Note: Descriptions are shown in the official language in which they were submitted.
~17(~61
BACK_ROUND OF TH~ INVENTION
This invention is concerned with a process for con-
verting coal and otAer solid, highly viscous liquid, or gaseous
fossi.l fuels to liquid petroleum products, particularly hydro-
carbon fuels. It more especially is concerned with convertingsuch materials to high quality gasoline.
The increasing de~and for high octane gasoline has
been met, until now, by advanced petroleum refining technology.
The two processes which have made it possible to satisfy the
demand are: catalytic cracking, which serves to increase the
fraction of crude petroleum which can be brought into the gas-
oline boiling range; and catalytic reforming, which serves to
upgrade the octane number of low grade gasoline. Obviously,
the cracking process increases the gasoline yield at the
expense of the heavier fuel fractions, such as No. 2 fuel oil,
which also is su~ject to growing demand. The increasing cost
of petroleum, together with the foreseen increase in demand
for gasoline and other petroleum fractions, makes it necessary
to seek other fuel sources from which to make high quality gas-
oline.
Coal, for example, is an obvious alternative rawmaterial, since there are abundant deposits of this fuel.
Gasoline has been made from coal by gasiflcation and conversion
of the gases to gasoline by the Fischer-Tropsch process. How-
ever, this process, not presently used in this country, pro-
duces an extremely poor quality of gasoline, with an octane
1117~61
number of about 55, which cannot be efficiently upgraded by
the known catalytic reforming processes because it consists
predominantly of straight-chain aliphatic hydrocarbons.
Processes for the conversion of coal and other hydro-
carbons to a gaseous mixture consisting essentially of hydrogen
and carbon monoxide, or of hydrogen and carbon dioxide, or of
hydrogen and carbon monoxide and carbon dioxide, are well known.
Such gaseous mixture hereinafter will be referred to simply
as synthesis gas.
Although various processes may be employed for the
gasification, those of major interest for the present invention
depend either on the partial combustion of the fuel with oxygen,
or on the high temperature reaction of the fuel with steam, or
on a combination of these two reactions. In one known variant,
for example, coal may be completely gasified by first coking,
and then subjecting the coke to a cyclic blue water gas process
in which the coke bed is alternatively blasted with air to
increase the bed temperature and then reacted with steam to
; produce the synthesis gas ~he gases generated by the coking
step may be used as fuel or steam-reformed to additional syn-
thesis gas. In another process coal or coke may be reacted
with highly superheated steam or with oxygen and steam to
produce synthesis gas. Regardless of the process variants
chosen, oxygen rather than air is oftern used in the chemical
step in which the fuel is converted to synthesis gas since
the use of air would result in a gas that contained excessive
amounts of inert nitrogen.
~117~6~
An excellent summary of the art of gas manufacture,
including synthesis gas, from solid and liquid fuels, is
given in Encyclopedia of Chemical Technology, Edited by
Kirk-Othmer, Second Edition, Volume 10, pages 353-433,
(1966), Interscience Publishers, New York, N.Y. The
techniques for gasification of coal or other solid,
viscous or gaseous fuel are not considered to be per se
inventive here.
It is known that raw synthesis gas contains one or
more of the following impurities: sulfur compounds,
nitrogen compounds, particulate matter and condensibles.
The art of removing these contaminants is known, and
is described in the above reference and elsewhere.
Particular attention is called to the sulfur compounds.
It is desirable to reduce this contaminant below a
prescribed level for ecological purposes.
Purified synthesis gas ordinarily contains a volume
ratio of hydrogen to carbon monoxide plus carbon dioxide
of from as little as about 0.10 to as much as 1.1,
depending on the particular fuel and process used; in
most instances, the composition has a volume ratio from
about 0.30 to about 0.65. It is well known that this
ratio may be increased by the catalytic carbon monoxide
shift reaction described by the equation:
with subsequent removal of at least part of the produced CO2
--4--
F~
~117~1
to bring said volume ratio into a desired high range. The
catalytic carbon monoxide shift reaction is commonly conducted
with a chromia promoted iron oxide catalyst at a flow rate of
about 300-1000 standard cubic feet of gas per cubic foot of
catalyst bed per hours, and at sufficiently elevated temperature
to allow quasi-equilibrium, which us usually about 700F.
