CA2354343A1 - Integrated selective oxidation reactor apparatus and process - Google Patents

Abstract

An integrated selective oxidation reactor apparatus for selectively oxidizing carbon monoxide in the presence of high concentrations of hydrogen comprises an array of catalyst packed tubes disposed within a waste heat recovery steam generator operating at a selected pressure corresponding to the optimum temperature for conducting the catalytic oxidation reaction and a process for useful recovery of the exothermic heat of reaction to generate steam that is used in a process for the conversion of hydrocarbon feedstock into useful gases such as hydrogen.

Description

INTEGRATED SELECTIVE OXIDATION REACTOR APPARATUS AND PROCESS
FIELD OF THE INVENTION
The present invention relates to a process for recovering useful heat from a selective oxidation apparatus and a reactor apparatus for selectively oxidizing carbon monoxide contained in a hydrogen-rich stream at optimally controlled temperature conditions to produce a useful gas for a fuel cell power system.
BACKGROUND OF THE INVENTION
Catalytic reaction apparatus and processes for converting hydrocarbon feedstocks to useful gases, such as hydrogen, are well known in the art. Hydrogen-rich gases formed by these catalytic processes, such as the sequence of steam reforming followed by high and low temperature shift reactions, typically contain residual carbon monoxide concentrations in the range of 0.5 to 1.0 volume percent. Hydrogen-rich gas streams can be beneficially used to generate electrical power in a fuel cell system. The use of proton exchange membrane (PEM) fuel cells has particular economic advantage for certain electric power applications. However, when hydrogen-rich fuel is produced for use in PEM fuel cells, the carbon monoxide concentration must be reduced to low levels, preferably below 10 ppm, in order to prevent poisoning of the anode electrocatalysts typically used in these fuel cell systems.
While several methods are known in the art for purification of hydrogen-containing gas streams, selective catalytic oxidation of carbon monoxide by oxygen is a favored method for use in small fuel cell applications because of its simplicity and potentially low cost. The principal obj ective of a selective catalytic oxidation reactor apparatus for fuel cell applications is to oxidize carbon monoxide to the fullest extent possible to form carbon dioxide by reaction with molecular oxygen, while simultaneously minimizing the quantity of hydrogen that is oxidized to form HZO.
The oxidation process is exothermic and the process generates useful heat.
A selective process can be quantified by a conversion efficiency factor and a selectivity factor. Conversion efficiency is defined herein as the ratio of the quantity of COZ produced by oxidation to the initial quantity of CO in the feed stream, and selectivity is defined here as the ratio of the quantity of oxygen consumed by CO oxidation to the total quantity of oxygen consumed during reaction. For PEM fuel cell applications, it is desirable that the conversion

2 efficiency be at least in the range of 99% to 99.9%, in order to met PEM fuel cell CO tolerance limitations, and that the selectivity be at least in the range of 25% to 50%, in order to prevent unnecessary consumption of hydrogen that would otherwise limit fuel cell electric power generation efficiency. It is desirable that these objects be met when the selective oxidation reactor is operated in a temperature range where the exothermic heat of reaction can be beneficially used within the process.
Oh and Sinkevitch~'~ reported the results of CO oxidation tests conducted in the presence of hydrogen using a variety of catalytic materials including platinum, palladium, rhenium, ruthenium, silver, mixtures of cobalt and copper, and mixtures of nickel, cobalt and iron. Both RulAl203 and Rh/A1z03 catalysts were found to be active for CO oxidation at temperatures above about 200 °F, and to exhibit acceptable conversion efficiencies and selectivities for PEM
applications at temperatures up to about 350 °F. Pt/A1203 was found to be active only at temperatures above about 375 °F. Other catalyst materials tested required even higher activation temperatures and were generally less selective than the Ru/A1z03 and Rh/A1z03 catalysts.
Yasumoto et at (U.S. Patent No. 5702838) describe a catalyst material comprising an A-type zeolite carrying at least one metal selected from the group consisting of Pt, Pd, Ru, Au, Rh, and Ir, or an alloy of two or more metals. The catalyst material is claimed to have high activity and selectivity suitable for PEM fuel cells in the temperature range from about 122 °F to 392 °F.
Sato et at (U.S. Patent No. 5658681) describe a selective oxidation system that uses catalytic material comprising Au supported on an oxide carrier consisting of at least one oxide selected from Fe203, CO, NiO, A1203, TiOz, Zr02 and SiOz. The catalytic material is claimed to have activity and selectivity suitable for PEM fuel cells in the temperature range of 194 °F to 284 °F. The invention teaches the use of circulating cooling water to maintain the selective oxidation catalyst in the desired temperature range using a reactor comprising partitioned plates.
Vanderborgh et at (U.S. Patent No. 5271916) describe a method and apparatus for selectively oxidizing carbon monoxide in a hydrogen rich stream using two or more reactors operated at progressively higher temperatures. The main portion of oxidizing air is fed to the first reactor and a smaller portion of oxidizing air is fed to the second reactor. The invention claims to use a catalyst, for instance Pt/A1z03, in the first reactor that is fed gases at a preferred temperature range of 320 °F to 347 °F. The inlet temperature of the first reactor is controlled by exchanging heat against a two-phase fluid, such as 1,3,5 - trimethyl benzene, that boils at

