METHOD AND SYSTEM FOR PRODUCING A SYNTHESIS GAS IN AN OXYGEN TRANSPORT MEMBRANE BASED REFORMING SYSTEM USING A COMBINED FEED STREAM

Abstract

A method and system for producing a synthesis gas in an oxygen transport membrane based reforming system that utilizes a combined feed stream having a steam to carbon ratio between about 1.6 and 3.0 and a temperature between about 500 C. and 750 C. The combined feed stream is comprised a pre-reformed hydrocarbon feed, superheated steam, and a reaction product stream created by the reaction of a hydrogen containing stream reacted with the permeated oxygen at the permeate side of the oxygen transport membrane elements.

Claims

1. A method for producing a synthesis gas in an oxygen transport membrane based reforming system, wherein said system comprises two distinct reactors: i.) at least one catalyst containing reforming reactor configured to produce a synthesis gas stream by reacting a combined feed stream in the presence of the catalyst and heat; and ii.) a reactively driven oxygen transport membrane reactor proximate the at least one catalyst containing reforming reactor, the reactively driven oxygen transport membrane reactor comprising a plurality of oxygen transport membrane elements configured to separate oxygen from an oxygen containing feed stream and produce an oxygen permeate at a permeate side of the oxygen transport membrane elements and an oxygen depleted retentate stream at a retentate side of the oxygen transport membrane elements, the method comprising the steps of: separating an oxygen containing stream into an oxygen permeate and an oxygen depleted retentate stream using a plurality of oxygen transport membrane elements disposed in the oxygen transport membrane based reforming system; reacting a hydrogen containing stream fed to a permeate side of the oxygen transport membrane elements with the oxygen permeate to generate a reaction product stream and heat; transferring the heat via convection to the oxygen depleted retentate stream and via radiation to at least one catalyst containing reforming reactor configured to produce a synthesis gas stream; pre-treating a hydrocarbon containing feed stream by adding steam to form a pre-treated reformer feed stream; combining the pre-treated reformer feed stream with the reaction product stream, wherein steam is added to said pre-treated feed stream in an amount to adjust the carbon ratio and final temperature of the combined feed stream to a carbon ratio of about 1.6 to 3.0 and a temperature of from about 500 C. to 750 C.; reforming the combined feed steam in the at least one catalyst containing reforming reactor in the presence of the catalyst and the heat generated by the reaction of the hydrogen containing stream and permeated oxygen to produce a synthesis gas stream, the at least one catalyst based reforming reactor disposed proximate the oxygen transport membrane elements.

2. The method of claim 1 wherein the combined feed stream has a steam to carbon ratio between about 2.0 and 2.8 and a temperature between about 600 C. and 750 C.

3. The method of claim 1 wherein the step of pre-treating the hydrocarbon containing feed stream further comprises combining the hydrocarbon containing feed stream and superheated steam to form the pre-treated reformer feed stream.

4. The method of claim 3 wherein the superheated steam is at a pressure of between about 15 bar to 80 bar and a temperature of between about 300 C. and 600 C.

5. The method of claim 3 wherein the superheated steam is produced by heating the steam via indirect heat exchange with the oxygen depleted retentate stream.

6. The method of claim 3 further comprising the step of feeding the pre-treating reformer feed stream to a pre-reformer to produce a pre-reformed feed stream comprising methane, hydrogen, and carbon monoxide.

7. The method of claim 1 wherein the step of pre-treating the hydrocarbon containing feed stream further comprises combining a source of hydrogen to the hydrocarbon containing feed stream and removing sulfur from the hydrocarbon containing feed stream.

8. The method of claim 1 where the steam to carbon ratio of the pre-treated reformer feed stream is greater than about 0.8.

9. The method of claim 1 further comprising the step of pre-heating the hydrocarbon containing feed stream via indirect heat exchange with the oxygen depleted retentate stream.

10. The method of claim 1 further comprising the step of heating the pre-treated reformer feed to a temperature between 450 C. and 650 C. via indirect heat exchange with the oxygen depleted retentate stream.

11. The method of claim 1 further comprising the step of recycling a portion of the synthesis gas stream to the permeate side of the oxygen transport membrane elements to form all or a part of the hydrogen containing stream.

12. The method of claim 1 wherein the step of combining the pre-treated reformer feed stream with the reaction product stream further comprises mixing the reaction product stream with the pre-treated reformer feed stream using an ejector, eductor, or venturi based device configured to suction the reaction product stream at the permeate side of the oxygen transport membrane elements into the ejector, eductor, or venturi based device with a motive fluid comprising the pre-treated reformer feed stream proximate an inlet to the catalyst containing reforming reactor.

