Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD AND SYSTEM FOR PRODUCING A SYNTHESIS GAS
USING AN OXYGEN TRANSPORT MEMBRANE BASED REFORMING SYSTEM
WITH SECONDARY REFORMING AND AUXILIARY HEAT SOURCE
Field of the Invention
(0001) The present invention relates to a method and system for producing a
synthesis gas in
an oxygen transport membrane based reforming system, and more particularly, a
method and
system for producing a synthesis gas in an oxygen transport membrane based
reforming
system that provides both primary and secondary reforming and an auxiliary
heat source.
Backuound
(0002) Synthesis gas containing hydrogen and carbon monoxide is used for a
variety of
industrial applications, for example, the production of hydrogen, chemicals
and synthetic fuel
production. Conventionally, the synthesis gas is produced in a fired reformer
in which natural
gas and steam is reformed in nickel catalyst containing reformer tubes at high
temperatures
(e.g., 850 C to 1000 C) and moderate pressures (e.g., 16 to 30 bar) to
produce the synthesis
gas. The endothermic heating requirements for steam methane reforming
reactions occurring
within the reformer tubes are provided by burners firing into the furnace that
are fueled by
part of the natural gas. In order to increase the hydrogen content of the
synthesis gas
produced by the steam methane reforming (SMR) process, the synthesis gas can
be subjected
to water-gas shift reactions to react residual steam in the synthesis gas with
the carbon
monoxide.
(0003) A well established alternative to steam methane reforming is the non-
catalytic partial
oxidation process (P0x) whereby a sub-stoichiometric amount of oxygen is
allowed to react
with the natural gas feed creating steam and carbon dioxide at high
temperatures. The high
temperature residual methane is reformed through reactions with the high
temperature steam
and carbon dioxide.
(0004) An attractive alternative process for producing synthesis gas is the
autothermal
reforming (ATR) process which uses oxidation to produce heat with a catalyst
to permit
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reforming to occur at lower temperatures than the POx process. Similar to the
POx process,
oxygen is required to partially oxidize natural gas in a burner to provide
heat, high
temperature carbon dioxide and steam to reform the residual methane. Some
steam needs to
be added to the natural gas to control carbon formation on the catalyst.
However, both the
ATR and POx processes require separate air separation units (ASU) to produce
high-pressure
oxygen, which adds complexity as well as capital and operating cost to the
overall process.
(0005) When the feedstock contains significant amounts of heavy hydrocarbons,
SMR and
A.TR pmcesses, are typically preceded by a pre-reforming step. Pre-reforming
is a catalyst
based process for converting higher hydrocarbons to methane, hydrogen, carbon
nionoxide
and carbon dioxide. The reactions involved in pre-reforming are generally
endothermic.
Most pre-reformers on natural gas steams operate in the endothermic area and
operate
adiabatically, and thus the pre-reformed feedstock leaves at a lower
temperature than the
feedstock entering the pre-reformer. Another process that will be discussed in
this invention is
the secondary refolining process, which is essentially an autothermal process
that is fed the
product from a SMR. process. Thus, the feed to a secondary reforming process
is primarily
synthesis gas from steam methane reforming. Depending on the end application,
some natural
gas may bypass the SMR process and be directly introduced into the secondary
reforming step.
Also, when a SMR process is followed by a secondary reforming process, the SMR
may
operate at a lower temperature, e.g. 650T to 825 C versus 850 C to 1.000 C.
(0006) As can be appreciated, the conventional methods of producing a
synthesis gas such as
have been discussed above are expensive and require complex installations. To
overcome the
complexity and expense of such installations it has been proposed to generate
the synthesis
gas within reactors that utilize an oxygen transport membrane to supply oxygen
and thereby
generate the heat necessary to support endothermic heating requirements of the
steam
methane reforming reactions. A typical oxygen transport membrane has a dense
layer that,
while being impervious to air, will transport oxygen ions when subjected to an
elevated
operational temperature and a difference in oxygen partial pressure across the
membrane.
(0007) Examples of oxygen transport membrane based reforming systems used in
the
production of synthesis gas can be found in United States Patent Nos.
6,048,472; 6,110,979;
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6,114,400; 6,296,686; 7,261,751; 8,262,755; and 8,419,827. There is an
operational problem
with all of these oxygen transport membrane based systems because such oxygen
transport
membranes need to operate at high temperatures of around 900 C to 1100 C.
Where
hydrocarbons such as methane and higher order hydrocarbons are subjected to
such high
temperatures within the oxygen transport membrane, excessive carbon formation
occurs,
especially at high pressures and low steam to carbon ratios. The carbon
formation problems
are particularly severe in the above-identified prior art oxygen transport
membrane based
systems. A different approach to using an oxygen transport membrane based
reforming
system in the production of synthesis gas is disclosed in United States Patent
No. 8,349,214
which provides a oxygen transport membrane based reforming system that uses
hydrogen and
carbon monoxide as part of the reactant gas feed to the oxygen transport
membrane tubes and
minimizes the hydrocarbon content of the feed entering the permeate side of
the oxygen
transport membrane tubes. Excess heat generated within the oxygen transport
membrane
tubes is transported mainly by radiation to the reforming tubes made of
conventional materials.
