Mechanical Operations (CH 31007)
Unit operations • There are many physical operations that are common to a number of the individual industries, and may be regarded as “unit operations” • Some of these operations involve particulate solids – many of them are aimed at achieving a separation of the components of a mixture
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Mechanical operations • Chemical Engineering unit operations : – Fluid flow processes: fluids transportation, filtration, and solids fluidization – Heat transfer processes: evaporation, condensation, and heat exchange – Mass transfer processes: gas absorption, distillation, extraction, adsorption, and drying – Thermodynamic processes: gas liquefaction, and refrigeration – Mechanical processes: solids transportation, crushing and pulverization, and screening and sieving 03-09-2015
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Why mechanical operations!
Foundation of designs of chemical plants, factories, and equipment used
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Particulate Solids • Solids are difficult than Fluids ! – complex geometrical arrangements – basic problem of defining completely the physical state of the material
• The most important characteristics of an individual particle – its composition, properties as density and conductivity [provided the particle is completely uniform] – size and shape 03-09-2015
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Particulate Solids • Particle size affects properties such as the surface per unit volume and the rate at which a particle will settle in a fluid • Particle shape ? • Industrial scale: Large quantities of particles are handled and it is frequently necessary to define the system as a whole – Not the particle size – But the particle size distribution – Mean size
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Particulate Solids • To reduce the size of particles • To enlarge the size of particles or form them into aggregates or sinters
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What will I cover? •
Crushing and grinding
•
Mean particle size
•
Size distribution
•
Crushers and mills
•
Screening –
Sieve or membrane: Screen of filter
–
Settling: different rate of sedimentation of particles or drops as they move through gas or liquid
–
Special cases: Electrostatic, magnetic etc.
•
Slurry Transport
•
Mixing and Segregation
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Few things to remember! Mid-Sem
30
Sep 14 - 22, 2015
End-Sem
50
Nov 18, 2015 onward
TA
20
??
Coulson & Richardson’s Chemical Engineering Vol. 2: P article
Technology and Separation P rocesses: Richardson, J. F., Harker, J. H., and Backhurst, J. R.) [Butterworth-Heinemann] Unit Operations of Chemical Engineering: McCABE, W. L., SMITH, J. C. and HARRIOTT, P. [McGraw-Hill]
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Particle Size Reduction Mechanical Operations (CH 31007)
Background • Materials are rarely found in the size range required • Often necessary either to decrease or to increase the size • While decreasing in size – particle size will have to be progressively reduced in stages – Most appropriate type of machine at each stage depends • size of the feed and of the product • properties as compressive strength, brittleness and stickiness
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Background • Sometimes very fine powders are too difficult to handle • Hazardous dust clouds during transportation • Size enlargement processes include – granulation for the preparation of fertilisers – compaction using compressive forces to form the tablets in pharmaceuticals
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Single particles •
The sphere of the same volume as the particle.
•
The sphere of the same surface area as the particle.
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The sphere of the same surface area per unit volume as the particle.
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The sphere of the same area as the particle when projected on to a plane perpendicular to its direction of motion.
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The sphere of the same projected area as the particle, as viewed from above, when lying in its position of maximum stability such as on a microscope slide for example.
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The sphere which will just pass through the same size of square aperture as the particle, such as on a screen for example.
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The sphere with the same settling velocity as the particle in a specified fluid.
