Method and system for producing a liquid hydrocarbon product from a Fischer-Tropsch process using a synthesis gas produced from an oxygen transport membrane based reforming reactor

A system and method for producing a liquid hydrocarbon product from a Fischer-Tropsch process using a synthesis gas feed produced in an oxygen transport membrane based reforming reactor. The system and method involve reforming a mixed feed stream comprising natural gas, hydrogen and the Fischer-Tropsch tail gas, in a reforming reactor in the presence of steam, radiant heat from oxygen transport membrane elements and a reforming catalyst to produce a reformed synthesis gas stream comprising hydrogen, carbon monoxide, and unreformed hydrocarbon gas. The reformed synthesis gas stream is further reformed in an oxygen transport membrane based reforming reactor and conditioned to produce a synthesis gas product stream preferably having a H2/CO ratio of from about 1.7 to about 2.2. The synthesis gas product stream is then synthesized using a Fischer Tropsch process to produce the liquid hydrocarbon product and a Fischer-Tropsch tail gas.

FIELD OF THE INVENTION

The present invention relates to the production of liquid hydrocarbon products in a Fischer-Tropsch process, and more particularly to a method and system for producing liquid hydrocarbon products using synthesis gas produced from an oxygen transport membrane based reforming reactor.

BACKGROUND

The catalytic hydrogenation of carbon monoxide to produce light gases, liquids and waxes, ranging from methane to heavy hydrocarbons (C80and higher) in addition to oxygenated hydrocarbons, is typically referred to as Fischer-Tropsch or FT synthesis. Traditional FT processes primarily produce a high weight percent FT wax (C25and higher) from the catalytic conversion process. These FT waxes are then hydro-cracked and/or further processed to produce diesel, naphtha, and other fractions. During this hydro-cracking process, light hydrocarbons are also produced, which may require additional upgrading to produce viable products. These processes are well known and described in the art.

As indicated above, the costs associated with the production of synthesis gas for use in an FT process, such as liquid fuel production, represent a significant portion of the total cost of the plant and the quality characteristics of the synthesis gas is critical to the efficient operation of the plant. The synthesis gas used in the FT synthesis is typically characterized by the hydrogen to carbon monoxide ratio (H2:CO). A H2:CO ratio of from about 1.8 to about 2.1 defines the desired ratio of synthesis gas used in many gas to liquids production process.

Synthesis gas containing hydrogen and carbon monoxide is produced for a variety of industrial applications. Conventionally, the synthesis gas is produced in a steam methane reforming (SMR) process using a fired reformer in which natural gas and steam are 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). 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.

A well-established alternative to steam methane reforming is the non-catalytic partial oxidation process (POx) 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 catalytic reactions with the high temperature steam and carbon dioxide. Yet another attractive alternative process for producing synthesis gas is the auto-thermal reformer (ATR) process which uses oxidation to produce heat with a catalyst to permit reforming to occur at lower temperatures than the POx process. However, similar to the POx process, the ATR process requires oxygen to partially oxidize natural gas in a burner to provide heat, as well as high temperature carbon dioxide and steam to reform the residual methane. Normally some steam needs to be added to the natural gas to control carbon formation on the catalyst. However, both the ATR as well as POx processes require an air separation unit (ASU) to produce high-pressure oxygen, which adds complexity as well as capital and operating cost to the overall process.

When the feedstock contains significant amounts of heavy hydrocarbons, the SMR and ATR processes are typically preceded by a pre-reforming step. Pre-reforming is a catalyst based process for converting higher hydrocarbons to methane, hydrogen, carbon monoxide and carbon dioxide. The reactions involved in pre-reforming are typically endothermic. Most pre-reformers operate adiabatically, and thus the pre-reformed feedstock typically leaves at a much lower temperature than the feedstock entering the pre-reformer. Another process that will be discussed in this invention is the secondary reforming process, which is essentially an ATR type process that is fed the product from a steam methane reforming 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. 650° C. to 825° C. versus 850° C. to 1000° C.

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 reforming reactions. A typical oxygen transport membrane has a dense layer that, while being impervious to air or other oxygen containing gas will transport oxygen ions when subjected to an elevated operational temperature and a difference in oxygen partial pressure across the membrane.

Examples of oxygen transport membrane based reforming systems used in the production of synthesis gas can be found in U.S. Pat. Nos. 6,048,472; 6,110,979; 6,114,400; 6,296,686; and 7,261,751. There is an operational problem with some or 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 U.S. Pat. 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.

