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SEDIMENTATIONPRACTICALGROUP BShezi S 21706059Khusi M 21733534Mgobhozi S 21810139Cele P 21732286Mthiyane A 21814149List of Tables and Figures Table 3. 1 Nomenclature Symbols and units Table 3. 1 Nomenclature Suffixes Used Table 6. 1 The Continuous Sedimentation Apparatus Table 11. 1 Raw Data Obtained...

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SEDIMENTATION PRACTICAL GROUP B1

Shezi S 21706059 Khusi M 21733534 Mgobhozi S 21810139 Cele P 21732286 Mthiyane A.N 21814149

Contents 1.

Abstract .................................................................................................................................... 1

2.

Introduction ............................................................................................................................. 2

3.

Nomenclature ........................................................................................................................... 3 3.1

Symbols and Units ............................................................................................................ 3

3.2

Suffixes Used .................................................................................................................... 3

Theory ...................................................................................................................................... 4

4.

4.1

Principles of Sedimentation ............................................................................................. 4

4.2

Particle Properties ............................................................................................................ 5

4.3 Principles of Momentum and Sedimentation ........................................................................ 5 Experimental Method .............................................................................................................. 6

5.

5.1

Process Descriptions ......................................................................................................... 6

5.2

Experimental Deviations .................................................................................................. 7

5.3

Experimental Limitations and Constraints ..................................................................... 7

6.

Experimental Devices .............................................................................................................. 8

7.

Results .................................................................................................................................... 10 7.1

Tabulation of Results ..................................................................................................... 10

7.2

Graphical Representation of Results ............................................................................. 11

8.

Discussions ............................................................................................................................. 14

9.

Conclusion.............................................................................................................................. 15

10.

References .......................................................................................................................... 16

11.

Appendices ......................................................................................................................... 17

11.1

Raw Data ........................................................................................................................ 17

11.2

Technical Data................................................................................................................ 17

11.3

Sample Calculations ....................................................................................................... 18

List of Tables and Figures Table 3.1. 1 Nomenclature Symbols and units ................................................................................... 3 Table 3.2. 1 Nomenclature Suffixes Used .......................................................................................... 3 Table 6. 1 The Continuous Sedimentation Apparatus ........................................................................ 9

Table 11.1. 1 Raw Data Obtained for the Continuous Sedimentation Test........................................ 17 Table 11.1. 2 Raw Data Obtained for the Batch Sedimentation Test ................................................ 17 Table 11.2. 1 The Table of Technical Data ...................................................................................... 17

Figure 6. 1 Batch Settling Test Apparatus ......................................................................................... 8 Figure 6. 2 Sedimentation Zones ....................................................................................................... 8 Figure 6. 3 Continuous Sedimentation Apparatus .............................................................................. 9 Figure 7.2. 1 Batch Sedimentation Graph for Cylinder A ................................................................ 11 Figure 7.2. 2 Batch Sedimentation Graph for Cylinder B................................................................. 11 Figure 7.2. 3 Batch Sedimentation Graph for Cylinder C................................................................ 12 Figure 7.2. 4 The Removal Efficiency Against Volumetric Flowrate Graph..................................... 12 Figure 7.2. 5 The Sedimentation Rate Against Volumetric Flowrate Graph ..................................... 13 Figure 11.3. 1 Hand Drawn Batch Sedimentation Graph for Cylinder B .......................................... 19