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Sphericity
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Size Reduction of Solids (Comminution) • To increase the surface area – most reactions involving solid particles, the rate of reactions is directly proportional to the area of contact with a second phase – drying of porous solids, where reduction in size causes both an increase in area and a reduction in the distance the moisture must travel within the particles in order to reach the surface
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Size Reduction of Solids (Comminution) • Necessary to break a material into very small particles in order to separate two constituents, especially where one is dispersed in small isolated pockets • Colour and covering power of a pigment is considerably affected by the size of the particles • More intimate mixing of solids can be achieved if the particle size is small
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Mechanism of Size Reduction • A single lump of material is subjected to a sudden impact/blow – few relatively large particles – a number of fine particles – relatively few particles of intermediate size
• Energy in the blow is increased – the larger particles will be of a rather smaller size & more numerous – the number of fine particles will be appreciably increased, but their size will not be much altered
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Mechanism of Size Reduction • Size of the fine particles: closely connected with the internal structure of the material • Size of the larger particles: more closely connected with the process by which the size reduction is effected • Grind limit: After some time there seems to be little change in particle size but may result in a change in shape rather than in size
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Effect of progressive grinding on size distribution persistent mode
transitory mode
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Method of Application of the Force • Impact: particle concussion by a single rigid force • Compression: particle disintegration by two rigid forces • Cutting or Shear: produced by a fluid or by particle– particle interaction • Attrition or Rubbing: arising from particles scraping against one another or against a rigid surface
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Method of Application of the Force • Impact: Coarse, medium, or fine particles • Compression: Coarse and relatively few fine particles • Cutting / Shear: definite particle size & shape, no fines • Attrition / Rubbing: very fine particles
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Crushing + Attrition • Stress applied between two surfaces (either surface– particle or particle–particle) at low velocity, 0.01–10 m/s
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Jaw Crusher
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Gyratory Crusher
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Crushing Rolls
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Impact + Attrition • Stress applied at a single solid surface (surface–particle or particle–particle) at high velocity, 10–200 m/s
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Hammer Mill
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Fluid Energy Mill
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Shear + Attrition • Stress applied by carrier medium–usually in wet grinding to bring about disagglomeration
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Sand Mill
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Colloid Mill
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Ball Mill
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Ball mill • May be used wet or dry although wet grinding facilitates the removal of the product. • The costs of installation and power are low. • May be used with an inert atmosphere and therefore can be used for the grinding of explosive materials. • The grinding medium is cheap. • Suitable for materials of all degrees of hardness. • May be used for batch or continuous operation. 3-Sep-15
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Hammer Mill
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Hardinge conical ball
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Question! • If a ball mill, 1.2 m in diameter, is operating at 0.80 Hz, suggest the modification in operating condition to achieve its improved efficiency.
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Ideal Crusher or Grinder !! • Large capacity • Product of single size or size distribution • Requirement of small power input per unit of product
Size reduction is a very inefficient process and only between 0.1 and 2.0 per cent of the energy supplied to the machine appears as increased surface energy in the solids 3-Sep-15
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Energy Requirements
for p = -2
Rittinger’s law fc is thefor crushing strength µ of the the increase material in surface The energy required size reduction 3-Sep-15
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Energy Requirements for p = -1
Kick’s law The energy required for size reduction µ the reduction ratio
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Energy Requirements •
Neither of these two laws permits an accurate calculation of the energy requirements
•
Rittinger’s law is applicable mainly to that part of the process where new surface is being created and holds most accurately for fine grinding where the increase in surface per unit mass of material is large
•
Kick’s law, more closely relates to the energy required to effect elastic deformation before fracture occurs
•
Kick’s law is more accurate than Rittinger’s law for coarse crushing where the amount of surface produced is considerably less
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Energy Requirements for p = -3/2
Bond’s law
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CH 31007
Work index: the amount of energy required to reduce unit mass of material from an infinite particle size to a size L2 of 100 μm
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Problem
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Classification of size reduction equipment
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Nature of the material to be crushed •
Hardness –
The hardness of the material affects the power consumption and the wear on the machine. With hard and abrasive materials it is necessary to use a low-speed machine and to protect the bearings from the abrasive dusts that are produced.
•
Structure –
Normal granular materials such as coal, ores and rocks can be effectively crushed employing the normal forces of compression, impact, and so on. With fibrous materials a tearing action is required
•
Moisture content –
It is found that materials do not flow well if they contain between about 5 and 50 per cent of moisture. In general, grinding can be carried out satisfactorily outside these limits.
