Note: Descriptions are shown in the official language in which they were submitted.
CA 02479408 2004-10-O1
SYNT~iETIC JET FUEL AND
PROCESS FOR ITS PRODUCTION
That this application is a division of application number 2,277,974, filed in
Canada on
July 21, 1999 (International Filing Date: January 27, 1998, under
PCT/US98101669).
FIELD OF THE INVENTION
'This invention relates to a distillate material having excellent
suitability as a jet fuel with high lubricity or as a blending stock therefor,
as well
as the process for preparing the jet fuel. More particularly, this invention
relates
to a process for preparing jet fuel from a Fischer-Tropsch wax.
BACKGROUND OF THE INVENTION
Clean distillates streams that contain no or nil sulfur, nitrogen, or
aromatics, are, or will likely be in great demand as jet fuel or in blending
jet fuel.
Clean distillates having relatively high lubricity and stability are
particularly
valuable. Typical petroleum derived distillates are not clean, in that they
typically contain significant amounts of sulfur, nitrogen, and aromatics. In
addition, the severe hydrotreating needed to produce fuels of sufficient
stability
often results in a fuel with poor lubricity characteristics. These petroleum
derived clean distillates produced through severe hydrotreating involve
significantly greater expense than unhydrotreated fuels. Fuel lubricity,
required
for the e~cient operation of the fuel delivery system, can be improved by the
use of approved additive packages. The production of clean, high cetane number
distillates from Fischer-Tropsch waxes has been discussed in the open
literature,
but the processes disclosed for preparing such distillates also leave the
distillate
lacking in one or more important properties, e.g., lubricity. The Fischer-
Tropsch
distillates disclosed, therefore, require blending~with other less desirable
stocks
or the use of costly additives. These earlier schemes disclose hydrotreating
the
total Fischer-Tropsch product, including the entire 700°F- fraction.
This hydro-
treating results in the complete elimination of oxygenates from the jet fuel.
By virtue of this present invention small amounts of oxygenates are
retained, the resulting product having high lubricity. This product is useful
as a
jet fuel as such, or as a blending stock for preparing jet fuels from other
lower
.grade material.
CA 02479408 2004-10-O1
SUMMARY OF THE INVENTION
In accordance with this invention, a clean distillate useful as a jet
fuel or as a jet fuel blend stock and having lubricity, as measured by the
Ball on
Cylinder (BOCLE) test, approximately equivalent to, or better than, the high
lubricity reference fuel is produced, preferably from a Fischer-Tropsch wax
and
preferably derived from cobalt or ruthenium cataiysts, by separating the waxy
product into a heavier fraction and a lighter fraction; the nominal separation
being, for example, at about 700°F. Thus, the heavier fraction contains
primarily
700°F+, and the lighter fraction contains primarily 700°F-.
The distillate is produced by further separating the lighter fraction
into at least two other fractions: (i} one of which contains primary C~.,2
alcohols
and (ii) one of which does not contain such alcohols. The fraction (ii) is a
550°F+ fraction, preferably a 500°F+ fraction, more preferably a
475°F+
fraction, and still more preferably a n-C,4+ fraction. At least a portion,
preferably the whole of this heavier fraction (ii), is subjected to
hydroconversion
(e.g., hydroisomerization) in the presence of a bi-functional catalyst at
typical
hydroisomerization conditions. The hydroisomerization of this fraction may
occur separately or in the same reaction zone as the hydroisomerization of the
Fischer-Tropsch wax (i.e., the heavier 700°F+ fraction obtained
from the
Fischer-Tropsch reaction) preferably in the same zone. In any event, a portion
of
the, for example, 475°F+ material is converted to a lower boiling
fraction, e.g.,
475°F- material. Subsequently, at least a portion and preferably all of
the
material compatible with jet freeze from hydroisomerization is combined with
at
least a portion and preferably all of the fraction (i) which is preferably a
250-475
°F fraction, and is further preferably characterized by the absence of
any
hydroprocessing, e.g., hydroisomerization. The jet fuel or jet fuel blending
component of this invention boils in the range of jet fuels and may contain
hydrocarbon materials boiling above the jet fuel range to the extent that
these
additional materials are compatible with the jet freeze specification, i.e., -
47°C
or lower. The amount of these so-called compatible materials depends on the
degree of conversion in the hydroisomerization zone, with more
hydroisomerization leading to more of the compatible materials, i.e., more
highly
branched materials. Thus, the jet fuel range is nominally 250-550°F,
preferably
CA 02479408 2004-10-O1
_3e
250-500°F, more preferably DSO-475°F and may include the
compatible
materials, and having the properties described below.
