Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE OF THE INVENTION
Method and apparatus for selective deep catalytic cracking of hydrocarbons
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and related apparatus for catalytic
steam cracking
of hydrocarbons.
2. Description of the prior art
Steam cracking of hydrocarbons is one of the core processes in the
petrochemical industry.
Current world production of stream cracking products is estimated to reach 100
million
metric tons/year of ethylene and propylene.
Basically, steam cracking comprises a step in which the hydrocarbon mixture to
be
transformed is mixed with steam and submitted to elevated temperatures in a
tubular
reactor, usually in the presence of one or more catalysts. The reaction
temperature usually
ranges from 700 to 900°C according to the type of feedstock treated
(the longer the
hydrocarbon molecular structure, the lower the required temperature for
cracking) while
the residence time ranges from a few seconds to a fraction of second. The
different resulting
products, gaseous or liquid are then collected and separated. Thus, product
distribution
depends on the nature of the initial hydrocarbon mixture and the reaction
conditions.
During steam cracking, light paraffins (ethane, propane and butane, obtained
mainly by
extraction from various natural gas sources) naphthas and other heavier
petroleum cuts are
broken down (cracked) into mainly:
i) light olefins: primarily ethylene and propylene,
ii) secondarily, depending on the feedstock employed, a C4 cut rich in
butadienes and
a CS+ cut with a high content of aromatics, particularly benzene,
iii) and finally hydrogen.
Since enormous quantities of hydrocarbons are steam cracked throughout the
world, even
small yield or product selectivity improvements may lead to substantial
commercial
advantages.
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Common feedstocks in steam cracking operations are ethane and LPG in the
U.S.A. and
naphthas or gas oils in Europe. However, in recent years, the situation has
changed
dramatical 1y with the U.S.A. moving towards the use of heavier hydrocarbon
feedstocks.
Market demands are currently focussed on propylene and on some longer
isolefins such as
isobutene and isopentenes. The latter enter in the synthesis of alkyl ethers
used as octane
boosters for transportation fuels. However, currently available steam cracking
technology is
not sufficiently flexible to respond to these or other market trends.
More than ten years ago, the present inventor had developed a method for
upgrading the
products of propane steam cracking, see US Patent 4,732,881. This process
comprised
adding a small catalytic reactor to a conventional propane steam cracker. The
catalysts
used were based on hybrid zeolite catalysts, namely ZSM5 zeolite modified with
AI and Cr.
Significant increases in the yield of ethylene and aromatics were obtained.
However, the prior art has so far partially failed to develop a method
providing
simultaneously:
(a) enhanced production of commercially valuable products light olefins
(ethylene and
propylene) and aromatics, and
(b) higher production flexibility and selectivity for olefins, i.e. a wider
range of variation
for the ethylene/propylene ratio.
It is therefore an object of the present invention to provide a novel method
meeting these
req a i rements.
Other objects and further scope of applicability of the present invention will
become
apparent from the detailed description given hereinafter. It should be
understood, however,
that this detailed description, while indicating preferred embodiments of the
invention, is
given by way of illustration only, since various changes and modifications
within the spirit
and scope of the invention will become apparent to those skilled in the art.
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SUMMARY OF THE INVENTION
In general terms, the present invention provides a method and apparatus for
selective deep
catalytic cracking of petroleum naphthas or other hydrocarbon feedstocks.
Thus, the invention provides an apparatus for catalytic steam cracking of
hydrocarbons
comprising a reactor having first and second main reaction zones. The first
reaction zone
being heated to a first temperature between 500 to 900°C, preferably
between 660 and
720°C, comprises a low-surface area, porous and thermally resistant
catalytic material. The
second reaction zone being heated to a second temperature, being the same or
different
from the first temperature and also being between 500 to 900°C,
preferably between 660
and 720°C, comprises a zeolite-based catalyst.
Also provided is a method for selective steam cracking of hydrocarbons
comprising the
steps of:
a) feeding steam and the hydrocarbons to a first zone of a steam cracking
reactor, said
first zone comprising a low-surface area, porous and thermally resistant
catalytic material
and being maintained at a given first temperature between 500 to 900°C,
preferably
between 660 and 720°C, to obtain a partially cracked hydrocarbon output
stream
comprising selectivity modifiers capable of modifying the steam cracking
reactions
occurring downstream from said first zone;
b) directing the output stream from said first zone to a second zone of said
steam
cracking reactor, said second zone being heated to a second temperature, being
the same
or different from the first temperature and also being between 500 to
900°C, preferably
between 660 and 720°C, comprising a zeolite-based catalyst responsive
to said selectivity
modifiers present in the output stream from said first zone, to obtain a
cracked hydrocarbon
output stream rich in light olefins; and
c) recovering said hydrocarbon output stream rich in light olefins.
