Date Started: Date Finished: SEDIMENTATION I.
INTRODUCTION
According to Foust (1980), Sedimentation is the separation of a dilute slurry by gravity settling into a clear fluid and a slurry of higher solids content. According to Geankoplis (1993), it may also be defined as a form of separation where the particles are removed from the fluid by the action of gravitational forces acting on the particles. It has two functional operations, namely: thickening and clarification; thickening is increasing the concentration of suspended solids while clarification is the process of removing a relatively small quantity of suspended particles to produce a clarified effluent. Two types of sedimentation include gravity sedimentation and centrifugal sedimentation. Gravity sedimentation is defined as the separation of the diluted slurry and the clear fluid by means of gravity settling. Centrifugal sedimentation is defined as the separation of the diluted slurry and the clear fluid by means of a centrifugal force. Separation is dependent on the density difference of the two phases. It has two types of settleable solids namely: Discrete particles which are > 20μm and have little tendency to join together; and flocculent particles which are < 20μm. Sedimentation and Settling as a process is different from filtration. In filtration, the solid particles are detached from the slurry by forcing the liquid through a filter medium, which in turn blocks the flow of solid particles; the filtrate passes through freely. (Geankoplis, 1993) As any unit operation is important in the industry, so much more is Sedimentation in different areas. Common applications are removal of solids from liquid sewage wastes, 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 a slurry from a soybean leaching process. In some other significant processes, its purpose can be to remove the particles from the fluid stream so that the fluid is free of particle contaminants; in other processes the particles are recovered as the product, as in recovery of the dispersed phase in liquid-liquid extraction. In another cases, the particles are suspended in fluids so that the particles can be separated into fractions differing in size or in density. (Geankoplis, 1993) As observed by Foust (1980) on the mechanics of Sedimentation, several zones of concentration are established throughout the set-up; zone D of settled solids predominantly include the heavier-settling particles; zone C is a region of variable size distribution and non-uniform concentration; zone B is termed as a uniform-concentration zone which has approximately the same levels of concentration and distribution as
established initially; lastly, Zone A is a region above Zone B and is a boundary which has a composition of clear liquid. As the rate of sedimentation proceeds, the heights of each zone would tend to vary; whereas, zones A and D would eventually grow larger at the expense of Zone B. There will come a point where Zones B and C would disappear and all the solids would now appear in D; and this is now referred to as Critical Settling Point. The Critical Settling Point is a point at which a single distinct interface forms between clear liquid and the sediment. At this point in time, what can now be seen would be slow compression of solids, in which the liquid from the boundary layer of each particle is then being forced upward through the solids into the clear zone. (Foust, 1980) As labeled, these zones also have particular names, rather than just name these as Zone A, or Zone B. One zone or regime is the Particulate Settling; it is a zone where particles at low concentration are sufficiently far apart to settle freely. Next to Particulate Settling would be the Zone Settling Regime wherein the particles are constrained to settle as a mass; here, the settling rate of the mass of the particles can be a function of the solids concentration. These particles would then descend and are restrained by hydrodynamic forces and partially, by mechanical support from the particles from below. Nevertheless, Sedimentation relies on particle settling rates. There are as well types of Particle Settling, namely free settling and hindered settling; as defined, free settling involves particles which are at a sufficient distance from each other and from the container that these settle independently; hindered settling, on the other hand, involves particles which are close together that these continuously collide and these motions are impeded by other particles. According to McCabe (2001), Batch Sedimentation can be explained by a series of several stages in the settling of a flocculated suspension, and wherein different zones are formed as the sedimentation proceeds. As observed, the concentration of solids is usually high enough that the sedimentation of individual particles or flocs is blocked by other solids, reaching to a point where all the solids at a given level settle at a common velocity. In terms of the rate of sedimentation, there is a typical plot of interface height (the boundary between zones A and B) versus time. During the early stages of settling, the velocity would be constant. With the onset of zone B disappearing, the rate of settling starts to decline and then steadily drops until the ultimate height is reached. As stated by McCabe (2001), Slurries vary greatly in their settling rates and in the relative heights of various zones during settling. The initial concentration is a function of the feed concentration, but in the later stages, the settling rate would now be dependent on the initial height, Zo (where compression effects are more important with the thicker sludge layers).
SIGNIFICANCE OF THE STUDY After the experiment, The student can have better understanding of the settling characteristics of slurries; The student can interpret the equations used in the design of continuous thickener/clarifier; and The student can have knowledge on how to design continuous thickener/clarifier OBJECTIVES OF THE STUDY To determine the effect of initial slurry concentration on the slurry’s settling characteristics; To determine the effect of initial height on the slurry’s settling characteristics; and To gain knowledge on how batch data are used in the design of batch and continuous thickener/clarifier. MATERIALS AND EQUIPMENT (6) 1000 mL graduated cylinders CaCO3 (calcium carbonate) Analytical Balance Tap Water Ruler METHODOLOGY Prepare 15 g/L, 30 g/L, and 45 g/L of CaCO3 by weighing 15g, 30g, and 45g of CaCO3 using an analytical balance, placing them inside their respective graduated cylinders, and filling the cylinders up with water up to the 1000mL mark. *EFFECT OF INITIAL HEIGHT Prepare 800mL, 900mL, and 1000mL mixtures with the same concentration 15g/L, i.e., 12g, 13.5g, and 15g of CaCO3 for 800mL, 900mL, 1000mL of mixtures, respectively . *SEDIMENTATION Stir thoroughly the six mixtures until uniform distribution of CaCO3 is observed. Simultaneously stop the stirring before the initial heights of the mixtures were recorded at t=0. After every two (2) minutes, record the heights of the interface until no significant change in height is observed. REFERENCES: Geankoplis, C.J. (1993). Transport Processes and Unit Operations.3rd edition. Prentice-Hall International: Singapore. McCabe, W.L., Smith, J.C. & Harriott, P. (2001). Unit Operations of Chemical Engineering. 6th ed. McGraw-Hill: Singapore. Foust, A.S., et. al. (1980). Principles of Unit Operations. 2nd ed. John Wiley & Sons: Singapore.
