U.S. patent application number 13/224600 was filed with the patent office on 2013-04-11 for process for providing hydrogen to a synthesis gas conversion reaction.
This patent application is currently assigned to Chevron U.S.A Inc.. The applicant listed for this patent is Babak Fayyaz, Shabbir Husain, Charles Kibby, Lixin You. Invention is credited to Babak Fayyaz, Shabbir Husain, Charles Kibby, Lixin You.
Application Number | 20130090394 13/224600 |
Document ID | / |
Family ID | 48042471 |
Filed Date | 2013-04-11 |
United States Patent
Application |
20130090394 |
Kind Code |
A1 |
Husain; Shabbir ; et
al. |
April 11, 2013 |
PROCESS FOR PROVIDING HYDROGEN TO A SYNTHESIS GAS CONVERSION
REACTION
Abstract
A synthesis gas conversion process for carrying out the process
is disclosed. A hydrogen-containing sweep gas is caused to flow
across a water permselective membrane adjacent a synthesis gas
conversion reaction zone in which synthesis gas is contacted with a
catalyst and converted to effluent including water. Water is
removed from the reaction zone through the membrane. The sweep gas
has sufficient hydrogen partial pressure to cause hydrogen to pass
through the membrane into the reaction zone.
Inventors: |
Husain; Shabbir;
(Emeryville, CA) ; Kibby; Charles; (Benicia,
CA) ; You; Lixin; (Sugar Land, TX) ; Fayyaz;
Babak; (Danville, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Husain; Shabbir
Kibby; Charles
You; Lixin
Fayyaz; Babak |
Emeryville
Benicia
Sugar Land
Danville |
CA
CA
TX
CA |
US
US
US
US |
|
|
Assignee: |
Chevron U.S.A Inc.
San Ramon
CA
|
Family ID: |
48042471 |
Appl. No.: |
13/224600 |
Filed: |
October 7, 2011 |
Current U.S.
Class: |
518/711 ;
422/187 |
Current CPC
Class: |
C07C 7/144 20130101;
B01J 2219/2453 20130101; C07C 1/041 20130101; B01J 8/0257 20130101;
B01J 8/0214 20130101; B01J 8/009 20130101; B01J 2208/00212
20130101; B01J 2219/2481 20130101; B01J 2219/2462 20130101; C07C
7/144 20130101; B01J 2219/2459 20130101; C07C 1/041 20130101; C07C
9/04 20130101; C07C 9/04 20130101; B01J 8/067 20130101; B01J
2219/2475 20130101; B01J 19/249 20130101 |
Class at
Publication: |
518/711 ;
422/187 |
International
Class: |
C07C 1/04 20060101
C07C001/04; B01J 12/00 20060101 B01J012/00 |
Claims
1. A synthesis gas conversion process comprising: (a) contacting a
gas feed comprising hydrogen and carbon monoxide at a feed flow
rate with a synthesis gas conversion catalyst in a reaction zone in
which hydrogen and carbon monoxide react to form reaction products
including water; (b) removing the water from the reaction products
through a membrane in communication with the reaction zone, said
membrane having a retentate side facing the reaction zone and a
permeate side opposite the retentate side; and (c) flowing a sweep
gas containing hydrogen at a sweep gas flow rate across the
permeate side of the membrane whereby hydrogen passes from the
permeate side to the reaction zone.
2. The process of claim 1, wherein the membrane is selected from
the group consisting of zeolite membranes, ceramic membranes,
polymeric membranes and composite membranes.
3. The process of claim 1, wherein the membrane has a water
permeance of at least about 1000 GPU.
4. The process of claim 1, wherein the membrane has a water/carbon
monoxide selectivity of at least about 10.
5. The process of claim 1, wherein a ratio of the sweep gas flow
rate to the feed flow rate is between about 0.10 and about 2.0.
6. The process of claim 1, wherein the reaction zone is within a
fixed bed reactor tube in which the synthesis gas conversion
catalyst occupies an annular volume bounded by the wall of the
reactor tube and by a membrane tube formed by the membrane, the
membrane tube having an outer retentate side and an inner permeate
side; and wherein the sweep gas is supplied through a conduit
extending into the membrane tube such that the sweep gas flows
inside the membrane tube between the membrane tube and the
conduit.
7. The process of claim 1, wherein the reaction zone is in the form
of a channel within a plate type reactor comprising alternating
layers of reaction zone channels, cooling channels, and
membranes.
8. The process of claim 1, in which the hydrogen and carbon
monoxide react at a single pass carbon monoxide conversion at least
10 mol % higher than a process not comprising steps (b) and
(c).
9. The process of claim 1, wherein the reaction zone has a length
at least 5% lower than required in a process not comprising steps
(b) and (c) to produce an equivalent amount of hydrocarbon
product.
10. The process of claim 7, wherein the reaction zone has a number
of reactor tubes at least 10% lower than the number of reactor
tubes required in a process not comprising steps (b) and (c) to
produce an equivalent amount of hydrocarbon product.
