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Capillary pore structure

All current MF membranes may be classified as either "tortuous-pore" or "capillary-pore" membranes (see Figure 2.1). The "capillary-pore" structure is distinguished by its straight-through cylindrical capillaries, whereas the "tortu-ous-pore" structure resembles a sponge with a network of interconnecting tortuous pores. [Pg.62]

Further, a new alumina capillary-pore membrane has just been introduced by Anotec Separations Ltd. The capillary-pore structure appears to result from controlled growth of alumina crystals. [Pg.66]

Hysteresis, which is invariably present, adds to the complications its interpretation is, if anything more complex than with capillary condensation, inasmuch as it can depend not only on the pore structure of the solid but also on the magnitude of the applied pressure. [Pg.190]

A microscopic description characterizes the structure of the pores. The objective of a pore-structure analysis is to provide a description that relates to the macroscopic or bulk flow properties. The major bulk properties that need to be correlated with pore description or characterization are the four basic parameters porosity, permeability, tortuosity and connectivity. In studying different samples of the same medium, it becomes apparent that the number of pore sizes, shapes, orientations and interconnections are enormous. Due to this complexity, pore-structure description is most often a statistical distribution of apparent pore sizes. This distribution is apparent because to convert measurements to pore sizes one must resort to models that provide average or model pore sizes. A common approach to defining a characteristic pore size distribution is to model the porous medium as a bundle of straight cylindrical or rectangular capillaries (refer to Figure 2). The diameters of the model capillaries are defined on the basis of a convenient distribution function. [Pg.65]

Coimectivity is a term that describes the arrangement and number of pore coimections. For monosize pores, coimectivity is the average number of pores per junction. The term represents a macroscopic measure of the number of pores at a junction. Connectivity correlates with permeability, but caimot be used alone to predict permeability except in certain limiting cases. Difficulties in conceptual simplifications result from replacing the real porous medium with macroscopic parameters that are averages and that relate to some idealized model of the medium. Tortuosity and connectivity are different features of the pore structure and are useful to interpret macroscopic flow properties, such as permeability, capillary pressure and dispersion. [Pg.69]

The following brief account is concerned with factors that affect the acces-sibihty of the OH groups of cellulose, since this is the determining factor for its dissolution, hence subsequent derivatization. Electron microscopy. X-ray scattering and porosimetry of cellulose fibers have clearly shown the presence of non-uniform pores, capillaries, voids and interstices in the fiber surface [25]. Consequently, the total surface area of cellulose fibers exceeds by far the geometrical outer surface. Pore structure determines the internal... [Pg.109]

Water Uptake. There is evidence to suggest that water uptake caused by capillary forces is the crucial factor in the disintegration process of many formulations. In such systems the pore structure of the tablet is of prime importance and any inherent hydrophobicity of the tablet mass will adversely affect it. Therefore, disintegrants in this group must be able to maintain a porous structure in the compressed tablet and show a low interfacial tension towards aqueous fluids. Rapid penetration by water throughout the entire tablet matrix to facilitate its breakup is thus achieved. Concentrations of disintegrant that ensure a continuous matrix of disintegrant are desirable and levels of between 5 and 20% are common. [Pg.302]

Vapor sorption onto porous solids differs from vapor uptake onto the surfaces of flat materials in that a vapor (in the case of interest, water) will condense to a liquid in a pore structure at a vapor pressure, Pt, below the vapor pressure, P°, where condensation occurs on flat surfaces. This is generally attributed to the increased attractive forces between adsorbate molecules that occur as surfaces become highly curved, such as in a pore or capillary. This phenomenon is referred to as capillary condensation and is described by the Kelvin equation [19] ... [Pg.394]

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. [Pg.22]

Determination of Pore Size Distributions. The shape and range of a GPC calibration curve are, in part, a reflection of the pore size distribution (PSD) of the column packing material. A consideration of the nature of PSDs for the ULTRASTYRAGEL columns to be used in this work is therefore appropriate. The classical techniques for the measurement of PSDs are mercury porisimetry and capillary condensation. The equipment required to perform these measurements is expensive to own and maintain and the experiments are tedious. In addition, it is not clear that these methods can be effectively applied to swellable gels such as the styrene-divinylbenzene copolymer used in ULTRASTYRAGEL columns. Both of the classical techniques are applied to dry solids, but a significant portion of the pore structure of the gel is collapsed in this state. For this reason, it would be desirable to find a way to determine the PSD from measurements taken on gels in the swollen state in which they are normally used, e.g. a conventional packed GPC column. [Pg.172]

Crossflow technology is increasing, as it proves practical. Micioliltration membranes are of an isotropic and homogeneous morphology, i.e., the pore structure is consistent throughout. There is some movement, however, toward ihe use of "skinned" anisotropic membranes. Microliltration membranes are available in a wide variety ol polymers, including some that arc quite chemically inert. They also tire available as tubular, hollow fiber, or capillary fiber elements. [Pg.977]

It is necessary to point out a difficulty with regard to the integration of the flux equations in a real membrane. If, for example, membranes with a pore structure are concerned, the final result which one calculates for a complicated network of capillaries which run in all directions and which are interconnected is different from what is calculated for the model which only contains pores which run perpendicularly to the membrane surface, but when proceeding from the local parameters (e.g. Oik s or diffusion coefficients) to the integral ones, an extra parameter occurs in the resulting expressions, which accounts for the nature of the pore structure (tortuosity factor). [Pg.317]

Dullien (1979) considered the rate of capillary rise of a fluid in a model three-dimensional network pore structure consisting of a repeating pore element with step changes in diameter. The effective diameter, /)clT, model is given by ... [Pg.135]


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