Conversion of synthesis gas to hydrocarbon mixtures utilizing dual reactors

The conversion of less than 1 H.sub.2 /CO ratio syngas to high yield of C.sub.3 plus product is accomplished with a CO reducing catalyst comprising shift characteristics and the product of Fischer-Tropsch synthesis is converted to premium gasoline and distillate fuels by contact with acidic ZSM-5 zeolite. The syngas conversion may be accomplished in any catalyst system permitting a very close temperature control on the exotherm encountered and the gasoline-distillate yield relationship may be varied as a function of temperature and pressure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is concerned with an improved method and combination process 
for converting synthesis gas, i.e. mixtures of gaseous carbon oxides with 
hydrogen generally CO rich, to hydrocarbon mixtures. 
Processes for the conversion of coal and other hydrocarbons such as natural 
gas 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. 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 contents of which are herein incorporated 
by reference. The techniques for gasification of coal or other solid, 
liquid or gaseous fuel are not considered to be a part of this invention. 
It is well known that synthesis gas comprising carbon monoxide and hydrogen 
will undergo conversion to form reduction products of carbon monoxide, at 
temperatures in the range of 300.degree. F. to about 850.degree. F. and 
pressures in the range of one to one thousand atmospheres, over a fairly 
wide variety of catalysts. The Fischer-Tropsch process, for example, which 
has been extensively studied, produces a range of hydrocarbons, waxy 
materials and some liquid materials which have been used as low octane 
gasoline. The types of catalysts that have been studied for this and 
related processes include those based on metals or oxides or iron, cobalt, 
nickel, ruthenium, thorium, rhodium and osmium. 
The range of catalysts and catalyst modifications disclosed in the art 
encompasses an equally wide range of conversion conditions for the 
reduction of carbon monoxide by hydrogen and provides considerable 
flexibility toward obtaining selected boiling range products. Nonetheless, 
in spite of this flexibility, it has not been possible heretofore to 
provide a catalyst for medium pressure operation (5-30 atm) which will 
produce particularly olefin compositions comprising primarily internal 
double bond characteristics and boiling in the gasoline boiling range. A 
review of the status of this art is given in "Carbon Monoxide-Hydrogen 
Reactions", Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, 
Second Edition, Volume 4, pages 446-488, Interscience Publishers, New 
York, N.Y. 
The conversion of synthesis gas to hydrocarbon mixtures is disclosed in 
copending application Ser. No. 583,353, filed June 2, 1975, and copending 
application Ser. No. 566,167, filed Apr. 4, 1975. Compositions of iron, 
cobalt or nickel deposited in the inner absorption regions of crystallize 
zeolites are described in U.S. Pat. No. 3,013,990. Attempts to convert 
synthesis gas over X-zeolite base exchanged with iron, cobalt and nickel 
are described in Erdoel und Kohle--Erdgas, Petrochemie; 
Brennstoff--Chemie, Volume 25, No. 4, pages 187-188, April 1972. 
One particularly desirable catalyst used in the conversion of syngas has 
been potassium promoted iron, which has been used in combination with 
special types of zeolites, such as ZSM-5, in order to produce valuable 
hydrocarbons. Thus, for example, copending Application Ser. No. 566,167, 
now abandoned, is directed towards the conversion of syngas with potassium 
promoted iron in admixture with HZSM-5. Although the process of this 
copending application is indeed effective in producing products having a 
substantial quantity of aromatics, nevertheless there are disadvantages 
associated with said process, primarily in the regeneration aspect of the 
catalyst. It is known that when processes of this type are operated under 
conditions which favor the production of aromatics, there are also 
produced substantial amounts of coke which are deposited about the acid 
ZSM-5 catalyst. This requires that the catalyst be subjected to frequent 
regeneration, and due to the fact that the process of said copending 
application Ser. No. 566,167 involved a catalyst mixture containing an 
iron catalyst and a ZSM-5 catalyst, the extent and amount of regeneration 
were limited by the effect that the regeneration would have on the iron 
component. Thus, although HZSM-5 by itself exhibits a remarkable stability 
with regard to regeneration of the same by burning off carbon deposits, 
the same is not true with respect to a Fischer-Tropsch catalyst in 
general, and iron promoted potassium in particular. 
