US20080019902A1

US20080019902A1 - Process for producing hydrogen

Info

Publication number: US20080019902A1
Application number: US11/779,500
Authority: US
Inventors: Min Hon Rei; Guan Ting Yeh; Hang Fu Wang; Chia Yeh Hung
Original assignee: Green Hydrotec Corp
Current assignee: Green Hydrotec Corp
Priority date: 2004-01-21
Filing date: 2007-07-18
Publication date: 2008-01-24
Also published as: US7252692B2; TW200524824A; TWI294406B; US20050158236A1; CN1644485A; CN100410167C

Abstract

A process for producing hydrogen is provided. The process comprises the introduction of reactants into a reactor with a steam reforming section containing a steam reforming catalyst to form a hydrogen-containing product. The process is driven by heat generated in a combustion section containing an oxidation catalyst, which comprises a noble metal and boron nitride. According to the process of the subject invention, the first combustion reaction can rapidly generate heat and is advantageous for conducting steam reforming reactions.

Description

RELATED APPLICATION

This application is a continuation-in-part of and claims priority to U.S. patent application Ser. No. 10/761,789, filed 21 Jan. 2004, which is herein incorporated by reference.

TECHNICAL FIELD

The subject invention relates to a process for producing hydrogen. More specifically, the invention relates to a process for catalytically quick-driving hydrogen production with an integrated catalyzed oxidation.

BACKGROUND OF THE INVENTION

Purified hydrogen is an important fuel source for many energy conversion devices. For instance, fuel cells normally require hydrogen with an extremely high purity and oxygen (or air) as the fuel to generate electricity. A widely known process for providing hydrogen is the steam reforming process. Particularly, the steam reforming process comprises reacting steam with a fuel such as an alcohol (e.g., methanol or ethanol) or a hydrocarbon (e.g., methane, gasoline, or hexane) over a steam reforming catalyst to form the main product hydrogen and other by-products (e.g., CO and CO₂). Since the steam reforming reaction is an endothermic reaction, it requires a substantial amount of heat from an external heating system to maintain the temperature of the steam reforming system. Moreover, an additional purification facility is also necessary to purify the product formed in the steam reforming reaction to attain the desired purity of hydrogen, typically at least 95% such as 95% to 99.995%. Obviously, the external facilities for the steam reforming system, such as heaters and purifying devices, occupy a larger share of the capital investment and plant space.
Since highly-purified hydrogen is desirable in the industry, numerous studies have been conducted to find an economic and simple way to efficiently purify hydrogen from steam reforming reactions. Membrane separators have been proposed to harvest purified hydrogen from the steam reforming process. Additionally, to reduce the space of the reaction system, the combination of the membrane separator with the steam reformer in a single device, such as a membrane steam reforming reactor, has also been proposed. For example, U.S. Pat. No. 5,861,137 discloses a steam reformer with internal hydrogen purification, which comprises a tubular hydrogen-permeable and hydrogen selective membrane therein.
Traditional processes normally use external flame in the endothermic steam reforming reaction mentioned above; however, the complicated control and the heat transfer efficiency of such system is not always desirable. It is believed that a rapid and stable supply of heat is critical in maintaining the desired temperature of the steam reforming zone and thus the reaction rate therein. If not, the transfer of heat to the reaction zone fails, causing the reaction temperature and conversation rate of hydrogen to drop. As a result, developments have been focused on in-situ heating via conventional combustion of fuel and/or spent gases from the reformer to provide the heat required for the endothermic steam reforming reaction. For example, U.S. Pat. No. 6,821,502 B2 provides a membrane steam reforming reactor using flameless distributed combustion for generating heat. The flameless burning can be provided by injecting a fuel and a preheated air stream to the reactor for automatic ignition. Obviously, said technical means needs to preheat the air with an additional heater prior to feeding it into the combustion zone. U.S. Pat. No. 5,861,137 discloses a small burner that is provided to burn the fuel or vent product gases to provide the needed thermal energy. Such manner, however, produces dangerous open flame and polluting products, e.g., nitrogen oxide. U.S. Pat. No. 6,585,785 B1 teaches a fuel processor apparatus comprising a catalyst tubular reactor which is heated using an infrared radiant burner to provide the endothermic heat of the reaction needed to reform a mixture of hydrocarbon and steam for the production of hydrogen. Nonetheless, according to the teachings of U.S. Pat. No. 6,585,785 B1, to provide an even distribution of thermal energy or temperature in the reactor chamber, complicated facility or device such as forced circulation of hot air is required.
Apparently, the above mentioned developments concerning the heat supply to the steam reforming system still have some shortcomings, such as the use of additional heaters and complicated devices, naked flames, and polluting products. The subject invention provides a process for producing hydrogen with high purity (99.99%) in a simple and economical way. By using the process of the subject invention, the heat generated from a catalytic combustion section can rapidly increase the temperature of a steam reforming section to a sufficient level to initiate an endothermic steam reforming reaction carried out therein in a very short time and to maintain the reaction temperature.

