Transport processes pore mechanism

It is now recognised that a wide range of organic molecules, collectively termed ionophores 185,186) or complexones 187), are able to facilitate ion (usually cation) transport. Two major mechanisms have been revealed for this process, namely the involvement of transmembrane ion carriers and transmembrane pores or channels (see Fig. 19). The majority of ionophores studied to date are natural antibiotics and their synthetic analogues which are, on a biological scale, comparatively small molecules lending themselves to study outside the biological system. In contrast far less is known about the molecular structures involved in normal transport processes. Such molecules are likely to be more complex or present in small amounts and may require... 

We have used CO oxidation on Pt to illustrate the evolution of models applied to interpret critical effects in catalytic oxidation reactions. All the above models use concepts concerning the complex detailed mechanism. But, as has been shown previously, critical. effects in oxidation reactions were studied as early as the 1930s. For their interpretation primary attention is paid to the interaction of kinetic dependences with the heat-and-mass transfer law [146], It is likely that in these cases there is still more variety in dynamic behaviour than when we deal with purely kinetic factors. A theory for the non-isothermal continuous stirred tank reactor for first-order reactions was suggested in refs. 152-155. The dynamics of CO oxidation in non-isothermal, in particular adiabatic, reactors has been studied [77-80, 155]. A sufficiently complex dynamic behaviour is also observed in isothermal reactors for CO oxidation by taking into account the diffusion both in pores [71, 147-149] and on the surfaces of catalyst [201, 202]. The simplest model accounting for the combination of kinetic and transport processes is an isothermal continuously stirred tank reactor (CSTR). It was Matsuura and Kato [157] who first showed that if the kinetic curve has a maximum peak (this curve is also obtained for CO oxidation [158]), then the isothermal CSTR can have several steady states (see also ref. 203). Recently several authors [3, 76, 118, 156, 159, 160] have applied CSTR models corresponding to the detailed mechanism of catalytic reactions. 

Many factors including partition characteristics, degree of ionization, molecular size etc. influence the transport of drugs across biological membranes. Permeation of intact mucosa may also involve passive diffusion, intercellular movement, transport through pores or other mechanisms. The objective of the studies reported here was to employ the dog model to investigate these factors in a systematic and experimentally well-controlled fashion. The non-steriodal anti-inflammatory drug, diclofenac sodium, was selected as a test compound in this evaluation process. 

The slow process of diffusion is insufficient to transport many needed molecules across cellular membranes, so cells have evolved a variety of mechanisms for speeding up diffusion. This process, called facilitated transport, includes pore-facilitated transport and carrier-facilitated transport. It is important to note, however, that though pores or carriers speed the diffusion process, the driving force for each process is still diffusion, with all of its built-in limitations. 

Shaped catalyst bodies with optimized geometries (e.g., wagonwheels, honeycombs) offer lower resistance to gas flow and lower the pressure loss in reactors. The mechanical and thermal stabihty of catalysts and supports is being improved. New support materials such as magnesite, silicon carbide, and zircon (ZrSi04) ceramics with modified pore structures offer new possibilities. Meso- and macropores can be incorporated into solids to accelerate transport processes, and the question of porosity will increasingly be the subject of interest. 

This article focuses on transport that proceeds by the solution-diffusion mechanism. Transport by this mechanism requires that the penetrant sorb into the polymer at a high activity interface, diffuse through the poljuner, and then desorb at a low activity interface. In contrast, the pore-flow mechanism transports penetrants hy convective flow through porous pol5uners and will not be described in this article. Detailed models exist for the solution and diffusion processes of the solution-diffusion mechanism. The differences in the sorption and transport properties of rubbery and glassy pol5uners are reviewed and discussed in terms of the fundamental differences between the intrinsic characteristics of these two types of polymers. 

Multiscale Modeling, Fig. 2 Typical relevant scales for the simulation of an electrochemical power generator elementary reactirais and transport processes at the atom-istic/molecular level (nanoscale), electrochemical interfaces (e.g., polymer + water/catalyst) at the microscale, transport processes (e.g., in the electrode pores) at the mesoscale, and mechanical/thermal stresses at the device level (macroscale)... 

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