Initial coke deposition on hydrotreating catalysts. Part 1. Changes in coke properties as a function of time on stream

Initial coke deposition on an industrial Mo/Al2O3 hydrodemetallization catalyst was studied as a function of operating time during hydrotreating of Kuwaiti atmospheric residue (T; 380 °C, LHSV; 1 h−1). The primary objective of the study was to examine the nature of the initial coke and to determine cokeʹs possible role in catalyst deactivation. The amount of coke on the catalyst increased rapidly and climbed up to 15 wt% within 12 h, but then, coke formation slowed down and reached about 20 wt% after 240 h. Used catalyst samples from experiments conducted at different time on stream periods (1, 3, 12, 48, 120 and 240 h) were subjected to temperature programmed oxidation and diffuse reflectance infrared Fourier transform spectroscopy to obtain information on the nature of the initial coke and changes in the coke properties with increasing run time. Additionally, the 1, 12, 48 and 120 h aged catalysts were extracted with tetrahydrofuran (THF). The results showed the presence of two types of coke on the catalyst, an easy removable soft coke and a more refractory surface coke. After 240 h of operation, a large fraction of the soft coke had been converted into refractory coke. The refractory coke contained substantial amounts of nitrogen and sulfur, whereas their concentration in soft coke was rather low. Nitrogen species, e.g. NHx, were found to deposit at the active metal sulfide crystallites, whereas coke was formed predominantly at the surface of the support. The THF extraction removed mainly heterocompounds and aliphatic coke components.