Solid separation processes - Ali...

transcript

Solid separation

processes

Ali Ahmadpour

Chemical Eng. Dept.

Ferdowsi University of Mashhad

Contents

Introduction

Physical properties of solids

Separation of particulates & powders

Air classification

References

Particle size measurements, T. Allen, 1997

Vol. 1: Powder sampling and particle size measurements

Vol. 2: Surface area and pore size determination

Powder surface area and porosity, Lowell & Shields,

Powder technology: fundamentals of particles, powder

beds, and particle generation, M. Hiroaki, H. Ko, Y.

Hideto, 2007

Air pollution control equipment, H. Brauner & Y.B.G.

Varma, 1981

Introduction

Separations involving solids, together with their properties which influence the separation will be discussed.

The removal of solids from gases will be illustrated, to show some of the difficulties in selecting solids separation methods.

Solids come in many forms, shapes and sizes, so some discussions of the main properties of solids which will influence different types of separation processes will be discussed.

Mechanical solid separation

techniques

Solids from liquids

Sedimentation:

Principles: gravity, centrifugal, electrostatic, magnetic centrifugation

Examples: gravity settlers, centrifugal clarifiers, hydrocyclones; use

of chemical flocculants or air flotation

Filtration:

Principles: gravity, vacuum, pressure and centrifugal

Examples: sand and cake filters, rotary vacuum filters, cartridge and

plate and frame filters, microfilters, use of filter aids

Solids from gases

Solids come in a wide variety of shapes and sizes.

Solids contain moisture ranging from <10% to > 90%.

Some operations where separations from solids is

involved, are:

Cleaning of products,

Sorting and size grading,

Fractionation or recovery of the main components

within the solid bulks.

Some separation operations is concerned with the fractionation of solids

(in their particulate or powder form), and their recovery from other

materials.

Separation of powders are based on factors such as:

size and shape,

density differences,

flow properties,

color and electrostatic charge,

An important pretreatment for many operations is size reduction, but in

some cases very fine powders provide processing problems, and

agglomeration may be used to improve flow characteristics and

wettability.

Classification of powders

Particle size and particle size distribution

Particle shape

Particle density

Forces of adhesion

Bulk properties

Bulk density and porosity

Flowability

Particle size and PSD

Operations that result in the production of a powder, e.g.

milling or spray drying, will give rise to a product with a

distribution of particle sizes and this distribution is of extreme

importance and will affect the bulk properties.

Particle size can be measured by measuring any physical

property which correlates with the geometric dimensions of

the sample.

geometric characteristics, such as linear dimensions, areas, volumes,

mass (microscopy or image scanning techniques);

settling rates (wet and dry sieving methods);

interference techniques such as electrical field interference and light or laser scattering or diffraction (electrical impedance methods such as the Coulter counter, laser diffraction patterns).

Sampling

Since particles can vary in both shape and size, different

methods of particle size analysis do not always give consistent

results.

different physical principles being exploited,

size and shape are interrelated.

Sampling is important to ensure that a representative sample is

taken, usually by the method of quartering.

The results are present in the form of a distribution curves:

Frequency distribution (histogram)

Cumulative distribution

Frequency (F) and Cumulative (C)

distributions

From the distribution curves, mean diameter, median diameter

and standard deviation can be calculated.

Mean diameter:

Median diameter: the diameter which cuts the cumulative

distribution in half.

Standard deviation:

Sauter mean particle diameter (d3/2):

Sauter diameter (d3/2):

Equivalent diameters: For particles with shapes other than

sphere, the diameter is calculated from the comparison of their

surfaces or volumes to sphere.

Volume

area Surface

241.16

The particle size and distribution has a pronounced effect on

interparticle adhesion, which will affect some of the bulk

properties, such as bulk density, porosity, flowability and

wettability.

• Feret's Diameter. This is depicted as dimension 'A', it is the overall length from

'tip-to-tail' of the particle.

• Martin's Diameter. This is depicted as dimension 'B', it is the length of a

theoretical horizontal line, which passes through the centre of gravity of the

particle, to touch the outer boundary walls of the particle.