In U.S. Pat. No. 4,086,262 issued Apr. 25, 1978, there is disclosed a 
process for the conversion of synthesis gas using a single stage process 
wherein the catalyst is a mixture of an iron containing Fischer-Tropsch 
catalyst and a ZSM-5 type zeolite. One of the examples, however, is 
directed towards a two bed operation wherein syngas is contacted over a 
first fixed catalyst bed containing an iron catalyst and the total product 
is thereafter contacted in a second fixed catalyst bed containing a ZSM-5 
type zeolite. The example resulted in poor aromatic production and 
excessive methane production. 
In copending Application Ser. No. 614,586, filed Sept. 18, 1975, now U.S. 
Pat. No. 4,046,830, issued Sept. 6, 1977, there is disclosed a process 
wherein the total effluent from a Fischer-Tropsch operation is upgraded 
over a ZSM-5 type zeolite. Although the process of said application is 
indeed a valuable one, it has been found that the process can be improved 
by operating within a more selective range of process conditions. Thus, 
the instant invention represents an improvement over the operation of U.S. 
Pat. No. 4,046,830 and copending Application Ser. No. 826,487 filed Aug. 
22, 1977 and now U.S. Pat. No. 4,159,995. 
SUMMARY OF THE INVENTION 
It has been discovered that a highly aromatic or a highly olefinic gasoline 
of enhanced octane number or a gasoline plus distillate mixture can be 
obtained in greater yield from synthesis gas, i.e. mixtures of hydrogen 
gas with gaseous carbon oxide or the equivalents of such mixtures 
utilizing a selected synthesis gas composition of low H.sub.2 /CO ratio in 
a relatively special Fischer-Tropsch syngas conversion operation and in a 
sequentially arranged dual reactor conversion process. In a first reactor 
of the sequence of reactors, the low H.sub.2 /CO ratio (.ltoreq.1.0) 
syngas mixture is reacted in the presence of a special Fischer-Tropsch CO 
reducing catalyst under severely restricted and preselected reaction 
conditions favoring the formation of C.sub.1 to C.sub.40 hydrocarbons and 
oxygenates. The product obtained from this first stage syngas conversion, 
all or a part thereof, is thereafter processed in a second reactor with a 
special crystalline zeolite catalyst such as HZSM-5 zeolite of desired 
acid activity (.alpha. activity &lt;90) to yield a synthetic hydrocarbon 
product wherein the methane plus ethane yield is restricted to less than 
about 20 weight percent and the C.sub.5 + hydrocarbon product fraction is 
at least 45 weight percent. A gasoline fraction boiling less than 
400.degree. F. at its 90% overhead is produced by the combination. 
The process combination of this invention allows for considerable 
flexibility with respect to both reaction conditions and product produced. 
The different catalysts used in the separate and sequentially arranged 
reactors can be used under more selective conversion conditions and each 
catalyst so used can be regenerated separately such that the process is 
capable of being operated at long on-stream times. The first reactor or 
Fischer-Tropsch syngas conversion operation is carried out under 
conditions such that coke formation is restricted. The Fischer-Tropsch 
catalyst can be separated and regenerated or replaced with fresh catalyst. 
A swing reactor system, for example, may be used for this purpose. It is 
known by those skilled in the art that regeneration conditions for a 
Fischer-Tropsch catalyst, whether iron, cobalt or other suitable CO 
reducing Fischer-Tropsch metal, differ from those necessary for 
regenerating an acidic zeolite catalyst used in the combination operation. 
The synthesis gas converted in the combination process of the invention may 
be prepared from fossil fuels by any one of the known methods, including 
in-situ gasification processes by the underground partial combustion of 
coal and petroleum deposits. The term fossil fuels, as used herein, 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 simple physical separations or more profound transformations 
of these materials, including coked coal, petroleum coke, gas oil, residue 
from petroleum distillation, coke oven gas rich in CO or any two or more 
of the foregoing materials in combination. Other carbonaceous fuels such 
as peat, wood and cellulosic waste materials also may be used. 
