Particle size internal surface

We see that at small particle sizes internal diffusion is no longer the sir step and that the surface reaction sequence of adsorption, surface reaction, a desorption Steps 3. 4, and 5) limit the overall rate of reaction. Consider nc one more point about internal diffusion and surface reaction. These steps through 6) are not at all affected by flow conditions external to the pellet. 

Particles consist of both internal and external surface area. The external surface area represents that caused by exterior topography, whereas the internal surface area measures that caused by microcracks, capillaries, and closed voids inside the particles. Since the chosen surface area technique should relate to the ultimate use of the data, not all techniques are useful for fine powders. The commonly used approaches are permeametry and gas adsorption according to the Brunauer, Emmet, and Teller (BET) equation [9]. Because of simplicity of operation and speed of operation, permeametry methods have received much attention. The permeametry apparatus consists of a chamber for placing the material to be measured and a device to force fluid to flow through the powder bed. The pressure drop and rate of flow across the powder bed are measured and related to an average particle size and surface area. Especially for porous powders, permeametry data include some internal surface area, thus decreasing their value. 

M. Bisrat and C. Nystrom, Physicochemical aspects of drug release. Vin. The relation between particle size and surface specific dissolution rate in agitated suspensions. International Journal of Pharmaceutics, 47(1-3), 223-231 (1988). 

White et al. [ 109] report that crystalline structure has little effect on the initiation of EMs by a shock wave, van der Heijden et al. [110], using the example of RDX, HMX and HNIW, state that the following crystal parameters play a role in determining the sensitivity towards a shock stimulus (a) internal product quality, (b) mean particle size, (c) surface smoothness/shape of the explosive particle. Like the impact sensitivity, also the shock sensitivity is affected by the density defect content (dislocations, grain boundaries, voids, impurities, inclusions). The shock initiation tests with HMX, for example, have clearly demonstrated a relationship between the average crystal density and shock initiation pressure. The findings from this area are very important for practice, because a modification of quality and crystal shape of, for example, RDX can give a product with increased resistance to impact and shock (I-RDX [110]). 

In immobilization, various carriers/supports have been used with enzymes. Porous glass and other derivatives (Cao et al., 1992 Marlot et al., 1985 Rucka and Turkiewicz 1990), activated carbon (Kandasamy et al, 2010), cellulose and other derivatives (Hwang et al., 2004), clay (de Oliveira et al., 2000 Lee and Akoh, 1998 Scherer et al., 2011), alumina (Bagi et al., 1997 Padmini et al., 1993), nylon (Pahujani et al., 2008), polyethylene and its derivatives (Watanabe et al., 1994), and polystyrene (Miletic et al., 2010) are the most common supports that have been used in lipase immobilization. The choice of a suitable support depends on its properties, such as its chemical composition, particle size, and surface area. These are essential for various potential applieations (Villeneuve et al., 2000). Generally, porous supports are better than nonporous ones due to their larger surface area. However, they must have a structure that allows the lipase to bind and access the substrate with a minimum internal diffusion limitation. Further details on the effect of diffusion restrictions on reaction efficiency are discussed in Chapter 4. 

Rawle, A., Basic principles of particle size analysis, Surface Coatings Intern., Part... 

To control their products and to calibrate their instruments many manufacturers of powders or porous materials produce standardised materials for their own needs (ref. 5). For public use such reference materials are certified, stored and offered by national and international standardisation institutions and by some commercial distributors (Tab. 1). Disregarding the difficulties in storage a variety of particle size and surface area reference materials are available (Tab. 2) and more are under consideration. Lists of the many industrial materials suitable for tests are included in ref. 9. The Institute for Inorganic and Analytical Chemistry of the University Mainz has developed single crystals of zeolites and aluminophosphates which may be used as reference materials in the micropore range (ref. 6). 

J) The extreme fineness of iadividual clay particles, which may be of colloidal size ia at least one dimension. Clay minerals are usually platy ia shape, and less often lathlike and tubular or scroU shaped (13). Because of this fineness clays exhibit the surface chemical properties of coUoids (qv) (14). Some clays possess relatively open crystal lattices and show internal surface colloidal effects. Other minerals and rock particles, which are not hydrous aluminosihcates but which also show colloidal dimensions and characteristics, may occur intimately intermixed with the clay minerals and play an essential role. 