11. The process of claim 1, wherein the membrane has a water
permeance that varies along at least one dimension of the
membrane.
12. The process of claim 1, wherein the membrane is in the general
shape of a cone having a smaller diameter at its upstream end
portion and a larger diameter at its downstream end portion such
that the membrane surface area varies along the length of the
reactor.
13. The process of claim 1, wherein the membrane comprises multiple
membranes segments, at least some of the membrane segments having a
different water permeance than other of the membrane segments.
14. The process of claim 1, wherein the feed of synthesis gas and
the sweep gas flow co-currently.
15. The process of claim 1, wherein the feed of synthesis gas and
the sweep gas flow counter-currently.
16. The process of claim 1, wherein the hydrogen partial pressure
in a downstream sweep zone adjacent the permeate side of the
membrane is greater than the hydrogen partial pressure in a
downstream reaction zone adjacent the retentate side of the
membrane.
17. A system comprising: a) a reactor including a housing and a
membrane within the housing which defines a reaction zone and a
sweep zone, the reaction zone adapted to convert syngas into
products including hydrocarbons and water vapor in the presence of
an appropriate catalyst, the membrane allowing the water vapor to
permeate from the reaction zone to the sweep zone and a sweep gas
to permeate from the sweep zone to the reaction zone; and h) a
control system including a controller, at least one pressure sensor
in communication with the controller for sensing the pressure in at
least one of the reaction zone and the sweep zone, and a valve in
communication with the controller for controlling sweep gas
introduced into the sweep zone.
18. The system of claim 18, further comprising a gas chromatogram
in communication with the controller and in communication with at
least one location within the reactor for determining hydrogen
concentration at the at least one location.
Description
FIELD
[0001] The present invention relates to methods wherein water and
hydrocarbon products are produced as part of a synthesis gas
conversion and the water is removed in situ from the reaction
products using a membrane.
BACKGROUND
[0002] Removal of water is a key issue to be addressed in synthesis
gas conversion reactions. For instance, water is a primary
by-product in a Fischer-Tropsch (FT) reaction and its presence is
generally detrimental to the overall efficiency of the FT reaction.
In an FT reaction, a synthesis gas mixture of carbon monoxide (CO)
and hydrogen gas (H.sub.2), referred to hereinafter as "syngas," is
converted in the presence of an FT catalyst (most commonly iron- or
cobalt-based) into hydrocarbon products, water and other
byproducts. The syngas may be generated from a number of carbon
containing sources such as natural gas, coal or bio-mass. It is
often desirable to convert these carbon sources into a liquid
hydrocarbon mixture from their original gas or solid states.
[0003] As the FT reaction occurs at relatively high temperature,
the water produced is generally in the form of water vapor.
Produced water vapor reduces the partial pressures of FT reactants,
thus affecting reaction kinetics and reducing reaction rates. Water
vapor is also detrimental to the life of FT catalysts, and
especially in high partial pressures, leads to the oxidation of the
catalyst and the sintering of the catalyst support, resulting in a
reduction in the catalyst activity. Due to these adverse effects of
water on the FT reaction, conventional FT fixed bed reactors have a
relatively low rate of per pass CO conversion to limit high water
partial pressures in the reactor. Conventional FT fixed bed
reactors separate water from other reaction products and unreacted
CO and H.sub.2 gas after they exit the reactor's outlet. The
unreacted CO is often recycled back to an FT reactor inlet so that
it may again potentially be converted into a hydrocarbon, at the
cost of increased throughputs, resulting in larger reactors.
[0004] Efforts with respect to in situ dehydration in conversion of
syngas to hydrocarbon products and water have been described. U.S.
patent application Ser. No. 12/342,799 (Fayyaz-Najafi et al.),
assigned to Chevron U.S.A. Inc., hereby incorporated by reference
in its entirety, describes improved designs for FT reactors, in
which water is removed in situ using a membrane and wherein heat
management issues are also addressed.
[0005] Another issue to be addressed in synthesis gas conversion
reactions is control of the ratio of hydrogen to carbon monoxide
(H.sub.2/CO) in the syngas, as this affects the product
distribution. When this ratio is too high, reaction products
include undesirably high levels of methane and light gas. When this
ratio is too low, reaction products include undesirably high levels
of olefin and oxygenates. Additionally, consumption of hydrogen in
the FT reactor occurs rapidly in the initial or upstream section of
the reactor thereby lowering the partial pressure of hydrogen and
thus the reaction rate and the H.sub.2/CO ratio in the downstream
section of the reactor. Although the downstream end of the reactor
has available heat removal capacity, this capacity remains unused
when this section of the reactor is hydrogen starved.
