U.S. patent application number 10/216706 was filed with the patent office on 2003-01-16 for conversion of c1-c3 alkanes and fischer-tropsch products to normal alpha olefins and other liquid hydrocarbons.
This patent application is currently assigned to Chevron U.S.A. Inc.. Invention is credited to Schinski, William L..
Application Number | 20030010677 10/216706 |
Document ID | / |
Family ID | 26867374 |
Filed Date | 2003-01-16 |
United States Patent
Application |
20030010677 |
Kind Code |
A1 |
Schinski, William L. |
January 16, 2003 |
Conversion of C1-C3 alkanes and Fischer-Tropsch products to normal
alpha olefins and other liquid hydrocarbons
Abstract
Processes for converting C.sub.1 to C.sub.3 alkanes into high
purity C.sub.6 to C.sub.24 normal alpha olefins and internal
combustion engine grade fuels and/or lubricating oils comprising a
sequence of fractionation and thermal cracking and/or hydrocracking
operations. The C.sub.6 to C.sub.24 normal alpha olefin fractions
generally have a purity of at least about 90 wt. %.
Inventors: |
Schinski, William L.; (San
Rafael, CA) |
Correspondence
Address: |
CHEVRON PHILLIPS CHEMICAL COMPANY LP
LAW DEPARTMENT - IP
P.O BOX 4910
THE WOODLANDS
TX
77387-4910
US
|
Assignee: |
Chevron U.S.A. Inc.
|
Family ID: |
26867374 |
Appl. No.: |
10/216706 |
Filed: |
July 1, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10216706 |
Jul 1, 2002 |
|
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09649767 |
Aug 28, 2000 |
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60171735 |
Dec 22, 1999 |
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Current U.S.
Class: |
208/58 |
Current CPC
Class: |
Y02P 30/40 20151101;
C10G 2/32 20130101; C10G 9/00 20130101; C10G 3/50 20130101; C10G
2400/22 20130101; C10G 9/36 20130101; C10G 69/14 20130101; Y02P
30/20 20151101; Y10S 208/95 20130101; C10G 2300/1022 20130101 |
Class at
Publication: |
208/58 |
International
Class: |
C10G 047/00; C10G
047/02 |
Claims
What is claimed is:
1. A process for converting a Fischer-Tropsch type reaction product
fraction comprising at least about 90 wt. % C.sub.16-C.sub.50
linear paraffins into high purity C.sub.6-C.sub.24 normal alpha
olefins which comprises the steps of: a) thermal cracking said
reaction product in the presence of at least 5 moles of steam per
mole of said reaction product at a conversion based on said
reaction product no greater than 30 wt. % thereby producing a
product mixture comprising a fraction boiling in the
C.sub.6-C.sub.24 normal alpha olefin range containing at least
about 90 wt. % C.sub.6-C.sub.24 normal alpha olefins; and b)
separating the product mixture of step a) to recover one or more
fractions boiling within the boiling range of C.sub.6-C.sub.24
normal alpha olefins having a normal alpha olefin purity of at
least about 90 wt. %.
2. The process according to claim 1 wherein said separation in step
b) is conducted by fractional distillation.
3. The process according to claim 1 wherein said product mixture is
fractionated in said step b) by extractive fractional distillation
to produce one or more normal alpha olefin fractions within the
range of C.sub.6-C.sub.24 and wherein said fractions have a normal
alpha olefin purity of at least about 95 wt. %.
4. The process of according to claim 1 wherein said reaction
product is thermal cracked to a conversion between about from 15 to
25 wt. %.
5. The process according to claim 1 wherein said product mixture is
fractionated in step b) by adsorption to produce one or more normal
alpha olefin fractions within the range of C.sub.6-C.sub.24 and
wherein said fractions have a normal alpha olefin purity of at
least 95 wt. %.
6. The process according to claim 1 wherein said Fischer-Tropsch
type reaction product is a Fischer-Tropsch reaction product.
7. A process for upgrading Fischer-Tropsch type reaction products
comprising a first hydrocarbon reaction product boiling above about
540.degree. F. (282.degree. C.) comprising C.sub.16-C.sub.50
paraffins liquid fuel hydrocarbons and oxygenates and a second
reaction product boiling below about 540.degree. F. (282.degree.
C.) comprising tail gases, paraffins, olefins and oxygenates which
process comprises the steps of: a) fractionating said first
hydrocarbon reaction product into separate fractions comprising a
fraction boiling in the liquid fuel boiling range, a wax fraction
boiling in about the range of about 540.degree. F.-1100.degree. F.
comprising at least about 90 wt. % C.sub.16-C.sub.50 linear
paraffins, and a fraction boiling above about 1100.degree. F.; b)
thermal cracking the wax fraction of step a) in the presence of
steam at a mole ratio of steam to said wax fraction of about from
3:1 to 5:1, under reactive conditions adjusted to provide a
conversion no greater than about 30 wt. % of said wax fraction
thereby yielding a reaction product mixture containing substantial
amounts of normal alpha olefins of varying chain length within the
range of C.sub.6-C.sub.24, without the production of significant
amounts of C.sub.6-C.sub.24 dienes and wherein the fraction of said
reaction product mixture boiling within the C.sub.6 to C.sub.24
normal alpha olefin boiling range contains at least 90 wt. %
C.sub.6 to C.sub.24 normal alpha olefins; c) fractionating the
reaction product of step b) into separate fractions comprising at
least one normal alpha olefin product fraction comprising normal
alpha olefins selected within the range of 6 to 24 carbon atoms and
a higher boiling fraction boiling above about 730.degree. F.
(388.degree. C.) comprising higher boiling olefins and paraffins;
d) separating said second Fischer-Tropsch reaction product into a
tail gas fraction and a condensate fraction boiling below about
540.degree. F. comprising C.sub.5 and higher carbon atom
hydrocarbons, e) hydrocracking said condensate fraction of step d),
the higher boiling fraction of step c) and the liquid fuel fraction
of step a) with hydrogen under hydrocracking conditions in the
presence of a catalyst comprising a hydrogenation component and an
acid catalyst cracking component in a hydrocracker under
hydrocracking conditions to produce a liquid reaction product
mixture comprising a liquid fuel boiling fraction and a higher
boiling fraction; and f) fractionating the liquid reaction product
mixture of step e) into separate fractions comprising at least one
liquid fuel boiling range fraction and at least one higher boiling
fraction and recycling at least one higher boiling fraction back to
said hydrocracker.
8. The process of claim 7 wherein said step b) is conducted at a
temperature of about from 1000.degree. F. (538.degree. C.) to
1600.degree. F. (871.degree. C.) in the presence of about from 0.2
to 1 part by wt. of steam per part by wt. of said wax fraction.
9. The process of claim 7 wherein said catalyst of step e) contains
at least one non-noble Group VIII metal and at least one Group VIB
metal and an acid catalyst component.
10. The process according to claim 7 wherein said first hydrocarbon
reaction product is contacted with hydrogen under hydrotreating
reactive conditions in the presence of a hydrotreating catalyst
thereby converting oxygenates and olefins into paraffins.
11. The process according to claim 7 wherein said wax fraction is
contacted with hydrogen under hydrotreating reactive conditions in
the presence of a hydrotreating catalyst thereby converting
oxygenates and olefins into paraffins.
12. A process for converting C.sub.1-C.sub.3 alkanes into liquid
hydrocarbon products comprising C.sub.6-C.sub.24 normal alpha
olefins and fuels which comprises the steps of: a) reforming said
C.sub.1-C.sub.3 alkanes into a syngas having a mole ratio of
hydrogen to carbon monoxide of about from 1 to 3 moles of hydrogen
per mole of carbon monoxide; b) contacting the syngas product of
step a) with a Fischer-Tropsch catalyst under Fischer-Tropsch
reaction conditions to yield a first hydrocarbon reaction product
boiling above about 540.degree. F. comprising a major amount of
C.sub.16-C.sub.50 linear paraffins and lesser amounts of oxygenates
and higher boiling hydrocarbons and a second reaction product
boiling below about 540.degree. F. comprising tail gases,
oxygenates and liquid fuel paraffins and oxygenates; c)
fractionating said first reaction product of step b) into separate
fractions comprising a fraction boiling in the liquid fuel boiling
range, a wax fraction boiling in the range of about 540.degree.
F.-1100.degree. F. comprising at least about 90 wt. %
C.sub.16-C.sub.50 linear paraffins and a high boiling fraction
boiling above about 1100.degree. F.; d) thermal cracking the wax
fraction of step e) in the presence of steam at a mole ratio of
steam to said wax fraction of about from 3:1 to 5:1, under reactive
conditions adjusted to provide a conversion no greater than about
30 wt. % of said wax fraction thereby yielding a reaction product
mixture containing substantial amounts of C.sub.6-C.sub.24, alpha
olefins of varying chain length without the production of
significant amounts of C.sub.6-C.sub.24 dienes and wherein the
fraction of said reaction product boiling within the C.sub.6to
C.sub.24 normal alpha olefin boiling range contains at least 90 wt.