• Projected Area Diameter. This is depicted as dimension 'C' and is the diameter

of a theoretical circle, which would contain the same projected area as the

irregular particle.

• Equivalent Diameter. This is the diameter of a sphere, which would contain the

same volume as the irregular particle.

• Aerodynamic Diameter. This is the diameter of a spherical particle that exhibits

the same settling velocity as the irregular particle.

Sampling technique(Coning and quartering process)

Sampling

devices

Sampling points

Sampling devices

Unit dose sampler

A popular sampler in the pharmaceutical

industry for taking a small volume

cohesive powder sample.

SLOT SAMPLER

For target, multilayer and

average sampling

Powder sampler

Sampling from falling streams

Sample splitter

Particle shape

Sphere has the lowest and a chain of atoms has the highest

surface/volume ratio.

The relation between particle’s surface area and shape can be

shown by assuming two particles with same weights one in

sphere and the other in cubic forms.

spherecubespherecubespherecube VVVVMM

sphere

spherecubecube3

sphere3cube

sphere

Porosity

Porosity is the summation of surfaces of those pores that their

depths are more than their diameters.

Surface area of non-porous sphere particles:

Particles with r = 0.01 m and = 3 g/cm3 have 100 m2/g surface area.

Particles with r = 0.1 m and = 3 g/cm3 have 10 m2/g surface area.

Particles with r = 1 m and = 3 g/cm3 have 1 m2/g surface area.

But, porous particles with r = 1 m and = 3 g/cm3 have >1000 m2/g surface

area. This shows the importance of porosity.

21t Nr4NrNrNr4S

4NrNrNr

Particle density

The density of an individual particle is important as it will

determine whether the component will float or sink in water or

any other solvent; the particle may or may not contain air.

Air has a density of 1.27 kg/m3.

Therefore, this equation is not applicable where there is a

substantial volume fraction of air in the particle.

An estimate of the volume fraction of air (Va) can be made

Differences in particle densities are exploited for several

separation techniques, e.g. flotation, sedimentation and air

classification.

Forces of adhesion

There are interactions between particles, known as

forces of adhesion and also between particles and the

walls of containing vessels.

These forces of attraction will influence how the

material packs and how it will flow.

Interparticle adhesion increases with time, as the

material consolidates.

Flowability may be time-dependent and decrease

with time.

Fractal geometry

To characterize rough or textured surfaces,

Mandelbrot suggested a new geometry in 1975.

According to him, there are new dimensions

between the common dimensions of 1, 2, and 3

known as fractal dimensions (D).

Brian Kaye (1991) has elaborated the importance

of fractal geometry in particle characterization.

If we put a irregular shape in a polygon with

length of “”, its perimeter (P) will be increased

by reduction of length.

Polygon with n sides:

Mandelbrot showed that:

Therefore, plotting logP vs. log gives a straight

line with 1-D slope.

Fractal in

nature

Bulk properties

In most operations, the behavior of the bulk particles

is very important.

The bulk properties of fine powders are dependent

Geometry,

Surface characteristics,

Chemical composition,

Moisture content, and

Processing history.

The behavior of powders influenced by forces of attraction (or

repulsion) between particles is called cohesiveness.

For cohesive powders, the ratio of the interparticle forces (F)

to the particles’ own weight is large.

F 1/d2 small particles adhere to each other more strongly

than large particles.

For majority of particles, when the particle size exceeds 100

m, they are non-cohesive (free flowing).

Increase in moisture content makes powders more cohesive.

Bulk density and porosity

The bulk density (b) is an important property,

especially for storage and transportation, rather than

separation processes.

b = (mass / total volume occupied by the material).

Total volume includes air trapped between the

particles.

The volume fraction trapped between the particles is

known as the porosity ().

True (Skeletal) density: measured with helium (mass / volume of the solid).

Apparent density: measured by liquid

displacement (mass / voids volume + solid volume).

Bulk densities:

Loose density: (mass / total volume occupied by the material).

Compact (tap) density: (mass / total volume occupied by the

material after mechanical compression).

The ratio of tapped bulk density to the loose bulk

density is referred to as the Hausner ratio.

