Cyclone Separator Challenge

Cyclone separator: Cyclones are devices that employ a centrifugal force generated by a spinning gas stream to separate particles from the carrier gas. The inflowing fluid rotates within the main body of the chamber and is constrained to follow a spiral flow path. Any particles suspended within the fluid are subjected to an enhanced radial acceleration. The larger particles migrate outwards to the cone wall where they travel in a downward spiral to the base of the chamber and exit at the underflow.

The four empirical models used for cyclone separator efficiency are:

1. 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 efficiency of collection of any size of the particle is given by

η_i = 1/ [1+(d_pc/d_pi)²]

2. Koch & 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 described particle motion in the entry and collection regions by the

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 uRⁿ= constant.

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

η_i = 1- exp {-2[(G τ_i Q)(n+1)/ D³)]^0.5/(n+1)}

3. Iozia & Leith Model: Iozia and Leith logistic model are based on force balance. The model assumes that a particle carried by the vortex endures the influence of two forces: a centrifugal force and flow resistance.

The collection efficiency η_i of particle diameter d_pi can be calculated from

η_i = 1/[1+(d_pc/d_pi)^β

β is an expression for slope parameter derived based on the statistical analysis of experimental data of a cyclone.

4. Li & 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.

The resultant expression of the collection efficiency for a particle of any size is given as

η_i = 1- exp{-λθ₁}

Geometry -

Fluid Domain extracted -

Meshing -

Cut cell Method is used to get structural mesh

Element Size 3.49mm

Nodes- 510659

Elements- 499179

Setup

Gravity - y-direction -9.81m/s

Viscous model - The swirl dominated RNG K-epsilon model.

The Discrete phase modelling is used to track the flow of the particles.

No of step for particle tracking = 50000

Injection Material: Anthracite (5microns in diameter)

Velocity Inlet: 3m/s

DPM Settings are

Inlet and wall:- Reflect

Outlet(Top):- Escape

Outlet(Bottom/dustbin) - Trap

Reflect: The particle rebounds off the boundary in question with a change in its momentum.

Escape: Particle escapes out when encountered by the boundary.

Trap: The trajectory calculation is terminated & fate of the particle is recorded as a trap.

Result -

CASE I - To perform an analysis on a given cyclone separator model by varying the particle diameter from 1 μm to 5 μm with constant velocity 3m/s

CASE II - To perform an analysis on a given cyclone separator model by varying the particle velocity from 1 m/sec to 5 m/sec. Using particle diameter size as 5 μm

CASE I.A -

particle diameter 5μm Velocity - 5m/s

Number of Particles Tracked - 435

Escaped particles - 112

Trapped particles - 323

CASE I.B

particle diameter 3μm Velocity - 5m/s

Particles

number tracked = 435, escaped = 186, trapped = 249

CASE I.C

particle diameter 1μm Velocity - 5m/s

Particles

number tracked = 435, escaped = 222, trapped = 212, incomplete = 1

CASE II.A -

Velocity - 1m/s particle diameter 5μm

Particles

number tracked = 435, escaped = 215, trapped = 114, incomplete = 106

Pressure near the top outlet - 0.00531525 [Pa]

CASE II.B -

Velocity - 3m/s, particle diameter 5μm

This case is similar to the CASE I.A

Pressure near the top outlet - 0.198554 [Pa]

CASE II.C -

Velocity - 5m/s, particle diameter 5μm

Particle

number tracked = 435, escaped = 34, trapped = 401

Pressure near the top outlet - 0.609071 [Pa]

Comparing CASE I results

Particle size	Tracked	Escaped	Trapped	Incomplete	Efficiency
5 μm	435	112	323	0	74%
3 μm	435	186	249	0	54%
1 μm	435	222	212	1	48%

Comparing CASE II results

Velocity	Tracked	Escaped	Trapped	Incomplete	Efficiency
5 m/s	435	34	401	0	92%
3 m/s	435	112	323	0	74%
1 m/s	435	215	114	106	26%

Velocity	Pressure
5m/s	0.609071 [Pa]
3m/s	0.198554 [Pa]
1m/s	0.00531525 [Pa]

Conclusion

1. With the increases in particle size the efficiency of the cyclone separator increases.
2. The velocity of the particles also plays an important role in cyclone separator efficiency. High Velocity provides high efficiency
3. The pressure drop can be visualised from the pressure contours plots.