SUMMARY OF THE INVENTION

The objective of the subject invention is to provide a process for producing hydrogen comprising a step of conducting a steam reforming reaction of reactants. The steam reforming reaction is driven by a heat generated from a first combustion reaction, and the first combustion reaction is catalyzed by a supported oxidation catalyst comprising a noble metal and boron nitride.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a reactor module for implementing the process of the subject invention.
FIG. 2 is a schematic view showing an assembly of reactor modules for implementing the process of the subject invention.
FIG. 3 is a schematic view showing a reactor for implementing the process of the subject invention.
FIG. 4 is a schematic view showing another reactor for implementing the process of the subject invention.
FIG. 5 is a temperature profile showing a first combustion reaction of methanol using various catalysts and oxygen/methanol ratios with WHSV=3.2 to start from room temperature, wherein T₁represents the temperature at the peak and T₂represents the temperature at the steady.
FIG. 6 and FIG. 7 show the temperature distributions of the combustion section and the membrane tube section at different conditions of WHSV and air/MeOH ratio exemplified in EXAMPLE 11.
FIG. 8 shows the temperature variations of the steam reforming section of the reactor exemplified in EXAMPLE 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the process of the subject invention, the production of hydrogen from a steam reforming reaction of reactants is driven by a first combustion reaction. The first combustion reaction is normally an oxidation of a first fuel over an oxidation catalyst comprising a noble metal and boron nitride. In particular, the first combustion reaction generates heat to allow a section for conducting the steam reforming reaction, i.e., a steam reforming section, to reach a desired temperature to initiate and maintain the steam reforming reaction carried out therein. Optionally, after the steam reforming reaction is initiated, a portion of a hydrogen-containing product obtained therefrom is directed back to the process system as at least a part of the first fuel for the first combustion reaction, so as to continuously provide heat for maintaining steam reforming section at the desired temperature. Accordingly, the steam reforming reaction can be continuously driven by the heat generated from the first combustion reaction of the hydrogen-containing product.
The first fuel may comprise a hydrogen-containing gas (such as the hydrogen-containing product obtained from the steam reforming reaction), one or more alcohols (such as C_1-4alcohols), one or more hydrocarbons (such as C_1-6alkanes), and combinations thereof. Specific examples of the first fuel include methanol, ethanol, propanol, isopropanol, butanol, methane, ethane, propane, butane, pentane, hexane, gasoline, liquefied petroleum gas, and combinations thereof, and methanol and hexane are preferred. Moreover, according to the subject invention, the first combustion reaction is normally carried out at a molar ratio of O₂(from such as air) to C (from the first fuel) ranging from about 1.0 to about 4.0.
Any noble metal suitable for oxidation can be used in the oxidation catalyst for the process of the subject invention. Generally, the noble metal is selected from a group consisting of Pt, Pd, Rh, Ru, and a combination thereof. It is preferred that the noble metal is Pt. In addition to the noble metal, the oxidation catalyst used in the subject invention comprises boron nitride. Moreover, in application, the oxidization catalyst comprising the noble metal and boron nitride is normally carried by a support. For example, in one embodiment of the supported oxidization catalyst used in the subject invention, the noble metal is dispersed on a horizontal boron nitride layer over a support. The material of the support should be inert and thermally-stable and the support is in a porous format. The material of the support can be, but is not limited to, alumina, titania, zirconia, silica, or a combination thereof. Preferably, the support is consisting essentially of a material selected from a group consisting of alumina, titania, zirconia, silica, and a combination thereof. A preferred embodiment of the support material is alumina because of its excellent thermal resistance. Moreover, commercial products such as DASH 220 (NE Chemtec, Inc. Japan) and N220 (Süd Chemie Catalysts, Japan, Inc.) can be used as the support. Generally, based the total weight of the oxidation catalyst system (including the noble metal, boron nitride, and support), the amount of the noble metal is from about 0.05 to about 1.0 wt %, and preferably from about 0.1 to about 0.5 wt % and most preferably from about 0.15 to about 0.25 wt %; and the amount of boron nitride is about 1 to about 20 wt %, preferably about 2 to about 10 wt %, and most preferably about 4 to about 6 wt %.
The oxidation catalyst can be prepared by any suitable methods. A method for preparing the oxidation catalyst is illustrated as followed. A noble metal salt (e.g., H₂PtCl₆) and boron nitride are first dissolved in a suitable solvent, such as a mixture of methanol and dimethyl formamide (DMF), and then the resulting mixture is stirred for a while to obtain a slurry. Next, the slurry is coated on a support (made of such as Al₂O₃). The coated support is dried and then sintered so as to obtain a supported oxidation catalyst useful in the subject invention.
The boron nitride in the oxidation catalyst serves two functions. Because the oxidization reaction will produce not only heat but also water, which is adverse to the catalyst system and will decline the catalyst efficiency, the hydrophobic character of boron nitride can prevent the generated water from chemisorbing on the active catalyst sites too long and facilitate turning over of the sites for new run of reaction. Additionally, since the thermal conductivity of boron nitride is high, this helps a rapid transfer of exothermic reaction heat away from the active catalyst center which avoids the formation of detrimental hot spots on the catalyst, and also allows an even dispersion of heat in the combustion section for more effective heat supply to the steam reforming section. This is particularly appreciated in the reactor scale up design.
In addition, the operation of the subject invention is relatively safe because the first combustion reaction supplies flameless heat. In other words, no dangerous open flames or harmful gases will be produced during the steam reforming process. Furthermore, because the steam reforming reaction is an endothermic reaction, a rapid and stable supply of heat is critical to the steam reforming reaction. Using the unique oxidation catalyst of the subject invention (i.e., comprising the noble metal and boron nitride), the generated heat can be evenly and directly transferred to the steam reforming section so as to avoid the complicated arrangement of forced air circulation for achieving an even distribution of reaction temperature in the steam reforming section. Consequently, the process of the subject invention can be carried out smoothly as a result of a stable and effective heat supply from the unique oxidation catalyst.
As mentioned above, the subject invention utilizes the oxidation of the first fuel (such as methanol) to generate heat for heating the steam reforming section to a desired temperature, i.e., the reaction temperature. The inventors also found that prior to the starting of the steam reforming reaction, a second combustion reaction can be carried out in the steam reforming section until the steam reforming section reaches the desired temperature. In this way, the time required for attaining the steam reforming temperature can be extensively shorten. In particular, the second combustion reaction involves an oxidation of a second fuel (e.g., methanol), which can be identical to or different from the first fuel. Preferably, the first combustion reaction and the second combustion reaction are started simultaneously. For example, in comparison with merely utilizing the heat from the oxidation of methanol in the combustion section, the utilization of the heat from the oxidation of methanol in the combustion section as well as that in the steam reforming section can cut the time that the steam reforming section reaches the desired temperature (i.e., the temperature of steam reforming reaction) much sooner as much as 50% of initiation time can be saved.
In the steam reforming reaction, the relevant technical contents are well known in the art. Any materials that can be converted into hydrogen in a steam reforming reaction can be used in the subject invention as the reactants. Normally, the reactants comprise water as well as one or more alcohols, one or more hydrocarbons, or combinations thereof. For example, the alcohol can be, but is not limited to, methanol, ethanol, propanol, isopropanol, ethylene glycol, glycerol, or a combination thereof, and the hydrocarbon can be, but not limited to, methane, hexane, gasoline, liquefied petroleum gas (LPG), naphtha oil, diesel oil, or a combination thereof. Preferably, the reactants comprise water and methanol, hexane, or a combination of methanol and hexane.
