Mean pore diameter

Let us now assemble the complete set of dimensionless parameters for the problem. These are set out in Table 11.1, where the last column indicates the nature of their dependence on the external pressure p, the mean pore diameter and the pellet radius a. Symbols ft and 0... 

Usually the plaques produced by either method are coined (compressed) in those areas where subsequent welded tabs are coimected or where no active material is desired, eg, at the edges. The uncoined areas usually have a Bmnauer-Emmet-TeUer (BET) area in the range of 0.25—0.5 m /g and a pore volume >80%. The pores of the sintered plaque must be of suitable size and intercoimected. The mean pore diameter for good electrochemical efficiency is 6—12 p.m, deterrnined by the mercury-intmsion method. 

Typical pore size distributions result in mean pore diameters of around 15 //m. Even long and intensive efforts did not succeed in decreasing this value decisively in order to enable production of micropo-rous pocketing material resistant to penetration [65, 66], In practice PVC separators prove themselves in starter batteries in climatically warmer areas, where the battery life is however noticeably reduced because of increased corrosion rates at elevated temperature and vibration due to the road condition. The failure modes are similar for all leaf separator versions shedding of positive active mass fills the mud room at the bottom of the container and leads to bottom shorts there, unless — which is the normal case — the grids of the positive electrodes are totally corroded beforehand. 

Concentration of gel1, % Soil porosity, % Total Micro/total Mean pore diameter, pm Hydraulic conductivity, mday 1 Avialable moisture1, % Rate of evaporation 2, arb. units Transpiration ratio3, g g-1 Water use efficiency4, g kg-1... 

In which the ratio m/n is close to 3. The silane was produced by free radical copolymerization of vinyltriethoxysilane with N-vinylpyrrolidone. Its number-average molecular weight evaluated by vapour-phase osmometry was 3500. Porous silica microballs with a mean pore diameter of 225 A, a specific surface area (Ssp) of 130 m2/g and a pore volume of 0.8 cm3/g were modified by the silane dissolved in dry toluene. After washings and drying, 0.55% by weight of nitrogen and 4.65% of carbon remained on the microballs. Chromatographic tests carried out with a series of proteins have proved the size-exclusion mechanism of their separation. 

The column used in the separation depicted in figure 1 was 25 cm long and 6.2 mm in diameter packed with silica gel having a mean pore diameter of 100 A and a particle diameter of 5 jum. Thus, the column would have an HETP of approximately 0.001 cm (twice the particle diameter). Consequently, a column 25 cm long would have an 25... 

Figure 1. Influence of T1O2 content of LT-aerogels on relative proportion of Si-O-Ti connectivities R = [Si-0-Ti]/(Si-0-Si], mean pore diameter, and initial rate (to) of a-iso-phorone ep>oxidation with t-butyl hydroperoxide at 60 C. Data taken from ref. [18].

Aluminum oxides, similar to silica gels, are available as bulk materials and as precoated plates, to be used not only for straight phase adsorption chromatography, but also for partition PLC (see Table 3.3 and Table 3.4). In particular, the aluminum oxide type 150 (i.e., mean pore diameter 150 A [15 tun]) is suitable for partition chromatographic purposes. 

The silica gel matrix for the RP-18 PLC precoated plates consists either of a type 60 or 150 (indicating the respective mean pore diameter). The manufacturers and the properties of the PLC plates RP-18 available at present are listed in Table 3.8. 

Silica gels with mean pore diameters of 5-15 nm and surface areas of 150-600 m /g have been preferred for the separation of low molecular weight samples, while silica gels with pore diameters greater than 30 nm are preferred for the separation, of biopolymers to avoid restricting the accessibility of the solutes to the stationary phase [15,16,29,34]. Ideally, the pore size distribution should be narrow and symmetrical about the mean value. Micropores are particularly undesirable as they may give rise to size-exclusion effects or irreversible adsorption due to... 

Designation Mean Pore Diameter (nm) Pore Volume (ml/g) Specific Surface Area (m /g)... 

The mean pore diameter and the pore volume decrease respectively from 67 A and 0.46 cm7g for calcined SBA-15 to 56 A and 0.32 cm3/g for GFP/SBA-15. A decrease of SSA was also observed from 648 m2/g for SBA-15 to 456 m2/g for the hybrid material. These values suggest that GFP molecules can be embedded within the SBA-15 pores/channels and not simply adsorbed on the external surface of the mesoporous silica. 

