Week 8 - Simulating Cyclone separator with Discrete Phase Modelling

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

Introduction -

Cyclone separator is a 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.

• 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 1.21.5 meters tall (about 4-5 feet) to around 9 meters (30 feet)-which is about as tall as a three-story building.

Working Principle -

Cyclone is a centrifugal separator in which particles, due to their mass, are pushed to their outer edges as a result of centrifugal force. Incoming Air is automatically forced to adopt a fast revolving spiral movement - the so called double vertex. This double spiral movement consists of an outer stream, which flows downwards in spiral and inner stream, which flows upwards in spiral. At the interchange between both streams, air passes from one stream to another. The particles which are present in the air re forced to the outer edges and leave the separator via collection device fitted to the bottom of the separator.

• Air speed of the cyclone lies between 10 to 20 m/s, and the most common speed is 16 m/s. If there are fluctuations in speeds (with lower speed ) separation yield falls quite drastically.
• 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 ne 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. 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.

Variants -

• "High throughput" cyclones have a diameter greater than 1.5 m and are suitable for separating particles that are 20m or larger.
• "High efficiency" cyclones (pencil cyclones) have a diameter which lies between 0.4 and 1.5m and can be used to separate particles that are 10m or larger.
• Cyclones with diameter between
• Cyclones with diameter between 0.005 and 0.3 m are no longer used independently but are constructed into multi-cyclones. In this case the inflow of gas does not take place tangential as in regular cyclones, but axially, after which the gas is made to spiral via splitter blades. A multi-cyclone requires gas to be well-distributed across the smaller cyclones. If there is insufficient distribution, this may lead to gas back-flow and blockages.

Empirical Model -

• lozia and Leith Model -

lozia and Leith 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: centrifugal force Z and flow resistance W

`eta_i = 1/(1+(d_(pc)/d_(p_i))^beta`

`beta`is an expression for slope parameter derived based on the statistical analysis of experimental data of a cyclone with D = 0.25 m given as

``beta` = 0.62 - 0.87 ln (d_(pc)/100) + 5.21 ln((ab)/D^2) + 1.05[ln((ab)/D^2)]^2`

and `d_(pc)` is the 50% cut size given by birth

`d_(pc)=[(9muQ)/(pirho_pz_cnu_(t max)^2)]^0.5`

Where core length `z_c` and core diameter `d_c` are given as

`z_c = (H-S) - [(H-S)/((D/B)-1)] [(d_c/B)-1] ` for `d_c > B`

`z_c = H-S` for `d_c < B`

`d_c = 0.47D((ab)/D^2)^-0.25(D_e/D)^1.4`

• Li and 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. 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:

`c = c_0` at `theta = 0`

`D_r(delc)/(delr) = (1-alpha)wc` at `r =D/2`

The tangential velocity is related to the radius of cyclone by:

`uR^n = Constant`

The concentration distribution in a cyclone is given by

`c(r,theta) = (c_0(r_w - r_n)exp {-lamda[1/(k(1+n))r^(1+n)]})/(int_(rn)^(r_w) exp {1/(k(1+n))r^(1+n)} dr)`

`K = ((1-n)(rho_p-rho_g)d^2Q)/(18mub(r_w^(1-n) -r_n^(1-n)))`

`lamda = ((1-alpha )Kw_w)/(D_r r_w^n)`

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

`eta_i = 1-exp{-lamdatheta_1}`

`theta_1 = (2pi(S+L))/a`

• Koch and 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 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 `uR^n` =constant

A force balance and an equation on the particle collection yield the grade sfficiency `eta`.

`eta_0 = 1-exp{-2[(Gtau_iQ)/D^3(n+1)]^(0.5/(n+1))}`

`G = (8K_c)/(K_a^2K_b^2)`

`n = 1-{1 -((12D)^0.14)/2.5}{(T+460)/530}^0.3`

`tau_i = (rho_pd_(p i)^2)/(18mu)`

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

• 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

`d_(pc) = [(9mub)/(2piN_enu_i(rho_p-rho_g))]^(1/2)`

where `N_e` is the number of revolution

`N_e = 1/a[h+(H-h)/2]`

The effficiency of collection of any size of particle is given by

`eta_i = 1/(1+(d_(pc)/(d_(p i)))^2`

Applications -

• Chemical and Petrocheical processes
• Industrial, metallurgical and power generation
• Food and pharmaceutical
• Cement industry

Methodology -

There are two different Phases as given below:

• Continuous Phase
• Discrete Phase

Continuous Phase -

• It consists of fluid flowing within the fluid volume

Discrete Phase -

• It consists of smaller particles interacting with Continuous Phase.
• It can also exchange momentum, mass and energy with Fluid Phase or Continuous Phase.

To carry out this analysis,Simulation will be done using DPM (Discrete Phase Modeling)

Discrete Pnase modeling:

• It is employed when there are solid Particles in Gas, or Liquid or Lquid Droplets in Gas, or Bubbles in Liquid.

There are two ways to carry out DPM

1. Un-coupled Flow
2. Coupled Flow

1.Uncoupled Flow -

• In this type of flow, Discrete Phase will not have any interaction with Continuous Phase.

2.Coupled Flow -

• It will influence the flow of Continuous Phase.

Since, the size of the particles injected in to Cyclone Separator is very small, the usage of both type of flow should not matter much.

