U.S. patent number 3,928,174 [Application Number 05/538,221] was granted by the patent office on 1975-12-23 for combination process for producing lpg and aromatic rich material from naphtha.
This patent grant is currently assigned to Mobil Oil Corporation. Invention is credited to John C. Bonacci, Henry P. Ireland, Thomas R. Stein.
United States Patent |
3,928,174 |
Bonacci , et al. |
December 23, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Combination process for producing LPG and aromatic rich material
from naphtha
Abstract
Processing of reformate product of catalytic reforming to
produce significant yields of LPG and BTX as well as aromatic
enriched reformate with a ZSM-5 type crystalline zeolite is
described.
Inventors: |
Bonacci; John C. (Cherry Hill,
NJ), Ireland; Henry P. (Woodbury, NJ), Stein; Thomas
R. (Cherry Hill, NJ) |
Assignee: |
Mobil Oil Corporation (New
York, NY)
|
Family
ID: |
24146002 |
Appl.
No.: |
05/538,221 |
Filed: |
January 2, 1975 |
Current U.S.
Class: |
208/80; 208/79;
585/475; 585/752; 208/111.35; 208/66; 585/300; 585/488 |
Current CPC
Class: |
C10G
59/02 (20130101) |
Current International
Class: |
C10G
59/00 (20060101); C10G 59/02 (20060101); C10G
037/06 (); B01J 029/28 () |
Field of
Search: |
;208/80,66,79 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gantz; Delbert E.
Assistant Examiner: Schmitkons; G. E.
Attorney, Agent or Firm: Huggett; C. A. Gilman; M. G.
Claims
We claim:
1. A method for redistributing the paraffin-aromatic components of
a reformate product of catalytic reforming comprising C.sub.5 and
higher boiling hydrocarbons which comprises,
separating C.sub.5 and lower boiling components of catalytic
reforming from C.sub.6 and higher boiling materials in a first
separation zone,
separating the C.sub.6 and higher boiling material in a second
separation zone into a first fraction comprising some C.sub.7
components and lower boiling paraffin and aromatic components and a
second fraction comprising C.sub.7 and higher boiling paraffins and
aromatic components,
passing said first fraction in contact with a first crystalline
zeolite catalyst having properties for cracking paraffins to form
LPG materials and redistribute the ratio between benzene and
toluene components in the feed, separating product material of said
first crystalline zeolite conversion operation into gaseous
material comprising C.sub.2 and lower boiling material from
material comprising C.sub.3 and higher boiling components, passing
the C.sub.3 and higher boiling material to said first separation
zone,
passing a portion of said second fraction comprising C.sub.7 and
higher boiling paraffins and aromatics in contact with a separate
second mass of crystalline zeolite catalyst having properties for
cracking said paraffins and disproportionating aromatic to form a
mixture of benzene, toluene and xylene; separating the product of
said second zeolite catalyst conversion operation to recover a
hydrogen rich stream, a stream rich in benzene, toluene and xylene
aromatics and an intermediate product stream lower boiling than
said aromatic rich stream, and recycling said intermediate product
stream to said first separation zone.
2. The method of claim 1 wherein hydrogen rich gas is passed to
each zeolite catalyst conversion zone.
3. The method of claim 1 wherein a C.sub.6 .sup.+ reformate
fraction enriched in toluene is withdrawn from said first
separation zone.
4. The method of claim 1 wherein a toluene-xylene rich fraction is
withdrawn from said second separation zone.
5. The method of claim 1 wherein a hydrogen rich gas is separated
from the product of each zeolite catalyst conversion operation and
is recycled to the catalyst operation.
6. The method of claim 1 wherein LPG product material of the
process combination is concentrated in the overhead product
withdrawn from the first separation zone.
7. The method of claim 1 wherein the temperatures relied upon in
the first zeolite catalyst conversion zone are within the range of
500.degree.F. to 800.degree.F.
8. The method of claim 1 wherein the temperatures and operating
conditions relied upon in the other zeolite catalyst conversion
zone are more severe than those used in the first catalyst
conversion operation.
9. The method of claim 1 wherein the formation of a benzene,
toluene, xylene rich mixture is particularly promoted by the
operating conditions relied upon in the second zeolite catalyst
operation.
10. The method of claim 1 wherein the zeolite catalyst is
preferably a ZSM-5 type crystalline zeolite.
