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particle technology

particle size analysis

Desalegn1
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particle technology

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  1. Mechanical Unit Operation (Cheg3111) JIMMA UNIVERSITY JIMMA INSTITUTE OF TECHNOLOGY School of Chemical Engineering By Instructor Desalegn A.

  2. OUTLINE OF THE COURSE • Classification of Coarsely Dispersed Material System • Size Reduction Operations • Agglomeration (Size Enlargement) • Transportation and Storage of Solids In Bulks • Mechanical Micro-processes in A Fluid • Mechanical and Hydro-mechanical Separations • Mixing Processes and Mixers By Instructor Desalegn A.

  3. CLASSIFICATION OF COARSELY DISPERSED MATERIAL SYSTEM • Particle Technology and Mixture of Particles • Measurement of granule metric state • Size Reduction Operations • Agglomeration (Size Enlargement) • Transportation and Storage of Solids In Bulks • Mechanical Micro-processes in A Fluid • Mechanical and Hydro-mechanical Separations • Mixing Processes and Mixers By Instructor Desalegn A.

  4. Introduction of Particles • Particles can be grown from embryos to desired dimensions and shapes of crystals, • Particles can be obtained or extracted from naturally occurring materials, generated by size reduction from much larger materials and aggregates, • May created by management and/or manipulationof phases of materials, and produced as products or by-products of chemical and biological transformations and atomic interactions. By Instructor Desalegn A.

  5. Particle Technology • Particle technology is a term used to refer to the science and technology related to the handling and processing of particles and powders. • Particle technology is also often described as powder technology, particle science and powder science. • Powders and particles are commonly referred to as bulk solids, particulate solids and granular solids. • Today particle technology includes the study of liquid drops, emulsionsand bubbles as well as solid particles. • The discipline of particle technology now includes topics involving particles and powder like particle formation processes (such as crystallization, precipitation, granulation, spray drying, extrusion and grinding), transportation processes. By Instructor Desalegn A.

  6. Many granular and particulate material can be found in variety of unit operations of industrial process both as raw material and finished goods. • Therefore, understanding the behavior of particulate material and industrial powder is very much essential especially from chemical engineering point of view. • Ignorance of particle technology may result in lost production, poor product quality, risk to health, dust explosion or storage silo collapse. By Instructor Desalegn A.

  7. Particle Characteristics • Several particle characteristics are very important with reference to product properties such as: Size, Shape, Density, Surface characteristics (smooth/hard, porous/nonporous, etc.) Particle Size • Particles are three-dimensional objects for which three parameters (the length, width, and height) are required in order to provide a complete description. Thus, it is not possible to describe a particle using single number that equates to the particle size. • Most sizing technique therefore assume that the material being measured is spherical, as sphere is the shape that can be described by a single number (its diameter). • This equivalent sphere approximation is useful as it simplifies the way particle size is represented. By Instructor Desalegn A.

  8. Thus, we utilize what is called the equivalent size or equivalent diameter of an irregular particle which can be broadly defined as the size of a spherical particle having the same controlling characteristic as the particle under consideration. • Naturally, to apply this definition, we must first specify what this controlling characteristic is depends on the system and the process in which the particle is involved. • For example, for catalyst particles, the surface area is the most controlling parameter. Therefore, for defining the size of a catalyst particle we can use the surface diameter (ds) which will thus be defined as the diameter of a spherical particle having the same surface area as the particle. By Instructor Desalegn A.

  9. If Sp is the surface area of the particle, then Or • The gravitational free settling velocity of a particle in a liquid is very much controlled by the mass of the particle (or, for a given density, its Volume). We can therefore define the particle size for such a case by the volumetric diameter (dv) which is once again defined as the diameter of spherical particle having the same volume as the particle under consideration. By Instructor Desalegn A.

  10. Thus, if Vp is the volume of the particle, Or • The dynamics of gas bubbles in a liquid or that of liquid drops in a liquid or gas depends not only on the bubble or drop volume but also on the interfacial tension at the gas-liquid or liquid-liquid interface. Thus, both the volume as well as the surface area of the bubble or drop are controlling parameters here. By Instructor Desalegn A.

  11. In such cases, the bubble size or drop size is defined using the volume-surface diameter or more commonly called Sauter diameter (dvs).this is accordingly defined as the diameter of spherical particle having the same specific surface (surface area per unit volume) as the particle (bubble or drop) under consideration. Thus, Or Where spis the specific surface (surface area per unit volume) of the particle (bubble or drop). By Instructor Desalegn A.

  12. Thus, once the controlling characteristic is specified, we can define the size of any irregular particle using the above methodology. • Another popularly employed definition of particle size is the screen size or the screen average size, davg. • This in fact is the aperture size of the standard screen through which the particle just passes or more correctly, the arithmetic average of the aperture sizes of two successive standard screens, one of which lets the particle pass through whereas the other retains it. By Instructor Desalegn A.

  13. Particle Shape • Measuring particle size alone is sometimes insufficiently sensitive to identify important properties of the samples. For example, consider the 3 shapes below: • All these three shapes have the same area=4 square units. When they are converted to circle equivalent diameter they give the same result-a circle equivalent diameter of 2.257 units. By Instructor Desalegn A.