3 about 328 °F. The first reactor is operated adiabatically so that the exit temperature is greater than the inlet temperature due to the heat of oxidation. The exit gas from the first reactor is cooled by heat exchange to a second temperature preferably about 374 °F
before entering a second adiabatic reactor.
Pow et at (U.S. Patent No. 5316747) describes use ofheat exchange coil apparatus packed with a selective oxidation catalyst, for instance Pt/A1203, for the removal of oxidation reaction heat to a pressurized thermal fluid that is circulated within the Apparatus.
The apparatus includes means to introduce oxidant at a primary inlet and a secondary or multiple secondary inlets. The invention teaches the importance of reactor temperature control to maintain high reaction selectivity.
SUMMARY AND OBJECTS OF THE INVENTION
An obj ect of this invention to provide a novel catalytic reaction apparatus and process for the selective oxidation of carbon monoxide contained in a hydrogen-rich gas stream, and to control the temperature of the catalytic reaction apparatus in a temperature range that is both optimum for conducting the oxidation reaction at high conversion efficiency and selectivity, and for recovering the exothermic heat of reaction in order to generate steam that is needed in a process for converting hydrocarbons feedstock to useful gases, such as hydrogen. The invention uses an array of catalyst packed tubes that are disposed within a waste heat steam generator that is operated at a pressure corresponding to the optimum temperature for conducting the catalytic oxidation reaction. The subject invention is particularly well suited for the production of hydrogen for fuel cells having low tolerance to carbon monoxide.
PEM fuel cells systems based on the steam reforming of hydrocarbons to hydrogen-rich gases require that steam is available in sufficient quantities to conduct the steam reforming reactions under conditions that avoid carbon formation within the catalytic steam reforming apparatus. Typically, the conditions needed to avoid carbon formation within the catalytic steam reforming apparatus require the addition of approximately 3 moles of steam per mole of carbon contained in the hydrocarbon feed. If the PEM fuel cell system is to achieve high thermal efficiency, the required steam quantity must be generated from waste heat recovered from the hydrocarbon conversion process.

