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Enzyme Catalysis
Published in Harvey W. Blanch, Douglas S. Clark, Biochemical Engineering, 1997
Harvey W. Blanch, Douglas S. Clark
When ϕ is sufficiently small, (slow reaction, rapid diffusion), the effectiveness factor is practically unity for all values of β. In this situation, reactant molecules are able to diffuse far into the pore structure before they are converted to product; thus, the concentration of substrate is nearly uniform throughout the catalyst and the entire accessible surface area effectively promotes reaction. On the other hand, when ϕ is large, reactant molecules are consumed before they diffuse very far, and the reaction is limited to a thin region near the periphery of the particle. The concentration of substrate in the core of the catalyst is very low and ηt is small.
Clays and Clay Minerals
Published in Benny K.G. Theng, Clay Mineral Catalysis of Organic Reactions, 2018
Many clay-catalyzed reactions take place at external particle, or intradomain, surfaces (Theng 1974; Ovcharenko 1982; Solomon and Hawthorne 1983; Barrer 1984), domains being regions of oriented clay particles arranged in turbostratic array (cf. Figure 1.5) (Aylmore and Quirk 1960; Quirk 1968). Accordingly, the rate-limiting step of the reaction is determined by the diffusion of the reactants into, and that of the products out of, the interparticle pore and interlayer space (e.g., Breen et al. 1985). Thus, any pretreatment that increases the accessible surface area and porosity of the system would promote catalytic activity. Such textural improvements may be achieved through acid treatment (activation), heating (thermal treatment), and microwave/ultrasound irradiation of the clay material as described in Chapter 3 (Bannerjee and Sen 1974; Ovcharenko 1982; Mahmoud et al. 2003; Noyan et al. 2006; Steudel et al. 2009a, 2009b; Korichi et al. 2012; Heller-Kallai 2013; Komadel and Madejová 2013). Indeed, acid activation (using mineral acids) lies behind the formation and commercial production of catalytically active montmorillonites such as the K-catalysts (Süd-Chemie), the Filtrol clay catalysts (Filtrol Corporation and Laporte Industries), and the F-series catalysts (Engelhard) (Robertson 1948; Balogh and Laszlo 1993; Cseri et al. 1995a, 1995b; Chitnis and Sharma 1997; Li et al. 1997; Vaccari 1999; Nikalje et al. 2000; Flessner at al. 2001; Jang et al. 2005; Pushpaletha et al. 2005; De Stefanis and Tomlinson 2006; Dasgupta and Török 2008; Fernandes et al. 2012; McCabe and Adams 2013; Kumar et al. 2014).
Industrial Trickling Bed Biofilters for Abatement of Volatile Organic Compounds from Air Emissions
Published in Donald L. Wise, Debra J. Trantolo, Edward J. Cichon, Hilary I. Inyang, Ulrich Stottmeister, Remediation Engineering of Contaminated Soils, 2000
Vladimir O. Popov, Alexey M. Bezborodov, Phillip Cross, Adrian Murphy
Several methods of cell immobilization (physical adsorption, physical entrapment, chemical binding to derivatized carriers, etc.) and a number of commercially available carriers (sand, activated carbon, silica gel, various polymer structured packings and foams, various synthetic fibers) have been tested (29,30). As a result of extensive studies the following features were determined to be critical for efficient cell immobilization: (a) hydrophobicity of the cells; (b) surface charge of the carrier and the value of the free surface energy; and (c) accessible surface area and surface defects.
From μCT data to CFD: an open-source workflow for engineering applications
Published in Engineering Applications of Computational Fluid Mechanics, 2022
Kevin Kuhlmann, Christoph Sinn, Judith Marie Undine Siebert, Gregor Wehinger, Jorg Thöming, Georg R. Pesch
Although the difference between the two meshes seems to be quite low, the influence on simulations with catalytic surface reactions can be of importance. The amount of accessible surface area is of utmost importance for heterogeneous catalytic reactions, since it (together with the catalyst loading) determines the number of active sites. As a result, the overestimated surface area in mesh UP would lead to higher conversion. In exothermic reactions, higher temperatures and additionally accelerated reactions kinetics are the consequences. Thus, small differences in the surface area could have a significant influence on the whole system. Furthermore, the simulation of heterogeneous catalytic reactions is especially dependent on the surface fidelity and near-wall cells. Reactions take place at the interface between solid and fluid and, numerically, the heat of reaction is introduced into the fluid cells at the solid (this is the case for the OpenFOAM add-on DUO, Deutschmann et al., 2020). The resulting high gradients in temperature and species concentration require cells of high quality to guarantee convergence of the simulations and realistic results. Additionally, recesses like the ones we showed for the mesh UP can lead to additional problems, since transport of the reaction heat is limited due to missing convection. This makes complex reaction simulations more prone to errors. The improved discretization based on the smoothed STL is therefore beneficial for sound reactive CFD simulations.