Particle size, measurement purposes

However, this section pursues particle size measurement and evaluates its importance (as well as density) for the purpose of classifying the suitability of powders for long-distance pneumatic conveying applications. Initially, an appreciation of the fundamentals and the existing powder classification techniques is required. 

Allen, T., Particle Size Measurement (Powder Technology Series), 5th edn, Chapman Hall (November 1997), ISBN 0 41275 350 2 Bernhardt, 1. C., Particle Size Analysis—Classification and Sedimentation Methods, Chapman Hall (September 1994), ISBN 0 41255 880 7 Gy, P. and Royle, A. G. (translator). Sampling for Analytical Purposes, John Wiley Sons Ltd (abridged 153 pages) (July 1998), ISBN 0 47197 956 2 Kaye, B. H., Characterization of Powders and Aerosols, John Wiley Sons Ltd (January 1999), ISBN 3 52728 853 8... 

Gravitational settling of particles in liquids is an age-old process which can be used for a variety of purposes. For example, it is used for the classification of solids, washing, particle size measurement or mass transfer, and in solvent extraction. The majority of applications of gravity sedimentation, however, are in solid-liquid separation duty. The object here is to remove the solids from the liquid either because the solids and/or the liquid are valuable or because the two phases have to be separated before disposal. 

At the simplest level we use particle size measurements to monitor their concentration or to control the reproducibility of a product. Thus, we compare what we find with what we expect and if the two do not coincide we reject the product. The science of powder technology, however, is concerned to use the microscopic properties of the system, for example the particle size distribution, to interpret the bulk behaviour of the powder. If it is to be used in dilute circumstances, then the bulk behaviour can be derived by integrating the behaviour of the individual particles but usually this is not so and the relationship between the microscopic and macroscopic properties must take account of the particle interactions. By observing the difference in particle size distribution of samples which exhibit a different bulk behaviour, we begin to make a "correlation" between the two which, whether empirical or theoretical, quantitative or qualitative, involves interpretation of the mechanisms involved. Somewhere between these two purposes usually lies the purpose of a particle size measurement. There is, however, a far more ambitious level at which powder technology must eventually operate and, as yet, rarely does. That is to design the particles and the particle mixture to produce required properties, to use the relationships between microscopic and macroscopic properties in a predictive manner. It is the more rigorous use of particle size measurements which introduces the real diversity and which requires the measurements to be carefully matched to the problem. The increased diversity does not alter the basic needs which Heywood described. 

Particle size measurements must always be related to their purpose, particularly the accuracy necessary in different regions of the particle size range. The determination of the grade efficiency of separation equipment is particulary difficult because it represents, essentially, the difference in two particle size distributions. At least one of the distributions must be accurately known at the ends of the distribution. In this paper we report on... 

For the purpose of calibration and particle size calculations, it was decided to confirm the reported particle sizes of the various latices supplied by Dow and Polysciences, using scanning electron microscopy. Unfortunately, after subtracting the thickness of the gold layer with which the particles were coated from the size shown on the micrographs, some inconsistencies were noted with respect to the measured sizes of the particles and their elution behaviour. It was therefore decided to assume the reported sizes as true values with the exception of the 5T nm particle. 

The usual techniques for the determination of particle sizes of catalysts are electron microscopy, chemisorption, XRD line broadening or profile analysis and magnetic measurements. The advantage of using Mossbauer spectroscopy for this purpose is that one simultaneously characterizes the state of the catalyst. As the state of supported iron catalysts depends often on subtleties in the reduction, the simultaneous determination of particle size and degree of reduction as in the studies of Fig. 5.10 is an important advantage of Mossbauer spectroscopy. 

The prototype cotton-dust analyzer used in the initial study was designed to measure dust smaller than 100 pm, whereas a vertical elutriator in a card room measures only the dust that is smaller than about 15 pm. Differences in particle size distributions of dust from various types of cotton would likely affect the relationship between the two dust measurements. Therefore, we deemed it necessary to investigate the use of sizing screens with smaller openings i.e., openings whose size approximated the maximum size of particles collected by a vertical elutriator. The purpose of this report is to describe additional modifications to the cotton-dust analyzer and to present data on the performance of the machine when 17-, 50-, and 100-pm sizing screens were used. 

Given the complexity of particle size distributions in the atmosphere (see Chapter 9.A), as well as the large number of chemical components (Chapter 9.C) that are not distributed equally throughout the various sizes, characterizing a typical collection of particles in the atmosphere is not possible. However, some indication of particle levels in the atmosphere is provided by mass measurements of PMm (i.e., total mass less than 10 gm in diameter), for which extensive measurements have been made for regulatory purposes. 

Herein lies the value of these different averages the divergence between the averages calculated by different methods offers a clue as to the breadth of the distribution of particle sizes. Remember, the average, however evaluated, is only one measure of the distribution of sizes. A fuller description requires some measure of the width of the distribution as well. For classified data, the standard deviation (see Appendix C) is routinely used for this purpose. For characterizations based on macroscopic experiments such as we have been discussing it is quantities such as ds/d or dv/ds that quantify this spread. (The averages ds and dv are defined below and are also discussed in Appendix C.)... 

An in vitro release rate can reflect the combined effect of several physical and chemical parameters, including solubility and particle size of the active ingredient and rheological properties of the dosage form. In most cases, in vitro release rate is a useful test to assess product sameness between prechange and postchange products. However, there may be instances where it is not suitable for this purpose. In such cases, other physical and chemical tests to be used as measures of sameness should be proposed and discussed with the Agency. With any test, the metrics and statistical approaches to documentation of sameness in quality attributes should be considered. 

All the samples measured showed characteristic superparamagnetic behavior with a blocking temperature TB. An independent method of determining the parameters of the particle size distribution g(D) is by means of the analysis of magnetic measurements under equilibrium conditions, i.e. at temperatures above the superparamagnetic blocking temperature Tb- For this purpose we performed magnetization measurements as a function of field M(H) at different temperatures [4,5]. 

The active pharmaceutical ingredient in a low-dose formulation is typically a small molecule, designed to meet a small particle size requirement for uniformity purposes, and can be susceptible to effects of static charge and segregation. The impact of static charge on the accuracy of blend uniformity measurements (i.e., sampling bias) is discussed in the next section. 

Particle Temperature Overshoot. The temperature of the burning char particles will run hotter than that of the bed by amounts that depend upon particle size, reactivity, bed temperature. It is determined in part by the heat released at the particle surface due to reaction and in part to the additional heat released by carbon monoxide oxidation near the particle surface (54-58). Measurements for 1.8 to 3.2 millimeter size coke particles burning in a fluidized band of sand at 1173 K increased from the bed temperature at low oxygen concentrations to values 150 to 200 K above the bed temperature for oxygen concentrations approaching that of air (72). Estimation of this temperature rise is important for purposes of evaluating the NO/C reaction and also for prediction of the burnout times of fines. 

The current status of prediction and modelling in the area of fuel spray combustion requires, among other parameters, the measurement of droplet or solid particle size distribution and the relative velocity between the fuel and the surrounding gas. Many optical techniques, based on laser light scattering, have been investigated to this purpose (Refs.1,2,2,]+,, 6 and j), but the only system able to simultaneously determine the size and the velocity is the dual-beam laser Doppler velocimeter shown in Figure 1. 

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