L. Kiwi-Minsker

Microstructured catalytic reactors

Abstract This chapter is a summary of recent advances in microreactors, which have become increasingly important in process development and chemical research. Microstructured chemical reactors are characterized by dimensions in the submillimeter range and, thus, surface-to-volume ratios are roughly two orders of magnitude greater than those of conventional reactors. The fundamentals of design and operation of microreactors are explained, and the necessary heat and mass transfer considerations are presented. Various designs are discussed, their key features are illustrated, and examples of successful applications are given. Emphasis is placed on the methods of introducing a catalyst into a microstructured reactor (MSR). The advantages of microreactor technology—and also the difficulties in development—are highlighted. Because of the small reactor dimensions, diffusion times are short, and the influence of mass transfer on the rate of reaction can be efficiently reduced or even be avoided. Because the heat transfer is greatly improved relative to that in conventional reactors, higher reaction temperatures, pressures, and reactant concentrations are attainable, leading to reduced reactor volumes and amounts of catalyst. Therefore, MSRs are particularly advantageous for fast reactions that are highly exothermic or highly endothermic. Isothermal conditions combined with short residence times and narrow residence time distributions allow optimization of the contact time in the reactor and avoidance of unwanted consecutive reactions. The rational design of catalytic microreactors requires the simultaneous development of the catalyst and the reactor. The catalyst design should be closely integrated with the reactor design, with consideration taken of the intrinsic reaction kinetics and transport phenomena. The accurate tuning and control of the reaction conditions lead to impressively high product selectivity and energy efficiency. Thus, MSRs are versatile tools for the development of sustainable processes. Further objectives of the application of miniaturized reactors concern the generation of chemical information, facilitation of catalyst development, optimization, and characterization of reaction kinetics.

Experimental Study of Reaction Instability and Oscillatory Behavior during CO Oxidation over Pd supported on Glass Fiber Catalysts

The catalytic ignition-extinction limits and oscillatory behavior of the CO oxidn. at atm. pressure over Pd supported on glass fibers were investigated as a function of catalyst sp. surface area, surface concn. and dispersion of Pd. The ignition-extinction limits for each specific catalyst depend on the CO concn. and the gas flow rate. Three reactivity regions were found as a function of temp. and CO concn.: high reactivity region with conversion close to 100%, low reactivity region and the region of multiplicity of steady states. In the region of multiplicity the self-sustained oscillations with a period up to 6 h were obsd. for the catalyst with Pd loading of 0.2%wt. High and low reactivity states of the catalyst were assocd. with reduced and oxidized state of Pd on the glass fiber surface. It is believed that the reaction rate oscillations are due to the cycles of Pd partial oxidn.-redn. [on SciFinder (R)]