Self-regulating hydrogen generator

Details Self-regulating hydrogen generator Self-regulating hydrogen generator

1998-08-06

2001-08-14

Tran, Hien (Department: 1764)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

C422S113000, C048S061000

Reexamination Certificate

active

06274093

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to generation of hydrogen and, in particular, to a portable apparatus for generating hydrogen using a reactant having a positive vapor pressure when at ambient temperature.
BACKGROUND OF THE INVENTION
The generation of hydrogen has been commonly performed for over 100 years through the hydrolysis of hydrides or other solid reactants. Previously, hydrogen generation has been advanced by employing the adiabatic hydrolysis and thermal decomposition of the chemical hydride in a portable lightweight unit. Such hydrogen generation technologies are characterized by heating the chemical hydride to a predetermined temperature. The chemical hydride is preferably lithium aluminum tetrahydride (LiAlH
4
) and the predetermined temperature is greater than about 100° C. Only after the chemical hydride reaches the predetermined temperature is water supplied for hydrolysis of the chemical hydride.
With respect to such a portable hydrogen generator, it may be more suitable to employ a reactant other than water to avoid some of the requirements when water is utilized. In particular, use of water requires a controlled pump mechanism that pumps the water from the water supply for reaction with the chemical hydride at pressures greater than ambient atmospheric. Furthermore, as just noted, in connection with proper preparation for reaction with the water, the chemical hydride must be heated to a high temperature before allowing the reaction to occur. Such a system may only be operated above 0° C. This results in additional heating materials or components in order to implement a fully operational unit that outputs the desired hydrogen gas. Such a prior art hydrogen generator also has a buffer to handle excess hydrogen generated when the apparatus is shut down and can also serve to smooth hydrogen demand swings during normal operation. It is preferred that this system have a restart capability after a long (days to weeks) shutdown period. In considering these aspects, it would be advantageous to provide a hydrogen generating apparatus that is fully operational and satisfies all specified power demands, or other performance criteria, while eliminating one or more of the afore-noted hardware requirements that must be incorporated when water is utilized as the reactant with the chemical hydride.
SUMMARY OF THE INVENTION
In accordance with the present invention, a portable hydrogen generator apparatus is provided that produces hydrogen as a result of the chemical reaction between a solid reactant and a reactant or composition that is supplied to the solid reactant. The supplied or input reactant has a composition with a majority thereof, by at least one of weight and volume, being different from water. Preferably, the input reactant has a positive vapor pressure (greater than atmosphere) when exposed to the solid reactant at a temperature of between ambient and −40°. In one embodiment, the input reactant includes anhydrous ammonia (NH
3
) and the solid reactant is a hydride that includes lithium aluminum tetrahydride (LiAlH
4
)
The apparatus includes a tank for housing the supplied reactant, such as the ammonia. It is desired that the ammonia in the tank be available at a pressure greater than atmospheric pressure. The absolute, theoretical minimum operating temperature is that temperature at which ammonia's vapor pressure equals atmospheric pressure. At sea level, this occurs at −33° C. (−27° F.).
A flow control assembly communicates with the tank and is located downstream therefrom. The flow control assembly can include a valve member or check valve that opens or closes, as a function of the difference in pressure between the pressure in the tank due to the ammonia gas and the pressure in the reactor, primarily based on the hydrogen gas. More specifically, the valve member closes when the reactor pressure exceeds the ammonia pressure. The flow control assembly can also include a restrictor member that communicates with the output of the valve member. The restrictor member limits the maximum rate of ammonia injection and, accordingly, acts as a damper on the reaction rate. In one embodiment, the restrictor member can be a constriction in the supply tube that carries ammonia from the valve member to the reactor. The apparatus also includes a particle filter that may be included within the reactor adjacent its output end. The particle filter acts to prevent solid particles from escaping the reactor so that, essentially, only a combination of gases exits the reactor.
The reactor contains the lithium aluminum tetrahydride (LiAlH
4
), or other satisfactory solid reactant, from which hydrogen gas can be generated using the input reactant, such as the ammonia, that is supplied to the reactor at relatively low temperatures. In that regard, not only does the ammonia have a relatively lower temperature, for a given pressure, at which it becomes a vapor, but such ammonia has a relatively significantly lower latent heat of vaporization parameter than, for example, water. In particular, the magnitude of the latent heat of vaporization parameter for ammonia is about 5.58 kcal/mole, while the magnitude of the latent heat of vaporization parameter for water is about 9.7 kcal/mole. The latent heat of vaporization parameter relates to the amount of energy that is required to cause the particular reactant or composition, such as ammonia or water, to change from a liquid phase to a vapor.
The output from the reactor includes a combination of gases, commonly including hydrogen gas, together with trace ammonia and trace organic vapor. Since it is necessary that the apparatus only output the hydrogen gas, the trace ammonia and the trace organic vapor must be trapped or removed. The apparatus further includes a trap or ammonia (NH
3
) getter that communicates with the various gases output by the reactor. The ammonia getter substantially removes the ammonia from the gas stream output by the reactor. In one embodiment, the ammonia getter includes a sulfuric acid composition; however, other compositions could be used that have acidic properties such as sodium hydrogen sulfate or its monohydrate. A second trap or organic vapor removal unit is also provided. The second trap is typically separate from, but adjacent to, the ammonia getter. This second trap substantially removes organic vapors, such as the organic vapor, as well as ammonia or other reactants that still might be present in the gas stream after it exits the ammonia getter. In one embodiment, the second trap includes activated carbon, such as charcoal.
The apparatus also includes an output or manual valve that is in the gas flow path downstream of the organic vapor trap. The gas stream input to the output valve is essentially all hydrogen gas. When the user or operator of the apparatus wishes to use the generated hydrogen gas as a fuel source, such as to a fuel cell or other load, the operator opens the output valve to permit the release or input of the hydrogen gas to the fuel cell.
With regard to the method of operation of the present invention, in the embodiment in which ammonia is contained in the tank in a gaseous state and the gas in the tank is to be input to the reactor, when such gas pressure is sufficiently greater than the gas pressure in the reactor, the valve member, such as a check valve, opens and establishes a gas communication path from the tank to the reactor. In the reactor, the ammonia gas flows through the reactor and reacts with the solid reactant, such as the hydride, to produce hydrogen gas. In one embodiment, in addition to the hydride, a gas flow enhancing material, such as a vermiculite additive, is provided within the reactor and contributes to or otherwise enhances desired flow of the input gas through the reactor in order to prevent unwanted clogging. Preferably, such a gas flow enhancing material is greater than 5% by volume of the total volume occupied by the hydride(s) and the gas flow enhancing material. The maximum volume of such a gas flow enhancing material is

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