SEDIMENTATION, May 2019

1. Abstract In industrial plants separation processes are often undertaken to obtain the desired products in the preferred purity and form. Separation processes can be grouped based on their properties into: Diffusional separators, Membrane separators and Mechanical separators. This report is focussed around an experiment on batch and continuous sedimentation. Sedimentation is the process by which particles settle to the bottom of a liquid and form a sediment due to gravitational pull. The main objective of this study is the determination of the efficiency of sedimentation of calcium carbonate suspensions at various flowrates. For the batch sedimentation process three pre-prepared sedimentation cylinders, containing unknown masses of calcium carbonate in water were examined. They were shaken thoroughly and allowed to settle. The heights obtained are graphically represented against time and used to obtain the settling velocities of the particles. For the continuous sedimentation the volumetric flowrate was increased to determine the removal efficiency. The rate of sedimentation was also calculated. The results obtained are graphically represented against the volumetric flowrate. The size and type of particle to be removed has a major effect on the settling rate. The solid particles in the Cylinder A were larger. During the experiment the particles of the Cylinder A settled faster and therefore had the highest settling velocity. Sedimentation is achieved by decreasing the velocity of the mixture to a point which the solid particles will no longer remain in suspension. From the results obtained for continuous sedimentation it can be seen that the removal efficiency is decreasing with the increasing flowrate (and therefore increase in solid particles’ velocity), which means that lesser solids are settling. This proves that for sedimentation to occur properly when the velocity, which also goes to say the volumetric flowrate, must be kept minimal.

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SEDIMENTATION, May 2019

2. Introduction In most industrial chemical processes the desired product formed is usually in a mixture with other components. These components can be excess reactants, by-products, catalysts and components of solvents or reaction media. To obtain the products in the desired purity and form separation must be undertaken. The most common separation processes include evaporation, distillation, absorption, crystallization, filtration, centrifugation, drying and membrane processes. Separation processes are mainly based on physical and physio-chemical means (Chapter 3 Separation Processes 2009). Differences in physical properties allow components in a mixture to undergo separation. Based on these properties various separation processes can be grouped into: Diffusional separators, Membrane separators and Mechanical separators. In diffusional separators separations are based on molecular movement towards a favourable phase. Membrane separators make use of semi-permeable membranes to separate molecules with different sizes or other properties. In mechanical separators separations are based on size and/or density differences of the components in the mixture, for separation of solid from liquid (e.g. filtration, centrifugation, etc.) (Chapter 3 Separation Processes 2009). In heterogeneous mixtures, two or more phases combine but remain physically separate. They can include mixtures of two liquids or suspensions (liquid and solid). Liquid-gas mixtures also exist. Separation of such mixtures is achieved by using mechanical-physical forces. These forces act on the particles, liquids or mixtures of particles and liquids themselves and not necessarily on the individual molecule. Mechanical-physical forces include gravitational and centrifugal, actual mechanical and kinetic forces arising from flow (Lotta & Marja 2015). This paper is focussed around an experiment on batch and continuous sedimentation. Sedimentation can be described as the process of letting suspended material settle by the force of gravity (Sedimentation). The main objective of this study is the determination of the efficiency of sedimentation of calcium carbonate suspensions at various flowrates. Settling velocities were determined from the batch settling tests. Removal efficiencies and rates of sedimentation of the continuous test were also calculated. Further details about how the experiment was carried out and all of these calculations undertaken are included in this report. The data and results obtained have been captured and presented using tables and figures of the Microsoft Office applications

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SEDIMENTATION, May 2019

3. Nomenclature 3.1 Symbols and Units Table 3.1. 1 Nomenclature Symbols and units Quantity

Symbol

Units

𝑚𝐹

G

𝑚0

G

Removal efficiency

%𝐸

%

Sedimentation rate

𝑄󰇗𝑠

𝑚 3 /𝑚𝑖𝑛

U

mm/min

t

Min

T

Min

V

m3

Mass of solids in the feed stream Mass of solids in the overflow

Settling velocity Time interval for batch settling Time taken to fill the sedimentation tank Volume of sedimentation tank

3.2 Suffixes Used Table 3.2. 1 Nomenclature Suffixes Used Number 1 2 3 4

Quantity Overflow at 5 L/min Overflow at 10 L/min Overflow at 15 L/min Overflow at 20 L/min