% C.sub.6 to C.sub.24 normal alpha olefins; e) fractionating the
reaction product of step d) into separate fractions comprising at
least one normal alpha olefin product fraction comprising a normal
alpha olefin fraction selected within the range of 6 to 24 carbon
atoms and having a normal alpha olefin purity of at least 90 wt. %
and a higher boiling fraction comprising higher boiling olefins and
paraffins; f) hydrocracking said higher boiling fraction of step e)
the liquid fuel fraction and the high boiling fraction of step c)
and with hydrogen in the presence of a catalyst comprising a
hydrogenation component and an acid catalyst cracking component
under hydrocracking conditions to produce a liquid reaction product
mixture comprising a liquid fuel boiling fraction and a higher
boiling fraction; and g) fractionating the liquid reaction product
mixture of step f) into separate fractions comprising at least one
liquid fuel fraction and at least one higher boiling fraction and
recycling at least one said higher boiling fraction back to said
hydrocracker.
13. The process of claim 12 wherein said second reaction product of
step b) is separated into a tail gas fraction and a C.sub.5 and
higher carbon atom hydrocarbon fraction boiling below about
540.degree. F. and hydrocracking said C.sub.5 and higher carbon
atom hydrocarbon fraction with hydrogen in the presence of a
hydrocracking catalyst comprising a hydrogenation component and an
acid cracking component under hydrocracking conditions.
14. The process of claim 12 wherein said step b) in conducted in a
bubble slurry reactor.
15. The process of claim 12 wherein said step d) is conducted at a
temperature of about from 650 to 1900.degree. F. in the presence of
about 0.2 to 1 part per weight of steam per part by weight of said
wax fraction.
16. The process of claim 12 wherein said catalyst of step f)
contains at least one non-noble Group VIII metal and at least one
Group VIB metal and an acid catalyst component.
17. The process according to claim 12 wherein said first
hydrocarbon reaction product is contacted with hydrogen under
hydrotreating reactive conditions in the presence of a
hydrotreating catalyst thereby converting oxygenates and olefins
into paraffins.
18. The process according to claim 12 wherein said wax fraction is
contacted with hydrogen under hydrotreating reactive conditions in
the presence of a hydrotreating catalyst thereby converting
oxygenates and olefins into paraffins.
19. A process for upgrading a Fischer-Tropsch reaction product
boiling above about 540.degree. F. containing C.sub.16 to C.sub.50
linear paraffins, oxygenates, hydrocarbons boiling in the liquid
fuel ranges and linear boiling hydrocarbons and wherein said
reaction product contains at least 20 wt. % of C.sub.16 to C.sub.50
linear paraffins and less than 5 wt. % of said oxygenates, which
process comprises the steps of: a) fractionating said
Fischer-Tropsch reaction product into separate fractions comprising
a fraction boiling in the liquid fuel boiling range, a wax fraction
boiling in about the range of 540.degree. F. to 1100.degree. F.
comprising at least about 90 wt. % C.sub.16 to C.sub.50 linear
paraffins and a high boiling fraction boiling above about
1100.degree. F. b) thermal cracking the wax fraction of step a) in
the presence of steam at a mole ratio of steam to said wax fraction
of at least 5:1, under reactive conditions adjusted to provide a
conversion no greater than about 30 wt. % of said wax fraction
thereby yielding a reaction product mixture containing normal alpha
olefins of varying chain length within the range of C.sub.6 to
C.sub.24, without the production of significant amounts of C.sub.6
to C.sub.24 dienes and wherein the fraction of said reaction
product boiling within the C.sub.6 to C.sub.24 normal alpha olefin
boiling range contains at least 90 wt. % C.sub.6 to C.sub.24 normal
alpha olefins; c) fractionating the reaction product of step b)
into separate fractions comprising at least one normal alpha olefin
product fraction comprising normal alpha olefins selected within
the range of 6 to 24 carbon atoms having purity of said normal
alpha olefins of at least 90 wt. % and a higher boiling fraction
comprising higher boiling olefins and paraffins; d) hydrocracking
said higher boiling fraction of step c), and the liquid fuel
fraction of step a) with hydrogen in a hydrocracker in the presence
of a catalyst comprising a hydrogenation component and an acid
catalyst cracking component, under hydrocracking conditions to
produce a liquid reaction product mixture comprising a liquid fuel
boiling fraction; and e) fractionating the liquid reaction product
mixture of step d) into separate fractions comprising a liquid fuel
fraction, and at least one higher boiling hydrocarbon fraction and
recycling at least one of said higher boiling fraction back to said
hydrocracker.
20. The process of claim 19 wherein said step b) is conducted at a
temperature of about from 1000.degree. F. (538.degree. C.) to
1600.degree. F. (871.degree. C.) in the presence of about from 0.2
to 1 part by wt. of steam per part by wt. of said wax fraction.
21. The process of claim 19 wherein said catalyst of step d)
contains at least one non-noble Group VIII metal and at least one
Group VIB metal and an acid catalyst component.
22. The process according to claim 19 wherein said Fischer-Tropsch
reaction product is contacted with hydrogen under hydrotreating
reactive conditions in the presence of a hydrotreating catalyst
thereby converting oxygenates and olefins into paraffins.
23. The process according to claim 19 wherein after fractionation
said wax fraction is contacted with hydrogen under hydrotreating
reactive conditions in the presence of a hydrotreating catalyst
thereby converting oxygenates and olefins into paraffins.
24. A process for converting C.sub.1-C.sub.3 alkanes into liquid
hydrocarbon products comprising normal alpha olefins and fuels
which comprises the steps of: a) reforming said C.sub.1-C.sub.3
alkanes into a syngas having a mole ratio of hydrogen to carbon
monoxide of about from 1 to 3 moles of hydrogen per mole of carbon
monoxide; b) contacting the syngas product of step a) with a
Fischer-Tropsch catalyst under Fischer-Tropsch reaction conditions
to yield a liquid hydrocarbon reaction product boiling in about the
range of 68.degree. F. to 1300.degree. F. comprising at least about
20 wt. % C.sub.16 to C.sub.50 linear paraffins, and lesser amounts
of oxygenates; c) fractionating the reaction product of step b)
into separate fractions comprising a fraction boiling in the liquid
hydrocarbon fuel range; a wax fraction boiling in about the range
of 540.degree. F. to 1100.degree. F. comprising at least 90 wt. %
C.sub.16 to C.sub.50 linear paraffins; and a high boiling
hydrocarbon fraction boiling above 1100.degree. F.; and d) thermal
cracking the wax fraction of step c) in the presence of at least
five moles of steam per mole of said wax fraction under reactive
conditions adjusted to provide a conversion, based on said wax
fraction no greater than 30 wt. % thereby yielding a reaction
product mixture containing normal alpha olefins of varying chain
length within the range of C.sub.6 to C.sub.24, uncracked linear
paraffins without the production of significant amounts of C.sub.6
to C.sub.24 dienes and wherein the fraction boiling within the
boiling range of C.sub.6 to C.sub.24 normal alpha olefins contains
at least 90 wt. % C.sub.6 to C.sub.24 normal alpha olefins; e)
fractionating the reaction product of step d) into separate
fractions comprising at least one normal alpha olefin product
fraction comprising normal alpha olefins selected within the range
of 6 to 24 carbon atoms and a higher boiling fraction comprising
higher boiling olefins and paraffins and wherein said normal alpha
olefin fractions have a purity of at least about 90 wt. %; f)
hydrocracking the fuel fraction and higher boiling fraction of step
c) and the higher boiling fraction of step e) by contacting said
fraction with hydrogen in the presence of a catalyst comprising a
hydrogenation component and an acid catalyst cracking component,
under hydrocracking conditions to produce a liquid reaction product
mixture comprising a fuel boiling fraction; g) fractionating the
liquid reaction product mixture of step f) into separate fractions
comprising a liquid fuel boiling range fraction, and at least one
higher boiling hydrocarbon fraction and recycling at least one of
said higher boiling fraction back to said hydrocracker.
25. The process of claim 24 wherein said step b) in conducted in a
bubble slurry reactor.
26. The process of claim 24 wherein said step d) is conducted at a
temperature of about from 650 to 1900.degree. F. in the presence of
about 0.2 to 1 part per weight of steam per part by weight of said
wax fraction.
27. The process of claim 24 wherein said catalyst of step f)
contains at least one non-noble Group VIII metal and at least one
Group VIB metal and an acid catalyst component.
28. The process according to claim 24 wherein said liquid
hydrocarbon reaction product of step (b) is contacted with hydrogen
under hydrotreating reactive conditions in the presence of a
hydrotreating catalyst thereby converting oxygenates and olefins
into paraffins.
