New solid foam reactor packings for multiphase applications
Advancing the chemical engineering fundamentals
Keynote Lectures: Theme-2
Keywords: reactor packings, solid foam meterials
Jaap C. Schouten,
Charl P. Stemmet, Patrick W.A.M. Wenmakers, John van der Schaaf, Ben F.M. Kuster
Laboratory of Chemical Reactor Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, the Netherlands; Email: J.C.Schouten@tue.nl; Internet: www.chem.tue.nl/scr
The chemical industry continually strives for more cost-efficient processes, and in doing so, considers the use of more advanced materials to optimize and intensify processes to the desired conditions. Reactor packings such as monoliths, cloths, foams, and other structured packings are investigated and used for two- and three-phase flow operation due to their improved hydrodynamic performance compared with more conventional packings, e.g. spherical particles, Raschig rings, Sulzer packings, etc. The advantages of these structured reactor packings are a reduced pressure drop per packing height, improved hydrodynamic properties, and a greater window of stable operating conditions. The relatively high surface area ensures that adequate catalyst loadings may be applied. Improved gas-liquid contacting is advantageous to avoid, or at least to reduce, mass-transfer limitations under reaction conditions. In counter-current operation, flooding, the point at which flow reversal of the liquid occurs with increasing gas flow, is regarded as the limiting factor for using these packings in industrial processes.
Solid foam packings represent a generation of commercially available materials combining relatively high specific surface area with low pressure drop per unit height. This is largely due to the open-celled structure with pore sizes ranging from 5 mm to 0.25 mm (5-40 pores per linear inch (ppi)), with relatively high voidages (up to 97%). Solid foams may be produced in a variety of materials (metal, carbon, ceramics, SiC, polymers, etc.). Thus far, the applications in the chemical industry for these solid foam materials have been minimal. Only single-phase studies have been reported on chemical reaction, pressure drop, and axial dispersion. These materials can however also be considered for application in multiphase reactors where they may significantly increase the efficiency of the reactor volume. They show high rates of gas-liquid mass transfer at relatively low levels of energy dissipation per unit volume of the reactor. The choice of the foam material also allows the fine tuning of the reactor design; e.g. hydrophobic as opposed to hydrophilic packings for preferential conversion of components in the gas or liquid phase, metal foams for higher strength and improved heat conduction, or carbon foams for chemical inertness and electrical conductivity in fuel cell applications.
This presentation will outline the results obtained for two-phase gas-liquid flow through these solid foams in co-current and counter-current flow operation. The observed regimes of bubble flow, pulsing flow, and trickle flow will be demonstrated together with the flooding limits. Hydrodynamic characteristics, as frictional pressure drop and liquid holdup, will be presented for all three regimes. The overall gas-liquid mass transfer coefficient will be presented and correlated to the energy dissipation per unit of reactor volume. A comparison will be given between the solid foam characteristics and those of other packings, such as Katapak-S and Sulzer packings, monoliths, and micro-structured reactor packings.
In the presentation it will also be demonstrated that these solid foam materials are excellent carriers for immobilized active catalysts. For example, layers of entangled carbon nanofibers have been prepared on metallic as well as reticulated vitreous carbon foams. In this way the foam materials can be considered as the inverse of a packed bed, i.e. the void space and the packing have exchanged position, whereas entangled carbon nanofibers are the inverse of a porous catalyst particle. Finally, an outlook will be presented on the use of these solid foam materials as integrated parts of the reactor, such as rotating beds or stirrer blades. In addition to the role as catalyst support, the rotation of the foam packing should provide shear forces to break up gas bubbles and leads to enhanced mixing of the reactants and better mass transfer.
References
Stemmet et al., Chem. Eng. Res. Des., 84 (A12), 1134-1141, 2006
Stemmet et al., Chem. Eng. Sci., 60 (22), 6422-6429, 2005
Stemmet et al., Patent WO 065127-A1, 2006
Keynote lecture: Chemical Reaction Engineering
Presented Tuesday 18, 17:05 to 17:45, in session Keynote Lectures: Theme-2 (T2-K2).