Synthesis gas will undergo conversion to form reduc-
tion products of carbon monoxide, such as alcohols, at from
about 300F to about 850F, under from about 1 to 1000 atmos-
pheres pressure, over a fairly wide variety of catalysts. Thetypes of catalysts that induce conversion include ZnO, Fe, Co,
Ni, Ru, ~hO2, Rh and Os.
Catalysts based on ZnO are particularly suited for
the production of methanol and dimethyl ether. Catalysts
based on Fe, Co, and Ni, and especially Fe, are particularly
suited for the production of oxyg~enated and hydrocarbon products
; that have at least one carbon-to-carbon bond in their structure.
With the exception of ruthenium, all practical synthesis
catalysts contain chemical and structural promoters. These
promoters include copper, chromia, alumina and alkali. Alkali
is of particular importance with iron catalysts, since it
greatly enhances the converslon efficlency of the iron catalyst.
Supports such kieselguhr sometimes act beneficially.
~117(~61
The catalyzed reduction of carbon monoxide or carbon
dioxide by hydrogen produces various oxygenated and hydro-
carbon products, depending on the particular catalyst and
reaction conditions chosen. The products that are formed
include methanol, dimethyl ether, acetone, acetic acid, normal
propyl alcohol, higher alcohols, methane, gaseous, liquid , and
solid olefins and paraffins. It should be noted that this
spectrum of products consists of aliphatic compounds; aromatic
hydrocarbons either are totally absent or are formed in minor
quantities.
In general, when operating at the lower end of the
temperature range, i.e. from about 300F to about 500F, in
the reduction of carbon monoxide, and with pressures greater
than about 20 atmospheres, thermodynamic considerations suggest
that aliphatic hydrocarbons are likely to form in preference to
their aromatic counterparts. Furthermore, in some catalytic;
systems it has been noted that aromatic hydrocarbon impurities
in the synthesis gas inactivate the synthesis catalyst, and
one may speculate that a number of known synthesis catalysts
intrinsically are not capable of producing aromatic hydrocarbons.
The wide range of catalysts and catalyst modifications
disclosed in the art and an equally wide range of conversion
conditions for the reduction of carbon monoxide by hydrogen
provide considerable flexibility toward obtaining selected
products. Nonetheless, in spite of this flexibility, it has
.- .~
'~Y.`!
1~17~!61
not proved possible to make such selections so as to
produce liquid hydrocarbons in the gasoline boiling range
which contain highly branched paraffins and substantial
quantities of aromatic hydrocarbons, both of which are
required for high quality gasoline. A review of the
status of this art is given in "Carbon Monoxide-Hydrogen
Reactions," Encyclopedia of Chemical Technology, Edited
By Kirk-Othmer, 2nd Edition, Volume 4, pp. 446-488,
Interscience Publishersl New York, N.Y.
Oxygenated compounds and hydrocarbons are produced in
varying proportions in the conversion of synthesis gas.
This is understandable if, as proposed by some researchers
in the field, the hydrocarbons arise via oxygenated inter-
mediates such as alcohols. By selection of less active
catalysts such as zinc oxidé, it is possible to obtain
oxygenated compounds as the major product. One particular
commercial conversion is used to produce methanol from
synthesis gas with substantially no coproduction of hydro-
carbons. Suitable catalysts are those comprising zinc
oxide, in admixture with promoters. Copper or copper
oxide may be included in the catalyst composition.
Particularly suitable are oxide catalysts of the zinc-
copper-alumina type. Compositions of the type described
are those currently used in commercial methanol synthesis.
Contact of the synthesis gas with the methanol synthesis
catalyst is conducted under pressure of about 25 to 600
.~,, J
1117Q61
atmospheres, preferably about 50 to 400 atmospheres, and
at a temperature of about 400F to 750F. The preferred
gas space velocity is within the range of about 1,000 to
50,000 volume hourly space velocity measured at standard
temperature and pressure. It is noted that the conversion
per pass is from about 10% of the carbon monoxide fed to
about 30%, i.e. in this process the unconverted synthesis
gas must be separated from the methanol product and
recycled.
Crystalline aluminosilicate zeolites have been contac-
ted with methanol under catalytic conversion conditions.
