This invention relates to a process and an apparatus for the production of a product gas comprising hydrogen using precious metal and non-precious metal catalysts in the water gas shift reaction.
Hydrogen may be produced from carbon monoxide and steam via the water gas shift reaction: CO+H2O→CO2+H2 where the carbon monoxide and steam are reacted at elevated temperatures in the presence of a metal catalyst. The water gas shift reaction may be used to advantage in conjunction with other hydrogen production techniques to recover additional hydrogen using the reaction products of those techniques. For example, the output from the steam reforming of methane, CH4+H2O →CO+3H2 produces carbon monoxide and hydrogen. The carbon monoxide, when further reacted with steam in the water gas shift reaction produces carbon dioxide and hydrogen. Likewise, synthesis gas, produced by reforming hydrocarbons with steam, or by partial oxidation of hydrocarbons, and containing carbon monoxide and hydrogen, can be reacted further along with steam in a water gas shift reactor to increase the production of hydrogen.
The water gas shift reaction is mildly exothermic in nature, i.e., heat is liberated during the reaction. The heat liberated during the reaction needs to be removed from the reactor during the reaction. Because it is difficult to remove heat from the shift reactor, two different approaches have been used by the industry. In the first approach, feed gas is introduced into the reactor at substantially lower temperature than the temperature of the product gas. In the second approach, multiple reactors are used wherein heat is removed form the product of the first reactor by using a heat exchanger. The cooled product is introduced into the second reactor for further reaction. The first approach is commonly used by the industry because it is economical.
Two different catalysts are commonly used for the water gas shift reaction—a more expensive copper based catalyst and a less expensive iron-chromium based catalyst. The iron-chromium based catalyst can be promoted with low amounts of copper to enhance catalyst activity. There are no restrictions in terms of gas composition when using a copper based catalyst for the water gas shift reaction. However, there are a number of operational limitations when using a copper based catalyst for the water gas shift reaction. First, the catalyst needs to be pre-reduced with hydrogen to be effective for the water gas shift reaction. This means that a separate source of hydrogen needs to be provided to pre-reduce the catalyst prior to using it for the water gas shift reaction. Second, the operating temperature needs to be limited to a maximum of about 280° C. to avoid loss in catalytic activity due to sintering of the copper catalyst. Consequently, the use of copper based catalyst is limited to situations where iron-chromium based catalyst cannot be used.
Iron-chromium or copper promoted iron-chromium (also known as iron-chromium-copper) catalyst is widely used by the industry for the water gas shift reaction. It requires a slightly higher operating temperature (ranging from about 280° C. to about 450° C.) for the water gas shift reaction. Since it requires a higher operating temperature than the copper based catalyst, it is commonly termed as a high temperature shift (HTS) catalyst. The water gas shift reaction carried at higher temperatures with an HTS catalyst is called an HTS reaction, and the HTS reaction is commonly used by the industry for the water gas shift reaction.
Iron-chromium or iron-chromium-copper catalyst is used in an oxide form, and therefore does not require reduction with hydrogen prior to its use for the water gas shift reaction. In fact, it is desirable to avoid reduction of the iron-chromium-copper based catalyst because both iron-chromium and iron-chromium-copper catalysts in reduced form are very active for the methanation reaction (the reaction consumes hydrogen instead of producing it and concomitantly produces undesirable hydrocarbons such as methane). Consequently, when the water gas shift reaction occurs in the presence of a non-precious metal catalyst like iron-chromium or iron-chromium-copper catalysts two process parameters have a controlling effect on the reaction, as described in U.S. Pat. No. 6,500,403. These parameters are the ratio of carbon monoxide to carbon dioxide (CO/CO2) and the ratio of steam to other gases. If the CO/CO2 ratio is greater than 1.9, and/or, if the ratio of steam to other gases is less than 0.5, then the reaction that occurs will be reversed from the water gas shift reaction and hydrocarbons will be formed rather than hydrogen. The reverse reaction is believed to occur due to the reduction of the iron-chromium or iron-chromium-copper based catalysts, caused by the presence of either high concentrations of carbon monoxide (high CO/CO2 ratios) or low concentrations of steam (low steam to other gases ratios). Consequently, the use of non-precious metal catalysts like iron-chromium or iron-chromium-copper based catalysts is limited to treating water gas shift feed gas mixtures containing CO/CO2 ratios less than 1.9 and/or steam to other gas ratios more than 0.5.
