Cyclonic separation

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Cyclonic separation is a method of removing particulates from an air, gas or water stream, without the use of filters, through vortex separation. Rotational effects and gravity are used to separate mixtures of solids and fluids.

A high speed rotating (air)flow is established within a cylindrical or conical container called a cyclone. Air flows in a spiral pattern, beginning at the top (wide end) of the cyclone and ending at the bottom (narrow) end before exiting the cyclone in a straight stream through the center of the cyclone and out the top. Larger (denser) particles in the rotating stream have too much inertia to follow the tight curve of the stream and strike the outside wall, falling then to the bottom of the cyclone where they can be removed. In a conical system, as the rotating flow moves towards the narrow end of the cyclone the rotational radius of the stream is reduced, separating smaller and smaller particles. The cyclone geometry, together with flow rate, defines the cut point of the cyclone. This is the size of particle that will be removed from the stream with a 50% efficiency. Particles larger than the cut point will be removed with a greater efficiency, and smaller particles with a lower efficiency.

Large scale cyclones are used in sawmills to remove sawdust from extracted air. Cyclones are also used in oil refineries to separate oils and gases, and in the cement industry as components of kiln preheaters. Smaller cyclones are used to separate airborne particles for analysis. Some are small enough to be worn clipped to clothing and are used to separate respirable particles for later analysis.

Analogous devices for separating particles or solids from liquids are called hydrocyclones or hydroclones. These may be used to separate solid waste from water in wastewater and sewage treatment.

Contents 1 Cyclone theory 1.1 Steady state 2 Alternate models 3 See also 4 References 5 External links

[edit] Cyclone theory

[edit] Steady state

As the cyclone is essentially a two phase particle-fluid system, fluid mechanics and particle transport equations can be used to describe the behaviour of a cyclone. The air in a cyclone is initially introduced tangentially into the cyclone with an inlet velocity V_in. Assuming that the particle is spherical, a simple analysis to calculate critical separation particle sizes can be established.

Given that the fluid velocity is moving in a spiral the gas velocity can be broken into two component velocities, a tangential component, U_t, and a radial velocity component U_r. Assuming stokes' law, the drag force on any particle in this inlet stream is therefore given by the following equation:

F_d = 6πr_pμV_r.

If one considers an isolated particle circling in the upper cylindrical component of the cyclone at a rotational radius of r from the cyclone's central axis, the particle is therefore subjected to centrifugal, drag and bouyant forces. The centrifugal component is given by:

The bouyant force component is obtained by the difference between the particle and fluid densities, ρ_p and ρ_f respectively:

The force balance can be created by summing the forces together

This rate is controlled by the diameter of the particle's orbit around the central axis of the cyclone. A particle in the cyclonic flow will move towards either the wall of the cyclone, or the central axis of the cyclone until the drag, boyant and centrifugal forces are balanced. Assuming that the system has reached steady state, the particles will assume a characteristic radius dependent upon the force balance. Heavier, denser particles will assume a solid flow at some larger radius than light particles. The steady state balance assumes that for all particles, the forces are equated, hence:

F_b + F_c + F_b = 0

Which expands to:

This can be expressed by rearranging the above in terms of the particle radius. The particle radius as a function of cyclonic radius, fluid density and fluid tangential and rotaional velocities can then be found to be:

Experimentally it is found that the velocity component of rotational flow is proportional to r^2[1], therefore:

This means that the established feed velocity controls the vortex rate inside the cyclone, and the velocity at an arbitrary radius is therefore:

Subsequently, given a value for V_t, possibly based upon the injection angle, and a cutoff radius, a characteristic particle filtering radius can be estimated, above which particles will be removed from the gas stream.

[edit] Alternate models

The above equations are relatively simple and provide a basic approximation to the behaviour of a cyclone separator. These equations are, however, limited in many regards. For example, the geometry of the separator is not considered, the particles are assumed to achieve a steady state and the effect of the vortex inversion at the base of the cyclone is also ignored, all behaviours which are unlikely to be achieved in a cyclone at real operating conditions.

More complex differential equation based models exist, as many authors have studied the behaviour of cyclone separators ^[2]. Numerical modelling using computational fluid dynamics has also been used extensively in the study of cyclonic behaviour. ^[3][4]

[edit] See also

[edit] References

1. ^ Rhodes M. (1998). Introduction to particle technology. John Wiley and Sons. ISBN 0471984833. 
2. ^ Smith, J. L, Jr.. PhD thesis: Experimental and Analytical Study of the Vortex in the Cyclone Separator. http://dspace.mit.edu/handle/1721.1/11792. 
3. ^ "Evaluation of cyclone geometry and its influence on performance parameters by computational fluid dynamics (CFD)". Brazilian Journal of Chemical Engineering. http://www.scielo.br/scielo.php?pid=S0104-66322007000100008&script=sci_arttext. 
4. ^ PhD Thesis: On the Potiential of Large Eddy Simulation to Simulate Cyclone Separators. http://archiv.tu-chemnitz.de/pub/2007/0013/data/diss.pdf. 

[edit] External links

• Cyclone Separators -- An Overview (Link broken as of February 2008, archived version here)