To smooth out the steam reforming reaction, the reactants are normally preheated to a temperature slightly higher than the temperature of the steam reforming reaction, before being introduced into the steam reforming section. According to the process of subject invention, the reactants can be preheated by the heat generated from the first combustion reaction to effectively use the heat in the reaction system. If desired, the reactants can also be preheated using an external heater as described in the prior art, and then fed into the steam reforming section.
In addition to the heat supply, the steam reforming reaction also requires a steam reforming catalyst for lowering the activation energy of the steam reforming reaction to convert the reactants into hydrogen. The steam reforming catalyst is normally selected depending on the species of the reactants to be converted into hydrogen. Typical steam reforming catalysts that can be used in the subject invention include, but are not limited to, transition metals. Optionally, the steam reforming catalyst can be used in combination with a group IA metal such as potassium (K). It is noted that the use of the group IA metal reduce the coking of the catalyst. For example, the steam reforming catalyst used in the subject invention can comprise Cu, Zn, Pd, Re, Ni, or a combination thereof. Particularly, in steam reforming of methanol or glycerol, a combination of Cu and Zn can be used as the catalyst. On the other hand, in steam reforming of hexane, a combination of K and Ni can be used to catalyze the reaction.
Similar to the oxidation catalyst for the first combustion reaction of the subject invention, it is often desirable that the steam reforming catalyst is carried by a support, which is normally an inert compound. Suitable support for the steam reforming catalyst normally comprises one or more of elements of Group III and IV of the Periodic Table, for example, oxides or carbides of Al, Si, Ti, and Zr. A preferred embodiment of the support for the steam reforming catalyst is alumina. The method for producing a supported steam reforming catalyst is well known by persons skilled in the art, such as the sol gel technique or impregnation, and can be referred to “Production and thermal pretreatment of supported catalysts,” written by J. W. Geus (see Preparation of Catalysts III, ed. G. Poncelet, P. Grange and P. A. Jacobs, Elsevier, Amsterdam, 1983, 1-34). For example, the supported steam reforming catalysts such as CuOZnO/Al₂O₃, PdOCuOZnO/Al₂O₃, and K₂ONiO/Al₂O₃can be used in the subject invention.
The temperature of steam reforming reaction varies with many factors including the species of the reactants, the scale and module of the reactor for implementing the steam reforming process, and especially, the species of the steam reforming catalyst. For example, in the case of steam reforming of an alcohol (such as methanol, isopropanol, or glycerol), the steam reforming catalyst used typically comprises Cu and Zn, and the temperature should not go over about 330° C. to prevent the sintering and coking of the steam reforming catalyst. Hence, the temperature of steam reforming of an alcohol should stay within the range of about 200° C. to about 330° C., preferably, about 280° C. to about 300° C. On the other hand, for steam reforming of an alkane (e.g., hexane, methane, or gasoline), the reaction is generally carried out at a temperature of about 700° C. to about 900° C.
The steam reforming reaction of the subject invention provides a hydrogen-containing product. In industrial applications, such as in fuel cells, the hydrogen-containing product always needs to be further purified. As a result, the process of the subject invention preferably further comprises a step of purifying the hydrogen-containing product obtained from the steam reforming reaction to produce purified hydrogen and leave a spent product. Any proper purifying methods, such as catalytic adsorption, cryogenic cooling, pressure swinging adsorption, or polymer membrane, can be used to conduct the purification.
In the case of using a purifying step, a portion of the spent product obtained from the purifying step can be directed back to the combustion section as at least a part of the first fuel for the first combustion reaction, to continuously provide heat for maintaining steam reforming section at a desired temperature. Accordingly, the steam reforming reaction can be continuously driven by the heat generated from the first combustion reaction of the spent product.
One preferred embodiment of the process of the subject invention is to utilize at least one palladium membrane tube to purify the product of the steam reforming reaction. The palladium membrane tube can be formed by depositing a palladium-containing membrane with a thickness of about 3 μm to about 50 μm on a porous support. The palladium-containing membrane is normally made from one of the following materials: palladium, a palladium-silver alloy, and a palladium-copper alloy. The porous support can be made of such as ceramic material or stainless steel. Stainless steel is preferred because of its cost effectiveness and convenience in the fabrication of a reactor.
The palladium-containing membrane can be deposited on the porous support using an electroplating method, an electro-less plating method, a sputtering method, or a cold-rolled method. Many prior art references, such as TW 1232888, U.S. Pat. No. 6,152,987, JP 2002-119834, and JP 2002-153740, already describe the technology for depositing a palladium-containing membrane on a porous support and their contents are incorporated hereinto for reference.
As exemplified in Examples 3 and 4 below, in one embodiment of the palladium membrane tube suitable for the subject invention, the outside diameter of the tube is 9.525 mm, while the length of the tube is 150 mm. Furthermore, the palladium membrane tube has one sealed end, which is arranged upstream to the flowing path to speed up the flow of hydrogen permeating from the sealed end to the open end. The crude hydrogen, with a 60-75% purity, from the steam reforming reaction permeates through the palladium membrane tubes to yield hydrogen with a purity greater than 99%. The high purity of hydrogen is directly derived in the membrane tube side without any additional purification facilities.
Normally, the temperature of conducting the purification with the use of one or more membrane tubes is not higher than about 490° C., such as about 25° C. to about 490° C. Preferably, the purification is carried out at a temperature ranging from about 200° C. to about 380° C. The heat for maintaining the purification temperature can also be provided by the combustion section.
In the purification step, highly pure hydrogen is separated from the spent product. The spent product primarily contains CO and CO₂, and also contains H₂. As mentioned above, a portion of the spent product can be directed back to the combustion section to generate heat for continuously supplying heat to the endothermic steam reforming reaction. The spent product can also be used in many other applications, such as heating water. The highly pure hydrogen obtained may contain few undesired carbon-containing compounds such as CO and CO₂, which are unfavorable to many energy conversion devices, especially fuel cells, and will reduce their efficiency. Accordingly, it is preferred to further treat the highly pure hydrogen with a converter to convert the undesired carbon-containing compounds into an alkane, such as methane.
It is unexpectedly observed that the structure of the reactor will influence the temperature for conducting the steam reforming reaction. Particularly, it is observed that when the hydrogen purification is achieved by using palladium membrane tubes and the tubes are configured in the steam reforming section, the temperature necessary to conduct the steam reforming reaction can be reduced. For example, when gasoline is used as the reactant for the steam reforming reaction, the steam reforming temperature can be reduced from at least about 700° C. (e.g., from about 700° C. to about 900° C.) to less than about 650° C. (e.g., from about 500° C. to about 650° C.). Without limited by theory, it is believed that the reduction of hydrogen in the steam reforming section due to the hydrogen-permeable palladium membrane can break the limits of thermodynamic control on the conversion level and attain the same conversion of hydrogen at a lower temperature.
The apparatus for carrying out the process of the subject invention mainly comprises three sections, i.e., the steam reforming section, the hydrogen purification section, and the combustion section. It should be mentioned that the three sections can be arbitrarily arranged according to different needs of scales and temperature requirements given that the combustion section can be positioned inside, outside, or between the other two sections.
For implementing the process of the subject invention, a 3-in-1 reactor module can be used. In brief, the reactor used combines three sections, namely, a steam reforming section for producing a hydrogen-containing product, a membrane tube section containing at least one palladium membrane tube for purifying the hydrogen-containing product, and a combustion section for providing the heat required for driving the steam reforming reaction. For example, the membrane tube section can be arranged within the steam reforming section. In other words, the palladium membrane tube and the steam reforming section are positioned in the same compartment. Alternatively, the membrane tube section and the steam reforming section can be arranged in separate compartments of the reactor. As for the combustion section, as mentioned above, it can be configured inside, outside, or between the membrane tube section and the steam reforming section as required.
