BSc
Chemical Engineering Technology
Department
of Chemical Engineering Technology
Government
College University Faisalabad
Name: Sibtain Haider
Roll No: 8213
Submitted To: Engr. Syed Ejaz Haider
Subject: Particle Technology
Semester: 3rd
Department : Chemical Engineering
Technology
CHT-401:
Particle Technology
November
2020
Particle Characterization -
Assignment # 1
Contents
Theory
of sedimentation method
Determination
of Particle Size
DESCRIPTION
AND OPERATION OF SEDIMENTATION APPARATUS
Table of figure
Table of Equations
Elutriation Methods.
Elutriation is a process for separating particles based on their size,
shape and density using a stream of gas or liquid flowing in a direction
usually opposite to the direction of sedimentation.
This method is mainly used for
particles smaller than 1 μm.
The smaller or lighter particles
rise to the top (overflow) because their termina sedimentational velocities are
lower than the velocity of the rising fluid.
The terminal velocity of any
particle in any medium an be calculate using Stocs law if the particle’s Renold
numbers is below 0.2 Counter flow centrifugation eutration is related technique
to separate cells.
Air Classification:
An air elutriator is a
simple device which can separate particles into two or more groups. Material
may be separated by means of an elutriator, which consists of a vertical tube
up which fluid is passed at a controlled velocity.
When the particles are introduced, often through a
side tube, the smaller particles are carried over in the fluid stream while the
larger particles settle against the upward current. If one starts with low flow
rates, small less dense particles attain their terminal velocities, and flow
with the stream.
The particle from the stream is collected in overflow
and hence will be separated from the feed. Flow rates can be increased to
separate higher size ranges.Further size fractions may be collected if the
overflow from the first tube is passed vertically upwards through a second tube
of greater cross-section, and any number of such tubes can be arranged in
series.It is used in mineral processing for size classification.
The elutriation dust value is a usual measure for
quantification of dust, generated by testing wherein mechanical forces such as
vibration are applied to granules of e.g. a detergent agent.. Elutriation is a
common method used by biologists to sample meiofauna. The
sediment sample is constantly agitated by a flow of filtered water from
below, the action of which dislodges interstitial organisms embedded
between sediment grains. A very fine filter at the top captures.
Figure 0—1---------------------------------------------------------------------------------------------------------------
Elutration Method
Sieve analysis
A sieve analysis (or gradation test) is a practice or
procedure used (commonly used in civil engineering) to assess the particle size
distribution (also
called gradation) of a
granular material by allowing the material to pass through a series of sieves
of progressively smaller mesh size and weighing the amount of material that is
stopped by each sieve as a fraction of the whole mass.
Figure 0—2 -------------------------------Sieve
analysis
A gradation test is performed on a sample of aggregate in a laboratory. A typical sieve
analysis involves a nested column of sieves with
wire mesh cloth (screen). See the separate Mesh
(scale) page for details of sieve sizing.
A representative weighed sample is poured into the top
sieve which has the largest screen openings. Each
lower sieve in the column has smaller openings than the one above. At the base
is a round pan, called the receiver.
The column is typically
placed in a mechanical shaker. The shaker shakes the column, usually for some
fixed amount of time. After the shaking is complete the material on each sieve
is weighed. The mass of the sample of each sieve is then divided by the total
mass to give a percentage retained on each sieve. The size of the average
particle on each sieve is then analysed to get a cut-off point or specific size
range, which is then captured on a screen.
Results
The results of this test are
used to describe the properties of the aggregate and to see if it is appropriate
for various civil engineering purposes such as selecting the appropriate
aggregate for concrete mixes and asphalt mixes as well as sizing of water
production well screens.
The results of this test are
provided in graphical form to identify the type of gradation of the aggregate.
The complete procedure for this test is outlined in the American Society for
Testing and Materials (ASTM) C 136 and the American
Association of State Highway and Transportation Officials (AASHTO) T 27
A suitable sieve size for the
aggregate underneath the nest of sieves to collect the aggregate that passes
through the smallest. The entire nest is then agitated, and the material whose
diameter is smaller than the mesh opening pass through the sieves. After the
aggregate reaches the pan, the amount of material retained in each sieve is
then weighed.
