ChE 514L: Chemical Engineering Lab II Experimental Report Course Number and Name: ChE 514L Chemical Engineering Laboratory II Semester and Year: 1st Semester AY 2014-2015 Name of Team Leader: Name of Lab Instructor: Bilog, Josef Antonio Lab Section and Meeting Time:
Asst. Prof. Carol M. Encarnado Experiment Number:
5 ChE C, 8:00-11:00 am, Friday
E1
Title of Experiment: Sedimentation Date of Experiment Performed: August , 2014 Date of Report Submitted: August 29, 2014 Grade: Abstract (15 pts) Objectives (5 pts) Theory & Methods (15 pts) Results & Discussion (40 pts) Conclusions (5 pts) References (10 pts) Appendices (10 pts) Final Score:
Team No.: 4 Team Members: De Jesus, Medrell Faustino, Ma. Chamille B. Marquez, Mark Angelo S. Tuano, Hannah Mae C. Instructor Comments:
ABSTRACT [Primary Contributor: De Jesus, Medrell C.; Secondary Contributors: Faustino, Ma. Chamille ]
Batch sedimentation is the process of separating a liquid mixture of suspended particles into clear supernatant liquid and denser slurry having a higher concentration of solids. This is accomplished by decreasing the velocity of the solvent (water) being treated to a point below which particles will no longer remain in suspension. When solids diffuse through the interface, settling process starts from slurry and presumed to approach terminal velocities under hindered settling conditions. With this, several zones of concentration will be established. Through this experiment, the effect of initial concentration and initial height of the slurry on its settling characteristics is determined. Graphical demonstration is used in the experiment in order to analyze the relationship between the height of the interface and time. Based on the results provided, it shows that the height of the interface is inversely related with time. Also, the sedimentation rate can be affected by the different parameters like the initial concentration of solids (CaCO 3) and its average size particle. As the sedimentation continues, heights of each zone vary and the point at which a single distinct interface forms between the supernatant liquid and solids will be reached. OBJECTIVE AND INTRODUCTION [Primary Contributor: Faustino, Ma. Chamille B.; Secondary Contributors: Marquez, Mark Angelo S.]
The objective of this experiment is to determine the behavior of settling velocity as the sedimentation process proceeds. The experiment also intended to determine the effect of slurry concentration with particle settling velocity was also studied. The experiment aimed to observe the relationship of settling time with initial slurry height, as well as with slurry concentration. Introduction Sedimentation is one of the methods used in industry to separate liquid-liquid or solid-liquid mixtures. The separation of a dilute slurry or suspension by gravity settling into a clear fluid and slurry of higher solids content is called Sedimentation [1]. Applications of settling and sedimentation include removal of solids from liquid sewage wasters, settling of crystals from the mother liquor, separation of liquid-liquid mixture from a solvent-extraction stage in a settler. Settling of solid food particles from a liquid food and settling of slurry from a soybean leaching process. The particles can be solid particles or liquid drops [2]. In some processes of sedimentation purpose is to remove the particles from the fluid stream so that the fluid is free of particle contaminants. When a particle is at a sufficient distance from the walls of the container and from other particles so that its fall is not affected by them, the process is called free settling. When the particles are crowded, they settle at a lower rate and the process is called hindered setting [1].
1
THEORY AND EXPERIMENTAL METHODS [Primary Contributor: Marquez, Mark Angelo S.; Secondary Contributors: Tuano, Hannah Mae C.]
Theory Sedimentation is the process where the water has little or no movement and the suspended solids settle to the bottom under the force of gravity and form sediments. As sedimentation continuous, zones are formed [3].
