Compact fuel cell systems operating with hydrocarbon fuels possess high volumetric and gravimetric energy density and could outperform batteries in low-power, portable electronics [1]. Among the fuels considered, methanol is an attractive candidate since it is sulfur free, and most importantly, can be reformed at low temperatures (200-250°C). For a 20W fuel cell system operating at a 25% overall efficiency, about 3g of catalyst would be needed for the reformer operating isothermally at 230°C. In order to overcome the pressure drop, non-isothermality and repacking problems associated with micro-packed bed reactors, a wall-coated reactor offers a superior geometry [2, 3]. We have previously shown the feasibility of coating non-porous surfaces with a commercial BASF catalyst and reported coatings of up to 10µm thick in a 530µm fused silica capillary [4].

The challenge is to minimize the size of the wall-coated reactor, i.e. increase the catalyst loading per unit volume, which requires coatings thicker than 10µm. G.I. Taylor reported that the thickest coating possible occupies 57% of the total cross sectional area, independent of channel diameter. However, Taylor performed his study with a Newtonian fluid in channel diameters ranging from 1.5-4mm [5]. In this work we performed an experimental study to determine the maximum coating thickness feasible for non-Newtonian fluids in channels ranging from 250 to 530µm in diameter. In contrast to Taylor's findings, our data shows the fraction coated to be dependent on the channel diameter due to inertial effects within the coating fluid. The maximum fraction coated was drastically lowered, e.g. 12% in 250µm diameter channel.

It is imperative to minimize the effects of surface tension while concurrently minimizing the inertial effects within a given coating structure. We were able to increase the fraction coated by minimizing these effects through careful manipulation of the viscosity of the coating fluid. This was achieved using shear-thickening additives.

The presentation will show data on coating multi-channel ceramic microreactors. The microreactors were coated using a commercial Cu/ZnO/Al2O3 catalyst from BASF and the catalyst reactivity was measured and compared to that in a packed-bed.

This work has been funded by the U. S. Army Research Laboratory under the Collaborative Technology Alliance Program, Cooperative Agreement DAAD19-01-2-0010

References

1. Hu, J., Wang, Y., VanderWiel, D, Chin, C., Palo, D., Rozmiarek, R., Dagle, R., Cao, J., Holladay, J., Baker, E. Chem. Eng. J. 93, 5560 (2003)

2. Karim, A., Bravo J., Datye, A. Appl. Catal. A. 282, 101 (2005)

3. Karim, A., Bravo J., Gorm, D., Conant, T., Datye, A Catal. Today (in press).

4. Bravo, J., Karim, A., Conant, T., Lopez, G.P., Datye, A. Chem. Eng. J. 101, 113 (2004)

5. G.I. Taylor, Fluid. Mech., 10, 161 (1961)