[0006] It would be desirable to provide an improved process for the
in situ removal of water from a synthesis gas conversion reactor
such as an FT reactor. It would be further desirable to
simultaneously provide for the addition of hydrogen at a controlled
rate along the length of such a reactor to maintain sufficiently
high hydrogen to carbon monoxide ratio to overcome the
aforementioned current design constraints, thereby increasing the
productivity of the reactor.
SUMMARY
[0007] In one aspect, the present invention relates to a synthesis
gas conversion process in which a feed of synthesis gas comprising
hydrogen and carbon monoxide is contacted with a synthesis gas
conversion catalyst in a reaction zone in which the hydrogen and
carbon monoxide react to form reaction products including water.
The water is removed from the reaction products through a membrane
in communication with the reaction zone, said membrane having a
retentate side facing the reaction zone and a permeate side
opposite the retentate side. A sweep gas containing hydrogen is
caused to flow across the permeate side of the membrane at a
hydrogen partial pressure sufficient to cause hydrogen to pass from
the permeate side to the reaction zone.
[0008] In another aspect, the present invention relates to a system
including a reactor having a housing and a membrane within the
housing which defines a reaction zone and a sweep zone, the
reaction zone adapted to convert syngas into products including
hydrocarbons and water vapor in the presence of an appropriate
catalyst, the membrane allowing the water vapor to permeate from
the reaction zone to the sweep zone and a sweep gas to permeate
from the sweep zone to the reaction zone. The system further
includes a control system including a controller, at least one
pressure sensor in communication with the controller for sensing
the pressure in at least one of the reaction zone and the sweep
zone, and a valve in communication with the controller for
controlling sweep gas introduced into the sweep zone.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] These and other objects, features and advantages of the
present invention will become better understood with regard to the
following description, appended claims and accompanying drawings
where:
[0010] FIG. 1 is an illustration of a membrane tube employed in a
membrane reactor according to one embodiment of the invention;
[0011] FIG. 2 is a schematic drawing of a multi-tubular fixed bed
synthesis gas conversion reactor according to one embodiment of the
invention, including an in situ water removal membrane with a
hydrogen-containing sweep gas;
[0012] FIG. 3 is a schematic drawing of a multi-tubular fixed bed
synthesis gas conversion reactor, according to another embodiment
of the invention, including an in situ water removal membrane in
fluid communication with a hydrogen-containing sweep gas;
[0013] FIG. 4 is a schematic drawing of a plate type synthesis gas
conversion reactor according to yet another embodiment of the
invention, including an in situ water removal membrane in fluid
communication with a hydrogen-containing sweep gas;
[0014] FIG. 5 is a schematic drawing of a conventional
multi-tubular fixed bed synthesis gas conversion reactor, not
including an in situ water removal membrane;
[0015] FIG. 6 is a graph comparing the H.sub.2/CO ratios along the
length of a Fischer-Tropsch fixed bed reactor for three scenarios:
utilizing a water removal membrane with a hydrogen-containing sweep
gas, utilizing a water removal membrane with a nitrogen-containing
sweep gas and not utilizing a water removal membrane; and
[0016] FIG. 7 is a graph comparing the CO conversion rates along
the length of a Fischer-Tropsch fixed bed reactor for three
scenarios: utilizing a water removal membrane with a
hydrogen-containing sweep gas, utilizing a water removal membrane
with a nitrogen-containing sweep gas and not utilizing a water
removal membrane.
DETAILED DESCRIPTION
[0017] In one embodiment, the disclosed process provides a
synthesis gas conversion process including the in situ removal of
water from a reactor using a water permselective membrane. By
"water permselective membrane" is meant a membrane which allows
water to pass there through preferentially relative to other
components. A syngas feed is supplied to a reaction zone within a
reactor in which hydrogen and carbon monoxide, in the presence of a
synthesis gas conversion catalyst, react to form an effluent
including light gas, liquid hydrocarbon products and water. The
present process can be adapted to various syngas compositions,
including syngas with a relatively low H.sub.2/CO ratio, which
allows the use of the same design in different operations. The
syngas may, for example, be obtained from natural gas, and also
from peat, coal, biomass, or other hydrocarbon fractions by
processes like gasification, autothermal reforming, catalytic or
non-catalytic partial oxidation. The synthesis gas conversion
reaction can be, by way of example and not limitation, any reaction
in which water is produced, including, for example, a
Fischer-Tropsch reaction, a methanol synthesis reaction or a
dimethyl ether (DME) synthesis reaction.
[0018] The reaction zone can be within a fixed bed synthesis gas
conversion reactor tube loaded with synthesis gas conversion
catalyst. The effluent is then separated. The light gas, or tail
gas, can optionally be recycled to the reaction zone. The
hydrocarbons are removed from the reactor as product. The majority
of water vapor is removed from the reactor in situ through a water
permselective membrane in communication with the reaction zone. The
membrane has a retentate side facing the reaction zone and a
permeate side opposite the retentate side, i.e., facing away from
the reaction zone. The membrane allows water vapor to readily pass
there through from the reaction zone while inhibiting the passage
of other reactants and products.
