Environmental and Safety Aspects of Spent Hydroprocessing Catalysts - Spent Hydroprocessing Catalysts - page 113

for exposure to the environment during 40 to 70 days. After the exposure, the boxes were
removed to determine the change in the composition of both mineral and organic matter of
spent catalyst. After 14 months exposure, Al(III) and phosphate species became insoluble,
while sulfur was almost quantitatively converted to sulfate. The migration of Ni and Mo into
liquid phase increased as well. The Fe content in liquid phase increased due to corrosion of the
metal container. At the same time, the solubility of coke in dichloromethane was reduced
drastically, whereas the coke solubility in methanol increased. The increased solubility in
methanol was attributed to the formation of O-containing groups, such as acids, ethers, esters,
and phenols. After six months, the holes in metal boxes, presumably formed by corrosion with
H 2 SO 4 released from spent catalyst, were noted. Consequently, the contamination of ground
water with Ni, Fe, Mo, and sulfate was observed.
The active phase of the hydroprocessing catalysts comprises of Co(Ni)-Mo(W)-S entities [53] .
To various degrees, this phase may be still present in spent catalysts. As it was indicated
earlier, in this phase, promoters, such as Ni and Co, decorate MoS 2 and/or WS 2 crystallites.
Then, the O 2 will access the promoting atoms more readily compared with the sulfide
component of the active phase. This may be depicted by the general reaction such as:
Ni(Co)-Mo(W)-S X +O 2 =NiO(CoO) +Mo(W)-S X
The oxidation of the residual Mo(W)-S X entity would proceed according to the reactions
discussed above.
The presence of fluoride and phosphate may limit utilization options of spent hydroprocessing
catalysts [335] . A potential release of fluorine and phosphorus containing species from spent
catalysts received little attention. Because of their modifying effect on the support, it is
believed that both fluorine and phosphorus are associated with the Al 2 O 3 . In the absence of
any experimental data, only a speculative reaction may be proposed. Thus, for the fluorine, the
release of HF could be anticipated if sufficient concentration of H 2 SO 4 builds up on the
exposure of spent catalyst to air and water. Similarly, species containing Al 2 O 3 and a
phosphorus could be converted to Al 2 (SO 4 ) 3 as a more stable salt compared with Al
phosphate.
Various concentrations of arsenic in petroleum have been noted [336] . During
hydroprocessing, a portion of the arsenic may deposit on catalyst surface, most likely in a
sulfidic form, whereas another part may be released in refinery gases as AsH 3 . A similar set of
the oxidation reactions as postulated above for other metal sulfides can also be proposed for As
sulfides. It should be noted that a significant increase in the solubility of arsenic caused by the
conversion of arsenic sulfides to corresponding oxides, which may occur during the storage of
spent catalysts, should be of a concern because of the toxic nature of the As-containing
species. Consequently, leaching of arsenic to ground water could not be avoided unless
catalyst is handled in accordance with the regulations.