Week 8 - Simulating Cyclone separator with Discrete Phase Modelling

 

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

Objective:-

1. To write a few words about any four empirical models used to calculate the cyclone separator efficiency. 
2. To perform an analysis on a given cyclone separator model by varying the particle diameter from $1$$\mu m$to $5$$\mu m$ and calculate the separation efficiency in each case. Discuss the results. [Use both the velocity's as 3m/sec.]
3. 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 $\mu m\; for\; all\; cases\; \&\; keep\; flow\; velocity\; same\; as\; particle\; velocity$]

 

 

 

1.To write a few words about any four empirical models used to calculate the cyclone separator efficiency. 

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 1.21.5 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

 

Diagram:

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 spiral and inner stream, which flows upwards in spiral. At the interchange between both streams, air passes from one stream to another. The particles which are present in the air re forced to the outer edges and leave the separator via collection device fitted to the bottom of the separator. 

• Air speed 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  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, less 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 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.

 

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 which lies between 0.4 and 1.5m and can be used to separate particles that are 10m or larger.
• Cyclones with diameter 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.

 

Empirical models for Cyclone Separator Efficiency:

• Iozia and Leith Model:

Iozia and Leith logistic Model is a modified version of Barth (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 _i of particle diameter d_pi can be calculated by

• Li and Wang Model:

The Li and Wang model include 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:

1. The radial particle velocity and the radial concentration profile are not constant, for uncollected particles within the cyclones.
2. Boundary conditions with the consideration of turbulent diffusion coefficient and particle bounce re-entrainment on the cyclone wall.

 

• 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:

1. 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.
2. The tangential velocity is related to the radius of cyclone by uRⁿ =constant.

 

• 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 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, d_pc , is

 

Applications:

• Chemical and Petrochemical Processes
• Industrial, Metallurgical and Power Generation
• Food & Pharmaceutical
• Cement Industry

 

 

 

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 Continuous Phase.
• It can also exchange momentum, mass and energy with Fluid Phase or Continuous Phase.

 To carry out this analysis,

• 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 Lquid Droplets in Gas , or Bubbles in Liquid.

 There are two ways to carry out DPM

1. Un-coupled Flow
2. Coupled Flow

 

Uncoupled Flow:

• In this type of flow, Discrete Phase will not have any interaction with Continuous Phase.

 Coupled Flow:

• It will influence the flow of Continuous Phase.

 

Since, the size of the particles injected in to Cyclone Separator is very small, the usage of both type of flow should not matter much.

 

 

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.

 

Procedure:

Geometry:

• Import the geometry in fluent SpaceClaim.
• Extract the fluid volume within the geometry by selecting the inner edges at inlet and outlets.
• Hide the Geometry and suppress for physics.
• Save the fluid volume for further analysis.

 

Meshing:

• Here the geometry of Cyclone Separator is simple and we insert cartesian method with elemental size of5.5mm
•  structural Mesh gives the best results. 

Also we have to create Named selections for different kind of behavior at inlet and outlets.

Named selections are as per given below:

• Inlet
• Outlet_Top
• Outlet_bottom ( or Dustbin)

   

              

 

elemental size 10mm

nodes=134753

no. of elements=122301

Fluent set up:

Solver steps-

• Enable gravity in '-ve' Y direction and put value of 9.81m/s^2
• Select steady state pressure based solver
• Then select k-ε">k−ε,k-ε turbulent model
• Enable RNG and swirl dominated options also
• Then enable discrete phase modelling to track particle flow
• Then create injection with anthracite material and select injection type surface
• Then choose inlet face from reference surfaces
• As per conditions and case, change the velocity of anthracites and diameter of anthracite particles.

In fluent solver apply following boundary conditions-

• Inlet and wall:- Reflect
• Outlet(Top):- Escape 
• Outlet(Bottom/dustbin) - Trap

Here,

1. Reflect defines particles should rebound (reflect) from given surface
2. Trap defines trajectory calculation to be stopped after touching surface
3. Escape defines calculation stops as soon as particle escape from boundary

And then use standard initialization where take reference values from inlet.

                 

boundary conditions:

   

  

   

Discrete phase model:

            

 

2.           

case 1: for 1 Micrometer:with velocity 3m/sec.

 Residual plot:

 Flow contour and vortex:

 

case 2: for 3Micrometer:with velocity 3m/sec.

 Residual plot:

Flow contour and vortex:

 

case 3: for 5Micrometer:with velocity 3m/sec.

 Residual plot:

Flow contour and vortex:

 

 

3.

Case 1: for velocity 1m/sec:5 micrometer:

 Residual plot:

Pressure at inlet and outlet:

 

Flow contour and vortex:

 

Case 2: for velocity 3m/sec:5 micrometer:

 Residual plot:

Pressure at inlet and outlet:

Flow contour and vortex:

Case 3: for velocity 5m/sec :5 micrometer:

 Residual plot:

Pressure at inlet and outlet:

Flow contour and vortex:

 

• 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, `DeltaP`  = 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.

Observations from the case1:                       

Particle diameter	velocity	Particles tracked	Particles escaped	Particles trapped	incomplete	separation efficiency
1 micrometer	3	162	57	98	7	0.604
3micrometer	3	162	27	126	9	0.77
5 micrometer	3	162	22	140	0	0.864

 

 

Observations from the case2:

Particle diameter	velocity	Particles tracked	Particles escaped	Particles trapped	incomplete	separation efficiency	inlet pressure	outlet pressure in Pa	`DeltaP`
5micrometer	1	162	40	3	119	0.018	3.0326	0.278	2.754
5micrometer	3	162	25	137	0	0.845	31.34	2.579	28.761
5micrometer	5	162	22	140	0	0.864	90.85	7.30	83.55

 

 

 

Conclusions:

1. The separation efficiency of a cyclone separator at a particular velocity increases with the increase in the particle diameter, because the larger the particle diameter, the higher is its inertia, hence the chances of it escaping from the top outlet of the cyclone separator are reduced.
2. The inlet pressure of the cyclone separator remains the same for a given inlet velocity and the inlet pressure increases with the increase in the inlet velocity.
3. For particles with smaller diameter, and for particles with lesser velocity, it is more likely the major portion of particles do not complete the path in the cyclone separator to either get trapped or escape. That is if the flow velocity is less or the particle diameter is small or both, it will take more time to perform the separation.
4. For a particular particle diameter, the separation efficiency is the highest for a particular(optimal) inlet velocity. If the inlet velocity is lower than this then it will take more time for separation or the particles may endlessly revolve in the domain. If the inlet velocity is increased beyond this value, then these particles may get influenced by the vortex generated and may escape from the top outlet, hence reducing its separation efficiency.