U.S. Patent 3,036,134 shows a 98.4% conversion of methanol
to dimethyl ether over sodium X zeolite at 260C; 1.6
mole % of the product is a mixture of olefins through
pentene, with butene the predominant product. Conversion
of methanol over rare earth exchanged and zinc exchanged
X zeolite has been reported to produce some hexanes and
lighter hydrocarbons (see Advances in Catalysis, Vol. 18,
p. 309, Academic Press, New York, 1968). It has recently
been discovered that alcohols, ethers, carbonyl and their
analogous compounds may be converted to higher hydro-
carbons, particularly high octane gasoline, by catalytic
contact with a special type zeolite catalyst. This
conversion is described in Canadian Patent 1,032,558,
U.S. Patent 3,894,107 and Canadian Patent 1,037,972.
1117~61
It is an object of the present invention to provide an
improved method for converting fossil fuels to high quality
gasoline. It is a further object of this invention to
provide a method for converting a mixture of gaseous carbon
S oxides with hydrogen to high quality gasoline. It is a
further object of this invention to provide a novel method
of converting synthesis gas to high octane gasoline. It
is a further object of this invention to provide a process
for the manufacture of substantially sulfur-free liquid
hydrocarbon fuels. Further objects of this invention will
be apparent to those skilled in the art.
BRIEF SUMMARY OF THE INVENTION
In accordance with the stated objects, one aspect of
this invention provides a process for manufacture of liquid
hydrocarbon fuels boiling in the gasoline boiling range
comprising the following steps: converting the oil from
tar sands to a gaseous mixture of hydrogen and carbon
oxides and converting said gaseous mixture to normally
liquid hydrocarbons and oxygen-substituted hydrocarbons
comprising methanol; the improvement comprising the
additional step of contacting at least said oxygen-
substituted hydrocarbons including said methanol with
a catalytically active aluminosilicate zeolite having a
silica to alumina ratio of at least about 12, a constraint
index of about 1 to 12 and a crystal density, in the
hydrogen form, of not substantially below about 1.6 grams
per cubic centimeter at a temperature of about 500F to
1000F, a space velocity of about 0.1 to 50 liquid hourly
space velocity, and a pressure of about 1 to 50 atmos-
pheres; and, recovering liquid hydrocarbons.
~117Q61
DETAILED DESCRIPTION AND PREFERRED EM~O~IMENTS
The drawing of Fig. 1 will now be used to illustrate this
invention in certain of its aspects, without being limited thereto.
Coal, shale oil, or residua, or a combination thereof, is con-
veyed via line 1, 2 and 3, resp. and thence via line 4 to the
synthesis gas plant, 5, where it is converted to synthesis gas.
If hydrogen sulfide is produced in this plant, it may be separa-
ted and sent via line 6 to a treatment plant tnot shown) for
sulfur recovery. Synthesis gas, previously treated in a cataly-
tic carbon monoxide shift converter and then reduced in carbon
dioxide content be selective sorption, is conveyed via line 7 to
a first reaction zone 8, where it is at least partially converted
catalytically to produce a carbon monoxide reduction product that
contains at least 20~ by weight of oxygenated products. Part or
all of the unconverted synthesis gas may be separated from such
reduction product and recycled via line lO, but it is preferred
to convey the total mixture via line 9 to the second reaction
zone ll where catalytic conversion to hydrocarbons and steam oc-
curs. The reaction products from the second reaction zone ll are
conveyed via line 12 to a cooler, 13, and the cooled products are
then conveyed via line 14 to a separator 15; note that the
cooler 13 and line 14 and separator 15 may be one integral unit.
Water is removed from separator 15 via line 16, gases via line
17, and liquid hydrocarbon products via line 18. The liquid
hydrocarbon products are conveyed via line 18 to a distillation
tower 19. Propane and butanes (LPG) are recovered via line
20, and gasoline via line 21. The gases disengaged in the
separator 15 are conveyed via line 17 to storage 22, and
recycled via lines 23 and 25 to the synthesis.
Xl
1117061
gas plant, or via lines 23 and 24 and line 7 to the carbon
oxides converter 8.
