Development of a Novel Combined Catalyst and Sorbent for Hydrocarbon Reforming Justinus A

Chemical and Biological Engineering Publications Chemical and Biological Engineering 2005 Development of a novel combined catalyst and sorbent for hydrocarbon reforming Justinus A. Satrio Iowa State University Brent H. Shanks Iowa State University, [email protected] Thomas D. Wheelock Iowa State University, [email protected] Follow this and additional works at: http://lib.dr.iastate.edu/cbe_pubs Part of the Chemical Engineering Commons The ompc lete bibliographic information for this item can be found at http://lib.dr.iastate.edu/ cbe_pubs/220. For information on how to cite this item, please visit http://lib.dr.iastate.edu/ howtocite.html. This Article is brought to you for free and open access by the Chemical and Biological Engineering at Iowa State University Digital Repository. It has been accepted for inclusion in Chemical and Biological Engineering Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Development of a novel combined catalyst and sorbent for hydrocarbon reforming Abstract A combined catalyst and sorbent was prepared and utilized for steam reforming methane and propane in laboratory-scale systems. The am terial was prepared in the form of small spherical pellets having a layered structure such that each pellet consisted of a highly reactive lime or dolime core enclosed within a porous but strong protective shell made of alumina in which a nickel catalyst was loaded. The am terial served two functions by catalyzing the reaction of hydrocarbons with steam to produce hydrogen while simultaneously absorbing carbon dioxide formed by the reaction. The in situ er moval of CO 2 shifted the reaction equilibrium toward increased H 2 concentration and production. The oncc ept was proved by using both a thermogravimetric analyzer and a fixed-bed reactor loaded with the material to reform hydrocarbons. Tests conducted with the fixed-bed reactor at atmospheric pressure and with temperatures in the range of 520-650Â°C produced a product containing a large concentration of H 2 (e.g., 94-96 mol %) and small concentration of CO and CO 2. Therefore, the results achieved in a single step were as good as or better than those achieved in a conventional multistep reaction and separation process. Keywords fixed-bed reactors, reaction equilibrium, spherical pellets, catalysts, chemical reactors, hydrogen, phase equilibria, thermal effects Disciplines Chemical Engineering Comments Reprinted (adapted) with permission from Industrial and Engineering Chemistry Research 44 (2005): 3901, doi: 10.1021/ie040284m. Copyright 2005 American Chemical Society. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/cbe_pubs/220 Ind. Eng. Chem. Res. 2005, 44, 3901-3911 3901 Development of a Novel Combined Catalyst and Sorbent for Hydrocarbon Reforming Justinus A. Satrio, Brent H. Shanks, and Thomas D. Wheelock* Department of Chemical Engineering, Iowa State University, Ames, Iowa 50011 A combined catalyst and sorbent was prepared and utilized for steam reforming methane and propane in laboratory-scale systems. The material was prepared in the form of small spherical pellets having a layered structure such that each pellet consisted of a highly reactive lime or dolime core enclosed within a porous but strong protective shell made of alumina in which a nickel catalyst was loaded. The material served two functions by catalyzing the reaction of hydrocarbons with steam to produce hydrogen while simultaneously absorbing carbon dioxide formed by the reaction. The in situ removal of CO2 shifted the reaction equilibrium toward increased H2 concentration and production. The concept was proved by using both a thermogravimetric analyzer and a fixed-bed reactor loaded with the material to reform hydrocarbons. Tests conducted with the fixed-bed reactor at atmospheric pressure and with temperatures in the range of 520-650 °C produced a product containing a large concentration of H2 (e.g., 94-96 mol %) and small concentration of CO and CO2. Therefore, the results achieved in a single step were as good as or better than those achieved in a conventional multistep reaction and separation process. Introduction in the first stage while a temperature of 200-215 °C and a copper-zinc oxide catalyst are used in the second Hydrogen is an important commodity that is widely stage. The product mixture is subsequently separated used for the production of ammonia and methanol and by scrubbing it with a solvent for CO . in a variety of other applications. In the future hydrogen 2 may be utilized in fuel cells for the highly efficient It had been shown in several previous studies that it generation of electric power. Although there are several is possible to replace this continuous but complex ways of producing hydrogen, one of the most widely used multistep process by a much simpler single-step process methods is by steam reforming hydrocarbons. While which employs a mixed bed of catalyst and sorbent