13. The method of claim 1 wherein the produced synthesis gas stream has a module of between about 1.5 and 2.0.

14. The method of claim 1 wherein the synthesis gas stream has a methane slip of less than about 4 percent by volume.

15. The method of claim 1 wherein the methane slip in the synthesis gas stream is less than about 2 percent by volume.

16. An oxygen transport membrane based reforming system for producing synthesis gas which comprises two reactors, said system comprising: a reactor housing; at least one catalyst containing reforming reactor disposed in the reactor housing and configured to produce a synthesis gas stream by reacting a combined feed stream in the presence of the catalyst and heat; a reactively driven oxygen transport membrane reactor disposed in the reactor housing proximate the at least one catalyst containing reforming reactor, the reactively driven oxygen transport membrane reactor comprising a plurality of oxygen transport membrane elements configured to separate oxygen from an oxygen containing feed stream and produce an oxygen permeate at a permeate side of the oxygen transport membrane elements and an oxygen depleted retentate stream at a retentate side of the oxygen transport membrane elements; a hydrogen containing stream fed to the permeate side of the plurality of oxygen transport membrane elements, wherein the permeated oxygen reacts with the hydrogen containing stream to reactively drive the separation of oxygen from the oxygen containing feed stream and to generate a reaction product stream and heat that is transferred via convection to the oxygen depleted retentate stream and via radiation to at least one catalyst containing reforming reactor; and wherein the combined feed stream comprises a pre-treated reformer feed and the reaction product stream with a steam to carbon ratio between about 1.6 and 3.0 and a temperature between about 500 C. and 750 C.; and wherein the pre-treated reformer feed is at a pressure less than about 20 bar and comprises a mixture of a hydrocarbon feed stream and steam.

17. The system of claim 16 wherein the combined feed stream has a steam to carbon ratio between about 2.0 and 2.8 and a temperature between about 600 C. and 750 C.

18. The system of claim 16 wherein the step pre-treated reformer feed comprises a mixture of the hydrocarbon feed stream and superheated steam and wherein the superheated steam is at a pressure of between about 15 bar to 80 bar and a temperature of between about 300 C. and 600 C.

19. The system of claim 16 further comprising a pre-reformer configured to produce a pre-reformed feed stream comprising methane, hydrogen, and carbon monoxide from the pre-treated reformer feed stream.

20. The system of claim 16 further comprising an ejector, eductor, or venturi based device coupled to the permeate side of the oxygen transport membrane elements and the inlet side of the catalyst containing reforming reactor and configured to suction the reaction product stream into the ejector, eductor, or venturi based device with the pre-treated reformer feed stream as a motive fluid to produce the combined feed stream.

21. The system of claim 16 where the steam to carbon ratio of the pre-treated reformer feed stream is greater than about 0.8.

22. The system of claim 16 wherein the combined feed stream has a steam to carbon ratio between about 1.6 and 3.0 and a temperature between about 500 C. and 750 C.

23. The system of claim 16 wherein the produced synthesis gas stream has a module of between about 1.5 and 2.0.

24. The system of claim 16 wherein the synthesis gas stream has a methane slip of less than about 4 percent by volume.

25. The system of claim 16 wherein the synthesis gas stream has a methane slip of less than about 2 percent by volume.

26. The system of claim 16 further comprising: a heat exchanger network configured to: (i) cool the synthesis gas stream exiting the at least one catalyst containing reforming reactor to a temperature of less than about 400 C.; (ii) produce the steam, and (iii) pre-heat the hydrocarbon containing feed stream; and a recycle circuit coupling the cooled synthesis gas stream to the permeate side of the oxygen transport membrane elements to form all or a portion of the hydrogen containing stream and configured to recirculate a portion of the synthesis gas while maintaining the temperature of the recirculated synthesis gas stream at a temperature less than about 400 C.

