Rotating Flow Cyclone Separator

DEPOSITION FROM A ROTATING FLOW CYCLONE SEPARATOR 

An approximate analysis of the particle motion and cyclone performance can be carried out by setting up a force bulance for Stokesian particles in the radial direction  

For the motion in the 0 direction, it is assumed that the particle and gas velocities are equal, V0 = v f. It is thus possible to solve (4.42) for the particle trajectories, if the gas velocity distribution. Uy/. is known. 

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Separation based on rotating flow principles is one of the most common operations involved in gas-solid flows. This section describes the fundamental rotating flow principles and their applications to cyclone operation. The efficiency of dust collection in cyclones is also described. 

The aerodynamic diameter is one of the most common equivalent diameters. It can be defined as the diameter of a unit den.sity sphere with the same terminal settling velocity as the particle being measured. The aerodynamic diameter is commonly used to describe the mt)lioii of particles in collection devices such as cyclone separators and impactors. However, in shear flows, the motion of irregular particles may not be characterized accurately by the equivalent diameter alone because of the complex rotational and translational motion of inegular particles compared with spheres. That is, the path of the irregular particle may not follow that of a particle of the same aerodynamic diameter. It is of course possible that there may be a. sphere of a certain diameter and unit density that deposits at the same point this could be an average point of deposition because of the effects of turbulence or the. stochastic behavior of irregular particles. 

Observations of pneumatic dryers proved that a considerable amount of total moisture removed evaporates in the cyclone separator. Several new dryer types were constructed that apply the principle of vortex flow. In addition, the cyclone dryers have particle separation features. Centrifugal force keeps large and wet particles rotating whereas small and dry particles can be carried away with gas. Adjustment of gas velocity can thus change the critical particle size as well as residence time in the dryer. 

Centrifugal force can also be used to separate solid particles from fluids by inducing the fluid to undergo a rotating or spiraling flow pattern in a stationary vessel (e.g., a cyclone) that has no moving parts. Cyclones are widely used to remove small particles from gas streams ( aerocyclones ) and suspended solids from liquid streams ( hydrocyclones ). 

Fresh catalyst is normally delivered in hopper-bottom railroad cars. The catalyst may be withdrawn by gravity flow from the bottom of the car through a hose to a Fuller-Kinyon screw pump and transferred to the hopper by means of an air stream loss of catalyst is prevented by a cyclone on the air discharge from the hopper. Alternatively, the catalyst may be unloaded from the top of the ear by a vacuum lift. In this case, the suction line passes through a separator with bag filters, located above the storage hoppers. The catalyst collects in a chute and flows down through a rotating barrel-valve feeder into a screw conveyor which transfers the catalyst to the hopper (105). 

The cyclone, or inertial separation method, is a common industrial approach for segregating a dispersed phase from a continuous medium based upon the difference in density between the phases. The concept takes advantage of the velocity lag which occurs for dense particles with respect to a lower density medium when both phases are subject to an accelerating flow field, such as within a rotating vortex. The larger the acceleration, the smaller the particle which fails to follow the continuous phase streamlines and will migrate to the outer wall of the cyclone for collection. 

Hydrocyclones (see Figure 22.55) are closely related to centrifuges in that centrifugal forces effect the separation of particles. Rotational motion is effected by bringing the slurry radially into the upper periphery of the cyclone at high velocity. Solids are thrown out to the wall, flow down the inclined walls, and exit at the bottom. In general, hydrocyclones operate as classifiers with large particles in the underflow and small particles in the overflow. 

As illustrated in Figure 10.7, a cyclone consists of a vertical cylinder with a conical bottom, a tangential inlet near the top, and outlets at the top and the bottom, respectively. The top outlet pipe protrudes into the conical part of the cyclone in order to produce a vortex when a dust-laden gas (normally air) is pumped tangentially into the cyclone body. Such a vortex develops centrifugal force and, because the particles are much denser than the gas, they are projected outward to the wall flowing downward in a thin layer along this in a helical path. They are eventually collected at the bottom of the cyclone and separated. The inlet gas stream flows downward in an annular vortex, reverses itself as it finds a reduction in the rotation space due to the conical shape, creates an upward inner vortex in the center of the cyclone, and then exits through the top of the cyclone. In an ideal operation in the upward flow... 

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