Note: Descriptions are shown in the official language in which they were submitted.
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FLU I D CATALYTIC CRACKING PROCESS
Field of the invention
The present invention relates to a fluid catalytic
cracking process.
Background of the Invention
In fluid catalytic cracking processes a preheated
hydrocarbon feedstock of a high boiling point range is
brought into contact with a hot cracking catalyst in a
catalytic cracking reactor, usually a riser. The feed is
cracked into lower boiling products, such as dry gas,
LPG, gasoline, and cycle oils. Furthermore, coke and non-
volatile products deposit on the catalyst resulting in a
spent catalyst. The reactor exits into a separator
wherein the spent catalyst is separated from the reaction
products. In the next step the spent catalyst is stripped
with steam to remove the non-volatile hydrocarbon
products from the catalyst. The stripped catalyst is
passed to a regenerator in which coke and remaining
hydrocarbon materials are combusted and wherein the
catalyst is heated to a temperature required for the
cracking reactions. Hereafter the hot regenerated
catalyst is returned to the reactor.
As hydrocarbon feedstock a feedstock comprising a
large portion of paraffins can be cracked. However,
cracking such a paraffin rich hydrocarbon feedstock, such
as for example a Fischer-Tropsch product, is not
straightforward.
US-A-4684756 describes a process to prepare a
gasoline fraction by fluid catalytic cracking of a
Fischer-Tropsch wax as obtained in an iron catalysed
Fischer-Tropsch process. The gasoline yield is 57.2 wt9o.
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A disadvantage of the process disclosed in US-A-4684756
is that the yield to gasoline is relatively low.
EP-A-454256 describes a process to prepare lower
olefins from a Fischer-Tropsch product by contacting this
product with a ZSM-5 containing catalyst at a temperature
of between 580 and 700 C in a moving bed reactor at a
catalysts to oil ratio of between 65 and 86 kg/kg.
WO-A-2004/106462 describes a process wherein a
relatively heavy Fischer-Tropsch product and a catalyst
system comprising a catalyst, which catalyst comprises an
acidic matrix and a large pore molecular sieve, are
contacted, yielding a gasoline product having a high
content of iso-paraffins and olefins, compounds which
greatly contribute to a high octane number.
A disadvantage of processing such a paraffinic feed
in an FCC unit is that the coke make is too low. Coke on
the catalyst is removed by oxidation in a so-called FCC
regenerator. In such a process step the catalyst
temperature increases due to exothermic reactions and
reaches a temperature that makes it suitable for use in
the actual catalytic cracking step. If the coke content
of the catalyst is too low additional fuel is to be added
to the regenerator and this situation is obviously not
desired.
NL-A-8700587 describes catalytic cracking of water-
free butter to hydrocarbon products, like C4 gases and
lighter gases, gasoline (C5-216 C), light cycle oils and
coke, over a type RE-USY catalyst further comprising an
active crystalline aluminium oxide matrix.
It is the object of the present invention to achieve
a process which is better heat balanced than the prior
art processes.
7
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Summary of the Invention
It has now been found that the above can be achieved
by performing the fluid catalytic cracking of the
paraffinic feedstock in the presence of triglycerides.
Accordingly, the invention provides a fluid catalytic
cracking process for the preparation of cracked products by
contacting in a fluid catalytic cracking reactor a
hydrocarbon feedstock with a cracking catalyst, wherein the
hydrocarbon feedstock comprises triglycerides and a
paraffinic feedstock, which paraffinic feedstock is a
hydrowax or is a Fischer-Tropsch derived hydrocarbon
stream, wherein the weight ratio between the paraffinic
feedstock and the triglycerides present in the hydrocarbon
feedstock is in the range of from 20:1 to 1:5.
It has been found that by cracking a mixture of a
paraffinic feedstock and triglycerides, more coke is formed
on the cracking catalyst. An additional advantage of
cracking the mixture is that a gasoline is obtained having
a higher octane number. Applicant further found that by
choosing the right balance between the paraffinic feedstock
on the one hand and the triglycerides on the other hand, a
gasoline product may be obtained having a sulphur content
of less than 10 ppm, an aromatic content of lower than 35
vol%, preferably lower than 25 vol%, and an octane number
of higher than 87. The triglycerides present in the
hydrocarbon feedstock are cracked and the products formed
result in improved RON octane numbers of the total product.
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Detailed Description of the Invention
Triglycerides are glycerides in which the glycerol is
esterified with three fatty acids. Preferably, the
triglycerides that are being used in the process according
to the invention comprise fatty acids wherein the fatty
acid moiety ranges from 4 to 30 carbon atoms, the fatty
acids most commonly being saturated or containing 1, 2 or 3
double bonds. Triglycerides are the main constituent in
vegetable oil, fish oil and animal fat.