11. A process of upgrading petroleum reformate to an aromatics
concentrate comprising benzene, toluene and xylene (BTX),
liquifiable petroleum gas (LPG) comprising C.sub.3 and C.sub.4
components, and high octane gasoline comprising:
splitting at least a C.sub.6 .sup.+ portion of said reformate into
C.sub.7 .sup.+ and C.sub.7 .sup.- fractions having minimum and
maximum boiling points respectively of about 200.degree. to
240.degree.F;
contacting said C.sub.7 .sup.+ fraction with a crystalline
aluminosilicate zeolite having a silica to alumina ratio of at
least 12 and a constraint index of 1 to 12 in a first conversion
zone;
converting said C.sub.7 .sup.+ fraction in said first conversion
zone at at least about 500.degree.F. to a product comprising
C.sub.4 .sup.- gas and a substantially aromatic liquid
concentrate;
contacting said C.sub.7 .sup.- fraction with a crystalline
aluminosilicate zeolite, having the ability to crack substantially
only paraffinic components of said C.sub.7 .sup.- fraction to
C.sub.4 .sup.- gas, in a second conversion zone;
converting said C.sub.7 .sup.- fraction in said second conversion
zone at at least about 700.degree.F. to a product comprising
C.sub.4 .sup.- gas and an aromatics enriched liquid product;
and
recovering LPG from said C.sub.4 .sup.- gas, said aromatics
concentrate, and high octane gasoline comprising said aromatics
enriched liquid.
12. A process as claimed in claim 11 including admixing a portion
of said aromatics concentrate with said reformate prior to said
splitting.
13. A process as claimed in claim 11 including deleting a C.sub.5
.sup.- fraction from said reformate and splitting the remaining
C.sub.6 .sup.+ fraction.
14. A process as claimed in claim 11 including separating C.sub.3
and C.sub.4 components from said C.sub.4 .sup.- gas.
15. A process as claimed in claim 11 including splitting a portion
of the C.sub.6 .sup.+ portion of said reformate.
16. A process as claimed in claim 11, including admixing said
aromatics enriched liquid with said reformate prior to said
splitting.
17. A process as claimed in claim 11 including admixing said
aromatics enriched liquid, a portion of said aromatics concentrate
and said reformate; and splitting said admixture into said C.sub.7
.sup.+ and C.sub.7 .sup.- fractions.
18. A process as claimed in claim 11 including utilizing the same
zeolite respectively in both of said conversions.
19. A process as claimed in claim 18 wherein said zeolite is a
ZSM-5.
20. A process as claimed in claim 11 wherein said C.sub.7 .sup.-
conversion is carried out at about 500.degree. to 800.degree.F.,
about 200 to 1000 psig, about 1 to 4 LHSV and about 2 to 10 to 1
hydrogen to hydrocarbon ratio; and wherein said C.sub.7 .sup.+
conversion is carried out at about 750.degree. to 900.degree.F.,
about 200 to 1000 psig, about 0.5 to 2 LHSV and about 3 to 20 to 1
hydrogen to hydrocarbon ratio.
Description
BACKGROUND OF THE INVENTION
Reforming of hydrocarbons is a widely used process in petroleum
technology for upgrading hydrocarbon fractions such as naphthas,
gasolines and kerosines to improve the antiknock characteristics
thereof. Hydrocarbon fractions suitable for upgrading by reforming
are composed of normal and branched paraffins, naphthenic
hydrocarbons and even some aromatic hydrocarbons. During reforming
a multitude of reactions take place including dehydrogenation,
isomerization, dehydrocyclization, hydrocracking, and combinations
thereof to yield a product of increased aromatics content and
branched chain hydrocarbons. Thus in reforming it is desired to
dehydrogenate the naphthenic hydrocarbons to produce aromatics,
cyclize straight chain paraffins to form naphthenes, to convert
C.sub.5 ring compounds to C.sub.6 ring compounds which are
dehydrogenated to form aromatics, isomerize normal and branched
paraffin hydrocarbons to yield higher octane branched chain
hydrocarbons and effect a controlled hydrocracking of hydrocarbon
constituents which are of undesired octane characteristics.
Normal and slightly branched paraffin hydrocarbons found in the
above hydrocarbon fractions are generally of low octane rating.
Highly branched-chain paraffin hydrocarbons, on the other hand, are
characteristic of higher octane ratings. Therefore, one object of
reforming is to effect isomerization of the normal and slightly
branched chain paraffins to higher octane products by any one of
the aforementioned reactions. The production of aromatics during
reforming is accomplished by one or more of the above identified
reactions leading to the production of naphthenes which are then
dehydrogenated to aromatics such as benzene, toluene and xylene.
One method for producing aromatics involves the isomerization of
alkyl cyclopentanes to form cyclohexanes which thereafter are
dehydrogenated to aromatics.
