Patent Application
20070292342
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:
With reference to
Reactor 12 is employed in a heat transfer and steam generation environment of a type that is employed in connection with conventional steam methane reformers and autothermal reformers. In this regard, synthesis gas product stream 14 is cooled by passage through waste heat boiler 16, mixed feed heaters 18 and 20, a waste heat boiler 22, a natural gas preheater 24, a super heater 26 and a waste heat boiler 28. Steam stream 30 is divided into subsidiary steam streams 32 and 34. Subsidiary steam stream 32 is further divided into portions 36 and 38. Portion 38 passes through waste heat boiler 22 for export and portion 36 passes waste heat boiler 16 and is returned to steam drum 33. Subsidiary steam stream 34 passes through waste heat boiler 28 and is also redirected to steam drum 33. Steam drum 33 discharges a superheated steam stream 40 that passes through preheater 26 and forms a process steam stream 42.
Natural gas feed stream 10 is pretreated by a hydrotreater, not illustrated, to convert sulfur species contained within the natural gas to hydrogen sulfide. Natural gas feed stream 10 having been treated in such manner is passed through natural gas preheater 24 and sent to a convective section 44 of fired heater 46. The resultant heated natural gas feed stream 10 is then directed to a zinc oxide bed 48 for removal of the hydrogen sulfide to produce a treated stream 50.
The process steam stream 42 is divided into subsidiary process steam streams 52 and 54. Subsidiary process steam stream 52 is combined with treated stream 50 and is heated within feed heater 18. The resultant combined stream 56 is then passed into convective section 44 of fired heater 46.
Subsidiary process steam stream 54 is combined with a Fischer-Tropsch recycle stream 58 to produce a combined stream 59 which is preheated within mixed feed heater 20 and then finally heated in convective section 44 of fired heater 46. Preferably, the flow of subsidiary steam stream 54 is selected so that most of the steam from process stream 42 is combined with Fischer-Tropsch recycle stream 58 that contributes hydrogen which will allow for higher operational temperatures without carbon deposition. Combined stream 56 is subdivided into reactant streams 62, 64, 66, 68 and 70. Reactant stream 62 is combined with combined stream 59 to produce a combined reactant stream 60 that is fed into a reaction stage 72 of reactor 12. The steam to carbon ratio in reactant stream 60 is preferably between about 1.5 and about 2.0. This high steam to carbon ratio together with the contributed hydrogen will prevent carbon lay down in reaction stage 72. Additional hydrogen produced in this stage and in subsequent stages will also protect the subsequent stages from carbon deposition as outlined above. Reactant streams 64, 66, 68 and 70 are fed into reaction stages 74, 76, 78 and 80, respectively, also included in reactor 12. It is understood more or less said stages could be employed in a reactor in accordance with the present invention.
Each of the reaction stages 72, 74, 76, 78 and 80 has catalytic reaction zones 208 and separation zones 210. Combined reactant stream 60 and reactant streams 64, 66, 68 and 70 are subjected to partial oxidation and steam methane reforming reactions within catalytic reaction zones 208. As will be discussed, in practice there are several of such catalytic reaction zones 208 employed within each of the reaction stages 72, 64, 76, 78 and 80
Reaction stages 72, 74, 76, 78 and 80 are connected in series so that respective intermediate product streams 90, 92, 94 and 96 are produced that have a successively greater synthesis gas content. Aside from reaction stage 72, reactant streams 64, 66, 68 and 70 are combined with intermediate product streams 90, 92, 94 and 96 to serve as reactants fed to catalytic reaction zones 208.
Separation zones 210 are separated from the catalytic reaction zones 208 by oxygen transport membrane elements (to be discussed) that are capable of conducting oxygen ions at elevated temperatures occurring within the catalytic reaction zones 208. An oxygen containing stream that can be an air stream 100 is forced by a fan 102 through heat exchangers 104 and 106. This produces a heated air stream 198 that is subdivided into subsidiary air streams 110, 112, 116, 118 and 120 that are in turn fed into the separation zones 210 of the individual reaction stages 72, 74, 76, 78 and 80 to separate oxygen and thereby produce oxygen within the catalytic reaction zones 208. The oxygen reacts with hydrocarbons within the reactant streams 60, 64, 66, 68 and 70 to partially oxidize hydrocarbons and thereby to produce heat to support the steam methane reforming reactions and the elevated temperatures required by the oxygen transport membranes to separate the oxygen.
The separation produces retentate streams, depleted in oxygen, that are designated by reference numerals 122, 124, 126, 128 and 130. The retentate streams are combined into a combined retentate stream 132 that are passed into a duct fired heater 134. The resultant heated stream 135 is drawn through heat exchangers 106 and 104 by an induced draft fan 136.