Use of low hydrocarbon content high hydrogen and carbon monoxide feed to the
oxygen
transport membrane tubes addresses many of the highlighted problems with the
earlier oxygen
transport membrane systems.
(0008) Other problems that arise with the prior art oxygen transport membrane
based
reforming systems are the cost of the oxygen transport membrane modules and
the lower than
desired durability, reliability and operating availability of such oxygen
transport membrane
based reforming systems. These problems are the primary reasons that oxygen
transport
membranes based reforming systems have not been successfully commercialized.
Advances
in oxygen transport membrane materials have addressed problems associated with
oxygen
flux, membrane degradation and creep life, but there is much work left to be
done to achieve
commercially viable oxygen transport membrane based reforming systems from a
cost
standpoint as well as from an operating reliability and availability
standpoint.
(0009) The present invention addresses the aforementioned problems by
providing an
improved process for making synthesis gas using a reactively-driven oxygen
transport
membrane based system, which consists of two reactors that can be in the form
of sets of
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catalyst containing tubes ¨ reforming reactor and oxygen transport membrane
reactor. Partial
oxidation and some reforming occurs at the permeate (i.e. catalyst containing)
side of the
oxygen transport membranes and a reforming process facilitated by a reforming
catalyst
occurs in the reforming reactor in close proximity to the oxygen transport
membrane reactor.
The partial oxidation process, which is exothermic, and the reforming process,
which is
endothermic, both occur within the oxygen transport membrane based reforming
system and
thus have a high degree of thermal integration so that heat released in the
oxidation process
supplies the heat absorbed by the reforming process. Specifically,
improvements to the
reactively-driven oxygen transport membrane based system include modifications
to the
reactively-driven oxygen transport membrane based system to carry out both a
primary
reforming process in a catalyst filled reforming reactor as well as a
secondary reforming
process within the catalyst containing oxygen transport membrane reactor and
to provide a
source of auxiliary heat to balance the reforming duty between the oxygen
transport
membrane reactor and the auxiliary heat source.
Summary of the Invention
(00010) The present invention may be characterized as a method for
producing a
synthesis gas in an oxygen transport membrane based reforming system, which
may comprise
at least two reactors that can be in the form of sets of catalyst containing
tubes, including a
reforming reactor and an oxygen transport membrane reactor, the method
comprising the
steps of: (i) reforming a hydrocarbon containing feed stream in a reforming
reactor in the
presence of a reforming catalyst disposed in the reforming reactor and heat to
produce a
reformed synthesis gas stream; (ii) feeding the reformed synthesis gas stream
to a reactant
side of a reactively driven and catalyst containing oxygen transport membrane
reactor,
wherein the oxygen transport membrane reactor includes at least one oxygen
transport
membrane element configured to separate oxygen from an oxygen containing
stream at the
oxidant side of the reactively driven and catalyst containing oxygen transport
membrane
reactor and transport the separated oxygen to the reactant side through oxygen
ion transport
when subjected to an elevated operational temperature and a difference in
oxygen partial
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pressure across the at least one oxygen transport membrane element; (iii)
reacting a portion of
the reformed synthesis gas stream with oxygen permeated through the at least
one oxygen
transport membrane element to produce the difference in oxygen partial
pressure across the at
least one oxygen transport membrane element and generate reaction products and
heat; and
(iv) reforming unreformed hydrocarbon gas in the reformed synthesis gas stream
in the
presence of catalysts contained in the oxygen transport membrane reactor, the
reaction
products and the heat to produce a synthesis gas product stream. A first
portion of the heat
required for the initial or primary reforming step is provided by the
reactively driven and
catalyst containing oxygen transport membrane reactor and a second portion of
the heat
required for the primary reforming step is transferred from an auxiliary heat
source disposed
proximate the reforming reactor.
(00011) The invention may also be characterized as an oxygen transport
membrane
based reforming system comprising: (a) a reactor housing; (b) a reforming
reactor disposed in
the reactor housing and configured to reform a hydrocarbon containing feed
stream in the
presence of a reforming catalyst disposed in the reforming reactor and heat to
produce a
reformed synthesis gas stream; (c) a reactively driven oxygen transport
membrane reactor
disposed in the reactor housing proximate the reforming reactor and configured
to receive the
reformed synthesis gas stream and react a portion of the reformed synthesis
gas stream with
permeated oxygen and generate reaction products and heat, including a first
portion of the
heat required by the reforming reactor; and (d) an auxiliary heat source
disposed in the
reactor housing proximate the reforming reactor and configured to supply a
second portion of
the heat required by the reforming reactor to produce the reformed synthesis
gas stream.
(00012) The reactively driven oxygen transport membrane reactor is further
configured
to reform any unreformed hydrocarbon gas in the reformed synthesis gas stream
in the
presence of one or more catalysts and some of the heat generated by the
reaction of the
reformed synthesis gas stream and permeated oxygen to produce a synthesis gas
product
stream. At a temperature of about 730 C at the exit of the reforming reactor
and a
temperature of about 995 C at the exit of the OTM reactor, the module of the
synthesis gas
product stream is between about 1.85 and 2.15 or more and is dependent on the
amount of
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heat supplied to the reforming reactor from the auxiliary heat source. More
specifically, at the
specified temperatures, the module of the synthesis gas product stream rises
from a minimum
of about 1.85 when the percentage of heat supplied to the reforming reactor
from the auxiliary
heat source is less than 15% to a maximum of about 2.15 when the percentage of
heat
supplied to the reforming reactor from the auxiliary heat source is greater
than about 85%.