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Nature of the material to be crushed • Crushing strength – The power required for crushing is almost directly proportional to the crushing strength of the material.
• Friability – The friability of the material is its tendency to fracture during normal handling. In general, a crystalline material will break along well-defined planes and the power required for crushing will increase as the particle size is reduced.
• Stickiness – A sticky material will tend to clog the grinding equipment and it should therefore be ground in a plant that can be cleaned easily.
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Nature of the material to be crushed • Soapiness – In general, this is a measure of the coefficient of friction of the surface of the material. If the coefficient of friction is low, the crushing may be more difficult.
• Explosive materials – must be ground wet or in the presence of an inert atmosphere.
• Hazardous materials – Materials yielding dusts that are harmful to the health must be ground under conditions where the dust is not allowed to escape. 3-Sep-15
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Types of Crushing Equipment Crushers (coarse & fine)
Grinders (intermediate & fine)
Ultrafine grinders
Cutting machines
Jaw crushers
Hammer mills
Hammer mills with internal classification
Knife cutters
Gyratory crusher
Bowl mills, Roller mills Fluid-energy mills
Dicers
Crushing rolls
Attrition mills
Slitters
Agitated mills
Tumbling mills (Ball mill, Rod mill, pebble mill, Tube mill)
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Methods of operating crushers • Free crushing – feeding the material at a comparatively low rate so that the product can readily escape – short residence time prevents the production of appreciable quantities of undersize material
• Choke feeding – the machine is kept full of material – discharge of the product is blocked so that the material remains in the crusher for a longer period 3-Sep-15
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Methods of operating crushers • Open circuit grinding – choke feeding • Closed circuit grinding – free crushing
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Recap
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Particle Size Distribution
Single particle size !!
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It is important to use the method of size measurement which directly gives the particle size which is relevant to the situation or process of interest.
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Why! • Quantitative indication of the mean size and of the spread of sizes • Results of a size analysis can most conveniently be represented by means of a cumulative mass fraction curve
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Size frequency curve • Size frequency curve
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Cumulative distribution
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Differential frequency distribution
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Different distributions
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Conversion between distributions • Many modern instruments actually measure a number distribution, which is rarely needed in practice
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Conversion between distributions • If N is the total number of particles in the population, the number of particles in the size range
• the surface area of these particles
aS is the factor relating the linear dimension of the particle to its surface area 3-Sep-15
Conversion between distributions S is the total surface area of the population of particles
For a given population of particles, the total number of particles, N, and the total surface area, S are constant.
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Conversion between distributions • assuming particle shape is independent of size, i.e. aS is constant
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Conversion between distributions • Similarly, for the distribution by volume
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Conversion between distributions
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Describing the mixture by a single number • The mode – Most frequently occurring size in the sample – For the same sample, different modes would be obtained for distributions by number, surface and volume
• The median – Easily read from the cumulative distribution as the 50% size – The size which splits the distribution into two equal parts
• Different means including arithmetic, geometric, quadratic, harmonic, etc. 3-Sep-15
Different means
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Mean sizes based on volume Considering unit mass of particles consisting of n1 particles of characteristic dimension d1, constituting a mass fraction x1 volume mean diameter
mean volume diameter
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Different means
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Problem
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Solution
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Problem
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Solution
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0
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Quiz! •
The size analysis of a powdered material on a mass basis is represented by a straight line from 0% mass at 1 micron particle size to 100% mass at 101 micron particle size. What is the surface mean diameter of the particles constituting the system?
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Recap • Frequency distribution curves • Cumulative curves • Cumulative distribution is the integral of the frequency distribution • Distributions can be by number, surface, mass or volume
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Methods of particle size measurement • Sieving (>50 μm) – dry sieving using woven wire sieves is a simple, cheap method – gives a mass distribution and a size known as the sieve diameter – sieve series are arranged so that the ratio of aperture sizes on consecutive sieves is 2, 21/2 or 21/4 according to the closeness of sizing – horizontal & vertical vibration – lower limit of size!!