[0019] In operation, a syngas feed is introduced at the upstream
end of the reactor into the reaction zone. Under suitable reaction
conditions, desired synthesis gas conversion reactions, e.g.,
Fischer Tropsch reactions, occur. Reaction products can include
hydrocarbon products of varying carbon chain lengths, CO.sub.2 and
water and a variety of other compounds. Under these conditions, the
water is in the form of water vapor. Accordingly, water vapor
preferentially passes through the water permselective membrane as a
permeate stream while the other reaction products and unreacted
feed preferentially remain in the reaction zone and are eventually
discharged as a part of a retentate stream through the downstream
end of the reactor. Ideally, unreacted H.sub.2 and CO will be
separated from the discharged retentate stream and recycled to the
upstream portion of reaction zone and/or to a syngas reformer using
processes known in the art.
[0020] Suitable water permselective membranes can be selected from
zeolite membranes, ceramic membranes, polymeric membranes and
composite membranes. Composite membranes include composites of
ceramic and polymeric materials, composites of metallic and
polymeric materials, and composites of ionic liquids and porous
supports. In another embodiment zeolite membranes can be used, for
instance Linde type 4A zeolite membranes such as those available
from Mitsui Engineering & Shipbuilding Co., Ltd, Japan, and
Fraunhofer Institute for Ceramic Technologies and Systems IKTS,
Germany. Suitable membranes have a water/carbon monoxide
selectivity of at least 10, even at least 100. Suitable membranes
have a water permeance of at least 1000 GPU (gas permeation units),
even at least 4000 GPU. One GPU is defined as the gas or vapor flow
rate through a material per unit area and per unit of pressure
difference across the material, with the unit defined as 10.sup.-6
cm.sup.3(stp).cm.sup.-2s.sup.-1cmHg.sup.-1. The membrane can be
supported by a porous support, such as a ceramic, polymeric or
metal support. The water permeance can be constant across the
length of the membrane, or in some embodiments, the water permeance
can be caused to vary across at least one dimension of the membrane
in order to increase the water removal in the downstream sections
of the reactor, since most of the water accumulation occurs in the
downstream portion of the reactor. For example, the membrane may be
coaxial about a longitudinally extending axis and the permeance of
the membrane may increase in a downstream axial direction. In one
embodiment, this can be accomplished by employing multiple membrane
segments along the length of the reactor, each segment having a
different water permeance. Alternatively, the membrane can be in
the general shape of a cone having a smaller diameter at its
upstream end portion and a larger diameter at its downstream end
portion such that the membrane surface area varies along the length
of the reactor. The concept of a conical membrane in a membrane
reactor is disclosed in co-pending U.S. patent application Ser. No.
12/342,799 (Fayyaz-Najafi et al.), hereby incorporated by reference
in its entirety. In such an embodiment, a membrane assembly having
a porous generally conical shaped support member supporting a
membrane will replace the membrane tube of FIG. 1. The radius of
this membrane assembly increases from its upstream end to it
downstream end. By way of example and not limitation, the membrane
may be other shapes where the area perpendicular to the axis
increases in the downstream direction, such as a parabolic cone as
opposed to a purely frustoconical shape.
[0021] A sweep gas containing hydrogen is caused to flow across the
permeate side of the membrane at a pressure sufficient to cause
hydrogen to pass from the permeate side of the membrane to the
reaction zone along the length of the reactor, either continuously
or at discrete locations. The sweep gas contains a high partial
pressure of hydrogen gas and permeation of hydrogen to the reaction
zone occurs when the hydrogen partial pressure on the permeate side
is higher than the hydrogen partial pressure on the retentate side
facing the reaction zone. Not only is hydrogen inhibited from
"leaking" out of the reaction zone through the water removal
membrane, thus having a negative impact on productivity, but
hydrogen is actually added to the reaction zone, thus enhancing
conversion of the synthesis gas.
[0022] The flow rate of the sweep gas (volumetric or molar) can be
between about 10 and about 200%, even between about 40 and about
100%, of the flow rate of the syngas feed, i.e., the ratio of the
sweep gas flow rate to the feed flow rate can be between about 0.10
and about 2.0, even between 0.40 and 1.0. The flow rate of the
sweep gas is thus calculated based on the flow rate of the feed.
When the flow rate of the sweep gas is too low, water cannot be
removed effectively since the driving force causing water to cross
the membrane would be insufficient to remove water effectively. The
sweep gas and the syngas feed can flow in either co-current or
counter-current directions relative to one another. That is, the
sweep gas may flow downstream in the reactor while the hydrogen
passes upstream from the downstream portion of the reactor.
Alternatively, both the syngas feed and the hydrogen gas can be
introduced relative to the upstream portion of the reactor.