29. The process according to claim 24 wherein said wax fraction is
contacted with hydrogen under hydrotreating reactive conditions in
the presence of a hydrotreating catalyst thereby converting
oxygenates and olefins into paraffins.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to a method for converting
Fischer-Tropsch type reaction products rich in C.sub.16-C.sub.50
linear paraffins into high purity C.sub.6-C.sub.24 normal alpha
olefins ("NAOs") having a purity of at least about 90 wt. %. This
invention also relates to the conversion of C.sub.1-C.sub.3 alkane
rich gases to more useful liquid hydrocarbons. In a further aspect
it relates to the conversion of natural gas discharged in the
recovery of crude oil, commonly referred to as flare gas, and
excess C.sub.1-C.sub.3 alkanes produced as byproducts in various
refinery operations, into more useful liquid hydrocarbon products
such as normal alpha olefins, lubricating oil and liquid fuels.
(The term liquid refers to hydrocarbons which are liquid at ambient
conditions, including however, pentane.)
[0002] In the recovery of crude oil a large amount of natural gas
(methane) is frequently encountered. In the past, depending on the
location of the oil field, the value of the natural gas was
frequently not considered to be worth the cost of recovery and
transportation. Accordingly, in many cases, the natural gas which
was generated was simply burned off. As well as being wasteful this
practice is no longer considered acceptable from an environmental
standpoint and in many cases prohibited by governmental
regulations. A similar problem may also exist with respect to
excess C.sub.1-C.sub.3 alkanes produced during petroleum refining
operations or other chemical manufacturing operations to the extent
it exceeds the fuel requirements of the facility. Thus, a need to
convert natural gas or methane ethane and propane to more valuable
products has been recognized for a number of years. Efforts have
been undertaken since before World War II to convert methane to
synthesis gas and synthesis gas (CO+H.sub.2) into more desirable
liquid products and are still continuing today. Typically these
processes involve the use of the Fischer-Tropsch process, in which
a less valuable material, e.g. coal or methane, is first converted
to synthesis gas by incomplete oxidation and the synthesis gas
converted to liquid or solid hydrocarbon products, e.g., paraffins,
olefins and oxygenates. The Fischer-Tropsch products may in turn be
upgraded to more useful products by a variety of operations. For
example, U.S. Pat. Nos. 5,345,019 and 5,378,348 disclose a process
for hydrocracking paraffins produced by a Fischer-Tropsch to
produce kerosene, gas oil, and base oil. U.S. Pat. No. 4,943,672
discloses a process for producing lubricating oil from
Fischer-Tropsch waxes by hydroisomerization. U.S. Pat. No.
4,579,986 is directed to a process for making C.sub.10-C.sub.20
linear olefins which comprises thermal cracking, in the presence of
steam, C.sub.20+ paraffins obtained by a Fischer-Tropsch process
using certain Fischer-Tropsch catalysts containing cobalt and
zirconium, titanium and/or chromium. The patent also teaches that
in addition to being useful as a feed for the preparation of linear
C.sub.10-C.sub.20 olefins, the C.sub.20+ fraction is useful for
obtaining solid paraffins, lower olefins (primarily ethene), high
VI lubricating oil and middle distillates (Col. 4, lines 55-68) and
that the C.sub.19-fraction may be used to prepare lower olefins,
high VI synthetic lubricants, solvents and specialty oils (Col. 5,
lines 1-23). U.S. Pat. No. 4,594,172 discloses a process for
preparing high VI synthetic lubricants and U.S. Pat. No. 5,371,308
discloses a process for preparing lower olefins from a
hydroprocessed synthetic oil fraction such as may be obtained from
a Fischer-Tropsch synthesis. The general thermal cracking of
petroleum waxes to produce normal alpha olefins is described in
U.S. Pat. No. 4,042,488 and in The Oil and Gas Journal, pages
102-104, Dec. 13, 1965.
[0003] Many improvements have also been made in the basic
Fischer-Tropsch process since its origins in the 1923, such that
even though the Fischer-Tropsch process still produces a wide range
of molecular weight products, the selectivity of the process may be
directed between lighter paraffin and heavier paraffins (e.g.
C.sub.20+ waxes) by adjusting reaction conditions and/or using
different catalyst; see for example U.S. Pat. Nos. 4,041,097;
4,522,939; 4,579,986; and 5,378,348 and S. T. Sie, et al.
Conversion of Natural Gas to Transportation Fuels via The Shell
Middle Distillate Synthesis Process, Catalyst Today, Vol. 8 (1991)
pp. 371-394 B. Jager, Developments in Fischer-Tropsch Technology,
Studies in Surface Science and Catalysis, Vol. 107 (1997) pp.
219-224, and P. Chaumette, Gas to Liquid Conversion--Basic Features
and Competitors, Petrole et Techniques, No. 415 (July-August 1998)
pp. 83-85.
[0004] One of the problems with thermal cracking, at least where
high purity normal alpha olefins are desired, is that the purity of
the product is generally relatively poor because of the presence of
dienes and branched olefins. Thus in the past ethylene
oligomerization has been used where high purity normal alpha
olefins are desired. Therefore, it would be desirable to develop a
process embodying thermal cracking which produces a high purity
normal olefin product. Further, although much work has been done
with respect to Fischer-Tropsch processes and upgrading the
products therefrom, it would be desirable to develop improved
processes for converting Fischer-Tropsch reaction products into
more valuable products especially in locations where the
transportation costs associated with methane or other hydrocarbon
gases are economically unattractive.
SUMMARY OF THE INVENTION
[0005] The present invention provides an efficient process for
upgrading Fischer-Tropsch reaction products and for converting
natural gas and other gases containing large amounts of methane
ethane or propane or mixtures thereof into normal alpha olefins or
other liquid hydrocarbon products particularly normal alpha
olefins. The invention further provides a process embodying thermal
cracking which produces a high purity C.sub.6-C.sub.24 normal alpha
olefin product at least equal or better than that produced using
the more expensive ethylene oligomerization processes. The
C.sub.6-C.sub.24 normal alpha olefin products provided by the
present invention contain at least 90 wt. % and preferably at least
95 wt. % C.sub.6-C.sub.24 normal alpha olefins. Further by using
more rigorous separation processes purities of at least 98 wt. %
approaching 100% can be obtained.
[0006] In one embodiment the present invention provides a process
for upgrading Fischer-Tropsch products or product fractions
comprising at least about 90 wt. % C.sub.16-C.sub.50 linear
paraffins into high purity C.sub.6-C.sub.24 normal alpha olefin
products which comprises the steps of:
[0007] a) thermal cracking the 90 wt. % C.sub.16-C.sub.50 linear
paraffin mixture in the presence of steam at a mole ratio of steam
to said mixture of at least about 5:1, under thermal cracking
conditions adjusted to produce a cracking conversion of said
mixture of about 30% or less thereby yielding a reaction product
mixture comprising a fraction boiling within the C.sub.6-C.sub.24
normal alpha olefin boiling range, comprising at least 90 wt. %
C.sub.6-C.sub.24 normal alpha olefins.
[0008] b) fractionating the reaction product mixture of step a)
into separate fractions comprising at least one normal alpha olefin
product fraction comprising normal alpha olefins selected within
the range of 6 to 24 carbon atoms in which said fraction has a
normal alpha olefin purity of at least about 90 wt. % and a higher
boiling fraction boiling above about 740.degree. F. (393.degree.
C.) comprising higher boiling olefins and paraffins;
[0009] In another embodiment of the above process, full boiling
range Fischer-Tropsch products are separated into a fuel fraction
boiling below and about 540.degree. F. (282.degree. C.) a wax
fraction boiling between about 540.degree. F. to 1100.degree. F.
(593.degree. C.) containing at least about 90 wt. % linear
paraffins and a high boiling fraction boiling from above about
1100.degree. F. (593.degree. C.). The wax fraction is thermal
cracked as described above and one or more of the other fractions
are hydrocracked to more valuable liquid hydrocarbon products.
Similarly, the higher boiling fraction from step b) above may also
be upgraded by hydrocracking.
[0010] The present invention also provides a process for converting
C.sub.1-C.sub.3 alkane gases, e.g. natural gas, into more valuable
products such as higher molecular weight liquid fuels and normal
alpha olefins (NAO) which comprises the steps of:
[0011] a) reforming said C.sub.1-C.sub.3 alkanes into synthesis gas
for example, by steam reforming, partial oxidation or catalytic
oxidation;
[0012] b) contacting the synthesis gas with a Fischer-Tropsch
catalyst under reactive conditions to yield two hydrocarbon product
streams, one a wax containing product stream boiling above about
540.degree. F. (282.degree. C.) comprising C.sub.16-C.sub.50 linear
paraffins, and a second product boiling below about 540.degree. F.,
comprising hydrocarbons boiling in the vacuum gas oil and liquid
fuel ranges (e.g., paraffins, oxygenates and middle distillate,
gasoline) and tail gases;
[0013] c) distilling the wax containing product of step b) into
fractions comprising a linear C.sub.16-C.sub.50 paraffin fraction
boiling in about the range of 540.degree. F. (282.degree. C.) to
1100.degree. F. (593.degree. C.) containing at least about 90 wt. %
linear C.sub.16-C.sub.50 paraffins, a liquid fuel fraction boiling
below about 540.degree. F. (282.degree. C.) and a heavy fraction
boiling above about 1100.degree. F. (593.degree. C.);
[0014] d) thermal cracking the linear C.sub.16-C.sub.50 paraffin
fraction of step c) in the presence of steam at a steam to said
C.sub.16-C.sub.50 paraffin fraction mole ratio of at least about
5:1 under thermal cracking conditions adjusted to produce a
conversion no greater than about 30 wt. % to produce a reaction
product mixture comprising a substantial amount of C.sub.6-C.sub.24
NAOs without the formation of significant amounts of dienes;
[0015] e) fractionating the reaction product of step d) into NAO
product fractions of varying chain length within the range of
C.sub.6-C.sub.24 having a NAO content of at least 90 wt. % and a
higher boiling fraction boiling above about 1100.degree. F.
