2013 Che324 Chapter 6 Sedimentation.pdf - Che 324 Fluid and...

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Che 324 Fluid and Particle Processes Chapter 6 Sedimentation 1 Chapter 6 Sedimentation 6.1 Introduction Sedimentation is a process for the gravity separation of a dispersed phase from a fluid due to density difference. It is a simple process. The important factors influencing the process are the feed rate and the settling rate of the materials. Two general settlers are used for such a process, batch settler and continuous settles. (A) Batch settlers The batch settling can occur either in vessel or in ponds. They are simply filled and then allowed to stand until the desired degree of separation is achieved. Open or closed operations are all common. The above batch settler can allow the removal of the clear liquid from upper tapping points while waiting for the settling process to be completed.
Che 324 Fluid and Particle Processes Chapter 6 Sedimentation 2 (B) Continuous settlers Continuous settlers are used to treat a stream for clarification or slurry to be thickened. The fluid flow through and the overflow from the thickener are continuous, however, the solids removal may be either continuous or intermittent. The geometry of continuous settlers may be vertical, inclined or horizontal. Horizontal separators Horizontal continuous settlers are usually used for clarification operations. Feed enters at one end and clarified liquid overflows at the other. The settled sludge is removed intermittently from the bottom of the vessel. Vertical thickeners
Che 324 Fluid and Particle Processes Chapter 6 Sedimentation 3 These are cylindrical vessels with an axial feed well. Clarified liquid overflows at the edge and the thickened solids are discharged from the conical bottom. Settling tests The fundamental piece of information required for the design of any settler is the solids settling velocity. These can be determined from various settling tests. Such tests must match plants conditions closely. Ideally, the settling tests should be carried out in near to the sampling location, to minimise the effects of temperature change and aging.
Che 324 Fluid and Particle Processes Chapter 6 Sedimentation 4 The settling test can supply all the information for the design of a thickener for separation. In this test, a suspension of particles of known concentration is prepared in a measuring cylinder. The cylinder is shaken to thoroughly mix the suspension and then placed upright to allow the suspension to settle. The positions of interfaces which form are monitored in time. H initial, constant period Falling rate period Settling finished Time
Che 324 Fluid and Particle Processes Chapter 6 Sedimentation 5 Two types of settling occur depending on the initial concentration of the suspension. (I) Three zones of constant concentration are formed A A B B S S H AB AS BS Time (II) A zone of variable concentration is formed in addition to the zones of constant concentration (A,B, and S). The suspension concentration within zone (E) varies with position. However, the minimum and maximum concentrations within this zone are constant.

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