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UEMK3223 Introduction to Particle Technology DR. SIM LAN CHING Email: [email protected] Office: KB 804 Assessment System and Breakdown of Marks • Course Work Assignments : 20% • Tests, Mid-term : 20% • Final Examination 60% Important Date • Teaching: Week 1,4 &5 (Dr Sim) Week 2,3,6 & 7 (Dr Lim) • Tutorial 1 to be covered by Prof. Tee on week 2. Tutorial 2 to be covered by Dr. Lim on week 3. Tutorial 3 to be covered by Dr. Steven on week 4 Tutorial 4 to be covered by Dr. Lee Tin Sin on week 5 Tutorial 5 to be covered by Prof. Tee on Week 6. Tutorial 6 to be covered by Dr. Lee on Week 7. • Assignments : TBA by Dr Lim • Mid-term : Week 5 (13/11 – Tues) References Main References: •McCabe, W. L., Smith, J. C. & Harriott, P. (2005). Unit operations of chemical engineering. (7th ed.). Mc-Graw Hill. (ISBN : 0-07-118173-3) •Rhodes, M. (2008). Introduction to particle technology. (2nd ed.). Wiley (ISBN : 978-0-470-01428-8) Additional References: • Perry, R. H. & Green, D.W. (1998). Perry’s chemical engineers’ handbook. (7th ed.). McGraw-Hill International Editions. (ISBN: 0-07-115982-7) • Geankoplis, C. (2003). Transport processes and separation process principles. (4th ed.). Prentice Hall PTR. (ISBN : 0-13-101367-X) • Online Journals and documents Objective of Unit: Acquire knowledge related to particle technology including • Particle characterisation, and • Particle interactions in different types of fluids • in order to identify and solve problems arising when handling particular material. Learning Outcome of Unit: By the end of this unit, students should be able to: 1. Determine the properties of solids. 2. Analyse storage, conveying, transport and mixing of solids in fluid systems. 3. Assess particle size reduction and enlargement systems. 4. Evaluate separation systems for solid from gases and liquids using screening, filtration and sedimentation methods. 5. Design solid handling equipments. What is Particle? • Nobody knows!!! • "Everything is a particle!" - Wallace H. Coulter What is Particle? • Simple Definition of PARTICLE • a very small piece of something • a very small amount of something • Physics : any one of the very small parts of matter (such as a molecule, atom, or electron) Particle Why do we study Particles? •The first is the interfacial properties of particulate systems. •Applied in paints, coatings, catalysts, taste and texture, dessicants, adsorbents, rapid dissolution, abrasives, cutting tools, composite materials, ceramics and powder metallurgy. •The second quality is that materials in particulate form: Example: powders, slurries, droplets, emulsions can often be handled, transported and processed with greater ease and economy than the same materials in bulk forms. •The third quality arises, as particles get so small that their size approaches molecular dimensions http://perc.ufl.edu/particle.asp What is Particle Technology? • Techniques for processing and handling particulate solids Why we need to learn Particle Technology? • Most chemical engineers will find themselves working with particles at some point in their professional life Subjects of particle technology • Chemical engineers meet particulate solids in carrying out many unit operations • • • • • • Bulk storage, Crushing and grinding, Filtration. Crystallization Solid Fluid Reaction Particle size separation, such as • sieving, tabling, flotation, • Qualitative separation such as • Magnetic separation, and/or electrostatic recipitation, • Fluidization, flocculation, Centrifugal separation, Liquid filtration, particle size analysis, powder metallurgy, • Nanotechnology, particle characterization by shape, and others. Describing the size of a single particle Regular-shaped particles Shape Sphere Cube Cylinder Cuboid Cone Dimensions Radius Side length Radius and height Three side lengths Radius and height Irregular-shaped particles • No single physical dimension can adequately describe the particle • Describing the size of a single particle. Some terminology about diameters used in microscopy to be used Laboratory method for particle size measurement Method Approximate Size (μm) Gravity Sedimentation 2-100 Type of Size Distribution By Mass Microscopy • Optical • Electron 0.8-150 0.001-5 By Number Sieving 37-4000 By Mass Size Terminology µm µm µm µm Unit • Coarse particles: inches or millimeters • Fine particles: screen size • Very fine particles: micrometers or nanometers • Ultra fine particles: surface area per unit mass, m2/g Characterization of Solid Particles Which Size can be measured? • Individual solid particles are characterized by their size, shape, and density oDiameter of a sphere which has the same property as the particle itself – that is the same volume, same settling velocity, etc. oDiameter of a circle which has the same property and the projected outline of the particle – that is the same projected area or same perimeter. oLinear dimension measured parallel to a particular direction. • Equivalent circle diameter. (projected area diameter (area of circle with