Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HYDROCRACKING OF HEAVY HYDROCARBON OILS WITH
IMPROVED GAS AND LIQUID DISTRIBUTION
Background of the Invention
This invention relates to a process and apparatus for the
treatment of hydrocarbon oils and, more particularly, to the
hydroconversion of heavy hydrocarbon oils in the presence of
particulate additives, e.g. iron and/or coal additives.
Hydroconversion processes for the conversion of heavy
hydrocarbon oils to light and intermediate naphthas of good
quality for reforming feedstocks, fuel oil and gas oil are
well known. These heavy hydrocarbon oils can be such
materials as petroleum crude oil, atmospheric tar bottoms
products, vacuum tar bottoms products, heavy cycle oils, shale
oils, coal derived liquids, crude oil residuum, topped crude
oils and the heavy bituminous oils extracted from oil sands.
Of particular interest are the oils extracted from oil sands
and which contain wide boiling range materials from naphthas
through kerosene, gas oil, pitch, etc., and which contain a
large portion of material boiling above 524 C equivalent
atmospheric boiling point.
As the reserves of conventional crude oils decline, these
heavy.oils must be upgraded to meet the demand for lighter
products. In this upgrading, the heavier materials are
converted to lighter fractions and most of the sulphur,
nitrogen and metals must be removed.
This can be done either by a coking process, such as
delayed or fluidized coking, or by a hydrogen addition process
such as thermal or catalytic hydrocracking. The distillate
yield from the coking process is typically about 80 wt% and
this process also yields substantial amounts of coke as by-
product.
Work has also been done on a new processing route
involving hydrogen addition at high pressures and temperatures
and this has been found to be quite promising. In this
process, hydrogen and heavy oil are pumped upwardly through an
empty tubular reactor in the absence of any catalyst. It has
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been found that the high molecular weight compounds
hydrogenate and/or hydrocrack into lower boiling materials.
Simultaneous desulphurization, demetallization and
denitrogenation reactions take place.
Additives have been developed which can suppress coking
reactions or can remove the coke from the reactor. It has been
shown in Khulbe et al, U.S. Patent No. 4,923,838 issued May 8,
1990 that the formation of carbonaceous deposits in the reaction
zone can be substantially reduced by mixing with a heavy oil
feedstock a finely divided particulate consisting of carbonaceous
particles and particles of an iron compound, e.g. an iron salt
or oxide such as iron sulphate. The particles typically have
average sizes of less than 10 um. Canadian Patent No.
1,202,588 describes a process for hydrocracking heavy oils in
the presence of an additive in the form of a dry mixture of
coal and iron salt, such as iron sulphate.
A problem in the hydroprocessing of heavy hydrocarbon oil
containing finely divided particulate, such as iron sulphate,
is to achieve a good gas-liquid distribution in a reaction
zone while avoiding coke formation and build-up. Bubble cap
distribution plates are commonly used for gas-liquid
distribution, e.g. as described in U.S. Patent No. 4,874,583
issued October 17, 1989, etc. However, when all gas, liquid and
particulate are introduced into a lower region of a reaction
zone below a bubble cap distribution plate, there is a problem
that the bubble caps are quickly plugged and flow is reduced.
It is the object of the present invention to provide
improvements to the mixing of hot hydrogen containing gas with
heavy hydrocarbon oil in a hydrocracker and to ensure that
additive particles are well mixed into the reactor contents
and no settling occurs in the bottom head of the reactor.
Summary of the Invention
According to the present invention, it has been
discovered that further improvements in the hydroprocessing of
heavy hydrocarbon oils containing additive particles to
suppress coke formation are achieved by the manner in which
the heavy hydrocarbon oil and additive particles are
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introduced into the bottom of a reactor and the manner in
which hot hydrogen-containing gas is introduced into the
mixture of heavy hydrocarbon oil and additive particles within
the reactor.
Thus, one embodiment of the present invention in its
broadest aspect relates to a process for hydrocracking a heavy
hydrocarbon oil in which (a) a slurry feed comprising a
mixture of a heavy hydrocarbon feedstock and from about 0.04
to 4.0% by weight (based on fresh feedstock) of coke-
inhibiting additive particles having an average particle size
of less than about 30 pm, preferably less than about 10 pm,
and (b) a hydrogen-containing gas, are passed upwardly through
a confined hydrocracking zone in a vertical, elongated,
cylindrical vessel with a generally dome-shaped bottom head.
