One of
the prevailing factors of the energy transition toward a low-carbon energy
system is that the decarbonization of transport will lead to the contraction of
the main markets in which crude oil refining products are sold.
Hydroconversion
technologies must continue to progress to allow the reduction of fuel oil as
well as the reduction of heavy distillates that are now used as precursors of
transport fuels. These can possiblly be replaced with light distillates that
can be used for petrochemicals—demand for these products is expected to grow.1
To
obtain a higher fraction of light distillates from the hydroconversion of heavy
oils—preferably
the production of (atmospheric) light distillates only—hydroconversion systems
with a much higher hydrogenation capacity than the systems now is use will be
required.
Regardless
of the type of catalyst used (supported or slurried), even using an excess of
catalyst and reactors of great height, it has been found that only ~10% of
the hydrogen (H2) fed to the base of a hydroconversion reactor is
incorporated into the conversion products. This clearly indicates that in a
hydroconversion process, due to the temperatures used and the limited diffusion
of H2 from the gaseous phase to the reaction liquid, hydrogenation
constitutes the slow stage. Once the pressure and temperature of the reactor
have been set, the hydrogenation rate can only be increased by increasing the
gas-liquid contact surface through which the H2 reaches the reaction
liquid.
In the
reactors now is use, most of the H2 feed (which does not diffuse
into the reaction liquid) does not act on the chemical processes of the
hydroconversion treatment. For the purposes of hydrogenation, the H2
is unnecessarily recycled at high surface velocity in the reactor. To expand
the gas-liquid contact surface, and therefore the hydrogenation capacity, it is
necessary to redesign the way in which H2 is distributed to the
reactor in hydroconversion systems now in use.
Features of heavy oil
hydroconversion systems.
Hydroconversion
systems using ebullated catalytic bed reactors constantly operate with a H2-deficient
reaction liquid that limits the degree of conversion and the hydrogenation
capacity.2 This condition is due to the H2 distribution mode and the
type of process used (i.e., open-cycle without recycling) and cannot be
overcome.
A
different hydroconversion technology that uses a slurry catalyst in a bubble
column reactor has had recent and multiple confirmations on a commercial scale
and achieves a complete chemical conversion. It injects H2 directly
into the reaction liquid and operates in a closed liquid circuit with
recycling, and can be transformed into a version with increased hydrogenation
capacity.
The
reasons it is necessary to operate in a closed liquid circuit with recycling is
clarified in the next section.
Upflow reactor variant
with expandable gas-liquid contact surface. In a bubble column reactor, the fluid
dynamic condition in which the gas bubbles—at the threshold of coalescence, they are
packed with a gap between two bubbles equal to the
diameter of a bubble—is known as bubble packing. Assuming that the bubbles all have the same diameter, the fluid dynamic
condition corresponding to the packing of the bubbles implies a theoretically
calculated gas holdup value equal to 0.296 ≃ 0.3, regardless
of the diameter of the bubbles. Experimentally, therefore, with the natural
distribution of the diameter of the bubbles, the bubble packing is found at a
gas holdup value of about 0.299, substantially equal to the theoretical one.3
The bubble packing condition is the
most favorable for a heavy oil hydroconversion process, as it allows maximum
liquid filling of the reactor while the gas-liquid unit surface (m2/m3
of reactor volume, measurement of the gas-liquid contact surface area) is at its
maximum.4 Incidentally, bubble packing is the fluid dynamic
condition in which the upflow reactor usually operates in a slurry
hydroconversion system.
In a liquid-gas mixture, under bubble
packing conditions, the gas-liquid unit surface is determined solely by the
average diameter of the bubbles and is inversely proportional to it. By
maintaining the packing condition of the bubbles, the progressive reduction of
the diameter of the bubbles continuously expands the gas-liquid unit surface,
with the hydrogenation rate reaching its maximum when the rise speed of the
bubbles in the reactor (due to the reduction of their diameter) is lowered
until it is equal to that of the liquid.
In an upflow reactor equipped with a
means of H2 distribution with a large number of orifices per m2
(e.g., 100 or more), if the surface velocity with which the H2 is
fed is reduced by a certain extent, depending on the density of the orifices,
the bubble packing condition is preserved while smaller diameter bubbles are
generated. Without the reduction of the surface velocity of the fed H2,
the coalescence of the bubbles would be induced with the corresponding
undesired increase in their diameter. For example, by using an upflow reactor
equipped with H2 distribution means with 400 orifices/m2
rather than 50/m2 as in a conventional upflow reactor, and by
suitably lowering the surface velocity of the H2, the packing of the
bubbles is preserved while the bubbles generated have a diameter halved
compared to the bubbles of a conventional upflow reactor, as roughly shown in FIG. 1.
The variant of the upflow reactor
equipped with a H2 distribution means with 400 orifices/m2
allows the generation of a liquid-gas mixture that has a gas-liquid unit
surface that is double to that of a conventional reactor, with the doubling of
the hydrogenation rate. More details are described in the literature.5
Paradoxically, to increase the
hydrogenation capacity of an upflow reactor, the surface velocity (i.e.,
flowrate) with which the H2 is fed to the reactor must be lowered.
However, this is precisely what is
required so the H2 distributor, despite having a high density of
orifices to produce small diameter bubbles, continues to generate a liquid-gas
mixture in which the packing of the bubbles is preserved.