There is a continuing need to improve the efficiency and cost-effectiveness of production of liquid hydrocarbon products from a Fischer-Tropsch process. Accordingly, there is a specific need to identify and develop advanced technologies that will improve the efficiency and reduce the cost of producing synthesis gas for use in applications for producing liquid fuels, as well as improving or customizing the characteristics of synthesis gas for such applications.

SUMMARY OF THE INVENTION

The present invention in one or more aspects can be characterized as a method for producing a synthesis gas in an oxygen transport membrane based reforming system configured for use in a Fischer-Tropsch or Fischer-Tropsch type process. Examples of oxygen transport membrane based reforming systems employable in the present invention are described in U.S. patent application Ser. Nos. 14/078,897, 14/508,297, 14/508,326, and 14/508,344, which are all incorporated herein by reference. In one embodiment the method comprising the steps of: (i) reforming a feed stream in a reforming reactor in the presence of steam, heat and a reforming catalyst disposed in the reforming reactor to produce a reformed synthesis gas stream comprising hydrogen, carbon monoxide, and unreformed hydrocarbon gas; and (ii) further reforming the reformed synthesis gas stream in the presence of one or more catalysts contained in an oxygen transport membrane based reforming reactor, reaction products and heat to produce a synthesis gas product stream; wherein a portion of the heat required for the reforming of the feed stream is transferred via radiation from the oxygen transport membrane based reforming reactor which is disposed proximate the reforming reactor; and wherein the feed stream comprises a methane containing feed and a tail gas feed wherein the tail gas feed is produced in the Fischer-Tropsch process. The synthesis gas product stream is converted into a liquid hydrocarbon product and a Fischer-Tropsch tail gas using a Fischer Tropsch process or Fischer Tropsch type process. The step of further reforming the reformed synthesis gas stream further comprises: (a) feeding the reformed synthesis gas stream to a reactant side of a reactively driven and catalyst containing oxygen transport membrane based reforming reactor, wherein the oxygen transport membrane based reforming reactor includes at least one oxygen transport membrane element configured to separate oxygen from an oxygen containing stream at an oxidant side of the reactively driven and catalyst containing oxygen transport membrane reforming reactor to the reactant side through oxygen ion transport when subjected to an elevated operational temperature and a difference in oxygen partial pressure across the at least one oxygen transport membrane element; (b) reacting a portion of the reformed synthesis gas stream at the reactant side of the reactively driven and catalyst containing oxygen transport membrane based reforming reactor 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, reaction products, and heat, including the radiant heat transferred to the reforming reactor for the reforming of the feed stream; and (c) reforming the unreformed hydrocarbon gas in the reformed synthesis gas stream in the oxygen transport membrane based reforming reactor in the presence of the catalysts, the reaction products and the heat to produce the synthesis gas product stream.

The present invention may also be characterized as a method for producing a liquid hydrocarbon product from a Fischer-Tropsch or Fischer-Tropsch type process, the method comprising the steps of: (i) reforming a feed stream in a reforming reactor in the presence of steam, heat and a reforming catalyst disposed in the reforming reactor to produce a reformed synthesis gas stream comprising hydrogen, carbon monoxide, and unreformed hydrocarbon gas; (ii) further reforming the reformed synthesis gas stream in the presence of one or more catalysts contained in an oxygen transport membrane based reforming reactor, reactions products and heat to produce a synthesis gas product stream; and (iii) synthesizing the synthesis gas product stream using a Fischer Tropsch process to produce the liquid hydrocarbon product and a Fischer-Tropsch tail gas. A portion of the heat required for the reforming of the feed stream in the reforming reactor is transferred via radiation from the oxygen transport membrane based reforming reactor which is disposed proximate the reforming reactor and the feed stream comprises a methane containing feed and a portion of the Fischer-Tropsch tail gas. The step of further reforming the reformed synthesis gas stream further comprises: (a) feeding the reformed synthesis gas stream to a reactant side of a reactively driven and catalyst containing oxygen transport membrane based reforming reactor, wherein the oxygen transport membrane based reforming reactor includes at least one oxygen transport membrane element configured to separate oxygen from an oxygen containing stream at an oxidant side of the reactively driven and catalyst containing oxygen transport membrane reforming reactor to the reactant side through oxygen ion transport when subjected to an elevated operational temperature and a difference in oxygen partial pressure across the at least one oxygen transport membrane element; (b) reacting a portion of the reformed synthesis gas stream at the reactant side of the reactively driven and catalyst containing oxygen transport membrane based reforming reactor 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, reaction products, and heat, including the radiant heat transferred to the reforming reactor for the reforming of the feed stream; and (c) reforming the unreformed hydrocarbon gas in the reformed synthesis gas stream in the oxygen transport membrane based reforming reactor in the presence of the catalysts, the reaction products and the heat to produce the synthesis gas product stream.

