Note: Descriptions are shown in the official language in which they were submitted.
PROCESS FOR TREATING GAS OILS
The present invention relates to a process for
treating gas oil feedstocks in order to produce
valuable fuel products. Particularly the present
invention involves a specific combination of two
treatments of gas oil feedstocks in order to favor
the prod~ction of diesel fuel and gasoline fractions~
The heavy gas oils (gas oils from vacuum
dis~illation, VG0 or cut between 370~ - 540~C~ are
generally sent directly to the catalytic cracking
unit in order to be converted into valuable lighter
hydrocarbons. However, it is desirable to increase
the yield of valuable products from gas oils, either
the atmospheric gas oils or the vacuum gas oils. It
has been recognized during the last few years that it
is possible to treat the gas oils before submitting
them to catalytic cracking in order to recover much
more valuable products then solely by catalytic
cracking.
It has heretofore been proposed to submit the
gas oils to mild hydrocracking before subjecting them
to catalytic cracking. This treatment enables the
recovering of additional fractions of diesel oils.
Gas oils may also be submitted to a dewaxing
process in order to reduce their pour point.
The combination of a hydrotreatment and a
dewaxing has heretofore been described in the art.
3~
U.S. Patent 4r394~249 to Shen discloses
desulfurization of a hydrocarbon feedstock over a
conventional hydrodesulfurization catalyst comprising
Group VA and Group VIIIA metals, or metal oxides or
sulfides, followed by dewaxing over ZSM-5 or other
ZSM-type catalysts. U.S. Patent 4,45~,024 to Oleck
et al discloses a process for hydrodewaxing and
desulfurization over a single catalyst composition
based upon a ZSM-5 type zeolite and Group VI and
1o Group VIII metals. The catalyst composition may be
formulated by mixing ZSM-5 with an alumina binder
followed by calcining, ion exchanging to low sodium
content, and impregnation with Group VI and Group
VIII metal salt solutions.
European patent specification 43,681 (Gorring)
discloses lube oil manufacturing involving dewaxing
gas oils over a Ni-exchanged zeolite such as ZSM-5 or
ZSM-11 in order to eliminate sulfur present in the
feed, and then submitting the effluent to
hydrocracking conditions. For feeds containing high
levels of deleterious nitrogen compounds, a
hydrotreating step may be interposed between the
dewaxing and hydrocracking steps.
In European patent specification 72,220 (Oleck
et al), base oils with low pour point are
manufactured by first dewaxing the feed over a Ni-
exchanged zeolite and then submitting the eff1uent to
hydrocracking over a Ni-Mo exchanged zeolite. The
zeolites may be ZSM-5, ZSM-11, ZSM-23 and ZSM-35.
U.S. Patent No. 4,229,282 to Peters discloses a
process for dewaxing hydrocarbon oil in the presence
of hydrogen over a Ni-W exchanged zeolite, preferably
ZSM-5.
3 ~;2 g3~
The aforementioned patents indicate that when
the dewaxing and hydrocracking are combined, it is
necessary to use nickel-exchanged zeolites to obtain
satisfactory results in tenms of pour point
reduction.
An object of the invention is to provide a
process for treating hydrocarbons boiling in the
range of heavy gas oils, to increase the recovery of
light }lydrocarbons.
Another object of the present invention is to
provide a two-step process for treating heavy gas
oils to increase the production of diesel oils and
gasoline over and above that generally obtained by
catalytic cracking of the same feed.
A further object of the present invention is to
provide a process for the treatment of hydrocarbons
boiling in the range of 370C - 540C in order to
obtain a significant amount of light hydrocarbons.
In accordance with the present invention, there
is provided a process for the treatment of a
hydrocarbon feedstock having a distillation curve
within the range of heavy gas oils in order to
recover a light hydrocarbon product. The process
comprises subjecting the hydrocarbon feed to a mild
hydrocrac~ing treatment and a dewaxing treatment.
