Particle Sizes and Shapes

The choice of particle size and shape of commercial ammonia catalysts is determined mainly by two factors  

From the standpoint of space-time yield, it is desirable to use the finest possible particle, which, practically speaking, is about 1-2 mm (Fig. 19) however, with decreasing particle size, the pressure drop and the risk of destructive fluidization of the catalyst increase (Fig. 20). 

Two effects cause the low production capacity of coarse-grained catalyst first, large grain size retards transport of the ammonia from the particle interior into the bulk gas stream, because this proceeds only by slow diffusion through the pore system. Slow ammonia diffusion inhibits the rate of reaction. At the high reaction rate typical for the converter inlet layer, only a surface layer of the catalyst grains, about 1-2 mm thick, participates in the reaction. 

The second effect is a consequence of the fact that a single catalyst grain in the oxidic state is reduced from the outside to the interior of the particle [314] the water vapor produced in the grain interior by reduction comes into contact with already reduced 

Depth of the catalyst bed, 7 m reaction pressure 27.1 MPa reaction temperature 450 °C 

In Table 3 are listed the spectrum of methods used and the size range of effectiveness and the properties measured in each case. Clearly there is neither a single measure of size nor a single tool with which to measure it. 

Over the last year or two, true second-generation automated instruments have been developed where the analysis of the data obtained by electronic means is fed directly into a small computer with much of its logic for processing several standard suites of information already built into its circuits. 

The following independent measurements can be made on any field of particulate matter (Bausch and Lomb QMS) (A) Entire field measurements (1) total particle count, (2) lower positive tangent 

Optical microscopy Scanning electron microscopy Transmission electron microscopy Sieving 

Light scattering Automatic image analyzing microscopy or SEM Gas adsorption Electrical resistivity Sedimentation Elutriation Centrifugation Impaction X-ray diffraction line broadening 

Other important parameters in particle analysis are surface area, pore size, and volume. The basic method for measuring surface area involves determining the quantity of an inert gas, usually nitrogen, required to form a layer one molecule thick on the surface of a sample at cryogenic temperature. Many techniques are used for pore size measurement impregnation with molten metal, particle beam transmission, water absorption, freezing point depression, microscopy, mercury intrusion, and gas condensation and evaporation. The last three techniques are most often utilized. 

The particle shape can generally be established by simple visual observation or by using a microscope. The transport characteristics of particulate solids are quite sensitive to the particle shape. Both the internal and external coefficient of friction can change substantially with variations in particle shape even if the major particle dimensions remain unchanged. Small differences in the pelletizing process can cause major problems in a downstream extrusion process. Variations in the ratio of regrind to virgin polymer can cause variations in the extrusion process. 

The montmorillonites consist of thin, platy crystals, so small that their shape is difficult to discern, even with the electron microscope. Estimates of their size range from 0.01 to in diameter. 

There is no doubt that the extreme smallness of clay particles is responsible for many of their intrinsic properties, such as plasticity. 

Illites are very fine-grained minerals, particles as small as 0.05[x in radius having been reported. They are said to be slightly plastic, although it is not certain whether this plasticity is due to contamination with clay mineral. Because of their alkali content, clays containing illite have a comparatively low refractoriness. 

An ellipsoidal/spherical particles blending system shows much stronger ER effect than one-component system [59]. A contradictory result was obtained in hydroxyl-zinc compounds such as plate shape particle of Zn5(011)s(N03)2 2II2O and rod-like particle of Zn5(011)8(Cll3C00)2 -21120 /silicone oil suspensions. The rod particle suspension showed weak ER cffcct.fOOl, which was attributed to the low dielectric constant of the rod particle suspension. It may be hard to draw such a conclusion as the particle materials are different. 

There are several single-particle characteristics that are very important to product properties (Davies, 1984). They include particle size, particle shape, surface, density, hardness, adsorption properties, and so on. From all these mentioned features, particle size is the most essential and important one. The term size of a powder or particulate material is very relative. It is often used to classify, categorize, or characterize a powder, but even the term powder is not clearly defined and the common convention considers that for a particulate material to be considered powder, its approximate median size (50% of the material is smaller than the median size and 50% is larger) should be less than 1 mm. It is also common practice to talk about fine and coarse powders several attempts have been made at standardizing particle nomenclature in certain fields. For example. Table 1.1 shows the terms recommended by the British Pharmacopoeia referred to standard sieves apertures. Also, by convention, particle sizes may be expressed in different units depending on the size range involved. Coarse particles may be measured in centimeters or 

Terms Recommended by the British Pharmacopoeia for Use with Powdered Materials 

Av Surface volume diameter Surface-to-volume ratio 

Xs Free-falling diameter Free-falling speed in the same liquid at the same particle density 

t Stokes diameter Free-falling speed if Stokes law is used (ROp 0.2) 

When suspensions are formulated to provide a stable system, the particle size becomes critical. Flocculated suspensions also require careful particle size control either in the process of manufacturing or in the starting material. Equally important is the crystal habit — the outward appearance of an agglomeration of crystals. Crystal structure can be altered during the manufacturing process, particularly if the product is subject to temperature cycling, and this can alter the stability of suspensions. 

