Week 8 - Simulating Cyclone separator with Discrete Phase Modelling

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

Objective:

To write a few words about any four empirical models used to calculate the cyclone separator efficiency. 

To perform an analysis on a given cyclone separator model by varying the particle diameter from 1 μm to  5 μm and calculate the separation efficiency in each case. Discuss the results. [Use both the velocity's as 3m/sec.]

To perform an analysis on a  given cyclone separator model by varying the particle velocity from 1 m/sec to 5 m/sec and calculate the separation efficiency and pressure drop in each case. Discuss the results. [Use particle diameter size as 5 μm for all cases & keep flow velocity same as particle velocity]

Theory:

Cyclone Separator:

Cyclone separators are separation devices that use principle of inertia to remove particulate or suspended matter from the gases. These are one of the many air pollution control devices known as pre-cleaners since they remove larges size particles easily, which helps finer filtration devices not dealing with large size particles. Cyclones that can operate in parallel are known as multicyclone system.

Size of cyclone depends largely on how much gas must be filtered, thus large operations tend to need large cyclones. For example, several different models of one cyclone type can exist, and sizes can range from a relatively small 1.2-1.5 meters tall to around 9 meters.

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 Figure 2. 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.

Most cyclones are built to control and remove particulate matter that is larger than 10 micro-meters in diameter. However, there do exist high efficiency cyclones that are designed to be effective on particles as small as 2.5 micro-meters. As well, these separators are not effective on extremely large particulate matter. For particulates around 200 micro-meters 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 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 par0ticles 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 in 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 micro-meters effectively and the machines are unable to handle sticky or tacky materials well.

Separation efficiency is the ratio of trapped particles to the total number of particles tracked.

Pressure drop is defined as the difference in total pressure between two points of fluid carrying network.

pressure drop = Total inlet pressure - total outlet pressure(top).

Geometry:

Mesh:

Mesh size: 8mm

Number of nodes: 37119

Number of elements: 185349

Physics setup:

General setup

Solver type: Pressure based

Simulation: Steady state

Gravity: Enabled in -y axis direction

Turbulence model: k-epsillon

Discrete Phase Modelling:

Boundary Conditions:

Inlet and wall:- Reflect

Outlet(Top):- Escape 

Outlet(Bottom/dustbin) - Trap

Different cases are setup as shown in the tables below: 

Result:

Case A1: 1e-6m particle size, vel 3m/s.

Basic Residuals for all the A cases:

A2: 2e-6m particle size, vel 3m/s

A3: 3e-6m particle size, vel 3m/s

A4: 4e-6m particle size, vel 3m/s

A5: 5e-6m particle size, vel 3m/s

Case B1: 5e-6m, vel 1m/s

Basic residuals for all the B cases:

Case B2: 5e-6 m, vel 2m/s

Case B3: same as A5 case

Case B4: 5e-6 m, vel 4m/s

Case B5: 5e-6, vel 5m/s

Conclusion: 

1.Seperation Efficiency depends on both diameter and velocity of particles.

2.Increase in diameter and velocity increases separation efficiency.

3.Velocity has greater impact as compared to diameter of particle.

4.Pressure drop is dependent on velocity. for constant velocity with varying diameter size the pressure drop remains almost constant.

5.Increase in velocity causes the pressure drop to rise exponentially.

Different Cyclone Efficiency Models:

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 a flow resistance,

The addition made by Iozia and Leith on the original Barth (1956) model are 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.

LI AND WANG MODEL:

The Li and Wang (1989) 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

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:

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 the cyclone by: uRn = constant.

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.