Definition pores

Asci inoperculate, opening by an irregular rupture or definite pore at the tip—.Order Geoglossales (Helotiales)... 

Abstract Makrolon , a commercially available polycarbonate with a glassy ultramicroporous structure (mean pore-volume 0.1 nm3), was often employed as sensitive layer for optical sensors in recent years. Due to the definite pore volume-distribution, it can be used as a size-selective sensitive layer. The interaction behaviour of Makrolon of different layer-thicknesses under the influence of methanol, ethanol and 1-propanol is characterized by Spectral El-lipsometry (SE), Surface Plasmon Resonance Spectroscopy (SPR)... 

The qualitative correspondence between the experimentally obtained foam dispersity vs. foam drainage rate and that of Eq. (5.60) can be seen in Fig. 5.19 [21]. The dispersity change was achieved by blowing air through sintered glass filters of definite pore radii. 

Finally, Asaeda and co-workers [52,53,64] reported separation results using membranes which are modified in such a way that pore sizes below the mesopore range (<2 nm) are obtained no definitive pore characteristics are given however. 

An important type of porous carbons is activated carbons. Granular activated carbons are prepared from different precursors and used in a wide range of industries. Their preparation, structure and applications were reviewed in different books and reviews [3,4,71-75]. In Table 8, some properties of different adsorbents (activated carbons, silica gel, alumina gel and zeolite) are compared with each other. High BET surface area and light weight are the main advantages of activated carbons. Usually activated carbons have a wide ran of pore sizes from micropores to macropores, which shows a marked contrast to the definite pore size of zeolites. 

Avery and Ramsay (129) prepared Si02 (also ZrOa) particles about 4 nm in diameter as a voluminous powder by a vapor condensation method and compacted this in stages up to 100 tons in. . In the original Duffy powder it was not possible to determine any definite pores but at the point beyond where a coherent body was obtained, the approximate relation between pressure and the coordination number of silica particles was as follows ... 

Depending on the final purpose of the material, the appropriate pillar can be chosen. This flexibifity in the PILC synthesis is one of the main advantages compared to other porous substrates, such as zeofites, which have one definite pore size. The technique not only focuses on clays, but other layered structures serve as host materials as well. Examples are layered double hydroxides (anionic clays), metal(IV)- phosphates and phosphonates,layered silicic acids, etc. [4,5]. 

Some porous materials may be reoccupied by the cells and invaded by a newly formed bone tissue. These materials should have at the beginning a definite pore size. Very large pores do not encoitrage bone growth, while the very small ones do not allow an adeqirate vascirlarization. The pore diameter should be a minimum of 100 pm, but coitld go up to 500 pm (see below). Moreover, tissue rehabitation requires intercormected pores. 

Adsorbents such as some silica gels and types of carbons and zeolites have pores of the order of molecular dimensions, that is, from several up to 10-15 A in diameter. Adsorption in such pores is not readily treated as a capillary condensation phenomenon—in fact, there is typically no hysteresis loop. What happens physically is that as multilayer adsorption develops, the pore becomes filled by a meeting of the adsorbed films from opposing walls. Pores showing this type of adsorption behavior have come to be called micropores—a conventional definition is that micropore diameters are of width not exceeding 20 A (larger pores are called mesopores), see Ref. 221a. 

Hugo s approach can be extended without difficulty to apply throughout the whole range of pore sizes, but to accomplish this a specific and complete flux model must be used. To be definite we will assume that the dusty gas model is adequate, but the same reasoning could be applied to certain other models if necessary. The relevant flux relations are now equations (5.4). Applied to the radial flux components In one of our three simple geometries they take the form... 

In order to maintain a definite contact area, soHd supports for the solvent membrane can be introduced (85). Those typically consist of hydrophobic polymeric films having pore sizes between 0.02 and 1 p.m. Figure 9c illustrates a hoUow fiber membrane where the feed solution flows around the fiber, the solvent—extractant phase is supported on the fiber wall, and the strip solution flows within the fiber. Supported membranes can also be used in conventional extraction where the supported phase is continuously fed and removed. This technique is known as dispersion-free solvent extraction (86,87). The level of research interest in membrane extraction is reflected by the fact that the 1990 International Solvent Extraction Conference (20) featured over 50 papers on this area, mainly as appHed to metals extraction. Pilot-scale studies of treatment of metal waste streams by Hquid membrane extraction have been reported (88). The developments in membrane technology have been reviewed (89). Despite the research interest and potential, membranes have yet to be appHed at an industrial production scale (90). 