Fossil fuel, as the term is used in this invention,
is intended to include anthracite and bituminous coal, lignite,
crude petroleum, shale oil, oil from tar sands, natural gas,
as well as fuels derived from physical separation, or more
profound transformations, of these materials, including coked
coal, petroleum coke, gas oil, residua from petroleum distilla-
tion, and two or more of any of the foregoing materials in
combination. It is an attribute of this invention that fuels
which usually contain ecologically undesirable levels of sulfur,
i.e. greater than 2~ organically bound sulfur, may be used to
produce products substantially free of sulfur. Organically
bound sulfur, as distinguished from hydrogen sulfide, for
example, is sulfur which is chemically bonded to one or more
carbon atoms, and such sulfur is ordinarily difficult to remove
from fossil fuels. Particularly preferred for the practice of
this invention is to use coal. Non-fossil carbonaceous fuels
also may be used, however; these include wood, cellulosic ma-
terials, organic animal waste, and any other organic mattercharacterized by significant fuel value.
Any of the described fuels may be converted to synthe-
sis gas by techniques whcih are known in the art, and which are
not regarded as constituting this invention. It is also con-
templated to include in gasification techniques in situ processessuch as the underground partial combustion of coal and petroleum
deposits. In any case, it is to be understood that the gasifi-
cation art employed shall be selected so as to produce raw
synthesis gas comprising a mixture of carbon monoxide, carbon
11
~117a!61
dioxide and hydrogen as the principal constltuents Synthesis
gas, as first produced, contains impurities, including hydrogen
sulfide and volatile organically bound sulfur compounds, inclu-
ding carbonyl sulfide. This mixture shall be referred to as
raw synthesis gas.
Raw synthesis gas is next treated to remove impurities.
Iron and nickel carbonyls, if present, should be removed since
they will adversely effect the long term bevavior of the cata-
l~sts used in the subsequent conversions. The purification
may be carried out, for example, by absorption on activated
carbon. Partlculates and hydrocarbon impurities may be removed
by sorption processes well known in the art, if so desired. It
is very important, however to remove a major portion of the
sulfur which may be present as hydrogen sulfide, organically
bound sulfur compounds or mixtures of these. The organically
bound sulfur compounds may be decomposed, for example, over a
mixture of alkali metal carbonate and sulfurized iron at ele-
vated temperature; the hydrogen sulfide, either initially
present in the raw synthesis gas, or formed by decomposition
of the organically bound sulfur compounds, may be reduced in con-
centration and substantially removed by scrubbing under pressure
with ethanolamines, for example. For the purpose of this
invention, it is preferred to remove at least 90~ of the sulfur
present initially in the raw synthesis gas to form purified
synthesis gas.
X
1117061
The purified synthesis gas consists essentially of a
mixture of hydrogen gas, with gaseous carbon oxides including
carbon monoxide and carbon dioxide. By way of illustration, a
typical purified synthesis gas will have the composition, in
volume percentages, as follows: hydrogen, 51; carbon monoxide,
40: carbon dioxide, 4: methane, 1: and nitrogen, 4. Depending
on the particular fuel and the particular gasification process,
the hydrogen to carbon oxides ratio may vary widely. It is
preferred to adjust the hydrogen-to-carbon oxides volume ratio
in the synthesis gas to from 1.0 to 6.0 prior to use in sub-
sequent conversions. Should the purified synthesis gas be
excessively rich in carbon oxides, it may be brought within the
preferred range by the well known water gas shift reaction; on
the other hand, should the synthesis gas be excessively rich
in hydrogen, it may be adjusted into the preferred range by the
addition of carbon dioxide or carbon monoxide. Purified syn-
thesis gas adjusted to contain a volu~le ratio of hydrogen to
carbon oxides of from 1.0 to 6.0 will be referred to as adjusted
synthesis gas.
It is desirable that the adjusted synthesis gas
contain not more than 20% inert nitrogen since the economic
cost for subsequent conversions are increased by excess diluent.
Low levels of nitrogen are easily achieved by supplying essen-
tially pure oxygen gas, in the quantities required, in the
fossil fuels gasification step.
13
~'
~17~61
It i5 an essential feature of this invention that the
adjusted synthesis gas is catalytically converted to oxygenated
compounds in a first reaction zone and the oxygenated compounds
are catalytically converted to liquid gasoline boiling range
hydrocarbons in a second reaction zone. A number of catalysts
are known that will cause the carbon monoxide to be reduced
by the hydrogen of the synthesis gas to form oxygenated com-
pounds, liquid hydrocarbons, and mixtures of these in varying
proportions, depending on the particular catalyst and reaction
conditions.
