Athabasca bitumen was hydrocracked over a commercial
NiMo/γ-Al2O3 catalyst in two
reactors,
a microbatch reactor and a 1-L continuous stirred tank reactor (CSTR).
Coke deposition on
catalyst was measured as a function of hydrogen pressure, time on
stream, and liquid composition
by measuring the carbon content of the cleaned spent catalyst. The
carbon content ranged from
11.3% to 17.6% over the pressure range 6.9−15.2 MPa in CSTR
experiments. Batch and CSTR
experiments showed a rapid approach to a constant coke content with
increasing oil/catalyst
ratio. Coke deposition was independent of product composition for
residue concentrations ranging
from 8% to 32% by weight. Removal of the coke by tetralin at
reaction conditions suggested
reversible adsorption of residue components on the catalyst surface.
A physical model based on
clearance of coke by hydrogen in the vicinity of metal crystallites is
presented for the coke
deposition behavior during the first several hours of hydrocracking
use. This model gives good
agreement with experimental data, including the effect of reaction
time, the ratio of total feed
weight to catalyst weight, hydrogen pressure, and feed composition, and
it agrees with general
observations from industrial usage. The model implies that except
at the highest coke levels,
the active surfaces of the metal crystallites remain exposed.
Severe mass-transfer limitations
are caused by the overall narrowing of the pore structure, which in
γ-Al2O3 would give very low
effective diffusivity for residuum molecules in
micropores.
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