Cyclone Separator Challenge

Aim:- To perform analysis on cyclone separator and calculate the separation efficiency and pressure drop.

Introduction:-

1. Cyclone Separator- Cyclone separators or simply cyclones are separation devices (dry scrubbers) that use the principle of inertia to remove particulate matter from flue gases. Cyclone separators is one of manyair pollution control device known as precleaners since they generally remove larger pieces of particulate matter. This prevents finer filtration methods from having to deal with large, more abrasive particles later on. In addition, several cyclone separators can operate in parallel, and this system is known as a multicyclone. A cyclone separator can be observed in Figure 1.

Figure 1. Cyclone Separator

It is important to note that cyclones can vary drastically in their size. The size of the cyclone depends largely on how much flue gas must be filtered, thus larger operations tend to need larger cyclones. For example, several different models of one cyclone type can exist, and the sizes can range from a relatively small 1.2-1.5 meters tall (about 4-5 feet) to around 9 meters (30 feet)—which is about as tall as a three story building.

2. Working- Cyclone separators work much like a centrifuge, but with a continuous feed of dirty air. In a cyclone separator, dirty flue gas is fed into a chamber. The inside of the chamber creates a spiral vortex, similar to a tornado. This spiral formation and the separation is shown in Figure 2. The lighter components of this gas have less inertia, so it is easier for them to be influenced by the vortex and travel up it. Contrarily, larger components of particulate matter have more inertia and are not as easily influenced by the vortex.

Figure 2. GIF showing how particles move through a cyclonic separator

Since these larger particles have difficulty following the high-speed spiral motion of the gas and the vortex, the particles hit the inside walls of the container and drop down into a collection hopper. These chambers are shaped like an upside-down cone to promote the collection of these particles at the bottom of the container. The cleaned flue gas escapes out the top of the chamber.

Most cyclones are built to control and remove particulate matter that is larger than 10 micrometers in diameter. However, there do exist high-efficiency cyclones that are designed to be effective on particles as small as 2.5 micrometers. As well, these separators are not effective on extremely large particulate matter. For particulates around 200 micrometers in size, gravity settling chambers or momentum separators are a better option.

Out of all of the particulate-control devices, cyclone separators are among the least expensive. They are often used as a pre-treatment before the flue gas enters more effective pollution control devices. Therefore, cyclone separators can be seen as "rough separators" before the flue gas reaches the fine filtration stages.

3. Effectiveness- Cyclone separators are generally able to remove somewhere between 50-99% of all particulate matter in the flue gas. How well the cyclone separators are actually able to remove this matter depends largely on particle size. If there is a large amount of lighter particulate matter, less of these par0ticles are able to be separated out. Because of this, cyclone separators work best on flue gases that contain large amounts of big particulate matter.

There are several advantages and disadvantages of using cyclone separators. First, cyclone separators are beneficial because they are not expensive to install or maintain, and they have no moving parts. This keeps maintenance and operating costs low. Second, the removed particulate matter is collected when dry, which makes it easier to dispose of. Finally, these units take up very little space. Although effective, there are also disadvantages in using cyclone separators. Mainly because the standard models are not able to collect particulate matter that is smaller than 10 micrometers effectively and the machines are unable to handle sticky or tacky materials well.

Theory:- Following are the empirical models used to calculate cyclone efficiency:

1. Iozia and Leith Model- Iozia and Leith's (1990) logistic model is a modified version of Barth's (1956) model which is developed based on force balance. The model assumes that a particle carried by the vortex endures the influence of two forces: a centrifugal force, Z, and flow resistance, W. Core length, zc, and core diameter, dc, are given as

`z_c = (H-S)-[frac{H-S}{(D/B)-1}]` for `d_c gt B`

`z_c = H-S` for `d_c lt B`

`d_c = 0.47*(frac{ab}{D^2})^-0.25 *(frac{D_e}{D})^1.4`

The addition made by Iozia and Leith on the original Barth (1956) model are the core length zc and slope parameter b expression which is derived based on the statistical analysis of experimental data of cyclone with D = 0.25 m.