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Screen efficiency • F = feed, D = overflow, B = underflow • xF , xD , xB mass fraction of material A • (1-xF), (1-xD), (1-xB), mass fraction of material C • F=B+D • F xF = D xD + B xB Þ
𝐷𝐷 𝐹𝐹
=
𝑥𝑥𝐹𝐹 −𝑥𝑥𝐵𝐵 𝑥𝑥𝐷𝐷 −𝑥𝑥𝐵𝐵
Þ
𝐵𝐵 𝐹𝐹
=
𝑥𝑥𝐹𝐹 −𝑥𝑥𝐷𝐷 𝑥𝑥𝐵𝐵 −𝑥𝑥𝐷𝐷
Screen efficiency • 𝐸𝐸𝐴𝐴 =
• 𝐸𝐸𝐵𝐵 =
𝐷𝐷𝑥𝑥𝐷𝐷 ; 𝐹𝐹𝑥𝑥𝐹𝐹
based on oversize
𝐵𝐵(1−𝑥𝑥𝐵𝐵 ) ; 𝐹𝐹(1−𝑥𝑥𝐹𝐹 )
• 𝐸𝐸 = 𝐸𝐸𝐴𝐴 𝐸𝐸𝐵𝐵 = Þ 𝐸𝐸 =
based on undersize
𝐷𝐷𝑥𝑥𝐷𝐷 𝐵𝐵(1−𝑥𝑥𝐵𝐵 ) ; 𝐹𝐹𝑥𝑥𝐹𝐹 𝐹𝐹(1−𝑥𝑥𝐹𝐹 )
overall effectiveness
(𝑥𝑥𝐹𝐹 −𝑥𝑥𝐵𝐵 )(𝑥𝑥𝐷𝐷 −𝑥𝑥𝐹𝐹 )𝑥𝑥𝐷𝐷 (1−𝑥𝑥𝐵𝐵 ) (𝑥𝑥𝐷𝐷 −𝑥𝑥𝐵𝐵 )2 1−𝑥𝑥𝐹𝐹 𝑥𝑥𝐹𝐹
Capacity and effectiveness of screens • The capacity of a screen is measured by the mass of material that can be fed per unit time to a unit area of the screen. • Capacity and effectiveness are opposing factors. • To obtain maximum effectiveness, the capacity must be small, and large capacity is obtainable only at the expense of a reduction in effectiveness.
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For coarse crushing, Kick’s law may be used mean diameter of feed = 45 mm, mean diameter of product = 4 mm, energy consumption = 13.0 kJ/kg, compressive strength = 22.5 N/m2
mean diameter of feed = 42.5 mm, mean diameter of product = 0.50 mm compressive strength = 45 MN/m2
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Methods of particle size measurement • Microscopic analysis (1–100 μm) – measurement of the projected area of the particle and also enables an assessment to be made of its 2-D shape
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Methods of particle size measurement • Sedimentation and elutriation methods (>1 μm) – the rate of sedimentation of a sample of particles in a liquid – The suspension is dilute and so the particles are assumed to fall at their single particle terminal velocity in the liquid (usually water) – Stokes’ law is assumed to apply – the method using water is suitable only for particles typically less than 50 mm in diameter
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• terminal falling velocity of a particle in a fluid increases with size
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Assumptions • The suspension is sufficiently dilute for the particles to settle as individuals (i.e. not hindered settling) • Motion of the particles in the liquid obeys Stokes’ law (true for particles typically smaller than 50 mm) • Particles are assumed to accelerate rapidly to their terminal free fall velocity UT so that the time for acceleration is negligible
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Sedimentation
Particles are assumed to accelerate rapidly to their terminal free fall velocity UT so that the time for acceleration is negligible 3-Sep-15
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Ergun Equation
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Permeametry • This is a method of size analysis based on fluid flow through a packed bed
• The pressure gradient across a packed bed of known voidage is measured as a function of flow rate • The diameter we calculate from the Carman–Kozeny equation is the arithmetic mean of the surface distribution 3-Sep-15
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Electrozone Sensing •
Particles are held in suspension in a dilute electrolyte which is drawn through a tiny orifice with a voltage applied across it
•
As particles flow through the orifice a voltage pulse is recorded
•
The amplitude of the pulse can be related to the volume of the particle passing the orifice
•
Thus, by electronically counting and classifying the pulses according to amplitude this technique can give a number distribution of the equivalent volume sphere diameter
•
The lower size limit is dictated by the smallest practical orifice and the upper limit is governed by the need to maintain particles in suspension
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Although liquids more viscous than water may be used to reduce sedimentation, the practical range of size for this method is 0.3–1000 mm 3-Sep-15