Put another way, at temperatures of about 730 C and 995 C at the exit of the
reforming
reactor and OTM reactor respectively, the module of the synthesis gas product
stream will be
between about 1.85 and 2.00 when the second portion of heat supplied to the
reforming
reactor from the auxiliary heat source is 50% or less of the total required
heat to be supplied
to the reforming reactor and between about 2.00 and 2.15 when the second
portion of heat
supplied to the reforming reactor from the auxiliary heat source is more than
50% of the total
required heat to be supplied to the reforming reactor. As indicated above, the
actual module
of the synthesis gas product stream is also dependent on the reforming
temperatures within
the oxygen transport membrane based reforming system, and in particular, the
temperature at
the exit of the reforming reactor. For example, if the temperature at the exit
of the reforming
reactor is raised to a temperature of between 800 C and 900 C, the range of
module for the
synthesis gas product stream would be expected to increase to perhaps between
about 1.90 to
2.25 or more depending on the amount of heat supplied to the reforming reactor
from the
auxiliary heat source.
(00013) In
addition to the variation of the module of the synthesis gas product stream
based on the reforming duty split between the first portion of heat and the
second portion of
heat designed into the oxygen transport membrane based reforming system, the
hydrogen to
carbon monoxide ratio (H2/C0) of the synthesis gas product stream is also
varied slightly
between about 2.95 and 3.10 at a reforming reactor exit temperature of about
730 C and
depending on the amount of heat supplied to the reforming reactor from the
auxiliary heat
source. The carbon monoxide to carbon dioxide ratio (CO/CO2) of the synthesis
gas product
stream also varies between about 2.50 and 3.30 at an exit temperature of about
730 C and
depending on the reforming duty split between the first portion of heat and
the second portion
of heat.
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(00014) The auxiliary heat source may be designed to provide between about
15% and
85% of the heat required for the reforming of the hydrocarbon containing feed
stream. The
auxiliary heat source may be in the form of one or more auxiliary oxygen
transport membrane
reactors or one or more ceramic burners disposed within the reactor housing
and in close
proximity to the reforming reactor.
Brief Description of the Drawin2s
(00015) While the specification concludes with claims distinctly pointing
out the
subject matter that applicants regard as their invention, it is believed that
the invention will be
better understood when taken in connection with the accompanying drawings in
which:
(00016) Fig. 1 is a schematic illustration of an embodiment of an oxygen
transport
membrane based reforming system designed to carry out both a primary reforming
process
and a secondary reforming process within the oxygen transport membrane reactor
using an
auxiliary heat source comprising a second oxygen transport membrane reactor;
(00017) Fig. 2 is a schematic illustration of the oxygen transport
membrane based
reforming system of Fig. 1 tailored for and integrated with a methanol
production process;
(00018) Fig. 3 is a schematic illustration of an alternate embodiment of
an oxygen
transport membrane based reforming system designed to carry out both a primary
reforming
process and a secondary reforming process within the oxygen transport membrane
reactor
using an auxiliary heat source comprising one or more ceramic burners;
(00019) Fig. 4 is a graph that depicts the module of the synthesis gas
produced in the
oxygen transport membrane based reforming system as a function of the percent
of primary
reforming duty attributable to the auxiliary heat source;
(00020) Fig. 5 is a graph that depicts the hydrogen to carbon monoxide
ratio (H2/C0)
of the synthesis gas produced in the oxygen transport membrane based reforming
system as a
function of the percent of primary reforming duty attributable to the
auxiliary heat source; and
(00021) Fig. 6 is a graph that depicts the carbon monoxide to carbon
dioxide ratio
(CO/CO2) of the synthesis gas produced in the oxygen transport membrane based
reforming
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system as a function of the percent of primary reforming duty attributable to
the auxiliary heat
source.
Detailed Description
(00022) Fig. 1 provides a schematic illustration of an embodiment of an
oxygen
transport membrane based reforming system 100 in accordance with the present
invention.
As seen therein, an oxygen containing stream 110, such as air, is introduced
to the system by
means of a forced draft (FD) fan 114 into a heat exchanger 113 for purposes of
preheating the
oxygen containing feed stream 110. Heat exchanger 113 is preferably a high
efficiency,
cyclic and continuously rotating ceramic regenerator disposed in operative
association with
the oxygen containing feed stream 110 and a heated oxygen depleted retentate
stream 124.
The incoming air feed stream 110 is heated in the ceramic regenerator 113 to a
temperature in
the range of about 850 C to 1050 C to produce a heated air feed stream 115.
(00023) The oxygen depleted air leaves the oxygen transport membrane
reforming
tubes as heated oxygen depleted retentate stream 124 at the same or slightly
higher
temperature than the heated air feed stream 115. Any temperature increase,
typically less than
about 30 C, is attributable to the portion of energy generated by oxidizing
reaction of
hydrogen and carbon monoxide in the oxygen transport membrane tubes and
transferred by
convection to the oxygen depleted retentate stream 124.