(593.degree. C.) containing branched olefin, paraffins and NAO's
having more than 24 carbon atoms;
[0016] f) hydrocracking the liquid fuel portion of the second
product of step b); the vacuum gas oil fraction of step c) and the
higher boiling fraction recovered in step e) with hydrogen in a
hydrocracker in the presence of a hydrocracking catalyst under
hydrocracking conditions to produce a mixture comprising gasoline
and middle distillate; and
[0017] g) fractionating the reaction product of step f) and
recovering at least one liquid fuel fraction, and at least one
higher boiling hydrocarbon fraction and recycling at least one of
said higher boiling hydrocarbon fractions back to said
hydrocracker.
[0018] In another embodiment the invention provides a process for
upgrading a substantially full boiling range Fischer-Tropsch
reaction product including tail gases through bright stock boiling
range hydrocarbons, which process comprises the steps of:
[0019] a) fractionating said Fischer-Tropsch reaction product into
separate fractions comprising a fraction boiling in the liquid fuel
boiling range, a wax fraction boiling in about the range of about
540.degree. F. to 1100.degree. F. comprising at least 90 wt. %
C.sub.16 to C.sub.50 linear paraffins and a high boiling fraction
boiling above about 1100.degree. F.;
[0020] b) thermal cracking the wax fraction of step a) in the
presence of steam at a mole ratio of steam to said wax fraction of
at least 5:1, under reactive conditions adjusted to produce a
conversion based on said wax fraction no greater than 30 wt. % to
yield a reaction product mixture containing a substantial amount of
C.sub.6-C.sub.24 normal alpha olefins without the fomation of
significant amounts of C.sub.6 to C.sub.24 dienes;
[0021] c) fractionating the reaction product of step b) into
separate fractions comprising at least one normal alpha olefin
product fraction comprising a normal alpha olefin fraction selected
within the range of 6 to 24 carbon atoms having a C.sub.6-C.sub.24
normal alpha olefin purity of at least 90 wt. % and a higher
boiling fraction comprising higher boiling olefins and
paraffins;
[0022] d) hydrocracking said higher boiling fraction of step c),
and the liquid fuel fraction of step a) with hydrogen in a
hydrocracker in the presence of a catalyst comprising a
hydrogenation component and an acid catalyst cracking component,
under hydrocracking conditions to produce a liquid reaction product
mixture comprising liquid fuel boiling hydrocarbons; and
[0023] e) fractionating the liquid reaction product mixture of step
d) into separate fractions comprising a liquid fuel fraction, and
at least one higher boiling hydrocarbon fraction and recycling at
least one of said higher boiling fraction back to said
hydrocracker.
[0024] In another embodiment the invention provides a process
comprising the steps of:
[0025] a) converting C.sub.1-C.sub.3 alkanes into synthesis gas for
example, by steam reforming, partial oxidation or catalytic
oxidation;
[0026] b) contacting the synthesis gas with a Fischer-Tropsch
catalyst under reactive conditions to yield a reaction product
mixture of hydrocarbons comprising linear C.sub.16-C.sub.50
paraffins, vacuum gas oil, middle distillate, gasoline light
oxygenates and light olefins;
[0027] c) fractionating the Fischer-Tropsch reaction product
mixture of step b) into separate fractions comprising a linear
C.sub.16-C.sub.50 paraffin fraction containing at least about 90
wt. % linear C.sub.16-C.sub.50 paraffin, at least one liquid fuel
fraction and at least one higher boiling fraction boiling above the
temperature of the C.sub.16-C50 rich fraction;
[0028] d) thermal cracking the linear C.sub.16-C.sub.50 paraffin
fraction of step c) in the presence of steam at a mole ratio of
steam under reactive conditions adjusted to produce a conversion
based on said linear C.sub.16-C.sub.50 paraffin fraction of about
30 wt. % producing a mixture of NAO's of varying chain length as a
substantial product without the formation of significant amounts
C.sub.6-C.sub.24 dienes;
[0029] e) fractionating the reaction product of step d) into NAO
product fractions of varying chain length within the range of six
to twenty-four carbon atoms having an NAO purity of at least 90 wt.
% and a higher boiling fraction containing NAO's having more than
24 carbon atoms and branched olefins and paraffins;
[0030] f) hydrocracking at least one of the liquid fuel fraction
and higher boiling fractions recovered in step c) and the higher
boiling fraction recovered in step e) with hydrogen in the presence
of a hydrocracking catalyst under hydrocracking conditions to
produce a reaction product comprising liquid fuel hydrocarbons;
and
[0031] g) fractionating the reaction product of step f) and
recovering at least one liquid fuel fraction and at least one
higher boiling hydrocarbon fraction and recycling at least one
higher boiling hydrocarbon fraction back to said hydrocracker.
[0032] Additional aspects of the invention will be apparent from
the description which follows:
BRIEF DESCRIPTION OF THE DRAWING
[0033] The drawing is a schematic flow sheet of a preferred
embodiment of the invention in which two different boiling range
products are recovered from the Fischer-Tropsch reaction and
upgraded.
FURTHER DESCRIPTION OF THE INVENTION
[0034] The present invention provides an efficient process for
upgrading Fischer-Tropsch reaction products and for converting
natural gas and other gases containing large amounts of methane,
ethane or propane or mixtures thereof into normal alpha olefins or
other liquid hydrocarbon products. In general the major component
of these gases is methane. The invention is especially applicable
to remote sites which produce a surplus of natural gas or other
C.sub.1-C.sub.3 alkanes gases, but which are too remote from
markets for these gases to justify the cost of transporting the
C.sub.1-C.sub.3 alkanes. The invention also provides process
flexibility to adjust between waxes, normal .alpha.-olefins, liquid
fuel products and lube oil base stocks depending on the relative
market demand for the products and provide a thermal cracking
process which produces a high purity C.sub.6-C.sub.24 normal alpha
olefin product without significant amounts of dienes and branched
olefins; typically less than about 10 wt. % and preferably less
than about 5 wt. %. As used herein, liquid fuel refers to
hydrocarbon fractions boiling with the gasoline range and/or middle
distillate range, (e.g., diesel fuel and jet fuel). Thus, for
example, the term liquid fuel fraction refers to a gasoline
fraction, a diesel fuel fraction, a jet fuel fraction or a fraction
including both gasoline and middle distillate.
[0035] Starting with the C.sub.1-C.sub.3 alkanes gases the alkanes
are reformed to a mixture of hydrogen and carbon monoxide.
Reforming is well known in the art, and includes a variety of
technologies including steam reforming, partial oxidation, dry
reforming, series reforming, convective reforming, and autothermal
reforming. All have in common the production of syngas from methane
and other light hydrocarbons, and an oxidant (steam, oxygen, carbon
dioxide, air, enriched air or combinations thereof). The effluent
typically contains some carbon dioxide and steam in addition to
syngas and unreacted feed gases. Series reforming, convective
reforming and autothermal reforming incorporate exothermic and
endothermic syngas forming reactions in order to better utilize the
heat generated in the process. These processes for producing
synthesis gas or syngas from C.sub.1-C.sub.3 alkanes are well known
to the art. Steam reforming is typically effected by contacting
C.sub.1-C.sub.3 alkanes with steam, preferably in the presence of a
reforming catalyst, at a temperature in the range of about
1300.degree. F. (705.degree. C.) to about 1675.degree. F.
(913.degree. C.) and pressures from about 10 psia (0.7 bars) to
about 500 psia (34 bars). Suitable reforming catalysts which can be
used include, for example, nickel, palladium, nickel-palladium
alloys, and the like. Additional information regarding steam
reforming C.sub.1-C.sub.3 alkanes, e.g., methane, to syngas can be
found in U.S. Pat. No. 5,324,335 hereby incorporated by reference
in its entirety.
