Apparatus for the conversion of hydrocarbons

Abstract

The invention relates to an apparatus for the conversion of hydrocarbons to hydrogen, and to the conversion process. Fuel processors, or reformers are often used for large-scale conversion of natural gas to hydrogen containing mixtures. Although large fuel processors perform well, small reformers still have some major drawbacks. The object of this invention is to provide an apparatus for the conversion of hydrocarbons to hydrogen rich gas mixtures, that operates at a relatively low temperature, uses air instead of pure oxygen, can be operated at atmospheric pressure, has a low pressure drop, is compact, has low thermal mass and having high thermal efficiency.

Description

[0001] The invention relates to a fuel processor for the conversion of hydrocarbons to hydrogen, and to the conversion process. Fuel processors, or reformers are used for large-scale conversion of natural gas to hydrogen. Hydrogen, or a mixture of hydrogen and other gasses like carbon dioxide and nitrogen produced by fuel processors can be used as a fuel for fuel cells, like polymer electrolyte membrane (PEM) fuel cells. Although large fuel processors perform well, small reformers still have some major drawbacks.

[0002] A process for the conversion of fossil fuel to hydrogen rich gasses is disclosed in U.S. Pat. No. 5,628,931. Although suitable for large-scale reforming operations, required reaction temperatures of more than 1000° C. make this process less suitable for small reactors like the reactors that are needed for feeding hydrogen to PEM-fuel cells.

[0003] E. R. Stobbe, Catalytic routes for the conversion of methane to synthesis gas, dissertatie Universiteit Utrecht, 22/311999, teaches that it is not possible to convert natural gas by catalytic partial oxidation (CPO) by using pre wetted gas and pre wetted air because the nitrogen dilution and the presence of water reduce the reaction temperature to much for the subsequent steam reforming reaction. Pre mixing of the gasses is not possible because of explosion hazards. Preheating and mixing contains the risk of side reactions in absence of the catalyst, leading to unwanted soot formation. For large scale CPO therefore pure oxygen is used instead of air, oxygen and hydrocarbon are cold mixed immediately before contacting this gas stream with a precious metal wire mesh. The wire mesh stabilizes the flame, and the reaction heat is used for reforming of the remaining hydrocarbons. Although very satisfactory for large scale processing, the method has serious drawbacks for small-scale fuel processors. The high reaction temperature causes insulation problems, and severe heat loss in small fuel processors. The total heat loss being the product of radiation losses, conduction loss and heat loss trough convection In addition the insulation problems necessary use of pure oxygen is unpractical.

[0004] In large installations it is possible, and even advantageous to use compressed gasses. A high-pressure drop over the catalyst bed is therefore acceptable. In small installations, as for example fuel processors that are used in transport applications and houses as feeding device for fuel cell systems, use of compressed gasses is has disadvantages. High-pressure operation requires (mechanical) compressors. Such gas compression equipment is noisy and requires energy, thus reducing system efficiency.

[0005] In installations were pressure drop; volume and weight of the reactor are not restricted, packed reactors are often used. In these reactors ceramic monoliths are placed that are impregnated with a suitable catalyst. Although large fuel processors using this type of catalyst carrier perform satisfactory, there weight, thermal mass and volume are not attractive for small fuel processors.

[0006] The object of this invention is to provide a apparatus for the conversion of hydrocarbons to hydrogen rich gas mixtures, that operates at a relatively low temperature, uses air in stead of pure oxygen, can be operated at atmospheric pressure, has a low pressure drop, is compact, has low thermal mass and having high thermal efficiency.