(00024) The temperature of this oxygen depleted retentate stream 124 is
heated back to
a temperature between about 1050 C and 1200 C prior to being directed to the
heat
exchanger or ceramic regenerator 113. This increase in temperature of the
oxygen depleted
retentate stream 124 is preferably accomplished by use of a duct burner 126,
which facilitates
combustion of a supplemental fuel stream 128 using some of the residual oxygen
in the
retentate stream 124 as the oxidant. Though not shown, an alternative means is
to combust the
supplemental fuel stream 128 with a separate air stream in duct burner 126 and
then mix the
hot flue gas with the oxygen depleted retentate stream 124. In the ceramic
heat exchanger or
regenerator 113, the heated, oxygen depleted retentate stream 124 provides the
energy to raise
the temperature of the incoming feed air stream 110 from ambient temperature
to a
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temperature between about 850 C to 1050 C. The resulting cold retentate
stream exiting the
ceramic heat exchanger, typically containing less than about 5% oxygen, leaves
the oxygen
transport membrane based reforming system 100 system as exhaust gas 131 at a
temperature
of around 150 C.
(00025) Although not shown in Fig. 1, an alternate embodiment of the
oxygen transport
membrane based reforming system 100 could dispose the duct burner and
supplemental fuel
stream upstream of the reactors in intake duct 116. Such arrangement would
allow use of a
smaller ceramic regenerator 113 and less severe operating conditions for the
ceramic
regenerator 113.
(00026) The hydrocarbon containing feed stream 130, preferably natural
gas, to be
reformed is typically mixed with a small amount of hydrogen or hydrogen-rich
gas 132 to form
a combined hydrocarbon feed 133 and then preheated to around 370 C in heat
exchanger 134
that serves as a feed pre-heater, as described in more detail below. Since
natural gas typically
contains unacceptably high level of sulfur species, a small amount of hydrogen
or hydrogen-
rich gas 132 is added to facilitate desulfurization. Preferably, the heated
feed stream 136
undergoes a sulfur removal process via device 140 such as hydro-treating to
reduce the sulfur
species to H2S, which is subsequently removed in a guard bed using material
like ZnO and/or
CuO. The hydro-treating step also saturates any alkenes present in the
hydrocarbon containing
feed stream. Further, since natural gas generally contains higher hydrocarbons
that will break
down at high temperatures to form unwanted carbon deposits that adversely
impact the
reforming process, the natural gas feed stream is preferably pre-reformed in
an adiabatic pre-
reformer, which converts higher hydrocarbons to methane, hydrogen, carbon
monoxide, and
carbon dioxide. Also contemplated but not shown is an embodiment where the pre-
reformer is
a heated pre-reformer that may be thermally coupled with oxygen transport
membrane based
reforming system.
(00027) Superheated steam 150 is added to the pre-treated natural gas and
hydrogen
feed stream 141, as required, to produce a mixed feed stream 160 with a steam
to carbon ratio
between about 1.0 and 2.5, and more preferably between about 1.2 and 2.2. The
superheated
steam 150 is preferably between about 15 bar and 80 bar and between about 300
C and
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600 C and generated in a fired heater 170 using a source of process steam
172. As seen in
Fig. 1, the fired heater 170 is configured to combust a supplemental fuel
stream 174 and
optionally a portion of the off-gas 229 produced by the oxygen transport
membrane based
reforming system using air 175 as the oxidant to heat the process steam 172 to
superheated
steam 150. In the illustrated embodiment, a source of air 175 is heated in the
fired heater 170
to produce a heated air stream 176 to be used as the oxidant in the fired
heated 170. The
mixed feed stream 160 is also heated in the fired heater 170 producing a
heated mixed feed
stream 180. The heated mixed feed stream 180 has a temperature preferably
between about
450 C and 650 C and more preferably a temperature between about 500 C and
600 C.
(00028) The illustrated embodiment of the oxygen transport membrane based
reforming system 100 comprises three reactors (200, 210, 220) disposed in a
single reactor
housing 201. The first reactor is a reforming reactor 200 which comprises
reforming catalyst
containing tubes configured to reform the heated mixed feed stream 180
containing a
hydrocarbon feed and steam in the presence of a conventional reforming
catalyst disposed in
the reforming tubes and heat to produce a reformed synthesis gas stream 205.
The
temperature of the reformed hydrogen-rich synthesis gas stream is typically
designed to be
between 650 C and 850 C.
(00029) The reformed synthesis gas stream 205 is then fed as an influent
to the second
reactor which is an oxygen transport membrane reactor 210. More particularly,
reformed
synthesis gas stream 205 is fed to a reactant side of a reactively driven and
catalyst containing
oxygen transport membrane reactor 210. The reactively driven, oxygen transport
membrane
reactor 210 includes one or more oxygen transport membrane elements or tubes
each having
an oxidant side and a reactant side that are disposed proximate to the
reforming tubes. Each
of the oxygen transport membrane elements or tubes are configured to separate
oxygen from
the heated oxygen containing stream 115 contacting the oxidant side to the
reactant side
through oxygen ion transport. The oxygen ion transport occurs when the oxygen
transport
membrane elements or tubes are subjected to elevated operational temperatures
and there is a
difference in oxygen partial pressure across the oxygen transport membrane
elements or tubes.