One reactor module for implementing the process of the subject invention is shown in FIG. 1. Reactor module 1 comprises a reactor 15 with a shell 11 that has an inlet 12, an outlet 13 and a vent 14; a flowing path 17 extending from the inlet 12 to outlet 14; and several palladium membrane tubes 16. The palladium membranes deposited on each of the tubes 16 are used for purifying hydrogen, wherein each tube 16 has one sealed end located upstream to the flowing path 17. The reactor module 1 further includes a combustion section 18 for heating the reactor 15. The inlet 12 is configured to receive reactants composed of steam and a fuel, such as gasoline, after the reactants are pumped from the feed tank 121 and is properly heated to the desirable reaction temperature by the heat generated in the combustion section 18. The outlet 13 is configured to discharge pure hydrogen, while the vent 14 is configured to discharge a spent product, including H₂, CO and CO₂. The spent product discharged from the vent 14 is passed through a pressure reducer 191 and forwarded into the combustion section 18 through a connection 19 for combustion. A proper amount of air is pumped first through a fuel reservoir 182 and a check valve 183 and then into the connection 19 to mix with the spent product. The gases from the combustion section 18 are further vented through an outlet 181 for discharge as waste gases or heat exchanged with the feed stream. Moreover, reactor module 1 can further include a heat conductive perforated metal plate 151 welded to the wall of the reactor 15. The heat conductive perforated metal plate 151 facilitates heat transfer from the warmer reactor wall to the steam reforming catalyst zone for the endothermic reaction. In the case of steam reforming of hexane or gasoline, the reactor module 1 is suitable.
It is observed that hydrogen flux through the palladium membrane is drastically decreased when the hydrogen concentration is low. This means that the palladium membrane tube is very inefficient in the low hydrogen concentration region. This surprising discovery practically limits the use of long length membrane tubes during hydrogen production on a large scale. Accordingly, another reactor module with a short palladium membrane tube can be used to conduct the process of the subject invention. Preferably, the length of the palladium membrane tube is about 3 cm to about 120 cm. Moreover, in order to avoid using a long tube, an assembly 2 of the reactor modules is useful as shown in FIG. 2.
Referring to FIG. 2, an assembly 2 includes two reactor sections 28 and 29, both with an extended common shell 21, an inlet 22, two vents 24 and 25, and two outlets 26 and 27. The two reactor sections 28 and 29 are assembled to share the common inlet side and have flowing paths extending from the inlet to the outlet opposite in direction to the reactor sections 28 and 29, respectively. Each reactor section, 28 or 29 has a plurality of palladium membrane tubes 30. The palladium membrane tube 30 is formed by depositing a palladium membrane on the porous support for purifying hydrogen, wherein each palladium membrane tube 30 has one sealed end located upstream to the flowing path. Assembly 2 further includes a combustion section 31 for heating the reactor sections 28 and 29. The inlet 22 is configured to receive reactants. The reactants can comprise ethanol, methanol, isopropanol, methane, hexane, gasoline, LPG, glycerol, or a combination thereof. The outlets 24 and 25 are configured to discharge pure hydrogen, and the vents 26 and 27 are configured to discharge spent products including H₂, CO, and CO₂. The spent products discharged from the vents 26 and 27 are introduced into the combustion section 31 through a connector (not shown) for combustion. The waste gases are discharged from the combustion section 31 via a vent (not shown). Moreover, the assembly 2 further includes a heat conductive perforated metal plate 23 welded into the reactor wall in each reactor section. The heat conductive perforated metal plate 23 facilitates heat transfer from the warmer reactor wall to the steam reforming catalyst zone in the endothermic reaction.
In addition to the above reactors having the configuration that the palladium membrane tubes are arranged inside the steam reforming section, other reactor types can be used in the subject invention. One embodiment of the reactors is depicted in FIG. 3. As shown in FIG. 3, a reactor 300 comprises a membrane tube section 340 with at least one palladium membrane tube 350 located in the central part of the reactor 300, a steam reforming section 310 located in the peripheral part of the reactor 300, and a combustion section 330 located between the membrane tube section 340 and steam reforming section 310. A first fuel, such as methanol, and air are first introduced into the combustion section 330 via a line 331 for combustion. The combustion section 330 is filled with the oxidation catalysts. In this aspect, the first fuel and air can be introduced into the combustion section 330 using different inlets. For example, the first fuel can be pumped into the combustion section 330 at the bottom and the middle of the reactor 300 and air can be fed at the bottom of the reactor 300. Then, the first fuel is reacted with the oxygen subject in the air over the oxidation catalyst in the combustion section 330 to rapidly generate heat. Then, reactants comprising a fuel and water are pumped into a preheating coil 320 located in the combustion section 330 via a line 311 that is heated to a predetermined temperature of about 20° C. to about 50° C. higher than the temperature of the steam reforming reaction. The space velocity of the reactants depends on many factors, such as the size of the reactor and the components of reactants. Generally, the reactants are introduced into the reactor 300 with a space velocity of about 0.9 to about 5.0 hr⁻¹, preferably about 2.0 to about 4.0 hr⁻¹. Then, the pre-heated reactants are introduced into the steam reforming section 310 via a line 312. Meanwhile, the steam reforming reaction will be quickly driven by the heat generated in the combustion section 330 and is conducted smoothly due to the stable heat supply.
After, the product formed in the steam reforming section 310 is fed into a membrane tube section 340 via a line 313. The product includes hydrogen, un-reacted reactants, and by-products. In the membrane tube section 340, the hydrogen in the product will permeate the palladium membranes on the palladium membrane tubes 350, and flow in and exit from the palladium membrane tubes 350. Purified hydrogen can then be obtained via a line 314. The palladium membrane tubes 350 have one sealed end, which is arranged upstream to the flowing path to speed up the flow of hydrogen from the sealed end to the open end. Also, since the presence of CO and CO₂in a fuel cell is undesirable, the purified hydrogen from the palladium membrane tubes 350 is further treated with a methanizer 360 to convert CO and CO₂in the purified hydrogen to CH₄. The heat for maintaining the methanizer 360 at a desired temperature can also be provided by the combustion section 330. By using the reactor 300 for implementing the process of the subject invention, the purity of the resulting purified hydrogen is above 99.98%.
The spent product, which does not permeate through the palladium membrane, exits from the membrane tube section 340 via a line 315. Some of the spent product can be directed back into the combustion section 330 for conducting catalytic oxidization so as to generate heat. Then, the spent gas which is generated in the combustion section 330 exits from the reactor 300 via a line 332. The reactor 300 is particularly suitable for use in steam reforming of methanol because the steam reforming section 310 deposited in the peripheral part can be maintained at a lower temperature (e.g., about 280° C. to about 300° C.).
FIG. 4 shows another reactor for use in conducting the process of the subject invention. The reactor 400 comprises a membrane tube section 440 deposited in the peripheral part of reaction 400 and a steam reforming section 410 positioned in a combustion section 430. The steam reforming section 410 is in tubular form. As described in FIG. 3, the first fuel and air for combustion are introduced into the combustion section 430 via lines 433 and 431, respectively. Then, the reactants are first fed into the preheating zone 420 via a line 411 and then into the steam reforming section 410. The product formed in steam reforming section is introduced into the membrane tube section 440 with the palladium membrane tubes (not depicted) via a line 412 for hydrogen purification. After hydrogen purification, the purified hydrogen is obtained via a line 414, while the spent product is directed back into the combustion section 430 via a line 415. Waste gases in the combustion section 430 exit the reactor 400 via a line 432. The reactor 400 further includes a heat conductive perforated metal plate 460 welded into the reactor wall to facilitate the uniform distribution of the generated heat.