The hydrocracking zone is maintained at a temperature between
about 350 C and 600 C and a pressure of at least about
3.5 MPa. From the top of the hydrocracking zone there is
removed a mixed effluent containing a gaseous phase comprising
hydrogen and vaporous hydrocarbons and a liquid phase
containing heavy hydrocarbons and particulates.
According to the novel features of this process, the
slurry feed mixture and a portion of the hydrogen-containing
gas (secondary gas) are fed into the hydrocracking zone
through a feed injector at the bottom of the dome-shaped
bottom head. The balance of the hydrogen-containing gas (main
gas) is fed into the hydrocracking zone through a plurality of
injection nozzles in the hydrocracking zone at a location
above the slurry-feed injector. The temperature of the main
.hydrogen-containing gas entering through the nozzles is higher
than the temperature of the combined slurry feed and hydrogen-
containing gas entering through the bottom feed injector, and
is generally sufficient to maintain the contents of the
hydrocracking zone at a desired operating temperature. The
main gas temperature is typically in the range of about 450 to
600 C, preferably about 450 to 540 C. The combined slurry
feed and secondary gas entering through the bottom feed
injector should enter at a velocity of at least 5 m/s whereby
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the additive particles are maintained in suspension throughout
the reactor vessel and coking reactions are prevented. The
combined slurry feed and secondary gas enters the reactor
typically at a temperature in the range of about 300 to 430 C,
preferably about 350 to 390 C. In a typical process according
to the invention, the temperature of the vessel contents
varies between about 440 C in a lower region and 465 C in an
upper region.
The hydrogen-containing gas preferably comprises a
recycle gas stream rich in hydrogen typically containing at
least 60% hydrogen, and an important feature of this invention
is the manner in which this hydrogen gas is introduced into
the hydrocracking zone. In order to achieve a good contact
between the main recycle hydrogen stream and the heavy
hydrocarbon oil in the hydrocracking zone, it is important
that the main recycle hydrogen stream be uniformly distributed
within the hydrocracking zone in the form of high velocity
jets, which provide high shear and mixing, producing small.
bubbles, to give large surface area for mass transfer from the
hydrogen in the bubbles to the bulk liquid above the
distributor.
In order to achieve these results, the main gas is
preferably injected into the hydrocracking zone through
injection nozzles that are arranged to assist in the uniform
distribution of the content of the hydrocracking zone. The
slurry feed and gas fed in through the bottom feed injector
tends to create some central channelling of the flow within
the hydrocracking zone. Thus, there is a tendency for much of
the gas to flow up the middle of the reactor, with liquid and
particulate flowing down the sides. It is, therefore,
preferable to provide a lower central set of gas injector
nozzles so that the gas flowing from the nozzles is adapted to
disperse the central channelling in an outward direction
toward the vessel walls. These lower nozzles are preferably
arranged in a central circle with the nozzles aimed in an
upward and outward direction.
It is also preferable to provide a second set of gas
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injection nozzles in the vessel at a location above the lower
nozzles. These second nozzles are arranged adjacent the wall
of the vessel, with the majority being aimed in an upward
direction. Some of these second nozzles are also aimed in a
5 downward and inward direction. The upwardly directed second
nozzles serve to inhibit flow of liquid and particulates down
the vessel walls.
The individual nozzles are typically in the form of small
tubes, preferably having an inner diameter of about 6 to 25 mm
and lengths of about 50 to 100 mm. The pressure drop in these
nozzles is.preferably quite substantial, e.g. in the order of
30% of the liquid head in the vessels, and they can be
operated at quite high velocities, e.g. in the order of at
least 120m/sec. and as high as 200 m/sec. Such high
velocities provide sufficient kinetic energy to cause
attrition of the particulate in the vessel. Attrition depends
on the square of the velocity and the upper limit of
200 m/sec. is to limit the production of very fine powder.
These high velocities do not appear to cause foaming within
the vessel.