2. Li and Wang Model- The Li and Wang (1989) model includes particle bounce or re-entrainment and turbulent diffusion at the cyclone wall. A two-dimensional analytical expression of particle distribution in the cyclone is obtained. Li and Wang model was developed based on the following assumptions:

• The radial particle velocity and the radial concentration profile are not constant, for uncollected particles within the cyclones.

• Boundary conditions with the consideration of turbulent diffusion coefficient and particle bounce re-entrainment on the cyclone wall are:

c=`c_0`, at `theta=0`

`D_rfrac{deltac}{deltar}=(1-alpha)wc`, at r=D/2

• The tangential velocity is related to the radius of cyclone by:

`uR^n`= constant

3. Koch and Licht Model- Koch and Licht (1977) collection theory recognized the inherently turbulent nature of cyclones and the distribution of gas residence times within the cyclone. Koch and Licht describe particle motion in the entry and collection regions with the additional following assumptions:

• The tangential velocity of a particle is equal to the tangential velocity of the gas flow, i.e. there is no slip in the tangential direction between the particle and the gas.

• The tangential velocity is related to the radius of cyclone by:

`uR^n`= constant

A force balance and an equation on the particles collection yields the grade efficiency ni.

`n_i=1-exp{-2[frac{Gtau_iQ}{D^3}(n+1)]^(0.5/(n+1))}`

where, G is a factor related to the configuration of the cyclone, n is related to the vortex and t is the relaxation term.

4. Lapple Model- Lapple (1951) model was developed based on force balance without considering the flow resistance. Lapple assumed that a particle entering the cyclone is evenly distributed across the inlet opening. The particle that travels from inlet half width to the wall in the cyclone is collected with 50% efficiency. The semi empirical relationship developed by Lapple (1951) to calculate a 50% cut diameter, dpc, is

`d_(pc)=[frac{9mub}{2piN_ev_i(rho_p-rho_g)}]^(1/2)`

where,

`N_e` = number of Revolutions

The efficiency of collection of any size of particle is given by

`n_i=frac{1}{1+(d_(pc)/d_(p i))^2}`

Geometry- The Geometry of the cyclone separator is imported into the Design Modeler which can be observed in Figure 3. The simulation to be performed is required for the behavior of particles inside cyclone separator. Hence it is important to obtain the volume enclosed by the geometry. The volume of the fluid available inside the cyclone separator is obtained by using the 'volume extract' option. Extracted volume can be observed in Figure 4.

Figure 3. Geometry of Cyclone Separator Figure 4. Volume of Fluid inside Geometry

Meshing- CutCell mesh is used for the extracted volume with 510607 nodes and 499135 elements which can be observed in Figure 5 for element size of 3 mm. CutCell meshing is a general purpose hex-dominant meshing technique. The CutCell meshing algorithm is suitable for a large range of applications, and due to the large fraction of hex cells in the mesh, often produces better results than regular tetrahedral meshes. This method can be used instead of tetrahedral or hexcore meshing, without requiring a very high quality surface mesh as a starting point. Also, this method uses a direct surface and volume approach without the need of cleanup or decomposition, thereby reducing the turnaround time required for meshing.

The CutCell meshing process involves the following approach:

1. Objects, material points (optional), and size functions are defined.
2. The initial size of the Cartesian grid is computed based on the minimum and maximum size set for the size functions.
3. A uniform Cartesian grid is created within the bounding box for the geometry.

Figure 5. Meshing of Volume

Simulation Conditions- For simulation, a steady-state pressure-based solver was selected. Gravity was enabled along the Y direction. RNG k-epsilon model with swirl dominated flow was chosen as it gives more accurate results for such simulation type. In DPM (Discrete Phase Model) Interaction with continuous phase was enabled. Injection of the discrete particles was defined in the continuous phase. Inert particle type with material as anthracite was chosen. For injection, inlet was selected as a surface from which the particles are supposed to be released into cyclone separator. Following tabulated conditions in Table 1 are applied to the wall of geometry which can be observed in Figure 6.