  14. This highlights the main disadvantage of measuring particle size only – very different shaped particle could be characterized as identical simply because they have similar projected 2D areas or similar spherical equivalent volumes. • Particle shape often has a significant influence on final product performance parameters such as flowability, abrasive efficiency, bio-availability, etc. so some way of characterizing particle is required. • The exact shape of an irregular particle is difficult to specify. One of the methods of defining particle shape is by using the term sphericity (). By Instructor Desalegn A.

  15. By Instructor Desalegn A.

  16. Typical values of sphericity for some common materials are given below: By Instructor Desalegn A.

  17. Measurement of Granule Metric State • In the previous section, we have considered the case of a single particle only. However, in actual industrial practices, we normally come across mixtures of particles of different sizes. • Thus, to ascertain a properties (such as surface area, specific surface area, shape factor, etc.) of the system of particles it is necessary that we should know the size and shape distribution of particles. • In such case, the mixture can be separated into a number of fractions each fraction consisting of particle of a given size davgi. Particle Size Distribution • Particle size distribution is a process of separating mixture of particle according to their size range before using them for any industrial operation. • Determination of particle size distribution (PSD) is essential in many chemical processes, such as crystallization, precipitation, polymerization, etc., in which product quality depends on measurement and control of PSD. By Instructor Desalegn A.

  18. There are a number of ways used to determine PSD of a mixture. Some of them are listed below • The easiest method to determine PSD is sieve analysis, where powder is separated on sieves of different sizes. • Thus, the PSD is defined in terms of discrete size ranges, e.g. “% of sample between 45 µm and 53 µm, when sieves of these sizes are used. By Instructor Desalegn A.

  19. Screening • A Screen can be called an open container usually cylindrical with uniformly spaced opening at the base. It is normally made of wire mesh cloth, the wire diameter and the interspacing between wire being accurately spaced. • The opening are commonly square, and is called the aperture size of the screen. By Instructor Desalegn A.

  20. Screens are usually designated by their mesh number. The mesh number indicates the number of apertures per linear length. For example, screen having 10 square openings per cm may be called a 10 mesh screen or in that case, the aperture size of the screen will be 0.1 cm minus the wire diameter. • Clearly higher the mesh number, the smaller will be the aperture size of the screen. This is the practice followed in British standard screens (BSS), American standard screens (ASTM and Tyler standard screens), German standard screens (DIN 1171) etc. • The Indian standard screen (ISS) however follow a different type of designation, where the mesh number is equal to its aperture size expressed to the nearest Deca-micron (0.01 mm). Thus an ISS of mesh number 50 will have an aperture width of approximately 500 microns. • Such a method has the simplicity that the aperture width is readily indicated from the mesh number. By Instructor Desalegn A.

  21. Sieve mesh chart By Instructor Desalegn A.

  22. Standard test screen are usually made of phosphor bronze wires. Brass or wild steel wires are also sometimes used. It is always preferable to maintain a standard screen interval between successive test screens. The screen interval is the factor by which the aperture size of the test screen is to be divided to get the aperture size of the next successive test screen. An internationally accepted standard screen interval is , that is 1.189. Screen analysis • In standard sieve shaker, test screens are stacked one above the other in the ascending order of their aperture size. Then, a weighted amount of the feed material is fed to the top-most screen and the whole assembly is shaken continuously. • After some period of time, the vibration is stopped and the screens are disassembled. The material retained on each screen is weighted. By Instructor Desalegn A.

  23. The material passing through a given screen is termed as undersize, fines or minus (−) material, while the material retained in a given size screen is called oversize, tails or plus (+) material. By Instructor Desalegn A.

  24. The average size of particles () for (-400+340) Fraction: • Mass fraction By Instructor Desalegn A.

  25. Cumulative screen analysis • The above methods of representing size distribution of a mixture is called the differential size analysis. Another popular method of reporting screen data is on the cumulative basis. • In cumulative analysis, our interest will be to estimate the total mass of that fraction in which all particles have sizes below (or sometimes above) a particular value. • For example, in case of cumulative undersize, cumulative mass fraction Xi corresponds to mass fraction of the material having size less than di, or all particles in that fraction pass through a screen of aperture di mm. By Instructor Desalegn A.

  26. By Instructor Desalegn A.

  27. Consider the following table. Since there is no +480 materials, it is evident that all material fed have passed through the 480 mesh screen. In other words the cumulative mass fraction for 480 mesh is 1.0. The mass fraction of material retained by 400 mesh screen is and therefore cumulative mass fraction for 400 mesh screen will be (1-). Similarly we can compute cumulative mass fraction corresponding to each screen. • Once the mixture size distribution is known, specific surface area as well as number of particles per unit mass of mixture can be predicted. By Instructor Desalegn A.

  28. Cumulative Undersize Vs. Cumulative oversize plot By Instructor Desalegn A.

  29. Ends of Lecture One! If Any questions u Well Come! By Instructor Desalegn A.

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