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SAMPLING • In practice, the size distribution of many tonnes of powder are often assumed from an analysis performed on just a few grams or milligrams of sample • The importance of that sample being representative of the bulk powder cannot be overstated • The powder should be in motion when sampled • The whole of the moving stream should be taken for many short time increments
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Methods of particle size measurement • Microscopic analysis (1–100 μm) – measurement of the projected area of the particle and also enables an assessment to be made of its 2-D shape
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Methods of particle size measurement • Sedimentation and elutriation methods (>1 μm) – the rate of sedimentation of a sample of particles in a liquid – The suspension is dilute and so the particles are assumed to fall at their single particle terminal velocity in the liquid (usually water) – Stokes’ law is assumed to apply – the method using water is suitable only for particles typically less than 50 mm in diameter
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• terminal falling velocity of a particle in a fluid increases with size
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Assumptions • The suspension is sufficiently dilute for the particles to settle as individuals (i.e. not hindered settling) • Motion of the particles in the liquid obeys Stokes’ law (true for particles typically smaller than 50 mm) • Particles are assumed to accelerate rapidly to their terminal free fall velocity UT so that the time for acceleration is negligible
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Sedimentation
Particles are assumed to accelerate rapidly to their terminal free fall velocity UT so that the time for acceleration is negligible 3-Sep-15
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Ergun Equation
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Permeametry • This is a method of size analysis based on fluid flow through a packed bed
• The pressure gradient across a packed bed of known voidage is measured as a function of flow rate • The diameter we calculate from the Carman–Kozeny equation is the arithmetic mean of the surface distribution 3-Sep-15
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Electrozone Sensing
Although liquids more viscous than water may be used to reduce sedimentation, the practical range of size for this method is 0.3–1000 mm 3-Sep-15
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Electrozone Sensing •
Particles are held in suspension in a dilute electrolyte which is drawn through a tiny orifice with a voltage applied across it
•
As particles flow through the orifice a voltage pulse is recorded
•
The amplitude of the pulse can be related to the volume of the particle passing the orifice
•
Thus, by electronically counting and classifying the pulses according to amplitude this technique can give a number distribution of the equivalent volume sphere diameter
•
The lower size limit is dictated by the smallest practical orifice and the upper limit is governed by the need to maintain particles in suspension
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SAMPLING • In practice, the size distribution of many tonnes of powder are often assumed from an analysis performed on just a few grams or milligrams of sample • The importance of that sample being representative of the bulk powder cannot be overstated • The powder should be in motion when sampled • The whole of the moving stream should be taken for many short time increments
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Particle in a Fluid • To develop an understanding of the forces resisting the motion of a single particle
• To provide methods for the estimation of the steady velocity of the particle relative to the fluid
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Motion of Solid Particles in a Fluid • The drag force resisting very slow steady relative motion (creeping motion) between a rigid sphere and a fluid of infinite extent
where U is the relative velocity, x is the sphere dia.
