H2Tech - Q2 2022 - 39

PROCESS/PROJECT OPTIMIZATION
Heavy oil hydroconversion
using an upflow reactor variant
L. PATRON, Contributing author, Milan, Italy
In heavy oil hydroconversion upflow
or gas containing H2
diffusOnce
the reactor operating pressure
reactors that use supported- or slurrytype
catalysts, H2
is fed into the reactor's bottom by means
of bubbling. A fraction of this H2
es from the gas into the reaction liquid
through the gas bubbles-reaction liquid
interface (hereafter referred to simply as
the gas-liquid interface). The reaction
liquid saturates free radicals, hydrogenates
unsaturated structures and removes
metals, sulfur and nitrogen; the H2
diffused
within it remains incorporated in
both organic and inorganic conversion
products. The H2
tom. This proves that the hydrogenation
capacity is limited by the H2
-diffusion
process, which is governed by the gas-liquid
interface and not further expandable
in a conventional upflow reactor. When
the amount of H2
that diffuses into the
reaction liquid is not sufficient to completely
fuel the reactions that the hydroconversion
process involves, the result is
H2
-deficient reaction liquid.
H2-deficient reaction liquid. In an
upflow reactor with gas fed at the bottom
by bubbling, the gas-liquid interface
increases with the surface velocity
of the gas until the bubble packing is
completed. For reasons of geometry, in
theory this happens when the gas holdup
reaches a value of 0.299 in unit fraction;1
due to onset of the coalescence phenomenon,
the gas-liquid interface gradually
stops expanding beyond this point. Conventional
upflow reactors of the type
indicated at the beginning of this article
typically use a gas holdup value not far
from 0.3 at which the maximum value of
/m3
the gas-liquid interface (m2
tor volume) is reached.
of reacincorporated
in the
fed to the reactor botproducts
is always a small fraction of the
total quantity of H2
has been set, the hydroconversion is
carried out with a fixed and not further
expandable flow of H2
which diffuses
into the reactions liquid. When the diffused
H2
needed H2
is insufficient to fuel all of the
-consuming reactions, in particular
the saturation of free radicals generated
by molecular cracking, catalytic
dehydrogenation of the reaction liquid
results. This catalytic dehydrogenation
makes the ratio of hydrogen to carbon
(H/C ratio) in the heavy fraction of the
reaction liquid lower than the H/C ratio
of the same heavy fraction of the oil feed.
Dehydrogenation of the reaction liquid
is consistently found in hydroconversion
systems that use ebullated catalytic
bed reactors (supported-type catalyst),
as well as in hydroconversion systems
using slurry bubble column rectors
(slurry-type catalyst) with recycling of
the unconverted residue. The lack of H2
results in the dehydrogenation of all the
hydrocarbons that make up the reaction
liquid, including asphaltenes. In turn, the
dehydrogenation of the asphaltene fraction
generates carbonaceous material
(i.e., coke) that limits both the degree of
conversion and the reactor's hydroconversion
capacity. Pseduo-polymerization
of asphaltenes, if present, also generates
carbonaceous residue.
Catalytic dehydrogenation of reaction
liquid in heavy oil hydroconversion.
Molybdenum, typically used as a
catalyst in heavy oil hydroconversion, is
a transition metal. In the relevant applications,
molybdenum acts as molybdenite
S = Mo = S and as HS-Mo-SH, the
form of molybdenite coordinated with
H2
; this latter form acts as a hydrogenating
agent as well as a radical scavenger.
The saturation of free radicals (R*)
generated by molecular cracking occurs
by transfer of a hydrogen atom from HSMo-SH
to the incomplete carbonic octet
of R* (Eqs. 1 and 2):
HS-Mo-SH + R* → HS-Mo =
S + RH
S = Mo-SH + R* → S = Mo=
S + RH
(1)
(2)
1
2
FIG. 1. Upflow reactor equipped with
H2
distribution means (1) in the form
of high orifice density (2).
H2Tech | Q2 2022 39
H2Tech - Q2 2022

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