[0036] Partial oxidation of C.sub.1-C.sub.3 alkanes to syngas is
also conducted at high temperature and while the partial oxidation
may be conducted without a catalyst it is more effectively
conducted in the presence of a catalyst. In general Group VIII
metals can be used as the catalyst typically supported on a mineral
oxide or synthetic support, e.g., alumina. Typically, the partial
oxidation is conducted at temperatures in about the range of
1500.degree. F. (815.degree. C.) to about 2000.degree. F.
(1093.degree. C.) pressures in about the range from atmospheric to
3000 psia (1 to 20.4 bars). Space velocities can vary over a very
wide range and typical range of 100 to 100,000 hr-1 and even higher
depending on the particular catalyst used and the type of reactor.
A discussion of nickel silica alumina and nickel/magnesium oxide
and cobalt/magnesium oxide and other oxidation catalysts may be
found in A. Santos et al., Oxidation of Methane to Synthesis Gas in
Fluidized Bed Reactor using MgO-Based Catalysts, Journal of
Catalysis, Vol. 158 (1996) pp. 81-91 hereby incorporated by
reference in its entirety.
[0037] The partial oxidation may also be conducted using a
peroskite catalyst partial oxidation process such as described in
U.S. Pat. No. 5,149,516 hereby incorporated by reference in its
entirety. Peroskites are materials having essentially the same
crystal structure as the mineral peroskite (Ca Ti O3) without
limitation as to the elemental constituents thereof. Such materials
can be represented by the formula XYO3 wherein X and Y can be
variety of elements. For example, X can be La, Ca, Sr, Ba, Na, K,
Ag, Cd and mixtures thereof and Y can be Ta, Co, Ti, Ga, Nb, Fe,
Ni, Mn, Gr, V, Th, Pb, Sn, Mo, Zn and mixtures thereof. Partial
oxidation reactions using a peroskite catalyst are typically
conducted at temperatures in the range of about from 600 to
900.degree. C., pressures, of about from 0.1 to 100 bar and gas
hourly space velocities of from 100 to 300,000 hr-1. (These space
velocities are determined using a gas volume based on NTP
conditions, i.e. room temperature (about 25.degree. C.) and one
atmosphere of pressure.) The mole ratio of lower alkane can vary
from 1:1 to 100:1 moles of alkane to oxygen. Regardless of the
system used to produce syngas it is desirable to remove any sulfur
compounds, e.g., hydrogen sulfide and mercaptans, contained in the
C.sub.1-C.sub.3 alkane feed. This can be effected by passing the
C.sub.1-C.sub.3 alkanes gas through a packed bed sulfur scrubber
containing zinc oxide bed or another slightly basic packing
material. If the amount of C.sub.1-C.sub.3 alkanes exceeds the
capacity of the synthesis gas unit the surplus C.sub.1-C.sub.3
alkanes can be used to provide energy throughout the facility. For
example, excess C.sub.1-C.sub.3 alkanes may be burned in a steam
boiler to provide the steam used in the thermal cracking step of
the present process.
[0038] The syngas product is converted to liquid hydrocarbons by
contact with a Fischer-Tropsch catalyst under reactive conditions.
Depending on the quality of the syngas it may be desirable to
purify the syngas prior to the Fischer-Tropsch reactor to remove
carbon dioxide produced during the syngas reaction and any sulfur
compounds, if they have not already been removed. This can be
accomplished by contacting the syngas with a mildly alkaline
solution (e.g. aqueous potassium carbonate) in a packed column. In
general Fischer-Tropsch catalysts contain a Group VIII transition
metal on a metal oxide support. The catalyst may also contain a
noble metal promoter(s) and/or crystalline molecular sieves.
Pragmatically, the two transition metals which are most commonly
used in commercial Fischer-Tropsch processes are cobalt or iron.
Ruthenium is also an effective Fischer-Tropsch catalyst but is more
expensive than cobalt or iron. Where a noble metal is used,
platinum and palladium are generally preferred. Suitable metal
oxide supports or matrices which can be used include alumina,
titania, silica, magnesium oxide, silica-alumina, and the like and
mixtures thereof.
[0039] Although, Fischer-Tropsch processes produce a hydrocarbon
product having a wide range of molecular sizes the selectivity of
the process toward a given molecular size range as the primary
product can be controlled to some extent by the particular catalyst
used. In the present process, it is preferred to produce linear
C.sub.16-C.sub.50 paraffins as the primary product, and therefore,
it is preferred to use a cobalt catalyst, although iron catalysts
may also be used. Also, by hydrotreating the product other linear
hydrocarbon products, e.g. oxygenates and olefins, can be converted
to the corresponding linear paraffins.
[0040] One suitable Fischer-Tropsch catalyst which can be used is
described in U.S. Pat. No. 4,579,986 as satisfying the
relationship.
(3+4R)>L/S>(0.3+0.4R),
[0041] wherein
[0042] L=the total quantity of cobalt present on the catalyst,
expressed as mg Co/ml catalyst,
[0043] S=the surface area of the catalyst, expressed as m.sup.2/ml
catalyst, and
[0044] R=the weight ratio of the quantity of cobalt deposited on
the catalyst by kneading to the total quantity of cobalt present on
the catalyst.
[0045] Preferably, the catalyst contains about 3-60 ppw cobalt,
0.1-100 ppw of at least one of zirconium, titanium or chromium per
100 ppw of silica, alumina, or silica-alumina and mixtures thereof.
Typically, the synthesis gas will contain hydrogen, carbon monoxide
and carbon dioxide in a relative mole ratio of about from 0.25 to 2
moles of carbon monoxide and 0.01 to 0.05 moles of carbon dioxide
per mole of hydrogen. In the present process we prefer to use a
mole ratio of carbon monoxide to hydrogen of about 0.4 to 1, more
preferably 0.5 to 0.7 moles of carbon monoxide per mole of hydrogen
with only minimal amounts of carbon dioxide; preferably less than
0.5 mole percent carbon dioxide.
[0046] In the present process the Fischer-Tropsch reaction is
typically conducted at temperatures of about from 300 to
700.degree. F. (149 to 371.degree. C.) preferably 400 to
500.degree. F. (204 to 228.degree. C.); pressures of about from 10
to 500 psia (0.7 to 34 bar), preferably 30 to 300 psia (2 to 21
bar), and catalyst space velocities of about from 100 to 10,000
cc/g/hr, preferably 300 to 3,000 cc/g/hr. The reaction can be
conducted in any suitable reactor, for example, fixed bed reactors
containing one or more catalyst beds, or slurry reactors, and/or
fluidized bed reactor.
[0047] The Fischer-Tropsch reaction product can be separated into
the desired product fractions e.g. a gasoline fraction (B.P. about
68-450.degree. F.) a middle distillate fraction (B.P. about
450-540.degree. F.) a wax fraction (B.P. about 540-1100.degree. F.)
primarily containing C.sub.16 to C.sub.50 normal paraffins with a
small amount of branched paraffins and a heavy fraction (B.P. above
about 1100.degree. F.). If higher normal alpha olefins product are
desired or more middle distillate is desired a higher wax fraction
can be recovered C.sub.20-C.sub.50 normal paraffins (BP about
650-1100.degree. F.) and a middle distillate fraction BP about
450-650.degree. F. Further, using a higher linear paraffin cut,
e.g. C.sub.26-C.sub.50 will facilitate the removal of uncracked
linear paraffins from the desired C.sub.6-C.sub.24 NAO product of
the thermal cracking step, discussed below. With the exception of
the wax fraction, the other fractions are largely a matter of
choice depending on the products desired and the particular plant
configuration; for example, a single liquid fuel fraction may be
taken off comprising both gasoline and middle distillate may be
taken off and multiple heavy cuts may be taken off. In some cases
tail gases will be exhausted from the reactor separate from the
C.sub.5 and higher hydrocarbons. The tail gas, primarily containing
hydrogen and C.sub.1 to C.sub.4 paraffins, can be used as fuel gas
or can be treated to remove carbon dioxide and used as a hydrogen
or alkane recycle stream.
[0048] In a preferred embodiment, the Fischer-Tropsch reaction is
conducted in a bubble column slurry reactor. In this type of
reactor synthesis gas is bubbled through a slurry comprising
catalyst particles in a suspending liquid. Typically the catalyst
has a particle size of about from 10-110 microns, preferably about
from 20-80 microns, more preferably about from 25-65 micron and a
density of about from 0.25 to 0.9 g/cc preferably about from
0.3-0.75 g/cc. The catalyst typically comprises one of the
aforementioned catalytic metals, preferably cobalt on one of the
aforementioned catalyst supports. Preferably the catalyst comprises
about 10 to 14 wt. % cobalt on a low density fluid support, for
example alumina, silica and the like having a density within the
ranges set forth above for the catalyst. Since, the catalyst metal
may be present in the catalyst as oxides the catalyst is typically
reduced with hydrogen prior to contact with the slurry liquid. The
starting slurry liquid is typically a heavy hydrocarbon having a
viscosity high enough to keep the catalyst particles suspended,
typically a viscosity between 4-100 centistokes at 100.degree. C.)
and a low enough volatility to avoid vaporization during operation,
typically an initial boiling point range of about from 350 to
550.degree. C. The slurry liquid is preferably essentially free of
contaminants such as sulfur, phosphorous or chlorine compounds.
