9 Impact Comminution in Jet Mills
307
x 50,3
Mean volume-weighted particle diameter [μm]
x c
Contact diameter on the particle surface after impact [μm]
x cut
Particle cut size for breakage probability [μm]
x T
Cut size classifier [μm]
z
Nozzle distance in jet mill [mm]
η
Dynamic viscosity [Pa s]
ν
Poisson ratio [–]
ξ
Product residue [–]
ρ p
Particle density [kg m
−3 ]
ψ i
Sphericity of individual particle [–]
ψ
Volume-based mean sphericity [–]
Indices
m
Milling chamber
p
Product
t
Process time
gas
Gas
1 Introduction
Dynamic processes are becoming increasingly important in the field of solid process engineering. The on-demand production, faster product cycles and fluctuating
energy supply of renewable (wind or solar) energy are the driving forces of this
development. First and foremost, before addressing the modelling of interconnected
solids processes, a well-founded understanding of the dynamics of individual unit
operations and overall processes is essential [1]. To date, in-depth information on
the dynamic behaviour of comminution processes is rare. Modelling of comminution processes is a challenging task [2]: Within the mill, very complex transport
phenomena prevail, resulting in mostly unknown stressing conditions. In particular,
classifier mills are highly interesting because they involve three coupled unit operations: The comminution in the jet area, the transport to the classification zone, and the
classification itself [3]. Although advanced CFD [4–6] or coupled CFD-DEM [2, 7]
approaches are available today, the highly turbulent two-phase flows in the mills are
far from being understood sufficiently. Other approaches use dynamic model systems
for the calculating of closed loop systems [8–10] or population balance methods [11,
12]. The high Reynolds numbers and the steep velocity gradients in combination
with the high solids concentrations limit the application of numerical methods [2].
One particular type of classifier mills is the fluidized bed opposed jet mill, which
is the method of choice for size reduction of hard, abrasive, and thermosensitive
307
x 50,3
Mean volume-weighted particle diameter [μm]
x c
Contact diameter on the particle surface after impact [μm]
x cut
Particle cut size for breakage probability [μm]
x T
Cut size classifier [μm]
z
Nozzle distance in jet mill [mm]
η
Dynamic viscosity [Pa s]
ν
Poisson ratio [–]
ξ
Product residue [–]
ρ p
Particle density [kg m
−3 ]
ψ i
Sphericity of individual particle [–]
ψ
Volume-based mean sphericity [–]
Indices
m
Milling chamber
p
Product
t
Process time
gas
Gas
1 Introduction
Dynamic processes are becoming increasingly important in the field of solid process engineering. The on-demand production, faster product cycles and fluctuating
energy supply of renewable (wind or solar) energy are the driving forces of this
development. First and foremost, before addressing the modelling of interconnected
solids processes, a well-founded understanding of the dynamics of individual unit
operations and overall processes is essential [1]. To date, in-depth information on
the dynamic behaviour of comminution processes is rare. Modelling of comminution processes is a challenging task [2]: Within the mill, very complex transport
phenomena prevail, resulting in mostly unknown stressing conditions. In particular,
classifier mills are highly interesting because they involve three coupled unit operations: The comminution in the jet area, the transport to the classification zone, and the
classification itself [3]. Although advanced CFD [4–6] or coupled CFD-DEM [2, 7]
approaches are available today, the highly turbulent two-phase flows in the mills are
far from being understood sufficiently. Other approaches use dynamic model systems
for the calculating of closed loop systems [8–10] or population balance methods [11,
12]. The high Reynolds numbers and the steep velocity gradients in combination
with the high solids concentrations limit the application of numerical methods [2].
One particular type of classifier mills is the fluidized bed opposed jet mill, which
is the method of choice for size reduction of hard, abrasive, and thermosensitive