Methane reforming steam

Considering methane steam reforming [see Eq. (2.12)] as a large-scale process, Xu and Froment found that only the outer 2 mm of the catalyst pellets actually participates in the reaction [41], thus theoretically allowing for a two orders of magnitude reduction in catalyst volume. However, the well-known pressure drop limitations have prevented practical applications in the industrial field so far. 

2 Development of Catalyst Coatings for Methane Steam Reforming in Micro Channels 

Find et al. [42] developed a nickel-based catalyst for methane steam reforming. As material for the micro structured plates, AluchromY steel, which is an FeCrAlloy (see Section 2.10.7) was applied. This steel forms a thin layer of alumina on its surface, which is less than 1 pm thick. This layer was used as an adhesion interface for the catalyst. I ts formation was achieved by thermal treatment of micro structured plates for 4 h at 1 000 °C. 

The catalyst itself was based on a nickel spinel (NiAl204) for stabilization. The active nickel was introduced as surplus of the stoichiometric content of the spinel to the catalyst slurry. The content of active nickel in the final catalyst could be adjusted via the pH during the precipitation. By XRD, a-alumina was identified as an additional phase in case the nickel was incompletely incorporated into the spinel. The sol-gel technique was then used to coat the plates with the catalyst slurry. Good catalyst adhesion was proved by mechanical stress and thermal shock tests. 

---

Na.tura.1 Ga.s Reforma.tion. In the United States, most hydrogen is presently produced by natural gas reformation or methane—steam reforming. In this process, methane mixed with steam is typically passed over a nickel oxide catalyst at an elevated temperature. The reforming reaction is... 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

In a number of publications, Rostrup-Nielsen discusses different mechanism of methane steam reforming over Ni catalysts [17]. The proposed simplified reaction sequence for reforming of methane is as follows ... 

Xu, Z. and Froment, G., Methane steam reforming, methanation and water-gas shift I. Intrinsic kinetics, AIChE., 35, 88,1989. 

Shu, ]., B.P.A. Grandjean, and S. Kaliaguine, Methane steam reforming in asymmetric Pd- and Pd-Ag/porous SS membrane reactors, Appl. Catal. A General, 119, 305-325,1994. 

Tsuru, T., K. Yamaguchi, T. Yoshioka, and M. Asaeda, Methane steam reforming by microporous catalytic membrane reactors, AIChE., 50(11), 2794-2805, 2004. 

Ethanol and methane steam reforming reactions were studied assuming that the exit composition of the ethanol reformer depends on the steam reforming of methane. The competition for the same active site for ethanol and methane reforming maximizes the H2 and C02 production and minimizes the CO formation Catalysts were prepared by incipient wet impregnation. 20 wt% Ni supported on ZnO exhibited better performance compared to that supported on La203, MgO and A1203... 

There is a need for low-cost methane steam reforming catalysts that are active at low temperature and resistant to coke formation under membrane reactor conditions. Low-cost (Ni-based) catalysts are also needed that can withstand regeneration conditions in a sorption-enhanced reformer. 

Choudhary, T.V., Goodman, D.W. 1999. Stepwise methane steam reforming a route to CO-free hydrogen. Catal Lett 59 93-94. 

Unlike the methane steam reformer, the autothermal reformer requires no external heat source and no indirect heat exchangers. This makes autothermal reformers simpler and more compact than steam reformers, resulting in lower capital cost. In an autothermal reformer, the heat generated by the POX reaction is fully utilized to drive the SR reaction. Thus, autothermal reformers typically offer higher system efficiency than POX systems, where excess heat is not easily recovered. 

The same catalyst compositions used in the more important methane steam reforming [Eq. (3.1), forward reaction], may be used in methanation, too.222 All Group VIE metals, and molybdenum and silver exhibit methanation activity. Ruthenium is the most active but not very selective since it is a good Fischer-Tropsch catalyst as well. The most widely used metal is nickel usually supported on alumina or in the form of alloys272,276,277 operating in the temperature range of 300-400°C. 

If methane is considered for reaction (3.13), a first order reaction rate is usually assumed, i.e. the coefficient n of expression (3.22) is equal to 1. According to Achenbach [22], the activation energy of methane steam reforming is 82 kJ mol-1 and the pre-exponential factor is 4274 mol m-2 bar-1 s-1. More recent experimental studies report an activation energy of 112 15 kJ mol-1 [23],... 

Dicks A.L., Pointon K.D., Siddle A., 2000. Intrinsic reaction kinetics of methane steam reforming on a nickel/zirconia anode. Journal of Power Sources 86, 523-530. 

One-dimensional models of a solid oxide fuel cell (see Chapter 9) and a methane-steam reformer [19, 20] were incorporated into the ProTRAX programming environment for transient studies. Lumped parameter ProTRAX sub-models were used for the remaining system components (heat exchangers, turbomachinery, valves, etc ). A schematic of the model is provided for reference in Figure 8.21. 

Catalytic membrane reactors are not yet commercial. In fact, this is not surprising. When catalysis is coupled with separation in one vessel, compared to separate pieces of equipment, degrees of freedom are lost. The MECR is in that respect more promising for the short term. Examples are the dehydrogenation of alkanes in order to shift the equilibrium and the methane steam reforming for hydrogen production (29,30). An enzyme-based example is the hydrolysis of fats described in the following. 

Dehydrogenations, e.g., ethane to ethene, ethylbenzene to styrene, methanol to formaldehyde Methane steam reforming Water-gas shift reaction... 

Figure 2.68 Results from numerical calculations for combustion-assisted methane steam reforming, (a) Outlet conversion dependence on channel half-height (b) wall temperature as a function of dimensionless reactor length. Calculation results determined at constant inlet velocity [108]...

Find, J., Lercher, J. A., Cremers, C., Stimming, U., Kurtz, O., Cramer, K., Characterization of supported methane steam reforming catalyst for microreactor systems, in Proceedings of the 6th International Conference on Microreaction Technology, IMRET 6 (11-14 March 2002), AIChE Pub. No. 164, New Orleans, 2002, 99-104. 

Zanfir M., Gavriilidis, A., Catalytic combustion assisted methane steam reforming in a catalytic plate reactor, Chem. Eng. Sci. 2003, 58, 3947-3960. 

---