The jet material recovered from the fractionator has the properties
shown in the following table:
paraffins at least 95 wt%, preferably at Ieast 96 wt%, more
preferably at least 97 wt%, still more preferably at
least 98 wt%
iso/normal ratio about 0.3 to 3.0, preferably 0.7-2.0
sulfur < 50 ppm (wt), preferably nil
nitrogen < 50 ppm (wt), preferably C 20 ppm, more
preferably nil
unsaturates < 2.0 wt%, preferably < 1.0 wt%, most preferably
(olefins and aromatics) < 0.5 wt%
oxygenates about 0.005 to less than about 0.5 wt% oxygen,
water free basis
The iso-paraffins are normally mono-methyl branched, and since
the process utilizes Fischer-Tropsch wax, the product contains nil cyclic
paraffms, e.g., no cyclohexane.
The oxygenates are contained essentially, e.g., > 95% of
oxygenates, in the lighter fraction, e.g., the 250-475°F fraction, and
are
primarily, e.g., > 95%, terminal, linear alcohols of C~ to C12.
CA 02479408 2004-10-O1
-3a-
In a further aspect of the present invention there is
provided a process for producing a jet fuel comprising (a)
separating a product of a Fischer-Tropsch process into a heavier
fraction and a lighter fraction; (b) further separating the
lighter fraction inta at least two fractions, (i) at least one
fraction containing primary C7 - C12 alcohols and having an end
point which excludes essentially all n-C14 paraffins and (ii) one
or more other fractions; (c) hydroisomerizing at least a portion
of the heavier fraction of step (a) at hydroisomerization
conditions and recovering a 700~F- fraction; and (d) blending at
least a portion of the fraction (b)(i) with at least a portion of
the 7001~F- fraction recovered in step (c) to form a recovered
product wherein the recavered product of step (d) contains 0.01 to
0.5 wt~ oxygen, water free basis.
DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic of a process in accordance with
this invention.
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DESCRIPTION OF PREFERRED EMBODIMENTS
A more detailed description of this invention may be had by
referring to the drawing. Synthesis gas, hydrogen and carbon monoxide, in an
appropriate ratio, contained in line 1 is fed to a Fischer-Tropsch reactor 2,
preferably a slurry reactor and product is recovered in lines 3 and 4,
700°F+ and
700°F- respectively. The lighter fraction goes through a hot separator
6 and a
475-700°F fraction is recovered in line 8, while a 475°F-
fraction is recovered in
line 7. The 475-700°F fraction is then recombined with the
700+°F material
from line 3 and fed into the hydroisomerization reactor where a percentage,
typically about 50%, is converted to 700°F- material. The 475°F-
material goes
through cold separator 9 from which C4- gases are recovered in line I0. A CS-
475°F fraction is recovered in line 1 l and is combined with the output
from the
hydroisomerization reactor, 5, in line 12.
Line 12 is sent to a distillation tower 13 where a C4-250°F
naphtha
stream line 16, a 250-475°F jet fuel line 15, a 475-700°F diesel
fi;~i line 18, and a
700 ° F+ material is produced. The 700 ° F+ material may be
recycled back via line 14 to
the hydroisomerization reactor 5 or used as to prepare high quality tube base
oils. Preferably, the split between lines 15 and I S is adjusted upwards from
475 °
F if the hydroisomerization reactor, 5, converts essentially alI of the n-C,4+
paraffins to isoparaffins. 'This cut point is preferably 500°F, most
preferably
550°F, as long as jet freeze point is preserved at least at -
47°C.
The hydroisomerization process is well known and the table below
lists some broad and preferred conditions for this step.