There exists a need for a process and an apparatus for economically generating hydrogen using the high temperature water gas shift reaction at a CO/CO2 ratio greater than 1.9 and/or a steam to other gas ratio less than 0.5 without promoting the formation of hydrocarbons.
The invention concerns a process for producing a product gas comprising hydrogen. The process comprises:
The feed gas mixture may be produced by reforming hydrocarbons with steam or partial oxidation of hydrocarbons. In such cases the feed gas mixture will comprise hydrogen.
The feed gas mixture may further comprise carbon dioxide and unreacted hydrocarbon in the form of methane, and the volumetric ratio of carbon monoxide to carbon dioxide in the mixture may be greater than about 1.9. Furthermore, the volumetric ratio of steam to other gases in the mixture may be less than about 0.5.
The precious metal catalyst may be platinum, rhodium, palladium, ruthenium, gold, iridium and combinations thereof. The non-precious metal catalyst may be iron-chromium, iron-chromium-copper and combinations thereof.
The invention also includes a reactor vessel for producing a product gas comprising carbon dioxide and hydrogen from a feed gas stream comprising carbon monoxide, hydrogen and steam. The feed gas may also contain low levels of carbon dioxide and methane. The reactor vessel comprises a chamber having an inlet duct for receiving the feed gas stream and an outlet for discharging the product gas. A support medium is position within the chamber. A non-precious metal catalyst is positioned on the support medium. A structural support is positioned in the feed gas stream upstream of the support medium. A precious metal catalyst is positioned on the structural support.
The structural support may comprise a plurality of plates arranged within the inlet duct so as to permit flow of the gas mixture over the plates and into the chamber, the precious metal catalyst being supported on the plates. Alternately, the structural support may comprise a plurality of plates arranged within the chamber so as to permit flow of the gas mixture over the plates and then through the support medium, the precious metal catalyst being supported on the plates.
Preferably, the precious metal catalyst is present on the plates at an area density between about 0.015 mg per square inch and about 15 mg per square inch.
In one embodiment of a reactor, the support medium comprises a granular medium formed of or supporting the non-precious metal catalyst. The granular material may be made by compressing iron-chromium, iron-chromium-copper or other non-precious metal catalyst powder into pellets. Alternatively, the granular material may be made of ceramic pellets and the concentration of iron-chromium, iron-chromium-copper or other non-precious metal catalyst on the ceramic material may vary between about 5% to about 50% by weight of the ceramic pellets. In another embodiment, the support medium comprises a plurality of plates arranged within the chamber so as to permit flow of the gas mixture thorough over the plates and through the chamber, the non-precious metal catalyst being supported on the plates.
Preferably, the non-precious metal catalyst is present on the plates at an area density between about 0.075 mg per square inch and about 75 mg per square inch.
In the embodiment illustrated in
Reactor vessels according to the invention configured so as to present a precious metal catalyst on a structural support upstream of a non-precious metal catalyst on a support medium are expected to have greater efficiency and economy than reactors according to the prior art. Due to its higher catalytic activity, the precious metal catalyst may be used in the water gas shift reaction at CO/CO2 ratios higher than 1.9 and/or steam to gas ratios less than 0.5 without forming undesired hydrocarbons. The precious metal catalyst is also used to bring the CO/CO2 ratio into the proper range (less than 1.9) so that the shift reaction will proceed as desired when reacted in the presence of the non-precious metal catalyst positioned downstream within the chamber of the reactor.
The precious metal catalyst volume may vary from about 5% to 50% of the non-precious metal catalyst volume, preferably from about 5% to about 35%, and more preferably from about 5% to about 25%. The overall conversion of carbon monoxide in the precious metal catalyst volume may vary from about 5% to about 30%, preferably from about 5% to about 25%, more preferably from about 5% to about 20% depending upon the concentration of carbon monoxide or ratio of CO/CO2 in the feed gas. In any case, the ratio of CO/CO2 entering the non-precious metal catalyst volume is limited to less than 1.9.
