Page 289 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Mechanical Separations 259

Note that these units should not be connected in lines Efficiency
larger than their pipe inlet, since inlet velocity conditions
are very important, the swaging down or reduction tends The efficiency of centrifugal units is:
to produce a jet effect by the gas upon the mist elimina-
tor unit. This may erode the unit and cause other erratic
performance _ Type Efficiency Range _
High Velocity 99% or higher, of entering liquid.
Stationary Vanes Residual entrainment 1 ppm or less
Dry-Packed Imf;ingement .Beds Cyclone 70-85% for LO micron, 99% for
40 micron and larger. For high
entrainment, efficiency increases
Although this type of unit is not used as frequently as with concentration.
98% for agglomerating particles
most of the others discussed, it does have some specific Rotary ===================:::::=:::====�=======
applications in sulfuric acid mist removal and similar very
difficult. applications. The unit con sis Ls of a bed of gran u-
lar particles or ceramic packing, sometimes graduated in Cyclone Separators
size, operating dry as far as external liquid application to
aid in the separation. The superficial velocities of 0.5 to 8 The cyclone »r= unit is well recognized and accept-
feet per seccnd through the unit are rather low for most ed in a wide variety of applications from steam con-
separators therefore the vessels become large. Due to the densate lo dusts from kilns. In this unit the carrier gas
packed heights of 2 feet (min.) and higher, the pressure and suspended particles enter tangentially or volutely
drop can be appreciable. Particle removal may be 0.5 Lo 5 into a cylindrical or conical body section of the unit,
microns al 99% efficiency for a good design. These units then spiral downward forcing the heavier suspended
will plug on dust service and must be back washed to matter against the walls. Solids tend to slide down the
regain operability at reasonable pressure drops. wall while liquid particles wet the wall, form a running
film and are removed at the bottom. Figure 4-41 gives a
good typical cyclone arrangement, but this is by no
Centrifugal Separators means the highest efficiency or best design. References
[ 43,45,51] provide good design and performance
analysis.
Centrifugal separators utilize centrifugal action for the Some commercial units are shown in Figures 4-42 to 4-45.
separation of materials of different densities and phases. The zone of most efficient separation is in the conical
They are built in (a) stationary and (b) rotary types. Vari- region designated by dimension Z; in Figure 4-41. The
ous modifications of stationary units are used more than larger particles have already been thrown against the wall
any other kind for separation problems. The cost is mod- before the outlet was reached. The finer particles are
erate; it is simple in construction, and is reasonably flexi- thrown out in the inner vortex as the direction of motion
ble in service, being useful for gas-liquid or gas-solid sys- is reversed. Here, the relative velocity difference between
tems. In addition Lo serving as finishing separators the particle and the carrier is the greatesL for any point in
centrifugal units are also used to take a "rough cut" into a the separator. Although the tangential velocity compo-
separation problem. They may be followed by some addi- nent predominates throughout the cyclone, the axial
tional unit of special cyclone action or filtration through velocity prevails in the turbulent center. Van Dongen and
woven cloth pads, etc., Lo completely remove last traces of ter Linden [20] measured pressure patterns in a typical
entrained particles.
cyclone and found the lowest total pressure at the
extreme bottom point of the cone, even lower than al the
gas exhaust. Their pressure profiles indicate considerable
Stationary Vane eddy or secondary gas movement in the unit near the ver-
tical axis.
The stationary vane type is quite popular and adapts to
many applications. It is used in vessels or pipe lines as Particle Size Separation
illustrated in Figures 4-34 to 4-40. They are usually of high
efficiency for both liquid and solid particles such as rust, The theoretical minimum diameter particle to be sep-
scale, dirt, etc. When the system is dry with dust a special arated in a cyclone of the basic type given by Figure 4-41
design is used. is given by the relation of Rosin [ 13].