Hayes (1987) quotes the following ranges:

Flowability

The flowability of powders is very important in their handling.

Flowability increases with increasing particle size and decreasing

moisture content.

Factors used to assess flowability are:

Compressibility

Cohesiveness

Slide angle: Placing the powder sample on a flat smooth horizontal

surface and then slow inclination until the powder begins to move

The angle at which movement occurs is the slide angle.

Angle of repose: This is useful in the design of powder handling

systems. Its value depends upon the method of determination (forming

a heap, bed rupture, or rotating drum method). It is affected by

frictional forces and interparticle attractive forces.

According to Carr:

Angles up to 35° free flowability;

35 - 45° some cohesiveness;

45 - 55° cohesiveness or loss of free flowability;

>55° very high cohesiveness, very limited or zero

Slide angle

Angle of repose

A more fundamental method for flow behavior of powders is

based on the work of Jenike.

A flow cell is used, where the powder is first consolidated to

a particular bulk density and porosity. It is then subjected to a

compressive force (N) and the shear force (S) required to

cause the powder to yield and shear is determined. These

readings are converted to a normal stress () (N/A) and a

shear stress () (S/A).

Solid characterization

(a) Jenike flow cell;

(b) normal stress against shear stress, for a non-cohesive powder, = angle of

friction;

(c) yield locus for a cohesive powder for powders compacted to different initial

porosities; porosity 1 > 3;

Unconfined yield stress (fc)

Major consolidation stress (l)

The ratio of l/fc which is called the Jenike flow

function, is an indicator of the flowability of

powders. Its values correspond to the following

characteristics:

Definition of stress

Types of stress

Shear Stress

Bending Stress

The flowability is extremely useful for designing hoppers,

bins, pneumatic conveying systems and dispensers.

The hydrodynamics of powder flow are different to that for

liquids. The pressure does not increase linearly with height,

rather it is almost independent.

They can resist appreciable shear stress and can, when

compacted, form mechanically stable structures that may halt

flow. Also, any pressure or compaction can increase the

mechanical strength and hence the flowability.

The behavior of bulk solids in silos

v : vertical stress

h : horizontal stress

: stress ratio

Pressures in fluids and stresses in bulk solids

Qualitative courses of wall normal stresses (w) and assumed

trajectories of the major principal stress (1)

Wall normal stress in funnel flow silos

a. steep border line b. flat border line

Separation of particulates and

powders

The separation or recovery of solids from within a

solid matrix or from a particulate system is

concerned.

The main emphasis will be in fine particulate form,

so the production of material in a form suitable for

separations is often crucial for the process. In this

respect, size reduction and milling equipment is

important.

Size reduction

Size reduction is a very important preliminary

operation for several separation processes, extraction

operations, or expression processes.

Crushing: reduction of coarse material down to a

size of about 3 mm.

Grinding: production of finer powdered material.

The degree of size reduction can be characterized by

the size reduction ratio (SRR).

Cont. The main forces involved in size reduction are:

compressive forces,

impact forces,

shear or attrition forces.

The fracture resistance increases with decreasing particle size.

In selection of appropriate equipment for size reduction, two things need to be considered: particle size range required,

hardness of the material.

Hardness can be measured in Mohs, whose scale ranges between 0 and 8.5. very soft ( < 1.5 Moh),

soft (1.5 to 2.5 Moh),

medium hard (2.5 to 4.5 Moh),

hard (4.5 to 8.5 Moh).

Different mills for processing grains include:

1) Hammer mills: general-purpose mills; impact forces; used for spices,

sugar and dried milk powder.

2) Roller mills: one or several sets of rollers; compressive forces; SRR is

<5; used for milling of wheat; size range 10-1000 m.

3) Disc attrition mills: two discs, one is stationary and the other moving;

peripheral velocity of 4-8 m/s; used for grindings; size range down to

100 m.

4) Ball mills: tumbling mills used for very fine grinding processes; a

horizontal slow-speed rotating cylinder contains steel balls (d=25-150

mm) of hard stones; impact and shear mechanism.