The dewaxing treatment is conducted over a
crystalline silica polymorph silicalite dewaxing
catalyst under temperature and pressure conditions
suitable to crack waxy paraffinic hydrocarbons in the
feedstock. The mild hydrocracking treatment is
carried out over a hydrocracking catalyst at
temperature and pressure conditions suitable to
produce hydrocarbons of a reduced boiling point
~39~i
range. The hydrocracking catalyst may be oE any
suitable type such as a mixture of Group VI~ and
Group VIII metal components as described in greater
detail below. Following the dewaxing and
hydrocracking treatments, a product of reduced
boiling point having an increased amount of light
hydrocarbons is recovered. The silicalite dewaxing
catalyst is present in an amount within the range of
15-25 volume ~ of the total catalysts (including the
silicalite~ employed in the process.
The dewaxing and mild hydrocracking treatments
may be carried out simultaneously over a blend
comprising a discrete physical mixture of the
silicalite dewaxing catalyst and the hydrocracking
catalyst or the dewaxing and mild hydrocracking
treatments may be carried out seq~entially.
In a preferred embodiment of the invention, a
hydrocarbon feedstock having a final boiling point in
excess of 450C and a 25 wt.~ boiling point in excess
2~ of 370C is passed to a reaction zone where it is
dewaxed over a silicalite dewaxing catalyst. The
dewaxed hydrocarbon fraction from this initial
reaction zone is passed into a subsequent reaction
zone where it is hydrocracked over a hydrocracking
catalyst under mild operating conditions including a
temperature within the range of 350~C-450C and a
pressure within the range of atmospheric to 80
bars. The resulting product of reduced boiling point
range, which is predominantly in the diesel oil range
or below, is withdrawn from this reaction zone.
In a further aspect of the invention, there is
provided an intermediate reaction zone between the
dewaxing and hydrocracking zones in which the
5 ~z~
-
hydrocarbon fraction is catalytically hydrotreated to
remove sulfur. Preferably, the initial, intermediate
and subsequent reaction zones are defined by
respective layers of catalysts within the same
reactor. m e reactor is operated in a downflow mode
in which the hydrocarbon feed passes in a liquid
phase through the successive catalyst layers,
contacting the silicalite first.
In the present invention by first submitting the
hydrocarbon feedstock boiling in the range of the
heavy gas oils to dewaxing over a crystalline silica
polymorph of the silicalite type under suitable
conditions, and submitting the resulting feed to mild
hydrocracking, production of light hydrocarbons,
particularly diesel oil and gasoline, is obtained, in
greatly i~proved amounts over those reasonably
expected in view of the prior art.
The feeds used in the process of the invention
are heavy gas oils or vacuum gas oils (VGO),
comprising the hydrocarbon fraction boiling in the
range of 370 to about 540C. These feeds may
contain at most 25~ by weight hydrocarbons boiling
below 370~C.
The process of the invention is particularly
adapted to heavy gas oils feedstocks having a sulfur
content up to 5% by weight. A preferred application
of the invention resides in the treatment of
feedstocks having a sulfur content of at least 1 wt%,
particularly within the range of 1-4 wt.%.
The best results are obtained when the dewaxing
step is carried out by passing the feed over a
crystalline silica polymorph of the silicalite type
as catalyst, under suitable conditions to crack the
straight chain paraffinic hydrocarbons.
The dewaxing catalyst used in the process of the
invention is a crystalline silica polymorph of the
silicalite type. Silicalite has no ion exchange
capacity in comparison with aluminosilicates o~ the
zeolite type which are silicates of aluminum and
~odium and/or calcium. Aluminum may be present in
silicalite, but in the fonm of impurity which comes
from the silica ~ource used to prepare the
silicalite. ~ilicalites are microporous materials
which are prepared hydrothermally by using a reaction
mixture comprising tetrapropylammonium cations,
alkali metal cations, water and a source of reactive
silica. Silicalite and its preparation are described
in U.S. Pa~ent 4,061,721 to Grose et al.