Circulating fluidized-beds do not contain any in-bed tube bundle heating surface. The furnace enclosure and internal division wall-type surfaces provide the required heat removal. This is possible because of the large quantity of soflds that are recycled internally and externally around the furnace. The bed temperature remains uniform, because the mass flow rate of the recycled soflds is many times the mass flow rate of the combustion gas. Operating temperatures for circulating beds are in the range of 816 to 871°C. Superficial gas velocities in some commercially available beds are about 6 m/s at full loads. The size of the soflds in the bed is usually smaller than 590 p.m, with the mean particle size in the 150—200 p.m range (81). 

Particle size distribution determines surface-to-mass ratios and the distance internal moisture must travel to reach the surface. Large pieces thus have higher critical moisture contents than fine particles of the same material dried under the same conditions. Pneumatic-conveyor flash dryers work because very fine particles are produced during initial dispersion and these have low critical moisture contents. 

Material and uses Shape"of particles Size range, U.S. standard mesh f Internal porosity, % Bulk dry density, kg/L Average pore diameter, nm Surface area, kmVkg Sorptive capacity, kg/kg (dry)... 

Example 4 Calculation of Sample Weight for Surface Moisture Content An example is given with reference to material with minimal internal or pore-retained moisture such as mineral concentrates wherein physically adhering moisture is the sole consideration. With this simphfication, a moisture coefficient K is employed as miiltipher of nominal top-size particle size d taken to the third power to account for surface area. Adapting fundamental sampling theory to moisture sampling, variance is of a minimum sample quantity is expressed as... 

When the catalyst is expensive, the inaccessible internal surface is a liabihty, and in every case it makes for a larger reactor size. A more or less uniform pore diameter is desirable, but this is practically reahz-able only with molecular sieves. Those pellets that are extrudates of compacted masses of smaller particles have bimodal pore size distributions, between the particles and inside them. Micropores have diameters of 10 to 100 A, macropores of 1,000 to 10,000 A. The macropores provide rapid mass transfer into the interstices that lead to the micropores where the reaction takes place. 

These reactors for hquids and liquids plus gases employ small particles in the range of 0.05 to 1.0 mm (0.0020 to 0.039 in), the minimum size hmited by filterability. Small diameters are used to provide as large an interface as possible since the internal surface of porous pellets is poorly accessible to the hquid phase. Solids concentrations up to 10 percent by volume can be handled. In hydrogenation of oils with Ni catalyst, however, the sohds content is about 0.5 percent, and in the manufacture of hydroxylamine phosphate with Pd-C it is 0.05 percent. Fischer-Tropsch slurry reac tors have been tested with concentrations of 10 to 950 g catalyst/L (0.624 to 59.3 IbiTi/fF) (Satterfield and Huff, Chem. Eng. Sci., 35, 195 [1980]). 

Effectiveness of selective adsorption of phenanthrene in Triton X-100 solution depends on surface area, pore size distribution, and surface chemical properties of adsorbents. Since the micellar structure is not rigid, the monomer enters the pores and is adsorbed on the internal surfaces. The size of a monomer of Triton X-100 (27 A) is larger than phenanthrene (11.8 A) [4]. Therefore, only phenanthrene enters micropores with width between 11.8 A and 27 A. Table 1 shows that the area only for phenanthrene adsorption is the highest for 20 40 mesh. From XPS results, the carbon content on the surfaces was increased with decreasing particle size. Thus, 20 40 mesh activated carbon is more beneficial for selective adsorption of phenanthrene compared to Triton X-100. 

The selectivity in a system of parallel reactions does not depend much on the catalyst size if effective diffusivities of reactants, intermediates, and products are similar. The same applies to consecutive reactions with the product desired being the final product in the series. In contrast with this, for consecutive reactions in which the intermediate is the desired product, the selectivity much depends on the catalyst size. This was proven by Edvinsson and Cybulski (1994, 1995) for. selective hydrogenations and also by Colen et al. (1988) for the hydrogenation of unsaturated fats. Diffusion limitations can also affect catalyst deactivation. Poisoning by deposition of impurities in the feed is usually slower for larger particles. However, if carbonaceous depositions are formed on the catalyst internal surface, ageing might not depend very much on the catalyst size. 

The restricted access principle is based on the concept of diffusion-based exclusion of matrix components and allows peptides, which are able to access the internal surface of the particle, to interact with a functionalized surface (Figure 9.2). The diffusion barrier can be accomplished in two ways (i) the porous adsorbent particles have a topochemically different surface functionalization between the outer particle surface and the internal surface. The diffusion barrier is then determined by an entropy controlled size exclusion mechanism of the particle depending on the pore size of adsorbent (Pinkerton, 1991) and (ii) the diffusion barrier is accomplished by a dense hydrophilic polymer layer with a given network size over the essentially functionalized surface. In other words, the diffusion barrier is moved as a layer to the interfacial... 

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