Patent Application
20050276746
The present invention relates to the design of a catalytic reactor used in a system for generating hydrogen from a fuel solution, such generation being promoted by contact of the fuel solution with catalytic material in the reactor.
Hydrogen is the fuel of choice for fuel cells, however, its widespread use is complicated by the difficulties in storing the gas. Many hydrogen carriers, including hydrocarbons, metal hydrides, and chemical hydrides are being considered as hydrogen storage and supply systems. In each case, specific systems need to be developed in order to release the hydrogen from its carrier, either by reformation as in the case of hydrocarbons, desorption from metal hydrides, or catalyzed hydrolysis of chemical hydrides.
One of the more promising systems for hydrogen storage and generation utilizes borohydride compounds as the hydrogen storage media. Sodium borohydride (NaBH4) is of particular interest because it can be dissolved in alkaline water solutions with virtually no reaction; in this case, the stabilized alkaline solution of sodium borohydride is referred to as fuel. Furthermore, the aqueous borohydride fuel solutions are non-volatile and will not burn. This imparts handling and transport ease both in the bulk sense and within the hydrogen generator itself.
Various hydrogen generation systems have been developed for the production of hydrogen gas from aqueous sodium borohydride fuel solutions. Such generators typically require at least three chambers, one each to store fuel and borate product, and a third chamber containing a catalyst to promote hydrolysis of the borohydride. Hydrogen generation systems can also incorporate additional components such as hydrogen ballast tanks, heat exchangers, condensers, gas-liquid separators, filters, and pumps.
The current technology for hydrogen generation from stabilized sodium borohydride solutions involves feeding the fuel solution at ambient temperature to a catalyst bed packed with a catalyst to promote hydrogen generation. The hydrogen gas and discharged fuel solution pass to a second chamber which acts as both a gas/liquid separator and as a small ballast tank to store hydrogen gas. The hydrogen gas can next be processed through heat exchangers to achieve a specified dew point, condenser elements to remove water from the gas stream, and filters to remove entrained mist before the gas is fed to fuel cell or internal combustion engine.
In order to deliver the necessary rapid hydrogen generation response required for the effective operation of a fuel cell, large volume catalytic reactors are typically required. Such large volume reactors obviously require corresponding quantities of catalyst. As the most reactive catalyst metals are the relatively expensive Group VIII metals such as platinum, palladium, and ruthenium, the catalyst is a major contributor to the cost of a hydrogen generating system.
In addition, these large reactors demonstrate significant fuel hold-up of about 80% of reactor volume. When the demand of the fuel cell rapidly changes from high H2 flow rates to low or zero H2 flow rates, a considerable amount of fuel remains in the reactor. Any hydrogen generated by this residual fuel that is not immediately needed by the fuel cell must be vented from the reactor chamber as it cannot remain in the chamber without posing a potential safety risk.
It is desirable to develop catalytic reactor technology for hydrogen generation that reduces the reactor volume and cost without sacrificing the fast dynamic system control, high fuel conversion and high reactor throughput (the amount of hydrogen generated per unit time and per unit reactor volume) of the larger reactors. High reactor throughput is necessary to reduce the overall size of hydrogen generation systems and improve control under cyclic or frequent load changing conditions. Reactor technology that contributes to minimal balance of plant while maximizing fuel concentration and conversion is essential to maximize overall hydrogen storage density.
Attempts to develop improved reactor technology for hydrogen generation from metal hydride fuels have not yet addressed all of these issues. For example, an integrated reactor is described in U.S. Patent Application Publications 2003/0194368 A1 and 2003/0194369 A1. This reactor includes membrane fabricated from polytetrafluorine ethylene or polyethylene/polypropylene composite material, such as those commercially available under the Gore-Tex® and Celgard® trade names, for separating the hydrogen generated from the liquid fuel wherein the membrane is disposed around a catalyst bed having a plated screen or baffled (divided) catalyst bed. Such reactors are intended for operation at low temperature conditions with an upper limit of between about 80 and 100° C. and pressure conditions below 50 psig. The loosely packed or baffled catalyst beds of those systems lead to considerable back mixing and channel leak that contribute to low fuel conversion and low reactor throughput. In contrast, practical hydrogen generation systems using chemical hydride fuels typically operate at elevated temperatures above 100° C. and pressures exceeding 50 psig as described in detail in “Water and heat balance in a fuel cell vehicle with a sodium borohydride hydrogen fuel processor,” Proceedings of Future Transportation Technology Conference, June 2003, Costa Mesa, (2003-01-2271).
