Catalytic elements length

In this work, the numerical experiment was carried out for y = 1, i.e., in the region where uniform oscillations in system (3,4) are impossible. Can the distributedness of the system act, under such conditions, as a factor capable of inducing a spatially nonuniform self-oscillatory process There is no strict criterion at the present time for the existence of this new type of self-oscillatory instability. Numerical experiments, however, provide a positive answer to this question. Figure 36 shows a time display of a self-oscillatory process discovered in the course of DS transformation as the catalytic element length / was varied. Other parameters were ... 

We consider the following physical situation. A first-order exothermic reaction runs on a smooth, nonporous catalytic element having a length L (thread, fiber, tube). Assume that there is no temperature distribution at the element cross-section, thus reducing the analysis to a one-dimensional case. Assume also that the reaction runs under conditions of transversal flow, and the heat and mass transfer between the catalyst surface and the bulk-flow are described by the effective coefficients a and respectively. Under these assumptions, the model can be written in the form of two equations, one describing the heat balance in the solid catalyst phase, and another the reactant balance in the gaseous phase of a certain characteristic layer adjacent to the catalyst surface ... 

If g approaches unity, IJh = 0.5, and if e decreases, the ratio IJl decreases too. An interesting property of DS is that this ratio is independent of the catalytic element size, as illustrated by Fig. 26. This is not true only in the vicinity of the critical lengths for the formation of DS and multiplication of elementary fragments. The ratio is also weakly dependent on e when g 1. 

First identified in 1986 as the catalytic active element in the replication cycle of certain viruses, the hammerhead ribozymes (HHRz) are the smallest known, naturally occurring RNA endonucleases They consist of a single RNA motif which catalyzes a reversible, site-specific cleavage of one of its own phosphodiester bonds . Truncation of this motif allowed a minimal HHRz to be constructed which was the very first ribozyme to be crystallized. HHRz minimal motifs are characterized by a core of eleven conserved nucleotides (bold font in Figure 20) from which three helices of variable length radiate. Selective mutation of any of these conserved residues results in a substantial loss of activity. In the absence of metal ions the structure is relaxed ( extended ), but upon addition of Mg +, hammerhead ribozymes spontaneously fold into a Y-shaped conformation (Figure 20 Color Plate 3). ... 

In the 1980s, Julia and Colonna discovered that the Weitz-Scheffer epoxidation of enones such as chalcone (4, Scheme 2) by alkaline hydrogen peroxide is catalyzed in a highly enantioselective fashion by poly-amino acids such as poly-alanine or poly-leucine (Julia et al. 1980, 1982). The poly-amino acids used for the Julia-Colonna epoxidation are statistical mixtures, the maximum length distribution being around 20-25 mers (Roberts et al. 1997). The most fundamental question to be addressed refers to the minimal structural element (i.e. the minimal peptide length) required for catalytic activity and enantioselectiv-ity. To tackle this question, we have synthesized the whole series of L-leucine oligomers from 1- to 20-mer on a solid support (Berkessel... 

Numeroxis reactions are performed by feeding the reactants continuously to cylindrical tubes, either empty or packed with catalyst, with a length which is 10 to 1000 times larger than the diameter. The mixture of unconverted reactants and reaction products is continuously withdrawn at the reactor exit. Hence, constant concentration profiles of reactants and products as well as a temperature profile are established between the inlet and the outlet of the tubular reactor (see Fig. 8.10). This requires, in contrast to the batch reactor, the application of the law of conservation of mass over an infinitesimal volume element, d V, or mass element, dW, of the reactor. For a tubular reactor with a fixed catalytic bed, it is more convenient to relate the production rates to the catalyst mass, rather than to the reactor volume. 

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