14
1 Introduction
frequency. Following this early interest, an increasing number of researchers have
explored the effects of pulsation on gas-solid fluidised beds. Several studies have
shown that pulsating gas mitigates gas channelling, and, in general terms, reduces
the size of bubbles [8, 33, 81].
Pulsation often refers to the oscillation of the superficial inlet gas velocity. This
is typically achieved through the control of inlet valves, but other approaches have
also been put forward. Köksal and Vural [59] introduced a moving double-plate
distributor with two stacked reciprocating distributor plates, to create an oscillation
in the inlet gas flow rate. Under this oscillation and set-up, they reported a 40% drop
in bubble size at the intermediate range of frequency 5–7 Hz, due to the frequent
breakup of bubbles. In addition, they also observed enhanced gas-solid contacts and
a lower bed expansion with a more uniform flow pattern in a planar bed filled with
Geldart D turnip seeds and sand.
It is not surprising that the changes in particle dynamics under a pulsating flow have
a direct impact on the transport processes of heat and mass. Bokun and Zabrodski [15]
demonstrated that similar heat transfer rates can be achieved under pulsation with a
30–40% smaller volumetric flow rate than in conventional beds. In drying applications, pulsation mitigates the formation of agglomerates and reduces the local mass
transfer resistance. Akhavan et al. [2] showed a significantly increased drying rate
and uniform drying for processing lactose and cellulose fillers under pulsation. They
attributed the improvement observed in mass and heat transfer to the better solids
mixing, which leads to a more homogeneous moisture distribution and more effective gas-solids contacts. The mixing time was reduced from a maximum of 70 min
in the constant flow fluidisation to a few seconds under pulsation, and the required
drying processing time was reduced by approximately 20% overall. Recently, Jia
et al. [50, 51] reported similar conclusions for drying and mixing Geldart D biomass
particles; following on the steps of Wong and Baird [114], the authors found out the
most significant improvement in the interphase transfer occurs when pulsating the
gas flow at the bed natural frequency.
Both pulsation and vibration have been shown to facilitate different aspects of gassolid fluidisation, including homogenisation of bubble flows, enhancement of solid
mixing and gas-solid contact. However, it is important to take note that such improvements are strongly associated not only with the specific particle types and operating
conditions, but also the unit geometry and scale. Subsequently, it is challenging to
reproduce the similar behaviour and enhancement observed in theoretical studies at
an industrially relevant scale. To formulate a robust approach for design and optimisation of bubbling fluidised beds, one would expect to create a scalable macroscopic
flow structure and apply it to alter the system hydrodynamics in a better-controlled
manner.
1 Introduction
frequency. Following this early interest, an increasing number of researchers have
explored the effects of pulsation on gas-solid fluidised beds. Several studies have
shown that pulsating gas mitigates gas channelling, and, in general terms, reduces
the size of bubbles [8, 33, 81].
Pulsation often refers to the oscillation of the superficial inlet gas velocity. This
is typically achieved through the control of inlet valves, but other approaches have
also been put forward. Köksal and Vural [59] introduced a moving double-plate
distributor with two stacked reciprocating distributor plates, to create an oscillation
in the inlet gas flow rate. Under this oscillation and set-up, they reported a 40% drop
in bubble size at the intermediate range of frequency 5–7 Hz, due to the frequent
breakup of bubbles. In addition, they also observed enhanced gas-solid contacts and
a lower bed expansion with a more uniform flow pattern in a planar bed filled with
Geldart D turnip seeds and sand.
It is not surprising that the changes in particle dynamics under a pulsating flow have
a direct impact on the transport processes of heat and mass. Bokun and Zabrodski [15]
demonstrated that similar heat transfer rates can be achieved under pulsation with a
30–40% smaller volumetric flow rate than in conventional beds. In drying applications, pulsation mitigates the formation of agglomerates and reduces the local mass
transfer resistance. Akhavan et al. [2] showed a significantly increased drying rate
and uniform drying for processing lactose and cellulose fillers under pulsation. They
attributed the improvement observed in mass and heat transfer to the better solids
mixing, which leads to a more homogeneous moisture distribution and more effective gas-solids contacts. The mixing time was reduced from a maximum of 70 min
in the constant flow fluidisation to a few seconds under pulsation, and the required
drying processing time was reduced by approximately 20% overall. Recently, Jia
et al. [50, 51] reported similar conclusions for drying and mixing Geldart D biomass
particles; following on the steps of Wong and Baird [114], the authors found out the
most significant improvement in the interphase transfer occurs when pulsating the
gas flow at the bed natural frequency.
Both pulsation and vibration have been shown to facilitate different aspects of gassolid fluidisation, including homogenisation of bubble flows, enhancement of solid
mixing and gas-solid contact. However, it is important to take note that such improvements are strongly associated not only with the specific particle types and operating
conditions, but also the unit geometry and scale. Subsequently, it is challenging to
reproduce the similar behaviour and enhancement observed in theoretical studies at
an industrially relevant scale. To formulate a robust approach for design and optimisation of bubbling fluidised beds, one would expect to create a scalable macroscopic
flow structure and apply it to alter the system hydrodynamics in a better-controlled
manner.