Kinetic-based separations

It is important to note that the kinetics of lanthanide complexation reactions in general involve rapid association and dissociation reactions, except for structurally complex ligands like edta. Generally, lanthanide complexation kinetics in aqueous media can be considered sufficiently rapid as to have minimal effect on separations. Phase-transfer rates may be important in some systems, and should be considered in the optimization of an analytical separation procedure. The kinetics of lanthanide complexation reactions has been discussed in a previous report (Nash and Sulhvan 1991). There has been some consideration of kinetics-based separations for f-elements (Nash 1994, Merciny et al. 1986), but no useful analytical applications based solely on differences in lanthanide kinetics are known. 

The separation process in both ion exchange and solvent extraction consists, in its most elementary form, of the transfer of a (typically) charged metal ion (or complex), from a polar aqueous phase to an immiscible phase (with different solvating properties) with concomitant charged naturalization. The effectiveness of any separation process is a function of the ability of the reactions to accompUsh phase transfer (because without phase transfer, there can be no separation), and the relative affinity of the counterphase for the species to be separated. In the case of the trivalent lanthanides and actinides, the latter aspect must exploit the slight differences in ionic radii and covalency of the metal ions. It is conceivable that differences in rate of reaction could be utilized, but such data are much more difficult to obtain, and few examples of kinetic-based separations are extant. 

The factors contributing to a successful separation procedure are not different from those relevant in more acidic solutions, i.e., the efficiency is dependent on the nature of the extractant and the aqueous complexant, the pH of the aqueous solution, and the strength of aqueous complexes. Two advantages provided by this approach are the relative ease of back extraction (contact with neutral salt solutions in usually sufficient), and the potential for kinetic-based separations. The disadvantage (from a practical standpoint) is the requirement of working with concentrated salt solutions, which create waste disposal problems. 

Whereas Hquid separation method selection is clearly biased toward simple distillation, no such dominant method exists for gas separation. Several methods can often compete favorably. Moreover, the appropriateness of a given method depends to a large extent on specific process requirements, such as the quantity and extent of the desired separation. The situation contrasts markedly with Hquid mixtures in which the appHcabiHty of the predominant distiHation-based separation methods is relatively insensitive to scale or purity requirements. The lack of convenient problem representation techniques is another complication. Many of the gas—vapor separation methods ate kinetically controUed and do not lend themselves to graphical-phase equiHbrium representations. In addition, many of these methods require the use of some type of mass separation agent and performance varies widely depending on the particular MSA chosen. 

Liquid clathrates offer a great advantage over solid-state separations (e.g. by formation of Hoffman-type inclusion compounds, Section 9.4) because of the extremely fast mixing kinetics, the avoidance of the need to wait for crystallisation to occur and the easy separation of the two liquid phases. It should also prove possible to run liquid clathrate separations in a continuous extraction manner. The avalues of a number of liquid clathrate-based separations have been reported and are summarised in Table 13.1. 

For example, when we consider the design of specialty chemical, polymer, biological, electronic materials, etc. processes, the separation units are usually described by transport-limited models, rather than the thermodynamically limited models encountered in petrochemical processes (flash drums, plate distillations, plate absorbers, extractions, etc.). Thus, from a design perspective, we need to estimate vapor-liquid-solid equilibria, as well as transport coefficients. Similarly, we need to estimate reaction kinetic models for all kinds of reactors, for example, chemical, polymer, biological, and electronic materials reactors, as well as crystallization kinetics, based on the molecular structures of the components present. Furthermore, it will be necessary to estimate constitutive equations for the complex materials we will encounter in new processes. 

Branched hydrocarbons are preferred to linear hydrocarbons as ingredients in petrol because they enhance the fuel octane number. By catalytic isomerisation linear hydrocarbons are converted into mono and di branched hydrocarbons, and it becomes necessaiy to separate the mixture. A variety of zeolites may be used for this purpose, either on the basis of sorption thermodynamics or on the basis of sorption kinetics. Such data are relevant to the development of sorption based separation methods, but also they provide key information regarding the catalytic isomerisation over zeolites themselves. 

In a kinetically controlled separation system using CMS or zeolite 4A as adsorbents, it is necessary to use more accurate rate model. Therefore, concentration dependent diffusivity model based on Darken equation combined with Langmuir-Freundlich isotherm was applied and each result was compared with the experimental data. 

The kinetic graph shown in Fig. 1.1 corresponds to mechanism (41 based on the isomorphism between the reaction mechanism and the graph. Here vertex 1 corresponds to the free site Z on the catalyst surface, vertex 2 corresponds to the ISC cis-1,2-ChH,6 j Z, and vertex 3 corresponds to the ISC trans-1, 2-CaH,6-. Z. All steps in the mechanism (4) are reversible. For every elementary reaction of the mechanism in the kinetic graph separate arcs appear and for every elementary step in the reaction graph two arcs with the opposite directions appear. 

The importance of these kinetic phase-separation effects will depend strongly on the relative concentrations of the components, of course, since the kinetics of every elementary step leading to gel formation are concentration dependent. From the point of view of creating functional materials that require site accessibility, the base-catalyzed situation is preferable. Unfortunately for those apphcations that rely on optical properties as well as site accessibihty (such as fabrication of optically based sensors), the use of base catalysis typically affords opaque materials. For catalysis and separations applications, however, the optical properties are of little importance (except for characterization of the... 

With that said, it was traditionally believed that residue curve equation (and the resulting maps) were only suitable for equilibrium-based separations and could not be used for the representation of kinetically based processes [3]. However, the differential equations which describe a residue curve are merely a combination of mass balance equations. Because of this, the inherent nature of residue curves is such that they can be used for equilibrium- as well as nonequilibrium-based processes. 

Kinetic resolution Separation of enantiomers based on their unequal rates of reaction with a chiral reactant. 

Hecht et al. have studied the reforming kinetics based on an elementary reaction mechanism, using experiments tailored to mimic a typical fuel cell operation [64]. In their separated anode experiments, two flow channels were separated by the anode of an SOFC. One of the two channels was fed with fuel and the other was fed with H2O or CO2 diluted in Argon. The experiment was designed in a way that permitted the transport of species between the two channels through the reactive porous media. In all cases the mechanism used by Hecht et al. reasonably well reproduces the experimental observations. 

Figure 9.14 illustrates another use of the Safe Zone construct and the concept of a threshold power. In this assessment, simulations were performed for two otherwise identical 2.6-Ah 18650 cells having different anode materials one with MCMB 2528 (25 pm mean diameter, low surface area), and one with MCMB 628 (6 pm mean diameter, high surface area). The surface heat transfer coefficient was assumed to be 11 W/m -K. The FEA model for this particular set of simulations also accounted for the kinetics of separator shutdown. In this simulation, the short, rather than being modeled as a constant power source, is represented as a constant 25-mf2 resistance in series with the cell s 50-mQ equivalent series resistance. The total power dissipated in the cell due to the short (both i R heating in the short and Joule heating in the jelly roll) was calculated as 220 W initially. The total power dissipated in the short was then set to decrease with time based on a function that... 

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