It is also possible to arrange the nozzles all at the
same level and equally spaced across the vessel. In this
arrangement, all of the nozzles are in the form of small
vertical tubes aimed upwardly. While this arrangement gives a
relatively flat gas profile across the hydrocracking zone,
there remains some tendency for the gas to channel upwardly in
the central region and liquid and particulate to flow down the
walls.
Another important feature of this invention is that a
portion of the hydrogen-containing gas required for the
process is combined with the slurry feed being fed into the
bottom of the hydrocracking zone. About 10 to 35% by volume
of the total hydrogen-containing gas being fed into the
hydrocracking zone is fed in as the secondary hydrogen-
containing gas with the slurry feed at the bottom of the
reactor. The purpose of adding the hydrogen-containing gas to
the slurry feed is to increase the flow velocity, to control
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coking in a fired liquid heater and to sweep the bottom of the
reactor. Introduction of the relatively cooler slurry feed
plus secondary hydrogen-containing gas keeps the bottom head
cooler to prevent coking reactions and keeps it free of
particles which could settle out from the reaction mixture.
To achieve this, it has been found that the combined liquid
plus gas velocity should be at least 5 m/s at the point of
entry. Addition of liquid alone does not create sufficient
turbulence.
In order to achieve the desired sweeping effect on the
bottom of the reactor, the combined slurry feed and secondary
hydrogen-containing gas is preferably fed into the reactor.
through an injector having a plurality of side openings which
direct flow in an outward direction. During upset conditions,
or if the reactor is in the coking mode, then mesophase
particles can grow and fall through the reactor to the bottom
head, where they accumulate. This mesophase is mixed and
cooled by the incoming liquid plus gas feed, and can be
maintained without coking problems for several hours. At some
point it can be removed by dragging. Additionally, in
conditions described above, feed flow can be increased,
temperature decreased; and cold gas flow can be increased as
well to resuspend stubborn solids and mesophase to facilitate
dragging and recovery.
A further aspect of the present invention is an apparatus
for carrying out the above hydrocracking process. The
apparatus includes a vertical, elongated, cylindrical pressure
vessel with a generally dome-shaped bottom head. This bottom
head includes a feed injector adapted to feed a mixture of
feed slurry and hydrogen-containing gas into the bottom of the
vessel in an outward and upward direction. A higher circular
array of nozzles is positioned adjacent the outer wall of the
vessel in the region of the bottom end of the cylindrical
portion and conduit means are provided for feeding a hydrogen-
containing gas through these nozzles. A lower, axial circular
array of nozzles is positioned within the dome-shaped bottom
head and having a diameter less than one-half the diameter of
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the vessel. Conduit means are provided for feeding a
hydrogen-containing gas through these nozzles. A reaction
product outlet is provided in the top of the vessel. The feed
injector and the gas inlet nozzles are arranged to move the
content of the vessel upwardly through the vessel in a
substantially plug flow with a minimum of settling or
channelling.
Brief Description of the Drawings
For a better understanding of the invention, reference is
made to the accompanying drawings in which:
Fig. 1 is a schematic illustration of a hydrocracking
vessel;
Fig. 2 is a partial sectional view of the bottom end of
the reactor;
Fig. 3 is a plan view of a gas distributor;
Fig. 4 is a side elevation of a gas distributor;
Fig. 5 is a sectional view of upper gas injecting
nozzles;
Fig. 6 is a sectional view of lower gas injecting
nozzles;
Fig. 7 is a partial sectional view of a slurry feed
injector;
Fig. 8 is a plan view of an alternative gas distributor;
and
Fig. 9 is a schematic flow sheet showing a typical
hydrocracking process.
Description of the Preferred Embodiments
The system includes a typical cylindrical pressure vessel
10 with a dome shaped bottom head 11 and a reaction product
outlet 12 at the top.
The feed inlets at the bottom of the reactor include an
outer tube member 13 and an inner concentric tube 15. This
inner tube 15 carries hydrogen-containing gas only (main gas)
while the annular space 23 between tube 13 and tube 15 carries
a mixture of heavy hydrocarbon oil, particulate additive and a
portion of hydrogen-containing gas (secondary gas).