[0023] The addition of hydrogen to the reaction zone is controlled
by the difference in hydrogen partial pressures between the
reaction zone and the sweep zone. To control the hydrogen/carbon
monoxide molar ratio within a desired range, hydrogen addition to
the reaction zone can be minimized near the reactor inlet and
maximized near the reactor outlet. A simple representation of a
tubular membrane element in FIG. 1 is used to illustrate the
relevant pressure conditions. The location generally near the
reactor inlet in the reaction zone 8 is described as the upstream
reaction zone 8A, while the location generally near the reactor
outlet in the reaction zone is described as the down-stream
reaction zone 8B. The location generally near the reactor inlet in
the sweep zone 22 is described as the upstream sweep zone 22A,
while the location generally near the reactor outlet in the sweep
zone is described as downstream sweep zone 22B. Syngas feed (also
referred to as feed) 1 is fed to the reaction zone. Sweep gas can
be fed to flow along membrane 9 either co-currently (i.e., in the
same direction as feed 1) as indicated by 3A or counter-currently
(i.e., in the opposite direction as feed 1) as indicated by 3B.
[0024] When the sweep gas is fed in a co-current configuration (as
indicated by 3A), the partial pressure of hydrogen in the upstream
reaction zone 8A is desirably roughly equivalent to the partial
pressure of hydrogen in the upstream sweep zone 22A, in order to
minimize loss or addition of hydrogen to the reaction zone in the
region near the reactor inlet. In order to maximize the addition of
hydrogen to the reaction zone in the region near the reactor
outlet, the partial pressure of hydrogen in the downstream sweep
zone 22B is desirably greater than the partial pressure of hydrogen
in the downstream reaction zone 8B. As a result of the reaction and
consumption of hydrogen in the reaction zone, the partial pressure
of hydrogen in the downstream reaction zone 8B is less than the
partial pressure of hydrogen in the upstream reaction zone 8A.
Similarly, the partial pressure of hydrogen in the sweep zone is
lower in the downstream sweep zone 22B than in the upstream sweep
zone 22A.
[0025] When the sweep gas is fed in a counter-current configuration
(as indicated by 3B), the above relative pressure conditions also
apply, with the exception that the partial pressure of hydrogen in
the sweep zone is higher in the downstream sweep zone 22B than in
the upstream sweep zone 22A. In either co-current or
counter-current sweep gas flow, the pressure of the sweep gas at
the reactor inlet is controlled to minimize loss or addition of
hydrogen to the upstream reaction zone 8A, while the pressure of
the sweep gas near the reactor outlet is controlled to maximize the
addition of hydrogen to the downstream reaction zone 8B.
[0026] In one embodiment, gas samples from reactor outlet 32A,
downstream sweep zone location (inlet or outlet, depending on
whether sweep gas flow is counter-current or co-current,
respectively) 32B, upstream sweep zone location (outlet or inlet)
32C and reactor inlet 32D can be analyzed by an analyzer (not
shown), for example a gas chromatogram (GC), to determine the
concentration of hydrogen at the inlet and outlet of both the
reaction zone and the sweep zone. The total gas pressure can also
be measured by pressure sensors at one or more of these four
locations, 32A-D. Information on the total pressure measurements
and the hydrogen concentration measurements can be sent to a
controller 30 using lines 31A-D. Using this information, the
controller 30 can determine the partial pressure of hydrogen at
each of the four locations. The controller can then send signals to
pressure control valves via lines 34A and/or 34B, for example, to
control a pressure control valve 33A and/or 33B in communication
with a sweep gas inlet (not shown) to control the pressure and
consequently the hydrogen partial pressure within the sweep zone 22
as desired. The pressure control valve(s) in the downstream portion
of the sweep zone can be sized to allow a sweep gas flow between
about 10% and 200% of the feed gas flow to the reaction zone.
Additional valves can be brought online as needed to accommodate
higher flows, can provide greater flexibility in allowing both
pressure and flow to be controlled in the sweep zone.
[0027] In one embodiment, as shown in FIG. 2 and described in
detail hereinafter, the reaction zone is located within a fixed bed
reactor tube in which the synthesis gas conversion catalyst
occupies an annular volume within the tube which is bounded on the
outside by the wall of the reactor tube and on the inside by a
membrane tube formed by the membrane. This is also referred to as a
double tube-in-tube reactor design (tube in a tube in a tube). The
membrane tube has an outer retentate side and an inner permeate
side. In this embodiment, sweep gas is supplied through a pipe or
conduit extending into the membrane tube such that the sweep gas is
caused to flow inside the membrane tube, between the conduit and
the membrane tube. In one embodiment, the disclosed process further
provides for the addition of hydrogen to the reactor. In this
embodiment, the sweep gas has a hydrogen partial pressure
sufficiently high to cause hydrogen to pass from the permeate side
through the membrane to the desired location in the reaction zone.