Condition Broad Range Preferred Range
temperature, °F 300-800 500-750
total pressure, psig 300-2500 500-1500
hydrogen treat rate, SCFB 5~-5000 1500-4000
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- ~J -
While virtually any bi-functional catalysts consisting of metal
hydrogenation component and an acidic component useful in hydroprocessing
(e.g., hydroisomerization or selective hydrocracking) may be satisfactory for
this
step, some catalysts perform better than others and are preferred. For
example,
catalysts containing a supported Group VIII noble metal (e.g., platinum or
palladium) are useful as are catalysts containing one or more Group VIII non-
noble metals (e.g., nickel, cobalt) in amounts of 0.5-20 wt%, which may or may
not also include a Group VI metals (e.g., molybdenum) in amounts of 1.0-20
wt%. The support for the metals can be any refractory oxide or zeolite or
mixtures thereof. Preferred supports include silica, alumina, silica-alumina,
silica-alumina phosphates, titanic, zirconia, vanadia and other Group III, IV,
VA
or VI oxides, as well as Y sieves, such as ultrastable Y sieves. Preferred
supports include alumina and silica-alumina.
A preferred catalyst has a surface area in the range of about 200-
500 m2/gm, preferably 0.35 to 0.80 m.~/gm, as determined by water adsorption,
and a bulls density of about 0.5-1.0 g/m~.
This catalyst comprises a non-noble Group VIII metal, e.g., iron,
nickel, in conjunction with a Group IB metal, e.g., copper, supported on an
acidic support. The support is preferably an amorphous silica-alumina where
the
alumina is present in amounts of less than about 50 wt°/~, preferably 5-
30 wt%,
more preferably 10-20 wt%. Also, the support may contain small amounts , e.g.,
20-30 wt%, of a binder, e.g., alumina, silica, Group IVA metal oxides, and
various types of clays, magnesia, etc., preferably alumina.
The preparation of amorphous silica-alurnina microspheres has
been described in Ryland, Lloyd B., Tamele, M.W., and Wilson, J.N., Cracking
Catalysts, Catalysis: volume VII, Ed. Paul H. Emmett, Reinhold Publishing
Corporation, New York, 1960, pp. 5-9.
The catalyst is prepared by co-impregnating the metals from solu-
tions onto the support, drying at 100-150°C, and calcining in air at
200-550°C.
The Group VIII metal is present in amounts of about 15 wt% or
less, preferably 1-12 wt%, while the Group iB metal is usually present in
lesser
CA 02479408 2004-10-O1
_6_
amounts, e.g., 1:2 to about 1:20 ratio respecting the Group VIII metal. A
typical
catalyst is shown below:
Ni, wt% 2.5-3.5
Cu, wt% 0.25-0.35
A1203-Si42 65-75
A1s03 (binder) 25-30
Surface Area 290-325 m2/gm
Pore Volume (Hg)0.35-0.45 mLlgm
Bulk Density 0.58-0.68 g/mL
The 700°F+ conversion to 700°F- ranges from about 20-80%,
preferably 20-70%, mare preferably about 30-60%. During hydroisomerization,
essentially all olefins and oxygen containing materials are hydrogenated. In
addition, most linear paraffins are isomerized or cracked, resulting in a
large
improvement in cold temperature properties such as jet freeze point.
The separation of the 700°F- stream into a CS-4.75°F stream
and a
475-700°F stream and the hydroisomerization of 475-700°F stream
leads, as
mentioned, to improved freeze point in the product. Additionally, however, the
oxygen containing compounds in the CS-47~°F have the eflfect of
improving the
Iubricity of the resulting jet fuel, and can improve the lubricity of
conventionally
produced jet fuels when used as a blending stock.
The preferred Fischer-Tropsch process is one that utilizes a non-
shifting (that is, no water gas shift capability) catalyst, such as cobalt or
ruthenium or mixtures thereofy preferably cobalt, and preferably a promoted
cobalt, the promoter being zirconium or rhenium, preferably rhenium. Such
catalysts are well known and a preferred catalyst is described in U.S. Patent
No.
4,568,663 as well as European Patent 0 266 898.
The products of the Fischer-Tropsch process are primarily
paraffinic hydrocarbons. Ruthenium produces paraffms primarily boiling in the
distillate range, i.e., Clo-C2o; while cobalt catalysts generally produce more
of
heavier hydrocarbons, e.g., C2a+, and cobalt is a preferred Fischer-Tropsch
catalytic metal.