Figure 1. Batch Sedimentation
The figure above shows the zones in batch sedimentation. At first, the solids are well distributed in the liquid. After a short period of time, suspension zone or suspension interface (B) is formed. It is the point separating the constant composition zone and the clear liquid. The clear liquid (A) is the zone free of particles. Clarification refers specifically to the function of a sedimentation tank in removing suspended matter from the water to give a clarified effluent. In a broader sense, clarification could include flotation and filtration. As time pass by, solids continue to settle and the thickened zone (D) and the transition zone (C) are formed. Concentrated impurities withdrawn from the bottom of sedimentation tanks are called sludge, while material that floats to the top of the tank is called scum. The clear liquid (A) and the thickened zone (D) continues to increase with time, until the transition zone disappears, and thickened zone (D) and the clear liquid (A) are left. The accumulation of solids puts stress on the material at the bottom which leads to compression of solids in the thickened zone. When the weight of the solids is balanced by the compressive force, the settling process stops. Note that as the interface of the particles moves down and sludge builds up, two levels becomes equal, critical point is reached. For a rigid particle moving in a fluid, there are three forces acting on the body: gravity, buoyant and resistance or drag force acting in opposite direction to the particle motion. (Geankoplis 2012). Buoyant force, Fb, is the upward force exerted by the fluid on the particle. The buoyant force, Fb in N on the particle is (Geankoplis 2012) where m/ρp is the volume Vp in m3 of the particle and g is the gravitational acceleration in m/s 2.
2
The gravitation or external force Fg in N on the particle is given by Newton’s Law as (Geankoplis 2012) The drag force FD on a body in N may be derived from the fact that, as in flow of fluids, the drag force or frictional resistance is proportional to the velocity head v2/2 of the fluid displace by the moving body and is given by the equation [1]
where CD is the dimensionless drag coefficient, and
is velocity head.
The drag coefficient is a function of Reynolds number. In the laminar-flow region, called the Stoke’s law region for NRe < 1, is given by [1] (1) where µ is the viscosity of the liquid in Pa·s or kg/m·s (lbm/ft·s) [1] The constant-velocity period is usually of more importance, as the accelerated fall period is very short relative to the constant velocity period. In the constant rate period, the particles reach a maximum settling velocity known as the terminal velocity, vt . The terminal velocity is determined by solving the velocity at which the sum of the three forces is equal to zero. Geankoplis gives the equation for the terminal velocity of spheres as [1]
For spherical particles m= πD3pρp/6 and A = πD2p/4. Substitute to obtain spherical particles,
(2)
where
is the Particle Diameter [1]
Equation 2 gives the terminal velocity for free settling wherein a particle is at a sufficient distance away from the wall and other particles. For such hindered flow, the settling velocity is less than would be calculated from Equation 2 for Stoke’s Law. The true drag force is greater in the suspension because of the interference of the other particles [1].
3
The terminal velocity becomes a function of ε, the volume fraction of the slurry mixture occupied by the liquid. Several correlations have been developed to analyze settling velocity for hindered settling, and their methods and derivations are beyond the scope of this experiment. Experimental Method
Figure 2. Armfield W2 Sedimentation Apparatus
Before starting the experiment, double check the height of liquid of the prepared solutions to make sure the solutions prepared are correct. Switch on the light of the apparatus and carefully remove the tubes with different concentrations of solutions from the stand. The light will help you in distinguishing the phases or zones. Tilt the tubes from side to side until the solutions become homogenous. it was noted to hold the lid or stopper of the tube to prevent the solution from spilling. After reaching uniformity, the tubes are mounted. The time and the height of the zones are noted every two (2) minutes and then adjusted to 10 minutes until height of the thickened zone doesn’t change. After 24 hours, the height of the thickened zone was also recorded. RESULTS AND DISCUSSION [Primary Contributor: Bilog, Josef Antonio M.; Secondary Contributors: De Jesus, Medrell C.]
Based on the gathered data, at a short period of two minute time intervals, particles in water phase are well dispersed at its flocculated suspension initial state based on the nature of the CaCO3 particles suspended in water. About 8 minutes, clear
4
supernatant above the surface is seen while in the middle interface, it remains cloudy and cake is formed at the bottom.
Figure 3. Height of interface versus time (with changing concentrations)
As shown in the figure above, one parameter that could affect the characteristic of the sedimentation process is the concentration of CaCO3. The curve with higher concentration has longer time to reach the critical point, where there is minimal difference in the height. The density and viscosity of the subsiding particles increases at this point, thus making the sedimentation hindered settling.
Figure 4. Height interface versus time (with different initial heights)
Figure shows the initial height and concentrations of the slurry could affect the settling characteristic. The higher the initial heights, the longer the free settling rates of the sediments are. This is a free settling process due to larger space for the particles with higher initial heights, thus providing larger area for the interaction or movement of particles. 5
Also, one factor affecting the sedimentation process is particle size diameter. The formed flocs will cause an increase in sedimentation rate due to increase in size of the sedimenting particles. Thus, the final volume of the sediment is relatively large. Figure below shows the relationship of concentration with the rate of sedimentation at constant height.