hydrogen can also be generated by gasifying coal or particles that operates in an intermediate temperature 2-5 biomass, the hydrogen content of the raw gas is low range. Laboratory-scale demonstrations of such a process were conducted with beds composed of crushed because of the presence of other gases such as CO, CO2, CH4, and N2. The hydrogen content of such mixtures commercial nickel-based reforming catalysts and sor- can be increased by applying a combination of the steam bent particles selected for removing CO2 from the reforming reaction, reaction mixture. Different sorbents were used including calcium oxide and a potassium carbonate promoted + a + ) hydrotalcite. Excellent results were achieved while CH4 H2O 3H2 CO ∆H25°C 210 kJ/mol (1) reforming methane in beds composed of the nickel and the water-gas shift reaction: catalyst and a lime sorbent with temperatures ranging from 450 to 725 °C and under a total pressure of 15 + a + )- atm.2,3 Good results were also realized with the hydro- CO H2O H2 CO2 ∆H25°C 42 kJ/mol (2) talcite sorbent at 450 °C and 4.7 atm.4,5 These studies Of course, these are also the reactions which are widely demonstrated that with removal of CO2 from the used for producing hydrogen from natural gas. Since system, reaction 2 is limited less by thermodynamic both reactions tend to be limited by thermodynamic equilibrium so it can be conducted at higher tempera- equilibrium as well as reaction kinetics, they require tures with a less active catalyst. Of course, the sorbent catalysts and appropriate reaction temperatures. Be- must be regenerated periodically, and if the process is cause reaction 1 is endothermic, the equilibrium conver- conducted under pressure, regeneration can be ac- 4 sion of methane by this reaction is favored by higher complished by pressure swing absorption. Depending temperatures. On the other hand, since reaction 2 is on the method of regeneration, relatively pure CO2 can exothermic, the equilibrium conversion of carbon mon- be produced as a by-product. oxide is favored by lower temperatures. In a conven- Another promising concept which has been suggested tional methane reforming process reaction 1 is carried for overcoming the reaction equilibrium limitation is to out with a nickel catalyst at 800-870 °C, whereas conduct steam reforming in a reaction system fitted with reaction 2 is carried out in two stages operating at lower semipermeable membranes that selectively remove temperatures.1 A reaction temperature of 350-450 °C hydrogen from the system so that the concentration of and an iron oxide/chromium oxide catalyst are employed hydrogen is maintained below the equilibrium value.6,7 A recent modeling investigation of methane reforming * To whom correspondence should be addressed. Tel.: (515) in a fast circulating fluidized-bed reactor compared the 294-5226. FAX: (515) 294-2689. E-mail: [email protected]. improvement in methane conversion and hydrogen yield 10.1021/ie040284m CCC: $30.25 © 2005 American Chemical Society Published on Web 05/03/2005 3902 Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 which can be achieved with different methods of overcoming the reaction equilibrium barrier.7 These methods included (a) removing H2 with permselective membranes, (b) removing CO2 by reaction with CaO, and (c) a combination of both methods. Solid particles of a reforming catalyst were also assumed to be present. The results showed that methods (a) and (b) would improve CH4 conversion equally well, while (b) would increase H2 yield much more than (a) would. The increases produced by method (c) would be equal to the sum of the increases produced by (a) and (b). While the previous laboratory studies produced highly encouraging results, they were conducted with fixed- bed reactors packed with mixtures of very small catalyst and sorbent particles. Such a reaction system could prove difficult to scale up to larger sizes in practice. To overcome this potential problem, a new material is being developed which combines the catalyst and sorbent materials into a single pellet. This development is an outgrowth of a previous study which led to the creation of an advanced calcium-based sorbent for desulfurizing hot coal gas.8-11 This material was in the form of small spherical pellets such that each pellet consisted of a CaO core surrounded by a protective shell made largely of porous alumina. Since alumina is widely used as a catalyst support, especially for nickel reforming catalysts, the new “core-in-shell” pellets appeared ideally suited as a means for combining a reforming catalyst with a sorbent for CO2. Furthermore, by varying the size of the pellets, it may become possible to adapt the material for use in either fixed-bed, moving-bed, or Figure 1. Methane conversion and H2 yield obtainable by steam fluidized-bed reactors. However, the overall objective of reforming CH4 under equilibrium conditions both with and without CO absorption.

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