Mass-transfer mechanisms and kinetics

Gritti, R and Guiochon, G, Influence of the degree of coverage of C-18-honded stationary phases on the mass transfer mechanism and its kinetics, J. Chromatogr. A, 1128,45,2006. 

In catalysis, molecular structure determines the catalytic activity in the kinetic regime, and supramolecular structure controls the degree of usage of this catalytic activity in applied catalysis, as well as heat and mass transfer, mechanical and other properties. In other words, the absence of proper molecular structure causes the absence of catalysis, but one is restricted in preparation of a catalyst by the necessity to improve the supramolecular structure.4... 

Asymptotic Solution Rate equations for the various mass-transfer mechanisms are written in dimensionless form in Table 16-13 in terms of a number of transfer units, N = L/HTU, for particle-scale mass-transfer resistances, a number of reaction units for the reaction kinetics mechanism, and a number of dispersion units, Np, for axial dispersion. For pore and sohd diffusion, q = / // p is a dimensionless radial coordinate, where / p is the radius of the particle, if a particle is bidisperse, then / p can be replaced by the radius of a suoparticle. For prehminary calculations. Fig. 16-13 can be used to estimate N for use with the LDF approximation when more than one resistance is important. 

Absorption rates of carbon dioxide were measured in organic solutions of glycidyl methacrylate at 101.3 kPa to obtain the reaction kinetics between carbon dioxide and glycidyl methacrylate using tricaprylylmethylammonium chloride(Aliquat 336) as catalysts. The reaction rate constants were estimated by the mass transfer mechanism accompanied by the pseudo-first-order fast reaction. An empirical correlation between the reaction rate constants and the solubility parameters of solvents, such as toluene, A-methyl-2-pirrolidinone, and dimethyl sulfoxide was presented. 

Mass transfer is a kinetic process, occurring in systems that are not at equilibrium. To understand mass transfer from a thermodynamic perspective, consider the isolated system shown in Figure 1. The system is bounded by an impermeable insulating wall which prevents the transfer of matter, heat, or mechanical energy between the system and the external environment. The system is subdivided into... 

Despite the importance of the ORR and long history of study, very little is known about the reaction mechanism.126,130,131 Mechanistic information has been derived almost exclusively from rotating disk electrode (RDE)131,132 and rotating ring disk electrode (RRDE)133-136,62,128 studies. The rotating electrode minimizes mass transfer effects and allows a kinetic current density to be extracted. In the RRDE setup, the ring surrounding the disk electrode detects species weakly adsorbed to the electrode that are ejected due to electrode rotation. The ORR reaction (eqn 4) is... 

The main goal of this chapter is to review the most widely used modeling techniques to analyze sorption/desorption data generated for environmental systems. Since the definition of sorption/desorption (i.e., a mass-transfer mechanism) process requires the determination of the rate at which equilibrium is approached, some important aspects of chemical kinetics and modeling of sorption/desorption mechanisms for solid phase systems are discussed. In addition, the background theory and experimental techniques for the different sorption/ desorption processes are considered. Estimations of transport parameters for organic pollutants from laboratory studies are also presented and evaluated. 

A gas-solid reaction usually involves heat and mass transfer processes and chemical kinetics. One important factor which complicates the analysis of these processes is the variations in the pore structure of the solid during the reaction. Increase or decrease of porosity during the reaction and variations in pore sizes would effect the diffusion resistance and also change the active surface area. These facts indicate that the real mechanism of gas-solid noncatalytic reactions can be understood better by following the variations in pore structure during the reaction. 

The preparative separations of certain polar (e.g., strongly basic) compounds and of many large molecular compotmds e.g., peptides and proteins) usually involve a complex mass transfer mechanism that is often slower than the mass transfer kinetics of small molecules. This slow kinetics influences strongly the band profiles and its mechanism must be accovmted for quantitatively. The accurate prediction of band profiles for optimization purposes requires a correct mathematical model of the various mass transfer processes involved. The piupose of the general rate model (GRM) is to accormt for the contributions of all the sources of mass transfer resistances to the band profiles [52,62,94,95]. The mass transfer of molecules from the bulk of the mobile phase percolating through the bed to the surface of an adsorbent or the mass of a permeable resin particle involves several steps that must be identified. 

As a basic in B LMs for the wastewater treatment the author presents two-phase partitioning bioreactors. He presents the main criteria which must be considered in the selection of the LM solvent biocompatibility (toxicity of the solvent to the microorganism), bioavailabihty (resistance of the solvent to biodegradation by the microorganism used), immiscibility in the aqueous phase, high solubility of pollutant in the solvent, favorable mass-transfer characteristics, etc. Biodegradation mechanisms and kinetics are discussed. Apphcations of bioreactors in wastewater treatment in laboratory, phot, and industrial scale are reviewed. Potential applications are considered also. 

Studies have been made on the mechanism of separation, process kinetics, mass transfer modehng, and engineering evaluation with metals hke copper, zinc, cadmium, cobalt, nickel, mercury, uranium, chromium, rhenium, and several others, including noble metals hke gold and silver, lanthanides and rare earths. Table 4.2 presents some examples. 

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