Particle Technology Lectures GIKI

CH241: Particle Technology

Spring – 2014

Introduction• Particle technology is a term used to refer to the science and

technology related to the handling and processing of particles.• Particle technology is also often described as powder technology,

particle science and powder science.• Particles are commonly referred to as bulk solids, particulate solids

and granular solids.• Today particle technology includes the study of liquid drops,

emulsions and bubbles as well as solid particles.• This course is however limited only to solid particles.• The discipline of particle technology now includes topics as diverse

as the formation of aerosols to the design of bucket elevators, crystallization to pneumatics transport, slurry filtration to silo design.

Importance• Solids used in chemical industries are most commonly in form of

particles.• Solids in general are more difficult to handle then liquid and gases.• In process industries solid appear in variety of forms, they may be hard

and abrasive, tough and rubbery, soft and fragile, dusty and cohesive, Free flowing or sticky.

• Particulate materials, powders or bulk solids are used widely in all areas of the process industries, for example in the food processing, pharmaceutical, biotechnology, oil, chemical, mineral processing, metallurgical, detergent, power generation, paint, plastics and cosmetics industries.

• So the knowledge of their properties, handling, storage, transportation, separation and processing is important from chemical engineering point of view.

Course Content• Introduction to the subject.• Characterization of solid particles (size, shape and density).• Fundamentals of solid handling (conveying and storage).• Mixing• Size reduction (crushing and grinding).• Size enlargement (crystallization, pelletization, and granualization).• Motion of particles in a fluid.• Separation techniques• Screening and Sieving (for solid – solid separation)• Sedimentation and Filtration (for solid – liquid separation)• Gas cleaning (for solid – gas separation)

Books to be consult

• Coulson & Richardson’s Chemical Engineering by J F Richardson & J H Harker with J R Backhurst. Volume 2, Fifth Edition.

• Units Operations of Chemical Engineering by Warren Lee McCabe, Julian Smith & Peter Harriott. Seventh Edition.

• Introduction to Particle Technology by Martin Rhodes. Second Edition.

Marks Distribution

• Assignments (Nos?): ?%• Quizzes (Nos?): ?%• Midterm exam: ?%• Final Exam: ?%• Attendance: ?%

1. Characterization of Solid Particle

Individual solid particles are characterized by their size, shape and density.

Size and shape are easily specified for regular particles, such as spheres and cubes, but for irregular particles ?

Why measure particle properties?

• Better control of quality of product (cement, urea, cosmetics etc)

• Better understanding of products, ingredients.• Designing of equipment for different operations such as

crushing, grinding, conveying, separation, storage etc.

Which particle properties are important to measure?

• In addition to chemical composition, the behavior of particulate materials is often dominated by the physical properties of the constituent particles.

• These can influence a wide range of material properties including, for example, reaction and dissolution rates, how easily ingredients flow and mix, or compressibility and abrasivity.

• From a manufacturing and development perspective, some of the most important physical properties to measure are:– Particle size– Particle shape– Surface properties– Mechanical properties– Charge properties– microstructure

1.1. Particle shape

• The shape of an individual particle is expressed in terms of the sphericity which is independent of particle size.

• Sphericity is the ratio of surface area of sphere of same volume as particle to the surface area of particle.

• So for spherical particle sphericity is equal to one.• For non-spherical particle it is defined by:

– Dp: equivalent diameter of particle

– Sp: surface area of one particle

– vp: volume of one particle

• The equivalent diameter is sometimes defined as the diameter of a sphere of equal volume.

• For fine particles, Dp is usually taken to be the nominal size based on screen analysis or microscopic analysis.

• The surface area is found from adsorption measurements or from the pressure drop in a bed of particles.

• For many crushed materials, Sphericity is between 0.6 and 0.8. For particles rounded by abrasion, their sphericity may be as high as 0.95.

• Exercise: Determine the sphericity of a particle of surface area 15 mm2 and volume 2 mm3.

1.2. Particle size• By far the most important physical property of particulate samples is

particle size.• Particle size measurement is routinely carried out across a wide range

of industries and is often a critical parameter in the manufacturing of many products.

