1.1 Granular Flow and Fluidisation
3
superficial gas velocity is much higher than the particle terminal velocity, at which
individual particle would be entrained out of the vessel, particles remain in a lean
dispersed form with gas. Particles fluidised in this flow regime can be transported
between different reactors conveniently, or recirculated back into the system, forming
a so-called circulating fluidised bed (CFB), which is commonly employed in oil
refining for the production of high-octane gasoline, and chemical looping processes
for the coal combustion and gasification [36, 63].
On the other hand, fluidisation behaviour is not only dependent on operating
conditions, but also on physical properties of particles, which influence macroscopic
dynamic flow structures. Geldart [40] classified granules into four different groups
(groups A to D), based upon their fluidisation behaviour. Group A corresponds to
particle sizes between 20 and 100 μm with a density less than 1400 kg/m
3 . Beds
filled with Group A particles will undergo expansion at incipient fluidisation, prior
to bubble formation. In addition, Group C comprises extremely fine particles usually
smaller 30 μm, which are highly cohesive. Their cohesiveness induces agglomeration
of the particles, which complicates fluidisation. In contrast, Group D lies in the region
where particles are sized above 600 μm with a relatively large density. The Geldart
B type classifies non-cohesive particles having a size between 40 and 500 μm and a
density in the region of 1400–4000 kg/m
3 .
Different from Group A, Group B particles manifest bubbling at incipient fluidisation, owing to a relatively larger size and density. Their minimum bubbling velocity is
thereby identical to U mf from a theoretical point of view, but often slightly higher than
U mf in real operations. In particular, bubbling fluidised beds of Geldart B particles
are broadly used in gas-solid processes where interphase heat and mass transfer are of
paramount interest, such as powder dryers or coaters, gasification, and polymerization
operations [90].
1.2 Challenges to Engineering of Fluidised Beds
The overall process performance of a bubbling fluidised bed, especially the efficiency
of solid mixing and interphase mass and heat transfer, relies significantly on the
characteristics of bubble flows [62]. Rising bubbles entrain solids upward inside their
wakes and drift toward the top surface, alongside frequent break-up and coalescence,
leading to local solid mixing, as shown in Fig. 1.2a. Bubbles moving towards the
central region also provide a preferable path for solids. Particles driven by bubbles
move upwards in the central region and downwards near the walls, developing a
long-range circulation inside a column, as shown in Fig. 1.2b.
Bubble propagation, growth, coalescence and split-up affect the movement of
particles, and therefore benefit local interphase contact and mixing, and reduce the
resistance to mass and heat transfer. On the other hand, gas bypassing and channelling,
induced by oversized and fast-rising bubbles, depress interphase contacts between
gas and solid species, resulting in poor interphase transport and inefficient use of
3
superficial gas velocity is much higher than the particle terminal velocity, at which
individual particle would be entrained out of the vessel, particles remain in a lean
dispersed form with gas. Particles fluidised in this flow regime can be transported
between different reactors conveniently, or recirculated back into the system, forming
a so-called circulating fluidised bed (CFB), which is commonly employed in oil
refining for the production of high-octane gasoline, and chemical looping processes
for the coal combustion and gasification [36, 63].
On the other hand, fluidisation behaviour is not only dependent on operating
conditions, but also on physical properties of particles, which influence macroscopic
dynamic flow structures. Geldart [40] classified granules into four different groups
(groups A to D), based upon their fluidisation behaviour. Group A corresponds to
particle sizes between 20 and 100 μm with a density less than 1400 kg/m
3 . Beds
filled with Group A particles will undergo expansion at incipient fluidisation, prior
to bubble formation. In addition, Group C comprises extremely fine particles usually
smaller 30 μm, which are highly cohesive. Their cohesiveness induces agglomeration
of the particles, which complicates fluidisation. In contrast, Group D lies in the region
where particles are sized above 600 μm with a relatively large density. The Geldart
B type classifies non-cohesive particles having a size between 40 and 500 μm and a
density in the region of 1400–4000 kg/m
3 .
Different from Group A, Group B particles manifest bubbling at incipient fluidisation, owing to a relatively larger size and density. Their minimum bubbling velocity is
thereby identical to U mf from a theoretical point of view, but often slightly higher than
U mf in real operations. In particular, bubbling fluidised beds of Geldart B particles
are broadly used in gas-solid processes where interphase heat and mass transfer are of
paramount interest, such as powder dryers or coaters, gasification, and polymerization
operations [90].
1.2 Challenges to Engineering of Fluidised Beds
The overall process performance of a bubbling fluidised bed, especially the efficiency
of solid mixing and interphase mass and heat transfer, relies significantly on the
characteristics of bubble flows [62]. Rising bubbles entrain solids upward inside their
wakes and drift toward the top surface, alongside frequent break-up and coalescence,
leading to local solid mixing, as shown in Fig. 1.2a. Bubbles moving towards the
central region also provide a preferable path for solids. Particles driven by bubbles
move upwards in the central region and downwards near the walls, developing a
long-range circulation inside a column, as shown in Fig. 1.2b.
Bubble propagation, growth, coalescence and split-up affect the movement of
particles, and therefore benefit local interphase contact and mixing, and reduce the
resistance to mass and heat transfer. On the other hand, gas bypassing and channelling,
induced by oversized and fast-rising bubbles, depress interphase contacts between
gas and solid species, resulting in poor interphase transport and inefficient use of