Lecture

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22 févr. 2014 (il y a 3 années et 4 mois)

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Lecture 4


SEDIMENTATION, CLARIFICATION,

FLOTATION, AND

COALESCENCE



INTRODUCTION


Suspended solid matter in a fluid media represents a heterogeneous system that

from a very
general viewpoint is a fundamental fluid dynamics problem that

virtually all
chemical engineers
are familiar with. It is so frequently encountered in

numerous industry applications that
considering only the problem of water

treatment simply does not do the subject justice.

Examples of operations where heterogeneous system are encou
ntered include

sedimentation of
dust in chambers and cyclone separators, separation

of suspensions

in settlers, separation of
liquid mixtures by settling and centrifuging, hydraulic and

pneumatic transport, hydraulic and air
classification, flotation, mixi
ng by air and

others. Each of these operations involves the
simultaneous flow of gas and solid,

or liquid and solid phases.



HOW PARTICLES SETTLE

If a particle at rest (with mass ‘m’

and weight ‘mg’) begins to fall

under the influence of gravity,
its

velo
city is increased initially over a

period of time. The particle is

subjected to the resistance
of the surrounding water through which it descends. This

resistance increases with particle
velocity until the accelerating and resisting forces

are equal. From
this point, the solid particle
continues to fall at a constant

maximum velocity, referred to as the
terminal velocity,
we called
the
settling velocity.
The force responsible for

moving a spherical particle of diameter ‘d’ can be
expressed by the difference

between its weight and the buoyant force acting on the particle.

The
buoyant force is proportional

to the mass

of fluid displaced by the particle, that

is, as the particle
falls through the surrounding water, it displaces a volume of fluid equivalent to
its own weight:



where
ρ
p

= density of the solid particle


ρ
= density of the fluid

The important factor for us to realize is that the settling velocity of a particle

is that velocity
when accelerating and resisting forces are equal




The

maximum size

particles that will settle out in the first two regimes of settling. If we take
Stokes

law for example, the maximum size particle whose velocity follows Stokes' law can

be
found by substituting
η
Re/
d
p

ρ

for the settling velocity into the firs
t sidebar

equation, and then
setting Re = 2 (the limiting Reynolds number value for the flow

regime). This then gives us the
following useful expression:




GRAVITY SEDIMENTATION


THICKENERS AND CLARIFIERS

Sedimentation involves the removal of suspended s
olid particles from a liquid

stream by
gravitational settling. This unit operation is divided into
thickening,
i.e., increasing the
concentration of the feed stream, and
cl
a
rific
a
tion
,
removal of solids

from a relatively dilute
stream. A thickener is a
sedimentation machine that operates

according to the principle of gravity
settling. Compared to other types of

liquid
/s
olid separation devices, a thickener’s principal
advantages are:



simplicity of design and economy of operation;



its capacity to handle ex
tremely large flow volumes; and



Versatility
, as it can operate equally well as a concentrator or as a clarifier.

In a batch
-
operating mode, a thickener

normally consists of a standard vessel filled

with a
suspension. After settling, the clear

liquid is dec
anted and the sediment removed

periodically.
The operation of a continuous

thickener is also relatively simple. Figure
4

illustrates a cross
-
sectional view of a

standard thickener. A drive mechanism

powers a rotating rake mechanism.
Feed

enters the apparat
us through a feed well

designed to dissipate the velocity and

stabilize the
density currents of the incoming

stream. Separation occurs when the heavy

particles settle to the
bottom of the tank.

Some processes add flocculants to the feed

stream to enhance p
article agglomeration to

promote
faster or more effective settling. The

clarified liquid overflows the tank and is sent

to the next
stage of a process. The underflow

solids are withdrawn from an underflow cone

by gravity
discharge or pumping.


Figure
4.
Cross
-
sectional view of a thickener.


Continuous clar
ifi
ers

handle a variety of process wastes,

domestic sewage and other dilute

suspensions. They resemble

thickeners in that they are

sedimentation
tanks
or basins whose

sludge removal is controlled by a

mechanical sludge
-
raking

mechanism. They differ from

thickeners in that the amount of

solids and weight of thickened

sludge are considerably lower.


