Supercritical process, reactor and system for hydrogen production

A reactor, system and method for producing hydrogen features a reactor containing a heating stream channel and a hydrogen channel with a reaction channel positioned there between. A heat transfer sheet separates the heating stream channel and the reaction channel and a porous support plate separates the reaction channel and the hydrogen channel. A membrane constructed from palladium, vanadium, copper or alloys thereof covers the porous support plate. The heating stream channel receives a heating stream so that heat is provided to the reaction channel through the heat transfer sheet. A catalyst is positioned in the reaction channel and the reaction channel receives a reaction stream including a mixture of supercritical water and a hydrocarbon fuel so that hydrogen is produced in the reaction channel and is passed through the membrane into the hydrogen channel. The hydrogen separation may alternatively be accomplished in a separator device distinct from the reactor via either a membrane or pressure swing adsorption.

TECHNICAL FIELD

The present invention relates generally to hydrogen production and, more particularly, to a process utilizing supercritical water and hydrocarbon sources and an associated reactor and system for generating hydrogen.

BACKGROUND

Hydrogen is required as an input for a variety of processes and various technologies. Examples of such processes and technologies include hydrogenation, ammonia synthesis and fuel cells.

Water is the most prevalent substance from which hydrogen may be obtained. Methane steam reforming (MSR), however, is the only prior art technology economically operable and commercially available for obtaining hydrogen from water. The MSR process, which requires a source of methane or natural gas, is a costly and complex one. For MSR, thermal control at high temperatures (such as above 800° C.) and catalyst deactivation are both technically difficult areas. A need therefore exists for an economical system and method whereby hydrogen may be obtained from water using a process other than the MSR process.

Electrochemical extraction of energy from hydrogen via fuel cells is an especially clean and efficient method of providing power. As a result, fuel cell development is very active for various applications. An example of such an application is powering automobiles. Governmental requirements regarding the maximum allowable harmful fuel emissions for vehicles in the United States are forcing vehicle manufacturers to design vehicles that run on fuels other than gasoline and diesel fuel or consider alternative types of engines, such as electric engines. This has led to the design of vehicles that use fuel cells that run on pure hydrogen. When pure hydrogen is mixed with oxygen via a fuel cell in the vehicle, water, heat and electricity are produced, ideally without emitting other chemicals that are harmful to the air or the environment.

In addition, a fuel cell system running on hydrogen can be compact, lightweight and has no major moving parts. Because fuel cells have no moving parts, in ideal conditions they can achieve a very high reliability with minimal downtime. As a result, fuel cells are also very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, rural locations and in certain military applications.

Current fuel cell technology requires high purity hydrogen for successful operation. The government has directed that fuel cell vehicles rely on stationary hydrogen dispensing stations for fueling, yet there is no established infrastructure for hydrogen distribution. Furthermore, many technical difficulties have been encountered during attempts to develop an on-board hydrogen generation system for other mobile applications. As a result, a need exists for a simple, lightweight and compact hydrogen generation system and process that may be used either on-board a mobile vehicle or in a stationary facility.

DETAILED DESCRIPTION OF EMBODIMENTS

In a preferred embodiment, the invention uses a supercritical process and a reactor for processing a mixture of supercritical water and a hydrocarbon fuel to generate hydrogen. Separation of the generated hydrogen is preferably accomplished in the reactor by a membrane, such as palladium, vanadium, copper or alloys thereof (an alloy is a homogenous mixture of two or more elements at least one of which is a metal and the resulting material has metallic properties) or a polymer. In an alternative embodiment of the invention the separation may be performed by a separator device separate from the reactor which may use either a membrane or a pressure swing adsorption (PSA) process for the hydrogen collection.

A schematic view of a portion of an embodiment of the reactor of the invention is indicated in general at10inFIG. 1. As illustrated inFIG. 1, the reactor features a number of reaction channels12a-12d. While four reaction channels are illustrated inFIG. 1, the reactor may have more or may have a lesser number of reaction channels or even one reaction channel. Each reaction channel is bounded on one side by a hydrogen channel,14aand14b, and on the other side by a combustion or heating stream channel,16aand16b. Each reaction channel and heating stream channel are separated by a heat transfer sheet20a-20d, preferably constructed of metal, upon which a dehydrogenation catalyst, such as nickel, platinum, ruthenium, rhodium, copper or other noble metal or alloys thereof, is coated on the reaction side. Each reaction channel and hydrogen channel are separated by a membrane containing palladium, vanadium or a polymer22a-22dmounted on a porous support plate24a-24don the reaction side.