Ever since the concept of catalytic reforming was developed and
commercially adopted, the refiner has been concerned with improving
upon the selectivity of the product obtained and thus has strived
to reduce yields of carbon and normally gaseous product materials
since such materials represent a loss in desired liquid product.
Thus small improvement in product selectivity has been gained with
difficulty since there is a limit to the quantity of normally
liquid constituents of desired octane rating that can be produced
from a given charge. Consequently increases in product selectivity
are viewed with considerable interest particularly if the
selectivity increases can be associated with products of economic
interest to the refiner. It has been found that the selectivity of
a particular product slate or composition can be considerably
enhanced by following the concepts and sequence of steps comprising
this invention.
THE INVENTION
This invention relates to a method and combination of processing
steps for effecting a selective conversion and a rearrangement of
petroleum hydrocarbon constituents to form aromatic enriched
products and improve yields of LPG materials. In one aspect the
present invention is concerned with one or more methods for
selectively conducting chemical reactions with an arrangement of
catalytic compositions possessing selective reaction properties
with respect to different hydrocarbon components existing in the
naphtha boiling range material. In yet another aspect the present
invention relates to effecting a selective catalytic conversion of
hydrocarbon components comprising, normal and isoparaffin
hydrocarbon components in a sequence of hydrogenating conversion
steps maintained under operating conditions selected to obtain
products rich in aromatics and LPG material. More specifically, the
invention is concerned with an arrangement and sequence of
catalytic reactions designed to manipulate the reaction of
hydrocracking, dehydrogenation, isomerization and
dehydrocyclization to improve upon the yields of LPG products and
aromatic components readily separated by distillation.
The present invention is concerned with contacting a relatively
wide boiling range naphtha hydrocarbon material boiling in the
range of C.sub.5 hydrocarbons up to about 380.degree. or
400.degree.F. under selective reforming conditions in the presence
of a platinum type reforming catalyst. In this reforming operation
the conditions employed lead to the production of relatively low
octane branched and normal paraffin compounds which are available
for conversion and production of additional LPG products. The
reforming catalyst may be relied upon to hydrocrack these low
octane compounds formed during the reforming operation but it is
preferred that the reformate product comprising any C.sub.5 and
higher boiling normal paraffin constituents be subjected to a
selective zeolite hydrocracking operation designed to convert low
boiling normal paraffins to LPG product. Thus the present invention
includes the selective cracking of low and high boiling normal
paraffin components comprising the naphtha boiling material
processed in the combination of catalytic contact steps comprising
this invention. It includes reforming a naphtha charge under
reforming conditions providing normal and branched chain
hydrocarbons component along with reactions of dehydrogenation and
dehydrocyclization comprising catalytic reforming. This
relationship between normal and branched chain hydrocarbon
components provides normal paraffin constituents suitable for
conversion to LPG material.
A platinum type reforming catalyst including bimetallic and
non-bimetallic reforming catalysts and those comprising platinum or
palladium in combination with another Group VIII metal component
such as rhenium, iridium, ruthenium and osmium promoted with a
halogen will indiscriminately effect hydrocracking under elevated
temperature reforming conditions of the normal and branched
paraffin components comprising the hydrocarbon material in the
reforming operation. Thus employing a platinum type reforming
catalyst under controlled isomerizing and hydrocracking severity
conditions may be relied upon to produce LPG type products or
products more easily converted to LPG products. That is,
hydrocracking reactions performed with platinum reforming catalysts
are more usually rate controlled reactions wherein, for example, a
normal C.sub.8 hydrocarbon will crack more easily than a C.sub.7
hydrocarbon or a lower carbon number paraffin and thus a high
severity reforming operation would be required to crack, for
example, a C.sub.5 paraffin. However, such a high severity
non-selective hydrocracking operation with the platinum reforming
catalyst is undesirable since cracking of branched C.sub.8 and
C.sub.7 hydrocarbons will be accomplished before cracking of normal
hexane. This will result in cracking desired high octane branched
chain hydrocarbons. Furthermore, such an operation produces an
undesired mixture of light gases particularly comprising C.sub.1
and C.sub.2 hydrocarbons rather than C.sub.3 and C.sub.4
hydrocarbons. On the other hand, using the small pore selective
hydrocracking catalyst described herein, cracking the lower boiling
C.sub.5 and C.sub.6 paraffins more effectively for the production
of LPG products. Thus by maintaining a selective balance in rate
control and equilibrium controlled hydrocracking reactions with the
different catalysts described herein and particularly suitable for
this purpose, an improved overall yield of LPG products can be
obtained along with an aromatic rich product by the present
invention.