• Particle size has a direct influence on material properties such as:– Reactivity or dissolution rate e.g. catalysts, tablets– Stability in suspension e.g. sediments, paints– Efficacy of delivery e.g. asthma inhalers– Texture and feel e.g. food ingredients– Appearance e.g. powder coatings and inks– Flowability and handling e.g. granules– Viscosity e.g. nasal sprays– Packing density and porosity e.g. ceramics.

• In general "diameter" may be specified for any equidimensional particles (e.g. emulsions or bubbles).

• Most of the solid particles used in industries are not equidimensional, therefore cannot be specified by a single dimension i.e. “diameter”.

• In order to simplify the measurement process, it is often convenient to define the particle size using the concept of equivalent spheres.

• In this case the particle size is defined by the diameter of an equivalent sphere having the same property as the actual particle such as volume or mass for example.

• The equivalent sphere concept works very well for regular shaped particles.

• However, it may not always be appropriate for irregular shaped particles, such as needles or plates, where the size in at least one dimension can differ significantly from that of the other dimensions.

• Such particles are often characterized by the second longest major dimension. For example needle like particles, Dp would refer to the thickness of the particle, not their length.

• Units used for particle size depend on the size of particles.– Coarse particles: inches or millimetres– Fine particles: screen size– Very fine particles: micrometers or nanometers– Ultra fine particles: surface area per unit mass, m2/g

1.3 Mixed particle sizes and size analysis

• In a sample of uniform particles of diameter Dp, the total volume of the particles is m/ρp, where m = mass of the sample, ρp = density. Since the volume of one particle is vp, the total number of particle in the sample is:

• The total surface area of the particles is:

• To apply the above two equations to mixtures of particles having various size and densities, the mixture is sorted into fractions, each of constant density and approximately constant size.

• Each fraction can then be weighed, or the individual particles in it can be counted or measured by any of the number of methods.

• Information from such a particle size analysis is tabulated to show the mass fraction in each size increment as a function of average particle size. The analysis tabulated in this way is called differential analysis.

• A second way to present the information is through a cumulative analysis obtained by adding, consecutively, the individual increments, starting with that containing the smallest particles, and tabulating or plotting the cumulative sums against the maximum particle diameter in the increment.

Differential Analysis

Cumulative Analysis

Mass Quantities of sample of particles

Mass fractions from data in previous figure.

Cumulative mass fraction plot of data from previous figure.

1.4. Specific surface of mixture

• If the particle density ρp and spericity Φs are known, the surface area of particles in each fraction can be calculated and added to give the specific surface, Aw (The total surface area of the unit mass of particles):

• Where xi = mass fraction in a given increment,Dpi = average diameter (taken as arithmetic

average of the smallest and largest particle diameters in increment).

1.5. Average particle size

• The average particle size for a mixture of particles is defined in several different ways.

• Volume surface mean diameter Ds:

If number of particle Ni in each fraction is known,instead of mass fraction xi, then:

• Arithmetic mean diameter:NT = number of particles

in the entire sample

• Mass mean diameter:

• Volume mean diameter:

• For sample consisting of uniform particles these average diameters are, of course, all the same. For mixture containing particle of various sizes, however, the several average diameters may differ widely from one another.

1.6. Number of particles in mixture

The volume of any particle is proportional to its "diameter" cubed.

a = volume shape factor Assuming that a is independent of size, then:

1.7 Screen analysis

•Testing sieves are made of woven wire screens.•Openings are square.•Screens are identified by Mesh No.•Mesh No. is the numbers of opening per linear inch.

•Area of opening in any screen = 2 x Area of opening in next smaller screen.•Mesh dimension of any screen = 1.41 x Mesh dimension of next smaller screen.

• Standard screens are used to measure the size (and size distribution) of particles in the size range between about 3 and 0.0015in (76mm and 38m m).

• Testing sieves are made of woven wire screens, the mesh and dimensions of which are carefully standardized.

• The openings are square.• Each screen is identified in mesh per inch, e.g. 10mesh, Dpi = 1/10 = 0.1in.• The actual openings are however smaller than those corresponding to the mesh

number, because of thickness of wire.• The area of the openings in any one screen in the series is exactly twice to that

of the openings in the next smaller screen. The ratio of the actual mesh dimension of any screen to that of the next smaller screen is =1.41.