The heating stream channel may provide heat to the reaction channel by heat transfer from a hot gas stream flowing through the heating stream channel. Alternatively, as will be explained in greater detail below, combustion catalysts may be optionally packed or coated in the heating stream channel, as illustrated at21inFIG. 1for heating stream channel16a, so that a combustion reaction occurs in the heating stream channel. The heat produced by the combustion reaction heats the reaction channel. A third option is to heat a fluid flowing through the heating stream channel by placing an auxiliary electric heating arrangement in the heating stream channel, such as the resistance element illustrated in phantom at23inFIG. 1for heating stream channel16b.

A reaction stream passes through each reaction channel where the coated catalysts are used. The reaction stream inlet portion for the reactor consist of a mixture of supercritical water and a hydrocarbon fuel. The critical point for water is a temperature of 374° C. at a pressure of 221 bars, which is therefore the minimum temperature and pressure for the reaction stream inlet portion. On the other side of each reaction channel the membrane, supported by the porous material, is applied to extract hydrogen from the reaction stream. The hydrogen generated in each reaction channel permeates through the membrane and then is collected in one of the hydrogen channels at the other side of the membrane. Membranes containing palladium or vanadium have a unique property of exclusively allowing hydrogen to permeate through their structures while other gases have molecules that are too large to pass through the membrane. High purity hydrogen can be collected on the other side of the membrane while the other gases are recycled or collected separately after the reaction from the outlet of the reaction channels.

As illustrated inFIG. 1, a heating stream, which may include steam, inert gas or liquid, flows through each heating stream channel and provides heat (Q) to the reaction channels for the supercritical process. If a combustion catalyst is coated, packed or otherwise present in the heating stream channel, a mixture of air or oxygen mixed with a hydrocarbon may serve as the heating stream inlet so that combustion occurs in the heating stream channel and provides the heat Q to the reaction channels.

A simplified illustration of a portion of the exterior of the reactor10ofFIG. 1without pipes, headers or manifolds is illustrated inFIG. 2. The reactor features a housing30which contains the heating stream channel16a, reaction channel12band hydrogen channel14a(in addition to the other channels of the reactor, including those illustrated inFIG. 1). While the heating stream, hydrogen and reaction channels are illustrated schematically inFIG. 1as running in parallel for ease of explanation, the heating stream and hydrogen channels may run perpendicular to, or at any other angle with respect to, the reaction channels. In the embodiment ofFIG. 2, the supercritical inlet and outlet portions of the reaction stream are indicated at32and34, respectively (see alsoFIG. 1for34). The inlet and outlet portions of the heating stream are indicated at36and38, respectively (see alsoFIG. 1for36). The hydrogen outlet stream is indicated at42in bothFIGS. 1 and 2.

For the situation where combustion catalysts are present in the heating stream channels of the reactor30, the reaction stream outlet34may serve as the heating stream inlet36, since the reaction stream outlet contains a residual hydrocarbon, or outlet stream after fuel cells contains residual hydrogen.

Suitable reactors for use as the reactor ofFIGS. 1 and 2are known in the art. An example of such a reactor is Chart Industries, Inc.'s SHIMTEC® reactor, which is described in U.S. Pat. Nos. 6,510,894 and 6,695,044, the contents of which are incorporated herein by reference. This compact heat exchange reactor has the capability to perform at the high temperature and high pressure required for a process using supercritical water. Moreover, it provides abundant surface area for heat exchange in order to control reaction temperature for increasing the hydrogen production and also abundant membrane surface area for greater hydrogen production in a small device.

While the embodiment ofFIGS. 1 and 2feature a catalyst that is a coating or an unsupported catalyst, the catalyst can be installed in various alternative forms such as a packed bed catalyst having either a supported or an unsupported catalyst, a wash coated catalyst or incipient wetness impregnated catalyst producing a thin film on one or more walls of the reaction chamber or an electroless plated catalyst. The catalyst can be from a range of metals including, but not limited to nickel, platinum, ruthenium, rhodium, copper or alloys thereof. The catalyst is used to break the carbon-carbon bonds and carbon-hydrogen bonds in the reaction stream.

