unit operation solid handling lecture one & two

unit operation solid handling lecture one & two

1. Unit Operations in Chemical Engineering
Course No. ChE 3107
Course Code: Unit Operations in Chemical Engineering
Lecture 01 & 02
Maisa Rahman
Lecturer
Department of Chemical Engineering
Rajshahi University of Engineering & Technology
Email: [email protected]
Contact No. 01926997916

2. Reference Books
•Unit Operation – I (fluid flow and mechanical operations) by K A
Gavhane
•Drive Link: (https://drive.google.com/file/d/1716Tdmn-
suLEHqJkpo4h0WIoNJrBhyz9/view?usp=sharing)
•Unit Operations of Chemical Engineering, 5
th
edition, by McCabe &
Smith
•Drive Link:
(https://drive.google.com/file/d/1QLU3MtEZDx7vMEHud3IyOLwskM
uxvfjm/view?usp=sharing)
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3. Course Contents
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4. Size Reduction
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What is the meaning of size reduction ?
•Size refers to physical Dimension of an object.
•Reduction refers to decrement or the process of decreasing the size.
•Size reduction is the operation carried out for reducing the size of
bigger particles into smaller one of desired size and shape with the
help of external forces.
•Definition: Size reduction or ‘comminution’ is the unit operation in
which the average size of solid pieces is reduced by the application of
grinding, compression or impact forces. It involves creating smaller
mass units from larger mass units of the same material.

5. Size Reduction
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6. Size Reduction
Objectives of Size Reduction are –
•Increase the surface area because, in most reactions involving solid particles,
the rate of reactions is directly proportional to the area of contact with a
second phase.
•Break a material into very small particles in order to separate the valuable
amongst the two constituents.
•Achieve intimate mixing.
• To dispose solid wastes easily .
• To improve the handling characteristics.
• To mix solid particle more intimately.
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7. Size Reduction - Advantages
•Uniformity of the contents
•Uniform flow
•Effective drying
•Uniform mixing and drying
•Improves rate of absorption . Smaller the particles greater is
absorption.
• Improves dissolution rate.
•Increases the specific surface (surface-area-to-volume ratio); this
facilitates several processes, such as heat exchange, extraction,
chemical and biological reactions
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8. Size Reduction - Disadvantages
•Increased energy requirement (e.g., the finer the size reduction, the
higher the energy requirement).
•The cost of equipment increases significantly with the size reduction
and the cutting precision.
•Quality degradation, like food, drug, etc.
•Higher Contamination chances.
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9. Size Reduction - Mechanism
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Fig. Stress vs Strain Diagram for various materials
Here,
E, elastic limit;
Y, yield point;
B, breaking point;
O-E, elastic region;
EY, inelastic deformation;
Y-B, region of ductility.
&
1.Hard, strong, brittle material.
2.Hard strong ductile material.
3.Soft, weak, ductile material.
4.Soft, weak, brittle material.

10. Size Reduction - Mechanism
•Elastic deformations are valueless in size reduction. Energy is used up but
no breakdown occurs. Point E is known as the elastic limit.
•Beyond this point, the material undergoes permanent deformation until it
reaches its yield point Y. Brittle materials will rupture at this point. Ductile
materials will continue to deform, or flow, beyond point Y until they reach
the break point B, when they rupture.
•Larger particles will fracture more easily.
•In the case of small particles, new crack tips may need to be created during
the milling operation. Thus, the breaking strength of smaller particles is
higher than the larger ones. The energy required for particle breakdown
increases with decrease in the size of the particles.
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11. Size Reduction - Principle
To bring about a required reduced size of a solid substance, the larger mass
units need to be subjected to stress by the application of force. Three types
of force may be applied, i.e.: i.
•Compression: Compressive forces are generally used for the coarse
crushing of hard materials. Careful application of compressive forces
enables control to be exercised over the breakdown of the material.
•Impact: Impact forces are used to mill a wide variety of materials, including
fibrous types of solid materials.
•Shear force: Shear forces are best applied to relatively soft materials, again
including fibrous types of solid materials.
All three types of force are generated in most types of mill, but generally one
predominates. For example, in most roller mills compression is the dominant
force, impact forces feature strongly in hammer mills and shear forces are
dominant in disc attrition mills.
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12. Size Reduction - Principle
•The main kind of forces applied in size reduction are-
i.Compression
ii.Shear or attrition
iii.Impact
iv.Cutting
•The application of forces applied:
i.Pressure and/or friction on material placed between tool surfaces
ii.Shear force on material
iii.Collision between particles or impact between particles and tools
iv.Friction through medium surrounding the particles
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13. Size Reduction - Mechanisms
•Impact —particle concussion by a
single rigid force (hammer).
•Compression—particle
disintegration by two rigid forces
(nutcracker).
•Shear —produced when the
particle is compressed between the
edges of two hard surfaces moving
tangentially.
•Attrition —arising from particles
scraping against one another or
against a rigid surface (a file).
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14. Size Reduction – Machines
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15. Size Reduction – Machines
•Ball Mill:
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16. Size Reduction – Machines
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Fig. Schematic diagram of Conical ball mill Fig. Schematic diagram of Laboratory Scale ball mill