Table 1. Boundary Conditions

Boundaries	Conditions
Inlet	Reflect
Outlet (top)	Escape
Outlet (bottom)	Trap
Wall	Reflect

Figure 6. Boundary Conditions

Results- As it is required to calculate separation efficiency and pressure drop, along with residuals pressure at inlet and outlet is also necessary to obtain. Separation efficiency and pressure drop can be defined as

Separation Efficiency is the ratio of concentration that has been removed from the feed stream to the initial concentration in the feed stream. In this case, it is the ratio of trapped particles to the total number of particles tracked.

Pressure drop is the pressure difference generated between two locations of fluid carrying path. For this case, Pressure Drop =Inlet Pressure - Outlet (top) Pressure

Following are the results of the simulation for different cases:

Case 1: Varying particle diameter from 1 micrometer to 5 micrometers and keeping particle velocity = 3 m/sec.

For 1 micrometer Diameter

Figure 7. Residuals

Figure 8. Pressure at inlet

Figure 9. Pressure at outlet

Figure 10. Vortex Core Region

Animation 1. Animation of Particle Time

For 2 micrometer Diameter

Figure 11. Residuals

Figure 12. Pressure at inlet

Figure 13. Pressure at outlet

Figure 14. Vortex Core Region

Animation 2. Animation of Particle Time

For 3 micrometer Diameter

Figure 15. Residuals

Figure 16. Pressure at inlet

Figure 17. Pressure at outlet

Figure 18. Vortex Core Region

Animation 3. Animation of Particle Time

For 4 micrometer Diameter

Figure 19. Residuals

Figure 20. Pressure at inlet

Figure 21. Pressure at outlet

Figure 22. Vortex Core Region

Animation 4. Animation of Particle Time

For 5 micrometer Diameter

Figure 23. Residuals

Figure 24. Pressure at inlet

Figure 25. Pressure at outlet

Figure 26. Vortex Core Region

Animation 5. Animation of Particle Time

The details about the particle data obtained after the simulation are tabulated in Table 2 along with the calculated value of Separation Efficiency. The behavior of Separation Efficiency with respect to increasing diameter with a constant velocity of 3 m/sec can be observed from the graph mentioned in Figure 27.

Table 2. DPM output for Case 1

Diameter	Velocity	Tracked	Escaped	Aborted	Trapped	Evaporated	Incomplete	Separation Efficiency
1 µm	3 m/sec	435	150	0	155	0	125	35.63 %
2 µm	3 m/sec	435	127	0	170	0	138	39.08 %
3 µm	3 m/sec	435	104	0	202	0	129	46.43 %
4 µm	3 m/sec	435	60	0	229	0	137	52.64 %
5 µm	3 m/sec	435	27	0	254	0	154	58.39 %

Figure 27. Graph of Diameter vs Separation Efficiency

All the computed values of Inlet Pressure and Outlet pressure are tabulated in Table 3 with Pressure Drop. The behavior of Pressure Drop with respect to increasing diameter with a constant velocity of 3 m/sec can be observed from the graph mentioned in Figure 28.

Table 3. Values of Pressure for Case 1

Diameter	Velocity	Inlet Pressure	Outlet Pressure	Pressure Drop
1 µm	3 m/sec	29.73995 Pa	4.639221 Pa	25.10073 Pa
2 µm	3 m/sec	29.73212 Pa	4.638732 Pa	25.10122 Pa
3 µm	3 m/sec	29.73212 Pa	4.638732 Pa	25.10122 Pa
4 µm	3 m/sec	29.73212 Pa	4.638732 Pa	25.10122 Pa
5 µm	3 m/sec	29.73212 Pa	4.638732 Pa	25.10122 Pa

Figure 28. Graph of Diameter vs Pressure Drop

Case 2: Varying particle velocity from 1 m/sec to 5 m/sec and keeping particle diameter = 5 micrometer

For 1 m/sec Velocity

Figure 29. Residuals

Figure 30. Pressure at inlet

Figure 31. Pressure at outlet

Figure 32. Vortex Core Region

Animation 6. Animation of Particle Time

For 2 m/sec Velocity

Figure 33. Residuals

Figure 33. Pressure at inlet

Figure 34. Pressure at outlet

Figure 35. Vortex Core Region

Animation 7. Animation of Particle Time

For 3 m/sec Velocity

The results for this velocity is same as For 5 micrometer diameter mentioned in case 1.