The double tube-in-tube reactor design as described above and shown
in FIG. 2 can be used. While the figure illustrates two tubes, it
will be understood by those skilled in the art that the reactor may
include many such tubes. By adding hydrogen to the reaction zone
along the length of the reactor, a more constant H.sub.2/CO ratio
can be maintained along the length of the reactor. The rate of
hydrogen addition can be controlled by adjusting the hydrogen
partial pressure driving force across a given membrane.
[0028] One embodiment of an FT reactor having a water removal
membrane and utilizing a hydrogen-containing sweep gas is shown in
FIG. 2. In operation, a syngas feed 1 is introduced into reactor
10A by way of reactant inlet 18 and into reaction zone 8. Reactants
(H.sub.2 and CO) come in from the top of the tubular reactor and
flow downward into the catalyst bed. FT conversions take place in
reaction zone 8 with FT products being produced including
hydrocarbon and water vapor. The reaction zone 8 is contained at
the lower end by perforated plate 13. The fluid FT products stream
2 is then allowed to exit FT reactor 10A by way of products outlet
20. These products can be then separated with unreacted CO and
H.sub.2 gas being recycled (not shown) back to reactant inlet
18.
[0029] In this embodiment, the FT reactor 10A has the capability of
providing a sweep gas to enhance the in situ water vapor removal
from FT reactor 10A. Reactor 10A has catalyst (not shown) packed
into a reaction zone 8 formed between tubes 23 and 9. Partially
mounted in the FT reactor is a membrane assembly 14 which has
multiple tubes with porous walls 9 and an end plate 24 which seals
the tubes, thereby defining a water vapor zone. Membrane materials,
such as a zeolite membrane, are affixed to a support wall to permit
water vapor to readily pass from reaction zone 8, through water
permselective material or membrane 9 and into the water vapor zone.
The top of membrane assembly 14 is a tube sheet (i.e., a circular
plate with multiple holes drilled with specific pattern to
accommodate the membrane tubes). By way of example and not
limitation, the outer diameter of tube 23 can be, for example, in
the range of 1.05-2.375 inches (2.7-6.0 cm), and even 1.315-1.9
inches (3.3-4.8 cm). The outer diameter of tube 9 can be in the
range of 0.675-1.9 inches (1.7-4.8 cm), and even 0.84-1.66 inches
(2.1-4.2 cm).
[0030] An outer shell 25 provides a water bath chamber 7,
surrounding reaction zone 8. Water 5 is introduced into cooling
water inlet 21 and surrounds reaction zone 8 to maintain the
temperature in reactor 10 at a predetermined temperature. Heat
supplied from reaction zone 8 transforms the water into steam 6
which exits the reactor by way of steam outlet 19. Water inlet 21
and steam outlet 19 are in fluid communication with water chamber
7. Controlling the water flow and the pressure and boiling
temperature of water in water bath chamber 7 allows the temperature
in reaction zone 8 to be controlled.
[0031] A sweep gas assembly 11 is provided for introducing a
hydrogen-containing sweep gas 3 into the water vapor zone within
each membrane tube 9. Sweep gas assembly 11 has multiple conduits
which are inserted into the water vapor zone within each membrane
tube 9 and serve to deliver sweep gas 3 to the lower end of the
water vapor zone. Sweep gas assembly 11 is in fluid communication
with an end cap 15 which has a sweep gas inlet 16.
[0032] A significant portion of the water vapor produced passes
through membrane 9 into the water vapor zone on the inner permeate
side of each membrane. The partial pressure of water in the water
vapor zone is maintained at a relatively low value compared to
reaction zone 8, in part due to the sweep gas. Sweep gas is
introduced into sweep gas inlet 16; passes inside the sweep gas
conduits of sweep gas assembly 11 to the lower end of each water
vapor zone; and then flows counter current to the syngas feed along
membrane 9 to assist in the removal of water vapor. The sweep gas 3
contains reactant hydrogen gas. The combined water vapor and sweep
gas steam 4 is then swept out of the reactor by way of water vapor
outlet 17.
[0033] In an alternative embodiment, as shown in FIG. 3, reactor
10B operates in a similar manner as reactor 10A described above and
illustrated in FIG. 2, except that in this embodiment, the location
of the cooling water and the sweep gas relative to the reaction
zone are reversed, such that the cooling water cools from an
internal space within each reaction zone 8 and the sweep gas passes
across the membrane tube 9 which is disposed about each reaction
zone 8. Sweep gas 3 enters through sweep gas inlet 28 and flows
through sweep zone 27, across the outer surface of membrane tube 9,
and exits as gas stream 4 through sweep gas outlet 29.
[0034] A cooling water assembly is provided for introducing cooling
water 5 into the internal space within each reaction zone 8.
Cooling water assembly conduits 26 are inserted into the internal
space within each reaction zone 8 and serve to deliver cooling
water 5 to the lower end of the reaction zone. The internal space
within each reaction zone 8 is defined by conduit 37 of tube sheet
assembly 38. Cooling water assembly 26 is in fluid communication
with an end cap 15 which has a cooling water inlet 35. Cooling
water stream 6 exits through cooling water outlet 36.