Figure 28a shows the result of SAXS on sample BrlOOO. We used Guinier s formula (see eq. 6) for the small angle scattering intensity, I(k), from randomly located voids with radius of gyration, Rg. Although Guinier s equation assumes a random distribution of pores with a homogeneous pore size, it fits our experimental data well. The slope of the solid line in Fig. 28b gives R - 5.5 A and this value has been used for the calculated curve in Fig. 28a. This suggests a relatively narrow pore-size distribution with an equivalent spherical pore diameter of about 14A. Similar results were found for the other heated resin samples, except that the mean pore diameter changed from about 12 A for samples made at 700°C to about 15 A for samples made at 1100°C.

Sample Method of preparation Ti loading, calcined samples (wt%) Specific surface area (m2/g) Total pore volume (mL/g) Mean pore diameter (nm)... 

By carefully choosing the starting material and the activating process, it has been possible in recent years to generate in carbon a pore system with a narrow span of pore sizes. With a mean pore diameter of perhaps 0.6 nm, such products are known as carbon molecular sieves. 

When a silicate solution such as sodium silicate is acidified, a gel of polymeric colloidal silicilic acid is formed as an agglomerate of micro-particles. When the gel is heated, water is expelled leaving a hard, glassy structure with voids between the micro-particles equivalent to a mean pore diameter of about 3 nm and an internal surface of about 500,000 m2/kg. As discussed by Everett and Stone(8) these properties may be varied by controlling the pH of the solution from which the gel is precipitated. 

This transition produces an isomorphous phase and the resulting y-alumina has the same morphology and texture as its boehmite precursor. With increasing temperature and time the mean pore diameter increases gradually and other phases appear (S-, 6-alumina). Due to the broad XRD lines, the distinction between y- and S-alumina cannot be made 6-alumina occurs at about 900°C while the conversion to the chemically very stable a-alumina phase takes place at T> 1000°C. Some typical results for alumina membranes synthesized without binders are given in Table 2.4. When PVA was used as a binder, thermogravimetric analysis showed that, provided the appropriate binder type was used, the binder could be effectively removed at T > 400°C. The ash residue is of the order of 0.01 wt.%. Mean pore size and... 

Characteristic microstructural properties of TiOj membranes produced in this way are given in Table 2.5. Mean pore diameters of 4-5 nm were obtained after heat treatment at T < 500°C. The pore size distribution was narrow in this case and the particle size in the membrane layer was about 5 nm. Anderson et al. (1988) discuss sol/gel chemistry and the formation of nonsupported titania membranes using the colloidal suspension synthesis of the type mentioned above. The particle size in the colloidal dispersion increased with the H/Ti ratio from 80 nm (H /Ti = 0.4, minimum gelling volume) to 140 nm (H /Ti " — 1.0). The membranes, thus prepared, had microstructural characteristics similar to those reported in Table 2.5 and are composed mainly of 20 nm anatase particles. Considerable problems were encountered in membrane synthesis with the polymeric gel route. Anderson et al. (1988) report that clear polymeric sols without precipitates could be produced using initial water concentrations up to 16 mole per mole Ti. Transparent gels could be obtained only when the molar ratio of H2O to Ti is < 4. Gels with up to 12 wt.% T1O2 could be produced provided a low pH is used (H /Ti + < 0.025). 

Kameyama, Fukuda and Dokiya (1980, 1981) reported separation experiments of H2-H2S mixtures with porous glass (mean pore diameter 4.5 nm) and an alumina membrane (mean pore diameter 100 nm) up to temperatures of 800°C. They used the separation of H2 from the mixture to shift the equilibrium in the decomposition reaction of H2S. H2S, however, diffused partly along the surface, especially at the lower temperature range, thereby decreasing the separation efficiency. No attempt was made to describe the transport. Shindo et al. (1983, 1984) reported data on the separation of He-C02 mixtures by means of a porous glass membrane over the temperature range 296-947 K. A theoretical analysis was made, studying the... 

In this last section some recent developments are mentioned in relation to gas separations with inorganic membranes. In porous membranes, the trend is towards smaller pores in order to obtain better selectivities. Lee and Khang (1987) made microporous, hollow silicon-based fibers. The selectivity for Hj over Nj was 5 at room temperature and low pressures, with permeability being 2.6 x 10 Barrer. Hammel et al. 1987 also produced silica-rich fibers with mean pore diameter 0.5-3.0nm (see Chapter 2). The selectivity for helium over methane was excellent (500-1000), but permeabilities were low (of the order of 1-10 Barrer). 

Porous Vycor glass tube (in double pipe configuration), wall thickness 3 mm, length 600 mm, outer diameter 15 mm, mean pore diameter 45 A. Feed enters the reactor at shell side, permeate at tube side. ... 

Inner layer. M0S2 film, thickness 100 m, mean pore diameter 0.5 (itn. 

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