Various types of structural supports 20 are feasible for use with reactor vessels according to the invention. The example shown in
The plates of such structural support means may be formed of high temperature iron-chromium-aluminum metal alloys such as fecralloy or ceramics such as zirconia, alumina, calcium aluminate, magnesium aluminate, magnesium aluminum silicate, titania, alumina silicate, berylia, thoria, lanthania, calcium oxide, magnesia as well as mixtures of these compounds. Other examples of structural support means include static mixing elements, honeycomb monolith structures as well as other configurations having longitudinal passageways. Such structural support means for the precious metal catalyst provide high gas flow rates with low pressure drop. The gas hourly space velocity through such materials may range between 5,000 per hour to about 50,000 per hour.
The resistance to fouling and large surface area provided by structural supports permits smaller amounts of precious metal to be used than would otherwise be present on a granular support medium. Area densities of the precious metal on the surface of the structural support may vary between about 0.015 mg per square inch to about 15 mg per square inch. Thus, the structural support makes the use of precious metal economically feasible. The precious metal catalyst positioned on the structural support may include platinum, rhodium, palladium, ruthenium, gold, iridium and combinations thereof.
The structural support made of a ceramic material may be deposited with the catalyst by any of various techniques including impregnation, adsorption, ion exchange, precipitation, co-precipitation, spraying, dip-coating, brush painting as well as other methods.
The structural support made of metal alloy may be deposited first with a ceramic washcoat. The ceramic washcoat may be selected from ceramics such as zirconia, alumina, calcium aluminate, magnesium aluminate, magnesium aluminium silicate, titania, alumina silicate, berylia, thoria, lanthania, calcium oxide, magnesia as well as mixtures of these compounds. The washcoat my be deposited with deposition and/or precipitation methods including sol-gel methods, slurry dip-coating, spray coating, brush painting as well as other methods. The washcoat may then be deposited with the catalyst by any of various techniques including impregnation, adsorption, ion exchange, precipitation, co-precipitation, spraying, dip-coating, brush painting as well as other methods.
In preparing the structural support by washcoating, a ceramic paste or washcoat is deposited on the surface of the structural support. The washcoat is then deposited or impregnated with one or more precious metals. The area density of washcoat may vary between about 15 mg per square inch and about 150 mg per square inch. The amount of precious metal may vary between about 0.1% to about 10% by weight of the washcoat. The amount of non-precious metal may vary between about 5% to about 50% by weight of the washcoat.
The non-precious metal catalyst 26 positioned on the support medium 28 shown in
In another embodiment of a reactor vessel 32, shown in
In all of the various embodiments described, the precious metal catalyst volume may vary from about 5% to 50% of the non-precious metal catalyst volume, preferably from about 5% to about 35%, and more preferably from about 5% to about 25%. The overall conversion of carbon monoxide in the precious metal catalyst volume may vary from about 5% to about 30%, preferably from about 5% to about 25%, more preferably from about 5% to about 20% depending upon the concentration of carbon monoxide or ratio of CO/CO2 in the feed gas. For all the embodiments, the ratio of CO/CO2 entering the non-precious metal catalyst volume is limited to less than 1.9.
The invention also encompasses a process for producing a product gas comprising hydrogen using the water gas shift reaction: CO+H2O→CO2+H2. As illustrated in
The feed gas mixture first encounters the structural support 20 supporting the precious metal catalyst 24 (see also
The feed gas mixture reacts in the presence of the precious metal catalyst thereby producing a resultant gas mixture comprising carbon monoxide, carbon dioxide, hydrogen, steam and unconverted methane. The CO/CO2 ratio of the resultant gas mixture is less than 1.9. By first passing the feed gas mixture through the precious metal catalyst, the CO/CO2 ratio of the feed gas mixture is brought within the proper limits so that the water gas shift reaction will continue as the resultant gas mixture passes through the support medium 28 which supports the non-precious metal catalyst. Having these parameters within the proper range ensures that hydrocarbons will not be produced, as would occur in the presence of the non-precious metal catalyst if the CO/CO2 ratio of the feed gas were greater than 1.9 and/or the steam to gas ratio were less than 0.5. The non-precious metal catalyst is also maintained at a temperature between about 280° C. and about 450° C. through the heat exchanger 17 or other heat exchangers, not shown. A product gas 40 exits the chamber through outlet 18, the product gas comprising the products of the water shift reaction, namely, carbon dioxide and hydrogen.
It is expected that the various reactor embodiments according to the invention will efficiently and economically handle feed gas mixtures with CO/CO2 ratios as high as 2.5 without promoting the formation of undesired hydrocarbons in a reversal of the intended reaction.