H2Tech - Q2 2022 - 40

PROCESS/PROJECT OPTIMIZATION
HS-Mo-SH, the hydrogenated form of
molybdenite, is restored by the H2
diffusing
from the gas phase into the reaction
liquid.
When the H2
consumption rate is
higher than the weight flowrate of H2
diffusing into the reaction liquid, however,
the hydroconversion catalyst (in
the form S = Mo = S) activates the catalytic
dehydrogenation of the hydrocarbons
making up the reaction liquid itself.
Eq. 3 is as follows:
S = Mo = S + > CH-CH < →
HS-Mo-SH + > C = C <
(3)
The hydroconversion catalyst can act
as both a hydrogenating and a dehydrogenating
agent in accordance with H2
availability. This holds true for any transition
metal used as a catalyst, not only
molybdenum.
Operating with H2 deficient reaction
liquid. As dehydrogenation proceeds (either
in the subsequent reaction stages or
because of the recycling of unconverted
residue), the H/C ratio of the asphaltene
hydrocarbons becomes lower, making
such hydrocarbons refractory to hydrogenation
and insoluble in the reaction
liquid and thus no longer convertible into
distillates. These low H2
-insoluble hydrocarbons
(collectively known as carbonaceous
residue) must be removed from
the hydroconversion system. This may
be accomplished through the final unconverted
stream in hydroconversion systems
devoid of recycling (typically using
ebullated catalytic bed reactors) or in hydroconversion
systems equipped with recycling
(typically using slurry bubble column
reactors), such as a purge stream. In
the former case, the degree of conversion
is limited to ~80%; in the latter, ~95%.
Operating with H2
-deficient reaction
liquid also limits the reactor's hydroconversion
capacity, which cannot increase
without a corresponding reduction in the
degree of conversion. While an increase in
the reactor's operating pressure would allow
for increased availability of H2
in the
reaction liquid, the increased cost for the
plant makes increasing pressure an unattractive
option.
Another way to avoid H2
is to lower the amount of H2
by limiting molecular cracking. This can
be accomplished by lowering the reactor
40 Q2 2022 | H2-Tech.com
deficiency
consumed
temperature until the H/C ratio of the
heavy fraction of the reaction liquid is
not lower than the H/C ratio of the same
heavy fraction of heavy oil feed. This
would involve severely derating the system's
hydroconversion capacity, however,
and increasing reactor pressure is not an
economically sustainable option.
Expansion of gas-liquid interface as
a countermeasure to avoid H2
ciency. To overcome H2
diffusing into the reaction liquid
defideficiency
without
raising operating costs or reducing
hydroconversion capacity, the flowrate
of H2
must be increased by expanding the gasliquid
interface. This is achievable using
a variant of an upflow reactor2
with a means of H2
equipped
a high density of orifices per m2
distribution with
of the
horizontal reactor section (FIG. 1). These
reactors are capable of creating a fluid
dynamic regime wherein the specific surface
of the gas-liquid interface (m2
of gas
bubble surface per m3 of liquid) increases
with the squared power of the gas holdup
value, bringing hydrogenation capacity
(per unit volume of reactor) up to values
more than double what is seen in
conventional upflow reactors. In hydroconversion
systems using ebullated catalytic
bed reactors, a fluid dynamic regime
with an expandable gas-liquid interface
2
3
Due to its H2
-distribution,
the upflow reactor has
a low hydrogenation
capacity, inadequate
for hydroconversion
of heavy oils, which
demands a lot of H2
.
Using an upflow reactor
variant equipped with
H2
distribution and a high
density of orifices means
hydrogenation capacity
can more than double.
is created in correspondence with the
catalytic bed, above the distributor plate;
in hydroconversion systems using slurry
bubble column reactors, a fluid dynamic
regime of this kind is preferably created
over the entire height of the reactor.
When using a hydroconversion system
equipped with a distillation unit for
high boiler extraction and recycling of
unconverted residue, the lack of H2
in the
reaction liquid can be mitigated by maximizing
the production of middle distillates
at the expense of H2
light hydrocarbons.3
-rich gases and
Achievable results. An increase in hydrogenation
capacity brought about by
expanding the gas-liquid interface can
strongly expand the capacity of a hydroconversion
system and extend the
lifetime of its catalyst. In systems that
utilize ebullated catalytic bed reactors,
the reduction of dehydrogenation of the
reaction liquid to zero will result in an
unconverted stream that is completely
recyclable to the reactor, thus allowing
the completion of hydroconversion with
reaction stages in series or preferably in
parallel for easier product extraction.
The reduction of carbonaceous residue
associated with dehydrogenation will
proportionally extend the useful life of
the supported-type catalyst. In systems
using slurry bubble column reactors,
the reduction or elimination of carbonaceous
residue production (which is
typically removed by a purge stream)
greatly decreases the consumption of
slurry-type catalyst.
LITERATURE CITED
1
Shaikh, J. and M. Al-Dahhan, " A review on
flow regime transition in bubble columns, "
International Journal of Chemical Reactor Engineering,
Volume 5, 2007.
International patent, application number PCT/
IT2021/050122: " Hydrotreatment upflow reactors
with high hydrogen-hydrocarbon liquid contact
surface and improved hydrogenation capacity. "
U.S. patent, application number 16952824: " Process
for the hydroconversion of heavy hydrocarbon oils
with reduced hydrogen consumption. "
LUIGI PATRON served as Chairman
and CEO of the engineering
company Snamprogetti between
1997 and 2005. Snamprogetti,
formerly a subsidiary of ENI and
later merged with Saipem, designed
and built the demonstration unit
used to validate the first commercially applied heavy
oil slurry hydroconversion technology. Mr. Patron
has since continued working on hydroconversion
technologies independently.
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