• For close sizing, intermediate screen are available, each of which has a mesh dimension = 1.189 times that of next smaller standard screen.

• Analysis using standard screen: Screens are arranged serially in a stack, with the smallest mesh at the bottom and the largest at the top. Materials are loaded at top and then shacked for a period of time (e.g. 20 minutes).

• The particles retained of each screen are removed, weighed and masses of individual screen increments are converted into mass fraction of total sample.

• Any particle that passed the finest screen are caught in the pan at the bottom of stack.

• The results of screen analysis are tabulated to show the mass fraction of each screen increment as a function of the mesh size range of the increment.

• The notation 14/20 means “through 14 mesh and on 20 mesh”.• Typical screen analysis is given in next slide.

– First column: mesh size,– second column: width of opening of screen,– third column: mass fraction of total sample that is retained on that

screen xi (where i is the number starting from the bottom of the stack),

– fourth column: averaged particle size Dpi (since the particle on any screen are passed immediately by the screen ahead of it, the averaged of these two screen are needed to specify the averaged size in that increment).

– Fifth column: cumulative fraction smaller than Dpi.

Exercise

Size measurement with fine particles

• Dry screening is useful for sizing particles with diameter greater than about 44 μm (325 mesh).

• Wet screen analysis can be used for diameters down to 10 μm.• Optical microscopy and gravity sedimentation are used with

particles 1 – 100 μm.• Coulter counter, a device used for sizing and measuring

particles by measuring change in resistivity of an electrolyte as it carry particle one by one through a small orifice.

• Light scattering techniques, sedimentation in centrifuges and electron microscopy are other useful method for measuring size of even smaller particles.

PARTICULATE SOLIDS IN BULK• Masses of solid particles, especially when they are dry and not sticky,

have many properties of a fluid.• They exert pressure on sides of walls of container.• They flow through opening or inclined plane / channel.• Depending upon the flow property particulate solids are divided into

two classes, cohesive (wet clay, reluctant to flow through opening) and non-cohesive (grains, dry sand, plastic chips etc readily flow out of bin or silo).

• They differ from liquid and gasses in several ways because of particles interlocked at high pressure.

• Before the mass of tightly packed particles can flow, it must increased in volume to permit interlocking grains to move past one another.

Voidage

Voidage is the fraction of the total volume which is made up of the free space between the particles and is filled with fluid.

• One of the most important characteristics of any particulate mass.

• Voidage is the fraction of the total volume which is made up of the free space between the particles and is filled with fluid.

• Voidage corresponds to density of packing of the particles.• In general, isometric particles, will pack more densely than long

thin particles or plates.• The more rapidly material is poured on to a surface or into a

vessel, the more densely will it pack.• If it is then subjected to vibration, further consolidation may

occur.• The packing density or voidage is important in that it determines

the bulk density of the material.• It affects the tendency for agglomeration of the particles.• It critically influences the resistance offers to the fluid flowing

through it as for example in filtration.

Agglomeration

Agglomeration arises from interaction between particles, as a result of which they adhere to one another to form clusters.

• The main mechanisms giving rise to agglomeration are:– Mechanical interlocking: This can occur particularly if the particles are long

and thin in shape, in which case large masses may become completely interlocked.

– Surface attraction: Surface forces, including van der Waals’ forces, may give rise to substantial bonds between particles, particularly where particles are very fine (<10 μm), with the result that their surface per unit volume is high. In general, freshly formed surface, such as that resulting from particle fracture, gives rise to high surface forces.

– Plastic welding: When irregular particles are in contact, the forces between the particles will be applied on extremely small surfaces and the very high pressures developed may give rise to plastic welding.

– Electrostatic attraction: Particles may become charged as they are fed into equipment and significant electrostatic charges may be built up, particularly on fine solids.

– Effect of moisture: Moisture may have two effects. Firstly, it will tend to collect near the points of contact between particles and give rise to surface tension effects. Secondly, it may dissolve a little of the solid, which then acts as a bonding agent on subsequent evaporation.

– Temperature fluctuations: give rise to changes in particle structure and to greater cohesiveness.