The results are presented in
a graph of percent passing versus the sieve size. On the graph the sieve size
scale is logarithmic. To find the percent of aggregate passing through each
sieve, first find the percent retained in each sieve. To do so, the following
equation is used,
%Retained = ×100%
where WSieve is
the mass of aggregate in the sieve and WTotal is the total mass
of the aggregate. The next step is to find the cumulative percent of aggregate
retained in each sieve. To do so, add up the total amount of aggregate that is
retained in each sieve and the amount in the previous sieves. The cumulative
percent passing of the aggregate is found by subtracting the percent retained
from 100%.
%Cumulative Passing = 100% -
%Cumulative Retained.
The values are then plotted
on a graph with cumulative percent passing on the y axis and logarithmic sieve
size on the x axis.
There are two versions of the
%Passing equations. the .45 power formula is presented on .45 power gradation
chart, whereas the more simple %Passing is presented on a semi-log gradation
chart. version of the percent passing graph is shown on .45 power chart and by
using the .45 passing formula.
.45 power percent passing
formula
% Passing = Pi = x100%
Where:
SieveLargest -
Largest diameter sieve used in (mm).
Aggregatemax_size - Largest piece of aggregate in the sample in
(mm).
Percent
passing formula
%Passing = x100%
Where:
WBelow -
The total mass of the aggregate within the sieves below the current sieve, not
including the current sieve's aggregate.
WTotal -
The total mass of all of the aggregate in the sample.
Figure 0—3----Sieve Analysis
Methods
Throw-action
Here a throwing motion acts
on the sample. The vertical throwing motion is overlaid with a slight circular
motion which results in distribution of the sample amount over the whole
sieving surface. The particles are accelerated in the vertical direction (are
thrown upwards). In the air they carry out free rotations and interact with the
openings in the mesh of the sieve when they fall back. If the particles are
smaller than the openings, they pass through the sieve. If they are larger,
they are thrown.. The rotating motion while suspended increases the probability
that the particles present a different orientation to the mesh when they fall
back again, and thus might eventually pass through the mesh. Modern sieve
shakers work with an electro-magnetic drive which moves a spring-mass system
and transfers the resulting oscillation to the sieve stack. Amplitude and
sieving time are set digitally and are continuously observed by an integrated
control-unit. Therefore, sieving results are reproducible and precise (an
important precondition for a significant analysis). Adjustment of parameters
like amplitude and sieving time serves to optimize the sieving for different
types of material. This method is the most common in the laboratory sector
Horizontal
In horizontal sieve shaker the sieve stack
moves in horizontal circles in a plane. Horizontal sieve shakers are preferably
used for needle-shaped, flat, long or fibrous samples, as their horizontal
orientation means that only a few disoriented particles enter the mesh and the
sieve is not blocked so quickly. The large sieving area enables the sieving of
large amounts of sample, for example as encountered in the particle-size
analysis of construction materials and aggregates.
Tapping
A horizontal circular motion
overlies a vertical motion which is created by a tapping impulse. These
motional processes are characteristic of hand sieving and produce a higher
degree of sieving for denser particles (e.g. abrasives) than throw-action sieve
shakers.
Figure 0—1-------------------------- Tapping sieving
Wet
Most sieve
analyses are carried out dry. But there are some applications which can only be
carried out by wet sieving. This is the case when the sample which has to be
analysed is e.g. a suspension which must not be dried; or when the sample is a
very fine powder which tends to agglomerate (mostly < 45 µm) – in a dry
sieving process this tendency would lead to a clogging of the sieve meshes and
this would make a further sieving process impossible. A wet sieving process is
set up like a dry process: the sieve stack is clamped onto the sieve shaker and
the sample is placed on the top sieve. Above the top sieve a water-spray nozzle
is placed which supports the sieving process additionally to the sieving
motion. The rinsing is carried out until the liquid which is discharged through
the receiver is clear. Sample residues on the sieves have to be dried and
weighed. When it comes to wet sieving it is very important not to change the
sample in its volume (no swelling, dissolving or reaction with the liquid).
