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3.8 T Fluid–Solid Fluidized Bed Reactors o-Phase, w 193

Table 3.11

Geldart’s (1973) classification of powders
Approximate A B C D
particle size Aeratable Bubbling Cohesive Spoutable
30–100 µm 100–1000 µm C 30 µm D 1000 µm
Main characteristic Ideal for Only b ubbling Difficult to Coarse solids
fluidization; fluidization fluidize
particulate
fluidization is
possible
Example Cracking catalyst, Building sand Flour, cement Gravel, cof fee
milk powder beans
xpansion Bed e High Moderate Low because Low
of channeling
Bubble properties ubble Maximum b No limit to size, ubbles, No b No limit to size,
size is less than limited only by formation of limited only by
10 cm vessel size channels vessel size
Solids mixing High Moderate Very low Low
Gas backmixing High Moderate Very low Low
Spouting No Only in No Yes
shallow beds

• Type A fluidization: This fluidization type consists of a nonbubbling region be ginning
at the minimum fluidization velocity ( u fm ), and a bubbling region as fluidizing
velocity increases. This velocity limit is called the minimum bubbling v elocity
( u bm u fm ). It is obvious that in the case of air as a fluidizing agent at ambient condi-
tions, Group vType A fluidization. It should be noted that for Type e a A particles will gi
A fluidization, the range of gas velocities oer which nonbubbling fluidization occurs v
is small, and the minimum bubbling velocity is higher than the minimum fluidization
velocity. Thus, the particulate phase is not under minimum fluidization condition when
bubbling begins and thus its porosity is different from the porosity at minimum
fluidization velocity ( ). To distinguish the bubbling region in TA fluidization ype
fm
ype
from the Type B fluidization, the term “TA bubbling fluidization” is used in
the following sections. The region before bubbling will be referred to as “particulate
fluidization”.
• Type B fluidization: This fluidization type consists solely of a bubbling region. Bubbles
are formed as soon as the gas velocity exceeds the minimum fluidization v elocity
( u bm u fm ). It is obvious that in the case of air as a fluidizing agent under ambient con-
ditions, Group B particles will gie a Type B fluidization. It should be noted that by
v
definition, the minimum bubbling velocity in Type B fluidization is equal to the mini-
elocity mum fluidization v. Thus, the particulate phase is at minimum fluidization con-
ditions when bubbling begins, and thus its porosity is equal to the porosity at minimum
fluidization velocity ( ). Type B fluidization is also referred to as “bubbling fluidiza-
fm
tion” in the follo wing sections.