Stokes’ law 3-Sep-15
Stokes’ law • Stokes’ law is found to hold for single particle Reynolds number, – almost exactly for Rep ≤ 0.1 – within 9% for Rep ≤ 0.3
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Drag Coefficient
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Particles falling under gravity through a fluid
• A particle falling from rest in a fluid will initially experience a high acceleration as the shear stress drag, which increases with relative velocity, will be small. • As the particle accelerates the drag force increases, causing the acceleration to reduce. • Eventually a force balance is achieved when the acceleration is zero and a maximum or terminal relative velocity is reached. This is known as the single particle terminal velocity. 3-Sep-15
Particle terminal velocity
where UT is the single particle terminal velocity
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Particle terminal velocity
in the Stokes’ law region
terminal velocity is proportional to the square of the particle diameter
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Particle terminal velocity • In the Newton’s law region
terminal velocity is independent of the fluid viscosity and proportional to the square root of the particle diameter
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Particle terminal velocity • In the intermediate region no explicit expression
• Generally, when calculating the terminal velocity for a given particle or the particle diameter for a given velocity, it is not known which region of operation is relevant.
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For a given particle size
Archimedes number produce a straight line of slope ─2 if plotted on the logarithmic coordinates (log CD versus log Rep) of the standard drag curve. The intersection of this straight line with the drag curve gives the value of Rep.
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For a given UT
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Non-Spherical Particles • Shape affects drag coefficient far more in the intermediate and Newton’s law regions than in the Stokes’ law region. • In the Stokes’ law region particles fall with their longest surface nearly parallel to the direction of motion, whereas, in the Newton’s law region particles present their maximum area to the oncoming fluid.
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Sand particles falling from rest in air (particle density, 2600 kg/m3)
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Effect of boundaries on terminal velocity • When a particle is falling through a fluid in the presence of a solid boundary the terminal velocity reached by the particle is less than that for an infinite fluid. • In practice, this is really only relevant to the falling sphere method of measuring liquid viscosity, which is restricted to the Stokes’ region.
3-Sep-15
Effect of boundaries on terminal velocity • In the case of a particle falling along the axis of a vertical pipe this is described by a wall factor,
the ratio of the velocity in the pipe, UD to the velocity in an infinite fluid, Ua. 3-Sep-15
•
A sphere of diameter 10 mm and density 7700 kg/m3 falls under gravity at terminal conditions through a liquid of density 900 kg/m3 in a tube of diameter 12 mm. The measured terminal velocity of the particle is 1.6 mm/s. Calculate the viscosity of the fluid. Verify that Stokes’ law applies.
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Separation • Separation can be divided into 2 classes: – Diffusional operations: transfer of material between phases – Mechanical separation: based on physical differences, e.g. size, shape, density
• Mechanical separation are applied to heterogeneous mixtures, NOT to homogeneous mixtures
3-Sep-15
Mechanical Separation • Separation of – solids from gases – liquid drops from gases – solids from solids – solids from liquids
• Mechanical separation can be achieved by: – Sieve or membrane: Screen of filter – Settling: different rate of sedimentation of particles or drops as they move through gas or liquid – Special cases: Electrostatic, magnetic etc. 3-Sep-15
Screening • Separating particles due to size ONLY • Single screen gives unsized fractions • Series of screens provides sized fractions • Commonly applied for large scale for the separation • Generally applicable for particles of a size as small as about 50 μm
3-Sep-15
