Mechanical Operation

1. Unit Operation: CHD 228
Instructor: Dr. Pratibha Biswal
Lecture 1
Introduction to Unit
Operations
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Introduction: Chemical Engineering
• Design and Operation of
Industrial Chemical Plants.
• Design of units that change
materials from one form to
another more useful (and so
more valuable) form,
economically, safely and in an
environmentally acceptable
way.
• Application of basic sciences
(Mathematics, Chemistry,
Physics & Biology) and
Engineering principles.
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3. Introduction: Chemical Engineering
• Petrochemicals, petroleum and natural gas processing
• Plastics and polymers
• Pulp and paper
• Fertilizers, Pesticides
• Alcohol Industry
• Cryogenics
• Instrumentation and process control
• Energy conversion and utilisation
• Environmental control
• Food processing
• Composite materials, corrosion and protective coatings
• Manufacture of microelectronic components
• Pharmaceuticals and many more….
Pulp Production from Wood Chips
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Applications:

4. Example: Paper Production
Size Reduction
Digestion
Washing
Evaporation
Heating
Bleaching
Refining
Drying
Cooling
Coating
Sheeting and more
Series of Units
Unit Operations
Unit Processes
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Unit Processes and Unit Operation

5. Unit Processes and Unit Operation
5
Unit Processes (chemical changes)
• Processes that involve making chemical changes to
materials as a result of chemical reaction. Unit processes
are also referred to as chemical conversions.
• Chemical Reactions
• Biochemical processes
• Nitration
• Oxidation
Unit Operations (physical changes)
• Processes that involve only physical changes to materials is
termed as Unit Operation
• Mechanical Separations
• Fluid Mechanics
• Process Heat transfer
• Mass Transfer
• Simultaneous Heat and Mass Transfer
 Unit operation (physical change) may be
tied with the unit process (chemical
change).
 Example: when heat flows into an
endothermic chemical reaction or out of
an exothermic reaction.
 The unit operation (physical change)
may also be distinctly separated from the
unit process (chemical change)
 Example: flow of fluid, liquid is moved
from one part of an industrial
establishment to another
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6. Unit Operations
Separation of solids based on size
Transportation and storage of solids
Size Reduction of Solid
Size enlargement
Solid-fluid mixing
Fluid-Solid Separation
Mass Transfer
Heat Transfer
Fluid Flow..
Mechanical
Operations
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Operations on
Solid, Liquid
and Gases

7. • Particulate solid
• Separation of solids from solids based on size
• Transportation and storage of solids
• Size reduction of solid
• Size enlargement of solid
• Solid-fluid mixing
• Fluid-solid systems: separation of solid from solid,
separation of solid from fluid
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Unit Operations I: Mechanical Operations
1. Credits: 4
2. Grading Scheme:
a. Attendance and internal assessment: 10%
b. Assignments: 10%
c. Quizzes: 10%
d. Mid-Term: 30%
e. End-Term: 40%

8. • Unit Operations of Chemical Engineering by
Warren McCabe, Julian Smith, Peter Harriott,
Publisher: McGraw Hill Education
• Coulson and Richardson’s Chemical Engineering
Volume 2: Particle Technology and Separation
Processes by J. F. Richardson, J. H. Harker and J.
R. Backhurst, Publisher: Butterworth-Heinemann
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Textbooks for Reference

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Particulate Solids
Wood Chips Oil Seeds Salt
Sugar Yeast Urea

10. Important Operations Relating to Systems of Solid Particles:
• Storage in hoppers
• Flow through orifices and pipes
• Reduce the size of particles
• Mix two or more solids
• Separate solid mixtures
• Interaction between the particles and the surrounding fluid
• Filtration
• Flow of fluids through beds of granular particles
• Sedimentation of particles in a liquid
• Fluidization etc..
Particulate Solids: Applications
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Particulate Solids: Characterization
Shape
• A particle shape may be regular, such as spherical or cubic
• It may be irregular such as a piece of broken glass
• Regular shapes are capable of precise definition by mathematical equations.
• Properties of irregular particles are usually expressed in terms of some particular
characteristics of a regular shaped particle.
Size
• Particle size is important in that this affects properties such as the surface
per unit volume and the rate at which a particle will settle in a fluid.
• Particle sizes ranges from few millimeter to microns
Composition
• Composition determines such properties as density and conductivity,
provided that the particle is completely uniform.
• In some cases, the particle is porous or it may consist of a continuous
matrix in which small particles of a second material are distributed.
Individual solid particles
are characterized by:

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 The simplest shape of a particle: Sphere
 Sphere is symmetric.
 Orientation of sphere does not have to be considered, since the spherical particle
looks exactly the same from whatever direction it is viewed and behaves in the
same manner in a fluid, irrespective of its orientation.
 No other particle shape has this characteristic.
 Frequently, the size of a particle of irregular shape is defined in terms of the size of
an equivalent sphere.
Characterization of Particulate Solids: Particle Shape