Suspensions are manufactured either by a precipitation or by dispersed methods requiring use of suspending agents whose characteristic can significantly change because of the presence of other components such as electrolytes. 

It will be assumed here for simplicity that one parameter r (the radius in the case of a spherical liquid droplet) is sufficient to specify the size and shape of a particle. For solid particles (or liquid droplets), this assumption will be valid in spray combustion when either the particles are geometrically similar or their shape is of no consequence in the combustion process. Liquid droplets will obey this hypothesis in particular if they are spherical, which will not be true unless (1) they collide with each other so seldom that collision-induced oscillations are viscously damped to a negligible amplitude for most droplets, and (2) their velocity relative to the gas is sufficiently low. An alternative parameter to the radius is the mass of the droplet [10] the choice between this, the droplet volume, or the radius of a sphere of equal volume is a matter of individual preference. 

For liquid droplets, requirement (1) typically means that the spray must be dilute (that is, the ratio of the volume occupied by the condensed phase to the volume occupied by the gas must be small) because collisions tend to be frequent when the volume of particles per unit volume of space becomes too large. Since the mass density of the particles greatly exceeds that of the gas in many sprays and the stoichiometry of most hydrocarbon-oxidizer systems is such that the mass of the fuel is considerably less than that of the gaseous oxidizer in stoichiometric mixtures, the hypothesis of a dilute spray often is valid in hydrocarbon spray combustion. 

It has been shown [11] that the degree of deformation and the amplitude of oscillation of a liquid droplet depend on the ratio of the dynamic force to the surface-tension force, which is given by the Weber number. We = 2rpg y — u /S. Here S is the surface tension of the liquid, Pg is the gas density, v represents the droplet velocity, and u is the velocity of the gas. When We 10, droplets are nearly spherical as We increases, the droplets deform and eventually break up at We w 10. Hence, for systems involving liquid droplets, the present formulation should be applied only 

It has been shown [11] that the degree of deformation and the amplitude of oscillation of a liquid droplet depend on the ratio of the dynamic force to the surface-tension force, which is given by the Weber number, We = 2rp — u /S. Here S is the surface tension of the liquid, p 

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Tear Resistance. The resistance of an elastomer to tearing is affected by the particle size and shape of the filler it contains. Tear resistance generally increases with decreasing particle size and increasing sphericity of fillers. 

Specific gravity is the most critical of the characteristics in Table 3. It is governed by ash content of the material, is the primary deterrninant of bulk density, along with particle size and shape, and is related to specific heat and other thermal properties. Specific gravity governs the porosity or fractional void volume of the waste material, ie. 

Tap Density. Tapping a mass of loose powder, or more specifically, the appHcation of vibration to the powder mass, separates the powder particles intermittently, and thus overcomes friction. This short-time lowering of friction results in an improved powder packing between particles and in a higher apparent density of the powder mass. Tap density is always higher than apparent density. The amount of increase from apparent to tap density depends mainly on particle size and shape (see Table 4). 

Pigments and Extenders. Pigments are selected for use in house paints based on thek appearance and performance quaUties. Appearance includes color and opacifying abiUty. Performance quaUties include ultraviolet light resistance, fade resistance, exterior weatherabiUty, chemical resistance, as well as particle size and shape. Toxicity profiles and safety and health related properties are also important criteria in pigment selection. 

The most commonly measured pigment properties ate elemental analysis, impurity content, crystal stmcture, particle size and shape, particle size distribution, density, and surface area. These parameters are measured so that pigments producers can better control production, and set up meaningful physical and chemical pigments specifications. Measurements of these properties ate not specific only to pigments. The techniques appHed are commonly used to characterize powders and soHd materials and the measutiag methods have been standardized ia various iadustries. 

The following variables can affect wall friction values of a bulk soHd. (/) Pressure as the pressure acting normal to the wall increases, the coefficient of sliding friction often decreases. (2) Moisture content as moisture increases, many bulk soHds become more frictional. (3) Particle size and shape typically, fine materials are somewhat more frictional than coarse materials. Angular particles tend to dig into a wall surface, thereby creating more friction. (4) Temperature for many materials, higher temperatures cause particles to become more frictional. (5) Time of storage at rest if allowed to remain in contact with a wall surface, many soHds experience an increase in friction between the particles and the wall surface. (6) Wall surface smoother wall surfaces are typically less frictional. Corrosion of the surface obviously can affect the abiUty of the material to sHde on it. 