4 The oxidation of either carbon monoxide or hydrogen releases heat energy in an amount greater than 60,000 calorie per g mole of reactant. This represents a substantial amount of heat energy that can be beneficially recovered to generate steam necessary for the conversion of hydrocarbon feedstocks using a steam reforming process. Thus, it is desirable that the exothermic heat of reaction released by the selective oxidation process be recovered at a temperature range suitable for generating steam needed for the hydrocarbon steam reforming process.
For certain PEM fuel cell applications, such as the generation of residential electrical power, it is highly desirable that the pressure of the steam reforming process be as low as possible, typically in the range of 3 psig to 10 psig, for reasons of safety and cost, and because residential fuels, such as natural gas, are typically available only at relatively low pressure. Also, for reasons of safety and cost, it is preferred that the steam generator operate at the minimum pressure needed to supply steam to the steam reforming process. Thus, the preferred pressure operating range for the steam generator is about 5 psig to 10 psig, corresponding to a steam saturation temperature range of approximately 230 °F to 240 °F.
In the present invention, the selective oxidizer reactor apparatus is integrated with a waste heat recovery steam generator operating at a pressure range of 5 psig to 10 psig. The steam generator contains a reservoir of boiling water maintained at a temperature range of about 230 °F to 240 °F. An array of tubes are packed with a suitable oxidation catalyst and immersed within the boiling water reservoir of the steam generator. Since the surfaces of the tubes are in thermal communication with the boiling water, once thermal equilibration has occurred between the boiling water and the catalyst packed tubes, the temperature of the selective oxidation apparatus is maintained at a temperature at least equal to the saturation temperature of the boiling water. Thus, the present invention provides a convenient means for heating the selective oxidation reactor to a predetermined minimum operating temperature range during start-up.
During normal operation, hydrogen-rich gases containing CO are mixed with a defined portion of oxidant and passed over the catalyst contained within the tubes of the selective oxidizer apparatus. As the mixture passes through the catalyst packed tubes, oxygen reacts with the carbon monoxide and a portion of the hydrogen to form carbon dioxide and Hz0 thereby releasing exothermic heat. As the heat is released by the oxidation reaction, the gas temperature within the catalyst bed rises until the temperature driving force is sufficient so remove the heat of reaction by heat transfer from the catalyst packed bed tube surfaces to the boiling water contained within the reservoir of the waste heat steam generator.
The ratio of the heat transfer surface to the catalyst volume can be controlled by design in order to maintain the maximum temperature differential between the catalyst bed and the boiling water to within a predetermined limit, such as between 10 °F
and 70 °F, corresponding to maximum catalyst bed operating temperatures ranging from about 240 °F to 300 °F. Thus, the present invention achieves the object of reliably controlling the selective oxidation reactor temperature in an optimum range of about 240 °F to 300 °F while simultaneously recovering useful heat.
In the present invention, the selective oxidizer uses a catalyst that achieves high conversion and selectivity in the optimum operating temperature range of 240 °F to 300 °F. It is apparent from the prior art that there are several catalysts available, for instance Ru/A1203, Rh/A1203, Pt/A-zeolite, and Au/Fe203, that have the desired activity and selectivity to met PEM
applications for use in a selective oxidation reactor apparatus operating at temperatures in the range of about 240 °F to 300 °F.
The foregoing and other objects, features, and advantages of the present invention will become more apparent in the light of the following detailed description of the preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the effect of temperature on the reduction of CO
concentration using a selective oxidation catalyst operating at a space velocity of 3000 hr-' and an oxygen to CO ratio of 2:1.
FIG. 2 is a process flow diagram for recovering heat from a selective oxidation process and to control the selective oxidizer temperature within an optimum range to achieve high conversion efficiency and selectivity.
FIG. 3 is a preferred embodiment of a selective oxidation reactor apparatus according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A process is described that makes beneficial use of the heat of reaction from a selective oxidation reactor that is employed to produce hydrogen-rich gases containing low concentrations of carbon monoxide from hydrocarbon feedstocks.

