Dry methanation

Dr. Blum As a further comment on pressure optimization, and as it relates to our system, I think the response of the slurry methanation system to pressure is somewhat different from that of dry methanation. It relates to the ability of the catalyst to methanate a given amount of gas. In our system, the effective pressure is the total pressure minus the vapor pressure of the liquid phase. This doesn t hold for the standard methanation catalyst in the dry system. There is a different pressure relationship so the optimum just might not work quite the same way. 

R. Remick, O. Spaldon-Stewart, K. Krist, Alternative mechanism for direct oxidation of dry methane on ceria-containing anodes, Proceedings Fuel Cell Seminar, San Antonio, Texas, 2004. Courtesy Associates, Washington DC, USA (2004). 

The coke deposition on a catalyst under operation of the latter Is a typical reason for the catalyst deactivation. This process also can be considered as a manifestation of nonselectivity in the conversion of various organic compounds. Hence, the practically important problem is to find the conditions of the coking prevention. As an example, let us identify the conditions of no coke deposition on catalysts during the "dry" methane reforming described by a stepwise transformation as follows ... 

Dry methane is supphed by a compressor and precooling system to the cooler of a Linde liquid-methane system (Fig. 9.6) at 180 bar and 300 K. The low-pressure methane leaves the cooler at a temperature 6 K (6°C) lower tlian the temperature of the incoming high-pressure methane. The separator operates at 1 bar, and the product is saturated hquid at this pressure. Wliat is the maximumfraction of the methane entering the cooler that can be hquefied Perry s Chemical Engineers Handbook (footnote 7) is a source of data for methane. 

M. Diara, A. Abudula, H. Komiyama, and K. Yamada. Anodic reaction mechanism determining the threshold current density for the CO2 production in SOFC with dry methane fuel. In B. Thorstensen, ed., 2nd European SOFC Forum, Oslo/Norway, volume 2. 1996 637-646. 

Tahkov, L, Arishtirova, K., Bueno, J. M. C., Damyanova, S. (2014). Surface and structural features of Pt/Pr02—AI2O3 catalysts for dry methane reforming. Applied Catalysis A General, 474, 135—148. 

Figure 15.15 Voltage and power density versus current density, the latter being proportional to 0 /CH4 for constant 30 seem of dry methane feed to the Ni—YSZ anode of SOFC eCMR. Reprinted with permission from Zhan et al. (2006).

Since stable coke-free operation results because of the cell current, a cessation of cell current will cause coking that could permanently damage the stack. Figure 5 shows the results of different length current interruptions for a SOFC operated in pure dry methane at 1.8 A/cm at 750 °C. For the 1.5 and 6 min interruptions, the voltage exceeded the pre-interruption value immediately after current resumption, before... 

C., and Irvine, J.T.S. (2008) Ce-Substituted LSCM as new anode material for SOFC operating under dry methane. Solid State Ionics, 179,1562 1566. 

Sierra, G., Gallego, J., Batiot-Dupeyrat, C., Barrault, J., and Mondragon, F. (2009) Influence of Pr and Ce in dry methane reforming catalysts produced from Lai-xAxNiOs-s. Appl Catal. A, 369, 97-103. 

Godini, H.R., Xiao, S., Kim, M., Gorke, O., Song, S., and Wozny, G. (2013) Dual-membrane reactor for methane oxidative coupling and dry methane reforming reactor integration and process intensification. Chem. Eng. Process. Process Intensif., 74, 153-164. 

Figure 7.3 Suggestion for variable combination (graphite, 1 bar) (a) temperature and carbon conversion, (b) cold gas efficiency and dry methane yield, (c) syngas yield and H2/CO ratio, (d) selectivity of CO/C and CHVC.

As an example. Figure 7.15 demonstrates how diagrams for cold gas efficiency and dry methane yield would like if H2O is replaced by CO2 as gasifying agent... 

One approach to overcoming the limitations of nickel anodes, which has met with some success, is to augment the oxidation activity of Ni/YSZ cermets through the addition of an oxide-based oxidation catalyst. For example, stable operation on dry methane has been reported at 650°C in an SOFC using an yttria-doped ceria interlayer between the YSZ electrolyte and the Ni/YSZ cermet anode [61]. Ceria is a well-known oxidation catalyst, and might be expected to increase the activity of the anode for the electrochemical oxidation of methane. This approach still requires, however, that the operating temperature be maintained below 700°C to suppress carbon deposition reactions that take place rai nickel. 

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