Thus initially, it may be desirable to use a synthetic hydrocarbon
fluid such as a synthetic olefin oligomer as the slurry fluid.
Ultimately, a paraffin fraction of the product having the desired
viscosity and volatility is typically recycled as the slurry
liquid. The slurry typically has a catalyst concentration of about
2-40 wt. % catalyst, preferably 5-20 wt. % and more preferably 7-15
wt. % catalyst based on the total weight of the catalyst, i.e.
metal plus support. The syngas feed typically has hydrogen to
carbon monoxide mole ratio of about from 0.5 to 4 moles of hydrogen
per mole of carbon monoxide preferably about from 1 to 2.5 and more
preferably about 1.5 to 2.
[0049] The bubble slurry reactor is typically operated at
temperatures within the range of 150-300.degree. C., preferably 185
to 265.degree. C. and more preferably 210-230.degree. C. and
pressures within the range of 1 to 70 bar, preferably 6-35 bar and
most preferably 10 to 30 bar (1 bar=14.5 psia). Typical synthesis
gas linear velocity ranges in the reactor from about 2 to 40 cm per
sec. preferably 6 to 10 cm per sec. Additional details regarding
bubble column slurry reactors can, for example, be found in Y. T.
Shah et al., Design Parameters Estimations for Bubble Column
Reactors, AlChE Journal, 28 No. 3 pp. 353-379 (May 1982);
Ramachandran et al., Bubble Column Slurry Reactor, Three-Phase
Catalytic Reactors Chapter 10, pp. 308-332 Gordon and Broch Science
Publishers (1983); Deckwer et al., Modeling the Fischer-Tropsch
Synthesis in the Slurry Phase, Ind. Eng. Chem. Process Des. Dev. v
21, No. 2, pp. 231-241 (1982); Kolbel et al., The Fischer-Tropsch
Synthesis in the Liquid Phase, Catal. Rev.-Sci. Eng., v. 21(n), pp.
225-274 (1980) and U.S. Pat. No. 5,348,982, all of which are hereby
incorporated by reference in their entirety.
[0050] The gaseous reaction product from the Fischer-Tropsch bubble
slurry reactor comprises hydrocarbons boiling below about
540.degree. F. (e.g., tail gases through middle distillates). The
liquid reaction product is recovered as or with the slurry and
comprises hydrocarbons boiling above about 540.degree. F., e.g.,
vacuum gas oil through heavy paraffins. The minus 540.degree. F.
product can be separated into a tail gas fraction and a condensate
fraction, i.e., about C.sub.5 to C.sub.16 normal paraffins and
higher boiling hydrocarbons, using a high pressure and/or lower
temperature vapor-liquid separator or low pressure separators or a
combination of separators. The tail gas fraction may be used as
described above. The condensate fraction can be fractionated into
the desired product fraction; e.g. gasoline, light middle
distillate or more preferably can be upgraded by hydrocracking. The
F-T fraction boiling above about 540.degree. F., is typically
separated into a wax fraction boiling in the range of about
540.degree. F.-1100.degree. F. primarily containing C.sub.16 to
C.sub.50 linear paraffins with relatively small amounts of higher
boiling branched paraffins, one or more liquid fuel fractions
boiling below about 540.degree. F. and one or more fractions
boiling above about 1100.degree. F. Typically, the separation is
effected by fractional distillation. Alternatively, if the
Fischer-Tropsch reaction is designed to produce a single process
stream, then the entire product stream may be fractionated,
generally after first removing hydrogen and preferably other tail
gases. This can be done by passing the product stream through one
or more vapor-liquid separators prior to fractionation.
[0051] Because the Fischer-Tropsch product typically contains
linear oxygenates and olefins boiling in the same range as the
desired linear paraffins, either the F-T liquid reaction product or
the fraction boiling within the C.sub.16 to C.sub.50 linear
paraffin range is preferably hydrotreated to convert the oxygenates
and olefins to paraffins. Thus, improving the yield of the desired
linear paraffins. Hydrotreating is well known to the art and can be
effected using any suitable hydrotreating procedure. Typically,
hydrotreating is conducted at temperatures in about the range of
650 to 800.degree. F. (427.degree. C.) and pressures in about the
range of 800 to 3000 psi (54 to 204 atms) in the presence of a
catalyst comprising at least one Group VIII or Group VI metal and
more typically containing one metal from each group, e.g.
colbalt-molybdenum; nickel-tungsten, on a neutral mineral oxide
support such as alumina and the like, at LHSVs in the range of
about from 0.25 to 2 hr.sup.-1. Typically, the liquid hydrocarbon
feed is contacted with hydrogen at a ratio of at least 50 SCF of
hydrogen per Bbl of feed and preferably between about 1,000 to
5,000 SCF/Bbl.
[0052] The C.sub.16 to C.sub.50 paraffin fraction, or if desired a
C.sub.20 to C.sub.50 or C.sub.26 to C.sub.50 fraction, is thermally
cracked into smaller chain length normal alpha olefins, e.g.
C.sub.6to C.sub.24. The thermal cracking can be conducted over a
wide range of temperatures and pressures but is typically conducted
at temperatures in the range of about from 950.degree. F.
(510.degree. C.) to 1900.degree. F. (1038.degree. C.) preferably
1000 to 1600.degree. F. (538 to 871.degree. C.) and pressures of
about from 0.5 to 10 bars (7 to 147 psia) preferably about from 1
to 5 bar (14.5 to 73.5 psia). Residence times or space velocity
will vary with the reactor temperatures and pressures. Typical
residence times may vary from about 0.1 to 2 seconds where high
temperatures, e.g. above about 1300.degree. F. (704.degree. C.) are
used, to space velocities (LHSV) of about from 0.3 to 20 hr.sup.-1
with lower temperatures. The reaction may be conducted by passing
the feed through a packed bed of inert material or by using tube
reactors or other types of reactors. Generally a catalyst is not
used. The thermal cracking is conducted in the presence of steam.
The steam serves as a heat source for the endothermic reactions and
also as a diluent to isolate ethylene free radicals and suppress
undesired side reactions and coke formation. The severity of the
thermal cracking conditions will vary with the carbon chain length
or molecular weight distribution of the feedstock and the carbon
chain length distribution desired in the reaction product and the
desired cracking conversion. Details of a typical steam thermal
cracking process be found in U.S. Pat. No. 4,042,488, hereby
incorporated by reference its in entirety.
[0053] One of the problems with producing normal alpha olefins by
paraffin thermal cracking is that a significant amount of undesired
dienes, which are not easily separated from the desired normal
alpha olefins, are also produced. However, in accordance with the
present invention, by using a high purity linear paraffin feed
steam, at least 90 wt. % linear paraffins, and keeping the
conversion low and preferably using a high steam to feed mole ratio
the amount of dienes produced can be very substantially reduced
thus permitting recovery of a high purity C.sub.6to C.sub.24 normal
alpha olefin product fraction. The desired normal alpha olefin is
separated from the reaction product (e.g. fractional distillation)
to remove unreacted starting material as well as any higher boiling
branched olefins and dienes. In practicing the present invention
the cracking conversion should be no greater than 30% based on
weight of feed and preferably no greater than 25 wt. %. In general
best results in terms of yield of high purity C.sub.16-C.sub.24
normal alpha olefins is obtained by controlling the conversion
within the range of 15 to 25 wt. %. This can be accomplished by
adjusting the reaction temperatures, pressures and residence time
(space velocity) within the ranges set forth above. Optimum
reaction conditions will also vary somewhat with the particular
feedstock and can be determined by routine process optimization.
Typically mole ratios of steam to hydrocarbon feed in about the
range of from 2:1 to 7:1 preferably about from 3:1 to 5:1 and more
preferably about 5:1 moles of steam per mole of hydrocarbon feed
can be used.
[0054] Because some higher dienes boiling in the C.sub.16-C.sub.50
normal paraffin boiling range will be produced, even though small,
it is preferred not to recycle the C.sub.16-C.sub.50 paraffin
fraction of the reaction product range back to the thermal cracking
reactor. This is preferable to risking ultimately increasing the
diene content of the C.sub.6-C.sub.24 normal alpha olefin fraction
product, although a carefully monitored and controlled single
recycle may be acceptable. Further information regarding general
thermal cracking can be had by reference to U.S. Pat. Nos.
5,146,022; 5,656,150; and 5,866,745 hereby incorporated by
reference in their entirety.