Screening • For very fine materials – difficulty of producing accurately woven fine gauze of sufficient strength – screens become clogged – other methods of separation are usually more economical
• Woven wire cloth is generally used for fine sizes and perforated plates for the larger meshes
3-Sep-15
Screening • Commonly done in dry mode, occasionally in wet mode • With coarse solids the screen surface may be continuously washed by means of a flowing stream of water – to keep the particles apart – to remove the finer particles from the surface of larger particles – to keep the screen free of adhering materials
3-Sep-15
Screening • Fine screens are normally operated wet, with the solids fed continuously as a suspension • Concentrated suspensions have high effective viscosities and frequently exhibit shear-thinning non-Newtonian characteristics – By maintaining a high cross-flow velocity over the surface of the screen, or by rapid vibration, the apparent viscosity of the suspension may be reduced and the screening rate substantially increased. 3-Sep-15
Multiphase systems Dissolved or dispersed phase
Continuous medium
Solution
Colloid
Coarse dispersion
Gas
Gas
Gas mixture: air (oxygen and other gases in nitrogen)
None
None
Liquid
Gas
None
Aerosols of liquid particles: fog, mist, vapor, hair sprays
Aerosol
Solid
Gas
None
Aerosols of solid particles: smoke, cloud, air particulates
Solid aerosol: dust
Gas
Liquid
Solution: oxygen in water
Liquid foam: whipped cream, shaving cream
Foam
Liquid
Liquid
Solution: alcoholic beverages
Emulsion
Emulsion: milk, mayonnaise, hand cream
Solid
Liquid
Solution: sugar in water
Liquid sol: pigmented ink, blood
Suspension: mud (soil, clay or silt particles are suspended in water)
Gas
Solid
Solution: hydrogen in metals
Solid foam: aerogel
Foam: dry sponge
Liquid
Solid
Solution: amalgam
Gel: agar, gelatin, silica gel, opal
Wet sponge
Solid
Solid
Solution: alloys
Solid sol: cranberry glass
Gravel, granite
3-Sep-15
Source: Wikipedia
Non-Newtonian fluid
3-Sep-15
Non-Newtonian fluid
Time-dependent viscosity
Rheopectic
Apparent viscosity increases with Printer ink duration of stress
Thixotropic
Yogurt, aqueous iron oxide gels, gelatin gels, Apparent viscosity decreases with some clays, some drilling muds, duration of stress many paints, colloidal suspensions
Shear thickening Apparent viscosity increases with Suspensions of corn starch in (dilatant) increased stress water, sand in water
Timeindependent viscosity
Shear thinning (pseudoplastic)
Nail polish, whipped cream, ketchup, molasses, Apparent viscosity decreases with syrups, paper pulp in water, latex increased stress paint, blood, some silicone oils, some silicone coatings
Viscosity is constant Generalized Stress depends on normal and Newtonian fluids shear strain rates and also the pressure applied on it
3-Sep-15
Custard, Water
Screening Equipment • In most cases, the particles drop through the openings by gravity • Coarse particles drop through easily, but with fine particles, screen must be agitated • Agitation can be done by – shaking – vibrating – mechanically or electrically
3-Sep-15
Stationary screen & Grizzly • Made of longitudinal bars up to 3 m long, fixed in a rectangular framework • Space between bars is 2 – 8 in. • Usually inclined at an angle to the horizontal • Greater the angle, the greater is the throughput BUT the screening efficiency is reduced • Effective for very coarse free-flowing solids containing few fine particles 3-Sep-15
Grizzlies
3-Sep-15
Source: Google Image
Electromagnetic screen
3-Sep-15
The screen itself is vibrated
Mechanical screen
The whole assembly is vibrated 3-Sep-15
Mechanical screen • As very rapid accelerations and retardations are produced, the power consumption and the wear on the bearings are high • Generally mounted in a multi-deck fashion with the coarsest screen on top, either horizontally or inclined at angles up to 45˚ • With the horizontal machine, the vibratory motion fulfils the additional function of moving the particles across the screen
3-Sep-15
Mechanical screen • The screen area which is required for a given operation cannot be predicted without testing the material under similar conditions on a small plant
• In particular, the tendency of the material to clog the screening surface can only be determined experimentally
3-Sep-15
Trommel A very large mechanically operated screen
3-Sep-15
Electrostatic separator
3-Sep-15
Cyclone separator
3-Sep-15