Crystalline aluminosilicate conversion catalysts identified with
the prior art which are not selective within the limits defined
herein or those particularly known as methane producers rather than
producers of propane and butane are of little interest in pursuing
the concepts of this invention. Furthermore, high methane producing
cyrstalline aluminosilicate catalysts generally small pore
crystalline zeolites promoted with Zn, Cd and Hg or other
hydrocracking catalyst compositions which non-selectively produce
gaseous streams rich in methane are of little interest for
practicing the concept of this invention unless they can be
controlled by operating conditions to exclude the undesirable
production of light gaseous hydrocarbon constituents particularly
methane and ethane.
In the interest of convenience to a better understanding of the
concepts of the present invention the platinum type of reforming
catalyst used will be referred to as catalyst A hereinafter, and
the described selective crystalline aluminosilicate hydrocracking
catalyst relied upon particularly for the production of LPG gases
will be referred to hereinafter as catalyst B.
The platinum type reforming catalyst, catalyst A, selected for use
in the sequence of process steps of this invention may be selected
from any one of a number of known prior art reforming catalysts
suitable for accomplishing the results desired. These catalysts
include generally, for example, alumina as the carrier material for
one or more hydrogenation-dehydrogenation components distributed
thereon with the alumina being in either the eta, chi, gamma or
mixed forms thereof. The alumina carrier is promoted with, for
example, one or more Group VIII metal components either with or
without an acidic promoter such as silica, boron or a halogen. The
platinum type of reforming catalyst is intended to include
platinum, palladium, osmium, iridium, ruthenium, rhenium and
mixtures thereof deposited on an alumina containing carrier or
support with the alumina components generally being in an amount up
to about 95% by weight. Other components such as magnesium,
zirconium, thorium, vanadium and titanium may also be combined or
distributed in the alumina carrier. The platinum type catalyst may
also include various amounts of halogen such as chlorine or
fluorine in amounts ranging from about 0.1 up to about 10%; usually
not more than 5 or 6%. The platinum reforming catalysts described
may be one of those described in the prior art as homogeneous
mixtures of metal components, alloys, and metal halide complexes
thereof. A bimetal catalyst composition suitable for the reforming
operation of this invention may be platinum combined with either
rhenium, ruthenium, osmium or iridium and an alumina carrier
promoted with chlorine to provide desired acid activity.
In reforming operations it is known that as the reforming severity
is increased to achieve higher and higher product octane number,
the octane number increase is obtained primarily by way of paraffin
aromatization and 5 carbon ring aromatization. At the relative high
severity conditions required for paraffin to aromatic
dehydrocyclization reactions to become important, these are
accompanied by progressive and non-selective elimination by
hydrocracking of remaining paraffins to light gaseous products thus
increasing octane number at the expense of substantial liquid
volume loss. It is, therefore, preferred to selectively control the
reforming operation severity to restrict the chemical reactions
encountered therein to minimize the production of low octane liquid
and undesired gaseous component in favor of producing branched
chain hydrocarbons in an aromatic enriched product of relatively
high octane rating. Accordingly, the method and combination of
process steps herein described provide significant and unusual
benefits by adjusting the reaction mechanisms to implement and
improve the production of LPG products as well as high octane
aromatic products without unusual sacrifice of yield of liquid
products.
The operating conditions employed in the process combination of
this invention and particularly those of the reforming operation
with type A catalysts are those conditions which promote
dehydrogenation of naphthenes along with reactions associated with
paraffin isomerization which establish a relationship between
normal paraffins to branched paraffins and include operating
temperatures selected from the range of about 800.degree.F to about
1000.degree.F and preferably from 850.degree.F up to about
980.degree.F., liquid hourly space velocity in the range of about
0.1 to about 10, preferably about 0.5 to about 5; a pressure in the
range of about atmospheric up to about 600 p.s.i.g. and preferably
about 100 to about 400 p.s.i.g. and a hydrogen to hydrocarbon ratio
of about 0.5 to about 20 and preferably about 1 to 10.
The type B catalysts referred to herein are members of a special
class of zeolites exhibiting some unusual properties. These
zeolites induce profound transformations of aliphatic hydrocarbons
to aromatic hydrocarbons in commercially desirable yields and are
generally highly effective in alkylation, isomerization,
disproportionation and other conversion reactions involving
aromatic hydrocarbons. Although they have unusually low alumina
contents, i.e., high silica to alumina ratios, they are very active
even with silica to alumina ratios exceeding 30. This activity is
surprising since catalytic activity of zeolites is generally
attributed to framework aluminum atoms and cations associated with
these aluminum atoms. These zeolites retain their crystallinity for
long periods in spite of the presence of steam even at high
temperatures which induce irreversible collapse of the crystal
framework of other zeolites, e.g., of the X and A type.
