Welcome to the second week of the course unit
operations of particulate matter.
This is the first lecture of week two and
here we will introduce the concept of filtration.
Now this particular topic that is filtration,
we will cover in three different lectures,
that is lecture 1, lecture 2, and lecture
3 where we will cover filtration as well as
batch filtration.
So in first lecture we will define the filtration,
we will discuss the principles of filtration,
and then we will cover the steps involved
in filtration and different types if filtration.
In second lecture of this session we will
describe the governing equations which are
used to design the filtration equipment filtration
unit.
And in third lecture of this session we will
discuss the examples, so that the concept
of filtration and the calculation etc., for
design of filtration unit can be illustrated.
So let us start with the first lecture of
week two which is on filtration.
Now what is the purpose and motivation of
this lecture is, the purpose is to provide
the basic technical information on the filters
and filter media.
So the purpose is to, we will define the filtration,
we will discuss about the filters and filter
media.
Now what is the motivation behind it?
The filtration if we consider filtration process
we use in our day-to-day life whether we are
aware with it or not, but we use it and how
we will translate the concept of filtration
which we use in our day-to-day life to the
industrial scale that will be the motivation
of this particular lecture.
So what is filtration?
If you see this particular figure what this
image shows that here we have the filter media
if you consider, and we have the solution
over there, the solution is continuously coming
over here, but the particle which are available
in the solution if their size is greater than
the pores of the media the solid will be deposited
or it will not be filtered from this, it will
not be penetrated through this.
However, the particle which are lesser than
they can penetrate, so that speaks about the
concept of filtration.
Now what is filtration is?
Filtration can be defined as the removal of
solid particles from a fluid by passing the
fluid through a filtering media on which solids
are deposited.
So we have to filter the slurry, so that solid
which is available in this that should be
deposited on the filter media, and liquid
which is available that should pass through
the media.
So that is basically the filtration.
So filtration is very much a part of our daily
life even if we are not aware of them.
Now to give an example of this when we carry
out when -- in our day-to-day life what we
do, I can say this that many of us have at
least strain the tea which we prepare for
us.
So that is nothing but the filtration process,
what we take, what we do in that we take the
strainer and then we pass the tea through
this, so the tea leaves which are available
in the tea that will be collected in the strainer
and rest of the liquid will be passed through
it.
So that is nothing but the filtration itself.
So similarly in many other day-to-day activity
we use filtration, but whether we are aware
this or not, but we use the filtration.
But the purpose of this lecture is how we
will use this technique to design the process.
Further as we all have gone through the labs
in school chemistry labs in school, so we
all have use the filtration process.
If you consider this image here we have the
test tube and over this test tube we put the
funnel, and inside the funnel we have the
filter paper, then the mixture will be entered
into this, so that the unwanted part or the
solid which is available in the mixture that
we will be collected in the filter and liquid
is pass through this.
So in lab also we have use the concept of
filtration, but obviously at very low scale.
So in laboratory the suspension is poured
into the conical funnel fitted with a filter
paper.
Now if we want to translate this concept to
industrial purpose in a industrial equivalent
difficulties are encountered in the mechanical
handling of much larger quantities of suspension
and solids.
A thicker layer of solids has to form and
in order to achieve high rate of passage of
liquid through the solids higher pressure
are needed and a far greater area has to be
provided.
So you see in lab scale in our day-to-day
life we use filtration at very low scale.
However, at industrial scale a significantly
large area is required, and because of that
media, sometime due to the accumulation of
solid over the filter media the liquid will
not pass through, so for that purpose we have
to increase the pressure so that liquid should
pass through the media.
So all these complexities we have to consider
in designing of filtration system for industrial
process or industrial purpose.
So here we have in this slide the principle
of filtration.
Now what is the principle of filtration?
First of all we should see what are the different
component in filtration system we have to
consider, and these components are first of
all we have to take the filter media through
which slurry should pass, and then that filter
media should be state over some about I say
some support like if we consider the example
of funnel also like in the previous slide
we have discussed.
So the funnel and inside this we put the filter
media, so that funnel works as a support to
the filter media.
In the similar line and in industry also below
the filter media we provide some support so
that it should be stayed over there, and over
the filter media we pass the slurry after
sometime of the operation what happens the
filter, the solid available in the slurry
that pass, that is deposited on the filter
media and prepare a thick cake which we call
as the cake of solid available in the slurry.
And due to formation of, due to accumulation
of solid over the filter media liquid can
be passed through that so we can have very
clear liquid as filtrate.
So what is the principle over here that we
have to take the filter media, we have to
take support to the filter media, and then
slurry should pass through the filter media?
So we can collect the solid in terms of cake
we can collect the clear water or clear solvent
in terms of filtrate.
So since the filter media is permeable only
to the liquid, only to the fluid it retains
the solid particles and permits only the fluid
to pass through which is collected as the
filtrate.
The volume of filtrate collected per unit
time is dV/dT is termed as the rate of filtration.
So how we have define the rate of filtration
the volume collected per unit time.