[0007] According to the invention this objective is achieved by a apparatus in which heating of the feed gasses, an exothermic reforming reactor, a shift reactor, and cooling of the hot reformed gas stream are integrated in counter flow, in such a way that the feed gasses are heated by the exhaust gasses and by the exothermic CPO reaction and/or the exothermic reaction between Carbon Monoxide and oxygen or the exothermic reaction between hydrogen and oxygen, this heat exchanger having the form of a large number of thermal radiation reflectors that are co-heated by the exothermal of the CPO reaction, and the integrated reactor having the form of a multi-blade spiral, having preferably 4 blades, and between these blades the channels for feed gas (hydrocarbon containing gas), an air containing gas mixture, reformed gas and an empty channel. The inner walls of the channels are coated partly or completely with a catalytically active layer or layers. The construction is elucidated in FIG. 1. Experiments showed that the reactor according to the invention has a reduced heat-loss, up to ten times less than reactors of a conventional design, and that heat loss through the exhaust gas stream is eliminated almost completely. The moistened feed gas and the moistened air containing gas are fed to the outer channels of the spiral reactor. Both gasses follow the spiraling gas channels, flow to the center of the spiral, and during their flow to the center these gasses are heated by the gas stream that has already reacted., and flows from the hart of the reactor outwards trough its spiraling channel. Thus the in going gas stream is heated, and the out going gas stream is cooled in counter flow. An other effect is cooling of the channel walls that act as radiation shields. The incoming gasses are kept separated until they have reached a temperature that is sufficiently high for a reaction to take place.

[0008] For small fuel processors that have to start and stop frequently, start-up time is should be limited to a minimum. To comply with this demand the blades of the spiral shaped reactor are made from thin sheet metal with a thickness that is preferably less than 1 mm and more than 25 micron, more preferred less than 250 micron, and more than 40 micron. The optimum thickness depends on the material used, the required pressure drop over the reactor, and the size of the reactor. Blade thickness typically lies between 49 and 126 micron. Heat resistant metals and metal alloys like tungsten and tantalum can be used but are expensive. Preferred are stainless steel, like AISI type 316, and alloys of Iron, Chromium, Aluminum and Yttrium. The spiral shaped seals between the blades can be welded, soldered or folded.

[0009] In high temperature reactors thermal stresses can lead to fatigue, plastic deformation, buckling, cracking and break down of the reactor. It is also an objective of this invention to have a reactor that is resistant to fast heating up and cooling down of the reactor. This objective is realized with the spiral shaped reactor by creating a low and one directional temperature gradient and linear, or almost linear temperature increase and decrease in the spiral shaped blades. The (unwound) length of the spiral shaped blades is such that at a given temperature difference between the center of the spiral shaped reactor and the low temperature end of the spiral, thermal stresses stay below a certain material dependent threshold, and the Euler buckling stress in the blades is not reached. This last requirement is particularly important when thin blades are used.

[0010] To avoid corrosion, and/or to improve adhesion of the catalytic active layer, the thin sheet metal can be coated with suitable coatings. Suitable coatings are corrosion resistant metal layers and for example metal oxide coatings. Preferred are metal oxide coatings like; SiO2 optionally comprising also other elements like, but not limited to Tin, Titanium Aluminum, Cerium, Phosphor or Borium. Thickness of the metal oxide layers is in the range of 10 to 1.000.000 nm, typically a few microns. Preferred precursors for these protective coatings are polymers, especially Silicone polymers, preferably cross-linkable Silicone elastomers. An example of such a protective coating is given in example 2.

[0011] The fuel processor according to the invention contains spiral shaped thin metal blades that are coated single sided or double sided with a catalytically active layer or layers to obtain a reactor with high heat exchange capacity and low pressure drop. Only the sides of the blades in direct contact with the hydrocarbon containing gas an/or the reformate are coated with catalyst. The catalyst layers are preferably applied prior to assembly of the reactor. Application was done by spay coating, brush coating and can be done by most known coating techniques. As precursor for the catalyst carrier silicone elastomers can be used that are subsequently crosslinked and pyrolized in an oxygen containing atmosphere. The active catalyst can be impregnated after pyrolises in the nano porous silicon oxide layer, can be applied directly as a component in the silicone elastomer coating liquid or can be applied in a two step process were first the silicone elastomer is coated and partially or completely cross linked and subsequently swollen by a suitable solvent containing the active catalyst or a precursor for this active catalyst, drying and pyrolizing.