WhileFIGS. 1 and 2illustrate a compact reactor within which hydrogen may be removed from the reaction stream, the removal of hydrogen from the reaction stream may alternatively be accomplished outside of the reactor. The process of separating hydrogen from a stream outside of a reactor is well known and devices are commercially available. For example, as illustrated inFIG. 3, the reaction and separation might be done in two separate devices52and54connected by a passageway, such as a tube, pipe or conduit to simplify the reactor construction. In such an arrangement, the first device52may be a compact reactor, such as the one illustrated in, and described with reference to,FIGS. 1 and 2, but without the membranes22a-22dand porous plates24a-24d(FIG. 1) and the hydrogen channels. The reactor52is used in a supercritical condition for hydrogen generation while the separator device54is used for hydrogen separation from the product stream56exiting the first reactor through the passageway connecting the reactor and separator. As with the embodiment ofFIGS. 1 and 2, the reaction stream input portion58and heating stream channel for the reactor52may have temperatures above 374° C. and pressures above 221 bars.

The conditions for the separator54depend on the membrane and support materials within the device. For example, if the separator54features channels divided by porous metal coated with palladium, as illustrated at22a-22dand24a-24dofFIG. 1, operating temperature could be below 374° C., and operating pressure could be below 221 bars for hydrogen separation. The hydrogen stream exiting the separator54is illustrated at62inFIG. 3, while the residual stream (which corresponds to the reaction stream outlet portion34inFIG. 2) is illustrated at64.

In an alternative embodiment of the invention, a process swing adsorption (PSA) process may be used by the separator54instead of a membrane to separate hydrogen from the product stream56. The construction of PSA devices is well known in the art. The PSA device54separates the hydrogen from the product stream gas56under pressure according to the hydrogen's molecular characteristics and affinity for an adsorbent material. The device cycles are to first adsorp hydrogen on the adsorptive material at high pressure and then desorp the hydrogen by lowering the pressure. Hydrogen collection occurs during the low pressure cycle. Using two adsorbent vessels allows near-continuous production of hydrogen. It also permits pressure equalization, where the gas leaving the vessel being depressurized is used to partially pressurize the second vessel. This results in significant energy savings and is a common industrial practice.

As with the embodiment ofFIGS. 1 and 2, for the situation where combustion catalysts are present in the heating stream channels of the reactor52, the residual stream64may serve as the heating stream inlet60, since the residual stream contains a hydrocarbon (as well as residual hydrogen).

As an alternative to the compact reactor52ofFIG. 3, a tube or channel reactor70could be used, as illustrated inFIG. 4. The tube reactor70is placed in a housing72that defines an interior chamber. The tube reactor serves as the reaction channel and therefore features a catalyst coating on its interior surfaces or is packed with a catalyst and receives a reaction stream inlet74. The chamber of housing72receives a heating stream76whereby heat is provided to the reaction channel in the tube reactor70. As with the embodiment ofFIG. 3, the product stream78from the reactor flows through a passageway, such as a tube, pipe or conduit to the separator82. As with the embodiment ofFIG. 3, a hydrogen stream exits the separator82, as illustrated at84, while the residual stream (which corresponds to the reaction stream outlet portion34inFIG. 2) exits the separator as illustrated at86. As with the embodiment ofFIG. 3, the separator82may used either a membrane for the hydrogen separation or a PSA process.

Similar to the embodiments ofFIGS. 1-3, for the situation where combustion catalysts are present within the chamber of housing72, the residual stream86may serve as the heating stream inlet76, since the residual stream contains hydrocarbons (as well as residual hydrogen). Under such conditions, combustion occurs in the chamber of housing72to provide heat for the reaction channel of the tube reactor70.

In all of the embodiments of the invention described above, hydrogen production can be increased by changing the operating conditions of the reactor. For example, increasing the inlet pressure of the reaction stream will increase the driving force for the hydrogen separation. As a result, reactors which are capable of sustaining higher pressures, such as the compact reactors of the embodiments ofFIGS. 1-3, will favor more hydrogen production.

It should be noted that an equilibrium shift occurs in the reaction stream favoring hydrogen production. More specifically, as the hydrogen concentration decreases in the reaction stream, the reaction shifts to produce more hydrogen. Also, the removal of the reaction product hydrogen lowers the necessary reaction temperature which increases the range of materials acceptable for the reactor. This results in lower cost, better performance and increased ease of manufacture for the reactor.

The embodiments ofFIGS. 1-4offer a number of unique benefits including the generation of high purity hydrogen efficiently and simply and the generation of a potentially valuable byproduct of high pressure CO2(present in the reaction stream outlet portion34ofFIG. 2or product streams64and86ofFIGS. 3 and 4, respectively). In addition to use as the heating stream for the reactor, the high pressure CO2produced may be used for power plant or petrochemical complex applications.