Temperature within each of the reaction stages 72, 74, 76, 78 and 80 is controlled. The temperature in each of the catalytic reactor sections 208 is dependent upon the ratio of the oxygen to the reactants that are available for reaction. Increasing such ratio will increase the temperature and decreasing the ratio will decrease the temperature. As indicated above, the control of temperature is important in order to allow ceramic membranes to function and to allow some of the reaction stages to be fabricated with inexpensive materials. This temperature control is provided by controlling or metering the flow of the combined reactant stream 60 and the other reactant streams 64, 66, 68 and 80. Flow control can be provided through appropriate pipe sizing. More sophisticated control can be provided by valves and temperature controllers reacting to temperature sensed within reaction stages 72, 74, 76, 78 and 80. However, the control to be accomplished is based upon the amount of reactant within the reaction stage. Within limits too much reactant will tend to lower temperature and too little reactant will tend to increase the temperature.
Preferably, these temperatures are controlled to be in a range of between about 650° C. and below about 900° C. to allow temperature critical components of reaction stages 72, 74 and 76 to be fabricated from relatively inexpensive high temperature alloys such as INCONEL® 693 alloy or HAYNES 214 alloy. Oxide dispersed strengthened metals can be utilized in reaction stages 78 and 80 or at least reaction stage 80 that operate at high temperature of between about 900° C. and about 1100° C. As such, the more expensive oxide dispersed strengthened metals do not have to be used throughout the reactor 12.
The temperature of the final reactor stage 80 can be at an operating temperature that closely approaches an equilibrium temperature necessary for the conversion of substantially all the methane and other hydrocarbons fed to such stage through reactant stream 70 and intermediate product stream 96 to be converted to synthesis gas and such stage preferably operates at a temperature of approximately 950° C. to 1000° C. and at a pressure of about 30 bar. The initial or first of the reaction stages 72 operates at between about 600° C. and about 700° C. The temperature increases monotonically through reaction stages 74, 76 and 78. In all cases reaction stage 72 will operate a temperature which is in the capability of conventional alloys, while reaction stage 80 will invariably require high temperature alloy such as an oxide dispersed strengthened metal. The metallurgy required for stages 74, 76 and 78 will depend upon the operational temperatures employed, namely a temperature requiring a conventional alloy or the high temperature alloy of reaction stage 80. In all cases, however, there are a sufficient number of total reaction stages that about 95 percent of the hydrocarbon content of the feed is converted into synthesis gas.
As indicated above, natural gas stream 10 or process steam stream 42, Fischer-Tropsch recycle stream 58 are all at pressure and as such, reactant streams 60, 62, 64, 66, 68 and 70 are also preferably at high pressure, for example 30 bar. This creates a pressure differential within the reaction stages and specifically on opposite sides of the oxygen transport membrane elements. The effects of the pressure differential are most severe for final reaction stage in the series, namely, reaction stage 80 that operates at the highest temperature. Since reaction stage 80 will use the most expensive, creep-resistant material, it is desirable to decrease resulting material stresses in such stage and thereby decrease the thicknesses and therefore the amounts of such material.
In order to reduce material stresses induced by pressure differential and temperature within high temperature reaction stages, such as reaction stage 80, arrangements such as shown in
With reference to
With reference to
Reaction stage 74 has catalytic reaction zones 208 alternating with oxygen separation zones 210. Catalytic reaction zones 208 can contain catalyst, similar to conventional secondary reforming catalyst, for example in the form of nickel supported on alumina to promote the steam reforming reactions. Subsidiary oxygen containing stream 112 after having entered oxygen containing gas inlet 202 flows through the oxygen separation zones 210 in which oxygen separated from the subsidiary oxygen containing gas stream 112 by oxygen transport membrane elements (to be discussed hereinafter) to produce a retentate that is discharged as retentate stream 124. The separated oxygen permeates through the oxygen transport membrane elements to the catalytic reaction zones 208 to support the partial oxidation reactions. The heat generated from the partial oxidation reactions supports the endothermic heating requirements of the steam methane reforming reactions occurring within catalytic reaction zones 208 to produce intermediate product stream 92 that is discharged to downstream reaction stage 76.
The reactant stream 64, together with the intermediate product stream 90 produced in reaction stage 72, is introduced into an inlet plenum portion 212 of housing 200 and intermediate product stream 92 is discharged from an outlet plenum portion 214 of housing 200. Inlet plenum portion 212 is attached to the outlet plenum portion 214 of reaction stage 72. Outlet plenum portion 214 is attached to the inlet plenum portion 212 of reaction stage 76. For such purposes, inlet plenum portion 212 and outlet plenum portion 214 of reaction stages 74, 76 and 78 are open to allow for the flow of intermediate product, for example intermediate product stream 90 and intermediate product stream 92.