[0012] In order to obtain a high (thermal) efficiency, the moistening of the feed gasses (hydrocarbon and air) is also important. Up to 10% of the total heat is necessary for supplying the required amount of water vapor. According to the invention the heat leak of the reformer is used. This is can be done even better if the whole system is operated at atmospheric pressure If the fuel processor is used in combination with a low temperature fuel cell, the moistening is done in two steps. First the heat generated in the fuel cell stack is used, to generate water vapor at a temperature that is below the temperature of the outer shell of the fuel processor, than the water vapor content is further increased by heating the gasses by passing them over the outer shell of the fuel processor that. The outer shell has typically a temperature between 80° C. and 100° C.

[0013] Although the fuel processor is preferably operated at or close to atmospheric pressure to obtain high system efficiency, it is according to the invention possible to operate the fuel processor at higher than atmospheric pressure. For specific applications were space limitations apply and/or fast startup is required, like in fuel cell systems in cars, operating at higher pressure makes the fuel processor more compact and can reduce startup time. However the spiral shaped reactor as such is not appropriate for operation with a large pressure difference between the spiral shaped reaction chamber and the surrounding atmosphere. This drawback is solved according to this invention by placing the spiral shaped reactor in a pressure resistant chamber as further explained in FIG. 4. In a preferred embodiment a pressure leveling connection between the air channel and the pressure resistant chamber is provided. The pressure in the spiral shaped reactor will be at the same level as the pressure in the surrounding pressure resistant chamber.

[0014] Startup of the reactor can be done by electrical heating the center of the spiral outside the gas channels, by placing a catalyst coated or catalytically active hot wire inside the gas channels in the center of the spiral or by direct heating of the center with a flame or catalytic burner.

[0015] In gas processing equipment were flammable gasses are processed; there is always the risk of uncontrolled explosion. Surprisingly it was found that it was not possible to create a gas explosion inside the reactor of this invention, even not after the reactor was filled with an explosive mixture of natural gas and air and ignition. After ignition the gas mixture burned rapidly but the speed was retarded by the cooling effect of the reactor walls. According to a preferred embodiment of the invention the reactor does not contain coordinates inside the gas channels were the distance to the nearest wall is more than 5 mm, this distance being preferably less than 3 mm.

EXAMPLE 1

[0016] The fuel processor of this example was designed for feeding a 1 kWe PEM fuel cell. The reactor/heat exchanger has a unwound length of 2 meter, a height of 0,3 meter, contains 3 spiral shaped gas slits between 4 spiral shaped 125 micron thick blades(see FIG. 1), one channel (A) with a height of 2 mm is for the in coming moistened hydrocarbon containing gas stream, one other (B) with a height of 4 mm for the incoming moistened air stream, and the third slit (C) is for the out going reformate having a height of 5 mm. The total height of the 3 slits, including the four blades is thus 9,5 mm. Between the out side blade of slit A and the outer blade of slit C some space (D) is left open. When rolled in the form a spiral the diameter is 200 mm. A shift catalyst was coated on the inner walls of the reformate containing gas slit performing the conversion of CO and excess H2O to CO2 and H2 at a, temperature that is decreasing towards the outlet of the reactor. The reaction heat is used for preheating the incoming gas streams and moistening of these gas streams. The incoming air stream has a dead end near the center of the spiral. All air has to flow trough a porous, catalyst containing material like wire mesh. The pressure drop over the porous catalyst containing material in this example is 20 Pa. Since there is an excess of oxidizable gasses, reactions between oxygen and these oxidizable gasses can only take place inside the porous catalyst containing material. The oxidizable gasses have to diffuse against the airflow in the porous material to react at the catalyst and the reaction heat is transferred to the porous material. The oxidizable gasses diffuse in the hot (800° C.) porous material, and react to CO and H2. For this reaction, at this temperature no catalyst is required, and the material will stay in a reduced form, and will be active in steam-reforming. The heat necessary for these reactions is conducted from the other side of the porous material. The reaction order in time is different from known fuel processors. As a first step the hydrocarbon is reformed to CO and H2, these gasses are partially oxidized in the porous material and shifted to CO2 and H2 in the rest of the reactor. Because the porous material is thin, the temperature difference between both sides is only approximately 18° C. A last reaction step is selective oxidation of the reformate to remove traces of CO.