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(00030) A portion of the reformed synthesis gas stream 205 fed to the
reactant side of
the oxygen transport membrane reactor 210 immediately reacts with oxygen
permeated
through the oxygen transport membrane elements or tubes to produce the
difference in oxygen
partial pressure across the oxygen transport membrane elements or tubes which
drives the
oxygen ion transport and separation. This reaction produces reaction products
and heat.
A portion of the heat produced by the reaction the reformed synthesis gas
stream 205 and the
permeated oxygen is transferred via convection to the oxygen depleted
retentate stream and
another portion of the heat is transferred via radiation to the reforming
reactor 200.
(00031) The oxygen transport membrane reactor 210 is further configured to
reform
unreformed hydrocarbon gas in the reformed synthesis gas stream 205 and
produce a
synthesis gas product stream 215. This secondary reforming occurs in the
presence of one or
more reforming catalysts contained in the oxygen transport membrane elements
or tubes,
reaction products (e.g. from the reaction of a portion of the reformed
synthesis gas stream 205
and oxygen permeate) and the third portion of the energy or heat produced by
the same
reaction. The synthesis gas product stream 215 leaving the oxygen transport
membrane
reactor 210 is preferably at a temperature between about 900 C and 1050 C.
(00032) The third reactor in the illustrated embodiment is an auxiliary
oxygen transport
membrane reactor 220 that is configured to provide an auxiliary source of
radiant heat to the
reforming reactor 200. This auxiliary reactor 220 or heat source preferably
provides between
about 15% and 85% of the heat required for the initial reforming of the heated
mixed feed
stream 180 that occurs in the reforming reactor 200. The auxiliary oxygen
transport
membrane reactor 220 is also a reactively driven oxygen transport membrane
reactor 220 that
comprises a plurality of oxygen transport membrane elements or tubes disposed
proximate to
or in a juxtaposed orientation with respect to the reforming reactor 200. The
auxiliary
oxygen transport membrane reactor 220 is configured to also separate or
permeate oxygen
from the oxygen containing stream 115 contacting the oxidant side of the
oxygen transport
membrane elements or tubes to the reactant side of the oxygen transport
membrane elements
or tubes through oxygen ion transport. The permeated oxygen reacts with a low
pressure
hydrogen containing stream 222, preferably less than about 3 bar, that is fed
via a valve 221 to
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the reactant side of the oxygen transport membrane elements or tubes to
produce the
difference in oxygen partial pressure across the oxygen transport membrane
element and to
produce an auxiliary reaction product stream 225 and heat.
(00033) In the illustrated embodiment, the low pressure hydrogen
containing stream
222 is a hydrogen and light hydrocarbon containing stream that preferably
includes a
recirculated portion 226 of the synthesis gas product stream and optionally a
supplementary
fuel 224. A portion of the reaction product stream 225 exiting the reactant
side of the
oxygen transport membrane elements or tubes of the oxygen transport membrane
reactor 220
is an off-gas 227 that may be mixed with a supplementary natural gas fuel 228
to the duct
burner 126. Another portion of the reaction product stream 225 exiting the
reactant side of
the oxygen transport membrane elements or tubes is an off-gas 229 that may be
mixed with a
supplementary natural gas fuel 174 to fired heater 170.
(00034) Preferably, the reforming reactor 200 and the oxygen transport
membrane
reactor 210 are arranged as sets of closely packed tubes in close proximity to
one another.
The reforming reactor 200 generally consists of reforming tubes. Oxygen
transport membrane
reactor 210 as well as the auxiliary oxygen transport membrane reactor 220
comprise a
plurality of ceramic oxygen transport membrane tubes. The oxygen transport
membrane tubes
are preferably configured as multilayered ceramic tubes capable of conducting
oxygen ions at
an elevated operational temperature, wherein the oxidant side of the oxygen
transport
membrane tubes is the exterior surface of the ceramic tubes exposed to the
heated oxygen
containing stream and the reactant side or permeate side is the interior
surface of the ceramic
tubes. Within each of the oxygen transport membrane tubes are one or more
catalysts that
facilitate partial oxidation and/or reforming, as applicable. Although only
three of the
reforming tubes are illustrated in Fig. 1 in close proximity to six of the
secondary reforming
oxygen transport membrane elements or tubes and four of the auxiliary oxygen
transport
membrane elements or tubes, there could be many of such oxygen transport
membrane tubes
and many reforming tubes in each oxygen transport membrane based reforming sub-
system or
assembly as would occur to those skilled in the art. Likewise, there could be
multiple oxygen
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transport membrane based reforming sub-systems or assemblies used in
industrial
applications of the oxygen transport membrane based reforming system 100.
(00035) The oxygen transport membrane elements or tubes used in the
embodiments
disclosed herein preferably comprise a composite structure that incorporates a
dense layer, a
porous support and an intermediate porous layer located between the dense
layer and the
porous support. Each of the dense layer and the intermediate porous layer are
capable of
conducting oxygen ions and electrons at elevated operational temperatures to
separate the
oxygen from the incoming air stream. The porous support layer would thus form
the reactant
side or permeate side. The dense layer and the intermediate porous layer
preferably comprise
a mixture of an ionic conductive material and an electrically conductive
material to conduct
oxygen ions and electrons, respectively. The intermediate porous layer
preferably has a lower
permeability and a smaller average pore size than the porous support layer to
distribute the
oxygen separated by the dense layer towards the porous support layer. The
preferred oxygen
transport membrane tubes also include a mixed phase oxygen ion conducting
dense ceramic
separation layer comprising a mixture of a zirconia based oxygen ion
conducting phase and a
predominantly electronic conducting perovskite phase. This thin, dense
separation layer is
implemented on the thicker inert, porous support.