17. Size Reduction – Machines
Hammer Mill:
•SizereductionisachievedbyImpact.
•Feedthematerialintothehammermillthroughthehopper
andpressthestartbutton.
•TheHammerassemblystartstorotate(with8000to15000
RPMspeed),andparticlesizereducesintosmallparticles
duetodynamicimpact.
•Afterthereductionofparticlesfallingwithinthecontainer
throughthescreen.
•Hammermillsareoftwotypes,Horizontalshaft(Moves
horizontally)andverticalshaft(MovesVertically)types.They
differonlybymovingconditions,althoughtheremaining
thingsarethesame.
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18. Size Reduction – Machines
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Fig. Schematic diagram of hammer mill

19. Size Reduction – Machines
Advantages of hammer mill
•It produced rapid action in grinding
•Hammer also useful for hard particles
•East and Fast to use
•Required less space and is easy to dismantle, and clean
Disadvantages of hammer mill
•Produced more sound.
•Be careful to use heat-sensitive materials because the
hammer produced more heat.
•Can’t be used to grind fiber materials
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20. Size Reduction – Machines
Roller Mill:
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21. Size Reduction – Machines
Attrition Mill:
•Themillisequippedwithanouterstationarymillingdiscandaninner
rotatingmillingdiscthatadjuststovarythegapbetweenthediscs.
Materialentersthegapthroughthecenteroftheouterdisc,andis
groundbetweenthediscsasittravelsradiallybycentrifugalforce
createdbytherotatingdisc,withgreaterreductionsachievedby
reducingthegap.
•Precisemachiningandalignmentofthediscs,togetherwithconstant
springpressure,
•maximizesizeuniformityofthereducedmaterial.
•Platepatternsareselectedtooptimizethereductionforspecifictypes
ofmaterials.
•Capacityisgenerallydependentonmeshsizeofthereducedmaterial,
withcoarserproductsproducedathigherrates.
•Quick-releasegrindingdiscseparationallowsforthoroughwashdown
betweenbatches.Singlerunnermillscomeequippedwithaccess
doorsforcleanoutandmaintenanceaccessibility.
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22. Size Reduction – Machines
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23. Factors Affecting Size Reduction
1. Hardness:
- It is a surface property of the material.
- It is frequently confused with a property named strength.
- Thus, it is possible for a material to be very hard, but if it is brittle also
then size reduction may present no special problems.
- An arbitrary scale of hardness has been devised known as Moh’s Scale;
• Moh’s Scale = 1 is for graphite
• Moh’s Scale < 3 is for soft material
• Moh’s Scale > 7 is for hard material
• Moh’s Scale = 10 is for diamond
The harder the material the more difficult it is to reduce in size
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24. Factors Affecting Size Reduction
2. Material structure:
•Some substances are homogeneous in character.
•Mineral substances may have lines of weakness along which
•the materials splits to form flake-like particles.
3. Abrasiveness:
•Abrasiveness is a property of hard materials (particularly those of
mineral origin).
•It may limit the type of machinery that can be used.
•During the grinding of some very abrasive substances the final
powder may be contaminated with more than 0.1 percent of
metal worn from the grinding mill.
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25. Factors Affecting Size Reduction
4. Softening temperature:
•During size reduction process sometimes heat is generated which
may cause some substances to soften, and the temperature at which
this occurs can be important.
•Waxy substances, such as stearic acid, or drugs containing oils or fats
are examples that may be affected.
•Some methods can be used to overcome this like cooling the mill,
either by a water jacket or by passing a stream of air through the
equipment.
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26. Factors Affecting Size Reduction
5. Moisture content:
•It is found that materials do not flow well if they contain between
about 5 and 50 per cent of moisture.
•Under these conditions the material tends to cake together in the
form of balls.
6. Crushing strength:
•The power required for crushing is almost directly proportional to
the crushing strength of the material.
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27. Size Reduction
Bulk Properties:
•An intensive property is a bulk property, meaning that it is the
physical property of the system that does not depends upon the size
or the amount of material in the system. E.g: temperature, density,
hardness of the object etc.
•Intensive Properties: Properties that does not depend upon the size
or the amount of material in the system, e.g: temperature, density,
hardness of the object etc.
•Extensive Properties: Properties that depend upon the size or the
amount of material in the system, e.g: mass, volume etc.
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28. Size Reduction
What is Particle?
•A sub-microscopic, localized matter
to ascribe several physical or
chemicalproperties such as
volume or mass.
•Such as granular, crushed,
powdered, subatomic,
microscopic, nanomaterial.
CHARACTERIZATION OF PARTICULATE
SOLID PROPERTIES
•CHARACTERIZATION means
Classification of particles on the
basis of their size, shape and
density.
•Size and shape can be specified for:
•Regular particles, e.g. spheres,
cubes, needles, crystals etc.
•Irregular particles e.g. uneven,
unequal, asymmetric etc.
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29. Size Reduction
Solid particles are
characterized by the
following;
shape, size, molecular weight,
structure and crystallography,
surface hydrophobicity and
density.
•Particles of homogeneous
solids have the same density as
the bulk material.
Some of the most important
physical properties to measure
are-
➢Particle Size
➢Particle Shape
➢Surface Properties
➢Mechanical Properties
➢Charge Properties
➢Microstructure
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30. Size Reduction
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Reactivity dependence on Particle Size Solubility dependence on Particle Size