The reaction stream inlet portions for the reactors ofFIGS. 1-4consist of a mixture of supercritical water and a hydrocarbon fuel. As mentioned previously, the critical point for water is a temperature of 374° C. at a pressure of 221 bars. Water at these conditions or at a higher temperature and/or a greater pressure (supercritical water) has desirable properties including a change in the capacity to dissolve liquid hydrocarbons. The hydrocarbon fuel may be any hydrocarbon-based fuel such as crude oil, liquid fuels such as jet fuel, diesel and gasoline, natural gas, liquid natural gas, coal, coal dust, saw dust, waste wood and/or biomass material. Other short chain (e.g. <C6) hydrocarbons may also be used in the reaction stream with the water. The temperature can be from 374° C. and up and the pressure from 221 bars and up for both the reaction and heating streams.

The supercritical water has the unique feature of high solubility for most organic liquids, powders or gases. Hydrocarbon fuels, not ordinarily soluble in water, become highly soluble in supercritical water thus permitting the possibility of a reaction between the fuel and water on a catalytic metal based surface, such as nickel, platinum, ruthenium, rhodium, copper or alloys thereof. Reaction conversion reaches 100% and the hydrogen yield can exceed 90%, implying the ability to control the selectivity of the reaction. Details can be seen in the following examples.

Two of the most significant benefits from this supercritical process are that additional hydrogen (for example, more than 60%) comes from water when using fossil fuel as a feed, and CO2production can be cut significantly (for example, in half) with same amount of hydrogen production compared to current fossil fuel combustion systems.

Examples of the process in embodiments of the invention using different fuel sources are described below.

Toluene as a model liquid hydrocarbon feedstock may be used for the supercritical process. The desired reaction between toluene and water is as follows:
C6H5CH3+14H2O⇄7CO2+18H2
The theoretical yield for this reaction is 39 grams of hydrogen per 100 grams of toluene, or 18 moles of hydrogen per mole of toluene.

Ruthenium on alumina (5 wt. % loading, 100 m2/g-cat surface area) may be used as the catalyst in one embodiment of the reactor. Such a catalyst may be obtained in unreduced form from commercial suppliers. The reaction channels of the reactor are each packed with Ru/Al2O3catalyst. Two-micron frits are placed at each end of each reaction channel, thus allowing reactants to freely pass through while the catalyst is retained.

Results from testing the reforming of toluene in supercritical water via Ru/Al2O3indicate that residence times on the order of seconds produce a good yield of hydrogen. For example, in a test using a catalytic test reactor consisting of a ¼ in. OD Inconel® tube packed with the catalyst, a 1.9 second reaction time gave a gas mixture of 65.5% H2, 0.9% CO, 5.3% CH4, and 28.3% CO2, with a hydrogen yield of 13.2 and a complete conversion of toluene to gaseous products.

Experiments were carried out at different temperatures ranging from 700 to 800° C. using a feed of 2 wt. % gasoline and 98 wt. % water. The test reactor pressure was kept constant at 3500 psi and the residence time in the catalyst was kept at 2 seconds for all the experiments. Effect of temperature is shown inFIG. 5, which shows moles of hydrogen yield per mole of toluene for varying residence time, with Ru/AL2O3catalyst, 800° C., 3500 psi, 2.1 wt. % toluene in water, based on calculated equivalent toluene from carbon outlet from the system.

The shorter residence time gives better hydrogen yield suggesting that the reactions are kinetically controlled. The reaction gives a very good yield of hydrogen; it is not too far from the theoretical yield of 18 moles hydrogen per mole of toluene. Further adjustment of the reaction conditions and moving to a compact reactor may improve the yield.

Octane as a model liquid hydrocarbon feedstock may be used for the supercritical process. The desired reaction between toluene and water is as follows:
C8H18+16H2O⇄8CO2+25H2
The theoretical yield of this reaction is 26.3 grams of hydrogen per 100 grams of octane, or 25 moles of hydrogen per mole of octane.

The same catalyst Ru/Al2O3was used for the reaction in the same test reactor described above for toluene. The experiment was conducted at 750° C. and 3500 psi. The results are shown in Table 1.

The results indicate that hydrogen can be effectively produced in the supercritical process. The yield reached 70% with complete octane conversion and further adjustment of the reaction conditions and moving to a compact reactor may improve the yield. Increasing the octane concentration in the feed stream reduces the hydrogen yield.

3. Model Gasoline

Gasoline is a mixture of several hydrocarbons comprising paraffins, iso-paraffins, naphthenes (cyclo-paraffin), and aromatic hydrocarbons with traces of sulfur compounds. The presence of sulfur might affect the performance of the catalyst and reduce the hydrogen yield. Hence for the comparative analysis, a sulfur-free gasoline was made by mixing iso-octane, methyl cyclohexane and toluene in the composition shown in Table 2.

