Batch Sedimentation
Batch Sedimentation
Batch Sedimentation
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Problem E1
BATCH SEDIMENTATION
1. OBJECTIVES
1.1 To determine the effects of initial concentration on sedimentation
characteristics.
1.2 To determine the effect of initial height on sedimentation characteristics.
1.3 To determine the average size particles using experimental settling data.
2. REFERENCES
Foust, Alan., et. al. Principles of Unit Operations, 2nd Ed. John Wiley and
Sons, In., 1980.
Geankoplis, Christie J., Transport Processes and Unit Operations, 3rd Ed.
Prentice-Hall International, Inc., 1993.
McCabe, Warren L., Smith, Julian C., and Harriott, Peter. Unit
Operations of Chemical Engineering, 5th Ed. New York: McGraw-Hill,
Inc., 1993.
Perry, Robert H. and Don Green (Editor), Perrys Chemical Engineers
Handbook, 6th Ed. New York: McGraw-Hill, 1984.
3. EQUIPMENT AND MATERIALS
W2 Sedimentation Apparatus (220 Volts)
Stopwatch
Calcium Carbonate (CaCO3)
Viscometer (220 V)
4. SAFETY
The sedimentation tubes are made of glass. There should be extra care
when covering them with rubber stoppers or when shaking them (to achieve
homogeneity of the slurry). Members of the group that are handling the tubes
must be given enough room to avoid any untoward incident. Likewise,
discarding the slurries must also be done properly. For this, follow the clean
up procedure carefully.
5. PRELIMINARY
To achieve the objectives of this experiment, it is important that one
reviews well the principles of Batch Sedimentation (Foust and Geankoplis).
In addition, one must be familiar with the difference between batch
sedimentation and continuous settling operations, and the equations
necessary in analyzing them.
Check if the light bulbs of the sedimentation apparatus are working. If
not, ask the technician to have the defective bulb replaced. Note also the
dimensions of the tubes. It may be useful to perform a trial run for one of the
slurries; this should provide a basis in deciding on the most convenient time
interval.
6. THEORY
Sedimentation is the method of separation of dilute slurry into a clear
supernatant liquid and concentrated slurry called sludge. Gravity effects this
separation. There are two general cases in sedimentation: one involves dilute
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slurry, the other involves more concentrated slurry. In the former case, the
particles settle individually and the settling characteristics depend on the
viscosity of the medium, relative density of the particles with respect to that of
the medium, and the particle size. An increase in the viscosity of the medium
will lower the settling velocities of the particles; while an increase in the
particle size and density will favor faster settling. Also, in this case, settling
by gravity proceeds such that after some time there will be a clear
demarcation line between the clear liquid and the slurry. The demarcation line
becomes more and more distinct as the mixture approaches the end of the
settling period.
More concentrated slurry behaves differently. It settles, or more correctly,
the particles settle as a single body with a very much lowered velocity due to
mutual interference among them. This is called hindered settling. At the start
there is only one zone, a layer of homogeneously dispersed solutes. At the
middle of the separation, four zones develop: zone A which is the region of
supernatant liquid, zone B which is the remains of the homogeneous zone
developed at the start, zone C which is the region of variable concentration,
also known as the transition zone, and zone D, the region of settled solutes.
As the operation continues, zones B and C gradually disappear while zones
A and D increase. After some time, the level of the sludge shrinks to a final
level known as the ultimate height.
The plot of height of demarcation line vs. time will give the settling
characteristics of the slurry. The derivative of the curve with respect to time is
the settling velocity. This is a function of the initial concentration and is
independent of the initial height of the slurry.
7. OPERATING PROCEDURE AND CONDITIONS
7.1 Start-Up
7.1.1
7.1.2
7.1.3
Bulk density
Bulk viscosity
7.2.2
7.2.3
7.2.4
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Note the height of the interface (between the clear liquid and the
subsiding dispersion) at a convenient time interval (2 or 3
minutes). If observable, note also the rise of the sludge at the
base of the tubes at each corresponding time interval. Do this until
there are no observable changes in the heights being measured.
Allow the slurries to stand for 24 hours to obtain the final
compaction reading, z. Be careful not to disturb the dispersions.
7.3 Clean Up
7.3.1
7.3.2
8. TREATMENT OF RESULTS
8.1 Graphical Treatment
8.1.1
8.1.2
8.1.3
fraction settled, f
z0 z
z0 z
8.1.4
8.1.5
zic = z0c0
Where:
c0 = initial concentration of the slurry
zi = intercept of the tangent drawn at a point in the
sedimentation curve
c = the corresponding concentration for zi obtained using
the equation above
v = slope of the tangent of the curves in graph 1 or the
settling velocity at time, )
= (zi z)/
Call this graph 5.
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8.2 Calculations
8.2.1
8.2.2
8.2.3
8.2.4