Week 8 - Simulating Cyclone separator with Discrete Phase Modelling

1. 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.]

• Cyclone Separator:

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.

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 the Figure below. 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.

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.

Description of the project statement:

1. With varying the particle diameter, at a constant velocity at inlet the flow is simulated. separation efficiency is found out.

2. With varying the velocity at the inlet, at constant diameter of the particles, the flow is simulated. separation efficiency and pressure drop in the cases is found out.

Case setup

• geometry
  • import geometry in SpaceClaim.
    
    imported geometry

extracted volume

• Meshing
  • create a baseline mesh
    Baseline mesh
    
  • refine the mesh.
    Refined mesh(5mm) setting
    

  • insert meshing method

    • select geometry
    • change method to cartesian
      
      Final mesh
      

Setup

• General setting
  
  • Solver type: pressure based
  • velocity formulation: Absolute
  • time: steady state
  • gravity enabled with the y-axis value -9.81ms-2">ms−2ms-2
    
• viscous model
  
  • RNG K-epsilon with swirl dominated flow with standard wall function
    
• Discrete phase model
  • Interaction with continuous phase enabled.
  • tracking parameters
    • max. no. of steps: 50000
    • step length factor: 5
      

Injections

• create a injection
  • injection name: injection 0
  • injection type: surface
  • injection surface: inlet
  • particle type: inert
  • material: anthracite
  • diameter distribution: uniform
  • velocity inlet: as per case requirement (for case-1 we have 3 m/s)
  • diameter: as per case requirement ( for case-1 we have 1 μm">μm)
  • Total flow rate: 1e-20kgsec-1">1e-20 kg/sec

Boundary conditions

• inlet_reflect type: velocity(3 m/s)
  
  
• DPM: Reflect
  
  

• outlet_top_escape: pressure outlet (0 Pa gauge pressure)
  
  
• DPM: escape
  
  

• outlet_bottom_trap: pressure outlet(0 Pa gauge pressure)
  
• DPM: Trap
  
• Wall type: wall
• DPM: Reflect
  
  

• Report definition
  • create a report definition for pressure at inlet, outlet-top-escape and outlet-bottom-trap
    

Initialization

• standard initialization from inlet.
  

Patch Z velocity = -3 m/s

Pressure velocity coupling

• Run simulation for 600 iterations

Case 1

a) Velocity = 3 m/s. Particle diameter = 1 micro meter

Scaled Residual

Inlet pressure graph

Seperation efficiency = No: of particles trapped/No: of particles tracked

= 125/378 = 33.068 %

Pressure drop = Inlet pressure - Total outlet pressure

= 26.175 - (-0.0114+0.02106) = 26.207 Pa

Or

Pressure drop = 31.683 - (2.46+4.313) = 24.91 Pa

Around 25 Pa

Result

Video: 1micro3msparticle.wmv

Vortex

Pressure distribution

Tangential velocity vector

This figure shows the eccentricity of vortex formation

Case 2

a) Velocity = 3 m/s. Particle diameter = 3 micro meter

Scaled Residual

Inlet pressure graph

Seperation efficiency = No: of particles trapped/No: of particles tracked

= 145/378 = 38.36 %

Pressure drop = Inlet pressure - Total outlet pressure

= 26.175 - (-0.0114+0.02106) = 26.207 Pa

Or

Pressure drop = 31.683 - (2.46+4.313) = 24.91 Pa

Around 25 Pa

Result

Vortex

Pressure distribution

This figure shows the eccentricity of vortex formation

Case 3

a) Velocity = 3 m/s. Particle diameter = 5 micro meter

Scaled Residual

Inlet pressure graph

Seperation efficiency = No: of particles trapped/No: of particles tracked

= 184/378 = 48.68 %

Pressure drop = Inlet pressure - Total outlet pressure

= 26.175 - (-0.0114+0.02106) = 26.207 Pa

Or

Pressure drop = 31.683 - (2.46+4.313) = 24.91 Pa

Around 25 Pa

Result

Vortex

Pressure distribution

This figure shows the eccentricity of vortex formation

1. 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]

Case 1

a) Velocity = 1 m/s. Particle diameter = 5 micro meter

Scaled Residual

Inlet pressure graph

Seperation efficiency = No: of particles trapped/No: of particles tracked

= 8/378 = 2.12 %

Pressure drop = Inlet pressure - Total outlet pressure

= 2.274 - (-0.0003-0.00007) = 2.274 Pa

Or

Pressure drop = 2.885 - (0.224+0.378) = 2.283 Pa

Around 2.28 Pa

Result

Vortex

Pressure distribution

This figure shows the eccentricity of vortex formation

Case 2

a) Velocity = 5 m/s. Particle diameter = 5 micro meter

Scaled Residual

Inlet pressure graph

Seperation efficiency = No: of particles trapped/No: of particles tracked

= 287/378 = 75.92 %

Pressure drop = Inlet pressure - Total outlet pressure

= 76.131 - (-0.07038-0.04488) = 76.246 Pa

Or

Pressure drop = 91.638 - (7.17+12.626) = 71.638 Pa

Around 75 Pa

Result

Vortex

Pressure distribution

This figure shows the eccentricity of vortex formation

SUMMARY & CONCLUSION & RESULT

If the velocity of the particle is kept constant and increase the particle size, the seperation efficiency increases but the pressure almost constant with the same velocity.

If the particle size is kept constant and increase the velocity, the seperation efficiency and pressure drop increases.

• 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, ΔP = Total inlet pressure - Total Outlet Pressure (trap & escape)

As the escape DPM conditions are given to the outlet-top and outlet-bottom in this case, so outlet-top and outlet-bottom is considered in pressure drop calculation.

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

1. Iozia and Leith Model

Iozia and Leith logistic model is a modified version of Barth 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 a flow
resistance, W. The collection efficiency ηi of particle diameter
d_pi can be calculated from

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

The tangential velocity is related to the radius of cyclone by:
The concentration distribution in a cyclone is given as:

3. Koch and Licht Model

Koch and Licht [2] collection theory recognized the inherently turbulent nature of cyclones and the distribution of gas
residence times within the cyclone. Koch and Licht described 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:

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

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

4. 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

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