Furthermore, carbonaceous deposits, when formed, may be removed by
burning at higher than usual temperatures to restore activity. In
many environments the zeolites of this class exhibit very low coke
forming capability, conducive to very long times on stream between
burning regenerations.
An important characteristic of the crystal structure of this class
of zeolites is that it provides constrained access to, and egress
from, the intra-crystalline free space by virtue of having a pore
dimension greater than about 5 Angstroms and pore windows of about
a size such as would be provided by 10-membered rings of oxygen
atoms. It is to be understood, of course, that these rings are
those formed by the regular disposition of the tetrahedra making up
the anionic framework of the crystalline aluminosilicate, the
oxygen atoms themselves being bonded to the silicon or aluminum
atoms at the centers of the tetrahedra. Briefly, the preferred
zeolites useful in type B catalysts in this invention possess, in
combination: a silica to alumina ratio of at least about 12; and a
structure providing constrained access to the crystalline free
space.
The silica to alumina ratio referred to may be determined by
conventional analysis. This ratio is meant to represent, as closely
as possible, the ratio in the rigid anionic framework of the
zeolite crystal and to exclude aluminum in the binder or in
cationic or other form within the channels. Although zeolites with
a silica to alumina ratio of at least 12 are useful, it is
preferred to use zeolites having higher ratios of at least about
30. Such zeolites, after activation, acquire an intracrystalline
sorption capacity for normal hexane which is greater than that for
water, i.e., they exhibit "hydrophobic" properties. It is believed
that this hydrophobic character is advantageous in the present
invention.
The zeolites useful as B catalysts in this invention freely sorb
normal hexane and have a pore dimension greater than about 5
Angstroms. In addition, their structure must provide constrained
access to some larger molecules. It is sometimes possible to judge
from a known crystal structure whether such constrained access
exists. For example, if the only pore windows in a crystal are
formed by 8-membered rings of oxygen atoms, then access by
molecules of larger cross-section than normal hexane is
substantially excluded and the zeolite is not of the desired type.
Zeolites with windows of 10-membered rings are preferred, although
excessive puckering or pore blockage may render these zeolites
substantially ineffective. Zeolites with windows of 12-membered
rings do not generally appear to offer sufficient constraint to
produce the advantageous conversions desired in the instant
invention, although structures can be conceived, due to pore
blockage or other cause, that may be operative.
Rather than attempt to judge from crystal structure whether or not
a zeolite possesses the necessary constrained access, a simple
determination of the "constraint index" may be made by continuously
passing a mixture of equal weight of normal hexane and
3-methylpentane over a small sample, approximately 1 gram or less,
of zeolite at atmospheric pressure according to the following
procedure. A sample of the zeolite, in the form of pellets or
extrudate, is crushed to a particle size about that of coarse sand
and mounted in a glass tube. Prior to testing, the zeolite is
treated with a stream of air at 1000.degree.F for at least 15
minutes. The zeolite is then flushed with helium and the
temperature adjusted between 550.degree.F and 950.degree.F to give
an overall conversion between 10% and 60%. The mixture of
hydrocarbons is passed at 1 liquid hourly space velocity (i.e., 1
volume of liquid hydrocarbon per volume of catalyst per hour) over
the zeolite with a helium dilution to give a helium to total
hydrocarbon mole ratio of 4:1. After 20 minutes on stream, a sample
of the effluent is taken and analyzed, most conveniently by gas
chromatography, to determine the fraction remaining unchanged for
each of the two hydrocarbons.
The "constraint index" is calculated as follows: ##EQU1##
The constraint index approximates the ratio of the cracking rate
constants for the two hydrocarbons. Catalysts suitable for the
present invention are those which employ a zeolite having a
constraint index from 1.0 to 12.0. Constraint Index (CI) values for
some typical zeolites including some not within the scope of this
invention are: CAS C.I. ______________________________________
ZSM-5 8.3 ZSM-11 8.7 TMA Offretite 3.7 ZSM-12 2 Beta 0.6 ZSM-4 0.5
H-Zeolon 0.5 REY 0.4 Amorphous Silica-alumina 0.6 Erionite 38
______________________________________
The class of zeolites defined herein is exemplified by ZSM-5,
ZSM-11, ZSM-12, ZSM-21 and other similar materials. Recently issued
U.S. Pat. No. 3,702,886 describing and claiming ZSM-5 is
incorporated herein by reference.