Hammer mill

Roller mill

Disc attrition mill

Pin mill

Ball mill

Cost of milling

The particle size affects the cost of milling and the energy

requirement. Energy is based on the following equation:

where dE is the energy required to produce a small change in

diameter dD and Km is a characteristic of the material. The

three main equations result from different values of n are:

Wet milling

Wet milling is achieved by wetting the material and the

feedstock is ground in a suspension in the liquid, which is often

water.

Energy requirements are usually slightly higher than for dry

milling but a finer powder is obtained and dust problems are

eliminated.

Often wet milling is useful as part of an extraction process,

whereby soluble components are transferred from the solid to

the liquid phase.

Sieving

Sieving is the easiest and most popular method for size analysis

and separation of the components within powders.

A sieve is an open container with uniform square openings in

the base.

The effectiveness of a sieving process depends upon:

amount of material placed on the sieve,

type of movement,

time of the process.

The sieving time can be affected by the

following factors:

the material characteristics, e.g. fineness, particle

shape, size distribution, density;

intensity of sieving;

nominal aperture size of the test sieve;

characteristics of sieving medium;

humidity of the air.

Air classification

Air classification is a means of using a gaseous

entraining medium, which is usually air, to separate a

particulate feed material (for particles <50 m) into a

coarse and fine stream, on a dry basis.

Separation is based mainly upon particle size,

although other particle properties, such as shape,

density, electric, magnetic and surface properties may

play a part.

Simple classifiers

(a)aspiration F = fan;

(b)fractionation L = large; S = small particles;

(c)zig-zag classifier.

Commercial air classifiers

In commercial air classifiers, the gravitational force is used

supplemented by a centrifugal force. This is essential for

separating small particles and speeds up the separation

process.

Air classifiers are categorized by factors, such as:

the forces acting upon the particles; e.g. the presence or absence of a

rotor, the drag force of the air and the presence of collision forces;

the relative velocity and direction of the air and particles, controlled

by their respective feed systems;

directional devices such as vanes, cones or zig-zag plates;

location of the fan and fines collection device (internal or external)

Other important features are:

capacity of the classifier,

energy utilization.

In processing coal dust and cement classifiers,

flow rates of over 100 tonnes/h can be handled.

Classifiers handling foods can process more

than 5 tonnes/h.

Commercial air classifiers

Cyclone separation

Cyclone

Hydrocyclones

Process characterization

In most cases, air classification work is empirical because of the

difficulties in quantifying the forces acting upon a particle.

One method of characterizing the separation is by means of the

cut size. Ideally, all particles below the cut size end up in the fines

and all particles above the cut size end up in the coarse stream.

The cut size is defined as that size where the weight of

particles below the cut size in the coarse fraction is the same

as the weight of coarse particles above that size in the fines

stream.

Factors which influence the cut size are:

dimensions of the classifying chamber,

peripheral forces

the spiral gradient.

The cut point can be adjusted by varying:

the rotor speed,

air velocity,

vane setting,

feeding rate.

By equating these forces when they are in equilibrium,

an equation for the cut size (d) can be derived. This is

based on Stokes’ equation:

= viscosity of air

a = radial speed of air

r = clearance of classifier wheel

= particle density

p = rotational speed

Cut size determination

(a) ideal separation;

(b)real separation, weight frequency distribution;

Grade efficiency

The cut size alone does not provide information on how sharp

the separation is.

An alternative method of evaluation is grade efficiency, which

also indicate the sharpness of the separation.

The particle frequency distribution is determined by weight for

the coarse stream (qc(x)) and feed material (qf(x)) .

The yield is determined for the coarse stream Yc.

The grade efficiency T(x) indicates for any particle size x, the

mass fraction of feed material appearing in the coarse fraction.

Grade efficiency vs particle size

(a) ideal separation; (b) and (c) decreasing sharpness.

The sharpness of the separation is measured by the

ratio k = [x25t/x75t], i.e. the ratio of the sizes giving

grade efficiencies of 0.25 and 0.75 respectively.

Ideally k = 1.0.

The best industrial air classifiers achieve k = 0.7, but

typically commercial air classifiers show k values

from 0.3 to 0.6

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