Refernng to FIG. 1 the test results for a selective oxidation reactor apparatus using a commercial catalyst and operating at a space velocity of 3000 hr-' and an oxygen to CO ratio of 2:1. The tests were conducted over a range of temperatures using a reactant gas mixture consisting of 1 % CO and 99% hydrogen. The test results show that the CO
concentration of the reactor effluent was less than 10 ppm over the temperature range of approximately 200 °F to 300 °F, corresponding to a CO conversion efficiency of greater than 99.9%
and a selectivity of at least 25%. The test results show that a selective oxidation apparatus can be operated within the optimum temperature range of about 200 °F to 300 °F necessary for the useful generation of steam in a low pressure steam generator.
FIG. 2 illustrates a method and process for integrating a selective oxidation reactor apparatus in a process to beneficially recovery the exothermic heat of reaction of a selective oxidation reaction apparatus operating in the temperature range of about 240 °F to 300 °F for the object of generating steam that is useful for converting hydrocarbon feedstock to hydrogen-rich gases and for controlling the temperature of the selective oxidation reactor in a predetermined range.
Refernng to FIG. 2, a reactant mixture 1 consisting of hydrocarbon feedstock 32 and steam 11 are preheated in an exchanger 2 and introduced into a tubular catalytic reactor 3 that is contained within a combustion chamber 4. The tubular catalytic reactor typically contains a supported Ni catalyst and is commonly referred to in the industry as a steam reformer. Fuel 5 and air 6 are combusted in the chamber to heat the reactant mixture so as to produce a hydrogen-rich stream 7 containing carbon monoxide concentrations typically ranging from 5%
to 15 %.
Combustion products 8 from the combustion chamber pass through a flue gas heat exchange coil 9 that is contained within a waste heat steam generator 10, wherein the combustion products are cooled and steam 11 is generated.
The hydrogen-rich stream is cooled in an exchanger 2 to a temperature typically in the range of 600 °F to 650 °F, whereupon the cooled stream 12 is introduced into a fixed catalyst bed reactor 13 to effect a water gas shift reaction that converts a portion of the carbon monoxide to hydrogen and carbon dioxide by reaction with steam. The fixed catalyst bed reactor typically contains a supported Fe/Cr catalyst and is commonly known in the industry as a high temperature shift reactor. The carbon monoxide concentration of the process gas 14 exiting the high temperature shift reactor typically ranges from 2% to 5%.
The products from the high temperature shift reactor are cooled in a process exchange coil 15 that is contained within the waste heat steam generator 10 thereby generating steam and cooling the process gas to a temperature typically in the range from 350 °F to 450 °F before it is introduced into a second fixed catalyst bed reactor 16 wherein the carbon monoxide concentration is further reduced by a water gas shift reaction. The fixed catalyst bed reactor typically contains a supported Cu/Zn catalyst and is commonly known in the industry as a low temperature shift reactor. The carbon monoxide concentration of the process gas 17 exiting the low temperature shift reactor is typically less than 1 %.
The process gas 17 is cooled in a second process exchange coil 18 that is contained within the waste heat steam generator 10 thereby generating steam and cooling the process gas. The second process exchange coil 18 is designed so that the temperature of the process gas 19 at the coil exit is close to the saturation temperature of the steam generated in the waste heat steam generator 10. Typically, the process gas temperature is 10 °F to 30 °F higher than the steam saturation temperature.
The process gas 19 enters an air eductor 20 that induces by motive force a flow of ambient air 21 that is mixed with the process gas. The quantity of ambient air introduced into the process gas is typically controlled by design of the eductor to provide an OZ:CO ratio in the range of 1:1 to 2:1.
The mixture 22 is introduced into an array of catalyst packed tubes 23, also known as the selective oxidizer, that are disposed within the waste heat steam generator operating at a pressure in the range of 5 psig to 25 psig, and preferably in the range of 5 psig, to 10 psig. The surface of the tubes are generally immersed within the water reservoir 25 of the waste heat steam generator. The tubes contain a selective oxidation catalyst having optimum activity and selectivity in the temperature range of approximately 240 °F to 300 °F. As the mixture passes through the catalyst packed tubes, oxygen reacts with the carbon monoxide to form carbon dioxide to release exothermic heat. A portion of the oxygen also reacts with hydrogen to release additional heat. As the heat is released by the oxidation reaction, the gas temperature within the catalyst bed rises until the temperature driving force is sufficient to remove the heat of reaction by heat transfer from the catalyst packed bed tube surfaces 24 to the boiling water 25 contained within the waste heat steam generator. The ratio of the heat transfer surface to the catalyst volume can be controlled by design in order to maintain the catalyst bed in the desired operating temperature regime.
The process gas 26 exiting the selective oxidizer typically contains less than 10 ppm carbon monoxide. The process gas is then cooled in an air cooler 27 wherein excess steam is condensed and recovered in a separator vessel 28. The hydrogen-rich gases 29, having been purified of carbon monoxide to low concentrations are available for use, for instance, in fuel cells having a low tolerance to carbon monoxide.
The condensed steam or condensate 30 is pumped by a pressurizing means 31 through a boiler feedwater preheater that receives heat by exchange against combustion products 10 to heat the water before it is introduced into the waste heat steam generator.
Refernng to FIG. 3 a design of a selective oxidation reactor apparatus is given that accomplishes the obj ect of useful energy recovery to generate steam and for control of the reactor apparatus temperature in an optimum regime during start-up and normal operating states. A
process gas and oxidant mixture 40 enters an inlet manifold 41 through an inlet means 42. The inlet manifold evenly distributes the mixture to the inlet means 43 of an array of tubes 44. The tubes are packed with a selective oxidation catalyst 45 having optimum performance in the temperature range of 240 °F to 300 °F.
The array of tubes are disposed in a waste heat steam generator 46 or compartment 47 thereof operating at a pressure of 5 psig to 25 psig, and preferably between S
psig and 10 psig.
The tubes are partially or fully immersed in a reservoir of boiling water 48.
The tube diameters typically range from 3/8" to 2.0" and preferably from'/2" to 1 %2". The catalyst particle diameters contained in the tubes typically range from 1/32" to 1/2" and preferably 1/16"
to'/4", however, the catalyst may take many forms including pellet, structured packings, and monoliths. The space velocity in the catalyst packed tubes is typically in the range of 1000 to 15000 hr -' and preferably from 2000 hr' to 6,000 hr-'. The catalyst bed may be diluted by inert packing to control the heat release profile within the tubes.
As the heat is released by the oxidation reaction, the gas temperature within the catalyst bed rises until the temperature driving force is sufficient to remove the heat of reaction by heat transfer from the catalyst packed bed tube surfaces to the boiling water 48 contained within the waste heat steam generator. The steam 52 generated from the process exits the waste heat steam generator at an exit means 53. The ratio of the heat transfer surface to the catalyst volume can be controlled by design in order to maintain the catalyst bed in the desired operating temperature regime of 190°F to 300°F and preferably 240 °F to 300 °F. Typically, this ratio is in the range of 10 ft-' to 150 ft~' and preferably from 30 ft-' to 100 ft-'.
The tube array 44 is connected to an exit manifold 49 to collect product gases that exit the apparatus through an outlet means 50. The product gases, having a CO
concentration reduced below 10 ppm, typically exit at a temperature in the range of 190 °F to 300°F and preferably from 240 °F to 300 °F.
Other embodiemtns of the invention will be readily apparent to theose skilled in the art and are meant to be within the scope of the claims appended hereto.