[0055] The reaction product from the thermal reactor is typically
fed to a fractional distillation column, although other suitable
separation procedures could also be used, to separate the product
into normal alpha olefins of the desired chain length range and to
remove higher boiling paraffins and branched olefins and any lower
boiling material. The C.sub.6-C.sub.24 normal alpha olefin fraction
has a normal alpha olefin content of at least about 90 wt. %
preferably at least about 95% wt. %. Further by using more rigorous
purification techniques such as extractive distillation and/or
adsorption, normal alpha olefin contents in excess of 95 wt. % up
to about 99 wt. % and approaching 100% can be obtained. The normal
alpha olefin fractions either with or without further treatment are
used as chemical intermediates for a variety of products, including
lubricants and surfactants.
[0056] The higher boiling paraffins and olefins fraction, e.g.
above about C.sub.24, from the thermal reaction product
fractionator, the condensate, the liquid fuel fractions and the
1100.degree. F.+ fractions are preferably upgraded by
hydrocracking. This may be effected by hydrocracking the respective
products individually or by combining one or more of the fractions.
Preferably, fractions having similar boiling point ranges are
combined to optimize hydrocracking conditions. For example, the
condensate fraction from the minus 700.degree. F. Fischer-Tropsch
product is preferably combined with the vacuum gas oil ("VGO")
boiling range fraction, and hydrocracked to higher quality liquid
products. The hydrocracking operation can be conducted as a block
operation wherein the hydrocracker is alternated between liquid
fuel fractions and heavier fuel fractions or parallel hydrocrackers
can be used each processing a different distillation range
feedstock. Hydrocracking can be effected by contacting the
particular fraction or combination of fractions, with hydrogen in
the presence of a suitable hydrocracking catalyst at temperatures
in the range of about from 600 to 900.degree. F. (316 to
482.degree. C.) preferably 650 to 850.degree. F. (343 to
454.degree. C.) and pressures in the range about from 200 to 4000
psia (13-272 atm) preferably 500 to 3000 psia (34-204 atm) using
space velocities based on the hydrocarbon feedstock of about 0.1 to
10 hr-1 preferably 0.25 to 5 hr-1. Generally, more severe
conditions within these ranges will be used with higher boiling
feedstocks and depending on whether gasoline, middle distillate or
lubricating oil is desired as the primary economic product. The
hydrocracking step reduces the size of the hydrocarbon molecules,
hydrogenates olefin bonds, hydrogenates aromatics, and removes
traces of heteroatoms resulting in an improvement in fuel or base
oil product quality.
[0057] As is well known the hydrocracking catalysts contain a
hydrogenation component and a cracking component. The hydrogenation
component is typically a metal or combination of metals selected
from Group VIII noble and non-noble metals and Group VIB metals.
The noble metals, particularly platinum or palladium, are generally
more active but are expensive. Non-noble metals which can be used
include molybdenum, tungsten, nickel, cobalt, etc. Where non-noble
metals are used it is generally preferred to use a combination of
metals, typically at least one Group VIII metal and one Group VIB
metal, e.g., nickel-molybdenum, cobalt-molybdenum, nickel-tungsten,
and cobalt-tungsten. The non-noble metal hydrogenation metal are
usually present in the final catalyst composition as oxides, or
more preferably, as sulfides when such compounds are readily formed
from the particular metal involved. Preferred non-noble metal
overall catalyst compositions contain in excess of about 5 weight
percent, preferably about 5 to about 40 weight percent molybdenum
and/or tungsten, and at least about 0.5, and generally about 1 to
about 15 weight percent of nickel and/or cobalt determined as the
corresponding oxides. The sulfide form of these metals is most
preferred due to higher activity, selectivity and activity
retention.
[0058] The hydrogenation components can be incorporated into the
overall catalyst composition by any one of numerous procedures.
They can be added either to the cracking component or the support
or a combination of both. In the alternative, the Group VIII
components can be added to the cracking component or matrix
component by co-mulling, impregnation, or ion exchange and the
Group VI components, i.e.; molybdenum and tungsten can be combined
with the refractory oxide by impregnation, co-mulling or
co-precipitation. Although these components can be combined with
the catalyst support as the sulfides, that is generally not the
case. They are usually added as a metal salt which can be thermally
converted to the corresponding oxide in an oxidizing atmosphere or
reduced to the metal with hydrogen or other reducing agent. The
non-nobel metal composition can then be sulfided by reaction with a
sulfur donor such as carbon bisulfide, hydrogen sulfide,
hydrocarbon thiols, elemental sulfur, and the like.
[0059] The cracking component is an acid catalyst material and may
be a material such as amorphous silica-alumina or may be a zeolitic
or non-zeolitic crystalline molecular sieve. Examples of suitable
hydrocracking molecular sieves include zeolite Y, zeolite X and the
so called ultra stable zeolite Y and high structural silica:alumina
ratio zeolite Y such as for example described in U.S. Pat. No.
4,401,556, 4,820,402 and 5,059,567. Small crystal size zeolite Y,
such as described in U.S. Pat. No. 5,073,530 can also be used. The
disclosures of all of which patents are hereby incorporated by
reference in their entirety. Non-zeolitic molecular sieves which
can be used include, for example silicoaluminophosphates (SAPO),
ferroaluminophosphate, titanium aluminophosphate and the various
ELAPO molecular sieves described in U.S. Pat. No. 4,913,799 and the
references cited therein. Details regarding the preparation of
various non-zeolite molecular sieves can be found in U.S. Pat. No.
5,114,563 (SAPO); 4,913,799 and the various references cited in
U.S. Pat. No. 4,913,799, hereby incorporated by reference in their
entirety. Mesoporous molecular sieves can also be included, for
example the M41S family of materials (J. Am. Chem. Soc. 1992, 114,
10834-10843), MCM-41 (U.S. Pat. Nos. 5,246,689, 5,198,203,
5,334,368), and MCM-48 (Kresge et al., Nature 359 (1992) 710.)
[0060] In general amorphous silica-alumina is more selective for
middle distillates, e.g., diesel fuel, whereas crystalline
molecular sieves are much more active and produce greater amounts
of lighter products, e.g., gasoline. The so-called high
(structural) silica-alumina ratio (Si2O3:Al2O3=about 50) Y zeolites
are less active than the conventional zeolite Y but, are more
selective for middle distillate and more active than amorphous
silica-alumina. The catalyst also typically contains a matrix or
binder material resistant to the conditions used in the
hydrocracking reaction. Suitable matrix materials include synthetic
or natural substances as well as inorganic materials such as clay,
silica and/or metal oxides. The latter may be either naturally
occurring or in the form of gelatinous precipitates or gels
including mixtures of silica and metal oxides naturally occurring
clays which can be composited with the catalyst include those of
the montmorillonite and kaolin families. These clays can be used in
the raw state as originally mined or initially subjected to
calumniation, acid treatment or chemical modification.
[0061] The catalyst may be composited with a porous matrix
material, such as alumina, silica-alumina, silica-magnesia,
silica-zirconia, silica-thoria, silica-berylia, silica-titania as
well as ternary compositions, such as silica-alumina-thoria,
silica-alumina-zirconia, silica-alumina-magnesia, and
silica-magnesia zirconia. The relative proportions of molecular
sieve component and inorganic oxide matrix or binder may vary
widely with the molecular sieve content ranging from between 1 to
99, more usually 5 to 80, percent by weight of the composite. The
matrix may itself possess catalytic properties generally of an
acidic nature, such as for example where amorphous silica-alumina
is used as a matrix or binder for a molecular sieve. In general it
is preferred to use a non-zeolite or low acidic zeolite catalyst,
e.g., high structural silica:alumina ratio Y zeolite, as the
catalyst where middle distillates is desired as the main commercial
product and an acidic zeolite catalyst, e.g., conventional or ultra
stabilized Y zeolite, where gasoline is desired as the main
commercial product.
[0062] Furthermore more than one catalyst type may be used in the
reactor. The different catalyst types can be separated into layers
or mixed.
[0063] The hydrocrackate is then separated into various boiling
range fractions. The separation is typically conducted by
fractional distillation preceded by one or more vapor-liquid
separators to remove hydrogen and/or other tail gases. The
fractions separated will typically include a gasoline fraction and
a high boiling bottom fraction and one or more intermediate boiling
range fractions. The high boiling fraction is preferably recycled
back to the hydrocracker. The light tail gas fraction, i.e.,
methane, ethane, proposal and any residual hydrogen is withdrawn
and can be for fuel gases or for hydrogen recovery which in turn
can be recycled back to the hydrocracker. Typical, liquid/vapor
separator systems which can be used to remove tail gases and
hydrogen are, for example, described in U.S. Pat. No. 3,402,122 and
4,159,937 hereby incorporated by reference in their entirety.
[0064] If desired the feed may be hydrotreated prior to
hydrocracking to remove impurities and heterorganics, e.g.
oxygenates. Hydrotreating may be conducted in a separate reactor
preceding the hydrocracking or may be conducted in the same
reactor, for example, as one or more hydrotreating catalyst beds
preceding one or more hydrocracking catalyst beds. The
hydrotreating bed may also serve as a screen to remove any
particulate matter in the feedstock or may itself be preceded with
guard beds of crushed rock or other suitable material.