ZSM-11 is more particularly described in U.S. Pat. No. 3,709,979,
the entire contents of which are incorporated herein by
reference.
ZSM-12 is more particularly described in U.S. Pat. No. 3,832,449,
the entire contents of which are incorporated herein by
reference.
U.S. application, Ser. No. 358,192, filed May 7, 1973, now
abandoned, the entire contents of which are incorporated herein by
reference, describes a zeolite composition, and a method of making
such, designated as ZSM-21 which is useful in this invention.
Recent evidence has been adduced which suggests that this
composition may be composed of two different zeolites, one or both
of which are the effective material insofar as the catalyst for
this invention is concerned.
The specific zeolites described, when prepared in the presence of
organic cations, are substantially catalytically inactive, possibly
because the intracrystalline free space is occupied by organic
cations from the forming solution. They may be activated by heating
in an inert atmosphere at 1000.degree.F for 1 hour, for example,
followed by base exchange with ammonium salts followed by
calcination at 1000.degree.F in air. The presence of organic
cations in the forming solution may not be absolutely essential to
the formation of this special type zeolite; however, the presence
of these cations does appear to favor the formation of this special
type of zeolite. More generally, it is desirable to activate this
type zeolite by base exchange with ammonium salts followed by
calcination in air at about 1000.degree.F for from about 15 minutes
to about 24 hours.
Natural zeolites may sometimes be converted to this type zeolite by
various activation procedures and other treatments such as base
exchange, steaming, alumina extraction and calcination, alone or in
combinations. Natural minerals which may be so treated include
ferrierite, brewsterite, stilbite, dachiardite, epistilbite,
heulandite and clinoptilolite. The preferred crystalline
aluminosilicates are ZSM-5, ZSM-11, ZSM-12 and ZSM-21, with ZSM-5
particularly preferred.
The zeolites used as catalysts in this invention may be in the
hydrogen form or they may be base exchanged or impregnated to
contain ammonium or a metal cation complement. It is desirable to
calcine the zeolite after base exchange. The metal cations that may
be present include any of the cations of the metals of Groups I
through VIII of the periodic table. However, in the case of Group
IA metals, the cation content should in no case be so large as to
substantially eliminate the activity of the zeolite for the
catalysis being employed in the instant invention. For example, a
completely sodium exchanged H-ZSM-5 appears to be largely inactive
for shape selective conversions required in the present
invention.
In a preferred aspect of this invention, the zeolites useful as
catalysts herein are selected as those having a crystal framework
density, in the dry hydrogen form, of not substantially below about
1.6 grams per cubic centimeter. It has been found that zeolites
which satisfy all three of these criteria are most desired.
Therefore, the preferred catalysts of this invention are those
using zeolites having a constraint index as defined above of about
1 to 12, a silica to alumina ratio of at least about 12 and a dried
crystal density of not substantially less than about 1.6 grams per
cubic centimeter. The dry density for known structures may be
calculated from the number of silicon plus aluminum atoms per 1000
cubic Angstroms, as given, e.g., on page 19 of the article on
Zeolite Structure by W. M. Meier. This paper, the entire contents
of which are incorporated herein by reference, is included in
"Proceedings of the Conference on Molecular Sieves, London, April
1967," published by the Society of Chemical Industry, London, 1968.
When the crystal structure is unknown, the crystal framework
density may be determined by classical pyknometer techniques. For
example, it may be determined by immersing the dry hydrogen form of
the zeolite in an organic solvent which is not sorbed by the
crystal. It is possible that the unusual sustained activity and
stability of this class of zeolites is associated with its high
crystal anionic framework density of not less than about 1.6 grams
per cubic centimeter. This high density of course must be
associated with a relatively small amount of free space within the
crystal, which might be expected to result in more stable
structures. This free space, however, seems to be important as the
locus of catalytic activity.
Crystal framework densities of some typical zeolites including some
which are not within the preview of this invention are:
Void Framework Zeolite Volume Density
______________________________________ Ferrierite 0.28 cc/cc 1.76
g/cc Mordenite .28 1.7 ZSM-5, -11 .29 1.79 Dachiardite .32 1.72 L
.32 1.61 Clinoptilolite .34 1.71 Laumontite .34 1.77 ZSM-4 (Omega)
.38 1.65 Heulandite .39 1.69 P .41 1.57 Offretite .40 1.55 Levynite
.40 1.54 Erionite .35 1.51 Gmelinite .44 1.46 Chabazite .47 1.45 A
.5 1.3 Y .48 1.27 ______________________________________
The heavier reformate fraction is overwhelmingly aromatic in
composition, the aromatics comprising at least about 80 volume
percent, preferably at least about 90 volume percent, thereof. At
least a portion, and preferably all, of this heavier reformate is
contacted with a distinct mass of type B catalyst under conditions
conducive to substantially eliminating any aliphatic components
thereof. These conditions also induce aromatic isomerization,
disproportionation, alkylation, dealkylation, transalkylation and
side chain splitting so as to produce a product which is
substantially benzene and methyl substituted benzenes up to about
C.sub.10. BTX preferably predominates in this product.