And this dV/dT we will use to design the,
we will use to derive the governing equations
also.
So as filtration proceeds solid particles
accumulate on the filter media forming a packed
bed of the solid and we call these as a filter
cake.
So what happens first we take the media, slurry
is passed through it, and after sometime the
thick layer of solid will be formed over the
media that we call as the cake.
So what we can understand over here that as
the thickness of this cake increases, you
can understand when we have the formation
of solid over the filter media what happens
it will offer a resistance to for a liquid
to pass through the media.
So as the thickness of cake increases resistance
to flow of filtrate increases, the rate of
filtration gradually decreases.
Now here due to increase thickness of cake
over the filter media the filtrate which is
collected over, collected through it that
will be decreased after, that will start decreasing
after sometime because of the resistance which
is offered due to the formation of cake.
So if rate is maintained to be constant then
pressure difference driving force will increase.
So what we have to do over here that due to
the formation of thick layer over the filter
media, the filtrate volume collected per time
will keep on decreasing.
If you want to maintain same rate of filtration
that is volume collected per time should be
same.
So we have to increase the pressure drop across
the filter media or across the filter media
as well as cake.
So therefore, the batch filter is operated
either by constant pressure or at constant
rate.
So as for as its principle is concerned, we
either go for the constant pressure process
or we can go for the constant rate process.
Now what happens when we consider these two?
So that can illustrated with this figure where
at Y axis we have pressure as well as Q that
is volume collected and here we have the time.
So as time proceeds if we want to maintain
the flow rate constant, maintain the flow
rate of filtrate constant, then continuously
we have to increase the pressure.
Pressure will keep on increasing, so we will
collect the constant volume of the filtrate,
and when the pressure will be reached to the
perusable value after that pressure should
not be increased because of the operational
difficulty.
So after that pressure will remain constant
and once we keep the pressure constant the
volume of filtrate collected per unit time
will keep on decreasing.
So what we have to do first of all we have
to increase the pressure to constant, the
rate of filtration and then we have to go
for the constant pressure, so that filtrate
volume will keep on, or rate of filtration
will keep on decreasing.
So to get the maximum output first of all
we have to operate at constant rate and then
we should proceed with constant pressure.
Now what happens when we consider vice versa
of this like if we maintain the pressure constant
and slurry and filtrate, rate of filtration
will keep on decreasing.
So you can see this section of the image appears
first and after that when we increase the
pressure the filtrate can be constant.
So you can see the filtrate can be constant
at this level.
So obviously the filtrate which has to be
collected its volume will be decreased, our
rate will be decrease significantly.
So for maximum output we have to operate with
constant rate and then with constant pressure.
Important factors which affect the rate of
filtration are the pressure drop across the
filter media, obviously because we have to
maintain the rate of filtration, so pressure
drop is an important factor across the media.
Secondly, area of filtering surface, when
we increase the area we can handle more and
more slurry at one time, but at the same time
we should take care of the filter media, because
accordingly we have to use larger filter media
and the larger system which will be difficult
to handle.
So but area of filtering surface is an important
factor, another factor is the viscosity of
the filtrate it has to be not so much viscous,
so that the penetration of liquid should be
easier, so its viscosity should not be much,
but viscosity affects the filtration process.
Next is resistance of the filter cake, the
filter cake resistance can be offered by the
thickness of the cake so we should take care
that after sometime these cake should be removed.
So the thickness of these cakes gives the
resistance and that will be the factor for
designing of filtration system.
Finally the resistance of the filter media
and initial layers of the cake, now initial
layer when the slurry contact the filter media
what happens initially a layer is formed.
So that layer works also as the resistance,
also as a filter media because it stops, it
blocks other particles to, or it blocks liquid
to pass through this so that will also work
as a filter media.
So resistance will be offered by the initial
layer, filter media, as well as total thickness
of the cake.
So here we will discuss different steps which
are involved in filtration process you see
first is the filtration where slurry is in
contact with the media, filtration takes place
and after that we have second step is draining
the liquor the filtration which is, filtrate
which is formed that we have to drain from
the system, then further the system should
be filled with wash water, because whatever
cake is formed over the filter media that
we should wash with the clear water or this
clear water we also call as wash water.
So third step is filling the system with wash
water, then washing takes place in the similar
line as the filtration takes place washing
also takes place in the same, following same
method that instead of slurry clear water
should pass through the cake, so that it can
wash the cake.
Next step is draining the wash water and finally
we have to open, dump, and reassemble the
whole system, so these are the steps involved
in batch process for filtration purpose.
And then after dumping, after reassembling
again we fill with the slurry so you see filtration
draining leaker filling with wash water, washing,
draining the wash water, opening, dumping
and reassembling this is one cycle of filtration.
Now if we consider the example from our day-to-day
life many of us used to prepare the cheese
at our houses.
So how we prepare this first we have the curdly
milk then we strain with the media that is
very fine cloth and through this we pass through
the whole mixture.
So the lump inside the mixture that will be
collected in the media and liquid pass through
this, okay.