Hydrotreating can be effected by the same general procedure as
described above with respect to hydrotreating of the
Fischer-Tropsch reaction product.
[0065] Although the invention is described herein in terms of a
Fischer-Tropsch reaction or process the invention also applies to
the various modifications of the literal Fischer-Tropsch process by
which hydrogen (or water) and carbon monoxide (or carbon dioxide)
are converted to hydrocarbons (e.g. paraffins, ethers etc.) and to
the products of such processes. Thus the term Fischer-Tropsch type
process or product is intended to apply to Fischer-Tropsch
processes and products and the various modifications thereof and
the products thereof. For example, the term is intended to apply to
the Kolbel-Engelhardt process typically described by the reactions
1
[0066] The present process can also be applied to upgrade
Fischer-Tropsch products generally by applying the steps discussed
above following the Fischer-Tropsch reaction to Fischer-Tropsch
type products. Where such Fischer-Tropsch type products do not
contain at least about 90 wt. % linerar C.sub.16 to C.sub.50
paraffins the Fischer-Tropsch type product may be concentrated or
purified by any suitable procedure, typically fractional
distillation, to produce a fraction having the desired C.sub.16 to
C.sub.50 linear paraffin concentration, preferably such
Fischer-Tropsch type products, or at least the liquid portion
thereof, should contain at least 20 wt. % linear C.sub.20 to
C.sub.50 paraffins and more preferably should contain between 30-80
wt. % C.sub.20 to C.sub.50 paraffins for optimum benefit in the
case of the fully integrated process. The Fischer-Tropsch type
product preferably contains less than about 10 wt. % oxygenates,
more preferably less than 5 wt. %.
[0067] For the purposes of further understanding of the invention
an embodiment of the invention will now be described with reference
to the drawing.
EXAMPLE 1
[0068] Referring to the Figure, an embodiment of the invention will
be described using a bubble slurry Fischer-Tropsch reactor. Natural
gas 1, is fed by line 2 to scrubber 3 containing a packed bed of
zinc oxide to remove any hydrogen sulfide or mercaptan gases
contained in the natural gas. A portion of the natural gas is split
off via line 2a to provide fuel for boiler 2b. The sulfur free
natural gas, is fed via line 4 to syngas reactor 6 where it is
reacted with oxygen provided by oxygen line 5 to effect partial
oxidation of the methane. Fixed bed reactor 6 contains a packed bed
of peroskite LaCoO.sub.3 catalyst and is operated at a temperature
of about 720.degree. C. and a pressure of about 1 bar (atmospheric
pressure) and a space velocity of about 27,400 hr.sup.-1 to produce
a syngas containing about 2 mol of hydrogen per mole of carbon
monoxide. If needed the mol ratio of hydrogen to carbon monoxide
may be adjusted by passing the syngas through a membrane separator
(not shown). The syngas reaction product having a mole ratio of
hydrogen to carbon monoxide of about 2 is fed via line 7 to
Fischer-Tropsch bubble column slurry reactor 8 containing a 12 wt.
% cobalt on low density alumina catalyst having a particle size of
about 25 to 65 microns and a density of about 0.4 to 7 g/cc in a 8
cs, at 100.degree. C., synfluid slurry liquid. Prior to mixing with
the slurry liquid the catalyst is reduced by contact with a 5 vol.
% hydrogen, 95 vol. % nitrogen gas at about 200-250.degree. C. for
about 12 hours and then increasing the temperature to about
350-400.degree. C. and maintaining this temperature for about 24
hours while slowly increasing the hydrogen content of the gas until
the reducing gas is essentially 100% hydrogen. Reactor 8 is
operated at a temperature of about from 210 to 230.degree. C., a
pressure of 25-30 bar and a syntheis gas linear velocity of about 6
to 10 cm/sec to produce a liquid hydrocarbon product containing a
high proportion of C.sub.20 to C.sub.50 paraffins (the wax product)
discharged via line 8a and a light product boiling below about
650.degree. F. (343.degree. C.) containing middle distillate and
tail gases discharged via line 8b. Tail gases are removed from the
light fraction, for example by using one or more liquid/gas
separators, not shown, operating at lower temperatures and/or
pressures and the remaining light product stream (condensate)
comprising C.sub.5 and higher hydrocarbons boiling below
650.degree. F. (343.degree. C.) is fed to hydrocracker 26. The F-T
wax product is fed via line 8a to hydrotreater 9 operated at about
700.degree. F. (371.degree. C.) to 750.degree. F. (399.degree. C.)
and a pressure of about 60-65 atms and a LHSV of about 1 hr.sup.-1
over a nickel-tungsten on alumina catalyst. The hydrotreated
product is fed via line 9a to fractional distillation column 10
where it is fractionated into a wax fraction boiling above about
700.degree. F. (371.degree. C.) primarily containing at least 90
wt. % C.sub.20-C.sub.50 linear paraffins, a high boiling bright
stock fraction boiling above about 1100.degree. F. and a liquid
fuel fraction boiling below about 700.degree. F. The wax fraction
is fed via line 14 to thermal cracking reactor 16. Prior to
entering the reactor the high boiling fraction feed is combined
with steam furnished by line 15 from boiler 2b at a mole ratio of
about 5 mol of steam per mole of the wax fraction feed. The thermal
cracking reaction is initially conducted in reactor 16 at a
temperature of about 1050.degree. F. (566.degree. C.) to
1150.degree. F. (621.degree. C.), a pressure of about 15 psia and a
space velocity of about 2 hr.sup.-1 and then adjusted to produce a
conversion of about 25%. The reaction product from thermal cracker
16 is fed via line 17 to fractional distillation column 18 where it
is fractionated into four normal alpha olefin fractions of varying
carbon chain length and correspondingly boiling points. Thus, the
lower boiling C.sub.6-C.sub.9 normal alpha olefins are taken off as
product fractions via line 19a, C.sub.9-C.sub.11 normal alpha
olefins via line 20, C.sub.11-C.sub.14 normal alpha olefins via
line 21, C.sub.15 to C.sub.19 normal alpha olefins via line 22 and
finally the higher boiling C.sub.20-C.sub.24 normal alpha olefins
via line 23. In accordance with the invention, the C.sub.6-C.sub.24
normal alpha olefin streams will have a purity of at least about 90
wt. %. Hydrocarbon gases having five carbon atoms or less are
discharged via line 19 and may be used as a fuel to supply energy
to other plant operations. The bottoms fraction comprising
uncracked material and larger chain length olefins, and higher
boiling branched olefins and paraffins is fed to hydrocracker 26
via line 24 instead of being recycled back to thermal cracker 16.
This avoids build up of dienes and branched olefins in the reactor
and correspondingly produces a purier normal alpha-olefin product
because the higher boiling dienes and branched olefins are not
cracked into lower boiling dienes and branched olefins which would
be taken off with the C.sub.6-C.sub.24 normal alpha olefin product
fractions. Instead the high boiling fraction containing higher
boiling dienes, branched olefins and paraffins and uncracked
paraffins are hydrocracked into more valuable products such as
gasoline and middle distillates.
[0069] Referring to distillation column 10, the liquid fraction is
taken off and fed hydrocracker 26 via line 10a. The bright stock
fraction boiling above about 1100.degree. F. (593.degree. C.), is
fed via line 10b to hydrocracker 26 or more preferably at least a
portion of the bright stock fraction is taken off via line 13 for
processing as a heavy lube stock. Similarly a portion of the of the
C.sub.20-C.sub.50 paraffin fraction from column 10 may be taken off
via line 42 for neutral lube oil processing. (Lube oil processing
involves separate hydrocracking not shown and optional
hydrofinishing not shown). Hydrogen is fed to the hydrocracker 26
via line 25.
[0070] Hydrocracker 26 is a fixed bed reactor containing a
nickel-tungsten silica-alumina catalyst and is operated at a
temperature of from 650 to 850.degree. F., a pressure of 500 to
3500 psia and a catalyst space velocity of 0.1 hr.sup.-1 to 10
hr.sup.-1. The reaction product from the hydrocracker is fed via
line 27 to a series of vapor-liquid separators, shown in the
drawing as a single box 28, to remove hydrogen from the reaction
product. The hydrogen recovered from separator 28 is combined with
fresh make up hydrogen 25 and recycled back to the hydrocracker via
lines 29 or alternatively fed directly to hydrocracker 26. The
liquid hydrocrackate from the vapor liquid separators 28 is fed via
line 30 to fractional distillation column 31 where it is
fractionated into a fuel fraction and a lube oil fraction and taken
off via lines 33 and 34 respectively. Lower boiling hydrocarbons
and any residual hydrogen is taken off via line 32 tail gases and
used as an energy source for other plant operations. The bottom
fraction containing uncracked feed and other higher hydrocarbons is
recycled back to the hydrocracker via line 35.
[0071] Obviously many modifications and variations of the invention
described herein can be made without departing from the essence and
scope thereof.
* * * * *