Week 8 - Simulating Cyclone separator with Discrete Phase Modelling

Cyclone separator

Introduction

A division of multiphase flows is the discrete phase model. The difference between the Lagrangian and Eulerian views is that the Eulerian perspective assumes a finite volume element in the fluid flow route while the Lagrangian view bases its analysis of fluid behavior on particle tracking of a particulate flow.

Navier-Stokes equations are used in the discrete phase model to solve the continuous phase. Countless particles, bubbles, or droplets are tracked at once to approximate the discrete phase. Specifically, those that cross the predicted continuous flow field.

Cyclone Separator

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 are 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 on. In addition, several cyclone separators can operate in parallel, and this system is known as a multi-cyclone.

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 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 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. Disadvantages in using cyclone separators are mainly because the standard models are not able to collect particulate matter that is smaller than 10 micrometers effectively

Efficiency of a cyclone

The efficiency of a cyclone represents the proportion of particles that the cyclone can retain:

Efficiency = (inlet loading –outlet loading)/inlet loading*100

Cyclone collection efficiency models

Lapple (1951) developed a theory for cut-point (d50) based upon a force balance and representation of residence time with the air stream number of turns within a cyclone. The Lapple model is easy to use, but it cannot accurately predict cyclone collection efficiency. In 1972, Leith and

Licht presented another theory (back-mixing) for the study of cyclone collection efficiency. Their back-mixing theory suggests that the turbulent mixing of uncollected

particles in any plane perpendicular to the cyclone axis produces a uniform uncollected dust concentration through any horizontal cross section of a cyclone. Based upon this theory, they developed a model to predict efficiency for any size particles. It has been reported that the Leith and Licht model for efficiency appears to work best compared with other theories in the literature (Leith and Mehta, 1973). However, this model has

not been tested with experimental data and it involves variables and dimensionless parameters not easily accounted for in practical applications. Stairmand (1951) and Barth (1956) first developed the “static particle theory” for the analysis of cyclone collection efficiency in the 50’s. Since then, this static particle theory based upon a force balance analysis has been adopted by many other researchers in their theoretical analyses for characterizing cyclone performance. Basically the “static particle theory” suggested that force balance on a particle yields a critical particle, which has 50% chance to be collected and 50% chance to penetrate the cyclone. The diameter of the critical particle is d50. The critically sized particle (d50) is smaller than the smallest particle, which is collected, and larger than the largest particle that penetrates the cyclone. The critical particle with diameter of d50 is theoretically suspended in the outer vortex forever due to the force balance.

Methodology

• Upload geometry in space claim
• Extract volume
• Meshing 20 mm element size due to license count cell limitation finer mesh is not allowed, then name selection inlet, bottom and top outlet.
• Setup parameters pressure based solver, gravity enabled Y axis, viscous model k epsilon, RNG, swirl, standard wall function,

• DPM, injection setting particle diameter 1,3,and 5 micro meter respectively

• Boundaries setup inlet velocity fixed at 3 meter per second velocity in x axis direction the inlet option reflect, top outlet escape, bottom outlet trap.
• Solution method
• Standard initialization from inlet and Run calculation for 600 iterations, the decision to extend the iterations or not depends on the user judgment if the efficiency achieved satisfy the required application or not.

1 micro meter particle size 500 iterations

3 micro meter particle size 500 iterations

5 micro meter particle size 600 iterations

#	Particle size micro meter	Velocity m/s	Cyclone efficiency	Particle tracked	Particle trapped	Escaped	Incomplete
1	1	3	63%	378	129	141	108
2	3	3	71%	378	151	108	119
3	5	3	89%	378	244	41	93

Conclusion

From the table above, the cyclone efficiency increased proportionately with the particle size increase due to the increase of the particle inertia, the minimum particle diameter size 3 micro meter and above to obtain efficient operation.