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.

Introduction

Many process consist of reaction stages and different stages in which process are separated and purified. Such separation includes physical principles based on difference in properties of constitutes. And heterogeneous mixtures consist of two or more phases, the separation process includes gas-liquid separation, gas-solid, liquid-liquid and solid-solid separation.

Cyclone separator will give a method of removing particulate matter from air or their gas streams at different parameters. Cyclones are designed in such a way that simple gravity settling systems is used for the removal efficiency better.

The basic idea is based on centrifugal force which consists of upper cylindrical part and lower conical part. The inertia force of particular flows to a s centrifugal force by means of vertex developed in the cyclone body.

There are different types of cyclones are shown in figure. Three types of cyclones have distinguished and widely used in many plants.

Modelling approach

Pre-processing

1. Geometry : The geometry is imported in Spaceclaim software and extract the volume required for the analysis. After the extracting the volume from the geometry, the outer body is supressed from the geometry.

3. Mesh : Here the mesh type is tetrahedron type with the element size is 5 mm with cartesian method.
4. Boundary conditions : Here I defined the boundary surface i.e. one is inlet second is outlet trap which is at top portion of the geometry and third one od outlet trap at bottom of the geometry.

Velocity for inlet 3 m/s with DPM as reflect. DPM for outlet escape is escaped for outlet trap is trap.

Here we use discrete phase model. The langrangian phase model in ANSYS fluent follows the Euler-Lagrange approach. The fluid phase is treated as a continues ny solving the Navier stokes equations, while the dispersed phase is solved by tracking a large number of particles, bubbles or droplets through the calculated flow field.

The dispersed phase can exchange momentum, mass and energy with the fluid phase. A fundamental assumption made in this model the dispersed second phase occupies a low volume fraction. The particle or droplet trajectory are computed individually at specified intervals during the fluid phase calculation.

Solver type: Pressure based

Solver time: steady state

Gravity: Enabled

Viscous Model: K-epsilon

DPM: Enabled

Injection (Anthracite particles) : Surface injection

Material: Air

Solution

Iteration 1

Diameter of particle : 5e^-6 m

Velocity : 1 m/s

Interaction with continuous phase

Iteration 2

Diameter of particle : 3e^-6 m

Velocity : 3 m/s

Interaction with continuous phase

Iteration 3

Diameter of particle : 5e^-6 m

Velocity : 1 m/s

Interaction with continuous phase

Iteration 4

Diameter of particle : 3e^-6 m

Velocity : 1 m/s

Interaction with continuous phase

The various analytical models to calculate the separation efficiency of cyclone separators are:

1) Iozia and Leith model:

Developed from the basis of force balance. The model assumes that the particle carried by the vortex endures the influence of two forces: a centrifugal force Z and a flow resistance W.

2)Koch and Licht model:

Koch and Licht inherently described the turbulent nature of cyclones and distribution of gas residence times within the cyclone. They attributed by the two following assumptions:

The tangential velocity of the particle is equal to the tangential velocity of the gas flow. There is no slip in the tangential direction between gas nad particle.

The tangential velocity is related to the radius of the cyclone by uR^n = constant

This is the formula for the grade efficiency

3) Li and Wang model:

The Li and Wand 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 the following assumptions:

1. The radial particle velocity and the radial concentration profile are not constant for uncollected particles within the cyclone.
2. 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 cyclone radius by uR^n=constant

The concentration distribution is given as :

The expression for collection efficiency for particles of any size is given as:

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

where NeNe is the number of revolutions

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

Conclusion

Dia of particle & Velocity	5e^-6 m & 1m/s	3e^-6 m & 3 m/s	5e^-6 m & 3 m/s	5e^-6 m & 1 m/s
Particle residence time	46.73	4.35	4.22	32.32
Number of particles trapped	378	378	378	376
Escape	56	33	202	61
Trapped	320	345	175	316
Efficiency	84.65	91.26	46.29	84.04

Dia of particle & Velocity	5e^-6 mm & 1m/s	3e^-6 mm & 3 m/s	5e^-6 mm & 3 m/s	5e^-6 mm & 1 m/s
Area weighted average static pressure
Inlet	1.914	24.56	24.563	1.914
Outlet escape	-0.00067	3.29	3.299	-0.00068
Outlet trap	-0.00011	3.12	3.125	-0.000064

Dia of particle & Velocity	5e^-6 mm & 1m/s	3e^-6 mm & 3 m/s	5e^-6 mm & 3 m/s	5e^-6 mm & 1 m/s
Pressure mini	-0.631	-3.62	-3.266
Pressure max	2.057	19.95	19.956
Velocity mini	0	0	0
Velocity max	1.66	5.266	5.66

The above table shows the varying parameter which includes the particle size variation and velocity. It is clearly seen that heavier particle is trapped at the bottom and lighter particles are escaped at the top outlet.

The efficiency of cyclones varies as a function of particle size. The efficiency of cyclone is greatly affected by size of particle, as soon as particle size increases the efficiency also increases. Also with the increase in velocity, the efficiency also increases.