CA 02479408 2004-10-O1
-' 7
Good jet fuels generally have the properties of high smoke point,
low freeze point, high lubricity, oxidative stability, and physical properties
compatible with jet fuel specifications.
The product of this invention can be used as a jet fuel, per se, or
blended with other less desirable petroleum or hydrocarbon containing feeds of
about the same boiling range. When used as a blend, the product of this
invention can be used in relatively minor amounts, e.g., 10% or more, for
significantly improving the final blended jet product. Although, the product
of
this invention will improve almost any jet product, it is especially desirable
to
blend this product with refiner jet streams of low quality, particuiariy those
with high aromatic contents.
By virtue of using the Fischer-Tropsch process, the recovered
distillate has essentially nil sulftu and nitrogen. These hetero-atom
compounds
are poisons for Fischer-Tropsch catalysts and are removed from the methane
containing natural gas that is a convenient feed for the Fischer-Tropsch
process.
Sulfur and nitrogen containing compounds are, in any event, in exceedingly low
concentrations in natural gas. Further, the process does not make aromatics,
or
as usually operated, virtually no aromatics are produced. Some olefins are
produced since one of the proposed pathways for the production of paraffms is
through an olefinic intermediate. Nevertheless, olefin concentration is
usually
quite low.
Oxygenated compounds including alcohols and some acids are
produced during Fischer-Tropsch processing, but in at least one well known
process, oxygenates and unsaturates are completely eliminated from the product
by hydrotreating. See, for example, the Shell Middle Distillate Process,
Eiler, J.,
Posthuma, S.A., Sie, S.T., Catalysis Letters, 1990, 7, 253-270.
We have found, however, that small amounts of oxygenates,
preferably alcohols, provide exceptional lubricity for jet fuels. Far example,
as
illustrations will show, a highly paraffinic jet fuel with small amounts of
oxygenates has excellent lubricity as shown by the B4CLE test (ball on
cylinder
lubricity evaluator). However, when the oxygenates were not present, for
example, by extraction, absorption over molecular sieves, hydroprocessing,
etc.,
CA 02479408 2004-10-O1
to a level of less than 10 ppm wt oxygen {water free basis) in the fraction
being
tested , the lubriciiy was quite poor.
By virtue of the processing scheme disclosed in this invention a
part of the lighter, 700°F- fraction, i.e., the 250°F-
475°F fraction is not subjected
to any hydrotreating. In the absence of hydrotreating of this fraction, the
small
amount of oxygenates, primarily linear alcohols, in this fraction are
preserved,
while oxygenates in the heavier fraction are eliminated during the hydro-
isomerization step. The valuable oxygen containing compounds, for lubricity
purposes, are C~+, preferably C~-C,2, and more preferably C9-C12 primary
alcohols are in the untreated 250-475°F fraction. Hydroisomerization
also serves
to increase the amount of iso- paraffins in the distillate fuel and helps the
fuel to
meet freeze point specifications.
The oxygen compounds that are believed to promote Iubricity may
be described as having a hydrogen bonding energy greater than the bonding
energy of hydrocarbons (these energy measurements for various compounds are
available in standard references); the greater the difference, the greater the
lubricity effect. The oxygen compounds also have a lipophilic end and a
hydrophilic end to show wetting of the fuel.
While acids are oxygen containing compounds, acids are corrosive
and are produced in quite small amounts during Fischer-Tropsch processing at
non-shift conditions. Acids are also di-oxygenates as opposed to the preferred
mono-oxygenates illustrated by the linear alcohols. Thus, di- or poly-
oxygenates
are usually undetectable by infra red measurements and are, e.g., less than
about
15 wppm oxygen as oxygen.
Non-shifting Fischer-Tropsch reactions are well known to those
skilled in the art and may be characterized by conditions that minimize the
formation of C02 by products. These conditions can be achieved by a variety of
methods, including one or more of the following: operating at relatively low
CO
partial pressures, that is, operating at hydrogen to C~ rarios of at least
about
1.7/1, preferably about 1.711 to about 2.511, more preferably at least about
1.9/1,
and in the range 1.9/1 to about 2.311, all with an alpha of at least about
0.88,
preferably at least about 0.91; temperatures of about 175-225°C,
preferably 180-
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,:9-
220°C; using catalysts c~mprising cobalt or ruthenium as the primary
Fischer-
Tropsch catalysis agent.