Silicalite in the as synthesized form and after
calcining to decompose the alkyl ammonium templating
agent employed in the synthesis procedure is in the
orthorhombic form. ~owever, as disclosed in U.S.
Patent No. 4,599,473 to Debras et al, silicalite of
orthorhombic symmetry can be converted to monoclinic
symmetry by calcining in air at a temperature of at
least 600-C for a period of 3 hours or more.
Monoclinic silicalite has certain advantages in
hydrocarbon conversion reactions, as disclosed in the
Debras et al patent. ~or a description of monoclinic
silicalite, its preparation and use, reference is
made to the aforementioned U.S. Patent No. 4,599,473
to Debras et al. The silicalite used in the
present invention can be of orthorhombic or
monoclinic symmetry.
7 ~ 3~-~6
The silicalite catalyst employed in the present
invention can be in the un~odlfied forrn; that is, in
the form as synthesized in accordance with the
procedure disclosed in U.S. Patent No. 4,061,724 to
Grose, al~hough as noted above the silicalite may be
of either monoclinic or orthorhombic symmetry. The
catalyst need not be chemically pretreated to
increase its sta~ility to sulf~lr contaminants, and
when used directly with metal catalyst components, it
is in the form of a discrete physical mixture, as
described in greater detail hereinater.
Preferably in the process of the invention, the
silicalite used for dewaxing has pore sizes Gf about
0.55 nm and is present in the form of crystallites of
a size which is less than 8 microns.
The dewaxing step may be carried out in any
apparatus comprising a reaction zone which contains
the silicalite catalyst.
In the preferred embodiment of the invention, by
2~ directly submitting the feed which results from the
dewaxing step to a mild hydrocracking, the final feed
obtained contains light hydrocarbons in greater
amounts then would be expected. The mild
hydrocracking reaction may be carried out over any
suitable hydrocracking catalyst. The classic
catalysts for mild hydrocracking are mixtures of
Group VIB and Group VIII metal components,
particularly the oxides of such metals. An example
of such catalysts is a Ni-Mo catalyst deposited on
silica-alumina support. Such catalyst may be
prepared by incorporating within the support Ni and
Mo in the form of oxides, drying the impregnated
support, and then submitting it to a stream of a
8 ~3~
mixture o H2 and E~2s (1-2 % vol.) at 200 C-250C
first and then at a temperature of 32~C-350~C. A
part of this catalyst may also be replaced by a Co-Mo
catalyst deposited on an alumina support, said
catalyst being prepared according to a similar method
as described above. As described below, the use of a
Co-Mo catalyst is desirablè where the feed contains
substantial sulfur, since the Co-Mo catalyst will
function in a hydrotreating function to remove
sulfur, as well as nitrogen components, in the
feedstock. In their oxide form, these catalysts
contain generally from 3 - 6% by weight of NiO or
CoO, and from 10 - 20% by weight of MoO3; these
catalysts have a specific surface generally comprised
between 150 - 300 m2/g, and a pore volume generally
comprised between 0.3 - 0.6 ml/g. These catalysts
are commercially available under the form of oxide.
Although the reactions may be carried out in two
different reactors in cascade and under temperature
and pressure conditions which do not have to be
necessarily identical, applicants have found that
both reactions may be carried out in the same
reactor. The proportion of the different catalysts
plays a role in obtaining significant results. Thus,
in a specific aspect of the invention the proportion
of silicalite should be between 15 - 25% by volume,
while the proportion of mild hydrocracking catalyst
should be between 85 - 75~ by volume. The catalysts
may be placed in one or several beds which may be
separated by layers of inert materials.