Broadly, the present invention improves the operational performance of catalytic reactors and the hydrogen generating systems in which such reactors are disposed by incorporating one or more performance enhancing elements in the reactor. These elements include:
As described hereinbelow, the use of a heat exchanging element to preheat the fuel solution enhances the rate of reaction between the fuel and the catalyst. The use of one or more fuel diffusing elements within the reactor enhances the contact of the fuel solution with the catalyst so as to increase the rate of hydrogen generation. The use of two or more different catalytic materials having different hydrogen generating capabilities within the reactor can enhance certain operational characteristics of the reactor, e.g., start-up response time. The use of a membrane capable of withstanding pressures of at least 50 psig enhances the operation of the reactor by providing separation of the generated hydrogen from the liquid fuel within the catalytic reactor, eliminating or reducing the size of downstream gas/liquid separation elements, while also providing the higher hydrogen generation rates attainable at such pressures. Each of the foregoing elements can be used singularly or in any combination, as desired, and the present invention is compatible with use in otherwise conventional hydrogen generation systems, including such systems which recycle the water output of the fuel cell to which the hydrogen is delivered. Such recycling of the water advantageously utilizes what is normally considered a waste product of fuel cell operation dilute highly concentrated fuel solutions that are stored. The storage of highly concentrated fuels reduces the size of the required fuel storage tank in a given application.
A complete understanding of the present invention may be obtained by reference to the accompanying drawings when considered in conjunction with the following detailed description, in which:
The chemical hydride fuel component useful in an exemplary hydrogen generation system employing a catalytic reactor is a complex metal hydride that is water soluble and stable in aqueous solution. Examples of suitable chemical hydrides are those borohydrides having the general formula MBH4, where M is an alkali or alkaline earth metal selected from Group 1 or Group 2 of the periodic table, such as sodium, potassium, and calcium. Examples of such compounds include without intended limitation NaBH4, KBH4, and Ca(BH4)2. These metal hydrides may be utilized in mixtures, but are preferably utilized individually. Preferred for such systems in accordance with the present invention is NaBH4.
Borohydrides react with water to produce hydrogen gas and a borate in accordance with the following chemical reaction:
MBH4+2H2O→MBO2+4H2+300 kJ Eqn. 1
Sodium borohydride is preferred in the present invention due to its comparatively high solubility in water, about 35% by weight as compared to about 19% by weight for potassium borohydride. Two molecules of water are consumed for each borohydride molecule during the reaction illustrated above, and a saturated 35 wt-% sodium borohydride solution contains a stoichiometric excess of water. In other words, sufficient water is present in the solution to allow for complete conversion of the sodium borohydride. Typically, the fuel solution is comprised of from about 10% to 35% by wt. sodium borohydride and from about 0.01 to 5% by weight sodium hydroxide as a stabilizer.
For sodium borohydride, this reaction shown in Eqn 1 occurs very slowly in the absence of a catalyst at alkaline pH, such as when a hydroxide base is added to the fuel solution. Effective borohydride conversion to hydrogen depends on the activity of the catalyst, and also is influenced by the hydrodynamic and pressure conditions of the catalytic reactor.
A schematic overview of a typical hydrogen generation system is provided in
The hydrogen ballast tank 126 for the system meets the instantaneous demand for hydrogen during initial startup of the hydrogen generation system. The size of this tank is dependent on the operating pressure and reactor throughput. Generally, lower operating pressures result in larger ballast tank volumes. Furthermore, large reactors which exhibit a low reactor throughput tend to require large ballast tanks. To avoid using a large tank, a portion of the residual hydrogen remaining in the reactor during the shutdown period can be released to environment. The improved reactor of the present invention enhances the hydrolysis reaction by increasing the reactor throughput by effective pressure, temperature, and water management, and reduces the balance of plant by eliminating or minimizing downstream features such as heat exchanger 122, ballast tank 126, gas/liquid separator 120, and condenser 124, and incorporating such functional elements within reactor 116.
The rate of hydrogen generation from sodium borohydride fuels is related to the reaction temperature which in turn depends on factors such as fuel concentration and flow rate, heat and mass transfer, and operating pressure. Typical reaction temperatures are between from about 100 to about 200° C. at an operating pressure between about 10 to about 200 psig with a fuel concentration of 20 wt % SBH and 3 wt % NaOH. The reactor can be operated in a self-sustainable fashion in which no heating of fuel or reactor is necessary for reactor startup and steady-state operation. When fuel is fed to the catalyst reactor at ambient temperature, there is a startup period necessary for the reactor to reach its normal steady-state operating temperature and the rate of hydrogen generation. This reactor startup time is a characteristic of the particular catalyst used and typically the more active, and more expensive, catalysts (such as ruthenium, platinum, and rhodium) have a reduced startup time. The startup time for the hydrogen generation system can be further reduced by providing a hydrogen ballast tank or pre-heated fuel to the reactor.