27. The system of claim 16 further comprising a synthesis gas recycle circuit disposed within the reactor housing between an outlet of the catalyst containing reforming reactor and the permeate side of the oxygen transport membrane elements and configured to recirculate a portion of the synthesis gas exiting the catalyst containing reforming reactor to the permeate side of the oxygen transport membrane elements to form all or a portion of the hydrogen containing stream while maintaining the temperature of the recirculated synthesis gas stream at a temperature greater than about 800 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] 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:

[0019] FIG. 1 is a schematic illustration of an embodiment of an oxygen transport membrane based reforming system in accordance with the present invention;

[0020] FIG. 2 is a schematic illustration of an alternate embodiment of an oxygen transport membrane based reforming system in accordance with the present invention;

[0021] FIG. 3 is a schematic illustration of an alternate embodiment of an oxygen transport membrane based reforming system in accordance with the present invention for a methanol production facility, where the oxygen transport membrane based reforming system is the only or primary source of synthesis gas supply; and

[0022] FIG. 4 is a graph of the temperature of a feed stream versus steam to carbon ratio of the feed stream and depicting various performance regimes of an oxygen transport membrane based reforming system.

[0023] For the sake of avoiding repetition, some of the common elements in the various Figures utilize the same numbers where the explanation of such elements would not change from Figure to Figure.

DETAILED DESCRIPTION

[0024] Turning now to FIG. 1, there is shown a schematic illustration of an embodiment of an oxygen transport membrane based reforming system 101 and assembly 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 blower or fan 114 into a heat exchanger 113 for purposes of preheating the oxygen containing stream 110. Heat exchanger 113 is preferably a high efficiency, cyclic and continuously rotating regenerator disposed in operative association with the oxygen containing stream 110 and the heated retentate stream 124. The heated and oxygen depleted retentate stream 124 can optionally be introduced into a duct burner region containing duct burner 126 and used to support combustion of a supplemental fuel stream 128 to produce supplemental heat introduced into the continuously rotating regenerator 113 to preheat the oxygen containing stream 110. Alternatively, the duct burner may also be disposed directly in the duct leaving heat exchanger 113 to pre-heat the oxygen containing stream 110. Exhaust stream 132 from heat exchanger 113 is discharged.

[0025] The heated oxygen containing stream 115 is then directed via the intake duct to the oxygen transport membrane elements 120 incorporated into the oxygen transport membrane based reforming system 101. Each of the oxygen transport membrane elements 120 are preferably configured as a multilayered ceramic tube capable of conducting oxygen ions at an elevated operational temperature, wherein the retentate side of the oxygen transport membrane elements 120 is the exterior surface of the ceramic tubes exposed to the oxygen containing stream and the permeate side is the interior surface of the ceramic tubes. Although only six oxygen transport membrane elements 120 are illustrated in close proximity to three catalytic reforming tubes 140, as would occur to those skilled in the art, there could be many of such oxygen transport membrane elements and many catalytic reforming tubes in each oxygen transport membrane sub-system. Likewise, there would be multiple oxygen transport membrane sub-systems used in an industrial application of the oxygen transport membrane based reforming system 101.

[0026] A hydrogen containing stream is also introduced into the permeate side of the oxygen transport membrane elements 120 and is oxidized though reaction with the permeated oxygen to produce a reaction product stream 198 and heat. As described in more detail below, the hydrogen containing stream is preferably a recycled portion of the produced synthesis gas 163. As a result of the separation of the oxygen and the reaction (i.e. combustion) occurring at the permeate side of oxygen transport membrane elements 120, a heated and oxygen depleted retentate stream 124 is also formed.

[0027] The reaction of the hydrogen containing stream or recycled synthesis gas stream 163 at the permeate side of the oxygen transport membrane element 120 produces heat. Radiation of this heat together with the convective heat transfer provided by heated retentate stream 124 heats the catalytic reactor tubes 140 to supply the endothermic heating requirements of the steam methane reforming occurring in catalytic reactor tubes 140. As the heated retentate stream 124 exits the oxygen transport membrane based reforming system 101, it also heats a reformer feed stream 138 to a temperature between about 450 C. and 650 C. via indirect heat transfer using one or more coils 191 disposed in the retentate duct such that the oxygen depleted retentate stream 124 heats the feed streams passing through the coils 191.