(00036) Oxidation catalyst particles or a solution containing precursors
of the oxidation
catalyst particles are optionally located in the intermediate porous layer
and/or in the thicker
inert, porous support adjacent to the intermediate porous layer. The oxidation
catalyst
particles contain an oxidation catalyst, such as gadolinium doped ceria, are
selected to
promote oxidation of the partially reformed synthesis gas stream in the
presence of the
permeated oxygen when introduced into the pores of the porous support, on a
side thereof
opposite to the intermediate porous layer.
(00037) The endothermic heating requirements of the reforming process
occurring in
the reforming reactor 200 is supplied through radiation of some of the heat
from the oxygen
transport membrane reactor 210 and auxiliary oxygen transport membrane reactor
220
together with the convective heat transfer provided by heated oxygen depleted
retentate
stream. Sufficient thermal coupling or heat transfer between the heat-
releasing ceramic
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oxygen transport membrane tubes and the heat-absorbing catalyst containing
reformer tubes
must be enabled within the design of the present reforming system. A portion
of the heat
transfer between the ceramic oxygen transport membrane tubes and the adjacent
or juxtaposed
reforming catalyst containing reformer tubes is through the radiation mode of
heat transfer
whereby surface area, surface view factor, surface emissivity, and non-linear
temperature
difference between the tubes (e.g., Totm4-Tref0.4) , are critical elements to
achieve the desired
thermal coupling. Surface emissivity and temperatures are generally dictated
by tube material
and reaction requirements. The surface area and surface view factor are
generally dictated by
tube arrangement or configuration within each module and the entire reactor.
While there are
numerous tube arrangements or configurations that could meet the thermal
coupling
requirements between the oxygen transport membrane tubes and the reformer
tubes, a key
challenge is to achieve a relatively high production rate per unit volume
which, in turn,
depends on the amount of active oxygen transport membrane area contained
within the unit
volume. An additional challenge to achieving the optimum thermal coupling
performance is
to optimize the size of the ceramic oxygen transport membrane tubes and the
catalyst
containing reformer tubes, and more particular the effective surface area
ratio, Areformer/Aotm 5
of the respective tubes. Of course, such performance optimization must be
balanced against
the manufacturability requirements, costs, as well as the reliability,
maintainability, operating
availability of the modules and reactor.
(00038) Advantageously, it has been found that the module of the synthesis
gas product
stream produced from the disclosed embodiments of the oxygen transport
membrane based
reforming system varies depending on the exit stream temperatures and the
amount of heat
supplied to the reforming reactor from the auxiliary heat source. For example,
as depicted in
Fig. 4, the module of the synthesis gas product stream produced from the
disclosed
embodiments, when the temperature at the exit of the reforming reactor is
about 730 C and
the temperature at the exit of the OTM reactor is about 995 C, is between
about 1.85 and 2.15
or more and is a function of the amount of heat supplied to the reforming
reactor from the
auxiliary heat source presented as a percentage of total primary reforming
duty that comes
from the auxiliary heat source. Similarly, as seen in Fig. 5, at a reforming
reactor exit
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temperature of about 730 C the hydrogen to carbon monoxide ratio (H2/C0) of
the synthesis
gas product stream is maintained with a small band generally between about
2.95 and 3.10
depending on the amount of heat supplied to the reforming reactor from the
auxiliary heat
source. Again, the amount of heat supplied to the reforming reactor from the
auxiliary heat
source is depicted in Fig. 5 as a percentage of total primary reforming duty
that comes from
the auxiliary heat source. Lastly, as seen in Fig. 6 and at a reforming
reactor exit temperature
of about 730 C, the carbon monoxide to carbon dioxide ratio (CO/CO2) of the
synthesis gas
product stream ranges between about 2.50 and 3.30 depending on the amount of
heat supplied
to the reforming reactor from the auxiliary heat source.
(00039) The actual module, H2/C0 ratio and CO/CO2 ratio of the synthesis
gas product
stream is very much dependent on the exit temperatures realized within the
oxygen transport
membrane based reforming system. The graphs of Figs. 4-6 represent a
temperature of about
730 C at the exit of the reforming reactor. If this temperature is raised to
a temperature of
between about 800 C and 900 C, the range of module for the synthesis gas
product stream
would be expected to also increase, perhaps to between about 1.90 to 2.25 or
more depending
on the amount or percentage of reforming duty heat supplied to the reforming
reactor from the
auxiliary heat source. Increasing the temperature at the exit of the OTM
reactor typically
results in a decrease in the module of the synthesis gas.
(00040) As indicated above, the auxiliary heat source is configured, or
more preferably
designed, to provide between about 15% and 85% of the total heat required for
the primary
reforming of the hydrocarbon containing feed stream in the reforming reactor.