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SEDIMENTATION, May 2019

4. Theory 4.1 Principles of Sedimentation As already explained, sedimentation is the process by which particles settle to the bottom of a liquid and form a sediment. Particles that experience a force, either due to gravity or due to centrifugal motion will tend to move in a uniform manner in the direction exerted by that force. For gravity settling, this means that the particles will tend to fall to the bottom of the vessel, forming a slurry at the vessel base. For settling particles, there are two main forces enacting upon any particle. The primary force is an applied force, such as gravity, and a drag force that is due to the motion of the particle through the fluid. The force applied is usually not affected by the particle’s velocity, whereas the drag force is the function of the particle velocity. As the particles increase in the velocity, eventually the forces will approximately equate, causing no further change in the particle’s velocity. This velocity is known as the settling velocity (Backhurst, Harker & Richardson 2002). For each of the batch settling tests the settling velocity can be determined from the slope of the height-time graph.

𝑈=

𝑦2 −𝑦1

(4.1.1)

𝑥2 −𝑥1

The rate of sedimentation can be calculated as follows: 𝑉 𝑄󰇗𝑠 = 𝑡

(4.1.2)

The removal efficiency can be defined as the ratio of the amount of solid material that settles/sediments per amount of solids fed. This ratio can also be expressed as a percentage as follows: %𝐸 =

𝑆𝑜𝑙𝑖𝑑𝑠 𝑆𝑒𝑡𝑡𝑙𝑒𝑑 𝑆𝑜𝑙𝑖𝑑𝑠 𝑖𝑛 𝐹𝑒𝑒𝑑

× 100

(4.1.3)

The solids settled can be expressed as the difference between the mass of solids fed and the mass of solids in the overflow. Therefore equation 4.1.3 becomes:

%𝐸=

𝑚𝐹 −𝑚0 𝑚𝐹

(4.1.4)

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SEDIMENTATION, May 2019

4.2 Particle Properties A batch settling test can supply all information for the design of the thickener for separation of particles from a fluid. The height of the suspension doesn’t generally affect either the rate of sedimentation or the consistency of sediment ultimately obtained. The suspensions of fine particles tend to behave rather differently from coarse particles (Backhurst, Harker & Richardson 2002) . The size and type of particle to be removed have a significant effect on the settling rate. Sand or silt tend to be more easily removed because of their density. On the contrary colloidal material (small particles that stay in suspension and make the liquid seem cloudy), will not settle until the material is coagulated and flocculated by adding a chemical. The particles shape also has an effect on the rate of settling. Rounded particles settle more readily than irregular shaped particles. All particles possess slight electrical charges. Particles with the same charge repel one another in in doing so hinder settling (Sedimentation).

4.3 Principles of Momentum and Sedimentation An object’s momentum is the product of its mass and velocity. An object can have a large momentum by having a large mass or a large velocity. It can have a large momentum by having either a small mass but a large velocity or a small velocity but a large mass. The same principles of momentum apply for the particles is suspension (Impulse and Momentum). Sedimentation is achieved by decreasing the velocity of the mixture to a point which the solid particles will no longer remain in suspension. When the velocity no longer supports the particles, gravity will remove them from the fluid (Sedimentation).

.