[0028] The hydrocarbon containing feed stream 182 to be reformed is preferably natural gas. Depending on the supply pressure, the natural gas is compressed or let down to the desired pressure via a compressor or valve arrangement (not shown) and then preheated in heat exchanger 150 that serves as a feed preheater. Also, since the natural gas typically contains unacceptably high level of sulfur species, the natural gas feed stream 182 undergoes a sulfur removal process such as hydro-treating, via device 190, to reduce the sulfur species to H.sub.2S, which is subsequently removed in a guard bed using material like ZnO and/or CuO. To facilitate the desulfurization, a small amount of hydrogen or hydrogen-rich gas (not shown) is added to stream 182 before heat exchanger 150. 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 182 is preferably pre-reformed in an adiabatic pre-reformer 192, which converts higher hydrocarbons to methane, hydrogen, carbon monoxide, and carbon dioxide. Pre-reformers are typically catalyst-based systems. Although not shown, this pre-reformed reformer feed stream 195 may be further heated via indirect heat exchange with heated retentate stream 124. Also contemplated, but not shown is an embodiment where the pre-reformer is a heated pre-reformer that is thermally coupled with oxygen transport membrane based reforming system.

[0029] In the illustrated embodiment, the above-described heated reaction product stream 198 is combined with the heated pre-reformed reformer feed stream 195 to produce a combined feed stream 200 that contains steam and hydrocarbons. This combined feed stream is introduced into the catalytic reactor tubes 140 where the combined feed stream 200 is subjected to steam methane reforming to produce a synthesis gas stream 142. The temperature of the combined feed stream 200 is between about 500 C. and 750 C., and more preferably between about 600 C. and 750 C. Additional steam 180 may also be added to the natural gas feed stream 182, or the preheated pre-reformed reformer feed stream 195, as required, to adjust the temperature of stream 200 as well as the steam to carbon ratio of the final combined feed stream 200 to between about 1.6 and 3.0, and more preferably to steam to carbon ratio between about 2.0 and 2.8. The steam is preferably superheated steam 180 between about 15 bar to 80 bar and between about 300 C. and 600 C. and heated by means of indirect heat exchange with the heated retentate stream 124 using steam coils 179 disposed in the retentate duct. The superheated steam 180 is preferably added to the hydrocarbon containing feed stream 182 upstream of the pre-reformer 192 to adjust the steam to carbon ratio and final temperature of the combined feed stream 200. Also, to optimize the economic performance of the oxygen transport membrane based reforming system in a methanol production process, the methane slip should be less than 4% by volume and preferably less than 2% by volume.

[0030] The synthesis gas stream 142 produced by the oxygen transport membrane based reforming system 101 generally contains hydrogen, carbon monoxide, unconverted methane, steam, carbon dioxide and other constituents. Heat exchange section 104 is designed to cool the produced synthesis gas stream 142 and recycle a portion of the synthesis gas stream 162A to form all or a part of the hydrogen containing stream 163. In this illustrated embodiment, the synthesis gas stream 142 is preferably cooled before recycling such stream using a synthesis gas recycle compressor 177 or other blower means. The heat exchange section 104 is also designed such that in cooling the synthesis gas stream 142, various feed streams are preheated and process steam is also generated.

[0031] The initial cooling of synthesis gas stream 142 is accomplished with steam generation in a process gas boiler (PG boiler) 149 coupled to steam drum 157 and designed to reduce the temperature of the cooled synthesis gas 144 to about 400 C. or less. As illustrated in FIG. 1, the initially cooled synthesis gas stream 144 is successively further cooled in a heat exchange network that includes hydrocarbon feed preheater 150, economizer 156, feed water heaters 158A and 158B, synthesis gas cooler 161 and water cooled heat exchanger 164. Specifically, the initially cooled synthesis gas stream 144 is directed to the feed preheater 150 to heat the natural gas feed stream 182 and then is directed to the economizer 156 to heat boiler feed water 188. The boiler feed water stream 188 is preferably pumped using a feed water pump (not shown), heated in economizer 156 and sent to steam drum 157.

[0032] The cooled synthesis gas stream 146 is then divided into a first portion 160 and a second or recycled portion 162A. First portion 160 is further cooled in a series of steps including a feed water heater 158A, used to heat feed water stream 159, followed by a synthesis gas cooler 161 and a subsequent water cooled heat exchanger 164 cooled via a separate cooling water stream 166. The heated feed water 159 is directed to a de-aerator (not shown) that provides boiler feed water 188. The resulting fully cooled synthesis gas stream 148 is then introduced into a knock-out drum 168 from which a condensate stream 170 is drained to produce a fully cooled synthesis gas stream 172. The fully cooled synthesis gas stream 172 is optionally compressed in a synthesis gas compressor 174 to produce a synthesis gas product 176.