The auxiliary
heat source may be an auxiliary oxygen transport membrane reactor as shown in
Figs. 1 and 2
or may comprise one or more ceramic burners as shown, for example, in Fig. 3
described in
more detail below. At the low end of the 15% to 85% range, the module of the
synthesis gas
product stream is around 1.90 whereas at the higher end of the range, the
module of the
synthesis gas product stream is between about 2.10 and 2.15 or more. An
alternative way to
characterize the graph of Fig. 4 and the synthesis gas product produced by the
presently
disclosed oxygen transport membrane based reforming systems is that the module
of the
synthesis gas product stream is between about 1.85 and 2..00 when the heat
supplied from the
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auxiliary heat source to the reforming reactor is 50% or less of the total
required heat to be
supplied to the reforming reactor, and between about 2.00 and 2.15 or more
when the heat
supplied to the reforming reactor from the auxiliary heat source is more than
50% of the total
required heat to the reforming reactor. As indicated above, if the temperature
at the exit of the
reforming reactor is raised, one would expect a corresponding increase in
module of the
synthesis gas product to 2.25 or more depending on the amount of heat supplied
to the
reforming reactor from the auxiliary heat source.
(00041) As a result, it is possible to design and/or tailor the present
oxygen transport
membrane based reforming system to produce a synthesis gas having desired
characteristics
by simply adjusting or modifying the heat duty split between oxygen transport
membrane
reactor and the auxiliary heat source as well as the exit temperatures. The
desired or targeted
synthesis gas characteristics will depend of course on the application of the
synthesis gas and
other system variables, such as stream exit temperatures, methane slip,
reactor pressures, etc.
(00042) Turning again to Fig. 1, the synthesis gas stream 215 produced by
the oxygen
transport membrane reactor 210 generally contains hydrogen, carbon monoxide,
unconverted
methane, steam, carbon dioxide and other constituents. A significant portion
of the sensible
heat from the synthesis gas stream 215 can be recovered using a heat exchange
section or
recovery train 250. Heat exchange section 250 is designed to cool the produced
synthesis gas
stream 215 exiting the oxygen transport membrane reactor 210. In this
illustrated
embodiment, the heat exchange section 250 is also designed such that in
cooling the synthesis
gas stream 215, process steam 172 is generated, the combined hydrocarbon feed
stream 133
is preheated, and boiler feed water 255 and feed water 259 are heated.
(00043) To minimize metal dusting issues, the hot synthesis gas product
stream 215,
preferably at a temperature between about 900 C and 1050 C is cooled to a
temperature of
about 400 C or less in a Process Gas (PG) Boiler 252. The initially cooled
synthesis gas
product stream 254 is then used to preheat the mixture of natural gas and
hydrogen feed
stream 133 in a feed pre-heater 134 and subsequently to preheat boiler feed
water 255 in the
economizer 256 and to heat the feed water stream 259. In the illustrated
embodiment, the
boiler feed water stream 255 is preferably pumped using a feed water pump (not
shown),
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heated in economizer 256 and sent to steam drum 257 while the heated feed
water stream is
sent to a de-aerator (not shown) that provides boiler feed water 255.
Synthesis gas leaving
the feed water heater 258 is preferably around 150 C. It is cooled down to
about 40 C using
a fin-fan cooler 261 and a synthesis gas cooler 264 fed by cooling water 266.
The cooled
synthesis gas 270 then enters a knock-out drum 268 where water is removed from
the bottoms
as process condensate stream 271 which, although not shown, is recycled for
use as feed
water, and the cooled synthesis gas 272 is recovered overhead.
(00044) The final synthesis gas product 276 is obtained from the
compression of the
cooled synthesis gas stream 273 in a synthesis gas compressor 274. Depending
on the
application, multiple stages of compression may be required. The inter-stage
cooling and
condensate knock out is not shown in Fig. 1. Prior to such compression,
however, a portion
of the cooled synthesis gas stream 226 may optionally be recirculated to the
reactor housing to
form all or part of the low pressure hydrogen containing stream 222. Depending
on the
operating pressures of the oxygen transport membrane based reforming system,
pressure of
the recovered synthesis gas is preferably in the range of about 10 bar and 35
bar and more
preferably in the range of 12 bar and 30 bar. The module of the final
synthesis gas product
produced in the described embodiment is typically about 1.8 to 2.3.
(00045) Fig. 2 is a schematic illustration of the oxygen transport
membrane based
reforming system of Fig. 1 tailored for and integrated with a methanol
production process.
In many regards, this embodiment is similar to the embodiment of Fig. 1 and,
for sake of
brevity, the description of the common aspects of the two embodiments will not
be repeated
here, rather, the following discussion shall focus on the differences. The
syngas is typically
compressed to between about 80 and 100 bar in syngas compressor 274. In the
embodiment
shown in Fig. 2, the final synthesis gas product 276 is mixed with a methanol
loop recycle
stream 310. This mixed stream 320 of compressed synthesis gas and methanol
loop recycle is
indirectly heated in heat exchanger 322 by the synthesized methanol stream 324
to a
temperature between about 175 C and 300 C. The heated stream 326 is directed
to the
methanol synthesis reactor 330. The exact heat arrangement will vary depending
on the type
of methanol synthesis reactor, technology vendor and approach to overall
process integration,
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i.e. integrating with the front-end or syngas generation section. In this
methanol synthesis
reactor 330, hydrogen, carbon monoxide and carbon dioxide are consumed to
produce
methanol and water in an exothermic process through the following reactions:
CO + 2H2 CH3OH
CO2 + 3H2 CH3OH + H20
(00046) The heat generated in the methanol synthesis reaction is used for
steam
production and/or for preheating of the synthesis gas feed. Temperature at the
outlet of the
methanol reactor is typically between about 200 C and about 260 C. This
methanol synthesis
stream 324 is cooled down to about 38 C in heat exchanger 322 and cooler 332
before
entering a separator 334 where the crude methanol stream 340 containing mostly
methanol,
water and trace amounts of other species (e.g. dimethyl ether, ethanol and
higher alcohols), is
separated in the bottoms and sent to further distillation steps for final
purification. Most of the
overhead stream 336 from the separator 334 is a methanol loop recycle stream
344 sent back
to the methanol synthesis reactor 330 via recycle compressor 345 to increase
the carbon
conversion to methanol. The recycle compressor 345 is required to compensate
for pressure
drop across the methanol synthesis reactor 330 and associated equipment, e.g.