EXAMPLES

Example 1

Hydrogen Permeation of a H₂—CH₄Mixture

The hydrogen mixture with different concentrations of hydrogen, i.e., 99.995%, 80%, 75%, and 66%, were used to study hydrogen permeation through the palladium membrane tube at 330° C. under a pressure of 5, 6, 7 and 8 bar at the shell side. The resultant hydrogen flux through the palladium membrane is shown in Table 1. The permeability was calculated in units of M3/M2-hr-bar1/2. The experiment was carried out in a stainless steel tubular reactor that was 25 mmOD×350 mL (outside diameter×length) with a palladium membrane tube of 9.525 mmOD×110 mmL. The hydrogen mixture is fed into the shell side of membrane, and then the pure hydrogen permeates through the membrane into the interior of the membrane tube. The permeation pressure is set by adjusting the back pressure regulator in the spent gas mixture stream before leaving the reactor system.

TABLE 1


Hydrogen permeation of a H₂/CH₄mixture
with a palladium membrane

			Perme-
	Flux,		ability,
% H in %	M³/M²-hr	P1, absol., bars	M³/M²-

H₂/CH₄	H₂purity	6	7	8	hr-bar^1/2

99.995	Flux,	18.30	21.28	24.00	12.6
	M³/M²-hr
	H₂purity		99.99999+
80	Flux,	13.85	16.66	18.96	11.85
	M³/M²-hr
	H₂purity		99.96
75	Flux,	12.71	14.83	16.94	11.45
	M³/M²-hr
	H₂purity		99.92
66	Flux,	10.86	12.96	14.28	10.8
	M³/M²-hr
	H₂purity		99.92

Example 2

Hydrogen Permeability of a H₂—Y Mixture with Y: N₂, CO₂and Cyclohexanol (CXL)

The palladium membrane tube, which was 9.525 mm×30 mm (outside diameter×length), was used for the hydrogen permeability test at 310° C. The results are shown in Table 2. The observed drop in hydrogen permeability was far more than that could be accounted for by the decrease of the partial pressure of hydrogen. Moreover, the dilution of hydrogen concentration brought about not only a decrease in hydrogen flux, but also a deterioration of the hydrogen purity via the permeation. Through the palladium membrane, an industrial grade of hydrogen with 99.995% purity can be purified into an electronic grade with 99.9999+purity. The purity was decreased to 99.9999% and 99.99% when the hydrogen concentration was decreased to 75% and 50%, respectively, as the Y was CXL.