Figure 5. Concentration versus sedimentation rate
CONCLUSIONS [Primary Contributor: Faustino, Ma. Chamille B.; Secondary Contributors: Marquez, Mark Angelo S.]
Based on all the data and graphs gathered from the experiment, we concluded that the initial concentration and height of the slurry affects its sedimentation characteristics. In particular, increasing the initial height of the slurry would also increase the settling time needed to reach the final height and increase the settling velocity. The relationship between the heights of interface of slurry versus time is inversely proportional. It can also be concluded that increasing the initial mixture concentration decreases the settling velocity of the particles before the compression settling zone. As the slurry go down, the operation goes to hindered settling as there is a transition period as the slurry and sludge become uniformly concentrated together. Two factors that also affect the rate between heights of interface versus time are the average particle size and bulk density and viscosity this is because of adsorption of particles. While in the compression settling zone, the higher concentrations would result to higher settling velocities. That is why CaCO3 sediment compacts much longer when there is higher concentration of it inside the cylinder such as the first cylinder which has 10% concentration of CaCO3 higher than the concentrations of other cylinders.
6
Recommendation During the course of the experiment, various problems were encountered that may lead to slight errors. These problems were usually problems of measurement. The rear panel should be translucent enough in order for the students to have a clear observation of the settling sediments in the sedimentation tube. REFERENCES [1] Geankoplis, C.J., Principles of Transport Processes and Separation Processes, Prentice Hall., 2012, pp. 919- 932. [2] McCabe, W.L. Unit Operations of Chemical Engineering, 7th ed., New York, N.Y.: McGraw-Hill, 2005, pp. 1055- 1065. [3] Engineering SL, “Engineering: It’s a Learning & Informative blog for Engineering,” http://engrsl.blogspot.com/2012/04/sedimentation.html
7
APPENDICES A. Sample Calculations (8 points) – Team Member 3 & 4 [Primary Contributor: Marquez, Mark Angelo S.; Secondary Contributors: Tuano, Hannah Mae C.]
B. Raw Data Tables [Primary Contributor: Bilog, Josef Antonio M.]
TIME 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
10% CaCO3 SLURRY SLUDGE 86 895 784 567 477 367 291 185 126 114 111 110 109 108 107 106 105 104 103 102 101 99 97 96.5 96
63 76 84 94 100 104 115 107 105 103 102 101 100 99 98 97 96 95 94 93.5 91 90.5 90
900 mm 7.5% CaCO3 SLURRY SLUDGE 770 624 507 399 298 194 85.5 81 80 78 77 76 75 74 73 72.5 72 71 70.5 70 69.5 69 68.5 68
40 50 59 66 73 80 86.5 75 73 70 69 69.5 68 67 66.5 66 65.5 65 64.5 64 63 63 62 61.5
8
5% CaCO3 SLURRY SLUDGE 680 450 268 99 31 30 30 30 29 29 29 28.5 28.5 28 28 28 28 29.5 29.5 29.5 29 29 29 27
24 28 31 31 31 26 26 23 23 23 23 23 23 25 25 25 25 25 25 23 23 23 23 23
700mm 5%CaCO3 SLURRY SLUDGE 470 228 39 37 35 35 34 34 32 32 32 32 32 32 32 32 32 32 32 32
42 40 34 31 29 28 27 26 26 26 26 27 27 27 27 27 27 28 28 28
500 mm 5% CaCO3 SLURRY SLU 410 278 178 88 28.5 27.5 27 27 27 27 27 27 27 26 26 26 26 26 26 26 26 26 24 24
1 2 2 2 1 2 2 2 2 2 21 2 2 2 2 2 2 2 2 2 2 2 2 2
PEER EVALUATION FORM Note: This form must be filled out by each student and submitted to the lab instructor separately from the lab report. Course Name: _____________________________________ Date of Experiment: ________________________________ A. Self Evaluation a. Rate Your Overall Contribution to this Project, (5=key contributions, 1=little contributions) 5
4
3
2
1
b. Explain briefly what you contributed to this project: __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________
9
B. Peer Evaluations
Comments: __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________
10