same area as the projected area )) • Martin’s diameter (length of the line which bisects the particle image) • Feret’s diameter. (distance between two tangents on opposite sides of the particle) • Shear diameter. (particle width obtained using an image shearing device) Equivalent Diameters 1) The sphere of the same volume as the particle 1 3 6 Volume-equivalent sphere diameter, xv Vp π where Vp is the volume of the particle 2) The sphere of the same surface area as the particle1 2 6 1/2 Surface-equivalent sphere diameter, xS =Sp Surface-equivalent sphere diameter, π where Sp is the surface of the particle 3) The sphere of the same surface area per unit volume as the particle ℎ 6 = = ℎ Example 1 Particle Distribution Description of populations of particles Description of populations of particles Description of populations of particles Description of populations of particles Typical differential frequency distribution Typical cumulative frequency distribution • Frequency distribution and cumulative are related mathematically • Cumulative distribution denoted by F, frequency distribution dF is written as f ( x) dx • Distributions can be by number, surface, mass or volume. Description of populations of particles D[1,0] = D[2,0] = 1 0 ( ) 2 0 ( ) If you are catalyst engineer and want to compare these spheres on the basis of surface area because higher surface area, higher activity of catalyst. Thus, 2 mean diameter = No. particles is inherent in formulae. Give rise the need to count large numbers of particles Thus can use, If you want to compare these spheres 4 on the basis of weight ( 3 ). Thus, D[3,0] = 3 0 ( ) 3 mean diameter = 3 3 D [4,3] – usually used in laser diffraction D [3,2] Description of populations of particles Distributions can be by number, surface, mass or volume. 1+4+9 Test Your Understanding, use D[1,0]……..or D[4,3] ?? • If we have a process stream we are not interested that there are 3.5 million particles in it, we are more interested that there is 1 kg or 2 kg of gold. • If you are in clean room making wafers of silicon or gallium arsenide. Here if one particle lands on our wafer it will tend to produce a defect. Description of populations of particles Description of populations of particles v v Assume 3 100 40 80 Volume (%) Number (%) 30 20 10 60 40 20 0 0 1 2 Size (nm) 3 1 2 Size (nm) 3 Description of populations of particles It’s important to select suitable method of measurement which directly gives particle size which is relevant to situation Consequences of interconversion between number, length and volume/mass means Imagine that our electron measurement technique is subject to an error of ± 3% ℎ . When we convert the number mean size to a mass mean size then as the mass mean is a cubic function of the diameter then our errors will be cubed or ± 27% ℎ Description of populations of particles • Cumulative distribution denoted by F, frequency distribution is written as dF f ( x) dx Typical cumulative frequency distribution fN(x) is the frequency distribution by number fS(x) is the frequency distribution by surface FS is the cumulative distribution by surface FM is the cumulative distribution by mass Comparison between distributions Fraction of number Fraction of volume Conversion between distributions Fraction of particles in the size range x to x+dx = f N ( x)dx • Cumulative distribution denoted by F, frequency distribution is written as dF f ( x) dx Fraction of the total surface of particles in the size range x to x+dx = fS ( x)dx Assume N is the total number of particles in the population, The number of particles in the size range Typical cumulative frequency distribution x to x+dx = N f N ( x)dx Surface area of these particles = 2 where S is the factor relating linear dimension of particle to its surface area. The fraction of the total surface area contained on these particles [ f S ( x)dx] is 2 where S is the total surface area of the population of particles. For a given population of particles : 1) 2) Total number of particles, N and total surface area S are constant. Assume particle shape is independent of size, so s is constant. where V is the total volume of the population of particles and is the factor relating the linear dimension of the particle to its volume. Example 2 Describing the population by a single number • Definitions of means where x is the mean and g is the weighting function. weighting function is different for each mean definition. Plot of cumulative frequency against weighting function g(x). Shaded area is Comparison between measures of central tendency. Adapted from Rhodes (1990). Comparison of the valuea of different means and mode and median for a given particle size distribution highlights : 1) The values of the different expressions of central tendency can vary significantly. 