In operation, the feed comprising the hydrocarbons to be cracked and steam, in
well-
defined proportions, is first sent into a pre-catalytic zone (Zone I) of a
steam cracking
reactor, preferably a tubular reactor. In a preferred embodiment, Zone I,
contains beads of
some catalytically mildly active porous material such as quartz or quartz
doped with Cr-AI,
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is set at a temperature T,. The gas stream flows then through a catalyst bed,
called Zone II,
where the catalytic reaction takes place. Zone II is set at temperature TZ and
contains a
zeolite type catalyst, preferably ZSM5 zeolite or, most preferably, a hybrid
zeolite catalyst.
It is shown that by moderately increasing the temperature T, of Zone I, it is
possible to have
an increased total conversion of n-hexane used as model molecule for petroleum
naphthas
and an increased yield of light olefins (and aromatics), when compared to the
parent ZSM-5
zeolite catalyst. The ethylene/propylene product ratio is closely dependent of
the
temperature T, if all other reaction parameters are kept constant and can be
widely varied
(from 1.0 to 2.0 for instance) for a limited range of variation of temperature
T,.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1: Flowchart schematically illustrating the method and apparatus of the
present
invention;
Figure 2: Graph showing the selectivity in light olefin output versus
temperature in pre
catalytic Zone I of the steam cracking reactor apparatus used in the present
invention (using
n-hexane as a model hydrocarbon molecule).
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Before describing the present invention in detail, it is to be understood that
the invention is
not limited in its application to the details of method steps and conditions
described herein.
The invention is capable of other embodiments and of being practised in
various ways. It is
also to be understood that the phraseology or terminology used herein is for
the purpose of
description and not limitation.
Referring now to Figure 1, a preferred embodiment of the method of the present
invention
will now be described. The method uses a tubular steam cracking reaction
comprising two
reaction zones: a pre-catalytic zone I containing a mildly active but robust
catalyst and a
catalytic zone II. Zone II contains a ZSM5 zeolite based catalyst, preferably
of the hybrid
configuration.
Still referring to Figure 1, the feed of hydrocarbons to be cracked and steam
are introduced
in well-defined proportions into Zone I, a pre-catalytic zone located at the
entrance of a
tubular reactor (not shown). Zone I will advantageously contain a pre-catalyst
composed of
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beads of some robust and mildly catalytic and porous material such as quartz,
or quartz
doped with Cr-AI. Zone I is set at a given temperature T, preferably between
500 to 900°C.
Variations of the temperature of Zone I and the textural properties and/or the
surface
composition of the pre-catalyst are surprisingly used to achieve: (a) an
increase the overall
5 conversion achieved by the method of the present invention, (b) an increase
the propensity
of the method towards the production of light olefins such as ethylene and
propylene and
(c) to vary the end product distribution, namely the ethylene/propylene ratio.
Exiting Zone I, the gas stream is partially cracked and surprisingly contains
selectivity
modifiers, i.e. compounds which will affect the end product distribution,
namely the
ethylene/propylene ratio. Therefore, the entire method will be more selective
than that
using only a catalyst bed as a reaction zone. In addition, under almost all
operating
conditions, the final conversion rate is increased owing to the partial
conversion of the feed
through Zone I.
The gas and/or liquid stream exiting from Zone I then proceeds deeper into the
tubular
reactor by flowing through a catalyst bed, called zone II, where the main
catalytic steam
cracking reaction takes place. Zone II which is set at temperature T2,
contains a catalyst
which is based on the ZSMS zeolite or, preferably, a hybrid zeolite catalyst.
Steam cracking
in the presence of a zeolite type catalyst is commonly referred to as deep
cracking.
As shown in Figure 2, it is surprising to note that by increasing the
temperature T, of Zone I,
it is possible to have an increased total conversion of n-hexane (used as
model molecule for
petroleum naphthas or heavier hydrocarbon feedstocks) and an increased yield
of light
olefins (and aromatics). The ethylene/propylene product ratio (wt/wt) is
closely dependent
of the temperature T, if all other reaction parameters are kept constant and
can be widely
varied from 1.0 to 2.0 (for instance) for a quite limited range of variation
of temperature T, .