The raw synthesis gas produced from fossil fuels will contain various 
materials and impurities such as particulates, sulfur, methane and metal 
carbonyl compounds, and will be characterized by a hydrogen-to-carbon 
oxides ratio which will depend on the fossil fuel and the particular 
gasification technology utilized. In general, it is desirable for 
improving the efficiency of subsequent conversion steps to purify the raw 
synthesis gas by the removal of impurities and provide a relatively clean 
mixture of hydrogen and carbon oxides. Techniques for such purification 
are known and are not part of this invention. However, it is preferred to 
adjust the formed hydrogen-to-carbon oxides volume ratio such as by the 
water-gas shift reaction to provide an H.sub.2 /CO gas ratio in the range 
of from 0.5 to about 1.0 and more usually within the range of 0.6 to 0.8 
for use in this invention. Should the purified synthesis gas be 
excessively rich in carbon monoxide, 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 monoxide or 
carbon dioxide. It is also contemplated charging water with low H.sub.2 
/CO ratio gas passed to the Fischer-Tropsch operation. Purified synthesis 
gas adjusted to contain a volume ratio of hydrogen-to-carbon monoxide 
within the above defined range will be referred to as adjusted synthesis 
gas. It is contemplated obtaining such syngas mixtures preferably by low 
cost syngas generation means. 
The synthesis gas used in this invention includes art-recognized 
equivalents to the already-described mixtures of hydrogen gas with gaseous 
carbon oxides. Mixtures of carbon monoxide and steam, for example, or of 
carbon dioxide and hydrogen, to provide adjusted synthesis gas by in-situ 
reaction, are contemplated. 
The catalysts employed in the first reactor of this invention include 
Fischer-Tropsch synthesis catalysts which contain (1) hydrocarbon 
synthesis activity and (2) activity for water-gas shift reaction. The two 
basic reactions accomplished with such catalysts are shown below in 
idealized form: 
EQU 2H.sub.2 +CO.fwdarw.(CH.sub.2).sub.n +H.sub.2 O (1) 
EQU H.sub.2 O+CO.revreaction.CO.sub.2 +H.sub.2 ( 2) 
where (CH.sub.2).sub.n stands for the hydrocarbons produced. Some 
Fischer-Tropsch catalysts possess activity for accomplishing both 
reactions, such as iron containing Fischer-Tropsch catalysts. Others 
catalyze essentially only the synthesis reaction of equation (1) above, 
such as Co or Ru. Cobalt or ruthenium catalysts can be used in the present 
invention when a separate shift catalyst component is added. Examples of 
some specific shift catalysts are Fe, Cu, Zn and Cr which may be used 
alone or in combination with one another to provide shift activity to, for 
example, cobalt and ruthenium. 
When mixtures of Fischer-Tropsch synthesis and shift catalysts are used, 
their ratio is preferably chosen such that reaction (2) above identified 
occurs at an equal or greater rate than reaction (1). 
In general, synthesis catalysts used in the first reactor are recognized as 
CO reduction catalysts and include iron, ruthenium, cobalt, rhodium, 
osmium and manganese. They may contain additional promoters such as 
alkali, alkaline earth (Group II), zinc oxide, vanadia, zirconia, copper, 
etc. Preferred catalysts include potassium promoted iron Fe(K) with and 
without copper. 
Prior to syngas conversion, Fischer-Tropsch synthesis catalysts are 
generally reduced with hydrogen or hydrogen containing gas at a pressure 
from 0 psig to synthesis operating pressure. In case that a 
co-precipitated Fe-K or Fe-K-Cu catalyst is used, the preferred 
pretreatment procedure involves carbiding with low H.sub.2 /CO gas or CO 
alone at a temperature in the range of 480.degree. to 610.degree. F. at a 
pressure up to synthesis operating pressure. The catalyst can also be 
pretreated by carbiding, followed by hydrogen reduction (or nitriding with 
NH.sub.3). Alternatively, it can be pretreated by nitriding with NH.sub.3 
alone. 
Optionally, ZSM-5 zeolites in the form of H.sup.+ or K.sup.+ can be admixed 
with a CO reduction catalyst and used as a Fischer-Tropsch synthesis 
catalyst. 
The crystalline aluminosilicate component used in the second reactor is a 
special crystalline zeolite such as ZSM-5 zeolite which is characterized 
by a pore dimension greater than about 5 Angstroms, i.e. it is capable of 
sorbing paraffins, it has a silica-to-alumina ratio of at least 12 and a 
constraint index within the range of 1 to 12. Zeolite A, for example, with 
a silica-to-alumina ratio of 2.0 is not useful in this invention, and it 
has no pore dimension greater than about 5 Angstroms. 
The crystalline aluminosilicates herein referred to, also known as 
zeolites, constitute an unusual class of natural and synthetic minerals. 
They are characterized by having a rigid crystalline framework structure 
composed of an assembly of silicon and aluminum atoms, each surrounded by 
a tetrahedron of shared oxygen atoms, and a precisely defined pore 
structure. Exchangeable cations are present in the pores. 
