(565a) Open-Cell Metal Foams As Enhanced Catalyst Supports for Heat Transfer Intensification in Tubular Reactors

In the context of process intensification, structured catalyst supports offer the advantage of a better control on the geometrical properties and consequently on the effective transport properties of the catalytic bed. In this regard, open-celled foams are of particular relevance. They represent a class of consolidated porous media which is characterized by high geometric surface areas and high void fraction [1]. Moreover, high conductive catalyst carriers can help to minimize the hot/cold spots and to prevent mechanical-strength and thermal shock limitations, thereby increasing the reactor and process efficiency. Thus, in this work the heat transport properties of metal foam supports suitable for highly exo- or endothermic catalytic gas phase reactions in tubular reactors are characterized adopting a combined experimental and modeling approach.

The experimental heat transfer data [2] was acquired with samples of different geometries and bulk solid materials in a lab-scale test rig which consists of a 10 cm long foam bed fitted in a stainless steel pipe of 28mm I.D. placed in a thermostatic chamber. Steady-state axial and radial temperature profiles were collected inside the foam beds at heating and cooling conditions and at different flow rates. Both the effects of the solid matrix (cell density, porosity, thermal conductivity) and of the saturating fluid (N2 and He) on the heat transfer have been investigated.

Afterwards, 3D representations of selected samples have been generated from the scanned images with X-ray micro computed-tomography (μCT), a non-destructive technique enabling resolutions in the order of microns, and afterwards investigated by finite volume analysis (FVA).

This detailed study points out the determinant role of the solid phase to the effective heat conductivity of the foam bed, allowing for identifying the dependency on the thermal conductivity of the bulk material, on the solid fraction and on the geometrical structure of the solid matrix. At the same heating condition, the supports with the highest thermal conductivity feature almost flat inner radial temperature profiles, with the whole T-gradient confined at the foam-tube wall interface. Because this seems to be the controlling resistance in highly conductive foam configurations, the region of coupling with the reactor tube is studied in detail [3]. The effects of the flow rate, the thermal conductivity of the saturating medium and the contact quality between the foam and the wall, have been modeled and examined in detail. Moreover, the role of the pore density as well as the solid strut thickness, which are of complementary importance to allow for a complete geometrical description of the foam, have been investigated in the simulations.

As a result of this detailed study, validated on the basis of experimental data and FVA, the dependence of the foam thermal properties on geometry and flow conditions has been clarified and correlations have been established. Such engineering correlations for the foam heat transfer processes can help to guide the design of optimized catalytic reactors. In summary, with open-cell foams as innovative catalyst supports process intensification can be achieved by taking advantage of the enhanced heat transport properties that are superior to conventional packed bed systems.

References

1.  Inayat A., Schwerdtfeger J., Freund H., Körner C., Singer R. F., Schwieger W., Chem. Eng. Sci., 2011. 66(6): p. 1179-1188.

2. Bianchi E., Heidig T., Visconti C.G., Groppi G., Freund H., Tronconi E., Chem. Eng. J., 2012. 198-199: p. 512-528.

3. Bianchi E., Heidig T., Visconti C.G., Groppi G., Freund H., Tronconi E., Catal. Today, 2013, (submitted).