Thus, the method of the present invention, using a pre-catalyst system Zone I
and a catalyst
system Zone II, each having adjustable temperatures and properties, achieves
dramatically
higher conversion to commercial valuable products and greater flexibility and
selectivity.
The method of the present invention operates as a catalytic lever (hereinafter
referred to as
"CatLever" configuration) in which the operating conditions in Zone I can
affect at will the
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composition of the products exiting Zone II. Hence, with such a novel method,
the cracking
activity of the ZSM5 zeolite based catalysts on various petroleum feeds (the
so-called deep
catalytic cracking) is dramatically improved and a considerable production
flexibility is
obtained. The novel method can be described as selective deep catalytic
cracking.
The term "ZSM5 zeolite materials" is to be understood to encompass any such
materials
known to those skilled in the art. Without restricting the foregoing, are
envisaged any
zeolite catalyst materials selected from the group of: microporous
aluminosilicates,
microporous silicoaluminophosphates and microporous aluminophosphates having
the
zeolite structure, and also mesoporous silica-containing materials. Also
envisaged are
desilicated and desilicated/silica-reinserted zeolite materials as described
in R. Le Van Mao,
S.T. Le, D. Ohayon, F. Caillibot, L. Gelebart and G. Denes, Zeolites 19 (4)
(1997), 270-278
and R. Le Van Mao and D. Ohayon, Proceedings 12th International Zeolite
Conference,
Baltimore
EXAMPLE
An example of the method of the present invention will now be described in
relation to the
catalytic steam cracking of n-hexane (used as a model molecule for petroleum
naphthas).
Although the present invention has been experimentally demonstrated based on n-
hexane,
this is an excellent model molecule for predicting the behaviour of other
hydrocarbons, in
particular longer chain hydrocarbons and their mixtures such as the ones found
in
petroleum naphthas, since the catalytic behaviors of these feeds are
analogous.
N-hexane when sent together with some steam through Zone I, undergoes partial
steam-
cracking and dehydrogenation. The products of this conversion include olefins
and diolefins
which are known to increase - by hydrogen transfer or olefin dissociation -
the selectivity
towards light olefins during the reaction over zeolite based catalysts.
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The probable light olefin selectivity modifiers generated in Zone I and
affecting the activity
and selectivity of the zeolite catalysts of Zone II are presently identified
as such:
Cracking: large paraffin molecules -~ to small paraffin molecules + HZ
Aromatization: olefins, diolefins and naphtene -~ aromatics + nH2
Hydrogen transfer: naphtene + olefins -~ aromatics + paraffins
diolefins + paraffins -~ olefins + olefins
Olefin dissociation: large olefins H 2 small olefins + Hz
Zone I contains a pre-catalyst porous material, preferably quartz having large
pores and low
surface area to avoid excessive coking. The surface of the porous material is
used to
enhance the contact effect and the pore system is used to lengthen the
diffusion path of the
feed in order to increase the steam-cracking conversion, thus, allowing the
reaction in Zone
I to be carried out at relatively low temperatures.
It is worth noting than Zone I is different from the feed preheating zone
which is used in the
prior art in many chemical or catalytic processes. Indeed, i) the role of Zone
I is well-
defined : to produce some selectivity modifiers and to help increase the final
total
conversion obtained at the outlet of the catalytic reactor (Zone II), and ii)
the temperature of
Zone I is normally (but not always) higher (at least in the case of the
present invention) than
the temperature of the catalyst bed.
Zone II contains a ZSM5 zeolite based catalyst or, preferably, a hybrid
zeolite catalyst. The
latter has been prepared by combining the ZSMS zeolite with a Cr-AI containing
cocatalyst
in accordance with the method of formation of a pore continuum as described in
US Patent
4,732,881.
The beneficial effects of the method based on two zones of conversion (with
the parent
ZSMS zeolite packed in Zone II) when compared to the conventional one-zone
catalytic
reaction (parent ZSM5 zeolite) are as follows
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i) generally higher total conversion and always higher selectivity to light
olefins (and
aromatics) as evidenced by the yield increases in light olefins and light
olefins +
aromatics of 24 % and 29 %, respectively;
ii) a wide variation of the ethylene/propylene product ratio, depending on the
temperature T1 of Zone I.
