Solid separation processes

Ali AhmadpourChemical Eng. Dept.Ferdowsi University of Mashhad

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Contents

Introduction Physical properties of solids Separation of particulates & powders Air classification Wet separation processes

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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, 1983

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

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Introduction Separations involving solid foods, together with the

properties of those solids which will influence the separation will be covered.

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 solid foods which will influence different types of separation processes will be discussed.

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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

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Physical properties of solids 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 agricultural products, Sorting and size grading, particularly for quality grading of

fruit and vegetables, Peeling of vegetables, dehulling of cereals and legumes and

deboning or shelling of meat and fish, Fractionation or recovery of the main components within the

foods, e.g. proteins, fat, carbohydrates and minerals.

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Cont. Special operations is concerned with the separation or fractionation of

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

Emphasis will be on the separation of powders, 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.

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Physical properties of solids

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Classification of powders

Particle size and particle size distribution Particle shape Particle density Forces of adhesion Bulk properties Bulk density and porosity Flowability

١٠

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).

١١

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

١٢

Frequency (F) and Cumulative (C) distributions

١٣

Cont. 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):

∑∑=

i

ii

n

dnd

( )n

ddn 2ii∑ −

=σ

∑∑= 2

ii

3ii

2/3 dn

dnd

١٤

Cont. 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.

2/3d6

Volumearea Surface

=

241.16d6dV

3/1

v

3v =

π

=⇒π

=382.16ddS2/1

s2s =

π

=⇒π=

sv3v

2s

v3v

2s

d6

dd6

S6/d

dVS

==⇒π

π=

١٥

Cont. 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.

١٦

١٧

• 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.

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Sampling technique(Coning and quartering process)

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Sampling devices

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Sampling points

٢١

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 =⇒ρ=ρ⇒=

3r

S6

lSr

34l sphere

spherecubecube3

sphere3cube =

⋅⇒π=

cube

sphere

sphere

cube

lr

2SS

=

٢٢

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 µm and ρ = 3 g/cm3 have 100 m2/g surface area. Particles with r = 0.1 µm and ρ = 3 g/cm3 have 10 m2/g surface area. Particles with r = 1 µm and ρ = 3 g/cm3 have 1 m2/g surface area. But, porous particles with r = 1 µm and ρ = 3 g/cm3 have >1000 m2/g surface

area. This shows the importance of porosity.

( ) ∑=

π=+⋅⋅⋅++π=1i

i2in

2n2

221

21t Nr4NrNrNr4S

( ) ∑=

π=+⋅⋅⋅++π=ρ

=1i

i3in

3n2

321

31 Nr

34NrNrNr

34MV r

3SNr

Nr3

MS

S

1ii

3i

1ii

2i

t

ρ=⇒

ρ==∑∑

=

=

٢٣

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.

The density (kg/m3) of the major components of foods are:

٢٤

Cont. Air has a density of 1.27 kg/m3. Therefore, the previous 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 from:

Differences in particle densities are exploited for several cleaning and separation techniques, e.g. flotation, sedimentation and air classification.

٢٥

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.

٢٦

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.

٢٧

Cont. If we put a irregular shape in a polygon with

length of “λ”, its perimeter (Pλ) will be increased by reduction of length.

Polygon with n sides:

Mandelbrot showed that:

Therefore, plotting logPλ vs. logλ gives a straight line with 1-D slope.

λ=λ nPD1kP −

λ λ=

٢٨

Cont.

٢٩

Cont.

٣٠

٣١

٣٢

Fractal in nature

٣٣

Bulk properties In most operations, the behavior of the bulk particles

is very important. The bulk properties of fine powders are dependent

upon: Geometry, Size, Surface characteristics, Chemical composition, Moisture content, and Processing history.

٣٤

Cont. 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 α 1/d2 ⇒ small particles adhere to each other more strongly than large particles.

For majority of food particles, when the particle size exceeds 100 µm, they are non-cohesive (free flowing).

Increase in moisture content makes powders more cohesive.