Pressure in particulate solids• The exerted pressure is not same in all directions. In general the

pressure applied in one direction creates some pressure in other directions.

• The minimum pressure in solid masses is in the direction normal to that of applied pressure.

• In homogenous masses the ratio of normal pressure to applied pressure is constant which is the characteristic of material which depends on:– shape and interlocking tendency of particles,– stickiness of grain surfaces,– and degree of packing.

• It is nearly independent of particle size until the grain become very small and material is no loner free-flowing.

Angle of repose

When the granular solid are piled up on a flat surface, the sides of the pile are at a definite reproducible angle with the horizontal. This angle is called angle of repose of that material.

• If solid is poured from a nozzle on to a plane surface, it will form an approximately conical heap and the angle between the sloping side of the cone and the horizontal is the angle of repose. When this is determined in this manner it is sometimes referred to as the dynamic angle of repose or the poured angle.

• The angle of repose may also be measured using a plane sheet to which is stuck a layer of particles from the powder. Loose powder is then poured on to the sheet which is then tilted until the powder slides. The angle of slide is known as the static angle of repose or the drained angle.

• Angles of repose vary from about 20◦ with free-flowing solids, to about 60◦ with solids with poor flow characteristics.

• Powders with low angles of repose tend to pack rapidly to give a high packing density.

• An angle which is similar to the static angle of repose is the angle of slide which is measured in the same manner as the drained angle except that the surface is smooth and is not coated with a layer of particles.

• A measure of the frictional forces within the particulate mass is the angle of friction.

• The angle of friction is important in its effect on design of bin and hoppers.

• If the pressure at the base of a column of solids is measured as a function of depth, it is found to increase approximately linearly with height up to a certain critical point beyond which it remains constant.

• For heights greater than Lc the mass of additional solids is supported by frictional forces at the walls of the hopper.

Storage of Solids

Coarse solid like gravel, sand and coal are stored outside in large pile unprotected from weather. Solids that are two valuable

and soluble on expose to outdoor piles are stored in bins, hoppers or silos.

• When hundred and thousands of tons of solids are involved then storing out door in a pile is the most economical method.

• Valuable solids are stored in bins, hoppers or silos.• These are cylindrical or rectangular vessel of concrete or metal.

– Silo is tall relatively small in diameter.– Bin is not very tall but fairly wide.– Hopper is small vessel with sloping bottom.

• Silos and bins are used storage for some period of time while hoppers are used for temporary storage before feeding solid to the process.

• All these container are loaded from top by some kind of elevator; discharging is from the bottom.

• The major problem in solid storage vessel design is to provide satisfactory discharge.

Flow of solids in hoppers

• Discharge from the hopper takes place through an aperture at the bottom of the cone, and difficulties are commonly experienced in obtaining a regular, or sometimes, any flow.

• Commonly experienced types of behavior are shown in Figure 1.15.• Bridging of particles may take place and sometimes stable arches (b) may

form inside the hopper. These can usually be broken down by vibrators attached to the walls.

• A further problem which is commonly encountered is that of “piping” or “rat-holing”(c), in which the central core of material is discharged leaving a stagnant surrounding mass of solids. As a result some solids may be retained for long periods in the hopper and may deteriorate.

• Ideally, “mass flow” (a) is required in which the solids are in plug flow and move downwards in masse in the hopper. The residence time of all particles in the hopper will then be the same.

• In general, tall thin hoppers give better flow characteristics than short wide ones and the use of long small-angle conical sections at the base is advantageous.

• The nature of the surface of the hopper is important and smooth surfaces give improved discharge characteristics.

Discharge rate, measurement and control of solids flowrate

• The discharge rate of solid particles is usually controlled by the size of the orifice or the aperture at the base of the hopper.

• The rate of discharge of solids through the outlet orifice is substantially independent of the depth of solids in the hopper, provided this exceeds about four times the diameter of the hopper.

• BROWN has developed an equation for flow through an orifice

• The flowrate of solids can be measured either as they leave the hopper or as they are conveyed. Methods include:– Fitting an orifice plate at the discharge point from the hopper. – Using a belt-type feeder in which the mass of material on the belt is

continuously measured, by load cells for example or by a nuclear densitometer which measures the degree of absorption of gamma rays transmitted vertically through the solids on the belt which is travelling at a controlled speed.