Claims (8)

WE CLAIM:

1. A process for selective oxidation of carbon monoxide contained in a hydrogen-rich gas stream, which controls the temperature of the catalytic reaction apparatus in a temperature range that is both optimum for conducting the oxidation reaction at high conversion efficiency and selectivity and for recovering the exothermic heat of reaction to generate steam that is useful for converting hydrocarbons feedstocks to hydrogen-rich gases.

2. The process of claim 1, wherein said selective oxidation reactor apparatus comprises an array of catalyst packed tubes immersed within a waste heat steam generator operating at a pressure range of 5 psig to 25 psig, and preferably between 5 psig and 10 psig.

3. The process of claim 2, wherein a waste heat steam generator also contains heat exchange coils to recover waste heat from a hydrocarbon steam reforming process.

4. The process of claim 1, a selective oxidation reactor apparatus using a catalyst having a conversion efficiency of greater than 99% to 99.9% and a selectivity of greater than 25%
in the temperature range of 190 °F to 300°F and preferably from 240 °F to 300 °F.

5. A selective oxidation reactor apparatus comprising an array of tubes that are packed with catalyst and immersed in the boiling water reservoir of a waste heat steam generator.

6. The apparatus of claim 5, wherein said catalyst having catalyst particle diameters from 1/32" to 1/2" and preferably from 1/16" to 1 /4" and packed tubes have diameters of 1/32"
to 1/2" and preferably from 3/8" to 2" and preferably from 1/2" to 1 1/2"
operating at temperatures of 190°F to 300°F and preferably 240 °F to 300 °F, space velocities of 1000 h-1 to 15,000 h-1 and preferably from 2000 h-1 to 6000 h-1, and oxygen to CO ratios of 0.7 to 3 and preferably from 1 to 2.

7. The apparatus of claim 5, wherein said selective oxidation reactor apparatus contains inert packing to control the heat release profile within the catalyst bed.

8. The apparatus of claim 5, wherein said selective oxidation reactor apparatus has a surface to catalyst volume ratio in the range of 10 ft-1 to 150 ft-1 to 100 ft-1.