[0035] In yet another alternative embodiment, as shown in FIG. 4,
the reaction zone 108 is in the form of a channel located within a
plate type reactor 110 having multiple alternating layers of
reaction zone channels 108, cooling channels 107 and water
permselective membranes 109. Each layer is kept separated by porous
spacers 122. Syngas feed is introduced to the reaction zone
channels 108 which contain catalyst. Reaction products including
water are produced in reaction zone channels 108, and water is
removed across membranes 109. In this embodiment, hydrogen sweep
gas is introduced across the face of membranes 109. Water or
another suitable coolant occupies cooling channels 107, thus
controlling the temperature of reaction zone channels 108. FIGS. 2,
3 and 4 illustrate only three of many possible configurations, as
would be apparent to one skilled in the art.
[0036] By controlling the rate of hydrogen addition, it has been
found that the present process provides for a number of advantages
as compared with a similar process with no hydrogen sweep gas used.
In some embodiments, the rate of carbon monoxide conversion is
increased as a result of added hydrogen in the reaction zone and a
higher ratio of hydrogen to carbon monoxide in the unconverted
syngas along the length of the reactor. In some embodiments, the
present process allows for a reduced reactor length, a reduced
volume of catalyst and/or a reduced number of reactor tubes since
the conversion rate is enhanced. In some embodiments, the present
process allows for a reduced amount of tail gas recycle since the
conversion is enhanced, thus desirably reducing the need for
compression of tail gas to be returned to the reactor.
[0037] In a Fischer-Tropsch process, typically, the reaction
conditions include using a suitable FT catalyst such as an
iron-based or cobalt-based catalyst or a mixture of both. In one
embodiment, the reaction occurs at a temperature between about
160.degree. C. and about 350.degree. C., even between about
200.degree. C. and about 250.degree. C. In another embodiment, the
temperature is kept at about 180-220.degree. C. when cobalt-based
catalysts are used and about 250-280.degree. C. when iron-based
catalysts are used. The pressure in the reaction zone is between
about 1 and about 100 atmospheres, even between about 10
atmospheres and about 30 atmospheres. The pressure on the permeate
side of the water removal membrane is maintained at a lower
pressure than that in the reaction zone where the FT conversions
take place. A hydrogen-containing sweep gas is used to further
reduce the partial pressure of water on the permeate side of the
water removal membrane and hence increase the driving force for the
water separation. The gaseous hourly space velocity of the reaction
is less than about 20,000 volumes of syngas per volume of catalyst
per hour. The syngas feed has a H.sub.2/CO ratio between about 0.5
and about 2.5, even between amount 1.0 and about 2.0. In general,
the upper limit of the H.sub.2/CO ratio will be the usage ratio of
the unit. The usage ratio is the ratio in which hydrogen and CO are
used in a reactor which varies depending on the nature of the
catalyst and the process conditions applied.
EXAMPLES
[0038] Computer modeling was used to compare an embodiment using a
pure hydrogen sweep gas with a comparable membrane reactor using a
nitrogen sweep gas and with a comparable non-membrane FT reactor. A
double tube-in-tube type reactor as illustrated in FIG. 2 was used
for the embodiments using sweep gas. A comparable non-membrane
reactor is defined as a reactor having an equivalent size and
configuration as the reactor shown in FIG. 2, and generally having
the configuration of reactor 210 as shown in FIG. 5. There is no
membrane tube and no provisions for a sweep gas. In operation, a
syngas feed 201 is introduced into reactor 210 by way of reactant
inlet 218 and into reaction zone 208 containing catalyst (not
shown). Reactants enter the top of the tubular reactor and flow
downward into the catalyst bed. FT conversions take place in
reaction zone 208 with FT products being produced and water vapor.
The reaction zone 208 is contained at the lower end by perforated
plate 213. The products stream 202 is then allowed to exit the
reactor by way of products outlet 220. Water bath chamber 207
surrounds reaction zone 208. Water 205 is introduced into cooling
water inlet 221 and surrounds reaction zone 208 to maintain the
temperature in the reactor at a predetermined temperature. Heat
supplied from the reaction zone transforms the water into steam 206
which exits the reactor by way of steam outlet 219. Water inlet 221
and steam outlet 219 are in fluid communication with water chamber
207. Controlling the water flow and the pressure and boiling
temperature of water in water bath chamber 207 allows the
temperature in reaction zone 208 to be controlled.
[0039] A simulation was conducted using software based on Aspen
Custom Modeler, commercially available from Aspen Technology Inc.,
Burlington, Mass. The membrane transport properties, operating
conditions and reactor dimensions assumed are listed in Table 1.
