Week 8 - Simulating Cyclone separator with Discrete Phase Modelling

Simulation of Cyclone separator with Discrete Phase Modelling

Objective-

To perform simulation and analysis on cyclone separator by analyzing the separation efficiency and pressure drop in different cases.

Introduction-

Cyclone separator:

Cyclonic separator is a means of separating different liquid phases (different liquid densities), or, separating particles from a gas stream. Cyclone separators often form part of a pre-cleaning stage prior to a gas or liquid being discharged.

Generally, smaller units are referred to as ‘dust’ separators or extractors, whilst large scale industrial separators are referred to as ‘cyclone separators’.
Gas cyclone separators are grouped into two main categories, reverse-flow and axial-flow.

In this project, we have reverse flow cyclone separators which are cone shaped. Gas enters the top of the separator body, flows downwards, then flows back upwards and is discharged.

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.

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.

Effectiveness:

Cyclone separators are generally able to remove somewhere between 50-99% of all particulate matter in the flue gas. How well the cyclone separators are actually able to remove this matter depends largely on particle size. If there is a large amount of lighter particulate matter, less of these particles are able to be separated out. Because of this, cyclone separators work best on flue gases that contain large amounts of big particulate matter.

There are several advantages and disadvantages of using cyclone separators. First, cyclone separators are beneficial because they are not expensive to install or maintain, and they have no moving parts. This keeps maintenance and operating costs low. Second, the removed particulate matter is collected when dry, which makes it easier to dispose of. Finally, these units take up very little space. Although effective, there are also disadvantages in using cyclone separators. Mainly because the standard models are not able to collect particulate matter that is smaller than 10 micrometers effectively and the machines are unable to handle sticky or tacky materials well.

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 (escape)

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

Empirical models used for cyclone separator efficiency:

1. IOZIA AND LEITH MODEL:

Iozia and Leith (1990) logistic model is a modified version of Barth (1956) 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 flow resistance, W. Core length, zc, and core diameter dc, are given as

The addition made by Iozia and Leith on the original Barth (1956) model is the core length zc and slope parameter β expression which is derived based on the statistical analysis of experimental data of cyclone with D = 0.25 m. The collection efficiency ηi of particle diameter dpi can be calculated from below-

2. LI AND WANG MODEL:

The Li and Wang (1989) 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. Li and Wang 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 cyclones.
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 radius of cyclone by:

u*Rⁿ = constant

The concentration distribution in a cyclone is given as:

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

3. KOCH AND LICHT MODEL:

Koch and Licht (1977) 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:

1. 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.
2. The tangential velocity is related to the radius of cyclone by u*Rⁿ = constant.

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

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

4. LAPPLE MODEL:

Lapple (1951) 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 (1951) to
calculate a 50% cut diameter, dpc is

Procedure-

Modelling-

The model of cyclone separator is shown below-

Now, we have to extract the volume for simulation in space claim. After volume extracting we have to suppress the outer part of the separator for physics before simulation. The extracted volume of the cyclone separator is shown below-

Meshing-

For meshing we will use cut cell meshing approach or Cartesian meshing for structure meshing since it gives the best result since it has very good mesh quality criteria. We can find the option for Cartesian meshing in ‘method’ option in meshing.

The element size is 5mm and total number of elements is 466428.

Below image shows the meshed cyclone separator-

Now, we have to name the inlet and outlet of the model. Here, we have one inlet and two outlets. The top outlet is named outlet-escape and bottom outlet is named outlet-trap.

Pre-Processing and Solver Setting:

Here, we are using solver type as pressure based and steady solver. Also, we have to enable gravity in –y direction. Then, select viscous model as K-epsilon model. Here, we have used coupled method for solving the equation.

Since in this simulation there is formation of swirling action of fluid so we have to select swirl dominated flow after selecting RNG model under K-epsilon model.

Here, we have to use discrete phase model for simulation of cyclone separator since it employs solving the tracks of large number of particles.