The zeolites utilized herein exhibit some unusual properties. 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 
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 
or aluminum atoms at the centers of the tetrahedra. Briefly, the preferred 
zeolites useful in this invention have 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 catalysts in this invention freely sorb normal 
hexane and have a pore dimension greater than about 5 Angtroms. 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 (C.I.) values for some typical zeolites, including some 
not within the scope of this invention, are: 
______________________________________ 
CAS C.I. 
______________________________________ 
Erionite 38 
ZSM-5 8.3 
ZSM-11 8.7 
ZSM-35 6.0 
TMA Offretite 3.7 
ZSM-38 2.0 
ZSM-12 2 
Beta 0.6 
ZSM-4 0.5 
Acid Mordenite 0.5 
REY 0.4 
Amorphous Silica-alumina 
0.6 
______________________________________ 
The above-described Constraint Index is an important, and even critical, 
definition of those zeolites which are useful to catalyze the instant 
process. The very nature of this parameter and the recited technique by 
which it is determined, however, admit of the possibility that a given 
zeolite can be tested under somewhat different conditions and thereby have 
different constraint indexes. Constraint Index seems to vary somewhat with 
severity of operation (conversion). Therefore, it will be appreciated that 
it may be possible to so select test conditions to establish multiple 
constraint indexes for a particular given zeolite which may be both inside 
and outside the above defined range of 1 to 12. 
Thus, it should be understood that the parameter and property "Constraint 
Index" as such value is used herein is an inclusive rather than an 
exclusive value. That is, a zeolite when tested by any combination of 
conditions within the testing definition set forth hereinabove to have a 
constraint index of 1 to 12 is intended to be included in the instant 
catalyst definition regardless that the same identical zeolite tested 
under other defined conditions may give a constraint index value outside 
of 1 to 12. 
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, 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 at least two (2) different 
zeolites, designated ZSM-35 and ZSM-38, one or both of which are the 
effective material insofar as the catalysis of this invention is 
concerned. Either or all of these zeolites is considered to be within the 
scope of this invention. ZSM-35 is described in U.S. application Ser. No. 
528,061, filed Nov. 29, 1974. ZSM-38 is described in U.S. application Ser. 
No. 528,060, filed Dec. 29, 1974. 
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 in the acid form, i.e. H-ZSM-5, being particularly 
preferred. 
In a preferred aspect of this invention, the initial 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 requirements are most desired. Therefore, the preferred catalysts 
of this invention are those comprising 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 
pycnometer 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 are 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 the catalytic activity. 
Crystal framework densities of some typical zeolites, including some which 
are not within the purview 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 
______________________________________ 
An importnt criticality in the improved processing combination of the 
invention resides in the operating conditions utilized. In the first 
reactor of the sequence of reactor and comprising a CO reducing component 
with water-gas shift activity, it is proposed to charge a syngas 
comprising an H.sub.2 /CO ratio in the range of 0.5 to 1.0, and preferably 
within the range of 0.6 to 0.8, in order to achieve an overall syngas 
conversion of at least 70% and more preferably greater than 80%. The 
conversion of the syngas feed is accomplished at a temperature selected 
from within the range of 400.degree. to 600.degree. F., it being preferred 
to use a Fischer-Tropsch temperature selected from within the range of 
420.degree. to 580.degree. F. An operating pressure within the range of 1 
to 1500 psig and more preferably in the range of 100 to 800 psig is 
employed. Within the above-defined operating constraints, the syngas WHSV 
is adjusted to achieve conversion thereof equal to or greater than 70%, it 
being preferred to achieve a conversion of the syngas on a once-through 
basis equal to or greater than 80%. 
In the second reactor stage of the combination operation and comprising the 
special crystalline zeolite conversion catalyst specifically identified as 
a ZSM-5 zeolite of desired acid activity, a temperature is selected from 
within the range of 530.degree. to 850.degree. F. and more preferably from 
550.degree. to 800.degree. F. to achieve a product selectivity favoring 
gasoline or a mixture of gasoline plus distillate. These products are 
favored by a pressure of 100 to 400 psig. On the other hand, when it is 
desired to emphasize production of higher boiling distillate material, a 
temperature within the range of 450.degree. to 650.degree. F. is selected 
and the pressure is chosen between 400 and 800 psig. In general, the 
pressure in the second reactor may be equal to or below that used in the 
first reactor. On the other hand, when it is desired to produce a diesel 
type fuel, the pressure in the second zeolite catalyst stage may be higher 
than the first or Fischer-Tropsch reactor preferred pressure range. Within 
these operating constraints, the syngas product of the first 
(Fischer-Tropsch) reactor, all or partially, is charged to the zeolite 
catalyst reactor at a rate within the range of 0.2 to 30 WHSV and more 
usually within the range of 1 to 12 WHSV, depending on the severity of the 
operation desired. 