According to a preferred embodiment of the
process of the invention, the dewaxing and
hydrocracking steps of the process are carried out in
9 ~293~
the same reactor, and the different catalysts are
placed in several beds. The firsk bed encountered
the hydrocarbon feed is a bed of crystalline silica
polymorph of the silicalite type. Where a
hydrotreating catalyst which is effective to remove
sulfur and nitrogen under the reactor conditions is
employed, it preferably will be placed immediately
below the silicalite catalyst bed. The hydrotreating
catalyst, such as the Co-Mo catalyst described above,
is separated from the silicalite catalyst by a layer
of inert material, and the hydrocracking catalyst,
such as the Ni-Mo catalyst described above t iS placed
in the reactor as a bottom layer. This catalyst will
normally also be separated from the
hydrodesulfurization catalyst by layer of inert
material. Typically, the hydrodesulfurization and
hydrocracking catalyst will be used in equal amounts,
each about 40 volume % of the total catalyst volume.
The feed is passed through the reaction zone or
zones containing the catalysts, at a temperature
between 350~C ~ 450VC, preferably between 380~C -
420~C, under a pressure between atmospheric pressure
and 80 bars, preferably between 35 - 65 bars, and at
a liquid hourly space velocity (LHSV) comprised
between 0.1 - 20 1/1 (calculated on both catalysts)
and preferably between 0.5 - 5 1/1 hr~1.
Simultaneously with the feed, hydrogen is
introduced into the reactor in an amount to provide a
volume of ratio hydrogen/hydrocarbons between 50 -
5000 and preferably between 250 - 1000 (the volume of
hydrogen being determined in the gaseous state and
under standard conditions). However, practically,
only a small amount of hydrogen is consumed and the
1 o ~Z~3~3 ~L6
gas recovered at the outlet of the reactor
~constituted of hydrogen and a minor amount of
gaseous hydrocarbons) is generally recycled. To
compensate for the hydrogen consumption, a part of
recycled gas is continuously withdrawn and is
replaced by hydrogen.
Applicants have also noted a synergistic effect
by carrying out another embodiment of the process of
the invention in which the feed is submitted to the
mild hydrocracking treatment before dewaxing. This
synergistic effect is much weaker when mild
hydrocracking is carried out after the dewaxing, but
the quality of the 250C-370C cut is better in this
latter case.
In the third embodiment of the invention in
which the dewaxing catalyst is mixed with the mild
hydrocracking catalyst, intermediate values are
obtained for the conversion rate and for the
properties of the 250~C-370C cut. In this
embodiment of the invention, the silicalite and
metallic catalysts may be physically mixed together
in any appropriate manner. The resulting mixture is
a discrete physical mixture in which the individual
catalyst components retain their chemical identity in
contrast with the catalyst systems such as disclosed
in the aforementioned U.S. patent to Peters et al or
sritish patent specification by Oleck et al in which
-catalysts are composited by chemical impregnation or
ion exchange with a zeolite.
The following examples are given in order to
better illustrate the process of the invention but
without limiting its scope.
3~ ~6
Example
The employed catalysts were silicalite
(available from Union Carbide and having mean pore
size of about O.55 nm and crystallite size of less
than 8 um) and a catalyst comprising Ni and Mo on
Al23/SiO2 and having the following characteristics:
specific area: 153 m~/g
pore volume: 0.53 ml/g
Nio: 3.6 weight %
MoO3 19.6 weight 96
This latter catalyst was pretreated by
subjecting it to a drying step at 130C and then to a
sulfuration treatment at 54 bars with a mixture H2 +
E~2S (1.1 vol.96), first at 250~C up to a partial
pressure of H2s higher than 0.03 bar at the reactor
exit, and then progressively up to 320~C, while
keeping the partial pressure of H2S higher than 0.03
bar at the exit. The culfided Ni-Mo catalyst
contained about 10 weight % of sulfur.
2~ A reactor having an inner diameter of 2.5 cm was
charged with 20 vol.% of silicalite (height: 7 cm)
and 80 vol.~6 (height: 28 cm) of sulfided Ni-Mo
catalyst, both being disposed between two layers of
inert material ~height of each layer: 40 cm).