The foUowing variables can affect a material s bulk density. (/) Moisture higher moisture content often makes a material mote compressible. (2) Particle size and shape often, the finer the bulk soHd, the mote compressible it is. The shape of the particles can affect how they fit together and thein tendency to break while being compacted. (3) Temperature some materials become mote compressible as thein temperature increases. This could be due, for example, to softening of the particles. (4) Particle elasticity elastic materials tend to deform significantly when they ate compressed. 

Because mass flow bins have stable flow patterns that mimic the shape of the bin, permeabihty values can be used to calculate critical, steady-state discharge rates from mass flow hoppers. Permeabihty values can also be used to calculate the time required for fine powders to settle in bins and silos. In general, permeabihty is affected by particle size and shape, ie, permeabihty decreases as particle size decreases and the better the fit between individual particles, the lower the permeabihty moisture content, ie, as moisture content increases, many materials tend to agglomerate which increases permeabihty and temperature, ie, because the permeabihty factor, K, is inversely proportional to the viscosity of the air or gas in the void spaces, heating causes the gas to become more viscous, making the sohd less permeable. 

Important physical properties of catalysts include the particle size and shape, surface area, pore volume, pore size distribution, and strength to resist cmshing and abrasion. Measurements of catalyst physical properties (43) are routine and often automated. Pores with diameters <2.0 nm are called micropores those with diameters between 2.0 and 5.0 nm are called mesopores and those with diameters >5.0 nm are called macropores. Pore volumes and pore size distributions are measured by mercury penetration and by N2 adsorption. Mercury is forced into the pores under pressure entry into a pore is opposed by surface tension. For example, a pressure of about 71 MPa (700 atm) is required to fill a pore with a diameter of 10 nm. The amount of uptake as a function of pressure determines the pore size distribution of the larger pores (44). In complementary experiments, the sizes of the smallest pores (those 1 to 20 nm in diameter) are deterrnined by measurements characterizing desorption of N2 from the catalyst. The basis for the measurement is the capillary condensation that occurs in small pores at pressures less than the vapor pressure of the adsorbed nitrogen. The smaller the diameter of the pore, the greater the lowering of the vapor pressure of the Hquid in it. 

The capacity of a pneumatic-conveying system depends on (1) produc t bulk density (and particle size and shape to some extent), (2) energy content of the conveying air over the entire system, (3) diameter of conveying hne, and (4) eqmvalent length of conveying hue. 

Solid particles have a distinct form, which can strongly affect their appearance, product quality and processing behaviour. Thus, in addition to chemical composition, particulate solids have to be additionally characterized by particle size and shape. Furthermore, particles can be generated at any point within the process. For example, nucleation occurs within a crystallization process and large particles are broken down to numerous smaller ones in a comminution process or within a drier. 

Additional purification of the product and improvement of particle size and shape can be achieved by re-ciystallization. The process consists of sequential dissolutions of potassium heptafluorotantalate in appropriate solutions at increased temperatures, filtration of the solution to separate possible insoluble parts of the product and cooling of the filtrated solution at a certain rate. The precipitated crystals are filtrated, washed and dried to obtain the final product. Re-crystallization can be performed both after filtration of the preliminary precipitated salt or after drying if the quality of the product is not sufficient. HF solutions of low concentrations are usually used for re-ciystallization. In general, even water can be used as a solvent if the process is performed fast enough. Nevertheless, practical experience suggested the use of a 30—40% HF solution within the temperature interval of 80-25°C, and a cooling rate of about 8-10°C per hour. The above conditions enable to achieve an acceptable process yield and good performance of the product. 

Effect of Filler Particle Size and Shape on the Rheological Properties of Composites... 

Particle size and shape have a strong effect on the rheological properties of materials as well, viscosity among them. 

The ability to produce threads, discs and spheres of defined size and structure will be of great importance when the very promising initial results from catalytic studies are applied on a larger scale. Processes using heterogeneous catalysts require the ability to control particle size and shape in order to ensure good mixing of all the reaction components, and separations after reaction. 

In industry, the emphasis is mainly on developing an active, selective, stable and mechanically robust catalyst. To accomplish this, tools are needed which identify those structural properties that discriminate efficient from less efficient catalysts. All information that helps to achieve this is welcome. Empirical relationships between those factors that govern catalyst composition (e.g. particle size and shape, and pore dimensions) and those that determine catalytic performance are extremely useful in catalyst development, although they do not always give fundamental insights into how the catalyst operates on the molecular level. 

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