To facilitate obtaining desired and selected products by the 
above-identified combination operation, the alpha (.alpha.) activity of 
the special zeolite catalyst is maintained below about 90 and preferably 
below 50. Zeolite catalysts as prepared often have .alpha. values 
exceeding 150. A reduction in the zeolite activity may be obtained by 
steaming, increasing the Si/Al ratio during synthesis, addition of alkali, 
etc. 
Cracking activity is obtained by a standard .alpha.-test which is fully 
described in a letter to the Editor entitled "Superactive Crystalline 
Aluminosilicate Hydrocarbon Catalysts" by P. B. Weisz and J. N. Miale 
appearing in "Journal of Catalysts", Volume 4, No. 4, August 1965, pages 
527-529, which is incorporated herein by reference, except that it is 
preferred for the zeolites contemplated herein to perform the measurement 
of the .alpha.-activity at a temperature of 1000.degree. F. 
The combination operation of the invention is highly versatile for varying 
product selectivity and this versatility may be amplified to some 
considerable extent by passing all or only a portion of the product of the 
first stage syngas conversion operation in contact with the zeolite 
catalyst of the second stage operation. For example, up to about 30% of 
the heaviest or higher boiling portion of the first reactor 
Fischer-Tropsch product may be withdrawn for separate catalytic treatment 
from lower boiling product. The hydrocarbon and oxygenated product of 
Fischer-Tropsch synthesis is catalytically modified by the special zeolite 
catalyst herein identified under selected operating conditions. Within 
this operating environment, it is contemplated performing the 
Fischer-Tropsch syngas conversion operation with the Fischer-Tropsch 
catalyst suspended in a liquid medium, or maintained as a fixed catalyst 
bed restrained within long reaction tubes of restricted cross-sectional 
area and/or diameter to achieve a desired indirect heat exchange with a 
heat exchange fluid adjacent the reaction tubes, that is, internal or 
external thereto. On the other hand, the catalyst may be employed in a 
dispersed phase reaction zone provided with indirect heat exchange coils 
or as a more dense fluidized catalyst bed arrangement provided with 
indirect heat exchange coil means suitably arranged for the purpose of 
removing undesired exothermic reaction heat. 
The combination operation of the invention lends itself to a variety of 
arrangements which can be relied upon to achieve the results desired. For 
example, it is contemplated employing more than one Fischer-Tropsch 
reactor in sequential or parallel flow arrangement or a combination 
thereof which will permit recovering, for example, CO.sub.2 by hot 
carbonate wash from the products of reaction and intermediate the 
sequential reaction stages. In this operating combination of 
Fischer-Tropsch reactors, it is proposed to obtain better than 60% 
conversion of CO in at least the first reactor of the sequence of reactors 
and this may be particularly accomplished by maintaining the H.sub.2 /CO 
ratio within the low limits herein identified. 
When using the Fischer-Tropsch catalyst slurried in a liquid product of 
Fischer-Tropsch synthesis as a liquid phase Fischer-Tropsch operation in 
apparatus such as a bubble column, it is contemplated using at least two 
such columns in sequence or parallel flow arrangement or a combination 
thereof. In such liquid phase systems, a portion of the liquid medium may 
be withdrawn and separated from catalyst particles so that a slurry of 
high catalyst particle concentration can be returned to the 
Fischer-Tropsch reactor. The liquid product freed of catalyst fines may 
then be passed to the zeolite catalyst conversion stage of the operation 
with or without entrained oxygenates of the Fischer-Tropsch operation. On 
the other hand, the liquid separated from catalyst fines may be passed to 
a catalytic operation designed particularly to form middle distillates, 
diesel fuels and jet fuels. 
In a particularly preferred embodiment, the processing technology of the 
invention is directed to processing H.sub.2 /CO gas in a ratio of 0.6 to 
0.8 under processing conditions providing at least 90% syngas conversion 
to hydrocarbon and oxygenate products which are thereafter converted with 
a ZSM-5 zeolite to more desirable products. 