Claims

1. A system for the catalytic conversion of hydrocarbons to hydrogen containing mixtures comprising, radiation shielding, heating of the incoming gasses and cooling of the converted gasses, and heat exchange characterized in that radiation shielding, heating of the incoming gasses and cooling of the converted gasses, and heat exchange are integrated in one apparatus.

2. An apparatus according to claim 1 wherein means for insulation against thermal conduction is integrated.

3. An apparatus according to any one of the preceding claims wherein the catalytic conversion of CO to CO2 is integrated.

4. An apparatus according to any one of the preceding claims being part of a fuel cell system, wherein an after burner for the hydrogen containing exhaust gasses coming from the fuel cell stack is integrated.

5. An apparatus according to any one of the preceding claims wherein the catalyst for the last conversion reaction is in the out going gas channel of the converted gasses.

6. An apparatus according to any one of the preceding claims wherein the gas conversion reactor or reactors and heat exchangers have the shape of a spiral.

7. An apparatus according to claims 6 wherein consists of 3 channels that are separated by blades.

8. An apparatus according to any one of the preceding claims wherein the air channel has a dead end and the air has to flow trough a porous material to one or both of the other gas channel.

9. An apparatus according to any one of the preceding claims wherein the porous material contains a suitable steam reforming catalyst.

10. An apparatus according to any one of the preceding claims wherein the in the porous material H2 and CO react with oxygen generating H2O, CO2 and heat.

11. An apparatus according to any one of the preceding claims wherein the flow control of the gases is based on temperature measurement in the fuel processor.

12. An apparatus according to any one of the preceding claims wherein the flow control of the gasses is based on temperature measurement in the afterburner.

13. An apparatus according to any one of the preceding claims wherein the reactor is made of thin metal sheet.

14. An apparatus according to any one of the preceding claims wherein the reactor walls have a thickness between 25 and 500 micron.

15. An apparatus according to any one of the preceding claims wherein the reactor walls have a thickness between 49 and 126 micron.

16. An apparatus according to any one of the preceding claims wherein the inner walls of the spiral shaped blades are coated with a suitable catalyst.

17. An apparatus according to any one of the preceding claims wherein the inner walls of the fuel channel is coated with a catalyst like koper, koperoxide, koper on zinc oxide, koper on cerium oxide or platinum on a suitable carrier.

18. An apparatus according to any one of the preceding claims wherein the inner walls of the channels are coated with a catalyst or a catalyst on a suitable carrier, and the amount op catalyst applied, and the composition of the catalyst is adjusted depending on its position in the reactor.

19. An apparatus according to claim 13 wherein the thin walled reactor is places in a chamber having the same or approximately the same pressure as the pressure inside the thin walled reactor, and the pressure outside this chamber is substantially higher than the pressure inside this chamber.

20. An apparatus according to claim 19 wherein there is a pressure leveling connection between the inside of the thin walled reactor and the surrounding chamber, this connection being preferably between the inlet of the air channel and the surrounding chamber.

21. An apparatus according to any one of the preceding claims containing means of heating the center of the spiral for startup.

22. An apparatus according claim 16 wherein an electrically heated element is used for start up, and the heating element is placed outside the gas channels in the hard of the spiral.

23. An apparatus according to claim 17 wherein the heating element is a hot wire that is coated with a suitable catalyst, and is placed inside a gas channel

24. An apparatus according to any one of the preceding claims wherein the pressure drop of the gasses inside the reactor is less than 100.000 Pa

25. An apparatus according to any one of the preceding claims wherein the pressure drop of the gasses inside the reactor is less than 10.000 Pa

26. An apparatus according to any one of the preceding claims wherein the pressure drop of the gasses inside the reactor is less than 1.000 Pa.

27. An apparatus according to any one of the preceding claims wherein the pressure drop of the gasses inside the reactor is less than 100 Pa.

28. An apparatus according to any one of the preceding claims, being part of a fuel cell system for micro co-generation, wherein cooling the outer wall with incoming cold tap water reduces heat leakage.