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Preferably, the hydrocarbon feedstock comprises
vegetable oil, animal fat or fish oil to provide the
triglycerides. The vegetable oil, animal fat or fish oil
does not need to be in anhydrous or pure form or to be
subjected to prior hydrogenation. The oil or fat may
contain variable amounts of free fatty acids and/or
esters both of which may also be converted to
hydrocarbons during the process of this invention. The
oil or fat may further comprise carotenoids,
hydrocarbons, phosphatides, simple fatty acids and their
esters, terpenes, sterols, fatty alcohols, tocopherols,
polyisoprene, carbohydrates and proteins.
Suitable vegetable oils include rapeseed oil, palm
oil, coconut oil, corn oil, soya oil, safflower oil,
sunflower oil, linseed oil, olive oil and peanut oil.
Suitable animal fats include pork lard, beef fat, mutton
fat and chicken fat. Mixtures of oils or fats of
different origins may be used as feed to the catalytic
conversion step. Thus, mixtures of the vegetable oils,
animal fats, fish oils, and mixtures which include
vegetable oil, animal fat and/or fish oil may be used.
Preferred oils are rapeseed oil and palm oil, in
particular palm oil. It has been found that the use of
palm oil results in a higher conversion to cracked
products and in higher yields to gasoline.
The hydrocarbon feedstock may further comprise
natural fatty acids and esters other than triglycerides,
for example fatty acid methyl esters derived from
transesterification of the above plant oils and animal
oils.
Without wishing to be bound to any theory, we found
that catalytic cracking of triglycerides seems to be a
stepwise process where in the first step fatty acids
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molecules and the glycerol backbone are being formed. The
fatty acid molecules are subsequently cracked into
lighter components. We found that, in the presence of a
cracking catalyst, the conversion of the triglycerides
into fatty acids is almost instantaneous, while the next
step, being the conversion of fatty acids, depends on
factors such as catalyst to oil ratio, type of catalyst,
temperature and residence time.
Typically, one expects that the oxygen present in
the triglycerides is being converted to CO2 in the
catalytic cracking step. We, however, found that most of
the oxygen is converted to water as by-product. This
water will already function as stripping gas and will be
separated from the valuable products in the stripping
step of the fluid catalytic cracking process.
Examples of suitable paraffinic feedstocks are a
Fischer-Tropsch derived hydrocarbon stream or hydrowax.
Hydrowax is the bottoms fraction of a hydrocracker.
With a hydrocracker in the context of the present
invention is meant a hydrocracking process of which the
main products typically are naphtha, kerosene and gas
oil. The conversion, expressed in the weight percentage
of the fraction in the feed to the hydrocracker boiling
above 370 C to hydrocarbons boiling below 370 C, is
typically above 50 wt%. Examples of hydrocracking proces-
ses which may yield a bottoms fraction that can be used
in the present process, are described in EP-A-699225,
EP-A-649896, WO-A-97/18278, EP-A-705321, EP-A-994173 and
US-A-4851109.
By "Fischer-Tropsch derived hydrocarbon stream" is
meant that the hydrocarbon stream is a product from a
Fischer-Tropsch hydrocarbon synthesis process or derived
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from such product by a hydroprocessing step, i.e.
hydrocracking, hydro-isomerisation and/or hydrogenation.
The Fischer-Tropsch reaction converts carbon
monoxide and hydrogen into longer chain, usually
paraffinic, hydrocarbons:
n(CO + 2H2) = (-CH2-)n + nH20 + heat,
in the presence of an appropriate catalyst and typically
at elevated temperature, for example 125 to 300 C,
preferably 175 to 250 C, and pressure, for example 5 to
100 bar, preferably 12 to 80 bar. Hydrogen:carbon
monoxide ratios other than 2:1 may be employed if
desired.
The carbon monoxide and hydrogen is typically
derived from a hydrocarbonaceous feedstock by partial
oxidation. Suitable hydrocarbonaceous feedstocks include
gaseous hydrocarbons such as natural gas or methane,
coal, biomass, or residual fractions from crude oil
distillation.
The Fischer-Tropsch derived hydrocarbon stream may
suitably be a so-called syncrude as described in for
example GB-A-2386607, GB-A-2371807 or EP-A-0321305. Other
suitable Fischer-Tropsch hydrocarbon streams may be
hydrocarbon fractions boiling in the naphtha, kerosene,
gas oil, or wax range, as obtained from the Fischer-
Tropsch hydrocarbon synthesis process, optionally
followed by a hydroprocessing step.