Now, select interaction with continuous phase under discrete phase model as shown in image below-

The maximum number of time steps by default is 50000 steps and if there are some incomplete particles, i.e., particles left in the separator, then we have to increase the number of time steps even after reaching convergence.

Now, we have to create injection method of the particles by selecting injections under the discrete phase model tab. We have to select surface option in the injection type and then select inlet as the surface.

Now, for case 1, change the diameter of particles as 5e^-6m and velocity as 3m/s. The material is selected as anthracite. For further 2 cases we are changing the diameter to 3microns and 1 micron keeping the velocity same and then for 2 cases, we are changing the velocity to 1m/s and 5m/s keeping the diameter to 5microns.

Now, we have to give the inlet boundary condition. The velocity of air will be same as particles velocity which is 3m/s and under DPM tab change the discrete phase boundary condition type to ‘reflect’.

Similarly, for the outlet-trap boundary conditions, change the discrete phase boundary condition type to ‘trap’.

Now, we will use hybrid initialization to initialize and run the calculation for 300 iterations.

After the calculation, we have to export the particle history data under file tab to load the file in the results since it doesn’t load the data automatically. From this file we can make animation video of the particle flow in the cyclone separator by selecting the file under animation generation tab. For importing in the results, click on import fluid particle track file under import.

Now, we have to run simulation for 4 more cases in which we have to change the velocity and particle diameter to record the changes in the parameters of the cyclone parameter.

Result-

The value of the particles escaped and trapped as well as the total pressure at inlet and outlet of the cyclone separator of each cases are shown below-

Case1- Particle diameter = 5microns, Velocity = 3m/s

Case2- Particle diameter = 3microns, Velocity = 3m/s

Case3- Particle diameter = 1micron, Velocity = 3m/s

Case4- Particle diameter = 5microns, Velocity = 1m/s

Case5- Particle diameter = 5microns, Velocity = 5m/s

The contour of all the particles’ time for cases 1, 2, 3, 4 and 5 are shown below-

The pressure contour of cases 1, 2, 3, 4 and 5 are shown below-

The animation video of the movement of the particles inside the cyclone separator of cases 1, 2, 3, 4 and 5 are shown below-

Conclusion-

Pressure drop across the cyclone separator in each cases is shown in below table-

Case no.	Particle diameter (microns)	Velocity (m/s)	Pressure at inlet (Pascal)	Pressure at outlet-escape (Pascal)	Pressure drop (Pascal)
1.	5	3	24.35	3.31	21.04
2.	3	3	24.89	3.35	21.54
3.	1	3	24.39	3.30	21.09
4.	5	1	2.53	0.34	2.19
5.	5	5	74.43	10.66	63.77

Pressure drop across the cyclone separator depends upon the velocity as can be seen from the table. The pressure drop is almost constant when the velocity is constant and it increases as the velocity increases.

Separation efficiency of cyclone separator when varying the particle diameter is shown in below table-

Case no.	Particle diameter (microns)	Velocity (m/s)	Particle tracked	Particle escaped	Particle trapped	Separation efficiency (%)
1.	5	3	378	24	354	93.65
2.	3	3	378	59	319	84.39
3.	1	3	378	159	219	57.94

Separation efficiency of cyclone separator when varying the velocity is shown in below table-

Case no.	Particle diameter (microns)	Velocity (m/s)	Particle tracked	Particle escaped	Particle trapped	Separation efficiency (%)
1.	5	3	378	24	354	93.65
4.	5	1	378	195	183	48.41
5.	5	5	378	26	352	93.12

Separation efficiency depends upon both the particle diameter and its velocity as can be seen from the above tables. But particle diameter has greater impact on the separation efficiency.

As the particle diameter is increasing, separation efficiency is also increasing confirming that it’s easier to separate large diameter particles than smaller ones.

Increasing the velocity increases the separation efficiency but after a point there is negligible change in the efficiency.