In all embodiments of the above-described methods, the ratio of H2/CO in the synthesis gas product stream is about 1.7 to about 2.9, and in another embodiment from about 1.9 to about 2.2. To achieve this relatively low H2/CO ratio, the feed stream generally comprises from about 20% to about 45% by volume of the tail gas feed and from about 55% to about 80% by volume of the methane containing feed. About 50% to about 80% by volume of the tail gas produced in the Fischer-Tropsch process is diverted to obtain the desired feed stream. Optionally the feed stream can be formed to also contain a hydrogen gas feed wherein the hydrogen gas feed is at most 20% by volume of the feed stream. An optional feature or process step in the above described methods is the diversion of a portion of the synthesis gas product stream to a hydrogen separation membrane to produce a synthesis gas stream with lower H2/CO ratio (also referred to as a carbon monoxide rich stream) and a hydrogen rich permeate. In the preferred embodiments, less than about 25% of the synthesis gas product stream is diverted to the hydrogen separation membrane. The synthesis gas with lower H2/CO ratio exiting the hydrogen separation membrane is then recombined with the synthesis gas product stream to produce a conditioned synthesis gas stream wherein the conditioned synthesis gas stream has a H2/CO ratio of about 1.7 to about 2.2. A portion of the hydrogen rich stream, typically after some compression, may be directed to the feed stream. Alternatively for the case of multi-stage reactors in the Fischer-Tropsch section, a portion of the hydrogen-rich stream may be used to increase the H2/CO ratio of the synthesis gas feed to the second or subsequent stage of Fischer-Tropsch synthesis. This portion of the hydrogen rich stream could also be upgraded to high purity H2 in a pressure swing adsorption unit (PSA), which would generate a hydrogen-containing tail gas as byproduct. High purity H2 could be used in Fischer-Tropsch synthesis as described above and/or used in the final upgrading step that converts Fischer-Tropsch liquids to finished products.

In all embodiments of the above-described methods the synthesis gas product stream from the oxygen transport membrane based reforming system is fed to a Fischer-Tropsch process also referred to as a Fischer-Tropsch type process to produce at least a hydrocarbon liquid product and a Fischer-Tropsch tail gas by product. The Fischer-Tropsch process employs a Fischer-Tropsch reactor selected from the group consisting essentially of a fixed bed reactor, a slurry phase reactor, a synthol reactor, or a microchannel reactor. The Fischer-Tropsch process can be configured as a multi-stage Fischer-Tropsch process comprising two or more Fischer-Tropsch reactors and a portion of the hydrogen-rich stream is fed to one or more of the Fischer-Tropsch reactors.

DETAILED DESCRIPTION

FIGS. 1 and 2provide schematic illustrations of the present system and method for producing liquid hydrocarbon products via a Fischer-Tropsch process using synthesis gas produced from an oxygen transport membrane based reforming subsystem. The illustrated system200preferably includes: (i) an air supply and preheating subsystem201; (ii) a reforming feed and conditioning subsystem202; (iii) an oxygen transport membrane based reforming subsystem203; (iv) a heat recovery subsystem204; (v) a synthesis gas conditioning subsystem206; and (vi) a Fischer-Tropsch synthesis subsystem208. As described in more detail below, the various subsystems are fluidically integrated in a manner that improves the overall efficiency and cost-effectiveness of liquid hydrocarbon production. In particular, the tail gas from the Fischer-Tropsch process is recycled and optionally used as both a supplemental fuel as well as part of the feed stream in the synthesis gas production.

The air supply and preheating subsystem includes a source of feed air or other oxygen containing feed stream210; a continuously rotating regenerative air preheater213configured to heat the source of feed air210; and conduits216for supplying the heated feed air stream215from the regenerative air preheater213to the oxygen transport membrane based reforming subsystem203. The air supply and preheat subsystem further includes return conduits225configured to return the heated, oxygen depleted air stream224from the oxygen transport membrane based reforming subsystem to the regenerative air preheater (e.g. ceramic regenerator)213to heat the source of feed air210and subsequently exhaust the cooled oxygen depleted stream as exhaust stream232.