So that will be a few step of the filtration
that is filtrating and then draining the liquor.
Now what we do we then wash the lump which
is collected over the cloth.
So we again fill this with water, clear water,
the water passes through this through the
lump and then we again, we keep that lump
one side and press with some heavy weight
so that it has the proper shape.
And whatever drainage, whatever wash water
we have used that we have already discharge.
So you see all these steps are involved even
when we do the filtration process in our houses
also.
So what happens here we have to, the cake
is formed we have to wash the cake and then
we have to discharge al the cake, so that
the system will be reassembled for the next
cycle.
Now it happens by some means if we continuously
remove this cake because the cake is main
huddle in the process.
So if continuously remove the cake the filtration
process can be translated into the continuous
system from bad system.
Now we have already seen the step involved
in filtration now we will see the different
types of filtration.
The very common filtration is the cake filtration,
now if you see what happens in the cake filtration
the particles from the suspension, which usually
has a high proportion of solids, is deposited
on the surfaces of a pours septum that is
filter media, which should ideally offer only
a small resistance to flow.
Now usually it has very less resistance because
that passes through the media.
So the resistances is very less, now what
happens after sometime the solid layer will
be formed over here that offers major resistance
for the liquid to pass, so as solids build
up on the septum the initial layer form the
effective filter medium, preventing the particles
from embedding themselves in the filter cloth,
and ensuring that a particle free filtrate
should be obtained.
So what happens that formation of layer of
solids it stops the other particle of solid
to pass through this, therefore we can get
the clear liquid, but because of this what
happens the solid will be deposited continuously
on the filter region and therefore cake formation
takes place.
Therefore this company process is called as
complete filtration it is called as cake filtration.
Another filtration we have is death or deep
bed filtration, what happens over here?
The particles penetrate into the pores of
the filter medium, where impacts between the
particles and the surface of the medium are
largely responsible for their removal and
retention.
This configuration is commonly used for the
removal of fine particles from very dilute
suspensions, where the recovery of the particles
is not of primarily importance.
So when the particle which are available in
the fine size then we go for this, and what
happens these particles penetrate through
the medium, these particle entered through
the media, but it cannot be penetrated through
this, liquid will pass through this, so after
in the case solid is entered into the media.
This takes place this type of filtration is
required when solid recovers the recovery
of solid is not only the aim, only getting
the clear liquid is the aim.
So in such type of filtration what happens?
Filter bed gradually becomes clogged with
particles and it resistance to flow eventually
reaches an unacceptably high level.
So you see because particle enters into filter
media it would not allow a liquid to pass
through this so after some time because of
clogging this complete system, we have to
stop because the further penetration of liquid
is not possible.
So that is the main disadvantages of this,
now to overcome the problem of clogging, it
is necessary to remove the accumulated solids,
and it is important that this can be done
easily.
Now how it can be done easily?
The filter commonly consists of a bed of particulars
solids, so instead of clot or very fine filter
media if we can keep a sand bed or bed of
a particular solid, if that bed of a particular
solid works as filter media so what happens?
Fine particle will be accumulated inside the
particulate bed and filtrate will pass through
this.
Now these particulate solid and the fine particle
deposited over here and that can be recovered
by back flushing process.
So you can use bed or particulate or other
particulate solid bed to carry out this deep
bed filtration.
Now here we have the filter media requirements,
so what of quality we should see as far as
the filter media is concerned and these are
the filter media should retain solids to be
filtered.
So filter media has to retain the solids that
is the 1st requirement, 2nd give a reasonable
clear filtrate, it should block the solid
particle to pass or retain the solid particle
and filtrate very clear filter we should get.
Does not plug or blind, the filter media should
be like that, it pores should be like that,
the solid will be deposited over here, it
should not penetrator though very fine particle
penetrates but we should take care the filter
media.
We should collect in such a way so that the
particle should not penetrate and should not
clog the filter media.
Filter media should be chemically and physically
resistant to process conditions.
So what happens it should, whatever slurry
we are using the filter media material should
not react with this?
So filter media material should chemically
and physically resistant to process.
It should permit cake to discharge cleanly
and completely.
Yes that is very important factor and it should
not be too expensive.
That is again the economic will involve over
here, so based on all these points we should
study the filter media.
Now the common type of filter media is shown
in this slide.
So if you see this image here we have different
types of filter media that is if we consider
this one will leave and this is the satin
wave and this is double layer weave also for
the filter media.
Filter media images you can see over here,
it gives the idea of that a filter media.
Filter media is basically the weed stuff of
cloth.
So a very thick cloth is there so that we
use as a filter media.
So these are different types of media.
And usually woven materials made of cotton,
wool, silk, nylon, polyester and glass fiber
and metal wires are most commonly used as
filter media.
Beds of granular solids such as sand are also
used when highly corrosive sludge is handled.
So you see there is different type of filter
media, not only cloth but sand can also used
as filter media.
Here we are stopping the 1st lecture on this
session, in the next session we will discuss
about the governing equation which are involved
in filtration process.
So that is all for now thank you.
