Page 152 - Chemical Process Equipment - Selection and Design

124 FLOW OF FLUIDS
.
104,

a 1 \ .^ . .A I
8 \
2 1000
5 \ \ A
(aeratable)
.- 3 500 c '\
v)
B (cohesive) \\
'\
200 1 I I 1
10 20 50 I00 200 500 1000 2000
Mean particle diameter c?,y (pm)

Fignre 6.12. Characteristics of four kinds of groups of particles classified by Geldart [Powder Technol. 6,
201-205 (1972); 7, 285-292 (1973)]. The boundary between A and-B is re resented by the equation
as = 44,000p~1&g/g(p, - pF) and that between B and D by (ps - pp) cf; = 10- F kg/m.

Feature Group C Group A Group B Group D

Distinguishing Cohesive aeratable bubble readily
word or phrase
Exam o le Flour fluid cracking sand wheat
catalyst
Particle size for s20 pm 20 < d, s 90 ym 90< as 5650 ym >650 pm
ps = 2.5 g/cm3
Channeling Severe little negligible negligible
Spouting - none shallow beds only readily
None
Collapse rate slow rapid rapid
Expansion Low because high; initially medium medium
of channeling bubble-free
Bubble shape channels, no flat base, rounded with rounded
bubbles spherical cap small
indentation
Rheological high yield apparent viscosity apparent viscosity apparent
character of stress of order 1 poise of order 5 poise viscosity of
dense phase order 10
poise
Solids mixing very low high medium low
Gas back mixing very low high medium low
Slugging mode flat raining axisymmetric mostly axi- mostly wall
symmetric
slugs
plugs
Effect of as unknown appreciable minor unknown
(within group)
on hydrodynamics
Effect of particle unknown appreciable negligible can cause
size distribution segregation

Solids of practical interest often are mixtures of a range of sphere with the same volume as the padcle. The sphericity is
particle diameters, but, for convenience, correlations are expressed defined as the ratio
in terms of a single size which is almost invariably taken as the
surface average diameter given by + = (surface of a sphere)/(surface of the
particle with the same volume) (6.130)
dp = 112 xidip (6.129)
and is always less than unity. Accordingly, the relation between the
where xi is the weight fraction of the material having a diameter dt effective particle size dp and that found by screen analysis is
measured by screen analysis. Particles that deviate substantially
from a spherical shape are characterized as having the diameter of a dp = +d%Iecn. (6.131)