DRIFTS particle size

Several factors affect the bandshapes observed ia drifts of bulk materials, and hence the magnitude of the diffuse reflectance response. Particle size is extremely important, siace as particle size decreases, spectral bandwidths generally decrease. Therefore, it is desirable to uniformly grind the samples to particle sizes of <50 fim. Sample homogeneity is also important as is the need for dilute concentrations ia the aoaabsorbiag matrix. 

Monomer compositional drifts may also occur due to preferential solution of the styrene in the mbber phase or solution of the acrylonitrile in the aqueous phase (72). In emulsion systems, mbber particle size may also influence graft stmcture so that the number of graft chains per unit of mbber particle surface area tends to remain constant (73). Factors affecting the distribution (eg, core-sheU vs "wart-like" morphologies) of the grafted copolymer on the mbber particle surface have been studied in emulsion systems (74). Effects due to preferential solvation of the initiator by the polybutadiene have been described (75,76). 

Several properties of the filler are important to the compounder (279). Properties that are frequentiy reported by fumed sihca manufacturers include the acidity of the filler, nitrogen adsorption, oil absorption, and particle size distribution (280,281). The adsorption techniques provide a measure of the surface area of the filler, whereas oil absorption is an indication of the stmcture of the filler (282). Measurement of the sdanol concentration is critical, and some techniques that are commonly used in the industry to estimate this parameter are the methyl red absorption and methanol wettabihty (273,274,277) tests. Other techniques include various spectroscopies, such as diffuse reflectance infrared spectroscopy (drift), inverse gas chromatography (igc), photoacoustic ir, nmr, Raman, and surface forces apparatus (277,283—290). 

Some concerns directly related to a tomizer operation include inadequate mixing of Hquid and gas, incomplete droplet evaporation, hydrodynamic instabiHty, formation of nonuniform sprays, uneven deposition of Hquid particles on soHd surfaces, and drifting of small droplets. Other possible problems include difficulty in achieving ignition, poor combustion efficiency, and incorrect rates of evaporation, chemical reaction, solidification, or deposition. Atomizers must also provide the desired spray angle and pattern, penetration, concentration, and particle size distribution. In certain appHcations, they must handle high viscosity or non-Newtonian fluids, or provide extremely fine sprays for rapid cooling. 

A preparative chromatograph fitted with an ST/2000-1 guard column (IX 2-in. ID) and an ST/2000B preparative column (25 X 2-in. ID), each packed with ST MACROBORE Cl8 (25-/xm particle size), was employed for separation of esters. The system was equipped with a differential refractometer with a flowing reference (as opposed to an air-filled reference) in order to minimize baseline drift. 

Particle mixing is caused by the bubbles, partly be shear displacement or drift but also by the bulk transport of particles in the bubble wake. Bubbles may also cause segregation if there are different kinds of particles present. Unlike other kinds of mixers, segregation is insensitive to particle size difference but particularly sensitive to density difference. In a binary system of particles segregation increases approximately as particle density ratio to the power 5/2 but with particle size ratio only to the power 1/5 (11). This can cause problems in, for example, coal combustion where char has a markedly lower density than ash and also in some ore reduction processes using coke. 

For DRIFT studies, a wood wafer, paper sheet, or milled wood sample dispersed in KBr (or KC1) is placed in a cup at the focal point of the concave, ellipsoidal mirror so that the incident light is focused on the sample. The scattered light coming from the sample is collected from the concave mirror and directed by a suitable mirror system to the detector of the FTIR instrument. The pressure used for smoothing the sample has to be adjusted so that reproducible results can be obtained (Yeboah et al. 1984). The contribution of specular reflectance can be diminished by reducing the particle size and by increasing the sample dilution. For powder samples, as indicated above, the diluent is KBr or KCI. Good results are normally obtained with alkali halide powders that contain 1-2% of sample. In certain cases, the sample concentration may be increased up to 10%. 

U average-particle migration velocity7 for a given size particle in an electrical precipitator also used as the drift velocity which is the average migration velocity7 for all particles sizes collected M/s... 

More realistic and complex tlieories consistent with the quite complex nature of the phenomena involved in the DR of light were developed later. Experimental studies have shown that this technique is affected by particle size, granulometric distribution and the refractive index of the particles, which has an important role when the particle size is near the wavelength of the 1R radiation. Diffuse reflectance Fourier transform (DRIFT) studies in the FIR region allow detection of the skeletal spectra of materials, such as mixed oxide catalysts, pigments and metal halides. 

Brown and Skrebowski [37] first suggested the use of x-rays for particle size analysis and this resulted in the ICl x-ray sedimentometer [38,39]. In this instrument, a system is used in which the difference in intensity of an x-ray beam that has passed through the suspension in one half of a twin sedimentation tank, and the intensity of a reference beam which has passed through an equal thickness of clear liquid in the other half, produces an inbalance in the current produced in a differential ionization chamber. This eliminates errors due to the instability of the total output of the source, but assumes a good stability in the beam direction. Since this is not the case, the instrument suffers from zero drift that affects the results. The 18 keV radiation is produced by a water-cooled x-ray tube and monitored by the ionization chamber. This chamber measures the difference in x-ray intensity in the form of an electric current that is amplified and displayed on a pen recorder. The intensity is taken as directly proportional to the powder concentration in the beam. The sedimentation curve is converted to a cumulative percentage frequency using this proportionality and Stokes equation. 

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