Conversion of oxygenated products of Fischer-Tropsch synthesis

Water-soluble oxygenates of Fischer-Tropsch synthesis separated from water and acids are sequentially upgraded by a dehydration catalyst and a special zeolite catalyst to produce gasoline, LPG and light fuel oil.

BACKGROUND OF THE INVENTION 
The conversion of oxygenated organic compounds to hydrocarbons including 
gasoline has been the subject of numerous prior-art disclosures. That is, 
U.S. Pat. No. 3,928,483 issued Dec. 23, 1975 discloses a process for the 
production of aromatic rich gasoline boiling range hydrocarbons from lower 
alcohols such as methanol, ethanol, propanol and corresponding ethers. In 
this patent, the process is carried out in two or more stages wherein the 
alcohol or ether is contacted with a condensation catalyst to produce 
aliphatic dehydration products and water. The dehydration product is 
thereafter converted to gasoline boiling hydrocarbon by contact with a 
special crystalline aluminosilicate zeolite providing a silica-to-alumina 
ratio greater than 12, a constraint index within the range of 1 to 12 and 
a dried crystal density of not less than about 1.6 grams per cubic 
centimeter. A ZSM-5 crystalline zeolite is representative of the special 
class of zeolite providing the above defining characteristics. U.S. Pat. 
No. 3,907,915 is directed to the conversion of aliphatic carbonyl 
containing compounds with the special zeolite above defined. U.S. Pat. No. 
3,998,898 is directed to converting a mixture of a difficult to convert 
aliphatic organic compound in combination with easily converted aliphatic 
alcohols, esters, acetals and analogs thereof over the special crystalline 
zeolite above defined to produce highly aromatic gasoline hydrocarbons and 
light aliphatic hydrocarbons. 
SUMMARY OF THE INVENTION 
The present invention is directed to an improved method and combination of 
processing steps for converting a wide spectrum of oxygenated products and 
particularly the water soluble oxygenates obtained from a Fischer-Tropsch 
operation to valuable hydrocarbon products including gaseous and gasoline 
boiling range liquid hydrocarbons. The liquid hydrocarbons formed by the 
process may comprise high octane gasoline and/or good quality light or 
middle distillate. 
The improved process of this invention generally involves collecting and 
passing the mixed water soluble oxygenates of a Fischer-Tropsch syngas 
conversion operation comprising low and higher boiling alcohols, ethers, 
aldehydes, ketones, acids and water, after separation of acids and some 
water, in contact with a dehydration catalyst under selectively restricted 
adiabatic temperature conditions to achieve at least 25% dehydration 
conversion of the feed and thereafter processing all or a part of the 
product of dehydration over a special crystalline aluminosilicate defined 
below to produce premium fuels comprising gases, high octane C.sub.5.sup.+ 
gasoline and/or some distillate boiling range hydrocarbons. In this 
combination operation, the dehydration of the charged oxygenates is 
important and effected under conditions to achieve an elevated conversion 
level normally falling short of complete conversion dehydration of the 
oxygenates. Thereafter a water phase and unconverted oxygenates are 
separated from converted material. The unconverted oxygenates and the 
water phase are thereafter recycled to the primary distillation zone 
upstream of the dehydration zone. The dehydrated oxygenates separated and 
substantially water free are thereafter converted by a special zeolite 
catalyst herein described in a separate conversion zone. 
The combination process of the invention achieves significant advantages at 
least with respect to the zeolite catalyst life by substantially reducing 
the amount of water and unconverted oxygenates contacting the zeolite 
catalyst. The special zeolite catalytic conversion of the water-free 
dehydrated oxygenates is more efficient, since the necessity for quenching 
of the zeolite catalyst conversion operation is virtually eliminated and 
this gasoline-components-forming operation can thus be sustained at a more 
optimum higher gasoline forming temperature. 
The processing combination of the invention is particularly concerned with 
processing C.sub.2.sup.+ oxygenates of a Fischer-Tropsch syng as 
conversion operation and dehydration products thereof, since conversion of 
such material will provide more aromatics and gasoline boiling compounds 
than can be derived from converting methanol. The charged C.sub.2.sup.+ 
oxygenate stream following dehydration will contain less water than is 
normally found in crude methanol and therefore any high temperature 
steaming of the special crystalline zeolite catalyst will be 
proportionately less. Thus the significant advantages of the processing 
combination reside in operating the gasoline component forming stage 
independently of the initial dehydration state, and any unconverted 
(undehydrated) oxygenates can be separated and recycled to the 
distillation operation upstream of the dehydration zone. 