An oxygen containing stream210, such as air, is preferably introduced to the system by means of a forced draft (FD) fan214into a high efficiency, cyclic, continuously rotating ceramic regenerative air preheater213disposed in operative association with the incoming air or oxygen containing feed stream210and the heated retentate stream224exiting the reforming subsystem for purposes of preheating the incoming air or oxygen containing feed stream210. The ceramic regenerator213heats the incoming air feed stream210to a temperature in the range of from about 850° C. to about 1000° C.

The heated feed air stream215is directed to the oxidant-side of the oxygen transport membrane based reforming subsystem203, and more particularly to the oxidant-side of the oxygen transport membrane elements or tubes220within the oxygen transport membrane based reforming subsystem203. As the heated feed air stream215flows across the oxidant-side surfaces of the oxygen transport membrane elements or tubes220, oxygen ions from the heated feed air stream permeate through the oxygen transport membrane elements or tubes220to the reactant side of the oxygen transport membrane elements or tubes220. The oxygen ions recombine at the permeate side of the oxygen transport membrane elements or tubes220and react with a hydrogen containing stream298at the permeate side to create the heat and a difference in oxygen partial pressure across the oxygen transport membrane element220which drives the oxygen transport.

As a result of the reactively driven oxygen ion transport across the membranes, the feed air stream215becomes generally depleted of oxygen and heated by the convective heat transfer between the oxygen transport membrane elements or tubes220and the passing air stream215. At the high temperatures within the oxygen transport membrane based reforming subsystem203, approximately 50% or more, in another embodiment 70% or more of the oxygen within the heated feed air stream215is transported or permeated across the oxygen transport membrane elements or tubes220. The oxygen depleted air224leaves the oxygen transport membrane reforming subsystem as a heated retentate stream224at a higher temperature than the heated air feed stream215. The heated, oxygen depleted retentate stream224is first used to heat the steam containing mixed feed stream238to a temperature from about 450° C. and 650° C., in another embodiment to a temperature from about 500° C. and 600° C., and may optionally be used to further heat steam to superheated steam (not shown). It is conceivable that the mixed feed heater279and optional steam superheater disposed within the return conduits225could alternatively be located in a separate fired heater (not shown). In that case, the fuel requirements of the duct burner described below will be substantially less.

The temperature of this oxygen depleted retentate stream224preferably needs to be then increased back to a temperature of from about 1000° C. to about 1200° C. prior to being directed to the ceramic heat exchanger or regenerator213. This increase in temperature of the oxygen depleted, retentate stream224is preferably accomplished by use of a duct burner226, which facilitates combustion of a supplemental fuel stream228using some of the residual oxygen in the retentate stream. In the ceramic heat exchanger or regenerator213, the re-heated, oxygen depleted retentate stream provides the energy to raise the temperature of the incoming feed air stream210from ambient temperature to a temperature of from about 850° C. to about 1050° C. The resulting cold retentate stream exiting the ceramic heat exchanger, typically containing less than 5% oxygen is exhausted at a temperature of around 150° C. as exhaust stream232. Alternatively, the duct burner226may be disposed directly in the air intake duct216downstream of the continuously rotating ceramic regenerator213to further pre-heat the incoming feed air stream210. Such an arrangement would allow use of a smaller regenerator and less severe operating conditions. It may also enable the use of a regenerator with conventional materials instead of ceramics. The supplemental fuel stream228can be a source of natural gas or a portion of the tail gas routed from elsewhere in the plant or a combination thereof. As described in more detail below, the preferred tail gas is typically associated with the Fischer-Tropsch synthesis subsystem.

The reforming feed and conditioning subsystem202is configured to include a feed conditioning section. More particularly, the feed stream292to be reformed in the oxygen transport membrane based reforming subsystem203is typically natural gas or associated gas based feed that is mixed with a portion of the Fischer-Tropsch tail gas and optionally a small amount of hydrogen or hydrogen-rich gas. Preferably, the feed stream292comprises from about 20% to about 45% by volume of the Fischer-Tropsch tail gas and from about 55% and 80% by volume of the methane containing feed (i.e. natural gas or associated gas). As shown inFIG. 1the feed stream292is preheated, if necessary, in a preheater250to a temperature of from about 300° C. to about 400° C. Since natural gas typically contains unacceptably high level of sulfur species, a small amount of hydrogen or hydrogen-rich gas, is added to the natural gas feed stream to facilitate desulfurization. Preferably, the heated feed stream282undergoes a sulfur removal process via device290such as hydro-treating unit to reduce the sulfur species to H2S, which is subsequently removed in a guard bed using material like ZnO or CuO. The hydrotreating step also saturates any alkenes present in the hydrocarbon containing feed stream. Alternately the feed stream292can be formed by first desulfurizing the methane containing feed, i.e. natural gas or associated gas in the hydro-treating unit290and then mixing the resulting desulfurized methane containing feed with a portion of the Fischer-Tropsch tail gas.

