Hydrocarbon Processing - December 2022 - 37

Catalysts
that while catalyst Package D offered the
lowest cycle benefit on paper, it exhibited
equivalent or better potential in pilot
plant testing when compared to catalyst
Packages A and B.
These results are supported by technical
factors. One is the choice of metals.
Principally, hydrocracking catalysts are
bi-functional, comprising cracking and
hydrogenation functions. For the cracking
function, various blending ratios of
amorphous silica-alumina (lower acidity/activity)
and zeolites (higher acidity/
activity) are possible. Concurrently, several
metal choices with different hydrogenation
strengths are also available, as
summarized in TABLE 2. Catalyst manufacturers
can adjust the ratio between
the catalyst's cracking and hydrogenation
strength to optimize overall catalyst
activity and selectivity. A higher hydrogenation-to-acidity
strength ratio results
in better middle distillate yield, while a
lower ratio produces more naphtha.1
In this example, catalyst Package B utilized
nickel-molybdenum (Ni-Mo) as the
hydrogenation function for the proposed
hydrocracking catalysts, while others relied
on nickel-tungsten (Ni-W). As Ni-W
is stronger than Ni-Mo when it comes to
hydrogenation, Ni-W-based hydrocracking
catalysts can better preserve primary
cracking products (e.g., kerosene and diesel)
by suppressing secondary cracking,
assuming the same type of zeolite and
zeolitic content, as illustrated in FIG. 3.
This observation is in line with the fact
that catalyst Package B achieved the lowest
middle distillate yield during the pilot
plant testing, among other factors. Additionally,
hydrocracking catalysts with
a stronger hydrogenation function help
improve product qualities [e.g., smoke
point, cetane number and unconverted
oil (UCO) viscosity index].
Conversely, the top performer, the
supplier of catalyst Package C, claimed
to have a high degree of mesoporosity in
the zeolite framework (FIG. 4). This structural
advantage aids molecular diffusion
and helps reduce the degree of secondary
cracking (i.e., the primary cracking
products are released more rapidly from
the zeolite network).2
Additionally, this
catalyst manufacturer employs a specially
engineered catalyst shape where the diffusion
path length is shorter than conventional
ones, therefore minimizing overcracking.
Lastly, this competitor utilizes
FIG. 4. Mesoporization of the zeolite structure can improve the acid site accessibility and reduce
the degree of over-cracking in heavy oil hydrocracking by shortening the diffusion path length
within the zeolite structure. Source: Zeopore Technologies.
Hydrocarbon Processing | DECEMBER 2022 37
stronger Ni-W as the hydrogenation function
for its hydrocracking catalysts. These
described catalytic features partially explain
why the company outperformed
other hydrocracking catalyst manufacturers
in terms of the total liquid and middle
distillate yields.
Although this is just one example, the
authors have seen several hydrocracking
catalyst evaluations with similar reFIG.
2. Relative cycle benefit of various hydrocracking catalyst packages for a 54,000-bpd
hydrocracker. Catalyst Package D was a baseline for a paper-based evaluation, while catalyst
Package B took the same role in pilot plant testing.
FIG. 3. Typical hydrocracking reaction pathways: both metal and acid sites are essential to
complete the hydrocracking process.

Hydrocarbon Processing - December 2022

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