H.sub.2 /CO Ratio 
The H.sub.2 /CO ratio of the synthesis gas or syngas employed in a 
Fischer-Tropsch operation may be varied over a wide range. That is, in the 
Fischer-Tropsch operation, the H.sub.2 /CO ratio used can vary from 0.2 to 
6.0 as taught by U.S. Pat. No. 4,086,262 and other sources. In a 
commercial Fischer-Tropsch operation now in operation, the H.sub.2 /CO 
syngas ratio used is normally greater than about 4. 
It has now been found, however, that high rates of conversion of a syngas 
feed can be accomplished to considerable advantage by using syngas 
providing an H.sub.2 /CO ratio in the range of 0.5 to about 1.0. The 
precise stoichiometry of the Fischer-Tropsch operation is a function of 
the composition of the reaction products and particularly the methane 
content of the reaction product. For synthesis products comprising up to 
about 25% methane, 100% syngas conversion can be achieved with a CO.sub.2 
free gas of H.sub.2 /CO ratio of 0.5 to 2.3, provided the Fischer-Tropsch 
synthesis catalyst has water-gas shift activity. However, generating 
synthesis gas from coal with an H.sub.2 /CO ratio greater than 1 is very 
costly and economically not particularly attractive to the processor. 
The prior art comprises several references which show that the rate of 
syngas conversion is greatest, the higher the H.sub.2 /CO ratio gas that 
is used. This suggests that, at a given set of processing conditions, a 
high conversion of the syngas feed can be achieved only with an H.sub.2 
/CO ratio at the high end of the stoichiometric range, e.g. about H.sub.2 
/CO ratio .perspectiveto.2. 
It has now been found, however, that there are some significant advantages 
in converting syngas providing an H.sub.2 /CO ratio in the range of 0.5 to 
1. That is, it has now been found quite unexpectedly that a high H.sub.2 
/CO ratio syngas is not essential to achieve a high rate of conversion as 
previously taught. On the contrary, it was found that at a high overall 
conversion of greater than 60% the rate of conversion of the syngas feed 
is especially high when processing a low H.sub.2 /CO ratio syngas, with 
the highest conversion rate achieved with about a 0.8 H.sub.2 /CO ratio 
gas when using a Fischer-Tropsch catalyst containing water-gas shift 
characteristics under selected operating conditions. This finding is 
clearly shown by FIG. 1. The use of a low H.sub.2 /CO syngas thus has the 
particular advantage of allowing for high conversion per pass; and this 
high conversion can be achieved with a higher rate of reaction than 
possible with gases outside this range. Furthermore, it was found that 
restricting the H.sub.2 /CO ratio to a value of 1 or less has a beneficial 
effect on the product distribution as shown in the following Table 1. The 
production of methane+ethane is &lt;20% and the yield of the more valuable 
C.sub.3 + products is at least 80%. 
In addition, it was found that the yield of liquid C.sub.5 + products is at 
least 45%. 
TABLE 1 
______________________________________ 
Effect of H.sub.2 /CO) . Ratio of Product Selectivity 
(265.degree. C., 200 psig, SV = 6 liters (STP)/Hr./g Fe) 
Rate of Syngas 
Conversion 
liters (STP) of 
Product Selectivity 
H.sub.2 /CO 
syngas/Hr./g Fe 
C.sub.1 + C.sub.2 in H. C. 
______________________________________ 
0.53 4.7 9.2 
0.60 5.2 11.4 
0.68 5.5 16.0 
1.0 5.4 17.0 
2.0 4.1 30.5 
______________________________________ 
It is known that the cost of producing a low ratio syngas of about 0.5 is 
significantly less than that required for producing syngas of 1 or higher 
H.sub.2 /CO ratio. Such low ratio syngas of about 0.5 can be adjusted if 
desired to within the most preferred range of 0.6 to 0.8 H.sub.2 /CO by an 
external shift reaction. Of particular interest is the realization that 
water can be charged with the low ratio syngas without encountering 
adverse effects particularly when using a water-gas shift Fischer-Tropsch 
catalyst. 