Saturated steam, or in another embodiment superheated steam280is then preferably added to the desulfurized and conditioned feed stream, as required, to produce a steam containing mixed feed stream238having a steam to carbon ratio of from about 1.0 to about 2.5, and more preferably from about 1.2 to about 2.2. The steam280is preferably from about 15 bar to about 80 bar and from about 300° C. to about 600° C. and may be generated in a fired heater (not shown) using a source of process steam or diverted from other portions of the system. The resulting steam containing mixed feed stream238is heated by means of indirect heat exchange with the heated retentate stream224to produce a heated mixed feed stream239at a temperature of from about 300° C. to about 650° C. and in another embodiment to a temperature of from about 450° C. to about 600° C.

Further, since the natural gas or associated gas based feed stream generally contains some higher hydrocarbons that will break down at high temperatures to form unwanted carbon deposits that adversely impact the reforming process, the steam containing mixed feed stream239may optionally be pre-reformed in an adiabatic pre-reformer. Although not shown in the illustrated embodiment, the pre-reformer converts the higher hydrocarbons present in the feed stream to methane, hydrogen, carbon monoxide, and carbon dioxide. An alternative pre-reformer suitable for use with the present embodiments would be a heated pre-reformer that is thermally coupled with the oxygen transport membrane based reforming subsystem. The pre-reformed feed stream is then directed to the oxygen transport membrane based reforming reactor, as described in the paragraphs that follow.

The oxygen transport membrane based reforming subsystem203generally comprises two reactors that can be in the form of sets of catalyst containing tubes—reforming rector and oxygen transport membrane reactor. As seen inFIG. 1, the OTM Combined Reforming Reactor comprises two reactor sections. A first reforming reactor section preferably consists of a plurality of reforming tubes240where the initial or primary reforming occurs. A second reactor section, namely an oxygen transport membrane based reactor, consists of catalyst containing oxygen transport membrane elements or tubes220where secondary reforming of the partially reformed stream occurs. Although only six secondary reforming oxygen transport membrane tubes220are illustrated in close proximity to three primary reforming tubes240, as would occur to those skilled in the art, there could be many of such secondary reforming oxygen transport membrane tubes and many primary reforming tubes in each OTM based reforming subsystem. Likewise, there could be multiple OTM subsystems in an industrial application of the OTM technology.

The heated air feed stream215is directed via an intake duct216to a plurality of catalyst containing oxygen transport membrane tubes220having an oxidant side and a reactive side that is capable of conducting oxygen ions at an elevated operational temperature. The oxidant side of the secondary reforming oxygen transport membrane tubes220is preferably the exterior surface of the ceramic tubes exposed to the heated oxygen containing stream and the reactant side or permeate side is preferably the interior surface of the ceramic tubes. Within each of the secondary reforming oxygen transport membrane tubes220are one or more catalysts that facilitate partial oxidation and reforming.

The heated mixed feed stream239first passes through the reforming tubes240, which contain conventional reforming catalyst which reforms a portion of the natural gas based feed stream239. The temperature of the partially reformed hydrogen-rich synthesis gas298leaving the reforming tubes is designed to be at a temperature of from about 650° C. to about 850° C. This partially reformed synthesis gas298is then fed to the oxygen transport membrane tubes220that are also filled with one or more catalysts, which facilitate further reforming and partial oxidation. Oxygen from the heated intake or feed air215permeates through the oxygen transport membrane tubes220and facilitates a reaction between the permeated oxygen and a portion of the hydrogen and carbon monoxide within the partially reformed synthesis gas298at the reactant side of the oxygen transport membrane tubes220. A portion of the energy or heat generated by this reaction is used for in-situ secondary reforming or further reforming of the residual methane in the partially reformed synthesis gas298. The rest of the energy or heat is transferred by radiation to the reforming tubes240to drive the primary reforming reactions in the reforming reactor and by convection to the oxygen-depleted retentate stream. The synthesis gas242leaving the oxygen transport membrane tubes220is at a temperature of from about 900° C. to about 1050° C.