The light reformate comprises substantial proportions of aliphatic
as well as aromatic components. It is contacted with a separate and
distinct mass of type B catalyst under conditions conducive to
cracking out the lower octane aliphatic components to produce LPG
components and to simultaneously increase the proportion of
aromatics at least partially by alkylation of existing aromatics
with fragments produced by such cracking.
The two separate and distinct type B catalyst conversions may be
carried out under substantially similar or widely different
reaction conditions depending upon the exact compositions of the
light and heavier reformate fractions and upon the desired product
distribution. It has been found, however, that if the reformate is
split in the manner set forth hereinabove, each of the type B
catalyst conversion performs its intended function admirably
whereas if the reformate is split differently, these conversion are
substantially less efficient at producing the desired product slate
of high quality, aromatic enriched gasoline, high quality
substantially aliphatic free aromatics concentrate, and large
quantities of LPG.
The operating conditions selected for processing the light
reformate fraction boiling below about 240.degree.F particularly
include an operating pressure within the range of 200 to 1000 psig;
a temperature within the range of about 500.degree.F to
800.degree.F; a volume hourly space velocity in the range of about
1 to 4 and a hydrogen to hydrocarbon ratio within the range of
about 1 to 10 to 1. Hydrogen consumption is about 100 or more SCF/B
depending on the charge composition and operating conditions
selected. Processing the heavy reformate fraction is preferably
accomplished under more severe operating conditions including a
temperature within the range of about 750.degree. to 900.degree.F;
a space velocity (vol. basis) in the range of about 0.5 to 2; a
hydrogen to hydrocarbon ratio in the range of 1-20/1 and a pressure
within the range of about 200 to 1000 psig. Hydrogen consumption
for the more severe operation is within the range of 200-800
SCF/B.
BRIEF DESCRIPTION OF THE DRAWING
The drawing is diagrammatic sketch in elevation of the process
combination comprising the separation of reformate product of
catalytic reforming into a light and heavy reformate product
fractions which are thereafter separately processed over type B
catalyst under lower and higher severity conditions as aforesaid to
particularly produce LPG, high octane gasoline and a
benzene-toluene-xylene rich fraction.
Referring now to the drawing, by way of example, a C.sub.5 .sup.+
full boiling range product of naphtha reforming is charged to the
process by conduit 2. The naphtha feed to a catalytic reforming
operation may be one boiling in the range of C.sub.5 hydrocarbon up
to about 380.degree. or 400.degree.F. Reforming of naphtha boiling
range hydrocarbons is well known in the prior art as discussed
above and is a part of this invention to the extent that the
material processed is a reformate product of catalytic reforming
comprising a C.sub.5 .sup.+ reformate. The reformate charge
introduced by conduit 2 to stabilizer 4 is separated in the
stabilizer to recover toluene enriched C.sub.6 .sup.+ reformate
material from the bottom thereof by conduit 8. C.sub.5 and lower
boiling components are recovered from the top of the stabilizer by
conduit 10 and then passed to gas recovery equipment not shown to
separate LPG materials from other gasiform materials.
A portion of the material withdrawn by conduit 8 from the bottom of
the stabilizer is passed to gasoline pool equipment. The remaining
portion of the C.sub.6 .sup.+ material is passed by conduit 12 to a
second separation zone 14, hereinafter sometimes referred to as a
reformate splitter. In separation zone 14, conditions are
maintained to separate the C.sub.6 .sup.+ reformate into an
overhead fraction comprising some toluene and primarily lower
boiling components withdrawn therefrom by conduit 16. A C.sub.7
.sup.+ toluene-xylene rich fraction is withdrawn from the bottom
portion of separation zone 14 by conduit 18. Separation of the
C.sub.6 .sup.+ reformate in zone 14 is accomplished to provide a
cut point between the fractions within the range of about
200.degree. to about 240.degree.F.