It is a preferred embodiment of this invention to
catalytically convert the adjusted synthesis gas in such a
manner that substantially all of the reaction product from
the first reaction zone is oxygenated product, and thus obtain
lS maximum benefit from this invention. The conversion to methanol,
to dimethyl ether, or to mixtures of these and other oxygenated
compounds is illustrative of this preferred embodiment.
Conventional methanol synthesis techniques are well
suited for the purpose of this invention. Contact of the
adjusted synthesis gas with a methanol synthesis catalyst,
preferably under about 50 to 400 atmospheres, at a temperature
from about 400F to 750F, and at a volume hourly space velocity
of from about 1000 to 50,000 volumes, serves to induce conver-
sion of from about 10% to about 30~ of the carbon monoxide
feed to oxygenates, mainly methanol. Methanol may be formed
as the almost exclusive product of the reduction, or it may
14
~1~75:~61
be contaminated by higher alcohols such as ethanol, propanol,
and butanols. In eit~er case, it is possible to pass the gas-
eous product mixture from the first reaction zone thrc~gh a
condenser, separate the crude oxygenated product ~methanol~,
and recycle the unconverted synthesis gas to the first reaction
zone. However, it is preferred to directly convey the mixture
of reduction product and unconverted synthesis gas to the
second reaction zone without separation since this provides
economies in handling and, in addition, the conversion ln the
second reaction zone is not adversely affected by the presence
of the unreacted synthesis gas.
It is to be emphasized that the conversion in the
first reaction zone may employ catalysts and reaction conditions
which lead to improved efficiencies of carbon monoxide conver-
slon, notwithstanding that the mixture of oxygenated compoundsformed may ordinarily be considered undesirable for ordinary
methanol synthesis because it contains substantial or even
major fractions of oxygenated compounds other than methanol.
This is so because the other oxygenated compounds such as
dimethyl ether, ethanol, propanol and butanol are converted in
the second reaction zone wit.h an efficiency at least equal to
pure methanol.
Thus, in a preferred embodiment of the present inven-
tion, crude methanol from the first reaction zone is fed to
the second reaction zone without separation of oxygenated
impurities. In this preferred embodiment full advantage is
taken of insensitivity of the second reaction zone to these
1~1706~
impurities, and furthermore the methanol synthesis catalyst
and reaction conditions in the first reaction zone may be
suita~ly modified to most efficiently affect the reduction of
carbon monoxide.
Although maximum benefit from this invention is
achieved by catalytic conversion of adjusted synthesis gas in
a first reaction zone under conditions such that substantially
all of the carbon monoxide reduction product is oxygenated
product, substantial benefits will accrue if at least 20 percent
by weight of the reduction product consists of oxygenated com-
pounds. Metallic catalysts of the iron, cobalt and nickel
variety are suitable for such conversions. Iron promoted by
alkali is especially useful. By way of example, pure iron,
roasted in an oxygen atmosphere in ~he presence of added
aluminum and potassium nitrates provides a composition that
contains 97% Fe304, 2.4% Al203, and 0.6% K20 with trace amount
of sulfur and carbon. This composition after reduction with hydro-
gen at about 850F catalyzes the conversion of synthesis gas
at from 360F to 430F, and at 20 Atm. pressure, such that 65%
of the carbon monoxide is reduced to a mixture consisting of
sbout one third by weight of hydrocarbons boiling in the range
of 200F to about 680F, and about two thirds of oxygenated
compounds, mostly alcohols, in the same boiling range. This
conversion is given by way of illustration only; other catalysts
and conversion conditions capable of producing at least 20 per-
cent by weight oxygenated compounds in the reduction product
will be evidence to those skilled in the art.
16
1117061
It is a feature of this invention that it is not
necessary to separate the oxygenated compounds from the liquid
hydrocarbons, prior to further conversion, when the two are
produced simultaneously in the first reaction zone. Although
the hydrocarbons produced in the first reaction zone are likely
to be linear paraffins and olefins and therefore undesirable
components of high octane gasoline, it is a remarkable attribute
of this invention that these hydrocarbons undergo conversion
to highly branched paraffins and aromatics along with the
alcohols when the mixture is converted in the second reaction
zone. It is a preferred embodiment of this invention to contact
the unresolved mixture of hydrocarbons and oxygenated com-
pounds with the catalyst in the second reaction zone, thereby
taking full advantage of the cooperative interaction of the
two reaction zones to produce maximum high octane gasoline.