The gas injection system can be seen in greater detail in
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Figures 2 - 6 and it will be seen that the main gas travels up
through tube 15 and into a gas distribution manifold. This
manifold includes four upper lateral tubes 16 connected to
four arcuate gas distribution tubes 17 adjacent the wall of
vessel 10. Mounted on these arcuate tubes 17 are a series of
nozzles 19a and 19b with the nozzles 19a being aimed in an
upward direction and the nozzles 19b being aimed in a downward
and inward direction. These arcuate tubes 17 are supported
within vessel 10 by means of brackets 18 connected to the
vessel walls.
Also as part of the gas distribution system, a pair of
tubes 21 extend down from a pair of the upper distribution
tubes 16 to deliver gas down into a second circular
distribution tube 20 having a diameter less than half the
diameter of the vessel. This distribution tube 20 has mounted
thereon a plurality of upwardly and outwardly directed nozzles
22 as well as two downwardly directed drain tubes 28.
The configurations of the nozzles are shown in greater
detail in Figures 5 and 6. The nozzles connected to
distribution tubes 17 are shown in Figure 5 and it will be
seen that the upwardly directed nozzles 19a have a central
bore 29 and are preferably directed slightly inwardly from the
wall of the vessel by about 6 . The downwardly and inwardly
directed nozzles 19b are preferably at an angle of about 45
to the wall of the vessel.
The upwardly and outwardly directed nozzles 22 on tube 20
have a central bore 34 and are preferably mounted at an angle
of about 45 to the vertical. The downwardly directed drain
tubes 28 have a central bore 51 extending down to a lateral
bore 52 for discharge of any accumulated fluid in the gas
distribution system.
An alternative gas distribution system is shown in
Figure 8. In this arrangement, the nozzles 62 are
substantially equally spaced across the reaction zone to give
a flat gas profile and are typically spaced at about 2 to 3
nozzles per square foot (about 20 to 30 nozzles per square
meter) of reactor cross section. The nozzle diameters and
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number of nozzles are designed such as to give a velocity of
at least about 120 m/sec. and generally the nozzles should
have a minimum diameter of about 6 mm to avoid plugging after
shutdown.
Usually, the pressure drop in the nozzles should be at
least 30% of the head of liquid in the vessel plus the head
differential between the two rings, or counterflow may result,
this being flow of liquid into the nozzles at the extreme ends
of the distributor and out of the nozzles close to the
hydrogen supply.
The bottom feed injector 14 for injecting the mixed
liquid/particulate/gas feed consists of a cylindrical wall.
portion 25 and a top plate 27. In the cylindrical wall are a
series of equally spaced slots 26 which direct flow in an
outward direction as shown in Figures 1 and 2.
A typical process to which the present invention is
applied is shown in Figure 9. The iron salt additive is mixed
together with a heavy hydrocarbon oil feed in a feed tank 30
to form a slurry. This slurry is pumped by a feed pump 31
through an inlet line 53 into the bottom of a cylindrical
reactor vessel 10. Recycled hydrogen 47 and make up hydrogen
from line 48 are simultaneously fed into the reactor as
recycle gas through line 50. This recycle gas stream 50 is
divided into a main gas stream 33 and a secondary gas stream
32. The secondary gas stream 32 is combined with oil/additive
feed slurry 30 and fed into the reactor through line 53 and
bottom feed injector 14 (Fig. 7). The main gas stream 33 is
fed into the reactor through line 15 and nozzles as shown in
Figs. 3 and 4 or Fig. 8. A gas/liquid mixture is withdrawn
from the top of the reactor through line 12 and introduced
into a hot separator 35. In the hot separator the effluent
from vessel 10 is separated into a gaseous stream 38 and a
liquid stream 36. The liquid stream 36 is in the form of
heavy oil containing particulate which is collected at 37.
The gaseous stream from hot separator 35 is carried by way of
line 38 into a high pressure-low temperature separator 39.
Within this separator the product is separated into a gaseous
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stream rich in hydrogen which is drawn off through line 42 and
an oil product which is drawn off through line 40 and
collected at 41.
The hydrogen-rich stream 42 is passed through a packed
5 scrubbing tower 43 where it is scrubbed by means of a
scrubbing liquid 44 which is recycled through the tower by
means of pump 45 and recycle loop 46. The scrubbed hydrogen
rich stream emerges from the scrubber via line 47 and is
combined with fresh make up hydrogen added through line 48 and
10 recycled through line 50 back to reactor 10.
