Rejuvenation - Spent Hydroprocessing Catalysts - page 219

reactions, e.g.:

H 2 SO 4 (2HCl) + MeO = MeSO 4 (MeCl 2 ) + H 2 O

H 2 SO 4 (2HCl) + MeS = MeSO 4 (MeCl 2 ) + H 2 S

These reactions suggest that for sulfided catalyst, attention has to be paid to the H 2 S emissions.
For oxidic catalyst, sulfur emissions were kept under control during decoking step. Jocker
[311] reported that during leaching of the oxidic catalyst more use was made of H 2 SO 4 than
of HCl. For the former, a greater flexibility and recirculation possibilities could be
achieved.

To various degrees, all metals in spent catalysts are soluble in strong acids. The solubility can
be monitored by the acid concentration and various additives added to the former. It is however
believed that it is more difficult to attain a desirable selectivity compared with the aqueous
solutions of organic agent.

Perhaps, the simplest case of the catalyst rejuvenation using inorganic agents is the removal of
the “skin” of inorganic solids, which is deposited on the exterior of catalyst particles. The
“skin” may comprise clays, alkali and alkali earth metals compounds, and V and Ni sulfides
formed via the reaction of porphyrins with H 2 S. The study of Gray et al. [243] showed that
such “skin” could be easily removed by the treatment with diluted HCl. Much more severe
conditions are necessary to remove inorganic solids deposited in pores. It is believed that little
contaminant metals would be removed in diluted HCl.

According to method disclosed by Sherwood et al. [494-496] , coked spent catalyst was steam
treated (550-680 ◦ C) before being sent to a rejuvenation unit where it was first washed with
water to fill the catalyst pores. Next, the catalyst particles were passed to an acid treating
column in which a solution of 5-50 wt.% H 2 SO 4 and 0-10 wt.% ammonium ion was
introduced into the lower section of the column. The acid solution was recovered from the top
of the acid treating column and recycled to the bottom of the column together with additional
make-up acid. Usually the acid treatment is conducted at temperatures ranging from room
temperature to 120 ◦ C. In a final step the catalyst particles are passed to a washing unit where
the acid-treated particles are washed with water to remove the acid following which the
recovered, washed catalyst particles are dried in air or in air diluted with nitrogen. Other
examples of treating acids include acetic acid, HCl, and HNO 3 . The acid treatment solution,
temperature and time duration are optimized to ensure that the active metal removal does not
exceed about 4 wt.%. Because the polar solvent and acid liquids used for the washing and
treatment steps are soluble in both oil and water, this catalyst rejuvenation process can utilize a
two-step water washing procedure for the solvent washed and the acid-treated catalyst, after
which the solvent and the acid liquids may be recovered by distillation for reuse in the
rejuvenation process.