The amount of oxygenates present, as oxygen on a water free basis
is relatively small to achieve the desired lubricity, i.e., at least about
0.01 wt%
oxygen (water free basis), preferably 0.01-0.5 wt% oxygen (water free basis),
more preferably 0.02-0.3 wt% oxygen (water free basis).
The following examples will serve to illustrate, but not limit this
invention.
Hydrogen and carbon monoxide synthesis gas (H2:C0 2.11-2.16)
were converted to heavy paraffins in a slurry Fischer-Tropsch reactor. The
catalyst utilized for the Fischer-Tropsch reaction was a titanic supported
cobaltlrhenium catalyst previously described in U.S. Patent 4,568,663. The
reaction conditions were 422-4.28°F, 287-289 psig, and a linear
velocity of 12 to
17.5 cmlsec. The alpha of the Fischer-Tropsch synthesis step was 0.92. The
paraffinic Fischer-Tropsch product was then isolated in three nominally
different
boiling streams, separated utilizing a rough flash. The three approximate
boiling
fractions were: 1) the CS-500°F boiling fraction, designated below as F-
T Cold
separator Liquids; 2) the 500-700°F boiling fraction designated below
as F-T
Hot Separator Liquids; and 3) the 700°F+ boiling fractian designated
below at
F-T Reactor Wax.
Example 1
Seventy wt% of a Hydroisomerized F-T Reactor Wax, 16.8 wt%
Hydrotreated F-T Cold Separator Liquids and 13.2 wt% Hydrotreated F-T Hot
Separator Liquids were combined and rigorously mixed. Jet Fuel A was the 250-
475°F boiling fraction of this blend, as isolated by distillation, and
was prepared
as follows: the hydroisomerized F-T Reactor Wax was prepared in flow
through, fixed bed unit using a cobalt and molybdenum promoted amorphous
silica-alumina catalyst, as described in U.S. Patent 5,292,989 and U.S. Patent
5,378,348. Hydroisomerization conditions were 708°F, 750 psig H2, 2500
SCFB H2, and a liquid hourly space velocity (LHSV~ of 0.7-0.8. Hydrotreated
F-T Cold and Hot Separator Liquid were prepared using a flow through fixed
CA 02479408 2004-10-O1
- 1~-
bed reactor and commercial massive nickel catalyst. Hydrotreating conditions
were 450°F, 430 psig HZ, 1000 SCFB H2, and 3.0 LHSV. Fuel A is
representative of a typical of a completely hydrotreated cobalt derived
Fischer-
Tropsch jet fuel, well known in the art.
Example 2
Seventy Eight wt% of a Hydroisomerized F-T Reactor Wax,
12 wt% Unhydrotreated F-T Cold Separator Liquids, and I O wt% F-T Hot
Separator Liquids were combined and mixed. Set Fuel B was the 250-
475°F
boiling fraction of this blend, as isolated by distillation, and was prepared
as
follows: the Hydroisomerized F-T Reactor Wax was prepared in flow through,
fixed bed unit using a cobalt and molybdenum promoted amorphous silica-
alumina catalyst, as described in U.S. Patent 5,292,989 and U.S. Patent
5,378,348. Hydroisomerization condirions were fi90°F, 725 psig H2, 2500
SCFB H2, and a liquid hourly space velocity (LHSV) of O.b-0.7. Fuel B is a
representative example of this invention.
Example 3
To measure the lubricity of this invention against commercial jet
fuel in use today, and its effect in blends with commercial jet fuel the
following
fuels were tested. Fuel C is a commercially obtained U. S. Jet fuel meeting
commercial jet fuel specifications which has been treated by passing it over
adapulgous clay to remove impurities. Fuel D is a mixture of 40% Fuel A
(Hydrotreated F-T Jet) and b0% of Fuel C (US Commercial Jet). Fuel E is a
mixture of 40% Fuei B (this invention) and 60% of Fuel C (US Commercial Jet}.