Preferably, the Fischer-Tropsch hydrocarbon stream
product has been obtained by hydroisomerisation of
hydrocarbons directly obtained in the Fischer-Tropsch
hydrocarbon synthesis reaction. The use of a hydro-
isomerised hydrocarbon fraction is advantageous because
it contributes to a high yield in gasoline due to the
high content of iso-paraffins in said fraction. A hydro-
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isomerised fraction boiling in the kerosene or gas oil
range may suitable be used as the Fischer-Tropsch derived
hydrocarbon stream. Preferably, however, a higher boiling
hydro-isomerised fraction is used as feed.
A particularly suitable hydro-isomerised hydrocarbon
fraction is a fraction which has a TlOwt% boiling point
of between 350 and 450 C and a T90 wt% of between 450
and 600 C and a wax content of between 5 and 60 wt%.
Such fraction is typically referred to as waxy raffinate.
Preferably, the wax content is between 5 and 30 wt%. The
wax content is measured by solvent dewaxing at -27 C in
a 50/50 vol/vol mixture of methyl ethyl ketone and
toluene. Examples of such a hydrocarbon streams are the
commercially available Waxy Raffinate product as is
marketed by Shell MDS (Malaysia) Sdn Bhd snf the waxy
raffinate product as obtained by the process described in
WO-A-02/070630 or in EP-B-0668342.
The paraffinic feedstock comprises preferably at
least 50 wt% paraffins, more preferably at least 70 wt%
paraffins. With paraffins both normal and iso-paraffins
are meant. The paraffin content of the paraffinic
feedstocks in the context of the present invention are
measured by means of comprehensive multi-dimensional gas
chromatography (GCxGC), as described in P.J.
Schoenmakers, J.L.M.M. Oomen, J. Blomberg, W. Genuit, G.
van Velzen, J. Chromatogr. A, 892 (2000) p. 29 and
further.
The hydrocarbon feedstock according to the present
invention comprises both a paraffinic feedstock and
triglycerides. Preferably, the weight ratio between the
amount of paraffinic feedstock and the amount of
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triglycerides present in the hydrocarbon feedstock is
between 20:1 to 1:5, more preferably between 5:1 to 1:2.
The hydrocarbon feedstock may optionally also
comprise a component not being a triglyceride or a
paraffinic feedstock. Suitable components are so-called
conventional FCC feedstocks, which are typically derived
from crude oil refining and which are less paraffinic
than the above described paraffinic feeds. The
conventional FCC feedstock that can be used in the
process according to the invention includes high boiling
non-residual crude oil fractions, such as vacuum gas oil,
straight run (atmospheric) gas oil, coker gas oils and
residues from atmospheric or vacuum distillation of crude
oil. These feedstocks have boiling points preferably
ranging from 220 C to 650 C, more preferably ranging
from 300 C to 600 C.
The quantity of the conventional FCC feedstock
relative to the paraffinic feedstock and triglycerides
may vary depending on feedstock availability and on the
quality of the desired product. In the process according
to the invention the hydrocarbon feedstock may comprise
up to 90 wt% of the conventional FCC feedstock,
preferably up to 70 wt% of the conventional FCC
feedstock, more preferably up to 50 wt% of the
conventional FCC feedstock, even more preferably up to
40 wt% of the conventional FCC feedstock. An advantage of
processing a mixture of conventional FCC feedstock,
paraffinic feedstock and triglycerides is for example
that gasoline with a reduced aromatic content is
produced. Another advantage is that when triglycerides
and a paraffinic feedstock are added to a heavy
conventional FCC feedstock, an increased yield of lower
olefins is obtained. The advantages of the present
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invention become more pronounced at lower content of the
conventional FCC feedstock in the feed.
Thus, by choosing the right balance between the
paraffinic feedstock and triglycerides on the one hand
and the conventional FCC feedstock on the other hand a
gasoline product may be obtained having the desired
properties such as an acceptable octane number, a low
sulphur content and a desired aromatic content. The
properties of the cracked products can be adjusted.
In the process according to the invention, the
cracking catalyst comprises a large pore zeolite. With a
large pore zeolite, a zeolite is meant comprising a
porous, crystalline aluminosilicate structure having a
porous internal cell structure on which the major axis of
the pores are in the range from 0.62 to 0.8 nanometer.
Axis of zeolites are depicted in the 'Atlas of Zeolite
Structure Types', of W.M. Meier, D.H. Olson, and Ch.