In particular, with the more active (Cr-AI) hybrid catalyst (tested with the
two zone
conversion set up) when compared to the parent ZSM5 zeolite alone (tested in a
conventional tubular reactor), the yield increases in light olefins and light
olefins +
aromatics reach the values of 35% and 41 %, respectively. This is shown in
Figure 3. In
addition, the ethylene/propylene product ratio varies from 1.0 (equal to the
value for the
parent ZSM5 zeolite, obtained using the conventional reactor) to 2Ø This is
also shown in
Figure 3.
In the following, are described in detail
i) the preparation of the porous precatalyst (Zone I) and of the zeolite
catalysts
(Zone II);
ii) the experimental set-up;
iii) the testing procedure, and
iv) the catalytic conversion results along with discussion.
PREPARATION OF THE POROUS CONTACT FILLER AND CATALYSTS
Porous precatalyst packed in the mildly catalytic zone (Zone 1):
Quartz (silicon oxide, fused from Aldrich, granules) was used as porous
precatalyst for the
conversion zone I without any further treatment. It has the following physical
properties
mesh size = 4 - 16 mesh (particle size = ca. 350 microns) ; surface area = 0.3
m2/g,
porosity = all mesopores and macropores.
Parent ZSMS zeolite catalyst (Zone II):
This catalyst (Zeocat PZ-2/50, H-form, 1/16 " extrudates) was purchased from
Chemie
Uetikon AG (Switzerland). It contains ca. 20 wt % of an unknown binder. Prior
to the
catalytic testing, it was activated in air at 700°C overnight. Its main
physical properties are:
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surface area = 389 m2/g, microporosity = 177 m2 /g and Si/AI = ca. 50. This
reference
catalyst is referred to as HZSMS.
Hybrid catalyst containing the ZSMS zeolite and the Cr-A1 cocatalyst:
i) The ZSM-5 zeolite used was the Zeocat PZ-2/50, H-form, powder, purchased
from
Chemie Uetikon AG (Switzerland). It was activated in air overnight at
700°C. Its main
physical properties are : surface area = 483 m2/g, microporosity = 277 m2/g,
and Si/AI =
ca. 50. This material is referred to as HZ.
The cocatalyst was prepared in the following way : the solid material (20 g)
obtained
by drying the Colloidal silica Ludox AS-40 from Dupont and then activated in
air at
700° overnight, was impregnated with an aqueous solution of Cr and AI
obtained by
dissolving 10.0 g of Cr(N03)3. 9 H20 and 9.0 g of sodium aluminate, all from
Aldrich, in 30 ml of water. After 10 min left at room temperature, the
solution was
slowly (and under stirring) evaporated to dryness on a hot plate. The
resulting solid
material was dried at 120°C overnight and finally activated in air at
700°C for
5 hours. This cocatalyst is referred to as Cocat.
iii) The final hybrid catalyst, referred to as Hyb. Cat., was obtained by
extrusion with
bentonite as follows : first, HZ and Cocat (70 wt % and 15 wt %, respectively)
were
carefully mixed (a hour stirring in dry conditions); then, bentonite clay used
as
binder (15 wt %) was added to the previous solid mixture and the al) was
stirred for
another hour. Water was then added dropwise until a malleable paste was
obtained.
The resulting catalyst extrudates were dried at 120°C overnight and
finally activated
in air at 700°C for 5 hours.
EXPERIMENTAL SET UP
Experiments were performed within a Lindberg triple zone series tubular
furnace coupled to
a Lindberg type 818 temperature control unit capable of regulating ,
individually, the
temperature of each zone. The reactor vessel consisted of a quartz tube 95 cm
in length and
2 cm in diameter. As mentioned in the previous section, Zone I (reactor inlet,
ca. 30 cm in
length) was packed with quartz granules. Zone II (reactor outlet, ca. 30 cm in
length) was
packed with catalyst extrudates. The zone which is in between Zone I and Zone
II, was
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used as cooling zone because the temperature T1 of Zone I was always set
higher than the
temperature T2 of the catalytic zone (Zone II).
TESTING PROCEDURE
5 Liquids, namely n-hexane and water, were injected into a vaporizer using two
infusion
pumps. In the vaporizer, nitrogen used as carrier gas, was mixed with n-hexane
vapors and
steam. The gaseous stream was then sent into the tubular reactor, first in
Zone I and then in
Zone II.