Air Circular Jet
Air jet sieving
machines are ideally suited for very fine powders which tend to agglomerate and
cannot be separated by vibrational sieving. The reason for the effectiveness of
this sieving method is based on two components: A rotating slotted nozzle
inside the sieving chamber and a powerful industrial vacuum cleaner which is
connected to the chamber. The vacuum cleaner generates a vacuum inside the
sieving chamber and sucks in fresh air through the slotted nozzle. When passing
the narrow slit of the nozzle the air stream is accelerated and blown against
the sieve mesh, dispersing the particles. Above the mesh, the air jet is
distributed over the complete sieve surface and is sucked in with low speed
through the sieve mesh. Thus the finer particles are transported through the
mesh openings into the vacuum cleaner.
Types of
gradation
A Dense gradation
A dense gradation
refers to a sample that is approximately of equal amounts of various sizes of
aggregate. By having a dense gradation, most of the air voids between the
material are filled with particles. A dense gradation will result in an even
curve on the gradation graph.
Narrow gradation
Also known as uniform gradation,
a narrow gradation is a sample that has aggregate of approximately the same
size. The curve on the gradation graph is very steep, and occupies a small
range of the aggregate.
Gap gradation
A gap gradation refers
to a sample with very little aggregate in the medium size range. This results
in only coarse and fine aggregate. The curve is horizontal in the medium size
range on the gradation graph
Open gradation
An open gradation
refers an aggregate sample with very little fine aggregate particles. This
results in many air voids, because there are no fine particles to fill them. On
the gradation graph, it appears as a curve that is horizontal in the small size
range
Rich gradation
A rich gradation refers
to a sample of aggregate with a high proportion of particles of small sizes.
Types of
sieves
Woven wire mesh sieves
Woven wire mesh sieves are
according to technical requirements of ISO 3310-1.[7] These
sieves usually have nominal aperture ranging from 20 micrometers to 3.55
millimeters, with diameters ranging from 100 to 450 millimeters.
Perforated plate sieves
Perforated plate sieves
conform to ISO 3310-2 and can have round or square nominal apertures ranging
from 1 millimeter to 125 millimeters.[8] The
diameters of the sieves range from 200 to 450 millimeters.
American standard sieves
American standard sieves also
known as ASTM sieves conform to ASTM E11 standard.[9] The
nominal aperture of these sieves range from 20 micrometers to 200 millimeters,
however these sieves have only 8 inches (203 mm) and 12 inches
(305 mm) diameter sizes
Limitations
of sieve analysis
Sieve analysis has, in general, been used for decades to
monitor material quality based on particle size. For coarse material, sizes
that range down to #100 mesh (150μm), a sieve analysis and particle size distribution
is accurate and consistent.
However, for material that is
finer than 100 mesh, dry sieving can be significantly less accurate. This is
because the mechanical energy required to make particles pass through an
opening and the surface attraction effects between the particles themselves and
between particles and the screen increase as the particle size decreases. Wet
sieve analysis can be utilized where the material analyzed is not affected by
the liquid - except to disperse it. Suspending the particles in a suitable
liquid transports fine material through the sieve much more efficiently than
shaking the dry material.
Sieve analysis assumes that
all particle will be round (spherical) or nearly so and will pass through the
square openings when the particle diameter is less than the size of the square
opening in the screen. For elongated and flat particles a sieve analysis will
not yield reliable mass-based results, as the particle size reported will
assume that the particles are spherical, where in fact an elongated particle
might pass through the screen end-on, but would be prevented from doing so if
it presented itself side-on.
Properties
Gradation affects many
properties of an aggregate, including bulk density, physical stability and
permeability. With careful selection of the gradation, it is possible to
achieve high bulk density, high physical stability, and low permeability. This
is important because in pavement design, a workable, stable mix with resistance
to water is important. With an open gradation, the bulk density is relatively
low, due to the lack of fine particles, the physical stability is moderate, and
the permeability is quite high. With a rich gradation, the bulk density will
also be low, the physical stability is low, and the permeability is also low.
The gradation can be affected to achieve the desired properties for the
particular engineering application.
Engineering applications
Gradation is usually
specified for each engineering application it is used for. For example,
foundations might only call for coarse aggregates, and therefore an open
gradation is needed. Sieve analysis determines the particle size distribution
of a given soil sample and hence helps in easy identification of a soil's
mechanical properties. These mechanical properties determine whether a given
soil can support the proposed engineering structure. It also helps determine
what modifications can be applied to the soil and the best way to achieve
maximum soil strength.