A fundamental difference exists between the assumptions of the homogeneous and porous membrane models. For the homogeneous models, it is assumed that the membrane is nonporous, that is, transport takes place between the interstitial spaces of the polymer chains or polymer nodules, usually by diffusion. For the porous models, it is assumed that transport takes place through pores that mn the length of the membrane barrier layer. As a result, transport can occur by both diffusion and convection through the pores. Whereas both conceptual models have had some success in predicting RO separations, the question of whether an RO membrane is truly homogeneous, ie, has no pores, or is porous, is still a point of debate. No available technique can definitively answer this question. Two models, one nonporous and diffusion-based, the other pore-based, are discussed herein. 

Figure 10 shows that Tj is a unique function of the Thiele modulus. When the modulus ( ) is small (- SdSl), the effectiveness factor is unity, which means that there is no effect of mass transport on the rate of the catalytic reaction. When ( ) is greater than about 1, the effectiveness factor is less than unity and the reaction rate is influenced by mass transport in the pores. When the modulus is large (- 10), the effectiveness factor is inversely proportional to the modulus, and the reaction rate (eq. 19) is proportional to k ( ), which, from the definition of ( ), implies that the rate and the observed reaction rate constant are proportional to (1 /R)(f9This result shows that both the rate constant, ie, a measure of the intrinsic activity of the catalyst, and the effective diffusion coefficient, ie, a measure of the resistance to transport of the reactant offered by the pore stmcture, influence the rate. It is not appropriate to say that the reaction is diffusion controlled it depends on both the diffusion and the chemical kinetics. In contrast, as shown by equation 3, a reaction in solution can be diffusion controlled, depending on D but not on k. 

We may begin by describing any porous medium as a solid matter containing many holes or pores, which collectively constitute an array of tortuous passages. Refer to Figure 1 for an example. The number of holes or pores is sufficiently great that a volume average is needed to estimate pertinent properties. Pores that occupy a definite fraction of the bulk volume constitute a complex network of voids. The maimer in which holes or pores are embedded, the extent of their interconnection, and their location, size and shape characterize the porous medium. 

FIG. 13 Average number of hydrogen bonds (for definition see text) as a function of p in five simulations at different levels of hydration in a Vycor pore. Full hues show the number of water-water bonds, long-dashed hnes show the number of bonds between water molecules and Vycor, and short-dashed lines denote the sum of the two. From top to bottom, the frames correspond to a water content of about 96, 74, 55, 37, and 19% of the maximum possible (corresponding to 2600, 2000,1500, 1000, and 500 water molecules in a cylindrical cavity of about 4nm diameter and 7.13 nm length). (From Ref. 24.)... 

The definitions above are an abbreviated version of those used in a veiy complex and financially significant exercise with the ultimate goal of estimating resei ves and generating production forecasts in the petroleum industry. Deterministic estimates are derived largely from pore volume calculations to determine volumes of either oil nr gas in-place (OIP, GIP). This volume when multiplied by a recovery factor gives a recoverable quantity of oil or natural gas liquids—commonly oil in standard barrels or natural gas in standard cubic feet at surface conditions. Many prefer to use barrels of oil equivalency (BOE) or total hydrocarbons tor the sum of natural gas, natural gas liquids (NGL), and oil. For comparison purposes 6,000 cubic feet of gas is considered to be equivalent to one standard barrel on a British thermal unit (Btu) basis (42 U.S. gallons). 

Permeability is defined as a measure of a rock s ability to transmit fluids. In addition to a rock s being porous, sedimentary rock can also be permeable. Permeability refers to the property of a rock that allows fluids to flow through its pore network at practical rates under reasonable pressure differentials. The quantitative definition of permeability was first given in an empirical relationship developed by the French hydrologist Henry D Arcy who studied the flow of water through unconsolidated sands [31]. 

As to the particular way in which solvent manages to creep through the membrane, to the exclusion of solute, nothing definite can be said. We might assume that the action of the membrane is of the character of a sieve, the smaller solvent molecules getting through its pores whilst the larger... 

As surface area and pore structure are properties of key importance for any catalyst or support material, we will first describe how these properties can be measured. First, it is useful to draw a clear borderline between roughness and porosity. If most features on a surface are deeper than they are wide, then we call the surface porous (Fig. 5.16). Although it is convenient to think about pores in terms of hollow cylinders, one should realize that pores may have all kinds of shapes. The pore system of zeolites consists of microporous channels and cages, whereas the pores of a silica gel support are formed by the interstices between spheres. Alumina and carbon black, on the other hand, have platelet structures, resulting in slit-shaped pores. All support materials may contain micro, meso and macropores (see text box for definitions). 

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