Catalyst Deactivation - Spent Hydroprocessing Catalysts - page 77

structure of the TIS coke formed within the first 120 h, whereas the “hard” coke that of
THFIS-coke formed after 6500 h, shown in Fig. 4.20 . As expected, the latter coke had very
low solubility. With time on stream, the “soft” coke was gradually converted to more
refractory coke. This was supported by the decrease of the low temperature and increase of the
high temperature CO 2 peak between 1 and 240 h. Also, more nitrogen and sulfur were
concentrated in the refractory coke than in the “soft” coke.
The optical microscopic techniques could characterize coke deposits according to their
reflectance, fluorescence and anisotropy. Micrographs usually reveal the presence of
meso-phase, i.e., the spherical domains, which exhibit characteristics of liquid crystals. The
meso-phase is denser, has a higher surface tension and wets catalyst surface better than the
phase from which it was originated. From the structural point of view, this is consistent with
the loss of long aliphatic chains from the coke precursors. These chains contributed to the
steric hindrance between the catalyst surface and coke precursor and as such inhibited the
wetting of catalyst surface. The mechanism of coke formation involving meso-phase as an
intermediate phase was proposed by Beuther et al. [241] . With time on stream, the liquid
crystals could be converted to coke whose structure was changing progressively. This involved
ordering and stacking of aromatic sheets. This may be considered as the very early stage of
graphitization, which tends to increase with increasing severity. Figures 4.20 and 4.21
[238,239] offer some support for this mechanism. Thus, the coke after 6500 h represents a
sheet, which possesses a high aromaticity. The stacking of such sheets into platelets may have
occurred particularly when the catalyst was approaching the end of its life, i.e., at this point,
the active surface hydrogen was very limited.
The optical microscopy of the polished cross-sections of a series of the spent catalysts after
heavy feed upgrading was investigated by Munoz et al. [242] and Gray et al. [243] . The
fluorescence due to the presence of the feed components and anisotropy due to the presence of
meso-phase were observed in addition to the high reflectance, which indicated the presence of
domains having higher aromaticity than surrounding matrix. This was an indication of the
gradual conversion of heavy components in the feed to meso-phase, which subsequently
converted to the high aromaticity species. This was supported by the absence of the feed
components and predominance of high aromaticity domains after more severe conditions, i.e.,
higher temperature and longer time on stream. These observations are in a good qualitative
agreement with the other studies [230,244-246] .
4.6.1.2 Physical Aspects
Physical properties of the petroleum feeds may be a contributing factor to deactivation. In this
regard, heavy feeds require much more attention than light feeds. Of particular importance is
colloidal stability of the system comprising oil, resin and asphaltenes phases. This stability
may be affected when resins are converted at a greater rate than asphaltenes. Similar effect
would have a high rate of the HYD of oil phase of the colloidal system. Compatibility is a