As described in more detail in U.S. patent application Ser. No. 14/078,897, which is incorporated herein by reference, the produced synthesis gas stream generally contains hydrogen, carbon monoxide, unconverted methane, steam, carbon dioxide and other constituents (See Tables). A significant portion of the sensible heat from the produced synthesis gas stream can be recovered using a heat recovery subsystem204designed to cool the produced synthesis gas stream242while preheating the natural gas based feed stream292and boiler feed water288as well as generating process steam281.

As shown inFIG. 1, the hot synthesis gas242exiting the oxygen transport membrane based reforming reactor is directly cooled to about 400° C. or less in a process gas (PG) boiler249operatively associated with the process stream drum257. The temperature is selected to minimize metal dusting issues. The initially cooled synthesis gas stream244is then used to preheat the mixed or conditioned feed stream292comprising the natural gas feed, the Fischer-Tropsch tail gas feed, and the hydrogen feed in a feed pre-heater250. Downstream of the feed pre-heater, the synthesis gas pre-heats boiler feed water288in an economizer256. Synthesis gas245leaving the economizer256is further cooled using synthesis gas cooler264fed by a cooling water stream266. A fin-fan air cooler (not shown) can be added ahead of the synthesis gas cooler264to minimize cooling water requirements. The cooled synthesis gas248then enters a knock-out drum268where water is removed from the bottoms as a process condensate stream270which, although not shown, can be recycled for use as feed water, and the cooled synthesis gas272is recovered overhead. In the illustrated embodiment, the feed water is sent from a plurality of sources via heat exchangers299,320to a de-aerator296that is configured to supply the boiler feed water and eventually the process steam281while exhausting a vent gas298. The boiler feed water288is preferably pumped from the de-aerator via pump294, further heated in the economizer256and sent to the process steam drum257. As can be appreciated by those skilled in the art, the illustrated heat recovery subsystem operatively couples or integrates the water and steam requirements of the synthesis gas production (i.e. synthesis gas island) with the Fischer-Tropsch liquid production. For example, the excess steam295produced in PG boiler249can be used to pre-heat the feed streams in the Fischer-Tropsch section208(seeFIG. 2) as shown in heat exchangers332A and332B. The condensed steam322and process water297generated in the Fischer-Tropsch section208are then returned to the de-aerator296(seeFIG. 1).

The present system also includes a synthesis gas conditioning subsystem206. In the illustrated embodiment, the synthesis gas conditioning subsystem206is configured to optionally divert a portion of the cooled synthesis gas302to a hydrogen separation membrane305to produce a hydrogen rich permeate304and a synthesis gas stream306with lower H2/CO ratio. Up to 25% of the synthesis gas may be diverted to the hydrogen separation membrane305. The exact amount depends on many process variables and operating conditions, such as synthesis gas composition, temperature, pressure, etc. For example, during start-up of the system200, a significant volume of the synthesis gas may need to be diverted to the hydrogen separation membrane305until the Fischer-Tropsch section has reached a steady operating point and sufficient flow of Fischer-Tropsch tail gas has been established, which can be recycled back to the reforming feed stream.

The main purpose of the synthesis gas conditioning subsystem206is to adjust, typically reduce, the H2/CO ratio of the synthesis gas306to meet the specifications and/or requirements of the Fischer-Tropsch process. This is partially achieved by recombining the synthesis gas stream with lower H2/CO ratio306exiting the hydrogen separation membrane305with the remaining synthesis gas product stream308to produce a conditioned synthesis gas stream310having a H2/CO ratio of between about 1.7 to about 2.2. Depending on the operating pressure of the oxygen transport membrane reformer203, a synthesis gas compressor (not shown) may be required to increase the pressure of the conditioned synthesis gas stream310to between about 350 and 450 psia. Also not shown, but known to those skilled in the art, further conditioning of the synthesis gas stream310may be required to reduce levels of contaminants such as ammonia, sulfur species and others, to below the threshold specifications for the catalysts used in the downstream Fischer-Tropsch reactors. The conditioned synthesis gas is subsequently cooled in synthesis gas cooler320and the final synthesis gas product315is directed to the Fischer-Tropsch process.