٣٥

Bulk density and porosity The bulk density (ρb) is an important property,

especially for storage and transportation, rather than separation processes.

ρb = (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 (ε).

s

b1ρρ

−=ε

٣٦

Cont. 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).

٣٧

Cont.

The ratio of tapped bulk density to the loose bulk density is referred to as the Hausner ratio.

Hayes (1987) quotes the following ranges:

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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.

٣٩

Cont.

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 flow.

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Slide angle

٤١

Angle of repose

٤٢

Angle of repose

٤٣

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 (σ) (N/A) and a shear stress (τ) (S/A).

٤٤

Solid characterization

(a) Jenike flow cell; (b) normal stress against shear stress, for a non-cohesive powder, α = angle of

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

porosities; porosity 1 > 3;

٤٥

Cont. Unconfined yield stress (fc) Major consolidation stress (σl) The ratio of σl/fc which is called the Jenike flow

function, is an indicator of the flowability of powders. Its values correspond to the following characteristics:

٤٦

Definition of stress

٤٧

Types of stress

Shear Stress

Bending Stress

٤٨

Cont. 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.

٤٩

The behavior of bulk solids in silos

σv : vertical stress σh : horizontal stress λ : stress ratio

٥٠

Cont.

Pressures in fluids and stresses in bulk solids

٥١

Cont.

Qualitative courses of wall normal stresses (σw) and assumed trajectories of the major principal stress (σ1)

٥٢

Cont.

Wall normal stress in funnel flow silos a. steep border line b. flat border line

٥٣

Cont.

٥٤

Cont.

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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.

٥٦

Size reduction Size reduction is a very important preliminary operation for

separation processes for many cereals, legumes and other commodity crops, as well as for extraction operations, e.g. tea and coffee, or expression processes, e.g. fruit juice expulsion or oil extraction.

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).

٥٧

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).

٥٨

Cont. Different mills for processing grain cereals, legumes, salt, and sugar

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 and refining of chocolate; size range 10-1000 µm.

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 µm.

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.

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Hammer mill

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Roller mill

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Disc attrition mill

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Pin mill

٦٣

Ball mill

٦٤

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:

٦٥

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. Wet milling is popular for corn milling.

٦٦

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.

٦٧

Cont.

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.

٦٨

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 µm) 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.

٦٩

Simple classifiers

(a)aspiration F = fan; (b)fractionation L = large; S = small particles;(c) zig-zag classifier.

٧٠

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)

٧١

Cont.

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.

٧٢

Commercial air classifiers

٧٣

Cyclone separation

٧٤

Cyclone

٧٥

Cont.

٧٦

٧٧

Hydrocyclones

٧٨

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.

Cut sizes of interest in food processing operations may range between 2 and 50 µm.

٧٩

Cont. 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.

٨٠

Cont. 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:

µ = viscosity of airυa = radial speed of airr = clearance of classifier wheelρ = particle densityυp = rotational speed

٨١

Cut size determination

(a) ideal separation;(b)real separation, weight frequency distribution;

٨٢

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.

٨٣

Grade efficiency vs particle size

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

٨٤

Cont.

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

٨٥

Air classifier applications Cereal separations (separation of starch and

protein from wheat, barley, … is based primarily on size and shape rather than density)

Legumes (fractionation of proteins in peas, lentil, beans, …)

Other applications (separation of oat-bran, removing gossypol from cottonseed protein, potato granules, rapeseed extracts, …)

٨٦

Wet separation processes Wet separation techniques are dependent upon differential

solubilities and precipitation methods. Applications are:

Protein recovery

Soya processing (flours, grits, concentrates and isolates)

Wheat protein (albumins, globulins, gliadins, glutenins)

Other applications (protein separation from fishes and animals) Of special interest is the recovery of protein from a solid

matrix.

٨٧

Protein recovery Objectives in protein recovery are:

Recovery of all the protein from foods to improve functional properties and reduce waste;

Separation of proteins from toxic components within the food;

Recovery of specific biologically active proteins, such as enzymes, insulin and hormones;

Fractionation of proteins; for example albumins are soluble in water and globulins in salt solutions.

