Regeneration - Spent Hydroprocessing Catalysts - page 177

With aim to predict regeneration difficulty on a scale 1 to 10, attempts have been made to
correlate various laboratory and pilot plant results [379] . In this regard, the best correlation
was established when the regeneration difficulty was related to the total amount of metals in
the catalyst. On the basis of this information, the final regeneration temperature for achieving
an optimal activity recovery can be identified. Murff [461] reported that TGA and differential
thermal analysis (DTA) techniques can also be used for predicting temperatures in a
commercial regeneration plant. It is believed that a similar set of tests prior to commercial
regeneration is conducted by other regeneration companies, although their methodology could
not be verified in the literature. In the case of these companies, similar information may be
considered proprietary.
6.4.3 Mechanical Separation of Spent Catalyst
The efficiency of regeneration may be improved if the catalyst particles of an unsuitable size
are removed. Also, for some spent catalysts (e.g., from ebullated bed reactors), a variable level
of deactivation has been noted. In this case, the spent catalyst represents a mixture of particles
comprising those still possessing a high activity (e.g., nearly fresh) and heavily fouled fraction
of particles that cannot be regenerated anymore. Because of its non-regenerability, it would
make little sense to subject the latter fraction to regeneration. Figure 6.35 [462] shows the
mass balance of spent catalyst particles separated to three fractions, i.e., heavy catalyst,
medium catalyst and light catalyst fractions, using a density grading method. This information
was the basis for a preliminary design of a rejuvenation process having a capacity of about
6000 tons of spent catalyst annually. According to this scheme, only light catalyst would be
subjected to regeneration (decoking). The medium and heavy fractions of the spent catalyst
required rejuvenation (metal leaching) prior to decoking. Thus, the preliminary design
represents the integration of the oxidative regeneration with rejuvenation involving metal
leaching. Without such integration, only a relatively small portion of the spent catalyst could
be regenerated to achieve a sufficient activity recovery unless the contaminant metals were
leached out via rejuvenation.
Density grading method can separate particles of a similar size but with the density differences
smaller than 10% [463] . The density increase may be caused by deposition metals, mainly
vanadium. For example, in the fixed bed reactor used for upgrading a heavy feed, the front bed
is contaminated by vanadium, whereas the end of the bed is not. After unloading, a mixture of
both contaminated and uncontaminated fractions of catalyst is obtained. The former fraction
can be easily separated from the uncontaminated fraction using density grading. The density
grading may be applied either to spent catalyst or to regenerated catalyst. The former case may
translate into a smaller amount of spent catalyst requiring regeneration by eliminating
non-regenerable catalyst from the mixture.
The fines that are part of the spent catalyst mixture can be simply removed by screening
providing that the spent catalyst was adequately de-oiled and dried on the refinery site.