The overhead fraction comprising primarily C.sub.6 and lower
boiling components withdrawn by conduit 16 is passed with added
hydrogen to a catalyst zone 20 containing a ZSM-5 type of
crystalline zeolite conversion catalyst. In zone 20, the fraction
comprising toluene and lower boiling components is subjected to
processing conditions comprising a pressure of about 400 psig at a
start of run temperature of about 550.degree.F. The reactant
material is passed in contact with the catalyst at a volume hourly
space velocity of about 2 and a hydrogen to hydrocarbon ratio of at
least 1/1. In zone 20 the light reformate comprising aromatic and
C.sub.7 minus saturated hydrocarbons are processed to particularly
produce LPG materials comprising propane and butane and some alkyl
aromatics comprising toluene. The product effluent of zone 20 is
then passed by conduit 22 to a high pressure separation zone 24. In
high pressure separator 24, a separation is made which permits the
recovery of C.sub.2 and lower boiling material comprising hydrogen
from an upper portion of the zone by conduit 26. Some of this
material may be withdrawn by conduit 28 and used as fuel. The
remaining portion of the material in conduit 26 is combined with
makeup hydrogen added by conduit 30, compressed in compressor 32 to
raise the pressure of this recycle stream about 50 pounds before
recycle by conduit 34 to zone 20.
A product material comprising C.sub.3 and higher boiling components
including benzene and toluene separated in zone 24 is withdrawn
essentially as a liquid stream from the lower portion of separation
zone 24 by conduit 36 and passed in its entirety to separation zone
4 wherein the C.sub.5 and lower boiling components are removed as
overhead material and processed as above briefly discussed.
The heavy C.sub.7 .sup.+ reformate material, enriched with C.sub.7
.sup.+ components from the separator 24, withdrawn from the lower
portion of separation zone 14 is a toluene-xylene rich stream
comprising C.sub.7 .sup.+ paraffins, having at most about 0.1
weight percent C.sub.7 paraffins, and boiling above about
225.degree.F which mixture is passed in a desired amount by conduit
38 to a second ZSM-5 zeolite catalytic conversion zone 40. A
portion of this toluene-xylene enriched reformate in conduit 18 may
be withdrawn as a product stream for use as desired. In zone 40,
the herein identified C.sub.7 .sup.+ reformate is processed in the
presence of added hydrogen which may come from conduit 28 over a
ZSM-5 type zeolite under relatively more severe processing
conditions designed to particularly crack paraffin components and
convert the aromatic containing reformate to a light aromatics
mixture comprising (BTX) benzene, toluene and xylene by a
combination of dealkylation and disproportionation reactions. In
this catalytic conversion operation it is proposed to employ an
operating pressure of about 400 psig and a start of run temperature
of about 750.degree.F. A hydrogen to hydrocarbon ratio of about 4/1
is used with a reactant volume space velocity of about 1. It is
particularly important that the processing conditions of zone 40 be
selected to particularly crack the paraffins to lower boiling
components comprising LPG products and disproportionate charged
aromatics to form a mixture of benzene, toluene, and aromatics
since these materials are of a boiling range easily separated by
simple distillation.
The product effluent of the aromatic forming zone 40 is passed by
conduit 42 to a high pressure separation zone 44. In separation
zone 44 a hydrogen rich gaseous stream is separated and recycled by
conduit 46 to conduit 38 communicating with zone 40. Hydrogen rich
make up gas is added to the recycled gas by conduit 48. The
remaining product separated in zone 44 comprising light aromatics
and lower boiling components of cracking are passed by conduit 50
to a separation zone 52 maintained at a pressure of about 100 psig
and a temperature of about 450.degree.F. In separation zone 52 a
separation is made between light aromatics comprising benzene and
higher boiling aromatics and reaction components boiling below
about benzene. Thus a light aromatic rich fraction is withdrawn by
conduit 54 from zone 52 for further processing by distillation and
other means not shown into desired components.
Materials generally lower boiling than benzene and comprising
hexane and lower boiling components is withdrawn from separator 52
by conduit 56 for recycle to stabilizer 4 as by conduit 36.
An important aspect of this invention resides in flashing the
product of shape selective conversion of light reformate,
preferably in a high pressure separator, to split it into a gas and
a higher octane gasoline liquid. The liquid is recycled to the
stabilizer along with the full range reformate. Thus the heavier
reformate from the splitter is enriched with C.sub.7 .sup.+
components (predominantly C.sub.9 .sup.+ alkyl benzenes) from this
higher octane gasoline liquid product.
It is most important that a portion of the toluene enriched C.sub.6
.sup.+ reformate material from the stabilizer be drawn down out of
the system. This product is excellent gasoline.
Having thus generally described the invention and presented a
specific example in support thereof, it is to be understood that no
undue restrictions are to be imposed by reason thereof except as
defined by the following claims.
* * * * *