Sedimentation

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K.C. Engineers

Sedimentation Studies Apparatus


Contents

Objective

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1

Aim

……………………………………

1

Introduction

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1

Theory

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1

Utilities Required

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2

Experimental Procedure

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2

Observation & Calculation

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3


SEDIMENTATION STUDIES APPARATUS OBJECTIVE: Study of batch sedimentation process.

AIM: 1. To plot a graph between the height of interface (z) and time (θ ) for slurries having five different concentration (5, 7.5, 10, 12.5, and 15 %) of CaCO3 2. To plot settling rate (dz/dθ )θ⇒ 0 vs initial concentration (Co) on a log-log-scale. 3. To determine the critical constant (θ c, Cc) for each concentration (Co). 4. Use these critical parameters of slurry for the determination of Area (A) for continuous thickener.

INTRODUCTION: The separation of dilute slurry by gravity settling into a clear fluid and slurry of higher solid concentration is known as Sedimentation. The mechanism of sedimentation observed during a batch settling set is shown in figure. As Sedimentation continues, the height of each zone varies as indicated in the figure. Both A and D grow larger at the expense of B and ultimately zone B & C disappear and all the solids appear in D. This is known as critical settling point. The same zones shall be present in a continuous thickener. Batch Sedimentation tests are used to determine the settling characteristics of a Slurry/Sludge. The analysis is further used for the design of continuous thickener.

THEORY: For any batch sedimentation experiment, on slurry of known concentration, the height of a liquid-solid interface is obtained as a function of time. Slopes of this curve at any point of time represent settling velocities of the suspension at that time and are characteristics of specific solid concentration.


1

( a S

)

A T r a n s iti o n

A

V a r ia b le C o n c e n tra tio n

U n ifo rm C o n c e n tr a tio n

B

B C D ( b

)

C

o S

t a r t

A

o

B C D

a r s e li d

( c )

A

C E

n

D

D

d ( d

)

( e

)

Figure- (1). zo Let initial conc. of solids in the cylinder be = Co Height of solid liquid interface

= zo

Cross sectional area of cylinder

=A

z Height of interface θ

0

Figure - 2 Let θ c be the time needed to reach the critical concentration C c (i.e. when the settling velocity decreases rapidly). From the settling curve data (Figure - 2) θ

c

and Cc are obtained

as: a)

Draw a tangent to the early part of the curve (representing free settling condition).

b)

Draw a tangent to the last portion of the curve (representing hindered settling condition).

c)

Extend the two tangents till they intersect.

d)

At the intersection, construct the angle bisector.

e)

The intersection of this angle bisector line with the settling curve gives an estimate of θ

c


2

The mass balance for solid is: Cozo = Cuzu Subscript u denotes the underflow or θ

zu = zoCo / Cu u

case obtained from :

(zc - zu) / (θ

u

- θ c) = (z1 - zc) / θ

c

and area of thickener is obtained from : A = Loθ

u

/ zo, Lo = flow to the Unit

SETTLING RATE-CONCENTRATION RELATIONSHIP: From the batch settling data the settling velocity at any θ tangent drawn at θ = θ

L

L

is obtained from the slope of the

and is given by:

Settling Velocity V = (zi - zL) / θ Concentration at θ

L

L

is given by:

CL = Co zo / zi

UTILITIES REQUIRED: 1. Graduated Cylinders (1m long, 50mm dia.) 2. Stop Watch 3. CaCO3 (Calcium Carbonate)

EXPERIMENTAL PROCEDURE: Prepare five samples of CaCO3 slurry (5, 7.5, 10, 12.5, and 15 %) in each cylinder. Mix the samples thoroughly before starting the experiment so that homogeneous slurry is obtained in each case. Note the initial height (zo). Start recording the time and interfacial height in each of the graduated cylinder separately continue to take the record till height of interface stops any further Note the final height (z∞).


3

OBSERVATION & CALCULATION: Concentration (Co, g/L) Time (θ )

Hight of Interface zi

Seconds

(cm)

Plot all the curves Z vs θ . Find the slope of initial settling rates (dz /dθ )θ =0 for each concentration (Co) and plot on a log-log graph.

log(dz/dθ )θ =0 15% cm

1 12.5% 10% 7.5% 5% θ sec

(log Co)

For a particular Co obtain settling rate concentration relationship. Determine the thickness A for any value of Lo.


4

Sedimentation

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