Week 8 - Simulating Cyclone separator with Discrete Phase Modelling
***Simulating Cyclone separator with Discrete Phase Modelling***
Aim:- To perform analysis on cyclone separator and calculate the separation efficiency and pressure drop.
Objective:-
- To write a few words about any four empirical models used to calculate the cyclone separator efficiency.
- To perform an analysis on a given cyclone separator model by varying the particle diameter from 1 μm to 5 μm and calculate the separation efficiency in each case. Discuss the results. [Use both the velocity's as 3m/sec.]
- To perform an analysis on a given cyclone separator model by varying the particle velocity from 1 m/sec to 5 m/sec and calculate the separation efficiency and pressure drop in each case. Discuss the results. [Use particle diameter size as 5 μm for all cases & keep flow velocity same as particle velocity]
Boundary Conditions:-
Inlet and wall:- Reflect
Outlet(Top):- Escape
Outlet(Bottom/dustbin) - Trap
Analysis of Cyclone Separator
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Introduction:
Cyclone separators are separation devices that use the principle of inertia to remove particulate matter from flue gases. Cyclone separator is one of many air pollution control devices known as pre-cleaners 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 multi-cyclone.
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.
Removed Components from Cyclone Separators:
- Dust
- Particles: Abrasive, Corrosive, Adhesive, Explosive
- Process Description:
Working Principle:
Cyclone is a centrifugal separator in which particles, due to their mass, are pushed to their outer edges as a result of centrifugal force. Incoming Air is automatically forced to adopt a fast revolving spiral movement - the so-called double vertex. This double spiral movement consists of an outer stream, which flows downwards in the spiral, and an inner stream, which flows upwards in a spiral. At the interchange between both streams, air passes from one stream to another. The particles which are present in the air are forced to the outer edges and leave the separator via a collection device fitted to the bottom of the separator.
- The airspeed of the cyclone lies between 10 to 20 m/s, and the most common speed is 16 m/s. If there are fluctuations in speeds (with lower speeds), separation yield falls quite drastically.
- 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 ne filtration stages.
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, fewer of these particles 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 to 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.
Variants:
"High throughput" cyclones have a diameter greater than 1.5 m and are suitable for separating particles that are 20m or larger.
"High efficiency" cyclones (pencil cyclones) have a diameter that lies between 0.4 and 1.5m and can be used to separate particles that are 10m or larger.
Cyclones with diameters between 0.005 and 0.3 m are no longer used independently but are constructed into multi-cyclones. In this case, the inflow of gas does not take place tangential as in regular cyclones, but axially, after which the gas is made to spiral via splitter blades. A multi-cyclone requires gas to be well distributed across the smaller cyclones. If there is insufficient distribution, this may lead to gas back-flow and blockages.
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- Empirical models for Cyclone Separator Efficiency:
- lozia and Leith Model:
lozia and Leith 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.
The collection efficiency of particle diameter dpi can be calculated by
- Li and Wang Model:
The Li and Wang 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's model was developed based on assumptions: on the following
- 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:
- Koch and Licht Model:
Koch and Licht 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 the cyclone by urn =constant.
A force balance and an equation on the particle collection yields the grade efficiency
G is a factor related to the configuration of the cyclone, n is related ti the vortex and tau is the relaxation term.
- Lapple Model:
Lapple 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 the inlet half-width to the wall in the cyclone is collected with 50% efficiency. The semi-empirical relationship developed by Lapple to calculate a 50% cut diameter, DPC, is
Applications:
- Chemical and Petrochemical Processes
- Industrial, Metallurgical, and Power Generation
- Food & Pharmaceutical
- Cement Industry
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- Methodology:
There are two different Phases as given below:
- Continuous Phase
- Discrete Phase
Continuous Phase:
- It consists of fluid flowing within the fluid volume
Discrete Phase:
- It consists of smaller particles interacting with the Continuous Phase.
- It can also exchange momentum, mass, and energy with Fluid Phase or Continuous Phase.
To carry out this analysis,
- The simulation will be done using DPM(Discrete Phase Modeling).
Discrete Phase Modeling:
- It is employed when there are solid Particles in Gas, or Liquid, or Liquid Droplets in Gas, or Bubbles in Liquid.
There are two ways to carry out DPM
- Un-coupled Flow
- Coupled Flow
Uncoupled Flow:
- In this type of flow, the Discrete Phase will not have any interaction with the Continuous Phase.
Coupled Flow:
- It will influence the flow of the Continuous Phase.
Since the size of the particles injected into the Cyclone Separator is very small, the usage of both types of flow should not matter much.