Reaction kinetics for a cobalt-based FT catalyst containing 7.5 wt
% Co and 0.19 wt % Ru on a support containing 80 wt % ZSM-12
zeolite and 20 wt % Al.sub.2O.sub.3 was assumed. The simulation was
conducted for a single tube-in-tube reactor, with the results
multiplied by 100,000, assuming a reactor with 100,000 reaction
tubes. The syngas (H.sub.2/CO=1.6) flow rate per tube was assumed
to be 0.99108 lb-mol/hr.
[0040] Sweep gas ratio is defined as the mole of sweep gas per
total mole of feed gas.
TABLE-US-00001 TABLE 1 Membrane properties H.sub.2O permeance 1.38
.times. 10.sup.-5 mol/cm.sup.2-bar-sec (4400 GPU) H.sub.2O/H.sub.2
selectivity 50 H.sub.2O/CO selectivity 125 H.sub.2O/CO.sub.2
selectivity 60 H.sub.2O/N.sub.2 selectivity 150 H.sub.2O/CH.sub.4
selectivity 200 Operating conditions H.sub.2/CO ratio of the feed
1.6 Pressure 20 bar (2000 kPa) Temperature 208.degree. C. Sweep gas
ratio 67% Sweep gas pressure 8 bar (800 kPa) Tube-in-tube reactor
design Reactor tube outer diameter 1.66 in (4.2 cm) Reactor tube
inner diameter 1.426 in (3.6 cm) Membrane tube outer diameter 1.05
in (2.7 cm) Membrane tube inner diameter 0.83 in (2.1 cm) Reactor
length 36 ft (11 m)
[0041] FIG. 6 is a graph comparing the H.sub.2/CO ratios along the
length of a Fischer-Tropsch fixed bed reactor for three scenarios:
utilizing a water removal membrane with a hydrogen-containing sweep
gas, utilizing a water removal membrane with a nitrogen-containing
sweep gas and not utilizing a water removal membrane, with a
H.sub.2/CO ratio of the feed of 1.6. The results indicate that a
higher H.sub.2/CO ratio is maintained in the hydrogen sweep
membrane reactor along the reactor length versus both the nitrogen
sweep membrane reactor and the non-membrane reactor. The higher
H.sub.2/CO ratio near the downstream end of the reactor is believed
to reduce the formation of olefins and oxygenates in the reactor.
Reduced formation of olefins may result in lower upgrade costs
since downstream hydrotreatment can be avoided or reduced. Reduced
formation of oxygenates may result in lower water treatment
costs.
[0042] FIG. 7 is a graph comparing the CO conversion rates along
the length of a Fischer-Tropsch fixed bed reactor for the same
three scenarios. The results indicate that hydrogen permeating from
the reaction zone into the membrane in the nitrogen sweep case
reduces the single pass CO conversion of the reactor for a given
reactor length. Considerably higher CO conversion is achieved for a
hydrogen sweep membrane reactor as compared with both the nitrogen
sweep membrane reactor and the non-membrane reactor.
[0043] Higher levels of CO conversion result in reduced need for
recycle, increased hydrocarbon product yields, higher carbon
efficiency and lower reactor feed rate. Table 2 lists the benefits
of the use of a water removal membrane with hydrogen-containing
sweep gas (also referred to as "hydrogen sweep membrane reactor")
when compared with a reactor not utilizing a membrane and sweep gas
(also referred to as "non-membrane reactor"). "Product" refers to
the amount of produced liquid hydrocarbons. "Recycle ratio" refers
to the ratio of recycle stream flow rate to the fresh syngas flow
rate. The recycle stream is the stream of unreacted CO, H.sub.2,
inert gases (N.sub.2, CO.sub.2) and light hydrocarbons from the
reactor outlet after condensing liquid products and water, the
recycle stream being recycled to the reactor inlet. "Carbon
efficiency" refers to the amount of carbon in the final product
divided by the carbon in the feed.
TABLE-US-00002 TABLE 2 % delta, hydrogen sweep membrane Hydrogen
sweep Non-membrane reactor vs. non- membrane reactor reactor
membrane reactor Reactor length, 36 (11 m) 39 (12 m) -7.7 feet
(meters) Number of 100,000 120,000 -16.7 reactor tubes Overall CO
90.9 85.8 +6.3 conversion, mol % CO single pass 60 51 +17.6
conversion, mol % Product, barrels 37,883 35,351 +7.2 per day
Recycle ratio 0.766 0.900 -14.9 Carbon 77.0 71.2 +8.1 efficiency, %
Reactor feed flow 99,108 123,597 -19.8 rate, lb-mol/hr Water
partial 1.3 (130 kPa) 5.6 (560 kPa) -76.8 pressure at reactor end,
bar
[0044] While in the foregoing specification this invention has been
described in relation to certain preferred embodiments thereof, and
many details have been set forth for purpose of illustration, it
will be apparent to those skilled in the art that the invention is
susceptible to alteration and that certain other details described
herein can vary considerably without departing from the basic
principles of the invention.
* * * * *