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SEDIMENTATION, May 2019

5. Experimental Method 5.1 Process Descriptions Batch Sedimentation 1. Three pre-prepared sedimentation cylinders, containing unknown masses and sample types of calcium carbonate in water, were dislodged from the sedimentation apparatus and shaken thoroughly. This was done until the mixture became homogenous. 2. The moment when they were put back onto the apparatus the timer was started and the heights of the three suspensions were recorded. 3. After two minute intervals the heights of the suspension interfaces, the separation between the zone of clear liquid and the zone of constant composition, were recorded each time. 4. After no further settling took place the tests were complete. Continuous Sedimentation 1. The slurry of calcium carbonate suspension and water was prepared, it was stirred until it became a homogenous mixture. The pump was also opened to ensure the thorough mixing of the slurry. Once the slurry was thoroughly mixed, a sample of 250 ml was taken directly out of the storage tank. 2. In the settling tank, a stopper was inserted to ensure that no fluid is passing out. The valve was opened and the rotameter was adjusted to 5 litres/min and the stopwatch was started to determine how much time is taken to fill the settling tank at that flowrate. Once the settling tank was full, the stopwatch was stopped and the time was recorded. The sample of 250 ml was taken when the settling tank overflows. 3. The valve was turned off and also the rotameter. The stopper was taken out to empty the settling tank via the output tap. 4. The same procedure was repeated for the rotameter readings of 10, 15 and 20 L/min. 5. The filter papers and petri dishes were weighed and marked. The samples were filtered using vacuum filter. 6. The filtered samples were placed in the oven for approximately 24 hours to ensure the thorough evaporation 7. Finally the samples out of the oven were weighed and discarded.

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SEDIMENTATION, May 2019

5.2 Experimental Deviations When the batch settling test was commenced already half of it had been pre-done. Supposedly, 50 g of calcium carbonate samples had been already weighed into the three sedimentation cylinders. Had this been done during the actual experiment much more information could’ve been drawn. Judging by the different settling patterns of the suspensions in the three cylinder it can be seen that the solids weighed out are different. However since these solids were already in water for quite some time it was difficult to see the physical differences. The continuous sedimentation experimental procedure written on the laboratory manual is exceptionally different compared to the one actually undertaken. One point that stands out is that many samples were taken. These samples were not only different in quantity but also the method with which they were taken. Apparatuses like Pipettes, which ensure more accuracy, were used. Another point to notice is that the rotameter readings in manual were reduced but as seen in the experimental procedure of this report this is the exact opposite.

5.3 Experimental Limitations and Constraints As explained in the experimental procedure 2 minute intervals were measured for batch sedimentation and time was also measured for the period taken to fill the sedimentation tank for the continuous sedimentation. This was not done by the use of standard stopwatches, but rather with cell phones. Cell phones have a relatively lower accuracy compared to standard stopwatches. Another major limitation was that the cylinders used for the batch sedimentation tests were not properly cleaned. This problem was mostly experienced with Cylinder C. When this suspension was shaken at the beginning of the experiment some mass of the sludge was stuck at the bottom of the cylinder. All attempts to move it upwards were a failure. Taking height readings was difficult. Compared to the other two cylinders this cylinder was the blurriest. The solid particles had suck on the surface of the cylinder refusing to settle. It cannot be said that the results obtained for it are accurate.

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SEDIMENTATION, May 2019

6. Experimental Devices

Figure 6. 1 Batch Settling Test Apparatus Source: https://www.perrytecheducational.com/product/sedimentation-studies-apparatus/

Figure 6. 2 Sedimentation Zones Source: https://www.google.co.za/search?q=batch+sedimentation+zones+labeled+pdf&sa=X&tbm=isch&im gil=9jlmdzonLFiHMM%253A%253Br-7UBfEd4TFqxM%253Bhttp%25253A%25252F%25252Fengrsl. blogspot.com%25252F2012%25252F04%25252Fsedimentation.html&source=iu&pf=m&fir=9jlmdzon LFiHMM%253A%252Cr-7UBfEd4TFqxM%252C_&usg=__qFVfGYoYxLPT2WPO23el9Y3lBi4%3D&biw =1366&bih=613&ved=0ahUKEwi_qa7LjZDWAhXJBcAKHdYMBf8QyjcIMQ&ei=_qyvWbyE8mLgAbWmZT4Dw#imgrc=9jlmdzonLFiHMM:

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SEDIMENTATION, May 2019

2

1

8 7

6

4

5 3

Figure 6. 3 Continuous Sedimentation Apparatus

Table 6. 1 The Continuous Sedimentation Appa...