31. Size Reduction
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1-) Regular shape particles can be described precisely by its shape and dimension.
2-) Irregular shape particle, can not be described conveniently due to irregularity of the
particle shape & dimension.
➢Dp = equivalent/nominal diameter
➢Vp = volume of one particle
➢Sp = surface area of particle

32. Size Reduction
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▪A sphere is a perfectly round geometrical
object in three dimensional space that is
the surface on a completely round ball.
▪For a perfectly spherical diameter, the
value for Sphericity equals to 1.
▪In case of Irregular shape particles,
nominal value of equivalent diameter is
taken
Here,
Vp = Volume of the Particle
Dp = Diameter of the Particle
Sp = Surface Area of the Particle

33. Size Reduction
•for statistical lengths, the following equivalent dimensions of particles
are used:
➢Feret’s Diameter
➢Martin’s Diameter
➢Volume- Surface Mean Diameter, etc.
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•Equivalent Spherical Diameter:
•Diameter of an irregular shape object is the diameter of a sphere of equivalent volume.

34. Size Reduction
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35. Size Reduction – Particle Size
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➢In general, diameter is specified for equidimensional particles.
➢Most of the particles are not equidimensional, therefore they can not be specified
by a single dimension “diameter”.
➢Therefore the concept of equivalent sphere has been introduced.

36. Size Reduction
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Mixed Particles and SIZE ANALYSIS
•If we have a sample of uniform particles of diameter = D
p,
•The total volume of the particles will be = m/ρp, (representing mass and density of
the particles),
•Since the volume of one particle = V
p,
•Total volume ‘V’ of N particles = V = V
p.N
•The total number of particles in sample = N = m/ρ
pV
p
•Total surface area of the particles is given by,

37. Size Reduction
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Specific surface area of mixture
•If the particle densityρ
pand SphericityΦ
sare known, the surface area of particles in
each fraction can be calculated and added to give the specific surface,A
w (The total
surface area of the unit mass of particles):
•For deriving this equation, it has been assumed that Sphericity and density of the
mixture is constant.
•Where x
i = mass fraction in a given increment,
D
pi = average diameter (taken as arithmetic average of the smallest and
largest particle diameters in increment).

38. Size Reduction
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Average particle size
•The average particle size for a mixture of particles is defined in several
different ways.
✓Volume surface mean diameter D
s:
If number of particle Ni in each fraction is known,
instead of mass fraction xi, then:

39. Size Reduction
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Arithmetic mean diameter:
N
T= number of particles
in the entire sample
Mass mean diameter:
Volume mean diameter:
•Total volume of the sample
•Divided by number of particles
•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.

40. Size Reduction
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Number of particles in mixture
✓The volume of any particle is proportional to its "diameter"
cubed.

a= volume shape factor
✓For sphere the value of a is 0.5236 and for short cylinder it is
0.785.
Assuming thatais independent of size, then:

41. Size Reduction - Screen analysis
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•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.

42. Size Reduction - Sieving
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•Smallest mesh at the bottom
•Largest mesh at the top

43. Size Reduction - Sieving
▪Screens or sieve analysis is used to measure the size (and size
distribution) of particles in size range of 0.0015 and 3 inch.
▪Woven wire screens, Silk, Plastic cloth, perforated or punched plate.
▪Openings are in the form of squares.
▪Each screen is identified in meshes per inch.
▪A stack of screens is arranged with the smallest mesh at the bottom
and the largest one at the top.
▪Vibratory motion is produced to cause better separation of particles.
▪Particles retained on each screen are then removed and weighed to
draw the Cumulative and Frequency distribution curves.
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44. Size Reduction - Sieving
•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 x
i (where i
is the number starting from the bottom of the stack),
•fourth column: averaged particle size D
pi (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 D
pi.
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45. Size Reduction
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46. Size Reduction - Example
•The screen analysis shown in the table applies to a sample of crushed
quartz.
•The density of the particles is 2,650 kg/m
3
(0.00265 g/mm
3
).
•The shape factor are a = 0.8 and φ
s = 0.571.
•For the material between 4-mesh and 200-mesh in particle size,
calculate
•A
w in square millimetres per gram and Nw in particles per gram.
•D
V, D
s, D
w and N
i for the 150/200 mesh increment.
•What fraction the total number of particles is in the 150/200-mesh
increment?
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47. Maisa Rahman, ChE, RUET