The crystalline aluminosilicate component used 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 zeolite 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 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 (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, and 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 at least 
two 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. Pat. No. 4,016,245 
(U.S. application Ser. No. 528,061, filed Nov. 29, 1974 as 
continuation-in-part of Ser. No. 393,767, a continuation-in-part of Ser. 
No. 358,192) ZSM-38 is described in U.S. application Ser. No. 528,060, 
filed Dec. 29, 1974, now U.S. Pat. No. 4,046,859. 
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 
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DISCUSSION OF SPECIFIC EMBODIMENTS 
Table 1 below identifies a typical oxygenated product stream of a 
Fischer-Tropsch syngas conversion operation. 
TABLE 1 
______________________________________ 
Oxygenated Product of Fischer-Tropsch Synthesis 
Components wt. % 
______________________________________ 
H.sub.2 O 15.0 
Acetaldehyde 2.4 
MEOH 5.1 
ETOH 44.3 
Acetone + C.sub.3 Alde. 12.2 
i-C.sub.3 OH 3.8 
1-C.sub.3 OH 6.2 
MEK + C.sub.4 Alde. 4.0 
2-C.sub.4 OH .5 
2Me-1-C.sub.3 OH .5 
C.sub.5 Ketones 1.0 
1-C.sub.4 OH 2.9 
C.sub.5 Alcohols 1.5 
C.sub.6.sup.+ Oxygenates 
.4 
100.0 
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Table 2 below identifies the boiling points of the major oxygenated 
components of the charge materials and their dehydration products. It will 
be noted from Table 2 that at the indicated "Proposed Cut Point" the 
unconverted portion of the charge will separate with the water phase and 
the dehydration product may be separated as vaporous material from a 
separation zone maintained under the identified temperature and pressure 
conditions. This vapor/liquid separation operation is maintained 
independent of the initial reactor dehydrating operating conditions. 
TABLE 2 
______________________________________ 
Boiling Point at 100 PSIA 
______________________________________ 
C.sub.2 = -79.9.degree. F. 
C.sub.3 = 42.3 
Dimethyl Ether 70.0 
1-C.sub.4 = 133. 
Acetaldehyde 168. 
1-C.sub.5 = 212. 
Proposed Cut Point 
Propionaldehyde 246. 
Methanol 252. 
Acetone 258. 
Ethanol 276 
Isopropanol 281 
1-C.sub.6 = 284 
n-Propanol 318 
Methyl Ethyl Ketone 
322 
Water 328 
______________________________________ 
Table 3 below identifies the product distribution obtained by processing 
the feed stream of Table 1 over the zeolite catalyst to obtain gasoline 
boiling components. 
TABLE 3 
______________________________________ 
Example 
Start After 
of Run 
4 Days 
______________________________________ 
Mass Recovery wt. % 
H.sub.2 O 47.9 47.0 
CO -- -- 
CO.sub.2 0.1 .2 
Hydrocarbons 52.0 52.8 
Hydrocarbon Product Distribution wt. % 
C.sub.1 -- 0.1 
C.sub.2 0.5 0.7 
C.sub.3 8.5 5.0 
C.sub.4 14.1 11.8 
C.sub.5 11.6 13.2 
C.sub.6.sup.+ Paraffins, Olefin, Naphthenes 
16.2 31.1 
Benzene 0.4 0.7 
Toluene 7.4 4.0 
Ethylbenzene 2.6 1.4 
Xylenes 12.3 7.2 
C.sub.9 Aromatics 15.0 13.4 
C.sub.10 Aromatics 6.9 7.4 
C.sub.11 Aromatics 3.1 2.9 
Naphthalenes .2 .2 
Unknown Hydrocarbons 1.2 0.9 
100.0 100.0 
C.sub.5.sup.+ wt. % 76.9 82.4 
C.sub.5.sup.+ R + 0 98.7 97.7 
C.sub.6.sup.+ R + 0 101.3 99.1 
______________________________________ 
The drawing is a schematic showing of the processing arrangement of this 
invention comprising a primary distillation zone, a dehydration zone, a 
dehydrated product separation zone, a crystalline conversion zone and a 
product separation zone.

Referring now to the drawing by way of example, a stream of oxygenated 
products and water separated from the product of a Fischer-Tropsch syngas 
conversion operation is charged to the process of this invention by 
conduit 2 to a distillation column or zone 4 maintained at a temperature 
and a pressure selected to achieve separation of water and acids from the 
remaining oxygenates. In distillation zone 4, a separation is made in the 
presence of relatively large amounts of water, a water phase and acids 
withdrawn from the bottom of the zone by conduit 6, with the remaining 
oxygenates and water being recovered from the top thereof by conduit 8. 