10 The testing conditions are as follows:
weight of catalyst = 5.0 g ; W.H.S.V. (weight hourly space velocity = g of
reactant, i.e. n-hexane injected per hour per g of catalyst) = 1.21 h-1 ;
water/n-hexane molar ratio = ca. 1.0 ;
Nitrogen flow rate = 10 ml/min, duration of a run = 3.5 h.
In this series of runs, the temperature TZ of Zone II was kept constant at
650°C while the
temperature T, of Zone I was varied from 660°C to 720°C.
Product liquid and gaseous fractions were collected separately using a system
of
condensers. The gas phase components were analysed using a Shimadzu Mini-3 FID
gas
chromatograph equipped with a 3 meter-long Haysep micropacked column while the
liquid
phase analysis was carried out using a Hewlett-Packard 8790 FID gas
chromatograph
equipped with a 50m PONA capillary column.
RESU LTS
The catalytic results are herein reported as : total n-hexane conversion and
product yields.
The total n-hexane conversion (mol % or wt %) is expressed as follows:
Ct = 100 x (moles of converted n-hexane / moles of n-hexane fed)
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The selectivity (to product i) is expressed in C atom % (or approximately in
wt %) as
follows:
Si = 100 x (number of carbon atoms of product i / number of carbon atoms
of converted products)
The yield in product i is expressed (in wt %) as follows:
Yi = 1/100 x Ct x Si
Steam-cracking conversion of n-hexane in the non-catalytic zone, i.e. Zone I
(Table 1):
Runs were performed with the zone I set at various temperature T1 (660°
- 720°C) which
were slightly higher than the temperature T2 of the catalytic bed
(650°C). In this series of
runs, the catalyst bed was empty and Zone II was not heated. The results are
reported in
Table 1.
The conversion was low at 660°C but it increased quite rapidly with the
increasing
temperature.
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12
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13
The yields in light olefins increased significantly while the yield in liquid
hydrocarbons
remained quite low. Within the C4 unsaturates, the yields in butadienes was
quite high and
increased with the temperature T,.
Performance of the catalysts tested in various conditions (Table 2):
i) All the catalysts tested under the Catlever conditions (both Zones heated)
showed a
n-hexane conversion almost equal or higher than that obtained with the HZSM5
zeolite
tested under conventional conditions (i.e. with the catalyst bed only, which
was heated at
650°C).
ii) It is interesting that heating Zone I at a temperature slightly higher
than the
temperature of the catalyst bed, increased significantly the yields in light
olefins and light
olefins + aromatics.
iii) Increasing the temperature T1 increased dramatically the yield in
ethylene while the
yield in propylene did not significantly change. As a consequence, the
ethylene/propylene
product ratio increased steadily. This ratio varied in a quite comparable way
as the ratio
reported for the conversion at Zone I alone (Table 1).
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14
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iv) The yield in butadienes of the zeolite based catalysts tested in the
Catlever
conditions was significantly decreased when compared with that of the pre-
catalyst tested
alone (Tables 1 and 2). This shows that the diolefins (butadienes) were used
by the zeolite
catalysts to significantly increase yields of ethylene and aromatics.
5 v) The Catlever configuration induced a higher production of liquid
hydrocarbons
which were richer in aromatics, when compared to the HZSM5 in normal
conditions of
testing and also to the pre-catalytic conversion alone (Table 1).
vi) The use of the hybrid catalyst increased further the yields in light
olefins and in light
olefins + aromatics.
DISCUSSION
The above results as ascribed to the combined effect of the two conversion
zones (the so
called Catlever effect. The selectivity enhancers created in Zone I,
advantageously improve
the performance of the catalyst in Zone II. In particular, the decrease of
butadienes seen in
the product stream was due to the hydrogen transfer within the zeolite network
(Table 1
versus Table 2) : this is indicative of the role of these substances in the
modification of the
activity and selectivity of the zeolite based catalysts. In addition, the data
obtained with the
hybrid catalyst (Table 2) when compared to the HZSM5 (normal conditions of
testing, i.e.
with no precatalytic zone, see Table 2), the increases in light olefins and
light olefins +
aromatics, are 35 °/o and 41 %, respectively. These increases are even
more important than
those obtained with the HZSM5 zeolite tested in the Catlever conditions (Table
2). This is
due to the dehydrogenating effect of the cocatalyst of the hybrid catalyst,
which results in
the formation of another activity and selectivity modifier.
It is to be noted that the apparatus of the present invention is not limited
to a tubular reactor
with two reaction zones. In fact, the catalytic zone (Zone II) may have
another configuration
such as that of a fluidized bed.