2) Two quite different distributions could have the same arithmetic mean or median. Equivalence of Means Means of different distributions can be equivalent Arithmetic mean of a surface distribution, _ The harmonic mean xhV of a volume distribution is defined as Same Expression Relationship between surface and volume Surface-volume mean can be calculated as then, Recalling, dFs x2ksdFN then, Note : Ks and Kv do not vary with size 1) 2) the arithmetic mean of the surface distribution, or the harmonic mean of the volume distribution Common methods of displaying size distributions Arithmetic-normal Distribution Log-normal Distribution z log x z: Arithmetic mean of z, sz: standard deviation of log x Arithmetic-normal distribution with an arithmetic mean of 45 and standard deviation of 12. Log-normal distribution plotted on linear coordinates Log-normal distribution plotted on logarithmic coordinates Methods of particle size measurements Sieving • Dry sieving using woven wire sieves is a simple, cheap method of size analysis • Suitable for particle sizes greater than 45 μm. Sieving gives a mass distribution and a size known as the sieve diameter. • Since the length of the particle does not hinder its passage through the sieve apertures (unless the particle is extremely elongated), the sieve diameter is dependent on the maximum width and maximum thickness of the particle. • The most common modern sieves are in sizes such that the ratio of adjacent sieve sizes is the fourth root of two (eg. 45, 53, 63, 75, 90, 107 μm). • Air jet sieving, in which the powder on the sieve is fluidized by a jet or air, can achieve analysis down to 20 μm. • Analysis down to 5 μm can be achieved by wet sieving, in which the powder sample is suspended in a liquid. Microscopy • The optical microscope may be used to measure particle size down to 5 mm. • The electron microscope may be used for size analysis below 5 mm. • Coupled with an image analysis system, the optical and electron microscopy can give number distribution of size and shape. • Such systems calculate various diameters from the projected image of the particles (e.g. Martin’s, Feret’s, shear, projected area diameters, etc.). • For irregular-shaped particles, the projected area offered to the viewer can vary significantly. Technique (e.g. applying adhesive to the microscope slide) may be used to ensure “random orientation”. Laser Diffraction How Laser Diffraction Works? • When light passing through a suspension, the diffraction angle is inversely proportional to the particle size. • An instrument consist of a laser as a source of coherent light of known fixed wavelength (typically 0.63 μm), a suitable detector (usually a slice of photosensitive silicon with a number of discrete detectors, and some means of passing the sample of particles through the laser light beam (techniques are available for suspending particles in both liquids and gases are drawing them through the beam). • This method gives a volume distribution and measures a diameter known as the laser diameter. • Particle size analysis by laser diffraction is very common in industry today. The associated software permits display of a variety of size distributions and means derived from the original measured distribution. Permeametry 1) Size analysis method based on fluid flow through a packed bed. 2) Carmen-Kozeny equation for laminar flow through packed bed of uniformly sized spheres of diameter x where -p is the pressure drop across the bed is the porosity U is the superficial fluid velocity. where: • is the total height of the bed; • is the superficial or "empty-tower" velocity; • is the viscosity of the fluid; • is the porosity of the bed; • x is the sphericity of the particles in the packed bed; • D is the diameter of the related spherical particle U = v/D2 Example 3 1.28 g of a powder of particle density 2500 kg/m3 are charged into the cell of an apparatus for measurement of particle size and specific surface area by permeametry. The cylindrical cell has a diameter of 1.14 cm and the powder forms a bed of depth 1 cm. Dry air of density 1.2 kg/m3 and viscosity 18.4 x 10-6 Pa s flows at a rate of 36 cm3/min through the powder (in a direction parallel to the axis of the cylindrical cell) and producing a pressure difference of 100 mm of water across the bed. Determine (i) the surface-volume mean diameter [20.08 mm] (ii) the specific surface of the powder sample [119.5 m2/kg] Electrozone sensing As particle flow through the orifice, a voltage pulse is recorded. The amplitude of the pulse can be related to the volume of particle the passing orifice. By electronically counting and classifying the pulses according to amplitude this technique can give a number distribution of the equivalent volume sphere diameter. Particle range: 0.3-1000 mm. Errors if more than 1 particle passes through the orifice at a time. Sedimentation Sedimentation Assumptions: 1. The suspension is sufficiently dilute for the particles to settle as individuals (i.e. not hindered settling). 2. Motion of the particles in the liquid obeys Stokes’ law (true for particles typically smaller than 50 μm). 3. Particles are assumed to accelerate rapidly to their terminal free fall velocity UT so that the time for acceleration is negligible. All particles travel at terminal velocity defined by Stokes’ law UT = h/t, thus