Example 4
Fuel A from Example 1 was additized with model compound
alcohols found in Fuel B of this invention as follows: Fuel F is Fuel A with
0.5% by weight of I-Heptanol. Fuel G is Fuel A with 0.5% by weight of 1-
Dodecanol. Fuel H is Fuel A with 0.05% by weight of I-Hexadecanol. Fuel I is
Fuel A with 0.2% by weight of 1-Hexadecanol. Fuel J is Fuel A with 0.5% by
weight of 1-Hexadecanol.
CA 02479408 2004-10-O1
~1~~
Example 5
Jet Fuels A-E were ail tested using a standard Scuffing Load Ball
on Cylinder Lubricity Evaluation (BOCLE or SLBOCLE), further described as
Lacey, P. I. "The U.S. Army Scuffang Load Wear Test", January 1, 1994. This
test is based on ASTM D 5001. Results are reported in Table 2 as percents of
Reference Fuel 2, described in Lacey, and in absolute grams of load to
scuffing.
TABLE 1
Scuffing BOCLE results for Fuels A-E. Results reported
as absolute scuffing loads and percents of Reference Fuel 2
as described in the above reference.
Jet Fuel Scuffing_Load % Reference Fuel
2
A 1300 19%
B 2100 34%
C 1600 23%
D 1400 21%
E 2100 33%
The completely hydrotreated Jet Fuel A, exhibits very low lubricity
typical of an all paraffin jet fuel. Jet Fuel B, which contains a high level
of
oxygenates as linear, C~-C 14 primary alcohols, exhibits significantly
superior
lubricity properties. Jet fuel C, which is a commercially obtained U. S. Jet
Fuel
exhibits slightly better lubricity than Fuel A, but is not equivalent to fuel
B of
this invention. Fuels D and E show the effects of blending Fuel B of this
invention. For Fuel D, the low lubricity Fuel A combined with Fuel C, produces
a Fuel with lubricity between the two components as expected, and
significantly
poorer than the F-T fuel of this invention. By adding Fuel B to Fuel C as in
Fuel
E, lubricity of the poorer commercial fuel is improved to the same level as
Fuel
B, even though Fuel B is only 40% of the final mixture. This demonstrates the
substantial improvement which can be obtained through blending the fuel of
this
invention with conventional jet fuels and jet fuel components.
CA 02479408 2004-10-O1
_ 12_
Example 7
An additional demonstration of the effect of the alcohols on
lubricity is shown by adding specific alcohols back to Fuel A with low
lubricity.
The alcohols added are typical of the products of the Fischer-Tropsch
processes
described in this invention and found in Fuel B.
TABLE 2
Scuffing BOCLE results for Fuels A and F-J. Results reported
as absolute scuffing loads and percents of Reference Fuel 2
as described the above reference.
Jet Fuel Scuffng Load % Reference Fuel
2
A 1300 19%
F 2000 33%
G 2000 33%
H 2000 32%
I 2300 37%
J 2700 44%
Example 8
Fuels from Examples 1-5 were tested in the ASTM D~S001 B4CLE test
procedure for aviation fuels. This test measures the wear scar on the ball in
millimeters as opposed to the scuffing load as shown in Examples 6 and 7.
Results for this test are show for Fuels A, B, C, E, H, and J which
demonstrate
that the results from the scuffing load test are similarly found in the ASTM
D5001 BOCLE test.
CA 02479408 2004-10-O1
_13_
TABLE 3
ASTM DSOOI BOCLE results for Fuels A, B, C, E, H, J.
Results reported as wear scar diameters as described in ASTM D5001
Jet Fuel Wear Scar Diameter
A 0.57 mm
B 0.54 mm
C 0.66 mm
E 0.53 mm
H 0.57 mm
J 0.54 mm
Results above show that the fuel of this invention, Fuel B, shows superior
performance to either the commercial jet fuel, Fuel C, or the hydrotreated
Fischer-Tropsch fuel, Fuel A. Blending the poor lubricity commercial Fuel C
with Fuel B results in performance equivalent to Fuel B as was found in the
Scuffing Load BOCLE test. Adding very small amounts of alcohols to Fuel A
does not improve Iubricity in this test as it did in the scuffing load test
(Fuel H),
but at higher concentration improvement is seen (Fuel J).