In addition, a portion of the hydrogen rich stream304A exiting the hydrogen separation membrane305may be used as a source of supplemental fuel or directed to the reformer feed stream to facilitate desulfurization of natural gas. Another portion of the hydrogen-rich stream304B may be optionally fed to one or more of Fischer-Tropsch reactors where the supplemental hydrogen is used to adjust the H2/CO ratio of the synthesis gas feed to the Fischer-Tropsch reactors in the second or subsequent stages in a multi-stage Fischer-Tropsch process. Alternatively,304B could be further upgraded to a high purity hydrogen stream using a pressure swing adsorption (PSA) system. This high purity hydrogen could be used in the Fischer-Tropsch process as described above and/or used in the product upgrading section to convert the Fischer-Tropsch liquids to finished products. An embodiment of the Fischer-Tropsch synthesis subsystem208is shown inFIG. 2as a multi-stage synthesis process with interstage compression of the intermediate product stream338using an interstage compressor336. Embodiments with a single stage or more than two stages are also possible. As seen therein, the conditioned synthesis gas stream315is synthesized into selected liquid hydrocarbon products in accordance with the general reaction ‘2H2+CO→—CH2-+H2O’ in a Fischer-Tropsch catalyst based reactors330A and330B (e.g. fixed bed reactors, slurry phase reactors, synthol reactors, or microchannel reactors) and subsequently purified into a final liquid hydrocarbon product340in a manner generally known to those skilled in the art. The liquid hydrocarbon product340generally produced by the Fischer-Tropsch gas to liquid (GTL) synthesis process heavily depends on temperature, catalyst, pressure and, more importantly, the synthesis gas composition. Typical FT processes include the use of preheaters332A and332B to heat the feed streams to each of the FT reactors using process steam295as well as a plurality of coolers335and separators337. The illustrated system further includes a separate steam processing section360, with steam drum362, steam turbine364, turbine condenser366, deaerator368, pump369, heat exchanger367, and boiler feedwater make-up361. The steam in steam processing section360is generated by the steam cooled reactors330A and330B. Although not explicitly shown, in some instances it may be preferable to superheat the saturated steam being generated in this section prior to sending to the steam turbine. A possible location for this steam superheater could be in the return conduit225of the OTM reforming system.

For example, at high temperature Fischer-Tropsch reactions (i.e. 330° C.-350° C.) the liquid hydrocarbon product predominantly comprises gasoline and light olefins whereas at low temperature Fischer-Tropsch reactions (i.e. 220° C.-250° C.) the liquid hydrocarbon product predominantly comprises distillates and waxes, with some gasoline. Catalysts used in many Fischer-Tropsch gas to liquid (GTL) synthesis processes include cobalt-based catalysts or iron-based catalysts. The synthesis gas composition, and in particular, the ratio of hydrogen to carbon monoxide (H2/CO ratio) is an important variable that affects the Fischer-Tropsch gas to liquid (GTL) synthesis process and can be controlled by aspects and features of the present invention. For Fischer-Tropsch reactors using iron-based catalyst, the target H2/CO ratio is around 1:1. For Fischer-Tropsch reactors using cobalt-based catalyst, the preferred embodiment for this invention, the target H2/CO ratio is around 2:1. The Fischer-Tropsch synthesis section208also generates a tail gas348comprising unconverted carbon monoxide, hydrogen, and water as well as light hydrocarbons such as methane and/or C2-C5hydrocarbons. A portion of the Fischer-Tropsch tail gas350is recycled to the reforming feed and conditioning subsystem202where it is mixed with the natural gas feed to be reformed in the oxygen transport membrane based reforming subsystem203. Another portion of the Fischer-Tropsch tail gas352can be used as a supplemental fuel source for the duct burner in the air intake subsystem201or other sections of the synthesis gas island. Any Fischer-Tropsch tail gas354that is not used elsewhere in the disclosed system200may be used for power generation or flared. One way to minimize the amount of unutilized or flared Fischer-Tropsch tail gas354and improve the overall process is to increase the steam to carbon ratio of the mixed feed stream238.

As indicated above, the H2/CO ratio in the synthesis gas product stream315is preferably from about 1.7 to about 2.9, and in another embodiment from about 1.9 to about 2.2. To achieve this relatively low H2/CO ratio in the synthesis gas product stream315, the feed stream generally comprises from about 20% to about 45% by volume of the Fischer-Tropsch tail gas350and from about 55% to about 80% by volume of the methane containing feed. Put another way, the amount of Fischer-Tropsch tail gas350and352recycled or diverted back to the oxygen transport membrane based synthesis gas production is from about 50% to about 80% by volume of the tail gas348produced in the Fischer-Tropsch process. The rest of the FT tail gas can be used as fuel in the overall process, e.g. fuel stream226to the duct burner228, or potentially recycled back to the FT reactors.