Baerlocher, Fourth Revised Edition 1996, Elsevier,
ISBN 0-444-10015-6. Examples of such large pore zeolites
are FAU or faujasite, preferably synthetic faujasite,
like zeolite Y, USY, Rare Earth Y (= REY) or Rare Earth
USY (REUSY). According to the present invention
preferably USY is used as the large pore zeolite.
The cracking catalyst preferably further comprises a
medium pore zeolite if a high yield of propylene is
desired. By a medium pore zeolite that can be used in the
present invention is understood a zeolite comprising a
porous, crystalline aluminosilicate structure having a
porous internal cell structure on which the major axis of
the pores are in the range from 0.45 to 0.62 nanometer.
Examples of such medium pore zeolites are of the MFI
structural type such as ZSM-5, the MTW type, such as
ZSM-12, the TON structural type such as theta one, and
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the FER structural type such is ferrierite. According to
the present invention preferably ZSM-5 is used as the
medium pore zeolite.
The weight ratio of large pore zeolite to medium
pore size zeolite in the cracking catalyst is preferably
in the range from 99:1 to 70:30, more preferably in the
range from 98:2 to 85:15.
The total amount of large pore size zeolite and/or
medium pore zeolite that is present in the cracking
catalysts is preferably in the range from 5 to 40 wt%,
more preferably in the range from 10 to 30 wt%, even more
preferably in the range from 10 to 25 wt% relative to the
total mass of the catalyst.
Next to the large or medium pore size zeolite, the
catalysts may comprise one or more porous, inorganic
refractory metal oxide binder materials or supports
and/or active matrix materials. These binder materials or
supports may or may not contribute to the cracking
reaction. Examples of such binder materials are silica,
alumina, titania, zirconia and magnesium oxide, or
combinations of two or more of them. Also organic binders
may be used.
The temperature at which the hydrocarbon feedstock
and the cracking catalyst are contacted is preferably
between 450 and 650 C. More preferably, the temperature
is above 475 C, even more preferably above 500 C. Good
gasoline yields are seen at temperatures above 600 C.
However, temperatures above 600 C will also give rise to
thermal cracking reactions and the formation of non-
desirable gaseous products like methane and ethane. For
this reason the temperature is preferably below 600 C.
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The process may be performed in various types of
reactors. In order to simplify catalyst regeneration,
preference is given to either a fast fluidised bed
reactor or a riser reactor. If the process is performed
in a riser reactor the preferred contact time is between
1 and 10 seconds and more preferred between 2 and 7
seconds. The catalyst to oil (hydrocarbon feedstock)
ratio is preferably between 2 and 20 kg/kg. It has been
found that good results may be obtained at a catalyst to
oil ratio above 6 kg/kg, since a higher catalyst to oil
ratio results in a higher amount of coke on the catalyst.
Examples
The invention is further illustrated by the
following Examples. The most important properties of
hydrowax are shown in table 1.
Table 1
Feed properties Hydrowax
Density (D70/4) 0.807
Nitrogen coul (ppmw) 2
Viscosity (100 C) (cSt) 6.73
Sulphur (wt%) 0.5
Total aromatics (wt %) 6.07
Carbon (wt%) 85.7
Hydrogen (wt%) 14.3
Initial Boiling Point ( C) 196
Final Boiling Point ( C) 608
Example 1
Catalytic cracking experiments were carried out in a
micro-riser reactor that operates in an isothermal plug-
flow regime. The micro-riser reactor is a once-through
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bench-scale fluid catalytic cracking reactor that
simulates the hydrodynamics of an industrial FCC reactor.
The reactor temperature was set to 525 C. The length of
the reactor was in these experiments 21.2 meters. The
catalyst used was a commercial silica sol based FCC
equilibrium catalyst (e-cat), containing 11 wt% USY
zeolite crystals. Before each experiment, the catalyst
was regenerated in a fluidised bed reactor, where coke
was combusted in air at 600 C for three hours. The
catalyst was fed to the reactor by means of a catalyst
feeder. Nitrogen was used to facilitate the catalyst
flow. The oil feed was fed through a pulse-free syringe
pump to the pre-heated oven where it was partially
evaporated. In the last part before the injection point
the oil was completely evaporated and adopted the
reaction temperature, as well as the catalyst. The feed
was injected perpendicularly into the catalyst stream.
The feed consisted of pure hydrowax, or hydrowax blended
with 20 wt% or 40 wt% of crude degummed rapeseed oil.