TABLE 2


Hydrogen permeability in a mixed feed of
H₂/Y (Y = CO₂, CXL, or N₂)^[a]

			Perme-
	Flux,		ability,
% H in %	cc/min	P1, absol., bars	M³/M²-

H₂/Y	Y	H₂purity	3	4	5	hr-bar^1/2

99.995		Flux,	87.3	122	152	8.66
		cc/min
		H₂purity		99.9999+
75	CXL^[b]	Flux,	81	113	140	9.30
		cc/min
		H₂purity		99.9999
50	CXL	Flux,	24	35	47	5.08
		cc/min
		H₂purity	99.992	99.994	99.996
50	N₂	Flux,	20	34	42	4.48
		cc/min
		H₂purity		>99.9^[c]
50	CO₂	Flux,	13	23	32	3.60
		cc/min
		H₂purity	99.94	99.95	99.96

^[a]The permeability test was conducted at 310° C. with a Pd-membrane of 9.575 mm × 30 mm (outside diameter × length). The products were analyzed with a GC-FID capable of detecting an impurity up to 1 ppm of COx (CO and CO₂) and other organic compounds.
^[b]CXL = cyclohexanol
^[c]Analyzed with a TCD that has a nitrogen sensitivity >0.5% in the permeation.

Example 3

Direction of Hydrogen Permeation in a Pd-Membrane Tube with Respect to the Membrane Sealing

A palladium membrane tube (9.525 mmOD×150 mL) with one sealed end was inserted into a tubular reactor (25.4 mmID×450 mL) via two modes of connections, [A] and [B]. In the [A]-mode, the sealed end of the membrane tube was arranged upstream to the hydrogen flow, while the hydrogen flowed into both the shell-side and the tube-side co-currently. In the [B]-mode, the sealed end of the membrane tube was arranged downstream to the hydrogen flow. The hydrogen flowed into both the shell-side and tube-side. When the permeation pressure in the shell side was set at 3 bar, the hydrogen flux in the tube side in the [A]-mode was 210 cc/min, while the corresponding hydrogen flux in the [B]-mode was 192 cc/min.

Example 4

Direction of Hydrogen Permeation in the Pd/Ag-Membrane Tube with Respect to the Membrane Sealing

A similar experiment to Example 3 was further tested with a 67/33 weight ratio of a Pd/Ag alloy membrane tube that was 25 μm thick. The membrane tube had a similar porous support (9.525 mmOD×150 mL) with the sealed end inserted into a tubular reactor of 25.4 mmID×450 mL via two modes of connections, [A] and [B] as described above. In the [A]-mode, the sealed end of the membrane tube was arranged upstream to the hydrogen flow, while the hydrogen flowed into the shell-side and tube-side co-currently. In the [B]-mode, the sealed end of the membrane tube was arranged downstream of the hydrogen flow, while the hydrogen flowed into the shell-side and tube-side. When the permeation pressure in the shell side was set at 3, 4, and 6 bar, the hydrogen flux in the tube side in [A]-mode was 95, 136, and 189 cc/min, respectively. The corresponding hydrogen flux in the [B]-mode was 80, 112, and 170 cc/min, respectively.

Example 5

Preparation of an Oxidation Catalyst on a Supporting Material

Five grams (5 g) of H₂PtCl₆and 50 grams of boron nitride were dissolved in a solvent comprising 800 ml of methanol and 200 ml of dimethyl formamide, and then stirred to obtain a slurry. The slurry was then coated on alumina (948 g), and the coated alumina was dried at a temperature of 100° C. to remove the solvent. Next, the dried alumina was sintered in an oven at a temperature of 450° C. with an air flow of 5 L/min for 8 hours to obtain the oxidation catalyst on alumina.

Example 6

Cold Start Heating with a PBN Oxidation Catalyst

Six grams (6 g) of Pt/BN/γ-Al₂O₃was used as the oxidation catalyst and was placed in a stainless steel tube with a ½-inch OD (outside diameter). The whole tube was insulated with mineral wool. Two sets of temperatures were measured by thermocouples as T_aand T_b. T_aindicated the temperature at the top of catalyst bed, while T_bindicated the temperature outside of the tube and adjacent to the top of catalyst. An appropriate amount of methanol was pumped into the stainless steel tube at a desired space velocity (WHSV, hr⁻¹) and air was introduced to provide a molar ratio of O₂/Methanol close to 1.65 or 1.80 (corresponding to 10% and 20% excess of theoretical demand). As a result, the reaction temperatures indicated as T_aand T_brose rapidly from room temperature to about 800° C. and then stabilized to a lower temperature of about 400° C. to 450° C. when a molar ratio of O₂/Methanol close to 1.65 or 1.80 was introduced at an appropriate space velocity of 2 hr⁻¹to 4 hr⁻¹(WHSV). In addition, the oxidation catalysts in the catalytic combustion section were either Pt/BN—N-220 and Pt/BN-Dash-220. The heating effect and cold start capability of Pt/BN—N-220 and Pt/BN-Dash-220 are shown in FIG. 5, wherein T₁represents the temperature at the peak and T₂represents the steady temperature.

Example 7

High Temperature from the Cold Started Catalytic Combustion of Hexane with Pt/BN—N-220

Six grams (6 g) of PtBN/N-220 were used as the oxidation catalyst, and was placed in the combustion section according to the subject invention. N-hexane, as the first fuel, was pumped onto the oxidation catalyst at a velocity of 1.66 gm/min. Then, airflow was introduced at a velocity of 2.35 L/min to give an O₂/hexane ratio close to 10.45 (10% excess of theoretical demand). The temperature indicated as T₃in the catalyst zone rose to 630° C. in 4 min, and then to 970° C. in another 5 min. The temperature indicated as T₄was maintained between 980° C. to 960° C. for the next 110 min until the reaction was terminated. Apparently, heat from the combustion of hexane for maintaining the steady temperature, T₄was much more than that of methanol. On the other hand, it was easier for methanol to initiate the oxidation reaction. Initially, T₁rose faster and higher than T₃.

