Is it possible to detect in situ the sulfur loading of a fixed bed catalysts with a sensor?

Abstract

Abstract. This study reports on a sensor concept to measure in situ sulfur poisoning (sulfidation) of refinery catalysts, in this case, of commercial silica pellets loaded with highly dispersed nickel. Catalyst pellets were poisoned in diluted H 2 S between 100 and 400 • C and the sulfidation of the catalyst was observed. During this process, nickel sulfides are formed on the catalyst according to X-ray diffraction spectra and energy dispersive X-ray spectroscopy data. The sulfidation kinetics was quantitatively described by a shrinking core model. Representative catalyst pellets were electrically contacted, and their impedance was recorded in situ during sulfidation. At the beginning, the particles are highly insulating and behave capacitively. Their conductivity increases by decades during sulfidation. At high temperatures, an almost constant slope in the double-logarithmic representation vs. time can be found. At low temperatures, the conductivity remains constantly low for a long time but changes then rapidly by decades, which is also indicated by the phase that drops from capacitive to ohmic behavior. Since nickel sulfides exhibit a lower conductivity than nickel, the conductivity increase by decades during sulfidation can only be explained by electrically conducting percolation paths that form during sulfidation. They originate from the increased volume of sulfides compared to the pure nickel metal.

Why does the Conductivity of a Nickel Catalyst Increase during Sulfidation? An Exemplary Study Using an In Operando Sensor Device

“…Since the highly dispersed small nickel particles do not have electrical contact to each other, this low value may mainly originate from the thermally activated conductivity of the insulating oxide ceramic carrier pellet. When the catalyst gets sulfidized, the nickel particles react with the hydrogen sulfide to form nickel sulfides in several stoichiometries with the end member NiS (XRD (X-Ray diffraction) and EDX analysis confirm that [3]). All of these nickel sulfides show the character of a metallic conductor [10,11,12] but with a markedly lower conductivity than pure nickel.…”

“…According to [1], a shrinking core model explains the timely behavior of the sulfidation. In [3], this assumption has been quantitatively validated and the shrinking core model was plotted against the time. Additionally, the conductivity measurement in this paper is plotted against the time.…”

“…Figure 2 illustrates the sensor setup. A more detailed description of the single catalyst sensor device and the lab test bench is given in [3]. The gas flow for the sulfidation experiments was 50 L/h (NTP) with an H 2 S concentration of up to 1000 ppm in N 2 .…”

“…The effective conductivity, σ , was derived from the resistance R with the geometry of the catalyst particle. For further details see [3]. …”

“…In a previous study, we showed initial data how the same principle can be also applied for sulfidation detection of commercial silica pellets loaded with highly dispersed nickel (commercial refinery catalysts for gasoline reforming) [3]. Since H 2 S chemisorbs on the catalyst surface and forms nickel sulfides, deactivation of the active centers follows [4,5,6,7].…”