Rather than incorporating a separate heating element to heat the fuel in the hydrogen generator, the system efficiency can be improved by incorporating heat exchange elements that utilize the heat generated by the hydrolysis reaction itself. Previous attempts to capture the heat from the hydrolysis reaction by integrating a heat exchanger with the catalyst chamber as described in U.S. Patent Application Publication No. 2003/0091876, sought only to transfer the heat of the hydrolysis reaction to the fuel cell stack to bring the fuel cell unit to the optimum operating temperature, rather than using the heat to improve the reaction efficiency of the hydrolysis reaction.
In the integrated reactor of the present invention, the heat generated by the hydrogen generation reaction is transferred to the incoming fuel solution. As a result, the discharged fuel and hydrogen product streams are cooled as they exit the reaction chamber and further downstream heat exchange elements can be removed. The increase in temperature of the incoming fuel feed results in a higher reaction rate in the inlet section of reactor as compared to a cool fuel feed. The rate of reaction in the inlet section of the reactor affects overall reactor throughput; high reactor throughput significant reduces the overall size of the hydrogen generator systems and improves the control in cyclic or variable load operating conditions.
For hydrogen generation systems of the present invention, the catalyst bed is preferably packed with a catalyst metal supported on a substrate. The preparation of such supported catalysts is taught, for example in U.S. Pat. No. 6,534,033 entitled “System for Hydrogen Generation,” the disclosure of which is incorporated herein by reference. Suitable transition metal catalysts for the generation of hydrogen from a metal hydride solution are known in the art and include metals from Group 1B to Group VIIIB of the Periodic Table, either utilized individually or in mixtures, or as compounds of these metals. Representative examples of these metals include, without intended limitation, transition metals represented by the copper group, zinc group, scandium group, titanium group, vanadium group, chromium group, manganese group, iron group, cobalt group and nickel group. Specific examples of useful catalyst metals include, without intended limitation, ruthenium, iron, cobalt, nickel, copper, manganese, rhodium, rhenium, platinum, palladium, and chromium. The catalyst may also be in forms of beads, rings, pellets or chips with a diameter ratio of reactor column to that of catalyst particle in a range of 8-100, preferably 10-50 and a ratio of packing height to column diameter in a range of 8-30, preferably 10-20 to ensure the desired flow pattern. It is preferred that structured catalyst supports such as honeycomb monoliths or metal foams are used in order to obtain the ideal plug flow pattern and mass transfer of the fuel to the catalyst surface. Such supports contain a plurality of liquid flow passages and will provide effective liquid fuel and catalyst contact and ensure desired fuel flow patterns as well as minimize the pressure drops across the catalyst bed.
The reactor can be orientated vertically or horizontally with various heat exchange configurations that allow efficient heat exchange between the reactor and the incoming fuel feed, including the use of a tube or coil in center of the reactor or a jacketed heat exchanger.
The operating pressure is one of the most important considerations in the design of borohydride based hydrogen storage system and the pressure directly affects the operating temperature of the reactor. The amount of liquid water present in a catalyst reactor relative to water vapor increases at higher operating pressures. In addition, the pressure significantly affects the contact time between the liquid fuel and the catalyst system. Inside the catalyst bed, the amount of generated hydrogen and water vapor formed in reactor takes up considerable reactor volume that reduces the contact time of liquid fuel to catalyst. For example, if the reactor operates at 10 psig, the contact time between the liquid fuel and catalyst is only about 10% of that at 180 psig. It is beneficial to operate the reactor at relatively high pressures to increase the liquid fuel contacting time.
Since sodium borohydride hydrolysis is an exothermic reaction, the reactor can be operated in a self-sustainable fashion without requiring external heating of the reactor or fuel for reactor startup and operation. A typical reaction temperature of about 150° C. is reached at an operating pressure of 55 psig for complete conversion of an aqueous fuel containing 20 wt-% NaBH4 and 3 wt-% NaOH. For a high reactor throughput, the reactor is preferably operated at pressures between 10 and 250 psig, preferably between 20 and 220 psig, and most preferably between 50 to 180 psig.