EXAMPLES

FIG. 3shows a modeled comparison of the targeted process and operating conditions using the system ofFIG. 1for a mixed feed stream having a steam to carbon ratio of 1.5; an oxygen transport membrane based reactor pressure of 460 psia; an oxygen transport membrane based reforming exit temperature of 1800° F.; and a fixed output of liquid hydrocarbon products of about 400 barrels per day. The amount of recycled Fischer-Tropsch tail gas added to the mixed feed stream is varied from 0% to 80% of the Fischer-Tropsch tail gas generated.

As seen inFIG. 3, for the same liquid production of about 400 barrels per day, the presently disclosed system and process provides clear cost and performance advantages. For example, the total natural gas required per barrel of FT product is 15330 scf per barrel with 0% recycle of the Fischer-Tropsch tail gas to the mixed feed stream but is only 10172 scf per barrel with 80% recycle of the Fischer-Tropsch tail gas to the mixed feed stream. This represents a reduction in natural gas consumption of over 33% by recycling most of the Fischer-Tropsch tail gas back to the reforming feed stream. In addition, the quality of the synthesis gas, as characterized by the H2/CO ratio (pre-membrane), is improved from 2.969 (at 0% recycle) to about 1.902 when 80% of the Fischer-Tropsch tail gas is recycled back to the reforming feed stream. Synthesis gas flow to the membrane decreases from 41% of total synthesis gas produced at 0% recycle to less than 15% at recycle rates of 60% or higher. Synthesis gas flow to membrane is not required when more than 74% of the tail gas is recycled back to the reforming feed stream. There are other advantages such as lower oxygen utilization, lower air utilization, lower steam to process rate, lower hydrogen separation, and lower power requirement as the amount of recycled Fischer-Tropsch tail gas is increased as can be seen inFIG. 3.

FIG. 4presents modeled data showing the composition of the synthesis gas fed to the Fischer-Tropsch process for the process and operating conditions described with reference toFIG. 3using the system ofFIG. 1for a mixed feed stream having a steam to carbon ratio of 1.5; an oxygen transport membrane based reactor pressure of about 460 psia; an oxygen transport membrane based reactor exit temperature of 1800° F.; and a varying percentage of Fischer-Tropsch tail gas added to the mixed feed stream.

FIG. 5shows another modeled comparison of the targeted process and operating conditions using the system ofFIG. 1for a mixed feed stream having a steam to carbon ratio of 2.0; an oxygen transport membrane based reforming reactor pressure of about 460 psia; an oxygen transport membrane based reforming reactor exit temperature of 1800° F.; and a fixed output of liquid hydrocarbon products of about 400 barrels per day. As withFIG. 3, the amount of recycled Fischer-Tropsch tail gas added to the mixed feed stream is varied from 0% to 80% of the Fischer-Tropsch tail gas generated.

As seen inFIG. 5, for the same liquid production of about 400 barrels per day, the presently disclosed system and process provides clear cost and performance advantages. For example, the total natural gas required per barrel of FT product is 16578 scf per barrel with 0% recycle of the Fischer-Tropsch tail gas to the mixed feed stream but is only 10595 scf per barrel with 80% recycle of the Fischer-Tropsch tail gas to the mixed feed stream. This represents a reduction in natural gas consumption of over 36% by recycling most of the Fischer-Tropsch tail gas back to the reforming feed stream. In addition, the quality of the synthesis gas, as characterized by the H2/CO ratio (pre-membrane), is improved from 3.285 (at 0% recycle) to about 2.052 when 80% of the Fischer-Tropsch tail gas is recycled back to the reforming feed stream. Synthesis gas flow to the membrane decreases from 50% of total synthesis gas produced at 0% recycle to less than 15% at recycle rates of 70% or higher. Synthesis gas flow to membrane is not required when more than 79% of the tail gas is recycled back to the reforming feed stream. There are other advantages such as lower oxygen utilization, lower air utilization, lower steam to process rate, lower hydrogen separation, and lower power requirement as the amount of recycled Fischer-Tropsch tail gas is increased as can be seen inFIG. 5.

FIG. 6presents modeled data showing the composition of the synthesis gas fed to the Fischer-Tropsch process for the process and operating conditions described with reference toFIG. 4using the system ofFIG. 1for a mixed feed stream having a steam to carbon ratio of 2.0; an oxygen transport membrane based reactor pressure of about 460 psia; an oxygen transport membrane based reactor exit temperature of 1800° F.; and a varying percentage of Fischer-Tropsch tail gas added to the mixed feed stream.

While the inventions herein disclosed have been described by means of specific embodiments and processes associated therewith, numerous modifications and variations can be made thereto by those skilled in the art without departing from the scope of the invention as set forth in the appended claims or sacrificing all of its features and advantages.