It is recognized that the Fischer-Tropsch synthesis reactions produce 
CO.sub.2 which may or may not be separated before passing the 
Fischer-Tropsch product in contact with additional Fischer-Tropsch 
catalyst or the zeolite conversion catalyst as herein provided. Separated 
CO.sub.2 may be recycled to the coal gasification operation for reaction 
with carbon. An important aspect of the invention is the indirect 
temperature control generation of steam in the highly exothermic 
Fischer-Tropsch synthesis operation which steam source is used to supply a 
substantial portion of the heat requirements of a low cost highly 
efficient coal gasification operation including the generation of oxygen 
for use in the coal gasification operation. The operating synergism 
between the processing steps of the combination contributes measurably to 
the economic and technological advance of the combination process, and 
such technological advance grasps from the morass of normal poor quality 
Fischer-Tropsch product wide spectrum carbon compounds, premium fuels by a 
selective conversion thereof with a special zeolite known as ZSM-5 
crystalline zeolite. 
In the combination operation of this invention, the Fischer-Tropsch 
synthesis gas conversion may be carried out in one or more reactors 
similar to that disclosed in the prior art. However, the special low 
H.sub.2 /CO ratio syngas used in the present invention combination 
requires a special arrangement and operating care to avoid any large 
and/or local significant temperature increases that will lead to low 
selectivity for premium products, significant methane make or an undesired 
increased carbon deposition. Therefore, adiabatic fixed catalyst bed type 
flow reactors are unsuitable for the purpose. Useful reactor types are 
those provided with adequate heat exchange means in fixed bed catalyst 
reactors or fluid catalyst bed reactors. Reactor arrangements of 
particular advantage are those comprising suspended Fischer-Tropsch 
catalyst particles in a liquid medium and broadly referred to as liquid 
phase-catalyst reactor systems that provide excellent temperature control 
with the catalyst suspending liquid medium. On the other hand, elongated 
tubes of restricted diameter filled with fixed or suspended catalyst and 
surrounded by a liquid may be employed. 
Especially preferred is a liquid phase bubble column reactor arrangement 
with suspended finely divided catalyst particles in a liquid medium such 
as hydrocarbon product medium. The catalyst particles suspended in the 
liquid medium are preferably iron particles with or without alkali 
promoter such as potassium of small particle size up to about 100 microns 
and preferably less than 40 microns. The alkali promoter is desirable to 
inhibit methane make. 
The liquid suspending medium desirably will possess enough thermal 
stability to be stable under the reaction conditions employed. Therefore 
it is desirable to employ a high boiling portion of the reaction product. 
Other examples are anthracene oil, heavy petroleum fractions and silicon 
oil. Preferred are synthetic hydrocarbons such as the heavier fraction 
produced in the Fischer-Tropsch synthesis reaction. A synthetic 
hydrocarbon oligomer, for example the trimer, tetramer or pentamer of 
1-decene, preferably after hydrogenation, may also be employed as the 
suspending liquid medium. 
The pressure utilized in the second or zeolite catalyst conversion reactor 
will be, in general, equal to, higher or less than the pressure used in 
the first reactor, i.e., from atmospheric to about 800 psig. The space 
velocity in the second reactor will vary from 0.2 to 30 WHSV and more 
preferably from 1 to 12 WHSV. The second reactor temperature is from about 
530.degree. F. up to about 850.degree. F. and more preferably from 
550.degree. F. to 800.degree. F. In general, operating temperatures in the 
range of from about 530.degree. F. to 650.degree. F. favor the formation 
of olefinic gasoline, whereas temperatures from about 600.degree. F. to 
800.degree. F. favor the production of aromatic gasolines, i.e. greater 
than 25 weight percent aromatics in the C.sub.5 + fraction. 
In general, the products of the present invention are considered premium 
fuels. Their composition can be regulated by the composition of the charge 
and operating conditions of the zeolite catalyst second stage operation. 
In one embodiment, a most desired product is a gasoline of relatively high 
octane number of at least about 85 research clear. Conditions to produce 
gasoline of either high olefin or high aromatic contents are identified 
herein. 
It has also been found that it is possible to coproduce by two separate 
catalyst stages of this invention a distillate fraction--in addition to a 
relatively high octane gasoline of at least 80 research clear--which 
distillate has a very low pour point generally below 0.degree. F. This is 
in contrast to a heavy waxy hydrocarbon fraction (pour point &gt;70.degree. 