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Some of the important sizes of equivalent spheres are:
• The sphere of the same volume as the particle.
• The sphere of the same surface area as the particle.
• The sphere of the same surface area per unit volume as the particle.
• The sphere of the same area as the particle when projected on to a plane
perpendicular to its direction of motion.
• The sphere of the same projected area as the particle, as viewed from above,
when lying in its position of maximum stability such as on a microscope
slide for example.
• The sphere which will just pass through the same size of square aperture as
the particle, such as on a screen for example.
• The sphere with the same settling velocity as the particle in a specified fluid.
Particle Shape: Equivalent Sphere

14. Measure of Particle Shape: Sphericity
Hakon Wadell, Volume, Shape and Roundness of Quartz Particles. Journal of Geology. 43 (1935): 250-280.
Sphericity (Φp)=
Surface area of sphere of same volume as particle
surface area of particle
Φ 𝑠 =
6
𝐷 𝑒
𝑆 𝑝
𝑉 𝑝
….(1)
De= equivalent diameter or nominal
diameter of particle
(The equivalent diameter is defined as the
diameter of a sphere of equal volume.)
Sp = surface area of one particle
Vp = volume of one particle
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 Sphericity is the measure of how closely the shape of an object approaches that of a
mathematically perfect sphere.
 Defined by Wadell in 1935 the sphericity, of a particle is the ratio of the surface area of
a sphere (with the same volume as the given particle) to the surface area of the particle.

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Measure of Particle Shape: Sphericity
Image Courtesy: Geological Society of America

16. Problems
Calculate the sphericity of a cylinder
of diameter 1 cm and height 3 cm.
Calculate the sphericity of a cube
with side 2 cm
Calculate the sphericity of a cube
with side “a” in general
1 cm
3 cm
2 cm
2 cm
a
a
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17. • Large quantities of particles are handled on the industrial
scale
• It is frequently necessary to define the system as a whole.
• It is necessary to know the distribution of particle sizes in
the mixture
• It is necessary to define a mean size which in some way
represents the behaviour of the particulate mass as a whole
in the system.
Why we need Particle Size Distribution?
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Particle Size Distribution

18. • In general, "diameters" may be specified for any equidimensional particle.
• Particles that are not equidimensional, i.e., that are longer in one direction than in
others, are often characterized by the second longest major dimension.
• For needle like particles, for example, Dp would refer to the thickness of the particles,
not their length.
• Conventionally, particle sizes are expressed in different units depending on the size
range involved.
• Coarse particles are measured in inches or millimetres.
• Fine particles in terms of screen size; very fine particles in micrometers or
nanometers.
• Ultrafine particles are described in terms of their surface area per unit mass.
Particle Size
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19. Particle Size
Particle size is characterized
using these terms:
i. Very coarse
ii. Coarse
iii. Moderately coarse
iv. Fine
v. Very fine
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Particle size can influence variety of important factors:
• Dissolution rate
• Suspendability
• Uniform distribution
• Penetrability
• Chemical Reaction
• Heating etc.
Effects of Particle Size

21. Mixed Particle Sizes And Size Analysis
• In a sample mass m, density ρt and uniform particles of diameter Dp, total volume of the sample is
• Number of particles in the sample is
• The total surface area of the sample is
𝑉𝑡 =
𝑚
𝜌𝑡
Mass of the sample
Density of the sample
….(2)
𝑁 =
𝑉𝑡
𝑉𝑝
Total Volume of the Sample
Volume of Single Particle
𝑁 =
𝑚
𝜌𝑡 𝑉𝑝 …(3)
𝐴 𝑡 = 𝑁𝑆 𝑝 =
6𝑚
Φ 𝑠 𝜌𝑡 𝐷𝑒 …(4)
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22. • Particles at a specified size range can be measured by screens.
• Testing sieves (screens) are constructed with square openings of woven wires.
• Coarse Screens: “Mesh” refers to the distance between adjacent wires.
• Fine Screen: “Mesh” is the number of openings per linear inch.
• Screens are stacked with the coarsest screen at the top and finest screen at the bottom.
• The sieves may either be mounted on a vibrator for vertical and horizontal vibration, or may be hand
shaken.
• Whether or not a particle passes through an aperture depends not only upon its size, but also on the
probability that it will be presented at the required orientation at the surface of the screen.
• Material retained in each screen is removed and weighed separately. Amount of material in each screen
is expressed as weight fraction.
• Sieves (Screens) are available in a number of standard series. (See Table 1.1 in Coulson and
Richardson; Appendix 20 in Mccabe, Smith and Harriot)
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Mixed Particle Sizes And Size Analysis

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Screen Analysis: Standard Screens