A hydrocarbon feed was passed through the
reactor, this feed passing successively through the
silicalite bed and the Ni-Mo catalyst bed.
This feed was a gas oil from a vacuum
distillation unit having the following
characteristics:
fraction up to 180C: 0.1 wt%
fraction 180C-250C: 2.55 wt%
fraction 250C-370C: 18.39 wt%
:~2~363'~6
fraction 370-500VC: 64.55 wt~
fraction 500~C + C: 14.41 wt~
specific gravity dl5/4: 0.91
sulfur content: 1.42 wt%
total nitrogen: 1010 ppm.
basic nitrogen: 267 ppm.
A hydrogen stream from a refinery (containing about
85% H2) was passed through the reactor at a H2
partial pressure of at least 40 bars, simultaneously
with the feed.
The run was carried out at 405C and a pressure
of 54 bars. The other working conditions and the
conversion rates (weight percentage of the 370 ~C
fraction which has been converted) are given in the
following Table 1. The ratio of recycled
gas/hydrocarbons was varied as a function of the LHSV
of the feed in order to keep constant the flow rate
of recycled gas.
TABLE 1
Run 1A 1B 1C
LHSV 0.6 1.01.5 based on
the whole
catalysts
Volume ratio recycled 750 450 300 liters of
gas/hy~rocarbons gas (under
normal
conditions)
per liter of
feed
1 3 ~ ~3~
Conversion (%) 51.1 36.6 21.8
Effluent conposition (wt~)
_
~ydrocarbons Cl_2 1.~6 1.48 0.91
13ydrocarbor~; ~ 1.73 1,04 0.47
~ rocarbo~; C4 3.78 2.0B 0.93
F~aceion C5-1804C 14.18 11.16 6~17
(gasoline)
Fraction 180C-250~C9.01 6.39 S.74
(kerosene)
~raction 250-C-370UC31.51 28.51 28.06
~diesel fuel)
Fraction 370-C 38.13 49.46 57.72
Properties of the fraction 180~C-250C
_________________________
$~ecific gravity d1s/4 0.844 0.847 0.843
Pour poi~t tC) -57 -45 -47
Cloud point ~C) -45 -45 -47
~operties of t}E~ fraction 250~C-370C
_________________.________
Specific gravity d15/4 0.893 0.890 0.890
Pour point (-C) -24 -15 -8
Clo~ poir~ C) -27 -11 -8
C~ne index 41.2 42.5 44.0
Example 2
The procedure of Example 1 was repeated, but by
replacing one half of the Ni-Mo catalyst with a Co-Mo
alumina catalyst (commercially available as ~etjen
* Trademark
~,
'~L
. 14 1~93~6
742). The eed was passed successively on the
silicalite, the Co-Mo catalyst and the Ni-Mo catalyst
beds.
The conversion yield was 48.7% with a LHSV of
0.6.
Example 3
The procedure of Example 1 was repeated, but by
inverting the catalysts, the feed passing first over
the Ni-Mo catalyst and then the silicalite bed.
The results are given in Table 2.
TABLE 2
Run 3A 3B 3C
LHSV 0.61.0 1.5
Conversion (%) 50.830.719.2
Effluent (wt%)
2~
Gaseous hydrocarbons 4~8
Fraction C5-180 C 11.9
Fraction 180C-250C 6.9
Fraction 250C-370C 21.7
Fraction 370+C 54.7
Properties of the fraction 180C-250C
Specific gravity d15/4 0.883
Pour point/cloud point (C) -45
15 ~2~
Properties oE the fraction 250-370~C
Speciic ~ravity d15/4 0.890
Pour point (C) -22
Cloud point (UC) -18
Cetane index 42.4
By comparison with run lB, it can be shown that
the properties of the diesel fuel fraction are
better.