However, the reduction product mixture may be separated from
the unconverted synthesis gas before contact with the catalyst
in the second reaction zone.
Small quantities of ammonia are sometimes produced
in the first reaction zone. Although not essential to this
invention, it is highly desirable to remove these from the
product mixture prior to contact with the catalyst in the
second reaction zone, thereby prolonging the effectiveness
of that catalyst. This may be done by brief contact with a
solid acidic adsorbent, such as acid-treated clay, for example.
d~`
11~7061
An essential step in the present invention is the
catalytic conversion of the oxygenated compounds to hlgh octane
gasoline in a second reaction zone in contact with a novel
class of zeolite catalysts. This recently discovered novel
class of zeolites has some unusual properties. These catalysts
induce profound transformations of aliphatic hydrocarbons to
aromatic hydrocarbons in commercially desirable yields.
Although they have unusually low alumina contents, i.e. high
silica to alumina ratios; they are very active even when the
silica to alumina ratio exceeds 30. The activity is surprising
since the alumina in the zeolite framework is believed respons-
ible for catalytic activity. These catalysts retain their crys-
tallinity for long periods in spite of the presence of steam at
high temperature which induces irreversible collapse of the
framework of other zeolites, e.g. of the X and A type. Fur-
thermore, carbonaceous deposits, when formed, may be removed
by burning at higher than usual temperatures to restore activity~
An important characteristic of the crystal structure
of this class of zeolites is that it provides constrained
access to, and agress from, this intracrystalline 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
18
X
~1~7061
or aluminum atoms at the centers of the tetrahedra. Briefly
the preferred type catalyst useful in this invention possess,
in combination: a silica to alumina ratio of at least about
12; and a structure providing constrained access to the crys-
talline free space.
The silica to alumina ratio referred to may be de-
termined by con~entional 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 form within the chan-
nels. Although catalysts with a silica to alumina ratio of
at least 12 are useful, it is preferred to use catalysts
having higher ratios of at least about 30. Such catalysts,
after activation, ac~uire 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 type zeolites useful in this invention freely
sorb normal hexane and have a pore dimension greater than
about 5 Angstroms. In addition, the structure must provide
constrained access to larger molecules. It is some~imes
possible to judge from a know crystal structure whether
such constrained access exists. For example, if the only
pore windows in a crystal are formed by eight membered rings
of oxygen atoms, then access to molecules of larger cross-
section than normal hexane is excluded and the zeollte is
not of the desired type. Windows of ten-membered rings are
19
~1~7Q61
preferred, although excessive puckering or pore blockage may
render these catalysts ineffective. Twelve-membered rings
do not generally appear to offer sufficient constraint to
produce the advantageous conversions, 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 catalyst possesses the necessary constrained
access, a simple determination of the "constraint index" may
be made by passing continuously a mixture of equal weight of
normal hexane and 3-methylpentane over a small, ap-
proximately l gram or less, of catalyst at atmospheric pres-
sure according to the following procedure. A sample of the
catalyst, 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 catalyst is treated with
a stream of air at 1000F for at least 15 minutes. The cata-
lyst is then flushed with helium and the temperature adjusted
between 550F and 950F 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 hydrocarbon
per volume of catalyst per hour) over the catalyst 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.
1117~61
The "constraint index" is calculated as follows:
Constraint Index = loglO (fraction of n-hexane remaining)
og1o raction of 3-methylpentane
remaining)
The constraint index approximates the ratio of the
cracking rate constants for the two hydrocarbons.
Catalysts suitable for the present invention are those
having a constraint index from 1.0 to 12.0, preferably
2.0 to 7Ø
The class of zeolites defined herein include those
of the ZSM-5 type and is exemplified by ZSM-5, ZSM-ll,
ZSM-12, ZSM-21, TEA mordenite and other similar
materials. Recently issued U.S. Patent 3,702,886 des-
cribes and claims ZSM-5 while ZSM-ll is more particularly
described in U.S. Patent 3,709,979. ZSM-12 is more
particularly described in West German Offenlagunschrifft
2,213,109, and ZSM-21 is more particularly described in
the Mobil, published French application 74-12078.
