, Andreas ; Fremerey, Peter ; Müller, Norbert ; Moos, Ralf ; Jess, Andreas:
Direct detection of sulfur deposits on fixed bed catalysts by electrical sensors.
2011
Veranstaltung: 8th European Congress of Chemical Engineering
, 25.-29.09.2011
, Berlin, Germany.
(Veranstaltungsbeitrag: Kongress/Konferenz/Symposium/Tagung
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Abstract
One mechanism for the deactivation of heterogeneous catalysts is catalyst poisoning stemming from sulfur compounds in the inlet, e.g. H2S. Poisoning might be caused by chemisorption on the catalyst surface or by the formation of bulk sulfides such as Ni3S2 and FeS. In some individual cases, “poisoning” is useful since it increases the catalyst activity, e.g., for CoMo-contacts to desulfurize mineral oil fractions through the formation of the active phases Co9S8 and MoS2. To optimize the processes, the knowledge of the actual sulfur loading can be helpful. Therefore, a method to detect insitu the sulfur poisoning level has been investigated. For that purpose, electrical contacts are applied to single catalyst pellets and the complex impedance of such devices representing the catalyst itself is measured during operation. Already performed studies using such a setup have shown that it is possible to detect both coking and coke burn-off not only of single catalyst pellets but also space-resolved in a complete fixed bed reactor. Sulfur poisoning is crucial with respect to nickel based catalysts. In theory, the nickel catalyst has a strongly reduced electrical conductivity compared to a pure metal conductor, because of highly dispersed nickel grains on the catalyst surface in combination with a lower nickel content of the catalyst. In a poisoning process with sulfur compounds, nickel gradually transforms to Ni3S2, which has a high conductivity. This should lead to a formation of conducting paths through the catalyst. As a consequence, the electrical resistance of the system should decrease. This agrees with the conducted impedance measurements. A non-poisoned catalyst pellet shows a high impedance. It drops when the sulfur loading gets increased. In addition, experiments with respect to regeneration of poisoned catalyst pellets with hydrogen have been performed. It could be shown that the impedance increases again when the sulphur desorbs. Further research deals with finding a correlation between the sulfur load and the electrical signal. Furthermore, it is important to investigate the dependence of the sensor signal with respect to varying test parameters (temperature, pressure, sulfurconcentration…) and to check cross-sensitivities to other deactivation mechanisms (e.g. catalyst fouling through coking).