heat
exchangers and coolers.
(00047) A small portion of the overhead stream 336, typically between
about 1% and 5%
is purged from the methanol synthesis loop 300 to prevent buildup of inerts in
the methanol
synthesis loop 300. The typical composition of the purge stream 350 is as
follows: 75%
hydrogen, 3% carbon dioxide, 12% carbon dioxide, 3% nitrogen, and 7% methane,
with a
higher heating value of about 325 BTU/scf. The methanol loop purge stream 350
is then split
into two streams, namely methanol purge stream 350A which is directed back to
the auxiliary
oxygen transport membrane reactor 220 as the hydrogen containing feed and
methanol purge
stream 350B which forms the hydrogen-rich gas that is combined with the
hydrocarbon
containing feed stream to form a combined hydrocarbon feed 133. In the
illustrated
embodiment, the low pressure hydrogen containing stream 222 is a mixture of a
portion of the
methanol purge stream 350A and a supplemental natural gas fuel stream 224.
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(00048) Fig. 3 is a schematic illustration of an alternate embodiment of
an oxygen
transport membrane based reforming system designed to carry out both a primary
reforming
process and a secondary reforming process within the oxygen transport membrane
reactor
using an auxiliary heat source comprising one or more ceramic burners. In many
regards, this
embodiment is similar to the embodiment of Fig. 2 and, for sake of brevity,
the description of
the common aspects of the two embodiments will not be repeated here, rather,
the following
discussion shall focus only on the differences.
(00049) The main difference between the embodiment shown in Fig. 2 with
the
embodiment of Fig. 3 is that the third reactor consisting of a reactively
driven oxygen
transport membrane reactor is replaced with one or more porous ceramic burners
(i.e.
flameless burners) disposed in the reactor housing 201 proximate to the
reforming reactor 200
and the reactively driven and catalyst containing oxygen transport membrane
reactor 210.
The one or more ceramic burners 555 are preferably configured to burn a light
hydrocarbon
containing stream using air or enriched air as the oxidant. When using a
porous ceramic
burner as the auxiliary heat source it is important to design the spatial
arrangement of the
ceramic burners vis-à-vis the oxygen transport membrane reactor and reforming
reactor so as
to maximize the thermal coupling and system efficiency while minimizing the
mechanical
complexity of the system. Unlike the use of oxygen transport membrane
reactors, the use of
porous ceramic burners within the reactor housing requires other design
challenges and
modifications of the system to fully integrate the burners as the auxiliary
heat source. Such
challenges and modifications may include providing a separate oxidant stream
and/or a
separate fuel source for the porous ceramic burners. In addition, the start-up
procedures as
well as the exhaust manifolding differences between the embodiments using the
low pressure
oxygen transport membrane reactor and those embodiments using one or more
porous
ceramic burners would be potentially significant and must taken into
consideration.
(00050) Although not shown in Fig. 3 the porous ceramic or flameless
burners may
preferably be radiant tube type burners having a similar tubular or
cylindrical configuration as
the oxygen transport membrane reactor tubes depicted in Figs. 1 and 2 with
combustion
occurring on the interior of the tube. Yet another ceramic burner
configuration is to arrange a
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plurality of porous tubular ceramic burners with the fuel transport from the
interior of the tube
to the exterior surface with the combustion occurring on the outside surface
using the oxygen
depleted retentate stream as the oxidant. Still other contemplated
arrangements of the
auxiliary heat source may include a radial or circumferential ceramic burner
arrangement or
perhaps even a circumferential arrangement of the burner.
(00051) Another difference between the embodiments shown in Fig. 2 and
Fig. 3 is that
the duct burner 126A is disposed upstream of the reactor housing 201 in intake
duct 116 and
coupled to a supplemental fuel stream and/or the recirculated methanol purge
stream 350B.
In such arrangement, the operating conditions for the ceramic regenerator 113
are less severe
and would save capital expense by allowing use of a smaller ceramic
regenerator 113. If
necessary, the duct burner 126A could instead be placed downstream of reactor
housing 201
as in Fig. 2.
(00052) While the present invention has been characterized in various ways
and
described in relation to preferred embodiments, as will occur to those skilled
in the art,
numerous, additions, changes and modifications thereto can be made without
departing from
the spirit and scope of the present invention as set forth in the appended
claims.
