Dynamic pore structure

The simple pore structure shown in Figure 2.69 allows the use of some simplified models for mass transfer in the porous medium coupled with chemical reaction kinetics. An overview of corresponding modeling approaches is given in [194]. The reaction-diffusion dynamics inside a pore can be approximated by a one-dimensional equation... 

In MCFCs, which operate at relatively high temperature, no materials are known that wet-proof a porous structure against ingress by molten carbonates. Consequently, the technology used to obtain a stable three-phase interface in MCFC porous electrodes is different from that used in PAFCs. In the MCFC, the stable interface is achieved in the electrodes by carefully tailoring the pore structures of the electrodes and the electrolyte matrix (LiA102) so that the capillary forces establish a dynamic equilibrium in the different porous structures. Pigeaud et al. (4) provide a discussion of porous electrodes for MCFCs. 

This sol-gel procedure is an elaboration on well established entrapment methods [29], but with the added advantage of stability and better flow properties. Interestingly, none of the examples presented thus far demonstrate competitive behavior between multiple ligands (i.e. displacement) in the FAC analysis of trimethoprim and pyrimethamine a reversed order of elution based on is described, but this could simply be due to the shift towards an on-rate limited situation for higher affinity compounds, as described earlier. Erosion of dynamic competition between ligands could occur if the sol-gel allows convective mixing of the entrapped protein however the bimodal pore structure of these materials would... 

The most important property of adsorbent materials, the property that is decisive for the adsorbent s usage, is the pore structure. The total number of pores, their shape, and size determine the adsorption capacity and even the dynamic adsorption rate of the material. Generally, pores are divided into macro-, rneso- and micropores. According to IUPAC, pores are classified as shown in Table 2.2. 

Flexibility recent reports on the dynamic properties of PCPs show that they are much more flexible than generally believed. Dynamic pores can form a type of soft framework with bistability, whose two states oscillate back and forth between one of two counterparts. A system can exist in either of two states for different parameters of an external field. The structural rearrangement... 

V. M. Gun ko, D. Palijczuk, R. Leboda, J. Skubiszewska-Zifba, and S. Ziftek, Influence of Pore Structure and Pretreatments of Activated Carbons and Water Effects on Breakthrough Dynamics of tert-Butylbenzene, J. Colloid Interface Sci. 294(1) 53-68 (2006). 

The selectivity and deactivation processes in pore fractals such as the Sier-pinski gasket were simulated by Gavrilov and Sheintuch (1997) and Sheintuch (1999). Their studies investigated, e.g., the effect of the fractal pore structure on the selectivity of a system that incorporates two parallel reactions. Geometrical factors, which influence dynamic processes in a porous fractal solid media, were also investigated by Garza-Lopez and Kozak (1999). 

The available transport models are not reliable enough for porous material with a complex pore structure and broad pore size distribution. As a result the values of the model par ameters may depend on the operating conditions. Many authors believe that the value of the effective diffusivity D, as determined in a Wicke-Kallenbach steady-state experiment, need not be equal to the value which characterizes the diffusive flux under reaction conditions. It is generally assumed that transient experiments provide more relevant data. One of the arguments is that dead-end pores, which do not influence steady state transport but which contribute under reaction conditions, are accounted for in dynamic experiments. Experimental data confirming or rejecting this opinion are scarce and contradictory [2]. Nevertheless, transient experiments provide important supplementary information and they are definitely required for bidisperse porous material where diffusion in micro- and macropores is described separately with different effective diffusivities. 

A heterogeneous pore structure with varying aspect ratio would increase the frequency of breakup and coalescence, which should increase the observed mobile ganglia size distribution. However, the basic flow mechanism should remain unchanged. Also the relative importance of snap-off as a breakup mechanism would be increased relative to dynamic splitting. Here too a detailed study seems desirable. 

The complex pore structure of MCM-22 is also reflected in the unique three step uptake profile of bulky 2,2-dimethylbutane (DMB) observed in the dynamic sorption experiment [12], shown in Figure 4. Each step is attributed to adsorption into different sections of MCM-22, but specific assignment is ambiguous. 

The relevance of the second approach stems from the possibility to use the same pore-structure model as used in description of the process in question (counter-current (isobaric) diffusion of simple gases, permeation of simple gases under steady-state or dynamic conditions, combined diffusion and permeation of gases under dynamic conditions, etc.). 

The melting point of nitrobenzene in the pore is always depressed. The linear relationship between the shift in the pore melting temperature and the inverse pore diameter is consistent with the Gibbs-Thomson equation for larger pore sizes. The deviations from linearity, and hence from the Gibbs-Thomson equation are appreciable at pore widths as small as 4.0 nm. The quantitative estimates of the rotational relaxation times in the fluid and crystal phases of confined nitrobenzene support the existence of a contact layer with dynamic and structural properties different than the inner layers. The Landau free... 

A combination of characterization techniques for the pore structure of mesoporous membranes is presented. Equilibrium and dynamic methods have been performed for the characterisation of model membranes with well-defined structure while three-dimensional network models, combined with aspects from percolation theory can be employed to obtain structural information on the porous network topology as well as on the pore shape. Furthermore, the application of ceramic membranes in separations of condensable from noncondensable vapors is explored both theoretically and experimentally. 

The evaluation of the commercial potential of ceramic porous membranes requires improved characterization of the membrane microstructure and a better understanding of the relationship between the microstructural characteristics of the membranes and the mechanisms of separation. To this end, a combination of characterization techniques should be used to obtain the best possible assessment of the pore structure and provide an input for the development of reliable models predicting the optimum conditions for maximum permeability and selectivity. The most established methods of obtaining structural information are based on the interaction of the porous material with fluids, in the static mode (vapor sorption, mercury penetration) or the dynamic mode (fluid flow measurements through the porous membrane). 

Dynamic methods rely on the study of fluid flow properties of porous membranes, which are extremely sensitive funetions of the pore structural characteristics like the pore size distribution, f(r) and the pore connectivity, z. The resulting data, if analyzed in combination with other measurements obtained by equilibrium methods, can offer important structural information, regarding the membranes performance evaluation. 

Knowledge of the properties of the texture and pore structure of solid materials is highly important for the development of catalytic materials and catalysts. It should, however, be mentioned that the assessment of the dynamic changes of the material under reaction conditions may be even more important than the assessment under pristine conditions, as the changes in material during use will determine its useful lifetime. Note also that physisorption markedly influences specific properties of porous solids, such as acid strength and catalytic activity which are crucial material parameters for sorption and catalysis. 

The critical moisture content depends upon the ease of moisture movement through the solid, and hence, upon the pore structure of the solid, sample thickness and (hying rate. Segment BC is the constant-rate peri(xl. During this pericxl, the drying is controlled simultaneously by heat and mass transfer applied to a liquid-gas interface in dynamic equilibrium with a bulk gas phase. 

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