Sedimentation ChE 155 – 1L
Date Performed:
Castro, Lyndon H. Medrano, Ivy Mae E. Villegas, Vanderville G.
Start: Finished:
Principles/Significance of the Experiment/Relevant Equations Sedimentation is a separation process wherein dilute slurry is separated by gravity into a clear fluid and slurry of higher solid concentration (Foust et al., 1980). According to Stokes Law, the terminal velocity is given by the equation 2
V t=
g d (ρ s−ρm ) 18 μ where:
d= particle size (diameter) ρs = density of the solid/particle ρm = density of the medium/fluid μ = viscosity of the liquid g = gravitational constant
Sedimentation contains several stages in the settling of flocculated suspension, and different zones are formed as sedimentation proceeds. These are the clear liquid zone, constant initial solids concentration zone, hindered settling zone and the compression zone. There are two kinds of sedimentation, the batch sedimentation and the continuous sedimentation. In batch sedimentation, heights of zones varies with time and often carried out in cylindrical tanks where dilute slurry is permitted to settle for a time and the clear liquid is decanted and sludge is removed. On the other hand, in continuous sedimentation, the heights of each zone are constant at steady state and the thickened slurry underflows and the clear liquid is drawn-off continuously. Clarification and thickening are the two types of sedimentation. Clarification virtually removes all particles from a given suspension. It removes a small quantity of the fine solids to produce a clear liquid effluent (Sinnott, 1996) and the desired product is called overflow. Clarification is widely used in sugar industry, steel mills, oil industries and wastewater treatment. Meanwhile, thickening is used for heavier-duty requirements imposed by a large quantity of concentrated pulp. It increases the concentration of a relatively large quantity of suspended solids
as it produces thickened or concentrated slurry in the bottom stream. Thickening is widely used in cement industries, mining industries and also in wastewater treatment. Nearly all the industrial separators provide the continuous removal of settled solids since it is very significant to remove particles from a stream to attain a particular purpose. Sedimentation has a great application in waste management just like in the treatment of industrial wastes which contains organic particles or solids from liquid wastes before dumping into the bodies of water. It is also used in the elimination of dusts ad fumes from air or flue gas. Also it is applied in the recovery of particles for recycle purposes as well as a mechanism for bacterial transport and other biological processes. There are two sizing methods in sedimentation, the Coe and Clevenger Approach and the Talmadge and Fitch Method. The Coe and Clevenger Approach was the oldest method and was proposed in 1916 by H.S. Coe and G.H. Clevenger where the rate of descent of solids-liquid interface is assumed to be a function of local concentration (i.e., v = f(c)). In this approach the total flux is given by the equation, F=F B + Fu =cv + where:
Lu c A
F = Total flux FB = Batch flux Fu = Flux associated with withdrawal of solids due to the underflow c = Layer composition v = settling velocity of solids at c Lu= underflow volumetric rate A= cross-sectional area normal to flux
The required area for the thickener is given by: L c A= o o FL where:
FL = limiting solid flux capacity Lo= specified feed volumetric rate co= specified feed concentration
In the Talmadge and Fitch Method, the time and concentration at which the critical flux is realized is calculated. This method is used to determine the minimum area for a continuous thickener given desired underflow concentrations feed rate. The required area for the thickener is given by:
A=
Lo t u zo
where: Lo = feed volumetric flowrate Zo = initial height tu = time to reach desired underflow concentration Objectives of the Experiment
To determine the effect of slurry concentration on the slurry’s settling characteristics. To determine the effect of initial height on the slurry’s settling characteristics. To use the batch settling data for the design of continuous thickener or clarifier.
Materials/Glassware to be borrowed • • • • • • • • •
stirring rods timer beaker plastic spoon ruler electronic top loading balance Water 310 g CaCO3 (calcium carbonate) 7 1L graduated cylinder
Materials and Methods Effect of the Initial Concentration 1. Weigh 25, 50, 75 and 100g of CaCO3 . 2. Put the weighted CaCO3 in 1L graduated cylinder and fill each up to 1L mark. 3. Mix the solution thoroughly using a stirring rod 4. Let each stand and record the height of the slurry every 3 minutes until the height becomes constant for 3 consecutive readings. 5. Let each stand for 10 minutes, and then record the change in height. *Time intervals may be widened after the critical point has been reached 6. Repeat the procedure for the second trial
Effect of the Initial Height 1. Weigh x g of CaCO3. 2. Put the weighted CaCO3 in 1L graduated cylinder and fill each up to y mL mark. 3. Mix the solution thoroughly using a stirring rod 4. Let each stand and record the height of the slurry every 3 minutes until the height becomes constant for 3 consecutive readings. 5. Let each stand for 10 minutes, and then record the change in height. *Time intervals may be widened after the critical point has been reached. 6. Repeat the procedure for the second trial
Data Tables/Blanks Table 1.1. Height of the interface at different concentrations. height (mm) time (mins)
25
g L
50
g L
75
g L
100
g L
trial 1 trial 2 trial 3 trial 1 trial 2 trial 3 trial 1 trial 2 trial 3 trial 1 trial 2 trial 3 0 2 4 6 8 10
Table 1.2. Height of the interface at different height of mixture. height (mm) 25 g 1000 L
time (mins)
22.5 g 900 L
20 g 800 L
17.5 g 700 L
trial 1 trial 2 trial 3 trial 1 trial 2 trial 3 trial 1 trial 2 trial 3 trial 1 trial 2 trial 3 0 2 4 6 8 10
References:
Foust, Alan S., et. al (1980). Principles of Unit Operations. 2nd ed. John Wiley and Sons, New York. Sinnott, R.K. (1996). Chemical Engineering: Chemical Engineering Design, Vol. 6. 2nd Ed. Oxford: Butterworth Heinemann. pp. 365-366. Weston, V. (2013). The Application of Mathematics, Physics, Chemistry and Engineering to Evaluate Solutions in Process, Environmental, and Mineral applications for Separating Suspended matter and Soluble Constituents from an Aqueous Phase. SLCC Science and Technology Paper. Retrieved from on November 23, 2013. Concha, Fernando and Bürger, Raimund (2002). A Century of Research in Sedimentation and Thickening. KONA No.2.
Retrieved from on November 23, 2013.