Figures
6
and
7
show examples of

cylindrical clarifiers. In this type of

sedimentation machine,
the feed

e
nters up through the hollow central

column or shaft, referred to as a

siphon feed
system.
The feed enters

the central feed well through slots or

ports
located near the top of the

hollow shaft. Siphon feed

arrangements greatly reduces the

feed stream veloci
ty as it enters the
basin proper.
This
tends to minimize

undesirable cross currents in the settling region of the
vessel.




COAGULATION


The term
co
a
gu
la
tion
refers to the first step in complete clarification. It is the

neutralization of
the
electrostatic charges on colloidal particles. Because most of the

smaller suspended solids in
surface waters carry a negative electrostatic charge, the

natural repulsion of these similar charges
causes the particles to remain dispersed

almost indefinitely.

To allow these small suspended
solids to agglomerate, the

negative electrostatic charges must be neutralized. This is
accomplished by using cationic polymers or polyelectrolytes. The most common and widely
used inorganic

coagulants are:



Alum
-
aluminum sul
f
ate

A
l
2
(S
O
4
)
3



Ferric
s
ulfate

F
e(SO
4
)
3



Ferric chloride

FeC1
3



Sodium aluminate
-
Na
2
Al
2
O
4


Inorganic salts
of
metals work by two mechanisms in water clarification. The

positive charge of
the metals serves to neutralize the negative charges on the

turbidity particles. The metal salts also
form insoluble metal hydroxides which are

gelatinous
and
tend to agglomerate the neutralized
particles. The most common

coagulation reactions are as follows:



The effectiveness
of inorganic coagulants is
dependent upon water chemistry, and

in particular
--

pH and alkalinity. Their

addition

usually alters that chemistry

Aluminum salts are most effective
as

coagulants when the pH range is between

5.5
and 8.0 pH. Because they react with the

alkalinity in the
water, it may be necessary

to add additional alkalinity (called

buffering) in the
form of lime or soda ash.

Use the values in Table 3 to guide you. Iron

salts, on the other hand, are most effective

as
coagulants at higher pH ranges (between

8 and 10 pH). I
ron salts also depress

alkalinity and pH
levels; therefore,

additional alkalinity must be added. Sodium

aluminate increases the alkalinity
of water,

so care must be taken not to exceed pH and

alkalinity guidelines.


AIR FLOTATION SYSTEMS


Air flotation is
one of the oldest methods for

the removal of solids, oil & grease and

fibrous
materials from wastewater. Suspended

solids and oil & grease removals as high as

99% + can be attained with these processes.

Air flotation is simply the production of

microscopic

air bubbles, which enhance the

natural
tendency of some materials to float by

carrying wastewater contaminants to the

surface of the
tank for removal by mechanical

skimming.

When the primary target is oil removal, we should distinguish between the forms

o
f oil. There
are two forms of oil that we find in wastewater.
Free oil
is oil that

will separate naturally and
float to the surface.
Emulsified oil
is oil that is held in

suspension by a chemical substance
(Detergents
-

Surfactants) or electrical energy.

W
hen making an evaluation, free oil will normally separate by gravity and float to

the surface in
approximately 30 minutes. Emulsified oil is held in a molecular

structure called a
micelle
and
will not separate on its own. Hence, there is the need for a

mor
e sophisticated method of treating

suspensions containing emulsified oils.



SEPARATION USING COALESCERS


A coalescer achieves separation of an oily phase from water on the basis of density

differences
between the two fluids. These systems obviously work best with nonemulsified

oils.

In this diagram the key features are:
A

Diffusion

baffle: this serves four roles. First to dissipate
the velocity head, thereby improving the overall hydraulic characteristics of the separator. Next,
to direct incoming flow downward and outward maximizing the use of the separator volume.
Third, to reduce

flow turbulence and to distribute the flow evenly over the separator’s cross
-
sectional area. Finally, to isolate inlet turbulence from the rest of the separator.
B
-

Internal
chambers;

C

PetroScreen
TMT
: his part of the design focuses on improved separatio
n efficiency.
D
-

Monitoring systems: Various monitoring and control instruments are included in a system for
level sensing and pumpout control purposes.