The specific zeolites described, when prepared in the
presence of organic cations, are catalytically inactive,
possibly because the intracrystalline free space is
occupied
-21-
1117~61
by organic cations from the forming solution. They may be
activated by heating in an inert~atmosphere at 1000F for
one hour, for example, followed by base exchange with ammonium
salts followed by calcination at 1000F in air. The presence
of organic cations in the forming solution may not be abso-
lutely essential to the formation of this type zeolite; how-
ever, the presence of these cations does appear to iavor the
formation of this special type of zeolite. More generally,
it is desirable to activate this type catalyst by base exchange
with ammonium salts followed by calcination in air at about
1000F for from about 15 minutes to about 24 hours.
Natural zeolites may sometimes be converted to this
type zeolite catalysts by various activation procedures and
other treatments such as base exchange, steaming, alumina
extraction and calcination, in combination. Natural minerals
which may be so treated include ferrierite, brewsterite,
stilbite, dachiardite, epistilbite, heulandite and clinoptilo-
lite. The preferred crystalline aluminosilicates are ZSM-5,
ZSM-ll, ZSM-12, ZSM-21 and TEA mordenite, with ZSM-5 particu-
larly preferred.
The catalysts of this invention may be in the hydro-
gen form or they may be base exchanged or impregnated to con-
tain ammonium or a metal cation complement. It is desirable
to calcine the catalyst 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 effectively inactiv~te the catalyst.
1~17~1
For example, a completely sodium exchanged H-ZSM-5 is not
operative in the present invention.
In a preferred aspect of this invention, the catalysts
hereof are selected as those having a crystal 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 because they tend to maximize the production
of gasoline boiling range hydrocarbon products. Therefore,
the preferred catalysts of this invention are those 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 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 11 of the
article on Zeolite Structure by W. M. Meier. This paper
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.
1117Q61
This high density of course must be associate.d 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, is important as the locus of
catalytic actlvity.
X
~1706~
Because the catalyst for the synthesis gas con-
version is a hydrogenation catalyst, the aromatization catalyst
is most preferably maintained in a separate reaction zone,
where it functions well even in the presenc~ of unconverted
synthesis gas.
In the practice of this invention, the conversion in
the second reaction zone is conducted at a temperature of about
500F to 1000F, preferably about 600F to 800~F, a pressure of
subatmospheric to about 50 atmospheres, and at a liquid hourly
space velocity of about 0.1 to 50 LHSV.
The conversion with this special, high silica to
alumina ratio, catalyst produces a s~ulfur-free, high quality
gasoline fraction boiling in the range about 82F to 415F
which has a research octane number of at least about 80
without the addition of lead. A minor fraction of valuable
liquefiable petroleum gas (e.g. propane) and a little dry gas
(e.g. ethane and methane) also are produced. If severe
reaction conditions are selected, the major fraction of the gas-
oline is aromatic hydrocarbons, and the paraffins are mostly
branched. Thus, this total mixture may be separated into a
small fraction suitably about l~, of "dry gas" comprising
methane, ethane, and ethylene, a small fraction, suitably
about 26~, of liquefiable petroleum gas comprising propane and
butanes, and a major fraction, sultably the remainder of
about 73~, of gasoline with a research octane number of at
least about 100 without requiring the addition of lead.
1117Q61
The liquefiable petroleum gas may be sold as a sulfur-
free fuel, or it may be recycled to the gasification plant. In
the latter case it need not be first separated from the "dry gas"
fraction. The dry gas fraction may be burned as fuel or sold
as such but preferably it is recycled to the fossil fuel
gasification operation. Hydrogen is sometimes produced in
the second, or aromatization, reaction zone. ~his hydrogen
is a valuable recycle product which can be most useful in
carbon oxide hydrogenation.
Tt will be seen by a consideration of this entire s
process that the individual steps are quite interrelated and
mesh very nicely with each other through thermal and/or
material conservation and recycle. The aromatization reaction
is quite exothermic and its heat is most useful to generate
steam used in other parts of the process.
26
X~