All of the above compounds are generally present in gasoline and represent isoparaffin, naphthene and aromatic hydrocarbons.

The desired reaction between these hydrocarbons and water during supercritical reforming is as follows:
C8H18+16H2O⇄8CO2+25H2
C6H11CH3+14H2O⇄7CO2+21H2
C6H5CH3+14H2O⇄7CO2+18H2

Hence, each mole of gasoline theoretically can give approximately 21.8 moles of hydrogen. Or 100 grams of gasoline can theoretically produce 43.6 grams of hydrogen. The same catalyst Ru/Al2O3was used for the reaction in the same test reactor described above for toluene.

A carbon input/output balance of mare than 95 percent was obtained for all of the above runs. Besides CO2, a small amount of carbon comes out as CO and CH4, as shown inFIG. 6, which shows the effect of temperature on gaseous product yields, for 2 wt. % gasoline in the feed, reactor pressure of 3500 psi, and 2 second residence time in the Ru/AL2O3catalyst bed. A hydrogen yield of 17 to 19 moles/mole-gasoline was obtained, which suggests near complete conversion of carbon to carbon dioxide. The hydrogen yield increased slightly as the temperature was increased from 700 to 800° C. The details of gaseous product distribution and carbon balance are shown in Table 3.

Further adjustment of the reaction conditions and moving to a compact reactor may improve the yield.

A system for producing hydrogen in accordance with the present invention is illustrated inFIG. 7. In the system ofFIG. 7, hydrogen separation from the reactor product stream is accomplished outside of the reactor. Liquid hydrocarbon fuel111and water121are fed into pumps110and120, respectively, to increase the pressure of each from approximately 1 bar to 240 bars. Suitable pumps are known in the art and are available, for example, from Agilent Technologies, Inc. of Santa Clara, Calif. and Milton Roy of Ivyland, Pa. Another water stream from water recycle stream516is fed into a pump130to increase the pressure to 240 bars and then mixed with fresh water in a mixer210to form the stream212. Both fuel112and water212streams pass through a heat exchanger310to increase the temperature of each to approximately 600˜800° C. via heat exchange with reactor product stream412. Stream312, supercritical water after the heat exchanger, is mixed with fuel stream314in mixer220to form a reaction stream222input for reactor410.

The product stream412exiting the reactor410is directed to the heat exchanger310where it heats incoming fuel and water streams112and212, respectively, and then is directed into a hydrogen separator510. Hydrogen as a product is collected from510and is distributed there from, as indicated at512, for use in fuel cells or hydrogenation. The rest of the stream514goes to a gas separator520via a pressure release process. All of the product gas except hydrogen is collected in the stream522leaving separator520. A stream of water516exits separator520and is recycled back to mixer210to mix with fresh water via pump130.

In the flow diagram, energy is imported to the system via streams113,123and133to power pumps110,120and130and stream411to provide the heating stream for reactor410(as described with reference toFIGS. 1-4for combustion in the heating stream channels) through burning residual hydrogen from a fuel cell or hydrocarbon from the gas separator520.

In addition, a process for fuel desulphurization may optionally be included in the hydrogen generation process of the invention. The purpose of such a process is to remove sulfur compounds which can poison the catalyst in the reactor410. A supercritical process provides a means of desulphurizing the fuel source as sulfur compounds may be separated due to unique properties achievable under supercritical conditions. More specifically, sulfur inorganic compounds normally are dissolved in water solution, but will form deposits in supercritical condition. In addition, some sulfur organic compounds form suspension in supercritical water condition. Either of these behaviors leads to the possibility of mechanically separating the sulfur from the fuel through the process of forming sulfur compounds which may be physically separated in the separator device, illustrated in phantom at600inFIG. 7. The separator device600may be, for example, a molecular sieve featuring a zeolite structure. The separator device600adsorbs the sulfur contamination as the fuel/reaction stream (222inFIG. 7) flows through the device. The usual practice is to place two sieves in parallel and alternate between the process flow and regeneration. One sieve,600inFIG. 7, is regenerated by desorption while the process flow goes through the other sieve602.

The supercritical process and reactor described above work well over a wide range of conditions and with various hydrocarbon fuel sources having a wide range of purities. In addition, the ratio of hydrogen fuel produced to the amount of CO2generated is much higher than if hydrocarbon fuel were burned by itself and the energy cost to operate the reactor and system is low for the amount of energy produced. The residence time for the reaction process is shortened due to the large heat exchange and separation surface areas provided in the reactor, which also facilitate the separation of hydrogen in the reactor.