Cyclone Separator Challenge

Objective

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

Introduction

Cyclone separator is a device that employ a centrifugal force generated by a spinning gas stream to separate particles from the carrier gas. Its simple design, low capital cost and nearly maintenance-free operation make it ideal for use as precleaners for more expensive final control devices such as bag-houses or electrostatic precipitators. Cyclone is particularly well suited for high temperature and pressure conditions because of its rugged design and flexible component materials. Cyclone collection efficiencies can reach 99% for particles bigger than 5 µm, and can be operated at very high dust loading.

Theory

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.

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. 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.

Cyclone Separator Efficiency Empirical Model

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`. Core length, `z_c`, and core diameter, `d_c`, are given as

`z_c = (H - S) - [{H - S}/ {(D / B) -1}][(d_c / B) -1] -> if d_c>B`

`z_c = H - S-> if d_c>B`

`dc = 0.47 ({ab}/{D^2}) ^0.25(D_e/ D)^1.4`

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

`c=c_o` at `theta=0`

`D_r{delc}/{delr}=(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 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.

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

`d_{pc} =[{9µb}/{2πN_ev_i (ρ_p -ρ_g )}]^{1/2}`

where N_e is the number of revolutions

`N_e =1/alpha[ h+{H - h}/2]`

Geometry

Geometry and dimension of cyclone separator

Meshing

The element size considered to mesh the model is 1mm with cutcell mesh technique.

Number of nodes=75,733

Number of element=72,758

Simulation Setup

The model is considered as viscous model and solved using RNG K-epsilon model and Standard wall fuction near wall treatment.

The model is in vertical plane and gravity is acting in negative y-direction. Solid particle is setup as discrete particle with same inlet velocity as fluid (air).

The wall of cyclone separater is made up of aluminium and fluid (air) flows inside it. There are two pressure outlet one at top and another at bottom. Filtered air come out from top outlet and solid particle is collected at bottom.

The simulation runs for 900 iteration considering it a steady state simulation.

Result

For Inlet velocity =3 m/s , the model is simulated for different diameter of solid particle

1. Diameter=1 ‎μm

Residual Plot

Pressure Contour

Cyclone Separator efficiency=Particle trapped/Particle traced

=37/120

=0.3083

Vortex Core

2. Diameter=3 μm

Residual Plot

Pressure Contour

Cyclone Separator efficiency=Particle trapped/Particle traced

=84/120

=0.7

Vortex Core

3. Diameter=5 μm

Residual Plot

Pressure Contour

Cyclone Separator efficiency=Particle trapped/Particle traced

=119/120

=0.9916

Vortex Core

Data Table 1

For diameter of solid particle= 5 μm, the model is simulated for different inlet velocity

1. Inlet Velocity=1 m/s

Residual Plot

Pressure Contour

Cyclone Separator efficiency=Particle trapped/Particle traced

=6/120

=0.05

Vortex Core

2. Inlet Velocity=2 m/s

Residual Plot

Pressure Contour

Cyclone Separator efficiency=Particle trapped/Particle traced

=86/120

=0.7167

Vortex Core

3. Inlet Velocity=5 m/s

Residual Plot

Pressure Contour

Cyclone Separator efficiency=Particle trapped/Particle traced

=115/120

=0.9583

Vortex Core

Data Table 2

Conclusion

Cyclone separator works on the inertia. 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 and collected at bottom. So, cyclone separator is more efficient at high inlet velocity. Simulation is too more stable at higher velocity.

Size of particulate matter have not significant effect on simulation but on efficiency. Efficiency of cyclone separator increses with increase in size of particulate matter.

From table 1 and table 2, we can observe that mass flow rate mainly depends on inlet velocity and size of particulate matter have negligible effect.