– Applying an impulse method in which a solids stream impacts vertically on a sensing plate orientated at an angle to the vertical. The horizontal component of the resulting force is measured by as load cell attached to the plate.

– Alternatively, the level of the solids in the hopper may be continuously monitored using transducers covered by flexible diaphragms flush with the walls of the hopper. The diaphragm responds to the presence of the solids and thus indicates whether there are solids present at a particular level.

Conveying of solids

• Belt conveyors—where the solids are conveyed horizontally, or at small angles to the horizontal, on a continuous moving belt.

• Screw conveyors—in which the solids are moved along a pipe or channel by a rotating helical impeller, as in a screw lift elevator.

• Bucket elevators—in which the particles are carried upwards in buckets attached to a continuously moving vertical belt.

• Vibrating conveyors—in which the particles are subjected to an asymmetric vibration and travel in a series of steps over a table. During the forward stroke of the table the particles are carried forward in contact with it, but the acceleration in the reverse stroke is so high that the table slips under the particles. With fine powders, vibration of sufficient intensity results in a fluid-like behavior.

• Pneumatic/hydraulic conveying installations—in which the particles are transported in a stream of air/water.

Mixing of solids• Achieving good mixing of particulate solids of different size and

density is important in many processes for example in pharmaceuticals.

• It is more art than science.• One can never achieve perfect mixing unlike that in fluid phase.• In the mixing of solid particles, the following three mechanisms

may be involved:– Convective mixing, in which groups of particles are moved from one

position to another,– Diffusion mixing, where the particles are distributed over a freshly

developed interface, and– Shear mixing, where slipping planes are formed.

• A trough mixer with a ribbon spiral involves almost pure convective mixing, a simple barrel-mixer involves mainly a form of diffusion mixing and a shear hoppers involves shear mixing.

Through mixer with ribbon spiral:Convective mixing

Barrel mixer:Diffusive mixing

Shear hopper:Shear mixing

The performance of an industrial mixer is judged by:•The time required•The power load•The properties of the product.

Both the requirements of the mixing device and the properties of desired material vary widely from one problem to another.•Sometime a very high degree of uniformity is required•Sometime a rapid mixing action is necessary•Sometime a minimum amount of power is needed.

Mixing Equipments

• For paste and heavy material– Kneader mixer– Mixing rolls– Muller mixer– Pug mill– Dispersers

• For granular solids– Paddle mixer– Ribbon mixer– Tumbling mixer– Internal screw mixer– Impact wheel

Kneader mixer• Two arm kneader handles suspensions, pastes and light plastic masses.• Kneading is a method of mixing used with deformable solid or pastes.• It involve squashing the mass flat, holding it over itself and squashing it

once again.• Sometime kneading machine tear the mass apart and shear it between

the moving blade and stationary surface.• Considerable energy is required even with fairly thin materials and as

the mass becomes stiff and rubbery, the power requirements become very large.

• Material to be kneaded is dropped into trough and mixed for 5 – 20 minutes.

• Sometime the mass is heated while in the machine.• Trough is often unloaded by tilting it so that it content spill out.• In many kneader mixer trough is open.

Muller Mixer• Mulling is the rubbing action similar to that in motor and pestle.• At large scale processing this action is given by wide heavy wheel of mixer

as shown in figure.

• In this particular design of muller the pan is stationary causing the central vertical shaft is driven, causing the muller wheel to roll in a circular path over the layer of solid or paste on the pan floor.

• The rubbing action results from the slip of wheel on the solid.• Plow guided the solid under the muller wheel or to the opening in the

pan floor at the end of cycle when the mixture is being discharged.

Mixing effectiveness• Quantitatively measured by analysis of number of spot samples taken randomly

from various location of a mixed product.• Can be done by adding some kind of tracer material for easy analysis.• If:

– ‘μ’ – be the overall average fraction of tracer in the mix.– ‘N’ – be the number of small samples taken from various location of the mix.– ‘xi’ – be the fraction of tracer in each sample.– ‘x(bar)’ – be the averaged value of measured concentrations.