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- Analysis:
In this project, we will perform an analysis on a given Cyclone model in two different manners.
- By varying particle diameters
- By varying particle velocities
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- Procedure:
Geometry:
Creation of fluid volume and geometry of Cyclone Separator as given below:
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Meshing:
Here the geometry of the Cyclone Separator is simple and so we can create hexahedron elements for the fluid volume. We can create a good mesh with help of the "Cut cell" method available in the assembly option in fluent, for the generation of structural mesh.
Mesh, settings, statistics, and cell arrangements are as given below:
- Statistics:
Element Size = 6.9895 mm
Assembly Method: Cutcell
Nodes: 66729
Elements: 64336
Also, we have to create Named selections for different kinds of behavior at inlet and outlets.
Named selections are as given below:
- Inlet
- Outlet Top
- Outlet_bottom (or Dustbin)
- Wall
Here we have given two Outlets purposely.
Outlet Top is used for light gas and Outlet_bottom is used for collecting heavy particles, which drop due to gravity at the bottom.
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Case Setup:
- After completing Meshing, we further go to the Setup in fluent.
- Here we will see the case set up for both Part 1 and Part_2 cases.
General Settings:
Solver Type: Pressure Based
Time: Steady
Gravity: -9.81 m/s2 (in -Y direction)
Viscous Model: K-Epsilon: RNG: Swirl Dominated Flow
Near Wall Treatment: Standard Wall Functions
- Here RNG is the slight modification of the Standard K-Epsilon model.
- This enhances the accuracy of the swirling flows.
DPM Settings:
Here we will see the setting for both Part_1 and Part 2 cases as given below:
- As per the above image, enable the "Interaction with Continuous Phase". This will enable the Coupled Flow.
- Also, enable "Update DPM Sources Every Flow Iteration". Enabling this option gives the chance to calculate the Particle Source Terms at every Iteration. (For Unsteady Simulation, it is recommended, and given by default.)
- Set "DPM Iteration Interval= 10". This option allows us to control the frequency at which particles attract.
- Here "Tracking Parameters = 150000". It is a maximum number of Time Steps, to compute a single trajectory of a Particle. In case, the fluent exceeds the maximum number of Time Steps, it will automatically cancel the report of Trajectory. For the Steady case, Time Steps number = 50,000 given by default. But for calculation of all possible particles, we have set it to 150000
- "Step Length Factor = 5". This is used to control the size of Time Step. The larger the Factor, the smaller the Time Step will be.
- Here, we have not used "Specify Length Scale", which is also used for controlling the Time Step size, by dividing Length Scale with Velocity of Discrete Particle and Velocity with Velocity of Continuous Phase.
Common settings for all cases are as given below:
Injection Type: Surface
Release From Surface: Inlet
Particle Type: Inert
Material: Anthracite
Flow Rate: 1e^-20 Kg/s
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Other settings for variables for cases of Part 1 & Part_2 are as given below:
| Division | Case | X velocity (m/s) | Diameter of the particle (m) |
| Part 1 | Case I | 3 | 1 e-6 |
| Case II | 2 e-6 | ||
| Case III | 3 e-6 | ||
| Case IV | 4 e-6 | ||
| Case V | 5 e-6 | ||
| Part 2 | Case A | 1 | 5 e-6 |
| Case B | 2 | ||
| Case C | 3 | ||
| Case D | 4 | ||
| Case E | 5 |
- As per the above table, the Diameter of the Particle and X-Velocity will be different for all cases.
- In the Cases of Part_1, X-Velocity = 3 m/s will be the same for all cases, and the Diameter of the Particle will be different.
- In Cases, Part 2, Diameter of the Particle = 5e^-6 m will be the same for all cases and the X-Velocity will be different.
- In all cases of Part_1& Part 2, the Velocity of the Particle and the Velocity of the Flow will be the same.
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Boundary Condition:
Boundary Conditions for analysis can be set as given below:
- As we can see in the above image, the value of velocity and DPM Type has been set according to the type of boundary.
Similarly, we can set all the parameters for different boundaries of Cyclone Separators given below:
Inlet: Type: Velocity-Inlet: Velocity
Magnitude: 3 m/s (For Part_1 Cases) & 1 to 5 m/s (For Part_2 Cases Respectively)
Here in above all boundary conditions, we have given different DPM conditions like Reflect, Escape and Trap.
- Reflect: It is the Particle Rebound off the boundary in question with a change in its momentum as defined by the coefficient of restitution.
- Escape: The particle is reported as having "escaped" when it encounters the boundary in question. Trajectories calculated are terminated.
