Patent Application
20160340187
1. Field of the Invention
The present invention relates to the production of hydrogen, and particularly to a steam methane reforming reactor using a hydrogen selective membrane to enhance hydrogen production.
2. Description of the Related Art
In recent years, there has been a large amount of interest in the usage of hydrogen as a fuel source, due to its potential advantages over hydrocarbon fuels, namely its clean combustion characteristics and higher calorific value. Hydrogen may be commercially produced by a number of different methods, such as electrolysis, steam methane reforming, auto thermal reforming, partial oxidation reforming, extensions of these processes and the like. Hydrogen production via electrolysis is a relatively expensive method due to high production costs, specifically in terms of the electricity requirements. Other processes use hydrocarbons as the main reactant for hydrogen production. Among these methods, the steam methane reforming (SMR) process is the cheapest, oldest and most widely used method for the worldwide commercial production of hydrogen. Steam reforming is, in industrial practice, typically carried out in reactors (referred to as “steam reformers”), which are essentially fired heaters with catalyst-filled tubes placed in the heater. The inlet feed is methane and steam (along with some traces of hydrogen), which enter from one end of the tube and leave as syngas at the other end, following the endothermic steam methane reforming reaction. Specifically, steam methane reforming (SMR) uses an external source of hot gas to heat tubes in which the catalytic reaction takes place that converts steam and lighter hydrocarbons, such as methane, into hydrogen and carbon monoxide (i.e., syngas). The carbon monoxide syngas reacts further to give more hydrogen and carbon dioxide in the reactor. The carbon oxides are removed before use by means of pressure swing adsorption (PSA) with molecular sieves for the final purification. The PSA works by adsorbing impurities from the syngas stream to leave a pure hydrogen gas.
This process may also be carried out in heat exchange reformers, where the heat required for the reaction is supplied predominantly by convective heat exchange. The tubes are filled with the catalyst and the heat required for the reaction is typically supplied by a flue gas, process gas or any other suitable supply of hot gas. The heat and mass balance is considered only on the process side (i.e., the tube side), thus presenting no difference between heat exchange reforming and fired tubular reforming. The process schemes differ only in the amount of latent heat in the flue gas or process gas and the way in which this heat is used.
Thus, a steam methane reforming reactor with a hydrogen selective membrane solving the aforementioned problems is desired.
The steam methane reforming reactor includes a substantially cylindrical housing or shell and a tube disposed concentrically within the substantially cylindrical shell. A hydrogen selective membrane extends through a central portion of the tube. The hydrogen selective membrane can be formed from a hydrogen selective material such as a palladium alloy or the like. The hydrogen selective membrane defines a central passage or permeate zone. A feed zone is defined by a space between an outer surface of the hydrogen selective membrane and an inner surface of the shell. The feed zone in the tube includes one or more catalysts, such as nickel, magnesium aluminate (MgAl2O4) or the like. The feed zone surrounds the permeate zone and receives reactant gases for methane conversion. A sweep gas, such as air, nitrogen or the like, may be injected in the permeate zone, via an inlet, and pass through the permeate zone, exiting via an outlet. An annular heated fluid passage is defined between an outer surface of the tube and an inner surface of the substantially cylindrical housing or shell. A heating medium, e.g., molten salt, may be injected into the heated fluid passage to pass therethrough and convectively heat reactant gases flowing through the feed zone.
These and other features of the present invention will become readily apparent upon further review of the following specification.
Similar reference characters denote corresponding features consistently throughout the attached drawings.
As shown in
In operation, the molten salt and/or other heating medium gets heated by circulating through a series of solar parabolic troughs to a temperature of about 600° C. Exhaust from a gas turbine unit, flue gas, and/or any other hot gas, may also be injected into the heated fluid passage 16 with temperatures ranging from about 370° C. to about 650° C. Reactant gases for reforming, i.e., a mixture of steam and methane (CH4), are injected into an inlet end 32 of the tube 14 including the catalyst. The tube 14 is convectively heated by the heating medium flowing through the heated fluid passage 16 and methane conversion takes place in the feed zone 15. The hydrogen formed during the methane conversion in the feed zone 15 is permeated through the hydrogen selective membrane 18 and high purity hydrogen is obtained. Preferably, the membrane 18 is positioned at a core or central portion of the tube 14. At least one syngas, such as carbon monoxide or carbon dioxide, exits the tube 14 through an outlet end 34 thereof, and a mixture of the sweep gas and the hydrogen gas exits the central passage 20 through outlet 30. It should be understood that the mixture of steam and methane may be injected, under pressure, into the inlet end 32 of the tube 14 by any suitable means, such as a pump, connection to an external pressurized supply, or the like. It should be further understood that the heated fluid may be injected, under pressure, into the inlet 24 of annular heated fluid passage 16 by any suitable means, such as a pump, connection to an external pressurized supply, or the like, with the heated fluid exiting through outlet 26.
When compared to conventionally used systems, higher conversions of methane are obtainable using the present reactor. This can be due to the equilibrium shifts which occur due to the removal of hydrogen from the product stream.
In an embodiment, as shown in
In order to test the efficacy of the steam methane reforming reactor with a hydrogen selective membrane 10, a simulation study was performed using a packed bed shell-and-tube type heat exchange reformer without a membrane for different ranges of inlet air temperatures (i.e., varying temperatures produced by a solar facility for the heated fluid, such as the solar parabolic trough 38 of
Because of the enhanced methane conversion of the present steam methane reforming reactor with a hydrogen selective membrane 10 at low temperatures, the reactor may be used in conjunction with heating systems which operate at relatively low temperatures, such as the solar parabolic trough 38 of
Today's world is rapidly moving towards green solutions for energy generation. Solar parabolic trough technology is both efficient and cost effective. Solar parabolic troughs, however, do not generate the temperatures required for the conventional steam methane reforming process, since the process is highly endothermic. The temperatures obtained by parabolic troughs are typically on the order of 300-600° C. The present steam methane reforming reactor with hydrogen selective membrane provides a significantly higher conversion rate of methane in the tube side of the reformer. The hydrogen selective membrane of the present heat exchange reformer allows hydrogen to be removed from the products such that the reaction tends to proceed toward the product side. Thus, high reaction temperatures are not required. This allows for higher methane conversion rates at low temperatures.
It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.
