Page 254 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Chapter

4

Mechanical Separations

PracticaJly every process operation requires the separa- data the range and distribution of particle sizes, or be in
tion of entrained material or two immiscible phases in a a position to intelligently estimate the normal and
process. This may be either as a step in the purification of extreme expectancies. Figures 4-1 and 4-lA give a good
one stream, or a principal process operation [64]. These overall picture of dimensions as well as the descriptive ter-
separations may be: minology so important to a good understanding of the
magnitude of a given problem. The significant laws gov-
l. liquid particles from vapor or gas erning particle performance in each range is also shown.
2. liquid particles from immiscible liquid Particle sizes are measured in microns, µ. A micron is
3. dust or solid particles from vapor or gas
1/1000 millimeter or 1/25,400 inch. A millimicron, mu,
4. solid particles from liquid is 1/1000 ofa micron, or 1/1,000,000 millimeter. Usually
5. solid particles from other solids particle size is designated as the average diameter in
microns, although some literature reports particle radius.
These operations may sometimes be better known as Particle concentration is often expressed as grains/ cubic
mist entrainment, decantation, dust collection, filtration, feet of gas volume. One grain is 1/7000 of a pound.
centrifugation, sedimentation, screening, classification,
scrubbing, etc. They often involve handling relatively The mechanism of formation has a controlling influ-
large quantities of one phase in order to collect or sepa- ence over the uniformity of particle size and the magni-
rate the other. Therefore the size of the equipment may tude of the dimensions. Thus, sprays exhibit a wide parti-
become very large. For the sake of space and cost it is cle size distribution, whereas condensed particles such as
important that the equipment be specified and rated to fumes, mists and fogs are particularly uniform in size.
operate as efficiently as possible [9]. This subject will be Table 4-1 gives the approximate average particle sizes for
limited here to the removal or separation of liquid or dusts and mists which might be generated around process
solid particles from a vapor or gas carrier stream (1. and plants. Figure 4-2 indicates the size ranges for some
3. above) or separation of solid particles from a liquid aerosols, dusts and fumes. Table 4-2 gives typical analysis
(item 4). Reference [56] is a helpful review. of a few dusts, and Table 4-3 gives screen and particle size
Other important separation techniques such as pres- relationships. Table 4-4 gives approximate mean particle
sure-leaf filtration, centrifugation, rotary drum filtration size for water spray from a nozzle.
and others all require technology very specific to the
equipment and cannot be generalized in many instances.
Preliminary Separator Selection
Particle Size
The Sylvan Chart [2] of Figure 4-3 is useful in prelimi-
The particle sizes of liquid and solid dispersoids will nary equipment selection, although arranged primarily
vary markedly depending upon the source and nature of for dust separations, it is applicable in the appropriate
the operation generating the particular particles. For parts to liquid separations. Perry [23) presents a some-
design of equipment to reduce or eliminate particles what similar chart that is of different form but contains
from a fluid stream, it is important either to know from much of the same information as Figure 4-1 and 4-lA.

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