F.) produced by the Fischer-Tropsch reaction itself in the absence of the 
second state ZSM-5 zeolite catalyst operation. The distillate material 
recovered from the zeolite conversion operation, generlly boiling above 
gasoline, may initially boil about 320.degree. F. and is suitable for use 
as a diesel fuel or for jet fuel production. For example, the distillate 
recovered may be a 350.degree.-570.degree. F. fraction which has an API 
gravity of about 59, a cetane number of about 80 and a pour point of about 
-40.degree. F. The operating conditions used in the zeolite catalyst 
conversion stage that favor the coproduction of gasoline and distillate 
fuels differ somewhat from those used for maximizing gasoline production 
and will vary with zeolite catalyst activity. They generlly include the 
use of a lower temperature and/or a higher pressure for increasing 
distillate yield. Distillate is intended to refer to material higher 
boiling than gasoline. 
A temperature within the range of 450.degree.-650.degree. F. may be used in 
the second zeolite conversion stage for coproduction of gasoline and 
distillate at a pressure in the range of 300-1500 psig. 
The second stage or zeolite catalyst conversion operation is an exothermic 
type of operation requiring relatively moderate temperature control means, 
whereas the first stage or Fischer-Tropsch catalyst conversion operation 
is a highly exothermic conversion operation requiring much more elaborate 
temperature control means. The reactions contemplated must occur in 
relatively close temperature control environments particularly for the 
Fischer-Tropsch operation in order to realize the desired conversion 
products of the operation. The second stage exothermic operation of the 
combination may be accomplished in either a fixed or fluid bed of catalyst 
provided with adequate indirect heat exchange means. The highly exothermic 
first stage Fischer-Tropsch catalyst operation, because it is processing 
syngas H.sub.2 /CO ratio of 1 or less, and as low as 0.5 or 0.8 H.sub.2 
/CO ratio gas, requires a very close temperature control reaction system. 
Systems which may be used for this purpose not necessarily with equal 
success include (1) a fixed bed of catalyst in a limited diameter tube and 
shell reactor arrangement with indirect heat exchange fluid on the 
opposite side of the tube from the catalyst, (2) a limited movement bed of 
catalyst particles in direct contact with circulating oil and provided 
with external heat exchange means, (3) random movement of fine catalyst 
particles suspended in a liquid phase medium such as a bubble column and 
provided with internal, external heat exchange means or a combination 
thereof, and (4) a fluid bed catalyst phase operation provided with 
internal heat exchange means suitable for retaining the exothermic 
temperature of the operation within relatively narrow prescribed limits. 
The preferred two stage operation of the invention comprises the use of a 
bubble column type of reaction system containing suspended finely divided 
catalyst particles in a liquid product medium of Fischer-Tropsch synthesis 
and maintaining close desired temperature control on the highly exothermic 
Fischer-Tropsch operation with the catalyst suspending liquid medium when 
particularly processing syngas of an H.sub.2 /CO ratio in the range of 0.5 
up to about 1.0. It is preferred, however, as herein identified, to 
maintain the H.sub.2 /CO ratio from about 0.6 to about 0.8 so as to 
achieve high rates of conversion within relatively economic processing 
conditions. This bubble column with suspended Fischer-Tropsch catalyst in 
a liquid medium is most appropriate for achieving restricted temperature 
variations about any given catalyst particle and such a temperature 
restriction is necessary to severely limit coke deposition. Severely 
limiting temperature excursions in the Fischer-Tropsch operation is 
particularly desirable during operations selected to maximize gasoline 
production because of the higher temperature requirements. 
The second stage of the combination operation comprising the special 
zeolite conversion catalyst herein identified used in a fixed or fluid bed 
of catalyst is relied upon to convert a highly olefinic product of the 
first stage with or without Fischer-Tropsch formed oxygenates under 
exothermic reaction conditions suitably temperature restricted to produce 
the desired product slate herein identified. In this zeolite catalyst 
conversion operation, it is contemplated using the special zeolite 
catalyst with an .alpha. (alpha) activity as high as about 90, it being 
more usually about 70 and preferably not above about 50.alpha. activity. 
In the two-stage syngas conversion, the product from the ZSM-5 reactor can 
be further upgraded by special downstream processing. For example, 
gasoline production can be maximized by alkylating isobutane with light 
olefins and polymerizing excess olefins (if any) by catalytic 
polymerization or olefination or dimerization. Ethylene, possibly 
propylene, can be recovered for chemical use. In addition, C.sub.2 
.dbd.-C.sub.4 .dbd. olefins can be utilized for production of middle 
distillate by processing over ZSM-5 zeolites at low temperature 
(450.degree. to 650.degree. F.) and high pressure (300 to 1500 psig).