- Trap: The Trajectory calculated is terminated and the fate of the particle is recorded as "trapped". In the case of the evaporating droplets, their entire mass instantaneously passes into the vapor phase and enters the Cell adjacent to the boundary. IN the case of combusting particle, the remaining volatile mass is passed into the vapor phase.
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Solution Methods:
Here, we have set the Momentum of "Turbulent Kinetic Energy" & "Turbulent Dissipation Rate" to the Second-Order Upwind scheme.
- This type of analysis can give us more accurate results.
Solution Initialization:
- Here, as per the above image, we have set the "Standard initialization". Also, the computing option has been set to "Compute From = Inlet".
- If we have Inlets and Outlets, then we can go for Standard Initialization, and it is best suited for this type of Analysis.
Run Calculation:
- Give Number of Iteration = 1000
- Select Calculate.
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Results:
Now we will see the results of all cases of Part 1 and Part_2 respectively.
Here we will see the results of Part 1 and Part 2 and calculate the Separation Efficiency and Pressure Drop for each case.
- Separation efficiency:
It is defined as the fraction of particles of a given size collected in the cyclone, compared to those of that size going into the cyclone. Experience shows that the separation efficiency of cyclone separator increases with increasing particle mean diameter and density; increasing gas tangential velocity; decreasing cyclone diameter; increasing cyclone length; extraction of gas along with solids through the cyclone legs.
In this case, depending upon the particle history data - Separation Efficiency is defined as the ratio of concentration that has been removed from the feed stream to the initial concentration in the feed stream. For this case ratio of the number of trapped particles to the total number of particles tracked.
Separation Efficiency= (No. of Particles Trapped)/(No. of Particles Tracked)
- Pressure Drop:
Pressure drop across the cyclone is of much importance in a cyclone separator. The pressure drop significantly affects the performance parameters of a cyclone. The total pressure drop in a cyclone will be due to the entry and exit losses, and friction and kinetic energy losses in the cyclone. Normally the most significant pressure drop occurs in the body due to swirl and energy dissipation.
Pressure drop is defined as the difference in total pressure between two points of a fluid-carrying network.
Pressure drop = Total inlet pressure - Total Outlet Pressure
As the "escape" DPM condition is given to the outlet-top in this case, so outlet-top is considered in pressure drop calculation.
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Results of Part 1:
CASE I:
Residuals:
Particle Resident Time:
Particle Tracking:
Separation Efficiency in % = (49 / 98)*100
Separation Efficiency = 50.00 %
CASE II:
Residuals:
Particle Resident Time:
Particle Tracking:
Separation Efficiency in % = (46 / 98)*100
Separation Efficiency = 46.93 %
CASE III:
Residuals:
Particle Resident Time:
Particle Tracking:
Separation Efficiency in % = (70 / 98) *100
Separation Efficiency = 71.42 %
CASE IV:
Residuals:
Particle Resident Time:
Particle Tracking:
Separation Efficiency in % = (95/98) * 100
Separation Efficiency = 96.93 %
CASE V:
Residuals:
Particle Resident Time:
Particle Tracking:
Separation Efficiency in % = (97 / 98) *100
Separation Efficiency = 98.98%
Pressure Drop:
For Pressure Drop, plotting of the Pressure curves for Inlet and Outlet_Top are as given below:
Inlet:
Outlet Top:
Total Pressure:
Now,
Pressure drop = Total inlet pressure - Total Outlet Pressure
= 31.632-3.894
Pressure drop=27.738 Pa
Pressure Contour:
Velocity Contour:
NOTE: The fluid velocity and particle velocity is 3m/sec for all particle sizes varied in Part 1; therefore the pressure drop remains
constant for all cases of Part_1.