Comparative experiments (hereinafter runs C1 to
C9) were carried out in order to evaluate the
synergistic effect resulting from the use of the
process of this invention. To this end, catalysts
given in the following Table 3 were tested and the
conversion yields were compared with those obtained
in the hereinabove described Examples.
TABLE 3
Run n~ Catalysts LHSV Conversion (%)
. . .
.
1A Silicalite/Ni-Mo 0.6 51.1
2 Silicalte/Co-Mo/Ni-Mo 0.6 48.7
3A Ni-Mo/silicalite 0.6 50.8
C1 Silicalite 3 5.6
C2 Ni-Mo 0.6 34.9
C3 Ni-Mo 0.75 26.9
1B Silicalite/Ni-Mo 1.0 36.6
3B Ni-Mo/silicalite 1.0 30.7
C4 Silicalite 5 5.0
16 3;~3~6
C5 Ni-Mo 1.0 24.7
C6 Ni-Mo 1.25 19.3
1C Silicalite/Ni-Mo 1.5 21.8
3C Ni-Mo/silicalite 1.5 19.2
C7 Silicalite 7.5 3.4
C8 Ni-Mo 1.5 18.2
C9 Ni-Mo ~.~7 15.3
These comparative runs clearly show that a
synergistic effect results from the combination of a
dewaxing treatment and a mild hydrocracking
treatment. For instance, the data of run 3A make it
possible to calculate the conversion rate resulting
from the mild hydrocracking step, takin~ into account
the conversion rate reached in run Cl for silicalite
alone, as follows:
50.8 x (1 _ 51U~6 ) = 47.9%
This result with the conversion rates of 34.9 and
26.9% obtained with runs C2 and C3 respectively.
The composition of some effluents and the
properties of some fractions are given in Table 4,
where they are compared with those of run lA.
~5
TABLE 4
~ 1A Cl C3
Effluent composition (wt4)
hydrocarbons Cl-C4 7.17 2.99 1.45
fraction C5-l8o~c 14.183.19 7.58
17 ~939 Ll ~i
fraction 180C-250~C 9.01 2.28 7.79
fraction 250C~370C 31.51 17.85 29.29
fraction 370~C 38.13 73.69 53.89
Properties of fraction 180~C-250C
specific gravity d15/4 O.B44 0.845
pour point (C) -57 -54
cloud point (C) -45 -45
Properties of fraction 250C-370C
- - - - - - - - - - - - - - - - - -
specific gravity d15/4 0.8g3 OD884
pour point (C) -24 -4
cloud point (C) -27 -4
cetane ir.dex 41.2 43.9
Example 4
A gas oil feed comprising
fraction 370~C 78.1 wt%
fraction 250C-370DC 19.1 wt%
fraction 180C-250C 2.8 wt%
was treated according to the process of this
invention and this treatment was followed by a usual
fluid catalytic cracking at 51 0Cr 1. 7 bar and
LHSV=40 on zeolite.
The recovered effluent contained (wt%)
10.6% gas (mainly C3 and C4)
35.8% gasoline (fraction C5-180C)
10.0% kerosene (fraction 180C-250C)
32.1~ diesel fuel (fraction 250C-370~C)
18 ~3~ ~6
7.1% light cycle oil
2.7~ residue
By way of comparison, a eed havlng the same
composition was subjected to a mild hydrocracking and
then to a catalytic cracking under the same working
conditions. The effluent contained (wt%):
8.6% gas (mainly C1-C3)
38.5% gasoline
8.5% kerosene
30.4% diesel fuel
9.5% light cycle oil
3.4% residue
This example shows that more kerosene and diesel
fuel are produced with the process of this
invention. Furthermore, the recovered gases are more
valuable.
Having described specific embodiments of the
present invention, it will be understood that
modification thereof may be suggested to those
skilled in the art, and it is intended to cover all
such modifications as fall within the scope of the
appended claims.
