Week 8 - Simulating Cyclone separator with Discrete Phase Modelling

Aim

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

Objectives

• Write about four empirical models used for the calculation of cyclone separator efficiency.
• To perform an analysis on the given cyclone separator by varying the particle diameter from 1 microns to 5 microns with velocity 3 m/s.
• To perform the analysis on the given cyclone separator with velocity varying from 1 m/s to 5 m/s with particle size 5 microns.

Introduction

Cyclone separators or simply cyclones are separation devices (dry scrubbers) that use the principle of inertia to remove particulate matter from flue gases. Cyclone separators is one of many air pollution control devices known as precleaners since they generally remove larger pieces of particulate matter. This prevents finer filtration methods from having to deal with large, more abrasive particles later. In addition, several cyclone separators can operate in parallel, and this system is known as a multicyclone.

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).

Cyclone separators work much like 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, like 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.

Out of all 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.

Theory

Models for the calculation of Cyclone separation efficiency

• Iozia and Leith model:

Iozia and Leith model is a modified form of the Barth model developed based upon the principle of force balance. The model assumes that the particle carried by the vortex endures two forces, a centrifugal force Z and a flow resistance W. The efficiency is calculated using the formulae:

Beta is the slope parameter.

Z_c is the core length and d_c is the core diameter

• 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 is:

N_e is number of revolutions

• Koch and Licht model

Koch and Licht collection theory recognized the inherently turbulent nature of the distribution of the gas residence time within the cyclone. Assumptions used are:

• The tangential velocity of a particle is equal to the tangential velocity of the gas flow.
• The tangential velocity is related to the radius of the cyclone as uRⁿ=constant.

G related to the orientation of the cyclone n related to vortex and tau

to relaxation time.

• 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. Assumptions used are:

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

Solving and modeling approach

• Open SpaceClaim from Ansys Workbench and load the geometry.
• Extract the fluid volume using the volume extract option from prepare menu.
• Suppress the cyclone separator body for the physics.
• A hex-dominant mesh is used for the analysis (cut cell meshing is not supported in my version of Ansys). The element size is 5 mm.
• Name the inlet, outlet1 and outlet2 of the geometry.
• A steady-state solver with absolute velocity formulation in a steady state is used.
• K-epsilon, RNG viscous model with a standard wall function is used for the analysis.
• Enable the Discrete Phase model, enable interaction with the continuous phase, and update the DPM source every flow iteration.
• DPM iteration interval is set to be 10.
• The Maximum number of steps is set to be 50000.
• Create injection at the inlet and choose anthracite as the material, the inlet velocity of the particle is defined as well as the diameter and flow rate.
• Close the window.
• In the boundary conditions the inlet velocity is set and DPM is set as Reflet, outlet1 is set to escape, outlet2 is set to trap and walls are set to reflect.
• The pressure velocity coupling is set as SIMPLE, second order used for pressure calculation, second order upwind for momentum, turbulent kinetic energy, and turbulent dissipation rate calculation.
• The setup is standard initialized and enabled computation from the inlet.
• The setup is run for 600 iterations.
• Export the particle history data for post-processing.
• Import the fluent particle track file and visualised the Anthracite particle time.
• Pressure drop and Separation efficiency are calculated from the data obtained after simulation.

Pressure drop = Inlet pressure – outlet1 pressure

Cyclone separation efficiency = Number of particle trapped/ Number of particles tracked

Pre-processing and solver settings

Results

Sl No.	Velocity	Particle size	Inlet Pressure	Outlet Pressure	Particle Trapped	Particle tracked	Pressure drop	Efficiency
1	3	1	26.915043	0.0022847	142	200	26.912758	71
2	3	3	4648.5052	0	170	200	4648.5052	85
3	3	5	47.981955	0	196	200	47.981955	98
4	1	5	211.83889	21.94894	197	200	189.88995	98.5
5	5	5	93.425819	0.042347	193	200	93.3834	96.5

Velocity: 3 m/s , Particle size : 1 microns

Velocity: 3 m/s, Particle size: 3 microns

Velocity : 3 m/s, Particle size : 5 microns

Velocity: 1 m/s , Particle size : 1 microns

Velocity: 5 m/s, Particle size: 5 microns

Conclusion

It is found that the separation efficiency of the cyclone separator depends upon the size of the particle, also the variation of velocity has more effect on the efficiency than the particle size. It can be concluded that higher the velocity higher is the efficiency and pressure drop. The simulation is caried out successfully and the results are obtained. Resident time taken by the particle is obtained and particle motion visualised.