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Results of Part_2:
CASE A:
Residuals:
Particle Resident Time:
Particle Tracking:
Separation Efficiency in % = (10/98) * 100
Separation Efficiency = 10.20%
Pressure Drop:
Inlet:
Outlet Top:
Total Pressure:
Pressure drop = Total inlet pressure - Total Outlet Pressure
= 2.832-0.349
Pressure drop=2.483 Pa
Pressure Contour:
Velocity Contour:
CASE_B:
Residuals:
Particle Resident Time:
Particle Tracking:
Separation Efficiency in % = (32/98) * 100
Separation Efficiency = 32.65%
Pressure Drop:
Inlet:
Outlet Top:
Total Pressure:
Pressure drop = Total inlet pressure - Total Outlet Pressure
= 13.13-1.64
Pressure drop=11.49 Pa
Pressure Contour:
Velocity Contour:
CASE C:
Residuals:
Particle Resident Time:
Particle Tracking:
Separation Efficiency in % = (97/98) * 100
Separation Efficiency = 98.98%
Pressure Drop:
Inlet:
Outlet Top:
Total Pressure:
Pressure drop = Total inlet pressure - Total Outlet Pressure
= 31.63-3.89
Pressure drop=27.74 Pa
Pressure Contour:
Velocity Contour:
CASE D:
Residuals:
Particle Resident Time:
Particle Tracking:
Separation Efficiency in % = (98/98) * 100
Separation Efficiency = 100%
Pressure Drop:
Inlet:
Outlet Top:
Total Pressure:
Pressure drop = Total inlet pressure - Total Outlet Pressure
= 57.976-7.141
Pressure drop=50.835 Pa
Pressure Contour:
Velocity Contour:
CASE E:
Residuals:
Particle Resident Time:
Particle Tracking:
Separation Efficiency in % = (97/98) * 100
Separation Efficiency = 98.98%
Pressure Drop:
Inlet:
Outlet Top:
Total Pressure:
Pressure drop = Total inlet pressure - Total Outlet Pressure
= 92.03-11.38
Pressure drop=80.65 Pa
Pressure Contour:
Velocity Contour:
Cyclone efficiency generally increases with the increase in inlet velocity. The efficiency of a cyclone separator is related to the pressure drop across the separator.
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Anthracite particle time animation:
Discrete particle velocity animation:
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COMPARISON:
Part 1:
| Part 1 | Size of the particle (in m) | Velocity of particle(m/s) | Particle Tracked | Particle Trapped | Particle Escaped | Separation Efficiency(%) | Pressure Drop(Pa) |
| CASE I | 1 e-06 | 3 | 98 | 49 | 40 | 50.00 | 27.74 |
| CASE II | 2 e-06 | 98 | 46 | 32 | 46.93 | ||
| CASE III | 3 e-06 | 98 | 70 | 28 | 71.42 | ||
| CASE IV | 4 e-06 | 98 | 95 | 3 | 96.93 | ||
| CASE V | 5 e-06 | 98 | 97 | 0 | 98.98 |
Part 2:
| Part 2 | Size of the particle (in m) | Velocity of particle(m/s) | Particle Tracked | Particle Trapped | Particle Escaped | Separation Efficiency(%) | Pressure Drop(Pa) |
| CASE A | 5 e-06 | 1 | 98 | 10 | 19 | 10.20 | 24.83 |
| CASE B | 2 | 98 | 32 | 15 | 32.65 | 11.49 | |
| CASE C | 3 | 98 | 97 | 0 | 98.98 | 27.74 | |
| CASE D | 4 | 98 | 98 | 0 | 100.00 | 50.85 | |
| CASE E | 5 | 98 | 97 | 0 | 98.98 | 80.65 |
- We can see from the above tables that in Part 1 cases, the efficiency of separation increases with increasing Particle Diameter, and in Part 2 cases, with an increment of Velocity.
- Also, we can observe that, in Part_1 cases, separation efficiency increases in a gradual manner with increases in Particle Diameter.
. But in Part 2 cases, separation efficiency increases rapidly with increases in velocity. It is because the Pressure Drop has more impact on cyclone separator efficiency.
- In Part_1 cases, Particle Resident Time decreases with increases in Particle Diameter.
- In Part 2 cases, Particle Resident Time decreases with increases in Particle Velocity.
- We can see from the Pressure contours in all cases that Velocity is high at the Inlet because the Particles are being injected forcefully by the Inlet.
- Also, Pressure is high at Inlet and at the Outer surface of the and low at the axial portion, because, when Particles travel in a swirl motion, they are being imparted on the outer surface of the cyclone. So most of the Particles are thrown from the axial to the radial direction and due to that pressure at the outer surface will always be high.
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Conclusion:
- The efficiency of a cyclone separator has a direct effect on the pressure drop.
- Higher efficiency cyclones have the highest pressure drops.
- Higher the velocity, the higher the pressure drop, and the highest the efficiency.
- Also, with an increase in the Diameter of particle and velocity of the particle, Particle resident time decreases.
- So, when less resident time is taken by the particles, the efficiency of the cyclone separator can be high.
- From the observation of PART1 & PART2 cases, we can conclude that the separation efficiency of cyclone separators depends on both diameters of particles and also on the velocity of the particles.
- Also, we can conclude that in the cases of part2 there are more efficient results. So the velocity is a more effective variable for higher efficiency.
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