Spent Catalysts from Dewaxing Operations - Spent Hydroprocessing Catalysts - page 259

Coke deposition and poisoning by N-bases are the main causes of catalyst deactivation. Under
optimal operating conditions (e.g., temperature, H 2 pressure, H 2 S/H 2 ratio, etc.), a continuous
operation lasting more than two years should be achieved. The properties of spent
hydroprocessing catalysts from dewaxing operations are identical as those of spent catalysts
used for upgrading distillate fractions. Detailed accounts of these properties were given in
preceding chapters. Environmental and safety aspects are identical as well.
10.2 Dewaxing Catalysts
The HIS/IS (isomerization) are among the most important reactions for converting n-paraffins
and n-olefins to corresponding isomers. For the latter, double bond IS may be part of the
overall mechanism. To various degrees, cracking reactions occur either in parallel with HIS or
as consecutive reactions because of the acidic nature of catalyst employed [574,575] . Such
situation is desirable in the case of paraffinic feeds with boiling range exceeding that of the
fuels of interest. The catalysts used for HIS and HCR reactions must exhibit a desirable acidity
[576] . For HCR, the acidity must be regulated to prevent excessive formation of the unwanted
gaseous by-products and coke. At the same time, the acid strength needed for HIS depends on
the length of chain. Thus, the HIS of light paraffins (e.g., butane and pentane) requires very
strong acidic sites, i.e., such as those encountered in the oxo-anion promoted zirconia. For
long chains, the acidic strength required for branching is lower. This suggests that a
bi-functional catalyst, exhibiting a good activity for simultaneous HIS and HCR, requires the
optimization of the acidic sites. For example, a catalyst with the dominance of medium and
weak acid sites may exhibit a high activity for HIS, but its activity for HCR may be rather low
[576] . To various degrees, other hydroprocessing reactions occur in parallel with HIS and
HCR reactions. This suggests that origin of the feed and properties of anticipated products
have to be taken into consideration while designing catalysts for HIS/HCR.
10.2.1 Composition of Dewaxing Catalysts
A wide range of catalysts have been developed and tested for IS and HIS of n-paraffins and
n-olefins. These catalysts were dominated by different combinations of active metals (e.g., Pt
and Pd) with zeolites, although during the early stages of research amorphous silica-alumina
and active clays were also receiving attention. Silica-alumina phosphates (SAPO) molecular
sieves and sulfated-zirconia-based catalysts have been attracting interests as well. In addition,
active metals supported on different supports (SiO 2 ,Al 2 O 3 , and various carbon supports) were
used in several studies. The activity determination involved both model compounds and real
feeds. The studies in which different types of catalysts were tested under identical conditions
are of a particular importance for comparison of catalyst performance [577,578] . The H 2
pressure varied between atmospheric up to about 6MPa, whereas temperatures was between