1.4 Methods for Structuring Bed Hydrodynamics
13
application is, nevertheless, restricted to specific conditions, such as adding an active
filler, that is either semi-insulating particles or ferromagnetic particles, and requires
additional energy demand, in the order of 40–80 W/m
3 for electric, and 100 kW/m
3
for magnetic actuation, respectively [47].
Mechanical stimulation, either rotation or agitation, is another promising design
that applies additional energy in a better-controlled and non-intrusive way. The rotational fluidised bed is a good example of modifying the solid phase dynamics by
acquiring particles with additional inertia. For example, Quevedo et al. [88] studied
the influence of centrifugal fluidisation on nanoparticles. Rotating the unit at different
velocities creating 0–40 g centrifugal acceleration increases the bed pressure drop
as well as the U mf , allowing a much larger superficial velocity without entrainment. Alternatively, the action of rotation can be introduced by injecting the gas
flow tangentially, without moving boundaries [31]. In this context, particles circulate and travel towards a central chimney, experiencing radially outward centrifugal
inertia. In practice, mechanical vibration and gas pulsation are the most broadly used
non-intrusive methods to improve gas-solid fluidisation for many types of particles.
Vibration becomes a common practice to break up agglomerates and improve
flowability for fine, cohesive powders. Vibrating the bottom distributor plate vertically transmits mechanical energy into the bottom layer of particles firstly, from
which it gradually propagates into the bulk. In general terms, vibration, at varying
amplitudes, facilitates fluidisation in reducing bubble size [119], improving gas-solid
mixing [52, 84], reducing the U mf [76], and increasing bubble rising velocity [120]
for various types of particles. Marco et al. [72] demonstrated the benefits of vibrofluidisation in terms of oxidative coupling of methane, showing a 50% lower U mf
than in the system without vibration, due to the reduced agglomeration of Li/MgO
catalysts.
Instead of through mechanical action, one can supply energy to the system by
manipulating the gas-solid interaction and, therefore, perturb particles continuously.
Pulsing the inlet gas flow in fluidised beds causes the fluid-particle interaction force
to oscillate. Different from vibration, which requires energy input from a moving
boundary to propagate through the bed media, the oscillation of the gas-particle interaction force acts simultaneously throughout the entire volume, resulting in much
less dissipation of energy and creating a range of influences on the flow pattern,
as shown in Fig. 1.5b. Pulsation-assisted fluidisation has also been widely applied
to improve the fluidisation of bubbling beds [3–5]. The earliest investigation documented in the literature can date from the 1960s by Massimilla et al. [74] who
created a pulsating airflow at frequencies 1–10 Hz using on-off solenoid valves. The
authors distinguished three different fluidisation regimes: intermittent for 1.2–2.7 Hz,
piston-like for 2.7–4.8 Hz and plain fluidisation for 4.8 Hz above. Kobayashi et al.
[57] followed this subject on a cylindrical bed, controlling the gas flow with the
on-off ratio of valves. Under pulsation, they showed considerably elevated pressure
drop and bed expansion, and more homogenous gas-solid contacts. Wong and Baird
[114] continued the study of pulsed beds, investigating the pressure fluctuation in the
frequency range 1–10 Hz. In comparison to a constant flow, the gas retention time
was shown to increase by up to 51% when the gas flow oscillated at the bed natural
13
application is, nevertheless, restricted to specific conditions, such as adding an active
filler, that is either semi-insulating particles or ferromagnetic particles, and requires
additional energy demand, in the order of 40–80 W/m
3 for electric, and 100 kW/m
3
for magnetic actuation, respectively [47].
Mechanical stimulation, either rotation or agitation, is another promising design
that applies additional energy in a better-controlled and non-intrusive way. The rotational fluidised bed is a good example of modifying the solid phase dynamics by
acquiring particles with additional inertia. For example, Quevedo et al. [88] studied
the influence of centrifugal fluidisation on nanoparticles. Rotating the unit at different
velocities creating 0–40 g centrifugal acceleration increases the bed pressure drop
as well as the U mf , allowing a much larger superficial velocity without entrainment. Alternatively, the action of rotation can be introduced by injecting the gas
flow tangentially, without moving boundaries [31]. In this context, particles circulate and travel towards a central chimney, experiencing radially outward centrifugal
inertia. In practice, mechanical vibration and gas pulsation are the most broadly used
non-intrusive methods to improve gas-solid fluidisation for many types of particles.
Vibration becomes a common practice to break up agglomerates and improve
flowability for fine, cohesive powders. Vibrating the bottom distributor plate vertically transmits mechanical energy into the bottom layer of particles firstly, from
which it gradually propagates into the bulk. In general terms, vibration, at varying
amplitudes, facilitates fluidisation in reducing bubble size [119], improving gas-solid
mixing [52, 84], reducing the U mf [76], and increasing bubble rising velocity [120]
for various types of particles. Marco et al. [72] demonstrated the benefits of vibrofluidisation in terms of oxidative coupling of methane, showing a 50% lower U mf
than in the system without vibration, due to the reduced agglomeration of Li/MgO
catalysts.
Instead of through mechanical action, one can supply energy to the system by
manipulating the gas-solid interaction and, therefore, perturb particles continuously.
Pulsing the inlet gas flow in fluidised beds causes the fluid-particle interaction force
to oscillate. Different from vibration, which requires energy input from a moving
boundary to propagate through the bed media, the oscillation of the gas-particle interaction force acts simultaneously throughout the entire volume, resulting in much
less dissipation of energy and creating a range of influences on the flow pattern,
as shown in Fig. 1.5b. Pulsation-assisted fluidisation has also been widely applied
to improve the fluidisation of bubbling beds [3–5]. The earliest investigation documented in the literature can date from the 1960s by Massimilla et al. [74] who
created a pulsating airflow at frequencies 1–10 Hz using on-off solenoid valves. The
authors distinguished three different fluidisation regimes: intermittent for 1.2–2.7 Hz,
piston-like for 2.7–4.8 Hz and plain fluidisation for 4.8 Hz above. Kobayashi et al.
[57] followed this subject on a cylindrical bed, controlling the gas flow with the
on-off ratio of valves. Under pulsation, they showed considerably elevated pressure
drop and bed expansion, and more homogenous gas-solid contacts. Wong and Baird
[114] continued the study of pulsed beds, investigating the pressure fluctuation in the
frequency range 1–10 Hz. In comparison to a constant flow, the gas retention time
was shown to increase by up to 51% when the gas flow oscillated at the bed natural