The required lowering of the surface
velocity of H2 can lead to a flowrate reduction of 50% or more. Such
a lowering of the H2 flowrate is feasible only when the extraction
of the conversion products from the reactor does not take place exclusively in
the vapor phase, but is completed by distillation of the reaction liquid
downstream of the reactor—this is the case with slurry hydroconversion systems
that operate in a closed-liquid circuit with recycling. In hydroconversion
systems that use ebullated catalytic bed reactors in an open-liquid circuit
without recycling, the extraction of the conversion products that is carried
out exclusively in the vapor phase prevents limiting the surface velocity of H2,
which is essential to expand the area of gas-liquid interface. The extraction
of the conversion products from the reactor exclusively in the vapor phase is
incompatible with a reaction mixture with a high gas-liquid contact surface.
H2 needed to
hydroconvert heavy oils into light hydrocarbons. In an example case, in the slurry
hydroconversion of a vacuum residue with a H2 content of 10.5%, a
set of conversion products (product slate) is obtained in which the average
weight content of H2 typically rises to 14%. This corresponds to an
amount of H2 incorporated in the products equal to 3.5% of the
converted charge.
Reference
is now made to a typical product slate consisting of:
If the heavy fraction [350°C–540°C (662°F–1,004°F)] with
a H2 content of 12.5% were to be converted into the light fraction [350°C (662°F)]
that, on average, has a H2 content
of 15.2%, the amount of H2 incorporated into the products would rise
from 3.5% (10.5% to 14%) to 4.7 (10.5% to 15.2%), an increase of 1.34x. For
product slates with a further increased H2 content (e.g., 16%) to
bring the H2 content of atmospheric distillates close to that of
paraffinic hydrocarbons, the hydrogenation capacity of the slurry reactor must
be increased in proportion of the greater quantity of H2 incorporated in the product slate, which will rise
from 3.5% (10.5% to 14%) to 5.5% (10.5% to 16%) of the converted charge, an
increase of 1.57x. The variant of the upflow reactor illustrated in FIG. 1,
which has a double hydrogenation rate compared to a conventional reactor, will
be able to supply all the H2 required.
The slurry catalyst bubble column reactor to convert
heavy oils to light hydrocarbons. The use of a slurry-type catalyst, characterized by a
limited molecular cracking activity and, therefore, with a limited consumption
of H2 for the saturation of radicals, allows more H2 to
be diverted to the hydrogenation of unsaturated structures.
The combination of a bubble column
reactor with the required gas-liquid unit surface with the use of a slurry
catalyst allows the complete conversion of heavy oils into [350°C (662°F)] hydrocarbons, or lighter, suitable for petrochemicals.
However, this combination alone is not enough. For the reasons described below,
a specific process variant must also be adopted.
Process requirements. Since the upflow
reactor with a high gas-liquid unit surface operates with a (sometimes
considerably) lower H2 surface velocity compared to the usual values—thus
reducing the extraction of the conversion products in the vapor phase—the
extraction of the same must be completed by extracting the liquids from the
reactor.
Furthermore, given the asphaltenic
nature of heavy oils, to avoid the separation of asphaltenes that would
otherwise make hydroconversion impractical, it is necessary to adequately
reduce the concentration of light hydrocarbons in the reaction liquid. By
extracting the conversion products as liquids from the reactor, the
concentration of light hydrocarbons can be lowered, as necessary.
Process variant for extracting the conversion products as liquids from the reactor. The conversion products in the liquid state are extracted by feeding an adequate flowrate of a heavy mixture consisting of feedstock and atmospheric residue to the reactor. Increasing the flowrate of this heavy mixture correspondingly increases the amount of conversion products withdrawn as liquids from the reactor and transferred to the distillation section for recovery. Further details are described in the literature.5
Slurry hydroconversion
technology version with high hydrogenation capacity. Implementing the variant of the
upflow reactor (FIG. 1), which allows the gas-liquid unit surface
to be expanded, and the process variant, which allows the conversion products
to be extracted from the reactor as liquids, transform the current slurry
hydroconversion technology into the high hydrogenation capacity version.
This
high hydrogenation capacity version converts heavy oils, including crude oils,
entirely to [350°C (662°F)] hydrocarbons, or lighter, producing products
with crescent H2 content by gearing up the reactor's hydrogenation
capacity. The new version of the slurry hydroconversion technology allows the
refinery to redesign the product slate with the flexibility that the shrinking
transport fuels market requires.
Since
this new version of hydroconversion technology does not require simultaneous production
of transport fuels, it also allows crude oil to be used exclusively for
petrochemicals, with the crude converted at the petrochemical site or at the
wellhead. This solution will allow the development of the petrochemical sector,
regardless of how the transport fuels market evolves.
Applications and achievable results. Hydroconversion of heavy oils entirely to [350°C (662°F)] hydrocarbons, or lighter, can be implemented in existing plants, based on slurry technology, through the limited adaptations describe above. In addition to allowing the replacement of heavy distillates with light distillates as a consequence of the greater quantity of H2 in the reaction liquid, the improved slurry technology version also brings advantages in terms of denitrification capacity, desulfurization capacity and reduced catalyst consumption of following the complete chemical conversion that is achieved.
Considering the market prospects mentioned above, the high hydrogenation capacity version of the slurry hydrogenation technology can make it profitable to transform the current open-liquid circuit systems, using ebullated catalytic bed reactors, into closed-liquid circuit systems, using slurry bubble column reactors.
For new installations, the high hydrogenation capacity version of the slurry hydroconversion technology, which reduces the recycling gas rate and does not require the vacuum section and the subsequent hydrocracking section, will allow for significant reductions in the cost of the plant. HP
LITERATURE
CITED
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.