Example 8

Cold Start of the Methanol Steam Reforming Reaction Using Catalytic Combustion of Aqueous Methanol

Twelve grams (12 g) of Pt/BN-γ-Al₂O₃, as the oxidation catalyst, was placed into the combustion section of the subject invention. 120 g of CuO-ZnOAl₂O₃, as the steam reforming catalyst, was placed in the reactor. The reactor was then wrapped with a thick layer of mineral wool for insulation. Both the inner catalyst zone and annular catalyst zone were independently connected with metering pumps for delivery of a methanol-water mixture for the reforming and combustion fuel, respectively. Initially, methanol, as the fuel, at a feeding rate of 7.5 mmol/min. with WHSV=1.2, was introduced into the combustion reactor at room temperature to set the oxidation reaction by reacting with air at a O₂/CH₃OH ratio close to 1.65 in 12 minutes. The temperature, T_OX, of the oxidation catalyst in the catalytic combustion section rose to 560° C. and almost simultaneously the temperature, T_SR, in the reactor reached 380-390° C. The endothermic steam reforming reaction of methanol was then started up by introducing liquid methanol (15 mmol/min) together with water (18 mmol/min) into the reforming catalyst bed. The reaction temperature, T_SR, dropped slightly to 350° C. and kept steady for the next 60 minutes. Hydrogen and carbon oxides were produced from the reforming side of the reactor. Thereafter, T_OXin the combustion section dropped to 420-460° C. and T_SRin the reformer decreased to 310° C. The lower temperature reactions were continued for another 30 minutes while the gaseous products evolved smoothly.

Example 9

Steam Reforming Reaction of Hexane

According to the subject invention, a steam reforming reaction of hexane was carried out at 500° C. under a pressure of 9 bars and VHSV of 10,000 to 30,000 hr⁻¹. Five steam reforming catalysts were tested for their carbon coking rate, conversion and their selectivity to hydrogen product. The characteristics of these catalysts are shown in Table 3 and their performance is presented in Table 4 for comparison. As shown in Tables 3 and 4, both commercial catalysts had a higher decaying rate by coking than the three homemade catalysts with a higher surface area. In addition, the commercial catalysts also brought about higher a hydrogen partial pressure and higher selectivity to the hydrogen product. With regard to the Y-2 catalyst, the use of palladium membrane tube showed a much higher conversion, hydrogen partial pressure and the selectivity to hydrogen.

TABLE 3


Characteristics of catalysts used for the steam reforming of hexane

	G-56H-1	FCR-4-02	Y1	Y2	Y3

Specific	2.3	3.0	2.0	2.0	2.0
gravity (kg/l)
Surface	27.41	6.73	145.32	133.6	131.44
area (m²/g)
Pore volume	0.054	0.014	0.365	0.338	0.32
(c.c./g)
Pore size (A)	78.8	87.6	100.33	101.1	97.3
Support	α-Al₂O₃	α-Al₂O₃	γ-Al₂O₃	γ-Al₂O₃	γ-Al₂O₃

Content	Ni	17.0	12.0	15.0	15.0	17.0
(%)	K₂O	0.4	-	0.4	1.0	0.4
	MgO	—	—	—	—	5.0

TABLE 4


The performance of steam reforming reaction of n-hexane^[a]

Av. Coking rate

n-Hexane

Partial Press

Gas composition (vol %)

Catalyst	(mgC/gCat-hr)^[b]	Conv. (% mol)	of H₂(%)	CO	CO₂	CH₄	H₂

FCR-4-02	128.3	48.02	14.93	0.59	22.55	38.69	38.18
G-56-H-1	13.29	65.84	12.15	0.56	21.64	50.23	27.57
Y-1	8.23	40.48	21.31	0.15	21.52	17.50	59.49
Y-2	3.20	42.39	21.16	0.97	22.35	12.67	64.00
Y-2/Membr	3.20	46.05	31.94	0.02	23.18	9.51	68.29
Y-3	7.70	39.69	18.38	1.51	21.92	3.57	72.99

^[a]VHSV = 20000 h⁻¹, H₂O/C = 1.5, 9 atm, 500° C.
^[b]Coking time of 6 hr under the conditions of [a]

Example 10

A reactor (Green Hydrotec Inc., Model: GHR500LPH100) with a structure as shown in FIG. 3 was used in this example. Methanol was introduced into the combustion section of the reactor. Upon achieving a temperature of 260° C., methanol and water in a molar ratio of 1:1.2 was introduced into the steam reforming section of the reactor with a feeding rate of 10 g/min. The amount of the purified hydrogen from the palladium membrane tubes was 7.5 L/min. The amount of the spent product (containing H₂and CO₂) was 10 L/min. After calculations, H₂recovery yield was 57.1%.
The spent product (10 L/min) was divided into two streams. One stream (5.2 L/min) was introduced back into the combustion section to generate heat for preheating coil and steam reforming. Another stream (4.8 L/min) was introduced into another oxidation zone to heat 162 g of water from 20° C. to 49° C. The thermal energy provided by the stream (4.8 L/min) was 19.6 KJ/min. The total thermal efficiency of the reactor was 78%.

Example 11

Different Values of WHSV and Air/MeOH Ratio on the Temperature Distribution of Combustion Section and Membrane Tube Section

In the case of GHT500LPH100, the space velocity, WHSV and Air/MeOH ratio were changed to test the optimal temperature distribution of the reactor. Table 5 lists the experimental conditions for this example. The temperature distribution of the combustion section and the membrane tube section are shown in FIGS. 6 and 7. All four conditions can have smooth temperature distributions without hot spots. The air flow rate was reduced to cut heat loss by the excess air vent as indicated by EXP1 and EXP3. Furthermore, EXP2 using the most methanol or fuel input with enough oxygen for combustion exhibited the highest temperature profile. EXP4 provides a high enough temperature profile for heat transfer with lower excess air, and is most efficient. The higher WHSV, on the other hand, provides a higher reactor temperature for faster heat time therefore, it is useful in bringing up the reaction temperature in the initial stage.