For 4 m/sec Velocity

Figure 36. Residuals

Figure 37. Pressure at inlet

Figure 37. Pressure at outlet

Figure 38. Vortex Core Region

Animation 8. Animation of Particle Time

For 5 m/sec Velocity

Figure 39. Residuals

Figure 40. Pressure at inlet

Figure 41. Pressure at outlet

Figure 42. Vortex Core Region

Animation 9. Animation of Particle Time

The details about the particle data obtained after the simulation are tabulated in Table 4 along with the calculated value of Separation Efficiency. The behavior of Separation Efficiency with respect to increasing velocity with a constant diameter of 5 µm can be observed from the graph mentioned in Figure 43.

Table 4. DPM output for Case 2

Velocity	Diameter	Tracked	Escaped	Aborted	Trapped	Evaporated	Incomplete	Separation Efficiency
1 m/sec	5 µm	435	18	0	214	0	203	49.19 %
2 m/sec	5 µm	435	23	0	235	0	177	54.02 %
3 m/sec	5 µm	435	27	0	254	0	154	58.52 %
4 m/sec	5 µm	435	41	0	283	0	111	65.05 %
5 m/sec	5 µm	435	52	0	314	0	69	72.18 %

Figure 43. Graph of Velocityvs Separation Efficiency

All the computed values of Inlet Pressure and Outlet pressure are tabulated in Table 5 with Pressure Drop. The behavior of Pressure Drop with respect to increasing velocity with a constant diameter of 5 µm can be observed from the graph mentioned in Figure 28.

Table 5. Values of Pressure for Case 2

Velocity	Diameter	Inlet pressure	Outlet pressure	Pressure Drop
1 m/sec	5 µm	24.63151 Pa	3.623586 Pa	21.00735 Pa
2 m/sec	5 µm	27.59602 Pa	4.001628 Pa	23.58974 Pa
3 m/sec	5 µm	29.73212 Pa	4.638732 Pa	25.09339 Pa
4 m/sec	5 µm	33.05155 Pa	5.224387 Pa	27.82716 Pa
5 m/sec	5 µm	36.12563 Pa	5.723415 Pa	30.40222 Pa

Figure 43. Graph of Velocity vs Pressure Drop

Conclusion-

From the above results, we can conclude that

1. Separation Efficiency is dependent on both velocity as well as diameter.

2. Pressure Drop is independent of diameter if velocity is constant which can be observed from the graph of Diameter vs Pressure Drop which shows linear behavior.

4. From the graph of Velocity vs Pressure Drop, we can say that Pressure drop is dependent on velocity when the diameter is constant. Pressure drop increases with increase in velocity.

5. By comparing graphs of Velocity vs Separation Efficiency and Diameter vs Separation Efficiency, we can say that both velocity and diameter are responsible for the value of separation efficiency but velocity is more responsible for increasing separation efficiency relative to the diameter.

References-

1. J.M.K.C. Donev et al. (2018). Energy Education - Cyclone separator [Online]. Available: https://energyeducation.ca/encyclopedia/Cyclone_separator

2. Jolius Gimbun, T.G. Chuah, Thomas S. Y. Choong, A. Fakhru’l-Razi, "Evaluation on Empirical Models for the prediction of Cyclone Efficiency", The Institution of Engineers, Malaysia, Vol. 67, No. 3, 54-58, September 2006.

3. https://cfd.ninja/ansys-meshing/ansys-meshing-cutcell/ "Ansys Meshing-CutCell"