The oxygenates and water in conduit 8 are heated in heat exchanger 10 to 
an elevated temperature within the range of 600.degree. to 1100.degree. F. 
and preferably about 900.degree. F. before being passed in contact with a 
dehydrating catalyst in zone 12. A portion of the material in conduit 8 
may be passed to vent as shown when required. In dehydrating zone 12, the 
oxygenates and retained water in conduit 8 are passed in contact with a 
dehydration catalyst suitable for the purpose. Several different 
dehydrating catalysts have been identified in the prior art, but it is 
preferred to employ gamma alumina for this purpose. Dehydration zone 12 is 
maintained under temperature conditions which will achieve a high 
conversion of the oxygenates to a dehydrated product suitable for passing 
upon recovery in contact with the gasoline forming special zeolite 
catalyst. Table 4 below identifies conditions which may be employed to 
achieve a desired conversion of dehydrated oxygenates preferably to within 
the range of 25 to 100%. 
TABLE 4 
______________________________________ 
ENDOTHERMIC HEATS OF DEHYDRATION AT 700.degree. F. 
ROH ---&gt; R.sup.= + H.sub.2 O 
Alcohol -H Kcal/Mole -H cal/gm 
______________________________________ 
Ethanol 11.20 243 
n-Propanol 8.93 149 
i-Propanol 12.15 203 
n-Butanol.sup.1 
5.37 72 
n-Pentanol.sup.2 
4.19 48 
n-Hexanol.sup.3 
4.31 42 
______________________________________ 
.sup.1 Olefin product taken as t2-butene 
.sup.2 Olefin product taken as 2M2-butene 
.sup.3 Olefin product taken as 
ESTIMATED ADIABATIC TEMPERATURE DROP 
FOR 900.degree. F. INLET 
Charge Mole Conversion .sup.T Adiabatic 
Composition 
% % .degree.F. 
______________________________________ 
Ethanol 84.3 
n-Propanol 8.3 100 420 
i-Propanol 3.0 * 50 575 
n-Butanol 3.4 25 745 
n-Pentanol 1.0 
______________________________________ 
.sup.T Adiabatic = 700.degree. F. at 31% conversion 
*Charge composition of each conversion level 
The product of the dehydration operation and comprising dehydrated 
oxygenates, water and unconverted oxygenates (not dehydrated) is passed by 
conduit 14 to a separation zone 16 maintained at a selected temperature 
and pressure designed to achieve a separation of dehydration product 
commensurate with a separation shown, for example, by Table 2 above. Thus 
a separation is made in zone 16 under selected temperature and pressure 
conditions which will achieve the recovery of water and unconverted 
oxygenates withdrawn by conduit 18 for recycle to the distillation zone 4. 
Separation of water from unconverted oxygenates before recycle is not 
essential. Light olefins and other dehydration products such as herein 
identified are recovered from separation zone 16 by conduit 20 for passage 
to heater 22 wherein the temperature of the light olefin stream is raised 
before contacting the special zeolite catalyst herein identified in zone 
24. In zone 24, the temperature is maintained within the range of 
400.degree. to 900.degree. F. and a pressure within the range of 
atmospheric to 1000 psig. In this special zeolite catalyst contacting 
operation, the light olefins and other products of dehydration are 
converted to premium fuel products of high octane value including C.sub.4 
to C.sub.10 olefin and aromatic boiling components and of product 
selectivity such as defined in Table 3 above. However, by using the 
combination of relatively low temperatures and high pressures, the product 
selectivity may be altered to improve the yield of middle distillate. 
The product of the zeolite catalyst conversion step is passed by conduit 26 
from zone 24 to a separator zone 28 wherein a separation is made in one 
embodiment to recover C.sub.5.sup.+ gasoline product of about 98 R+0 
octane which is withdrawn by conduit 30. Primarily C.sub.4 and lower 
boiling material is withdrawn by conduit 32. 
All or a portion of the separated C.sub.4 and lower boiling material may be 
recovered for further treatment in downstream equipment not shown. For 
example, a portion of this material after separation in appropriate 
equipment may be used as feed to an alkylation zone, a polymerization zone 
or a combination thereof. On the other hand, a portion of this material 
may be used to provide LPG or a fuel gas used to provide the heat 
requirements of the process. 
In yet another embodiment, it is contemplated passing all of the C.sub.4 
and lighter material through a compression zone 34 to raise the pressure 
therein sufficient for recycle by conduit 36 and admixture with the 
dehydrated feed in conduit 20 charged to heater 22. Recycle of the C.sub.4 
and lower boiling hydrocarbons recovered as above discussed is intended to 
take advantage of the restructuring characteristics of the special zeolite 
to produce longer chain olefins and/or aromatics, depending upon the 
particular combination of temperature and pressure operating conditions 
selected. 
Having thus generally described the method and processing combination of 
this invention and provided specific examples 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.