[0033] The second or recycle portion 162A of the initially cooled synthesis gas stream 144 is directed to a second feed water heater 158B, used to heat feed water stream 159, and this cooled recycle synthesis gas stream 163 is recirculated back to the permeate side of the oxygen transport membrane element 120 by means of a recycle compressor 177. Also note that any superheated steam not added or used in the natural gas feed 182 or recycle synthesis gas stream 163 is exported steam 181 that may be used for power generation.

[0034] When customized as a supplemental source of synthesis gas for a methanol production process, the oxygen transport membrane produced synthesis gas should have a module of between about 1.5 and 2.0. In addition, such produced synthesis gas stream ideally has a methane slip of less than about 4.5 percent by volume and more preferably, a methane slip of less than about 2.5 percent by volume.

[0035] Turning now to FIG. 2, there is shown a schematic illustration of an alternate embodiment of an oxygen transport membrane based reforming system. 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 between FIG. 1 and FIG. 2.

[0036] The primary difference between the embodiments in FIG. 1 and FIG. 2 is the use of a hot synthesis gas recycle 162B in FIG. 2 embodiment in lieu of the cold gas recycle 162A in the embodiment of FIG. 1. As a result, the heat exchange section 104 in FIG. 2 is designed to only cool the produced synthesis gas stream 142 and need not recycle a portion of the cold synthesis gas stream 162A. By using the hot synthesis gas recycle 162B, there is no need to use the synthesis gas recycle compressor 177 or the second feed water heater 158B, used to heat feed water stream 159 potentially further reducing the capital cost of the oxygen transport membrane based reforming system.

[0037] The hot synthesis gas recycle involves recycling a portion of the heated synthesis gas stream 142 exiting the catalyst containing reforming tubes or reactor 140 and directing the hot recycled synthesis gas 162B to the permeate side of the oxygen transport membrane elements 120 to react the portion of heated synthesis gas stream 162B with the oxygen permeate stream to generate the heated reaction product stream and radiant heat. The temperature of the hot synthesis recycled gas is preferably above 800 C. so as to avoid problems associated with metal dusting corrosion.

[0038] The hot synthesis gas stream 162B is driven or pulled to the permeate side of the oxygen transport membrane elements 120 by means of an ejector, eductor or venturi based device 199 operatively coupled to the permeate side of the oxygen transport membrane elements 120. By suctioning the streams at the permeate side of the oxygen transport membrane elements 120 into the ejector, eductor or venturi based device 199 with a motive fluid comprising the pre-reformed reformer feed stream 195, the reaction product stream 198 mixes with the pre-reformed reformer feed stream 195 to produce the combined feed stream 200, preferably having a steam to carbon ratio between about 1.6 and 3.0 and a temperature between about 500 C. and 750 C. Essentially, device 199 moves lower pressure reaction product 198 to higher pressure combined feed stream 200.

[0039] Turning now to FIG. 3, there is shown a schematic illustration of yet another alternate embodiment of an oxygen transport membrane based reforming system. In many regards, this illustrated embodiment is similar to the embodiments shown in FIG. 1 and FIG. 2. Thus, for sake of brevity; the description of the common aspects of the embodiments will not be repeated here. Rather, the following discussion shall focus on the differences present in the embodiment of FIG. 3. The primary difference between the embodiments in FIG. 2 and FIG. 3 is the addition of downstream processing of the initially cooled synthesis gas in the embodiment of FIG. 3. As seen therein, a portion of the cooled synthesis gas 350 is diverted to a synthesis gas conditioning system. This diverted portion of the cooled synthesis gas stream 350 is roughly between 5% and 25% of the synthesis gas stream 142.

[0040] The diverted portion of the cooled synthesis gas stream 350 is subjected to a water shift reaction 352 and subsequently cooled in heat exchanger 354 using cooling water or boiler feed water and then compressed in a synthesis gas compressor 360. The compressed gas feed stream 362 is directed to a hydrogen pressure swing adsorption (PSA) unit 370 which takes the compressed feed stream 362 and produces a higher purity hydrogen stream 372 at or near the feed pressure while the carbon oxides, methane and other impurities are rejected at lower pressure tail gas stream 374. Stream 374 may be recycled to use with the duct burners 126 while the higher pressure and higher purity hydrogen stream 372 is recombined with the non-diverted portion of the synthesis gas stream 172, preferably at some point mid-stage point within the synthesis gas compressor 174. By re-combining the higher purity hydrogen stream 372 with the non-diverted portion of the synthesis gas stream 172, one can adjust the module of the final synthesis gas product to about 2.0 to 2.2, the preferable range for methanol production or other synthesis gas characteristics such as hydrogen to carbon monoxide ratio, etc.