٨٨

Cont. The solubility of a protein in solution depends primarily upon the

properties of its exposed surface groups, the type of solvent, its temperature, pH and polarity level, i.e. dielectric constant, and the type and concentration of dissolved ions.

Methods for aggregation and precipitation of proteins are: lowering the temperature to reduce protein solubility; adjustment of pH to the isoelectric point; addition of non-polar solvents to reduce the attraction of surface polar

groups with water; unfolding (denaturation) and hydrophobic interactions; addition of large quantities of very polar solvents, (unfolding); increasing the levels of salts (salting out); raising the temperature to cause thermal denaturation to take place.

٨٩

Miscellaneous solids separations

Dehulling Peeling Cleaning of raw materials Sorting and grading

٩٠

Dehulling

Removal of the hull (seed coat) have the following advantages: a reduction in fibre and tannin content, improvements in appearance, cooking quality, texture, palatability and

digestibility.

Legumes are often soaked and dehulled manually and then dried.

Theoretical yield of dehulled product is between 85 and 95%. For most commercial dehulling applications, abrasion or

attrition mills are used. Factors most responsible for differences in dehulling

performance are seed hardness and resistance to splitting.

٩١

Peeling

Peeling is an important process for many processed fruit and vegetables.

Mechanisms involved in peeling are: Abrasion Chemical cleaning (caustic or brine) Thermal peeling

Often more than one mechanism is involved and often spray washing is required to remove any loosely attached peel.

٩٢

Abrasion peeling

The food is fed into a rotating bowl, which is lined with an abrasive material. Rollers and knives may also be used.

The abrasion rubs off the skin, which is removed by water. Advantages: low energy costs, minimal thermal damage, low

capital costs. Drawbacks: higher product losses (up to 25%), production of

large volumes of dilute wastes and relatively low throughputs. Some irregular shaped materials, for example potatoes with

eyes, may need some manual inspection and finishing.

٩٣

Chemical cleaning

A dilute solution of sodium hydroxide (1 to 2%) is heated to 100-120°C and contacted with the food for a short time period.

Water sprays are then used to dislodge the skin.

It has now been largely replaced by steam peeling.

The use of a more concentrated caustic solution (10%) reduces water consumption and produces a more concentrated waste for disposal.

Brine solutions are also sometimes used.

٩٤

Thermal peeling

The food is fed in batches into a pressure vessel, which rotates slowly.

High-pressure steam is fed into the vessel and rapid heating occurs at the surface, within 15-30 s.

The pressure is suddenly released, causing boiling of the liquid under the skin and flashing-off of the skin, which is removed with the condensed steam.

Additional water sprays may be required. This method produces good quality products with little damage

and high throughputs. There is minimum water utilization and minimum losses.

Flame peeling, using temperatures of 1000°C, has been used for onions.

٩٥

Cleaning of raw materials Contaminants on food raw materials can be of various origins:

Mineral: soil, stones, sand, metal, oil; Plant: twigs, leaves, husks, skins; Animal: hair, insects, eggs; Chemical: pesticides, fertilizers, contaminants; Microbial: yeasts, moulds, bacteria and metabolic by-products.

Screening is widely used for removing contaminants different in size.

Electrostatic methods based on differences in electrostatic charge of materials under controlled humidity conditions.

Wet methods are also widely used for cleaning purposes (soiled vegetables).

٩٦

Sorting and grading Sorting and grading are important preliminary operations. Sorting is used for separation of foods into categories based on a

single physical property, such as size, shape, weight or color. Grading is a quality separation and a number of factors (color,

flavor, texture) may be assessed. Food grading is usually done manually, by trained experts (meat

grading and inspection, fish grading, horticultural products, tea and cheese).

Equipment for size sorting is based on rollers and screens Sorting by weight is important for high value products such as

eggs, and some tropical fruits. Image analysis is recently used.