• Then:– x(bar) = μ for large value of N– xi = x(bar) for perfect mixing

• For incomplete mixing xi differ from x(bar) which is given by standard deviation:

• The deviation ‘s’ is the relative measure of mixing.• Low value of ‘s’ determines good mixing.• Its significance varies with amount of tracer.

• One index that increase as the mixing improves could be the reciprocal of the ratio of ‘s’ to the standard deviation at zero mixing ‘σ0’.

• For zero mixing the mix will be in two layer one in which xi = 0 and one in which xi = 1 then:

• Mixing index of the pastes ‘Ip’ is the ratio of ‘σ0’ to ‘s’:

• For solid the mixing index is based on standard deviation that would be observed with a fully blended mixture ‘σe’ (for pastes this factor is zero).

• Where ‘μp’ is the fraction measured by counting number of particles and ‘n’ is the number of particles in the spot sample.

• Ip is unity in start and increases as the mixing proceed.

• Theoretical Ip would have become infinite for very long mixing time.

• It does not happen in real because mixing never quite complete plus analytical method are not extraordinary precise.

• The Ip for sand rises rapidly and then level off and for other material the trend is same but mixing is slow and limiting value is lower.

• Ip typical fall in the range of 10 – 150.

• For granular solid mixing is initially rapid.• In tumbling blender mixing is never perfect.• Forces specially electrostatic are always at work which retard

mixing.

Mixing index at zero time for granular solid

• The standard deviation for complete mixing ‘σe’ is the used as a reference with mixing of granular solids.

• The standard deviation for zero mixing ‘σo’ is used as a reference with mixing of pastes.

• If ‘n’ is set equals to 1 then both equations become identical.• When n = 1 then xi = 1 or xi = 0 which is the same as in completely

unmixed material.• So for mixing index for granular solid for zero mixing willing be:

Rate of mixing• In mixing as in other processes the

rate is directly proportional to driving force.

• For the case of granular solid where ‘Is’ is the mixing index and ‘(1 – Is)’ is the driving force because mixing index at equilibrium will be 1.

• It is found that for short mixing time the rate of change of ‘Is’ is directly proportional to ‘(1 – Is)’.

• By solving we can get the time of mixing required for the desire mixing.

Exercise – from McCabe & Smith

Exercise – from McCabe & Smith

Assignment – from McCabe & Smith

CLASSIFICATION OF SOLID PARTICLES

• The problem of separating solid particles according to their physical properties is of great importance.

• For examples:– In the mining industry it is necessary to separate the valuable

constituents in a mineral from the adhering rocks which is usually of lower density. In this case, it is first necessary to crush the material so that each individual particle contains only one constituent.

– In coal washing plants in which dirt is separated from the clean coal.– The processing industries are more usually concerned with

separating a single material, such as the product from a size reduction plant or in obtaining a uniform material for incorporation in a system in which a chemical reaction takes place

• Separation depends on the selection of a process in which the behavior of the material is influenced to a very marked degree by some physical property.– If a material is to be separated into various size fractions, a sieving

method may be used because this process depends primarily on the size of the particles.

– Other methods of separation depend on the differences in the behavior of the particles in a moving fluid, and in this case the size and the density of the particles are the most important factors.

– Other processes make use of differences in electrical or magnetic properties of the materials.

• The maximum range of sizes that can be separated is calculated from the ratio of the sizes of the particles of the two materials which have the same terminal falling velocities.

• Gravity settling– The settling tank– The elutriator– The Spitzkasten– The double cone classifier– The mechanical classifier– The bowl classifier– The hydraulic jig– Riffled tables

• Centrifugal separators– The hydrocyclone

• Sieves or screens– Hummer electromagnetic

screen– Trommel

• Magnetic separators• Electrostatic separators• Flotation machine• Gas cleaning equipment

– Gravity separators– Centrifugal separators– Inertia or momentum

separators– Fabric filters– Electrostatic precipitators

The settling tank

The elutriator

The Spitzkasten

The double cone classifier

The mechanical classifier

The bowl classifier

The hydraulic jig

Riffled tables

The hydrocyclone

Hummer electromagnetic screen

Trommel