TABLE 5

Air MeOH WHSV

EXP No. (L/min) (g/min) (1/hr) O₂/C Excess air

1 60 5 0.375 3.29 119%

2 30 4.16 0.312 1.98 32%

3 30 3.52 0.264 2.34 56%

4 20 3.52 0.264 1.56 4%

Example 12

Introduction of a First Fuel and Air into the Steam Reforming Section and the Combustion Section

The conditions are as listed below:
For the steam reforming section (SRR):
WHSV (feeding rate of methanol/catalyst weight): 1.54 hr⁻¹
Catalyst weight: 50 g of CuOZnO/Al₂O₃
Feeding rate of air: 6 L/min
For the combustion section (OXD):
WHSV (feeding rate of mathanol/catalyst weight): 3.8 hr⁻¹
Catalyst weight: 20 g of Pt—BN/Al₂O₃
Feeding rate of air: 6 L/min
As shown in FIG. 8, the results show that introducing the methanol and air into the steam reforming section prior to feeding the raw material of the first fuel and water can shorten the time for heating the steam reforming section to the reaction temperature.
The above disclosure is related to the detailed technical contents and inventive features thereof. People skilled in this field may proceed with a variety of modifications and replacements based on the disclosures and suggestions of the invention as described without departing from the characteristics thereof. Nevertheless, although such modifications and replacements are not fully disclosed in the above descriptions, they have substantially been covered in the following claims as appended.

Claims

1. A process for producing hydrogen comprising a step of conducting a steam reforming reaction of reactants, wherein the steam reforming reaction is driven by a heat generated from a first combustion reaction, and the first combustion reaction comprises conducting an oxidation of a first fuel and is catalyzed by an oxidation catalyst comprising a noble metal and boron nitride.

2. The process according to claim 1, wherein the noble metal is selected from a group consisting of Pt, Pd, Rh, Ru, and a combination thereof.

3. The process according to claim 1, wherein the noble metal is Pt.

4. The process according to claim 1, wherein the oxidation catalyst is carried by a support consisting essentially of a material selected from a group consisting of alumina, titania, zirconia, silica, and a combination thereof.

5. The process according to claim 4, wherein the material is alumina.

6. The process according to claim 1, wherein the first fuel comprises a hydrogen-containing gas, an alcohol, a hydrocarbon, or a combination thereof.

7. The process according to claim 6, wherein the alcohol is selected from a group consisting of methanol, ethanol, propanol, isopropanol, butanol, and combinations thereof, and the hydrocarbon is selected from a group consisting of methane, ethane, propane, butane, pentane, hexane, gasoline, liquefied petroleum gas (LPG), and combinations thereof.

8. The process according to claim 6, wherein the first fuel comprises a portion of a hydrogen-containing product and the hydrogen-containing product is produced by the steam reforming reaction.

9. The process according to claim 6, wherein the first fuel comprises methanol.

10. The process according to claim 6, further comprising conducting a second combustion reaction in a steam reforming section for conducting the steam reforming reaction until the steam reforming section reaches a desired temperature prior to the starting of the steam reforming reaction.

11. The process according to claim 10, wherein the second combustion reaction comprises conducting an oxidization of a second fuel, wherein the second fuel is identical to or different from the first fuel.

12. The process according to claim 10, wherein the first combustion reaction and the second combustion reaction are started simultaneously.

13. The process according to claim 1, wherein the reactants comprise water as well as an alcohol, a hydrocarbon, or a combination thereof.

14. The process according to claim 13, wherein the alcohol is selected from a group consisting of methanol, ethanol, propanol, isopropanol, ethylene glycol, glycerol, and combinations thereof, and the hydrocarbon is selected from a group consisting of methane, hexane, liquefied petroleum gas (LPG), gasoline, naphtha oil, diesel oil, and combinations thereof.

15. The process according to claim 13, wherein the reactants comprise water as well as methanol, hexane, or a combination thereof.

16. The process according to claim 1, wherein the steam reforming reaction is catalyzed by a catalyst comprising Cu, Zn, Pd, Re, Ni, or a combination thereof.

17. The process according to claim 16, wherein the steam reforming reaction is catalyzed by a catalyst comprising K as well as Cu, Zn, Pd, Re, Ni, or a combination thereof.

18. The process according to claim 1, further comprising a step of purifying the product obtained from the steam reforming reaction to provide hydrogen with a relatively high purity and a spent product.

19. The process according to claim 18, wherein the first fuel comprises a portion of the spent product.

20. The process according to claim 18, wherein the purifying step is conducted with the use of at least one palladium membrane tube.

21. The process according to claim 20, wherein the palladium membrane tube is formed by depositing a palladium-containing membrane on a porous support, and the porous support is made of stainless steel or a ceramic material.

22. The process according to claim 21, wherein the palladium-containing membrane is made of palladium, a palladium-silver alloy or a palladium-copper alloy.

23. The process according to claim 20, wherein the hydrogen obtained from the purifying step has a purity of at least 99%.

24. The process according to claim 18, further comprising a converting step to convert any carbon-containing compounds contained in the hydrogen into alkane.

25. The process according to claim 20, the process is conducted in a reactor comprising a steam reforming section, a combustion section, and a membrane tube section, the steam reforming reaction is carried out in the steam reform section, the first combustion reaction is carried out in the combustion section, and the purifying step is conducted in the membrane tube section, wherein the steam reforming section is arranged in the peripheral part of the reactor, the membrane tube section is arranged in the central part of the reactor and comprises at least one palladium membrane tube, and the combustion section is located between the membrane tube section and the steam reforming section.

26. The process according to claim 1, wherein the reactants comprise water and methanol.

27. The process according to claim 26, wherein the steam reforming reaction is conducted at a molar ratio of methanol/water ranging from about 1.0 to about 1.5.

28. The process according to claim 27, wherein the molar ratio of methanol/water ranges from about 1.05 to about 1.25.

29. The process according to claim 26, wherein the steam reforming reaction is conducted at a temperature ranging from about 200° C. to about 330° C.

30. The process according to claim 29, wherein the steam reforming reaction is conducted at a temperature ranging from about 280° C. to about 300° C.

31. The process according to claim 20, wherein the purifying step is conducted at a temperature of not higher than about 490° C.

32. The process according to claim 31, wherein the purifying step is conducted at a temperature ranging from about 25° C. to about 490° C.

33. The process according to claim 31, wherein the purifying step is conducted at a temperature ranging from about 200° C. to about 380° C.