Example 1
Tests were conducted on a hydrocracking reactor using the
gas injection arrangement shown in Figure 8 having a nominal
throughput of 795 m3/day (5000 BPD). The reactor had a
diameter of about 2 m and a height of about 21.3 m and was
used with the process of Figure 9.
The gas distribution system had 60 nozzles spaced at a
distance of about 180 mm. Each nozzle had a height of 200 mm,
with a bottom inner diameter of about 9 mm and a top inner
diameter.of about 11 mm. The inner tapered portion extended a
distance of 50 mm.
The liquid injector included 12 injection slots, each
having an area of 8.3 cm2.
Conditions for a test run were as follows:
2.5 The fresh feedstock was Cold Lake refinery vacuum tower
bottoms containing 89 wt% of 524 C+ material and having an API
gravity 4.4 API. The additive particles were finely ground
iron sulphate monohydrate having average particle sizes less
than 10 pm, these particles being mixed with the feedstock to
form a feed slurry. The hydrogen-containing gas was a recycle
gas stream containing 85% H2. This gas was divided between a
main gas stream feeding directly into the reactor and a
secondary gas stream mixed with the feedstock/additive slurry.
(a) The process conditions were as follows:
Reactor Pressure 13.9 MPa
Temp. of liquid in reactor 451 C
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Temp. of liquid/additive/gas
feed to reactor 382 C
Temp. of main gas stream to reactor 493 C
Temp. of cold hydrogen quench 60 C
Fresh Feed Rate 3000 BPD
Cold hydrogen quench' 4,000,000 SCFD
Main gas flow 19,000,000 SCFD
Secondary gas flow 10,000,000 SCFD
Additive rate 3 wt% on fresh feed
This provided a 524 C+ conversion rate of 90%. After
running the above process for 20 days, there was little if any
coke build-up in the reactor.
*Cold hydrogen gas was fed directly into the reactor to
lower the reactor temperature.
(b) The above procedure was repeated with the secondary
gas flow being varied between 5,000,000 and 10,000,000 SCFD.
There was found to be poor distribution in the bottom for
secondary gas flows below 6,000,000 SCFD.
Example 2
A further test was carried out on the same reactor as in
Example 1. However, the flow sheet of Figure 9 was modified
to permit recycle of pitch and aromatic oil, as further
described in Benham et al., U.S. Patent No. 5,755,955 issued
May 26, 1998. Thus, in the flow sheet of Figure 9, the heavy
oil product 37, containing particulate, was fed to a
fractionator with a bottom pitch stream boiling above 524 C
and containing particulate being drawn off and recycled as
part of the feedstock to reactor 10.
The fractionator also served as a source of aromatic oil,
in the form of an aromatic heavy gas oil fraction removed from
the fractionator. This gas oil stream, preferably boiling
above 400 C, was also recycled as part of the feedstock to
reactor 10.
The fresh feedstock was visbreaker vacuum tower bottoms
from Flotta Crude having an API gravity of 8.5 API. The
additive particles were finely ground iron sulphate
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monohydrate having average particle sizes less than 10 pm,
these particles being mixed with the feedstock to form a feed
slurry. The hydrogen-containing gas was a recycle gas stream
containing 85% H2. This gas was divided between a main gas
stream feeding directly into the reactor and a secondary gas
stream mixed with the feedstock/additive slurry. A cold
hydrogen quench was also fed directly into the reactor to
lower the temperature.
The process conditions were as follows:
Reactor Pressure 13.9 MPa
Temp. of liquid in reactor 464 C
Temp. of liquid/additive/gas
feed to reactor 403 C
Temp. of main gas stream to reactor 516 C
Temp. of cold hydrogen quench 60 C
Fresh Feed Rate 3218 BPD
Aromatics feed 823 BPD
Pitch recycle 652 BPD
Main gas flow 26,000,000 SCFD
Secondary gas flow 10,200,000 SCFD
Cold hydrogen quench 1,500,000 SCFD
Additive rate 2.3 wt% on fresh feed
This provided a 524 C+ conversion rate of 89% with no
coke build-up in the bottom of the reactor.
Although this invention has been described broadly. and in
terms of various specific embodiments, it will be understood
that modifications and variations can be made and some
elements used without others all within the spirit and scope
of the invention, which is defined by the following claims.