Housing 200 of reaction stage 72 has a top wall 216 (illustrated in
With reference to
With continued reference to
With additional reference to
As discussed above the control of temperature allows planar members 228 and 236 of reaction stage 80 to be made of the more expensive high temperature materials, for instance, oxide dispersed strengthened metal as opposed to the upstream reaction sections 72, 74, 76 and 78 which can all be made out of a less expensive, high temperature alloy, can be formed of an oxide dispersed strengthened metal alloy to reliably function at high temperatures of up to 1200° C. Such metal alloys contain aluminum, chromium, and iron and yttrium oxide and can be obtained as MA956 alloy from Special Metals Corporation, Huntington, W. Va., United States or PM2000 alloy from Plancee Holding AG, an Austrian corporation having a place of business at A-6600 Reutte/Tirol Austria. Other possible materials are metal-reinforced intermetallic alloys, for example, Nb-reinforced Nb.sub.3Al (Nb/Nb.sub.3Al), Nb or TiNb/TiAl, Mo or Cr/NiAl, Nb/MoSi.sub.2, Boron-doped Mo.sub.5Si.sub.3-based intermetallic alloys and combinations thereof. Ceramic materials include, yttria toughened zirconia (such as Y-TZP, ZrO.sub.2-3% Y.sub.2O.sub.3), ceria toughened zirconia (such as Ce-TZP, ZrO.sub.2-xCeO.sub.2), or gadolinium doped ceria. All of the alloys and ceramics have attractive high temperature properties including good creep resistance and superior mechanical strength. These materials also have high fracture toughness.
The proceeding reaction stages 72, 74, 76 and 78 can be fabricated from less expensive high temperature alloys, discussed above. It is to be noted that the walls of the reactor stages formed by housing 200 of any reactor stage need not in any case be constructed of high temperature materials in that they are exposed to the ambient temperature and heat will thereby dissipate into the ambient environment.
With reference to
Dense layer 246 is preferably a mixture of three phases containing a mixed conductor, an ionic conductor and a metallic electronic conductor can contain about 40 percent by weight “LSFT” (La0.2Sr0.8Fe0.6Ti0.4O3), 40 percent by weight “CGO” (Ce0.8Gd0.2O), and about 20 percent by weight silver. Porous layer 248 can be similarly formed. The difference between the coefficient of thermal expansion of such material over a MA956 oxide disbursed strengthened metal is less than 1 ppm/° K. over the range of about 100° C. to about 1000° C. The proportions would be adjusted for upstream stages formed of high temperature alloys in a manner known in the art.
The material for the dense layer 246 and the porous layer 248 can be prepared by first obtaining about 40 grams of LSFT powder having particle sizes of between about 20 and about 30 microns agglomerated from primary particle sizes of between about 0.3 microns and about 0.5 microns, 40 grams of CGO powder having particle sizes of between about 20 and about 30 microns agglomerated from primary particle sizes of between about 0.5 and about 0.6 microns, and 20 grams of silver powder with particle size of between about 2 and about 3 microns. The foregoing components can be placed into a plastic vial with a few mixing zirconia balls. The powder mixture can then be ball milled for 20 minutes.
Dense layer 246 and a porous layer 248 can be applied to porous regions 240 of planar member 228 by plasma spraying with standard deposition conditions. Porous layer 248 can have a thickness of about 80 microns. The porosity within the porous layer 248 can be achieved using the above three-component mixed powder blended with 40 percent by weight of graphite having average particles size of 75 microns. The dense layer 246 can then be applied to porous layer 248. The thickness of dense layer 246 is preferably about 150 microns.
In order to prevent silver evaporation at high temperature, an additional thin porous layer 250 with a thickness of less than about 20 microns of LSFT coating containing no silver addition can be plasma sprayed on the surface of dense layer 246. This porous LSFT coating can be obtained by spraying the above LSFT powder mixed with 40 percent weight graphite having average particle size of 75 microns.
The following is a calculated example of a reactor 12 utilized within the environment of
Note that in this example case, the staged reactant streams have been arranged such that the outlet temperature from each of the first four stages is approximately 871° C., while the outlet temperature from the final stage is approximately 982° C. This is done to maximize the productivity of upstream stages by maintaining as high a temperature as possible (and consequently as high an oxygen flux as possible) consistent with the material properties. The final reaction stage employs oxide dispersed strengthened material and operates at a higher outlet temperature to secure sufficient conversion of methane.
It is to be further pointed out that the prior art has provided examples of oxygen transport membrane reactors that are capable of conducting autothermal reforming reactions. Such reactors can incorporate tubular oxygen transport membrane elements and are in any case different from reactor 12 described in detail hereinabove. As could be appreciated by those skilled in the art, such reactors could be linked together in a manner in which each reactor would function as a reaction stage in a manner that would at least enable a process of the present invention to be conducted. Furthermore, a reactor 12 could be constructed that would utilize a single dense layer on a planar porous support as opposed to the structure outlined above having an underlying porous layer 248 with interconnected pores and an overlying layer 250 to prevent silver evaporation. As could be appreciated, an oxygen transport membrane element having such structure would not incorporate the efficiency and potential longevity of the structure specifically described herein.
While the present invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, other changes, additions and omissions may be made without departing from the spirit and scope of the present invention as set forth in the appended claims.
This invention was made with United States Government support under Cooperative Agreement number DE-FC26-01NT41096 awarded by the U.S. Department of Energy, National Energy Technology Laboratory. The United States Government has certain rights in this invention.
