Process Technology: An Introduction - Haan A.B. 2015
11 Particle removal from gases
11.6 Particle interception mechanisms
Impingement separators require the transport of particles to a surface on which they are deposited by collision. Deposition requires a body for impact to be placed in the gas stream. This can be a wire, a fiber, or a droplet of washing liquid around which the gas can flow on all sides. However, as in the case of wave-plate separators, it can also be an impact surface which forces the gas flow to change its direction. If the particle contacts the surface of a fiber, it can be regarded as collected. The three important mechanisms for particle collection schematically drawn in Fig. 11.13 are inertial interception, Brownian diffusion, and flow-line interception. Inertial interception occurs when particles approach targets such as a baffle, impaction element, fiber, or droplet with sufficient velocity to cause a collision with the target by inertia of the particle. As a result of inertia, particle trajectories deviate from the streamline of the gas, so that the particles can strike the collecting surface of the target. This applies to all particles of large enough diameter which are present in the central filament of flow in the oncoming gas. Because sufficiently large particles will always move in a straight line, their collection efficiency will be close to unity. Brownian motion causes the particles to diffuse randomly across flow streamlines to be captured by a target. Only very small particles (< 1 µm) are subject to Brownian movement, which is therefore of secondary importance in most gas-solid separations. Flow-line interception is the striking of a target by a particle that passes the target in a streamline within one particle radius. While inertial interception separation increases with droplet diameter, diffusional deposition increases with decreasing droplet size. This results in a separation minimum at droplet sizes between 0.2 and 0.7 µm. It is mainly this minimum range where flow-line interception can predominate. The resulting fractional collection efficiency of a single fiber is shown in Fig. 11.14.
Fig.11.13: From top to bottom: inertial interception, flow-line Interception, and Brownian diffusion particle separation mechanisms.
Fig. 11.14: Collection efficiency of a single fiber.
For interception separators, the collection efficiency is usually estimated in terms of the target efficiency of the single baffle. The target or collection efficiency is described in terms of the separation number defined as
(11.15)
where g is the gravitational acceleration, Db the characteristic baffle dimension, vt the terminal settling velocity, and V0 the approach gas velocity. Fig. 11.15 assumes that Stokes’ law is applicable. The efficiency increases with increasing gas velocity because of particle inertia. Likewise, efficiency increases with increasing terminal velocity. Somewhat surprising is the decrease in efficiency with larger characteristic dimension. This is the result of the increased distance that the gas must be deflected.
Fig. 11.15: Target efficiency of single spheres, cylinders, and ribbons.