Sendimentation
method
Defination
The simplest method of sedimentation is
to use rectangular tanks with horizontal flow through them. The water with the
particles in suspension is introduced at one end of the tank, then as the water
flows to the other end of the tank settlement of particles in the water occurs.
Figure 0—1 ------------------sedimentation process
INTRODUCTION
The
fine particles of many finely divided materials are so small that it is
impractical to make effective size separations by pneumatic methods and
microscopic methods are not readily applicable. Since a knowledge of the
particle size of hyrdated lime, portland cement, and many other substances may
assist in explaining some of their properties, it was considered advisable to
attempt to apply a method devised by Oden * to determine the particle size
distributions of hydrated lime.
Theory of
sedimentation method
Defination
In the following discussion, "distribution
curve" 4 means the re- lation between the percentage of material and the
corresponding diameters. A " cumulative curve" shows the amount of
material which is coarser than a given size. When a fine material is uniformly
dispersed in a liquid and allowed to settle, the curve showing the weight of
material settled out as a function of time is a " sedimentation
curve." A distribution curve may be obtained from a sedimentation curve in
the following manner. The amount of material, P, settled on the pan at a given
time consists of two parts ; one part S, made up of all the particles coarse
enough to have settled the entire length of the column, and a second part,
consisting of those particles of smaller size which, though they may not have
settled the full length of the column, have nevertheless reached the pan. Each
size of this second part has a constant settling velocity and the rate of their
accumulation on the pan may be represented as dP/dt At time t, the weight of
this dP second part, amounts to t*dP/dt The total weight, P, settled at time t,
is therefore, P = S+t^
Equation 1------------------------------------------------------Sedimentation
Equation 2-------------------------------------------------------Sedimentation
Figure 0—1-------functional zone
Figure 0—2------------Determination of particle size
Determination
of Particle Size
Figure 0—1--------------------------------------------Determination
of particle size
Figure 0—2-----------------size of prticles
CONCLUSIONS
Microscopic
measurements on glass spherules have shown that changes in refractive index of
the medium surrounding the particle alter the apparent diameter of the
particle. The magnitude of this change is such that until it is known which
refractive index will give true particle size, the microscope certainly has
limited use as a means of determining particle size. When fractions of these
glass spherules were well dispersed in butyl alcohol, fair correlation of
particle size distribution was obtained between sedimentation measurements and
an average of microscopic measurements.
DESCRIPTION
AND OPERATION OF SEDIMENTATION APPARATUS
The apparatus with which sedimentation curves were
obtained is illustrated in figure 2. The material being analyzed is thoroughly
dispersed in a liquid and introduced into the sedimentation cylinder. As the
particles settle on the pan attached to the left arm of the balance, the beam
is rotated counterclockwise, thus deflecting the image of the lamp from the
opaque screen onto a photoelectric cell. This causes an increased current to
flow, which, after being amplified by the vacuum tube, operates the relay to
start the motor connected with the drum resistance. The motor runs until enough
resistance is cut out to increase the solenoid current sufficiently to restore
the equilibrium of the balance and throw the light off the photoelectric cell.
This is repeated automatically for each effective increment of weight produced
by the material settling from suspension. A record of the current flowing
through the solenoid circuit then corresponds to a time-weight record of the
sedimentation of the sample. The balance was inclosed in a double-wailed
cabinet provided with an air circulating system.
Potable water treatment
Sedimentation in potable water
treatment generally follows a
step of chemical coagulation and flocculation,
which allows grouping particles together into flocs of a bigger size. This
increases the settling speed of suspended solids and allows settling colloids.
Wastewater
treatment
Main
article: Wastewater treatment
Sedimentation has been used to treat wastewater for
millennia
Primary treatment of sewage is
removal of floating and settleable solids through sedimentation. Primary
clarifiers reduce the content of suspended solids as well as the
pollutant embedded in the suspended solids. Because of the
large amount of reagent necessary to
treat domestic wastewater, preliminary chemical coagulation and flocculation
are generally not used, remaining suspended solids being reduced by following
stages of the system. However, coagulation and flocculation can be used for
building a compact treatment plant (also called a "package treatment
plant"), or for further polishing of the treated water.
Sedimentation tanks called "secondary
clarifiers" remove flocs of biological growth created in some methods
of secondary treatment including activated
sludge, trickling filters and rotating biological
contactors
Comments
Post a Comment