[0041] One of the likely disadvantages of the previously disclosed oxygen transport membrane based reforming system and reactors is the potential for corrosion, and in particular metal dusting corrosion. Metal dusting is a severe form of corrosion that occurs when surfaces of certain metal and metal alloy components and piping are exposed to severe gas environments with a high carbon activity or content. The metal dusting corrosion is manifested by a disintegration of bulk metals, such as iron, nickel and cobalt to metal powders. The typical metal dusting process results from a series of sequential steps, including (i) rapid uptake of carbon into the metallic phase leading to saturation of the alloy matrix with carbon; (ii) formation of metastable carbides; and (iii) decomposition of these carbides into a loose film of carbon and metallic particles, which acts as catalyst for further carbon deposition. The exact mechanism may vary depending on the type of metal being used. The temperatures normally associated with metal dusting are about 400 C. to 800 C. (i.e., about 760 F. to about 1500 F.). At temperatures generally below 400 C. the rate of reaction to form the metastable carbide species is too low to be significant, while at temperatures above 800 C. the carbon formation is minimal.

[0042] To avoid the metal dusting corrosion in the oxygen based transport membrane based reforming system and associated components and piping, the system should be designed to avoid contact of any synthesis gas with metal surfaces having temperatures between about 400 C. to 800 C. While it is possible to provide corrosion resistant coatings on all high temperature metal surfaces exposed to synthesis gas, such solution would be cost prohibitive. The alternative corrosion prevention technique employed in the present embodiments is to manage the temperatures of the synthesis gas so as to avoid contacting bare metal surfaces with synthesis gas in the deleterious temperature range where metal dusting corrosion occurs.

[0043] As will be described in more detail below, the present embodiments of the inventions achieve this temperature control of the metal surfaces exposed to the synthesis gas in several ways. In particular for the embodiments in FIGS. 2 and 3, the oxygen based transport membrane based reforming system preferably recycles the synthesis gas in a high temperature state, generally above 800 C. where the non-recycled portion of the synthesis gas is cooled in the PG boiler until the synthesis gas is cooled below 400 C. The metal surfaces of the synthesis gas piping exiting the reactor are either maintained at a temperature above 800 C. or have refractory surfaces. The metal surfaces in the PG boiler and other elements of the heat recovery system that are exposed to the synthesis gas are generally maintained at temperatures below about 400 C.

[0044] In the embodiment of FIG. 1 where cooled synthesis gas is recycled back to the oxygen based transport membrane based reactor, the cooled recycled synthesis gas stream is maintained at temperatures below about 400 C. The recycled synthesis gas stream 163 can be preheated against steam or any other stream as long as the wall temperature is less than about 400 C. For this reason, the addition of superheated steam to the recycled synthesis gas stream should be avoided unless the addition of superheated steam maintains the temperatures below about 400 C.

[0045] Turning now to FIG. 4, region 310 on the feed temperature versus feed steam to carbon ratio graph generally corresponds to a combined feed stream feed at a temperature near or below about 500 C. and having a steam to carbon ratio of between 1.5 and 2.4. Because the present combined feed stream contains carbon monoxide produced in the pre-reformer, a combined feed stream having characteristics of region 310, may undergo unwanted Boudouard reactions thereby depositing excessive amounts of soot on the oxygen transport membrane based reforming system as well as associated components and piping. The Boudouard reaction is a redox reaction of a mixture of carbon monoxide and carbon dioxide at a given temperature and involves the disproportionation of carbon monoxide into carbon dioxide and carbon (i.e. soot).

[0046] On the other hand, region 330 generally corresponds to a region of high temperature and low steam to carbon ratios where the combined feed stream is subject to carbon lay down in the reforming catalysts. Region 330 is generally depicted as the region to the right of the curve or plot line on the feed temperature versus steam to carbon ratio defined by two points, namely a feed temperature of near 500 C. with a steam to carbon ratio of about 1.5 and a feed temperature of about 700 C. with a steam to carbon ratio of about 2.4 (See FIG. 4).

[0047] It has been found that conditioning the combined feed stream to a particular temperature range and steam to carbon ratio which avoids regions 310 and 330 translates into an optimum operating regime with noticeably less reliability problems in the oxygen transport membrane based reforming system due to carbon formation. This window of preferred operating characteristics for the combined feed stream is depicted generally as region 320 in FIG. 4.

[0048] 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.