Part 1: Setting the particle diameter `1 mum , 3mum and 5mum`respectively and velocity is `3m/s`

Ansys Fluent analysis -

Geometry -

Mesh -

Fluent setup -

General -

Viscous model -

Boundary Conditions

Discrete Phase -

Injection Properties for `1mum`diameter

Injection Properties for `3mum`diameter `

Injection Properties for `5mum`diameter `

Solution Method -

Initialization -

Run Caculation -

• for `1mum`diameter Results

Residuals -

Contour -

Vortex -

Separation Efficiency:

Partical Tracked in the Cyclone Separator = 378

Particle Escaped from the Cyclone Separator = 110

Particle Traped in the Cyclone Separator = 268

`text(Separation Efficiency) = text(Particle Traped in th cyclone separator)/(text(Particle Tracked in the cyclone separator))**100`

`nbsp; = 268/378**100`

= 70.89%

Animation -

• for `3mum`diameter ` resilts

Residuls -

Contour -

Vortex -

Separation Efficiency:

Partical Tracked in the Cyclone Separator = 378

Particle Escaped from the Cyclone Separator = 108

Particle Traped in the Cyclone Separator = 270

`text(Separation Efficiency) = text(Particle Traped in th cyclone separator)/(text(Particle Tracked in the cyclone separator))**100`

`= 270/378**100`

`= 71.42`%

Animation -

• for `5mum`diameter ` resilts

Residuls -

Contour -

Vortex -

Separation Efficiency:

Partical Tracked in the Cyclone Separator = 378

Particle Escaped from the Cyclone Separator = 103

Particle Traped in the Cyclone Separator = 275

`text(Separation Efficiency) = text(Particle Traped in th cyclone separator)/(text(Particle Tracked in the cyclone separator))**100`

`= 275/378**100`

`= 72.75`%

Animation-

Part 2: Setting the particle diameter `5mum` and velocity is varying 1m/s,3m/s and 5m/s respectively`

• For particle having 1m/s velocity

Residuls -

Pressure at the inlet -

Pressure at the outlet top -

Velocity contour -

Pressure Contour -

Separation Efficiency:

Partical Tracked in the Cyclone Separator = 378

Particle Escaped from the Cyclone Separator = 191

Particle Traped in the Cyclone Separator = 187

`text(Separation Efficiency) = text(Particle Traped in th cyclone separator)/(text(Particle Tracked in the cyclone separator))**100`

= (187/378)*100

= 49.40%

Pressure Drop -

Pressure at the inlet = 1.497 Pa

Pressure at the outlet = 0.19064 Pa

Pressure drop = Inlet pressure - outlet pressure

= 1.497 - 0.19064

= 1.3033 Pa

• For particle having 3m/s velocity

Residuls -

Pressure at the inlet -

Pressure at the outlet top -

contour -

Pressure Contour -

Separation Efficiency:

Partical Tracked in the Cyclone Separator = 378

Particle Escaped from the Cyclone Separator = 135

Particle Traped in the Cyclone Separator = 243

`text(Separation Efficiency) = text(Particle Traped in th cyclone separator)/(text(Particle Tracked in the cyclone separator))**100`

= (243/378)*100

= 64.28%

Pressure Drop -

Pressure at the inlet = 13.47 Pa

Pressure at the outlet =1.599 Pa

Pressure drop = Inlet pressure - outlet pressure

= 13.47 - 1.599

= 11.878 Pa

• For particle having 5m/s velocity

Residuls -

Pressure at the inlet -

Pressure at the outlet top -

Velocity contour -

Pressure contour -

Particle time contour -

Seperation Efficiency -

Partical Tracked in the Cyclone Separator = 378

Particle Escaped from the Cyclone Separator = 78

Particle Traped in the Cyclone Separator =300

`text(Separation Efficiency) = text(Particle Traped in th cyclone separator)/(text(Particle Tracked in the cyclone separator))**100`

= (300/378)*100

= 79.36%

Pressure Drop -

Pressure at the inlet = 29.53 Pa

Pressure at the outlet =4.89 Pa

Pressure drop = Inlet pressure - outlet pressure

= 29.53 - 4.89

= 24.64 Pa

Comparision -

• we can see from above tables that in Part 1 cases, efficiency of separation increases with increasing Particle Diameter and in Part 2 cases, with increment of Velocity.
• Also we can observe that, in Part1 cases, separation efficiency increases in gradual manner with increases in Particle Diameter.
• But in Part 2 cases, separation efficiency increases rapidly with increases in velocity. It is because the Pressure Drop has more impact in cyclone separator efficiency.
• In Part 1 cases, Particle Resident Time decreases with increases in Particle Diameter.
• In Part 2 cases, Particle Resident Time decreases with increases in Particle Velocity.
• We can see from the Pressure contours in all cases that Velocity is high at the Inlet, because the Particles are being injected forcefully by the Inlet.
• Also Pressure is high at Inlet and at the Outer surface of the and low at the axial portion, because, when Particles travels in a swirl motion, they are being imparted on the outer surface of the cyclone. So most of the Particles are thrown from axial to radial direction and due to that pressure at the outer surface will always be high.

Conclusion -

• From the observations of Part 1& Part 2 cases, we can conclude that Separation efficiency of Cyclone Separator depends on both Diameter of Particles and also on the Velocity of the Particles.
• Also we can conclude that in cases of Part 2, there are more efficient results. So the Velocity is more effective variable for higher efficiency.
• Efficiency of a Cyclone Separator has direct effect on the Pressure Drop.
• Higher efficiency Cyclones has highest Pressure Drops.
• Higher the Velocity, higher the Pressure Drop and highest the Efficiency.
• Also with increases in Diameter of Particle and Velocity of Particle, Particle Resident Time decreases.
• So, when less Resident Time taken by the particles, efficiency of the Cyclone Separator can be high.