Simultaneous removal of hydrogen in the reactor further improves the contact between the liquid fuel and solid catalyst, thus significantly increasing the reactor throughput. For example, if the overall rate is controlled by reaction kinetics, removal of 95% of the hydrogen produced from a reactor using a 20 wt-% sodium borohydride fuel could increase the reactor throughput more than 20 fold compared to a reactor operated without hydrogen removal. It is necessary that the membrane operate under elevated pressure and temperatures (up to 240° C.) and be hydrophobic to acts as a condenser and filter to prevent any entrained impurities and water from crossing into the hydrogen gas delivered to the fuel cell. Suitable materials include commercially available polytetrafluoroethylene (PTFE) membranes. The “dual use” membrane also contributes to the reduction of the balance of plant by eliminating downstream condenser 124 and gas/liquid separator 120. The membrane can be designed to withstand operating system pressures by judicious choice of material thickness. The pressure tolerance of a given membrane can also be strengthened by sandwiching it between sieved metal sheets/plates. Accordingly, PTFE membranes operably at pressures up to 200 psig are possible and the need for operating above these pressures is presently not considered desirable for safety and economic reasons. The operating temperature of a system is proportional to system pressure, and the preferred upper temperature limit is 220 C. Above this temperature, the cost of process elements operable at the such temperatures and associated pressures are prohibitive for most present system applications.
Accordingly, the present invention contemplates catalytic reactors for the hydrolysis of a fuel, such as metal borohydrides, having one or more of the exemplified elements—a) a heat exchanging mechanism for pre-heating the fuel prior to its contact with the catalyst, b) liquid distributors/re-distributors for providing fuel distribution over the catalyst that enhances the hydrogen generating capabilities resulting from the interaction of the fuel and catalyst, c) membrane which separate the hydrogen generated from the fuel and which are capable of operating at pressures greater than 50 psig and d) multiple catalytic materials having different hydrogen generating characteristics.
A reactor design to provide effective fuel/catalyst contact incorporating all elements described to improve reactor throughput and provide a controlled fuel flow pattern to maximize fuel conversion and reactor throughput is presented in
While
A schematic overview of a hydrogen generation system incorporating the integrated reactor design of the present invention and utilizing fuel cell water recycle is provided in
The borohydride fuel is metered from a storage tank 110 through a fuel concentrate conduit line 112 using a fuel pump 114. The fuel can be diluted with water from a water tank 132 to dilute the incoming fuel to a desired borohydride concentration.
The following examples further describe and demonstrate features of the improved reactor throughput according to the present invention. The examples are given solely for the illustration purposes and are not to be construed as a limitation of the present invention.
A tubular reactor having a 1.0″ outside diameter (“o.d.”) and a length of 7″ (volume of 60 mL) was used for reactor performance tests. The supported catalyst systems were prepared as described in U.S. Pat. No. 6,534,033. Two catalyst systems were tested: ruthenium-cobalt on nickel metal fiber (RuCo/Ni) with a nominal loading of 1.2 wt-% Ru and 3 wt-% Co and cobalt-zinc on nickel metal fiber (CoZn/Ni) with a nominal loading of 3 wt-% Co and 3 wt-% Zn.
Reactor A was packed with 55 g of RuCo/Ni catalyst; Reactor B was packed with 55 g of CoZn/Ni catalyst. Reactor C was packed with two catalyst beds in accordance with
The reactor startup time was measured at 55 psig and a feed fuel temperature of 22° C. under a constant fuel flow of 20 g/min. Steady-state performance of the reactor is assessed by measuring reactor throughput at a fuel conversion greater than 98% under a self-sustainable operation at 55 psig. The reactor throughput is defined as amount of hydrogen generated per unit time and per unit reactor volume.
Although Reactor B packed with a CoZn/Ni catalyst had a slow startup of 1250 s and an achievable reactor throughput of 332 standard liters per minute (SLPM) H2 per liter of reactor volume, Reactor C packed with two catalysts (RuCo/Ni—CoZn/Ni) had a startup time of 260 s and high reactor throughput close to that exhibited by Reactor A packed with only RuCo/Ni catalyst (Table 1).
(a) Throughput necessary for >98% fuel conversion and self-sustainable hydrogen generation.
Reactor A was further integrated with a heat exchange coil as illustrated in
While the present invention has been described with respect to particular disclosed embodiments, it should be understood that numerous other embodiments are within the scope of the present invention. First, for example, the present invention may be used in a catalytic reactor which operates with a fuel other than sodium borohydride. Second, while particular heat exchanger configurations have been disclosed, the present invention is applicable to numerous structures known in the art that are disposed so as to receive the transfer the heat from the hydrogen generation process to the incoming fuel solution. Similarly, various liquid diffusing elements known in the art, other than those shown, can be utilized to provide the desired distribution of the fuel across the surface of the catalyst(s). Finally, while the use of two catalysts in the reactor has been disclosed, the reactor may utilize more than two such materials.