Sample collection started when the system had
reached steady-state operation. Separation of the
catalyst and gaseous product took place by means of a
cyclone. During the steady-state operation the catalyst
was stored under reaction conditions and was afterwards
stripped with nitrogen. The effluent gas was led through
three condensers in series operating at 25, -60, and
-60 C, respectively. Any uncondensed products were
captured in a gas bag. The C1-C4 hydrocarbon components
in the gas bag were determined by means of gas
chromatography. The entrained C5 and C6 hydrocarbons were
detected as two separate lumps by this analysis method
and added to the gasoline fraction. The liquid product
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was analysed by simulated distillation. This gave the
amounts of product in terms of lumps of boiling ranges:
gasoline (C5-215 C), Light Cycle Oil (LCO, 215-325 C),
and Heavy Cycle Oil and Slurry Oil (HCO + SO, 325 + C).
The coke on the catalyst was determined with a LECO C-400
carbon analyser. The results are presented in table 2.
In comparison with 100% hydrowax, addition of
rapeseed oil (RSO) results in increasing amounts of coke
and LCO. Furthermore, a clear increase in the calculated
RON is observed for the catalytically cracked blend of
hydrowax with 40 wt% rapeseed oil as compared to 100%
hydrowax.
Table 2
Experiment 100% HWX + HWX +
HWX 20wt% 40wt%
RSO RSO
CTO (g cat/g oil) 3.8 4.5 4.6
Contact time (s) 4.4 4.4 4.5
Yields (wt%)
Coke 2.0 2.1 3.3
Gas 11.7 9.2 7.9
Gasoline (C5 - 215 C) 60.2 59.4 55.7
LCO (215 - 325 C) 15.7 18.1 20.7
HCO + SO (325+) 11 12
RON 85.7 89.2
Example 2
In a small-scale fluidised bed reactor the catalytic
cracking blends of hydrowax, with rapeseed oil and palm
oil (at 5, 10, 25 wt%) using a equilibrium catalysts,
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e-cat2, was performed. The experiments were done in a
reactor in which 10 grams of the commercial e-catalyst
was constantly fluidised with nitrogen. Dependent on the
cat/oil ratio an amount of 1.25 to 3.33 grams of oil was
injected in the reactor. During stripping the liquid
products were collected in glass vessels (receivers) in a
bath at a temperature of -15 C. The gas produced was
analysed online with a gas chromatograph. After stripping
for 660 seconds, the amount of coke formed on the
catalyst was determined by burning the coke from the
catalyst in a regeneration step. During 40 minutes the
temperature of the reactor was at 650 C in an air
environment. The coke was converted to CO2 and measured
online. After regeneration the reactor was cooled to the
reaction temperature and a new injection was started. The
results are presented in tables 3 and 4.
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Table 3. Product
distribution using e-cat2 at 500 C
(at Cat/Oil ratio 6.3) of hydrowax (HWX) and mixtures of
hydrowax and rapeseed oil (RSO).
HWX with HWX with HWX with
100% HWX 5% RSO 10% RSO 25% RSO
CTO (g cat/g oil) 6.3 6.3 6.3 6.3
Conversion 87.1 87.9 86.5 82.4
CO 0.0 0.0 0.0 0.1
CO2 0.0 0.1 0.1 0.2
H2O n/a 0.6 1.2 2.8
Coke 2.0 2.2 2.5 3.1
Drygas 1.0 1.0 1.1 1.2
LPG 24.2 25.2 24.7 21.6
Gasoline (C5 - 215 C) 59.9 59.5 58.2 56.5
LCO (215 - 325 C) 7.3 7.3 8.0 10.0
HCO + SO (325+) 5.6 4.2 4.2 4.5
GC-RON 86.3 86.7 87.3 87.7
GC-MON 77.4 77.8 78.4 78.5
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Table 4. Product distribution using e-cat2 at 500 C
(at Cat/Oil ratio 6) of hydrowax (HWX) and mixtures of
hydrowax and palm oil.
HWX + 5% HWX + 10 % HWX + 25%
palm oil palm oil palm oil
CTO (g cat/g oil) 5.8 5.7 5.5
Conversion 88.2 89.9 83.
CO 0.1 0.1 0.3
CO2 0.1 0.1 0.3
H2O 0.6 1.1 2.7
Coke 1.8 2.0 2.4
Drygas 0.9 1.0 1.0
LPG 23.6 23.4 21.3
Gasoline (C5 - 215 C) 61.8 63.5 58.9
LCO (215 - 325 C) 7.5 6.3 8.4
HCO + SO (325+) 3.7 2.5 4.6
GC-RON 88.0 87.8 88.3
GC-MON 79.4 79.4 79.7
