Biotechnological and Biomedical Applications of Magnetically Stabilized and Fluidized Beds - Porous Media: Applications in Biological Systems and Biotechnology - page 461

Heterogeneous bed
(preference paths)

U t

Bubbling
bed

Fluidized
expanded bed

U mf

Fluidized
stabilized bed

Fixed Bed

H c

H (A/m)

FIGURE 12.2
Regions of behavior of a MSB depending on the velocity and magnetic field
values. (Adapted from Augusto et al. 2008.)

magnetic distribution downcomer (MDD) (Zhang et al. 1984). Many authors
prefer to ally both definitions to avoid controversy, speaking of MSFBs or of
magnetic field assisted fluidization (Hristov 2002, 2003a,b, 2004, 2006b). In
this chapter we will adopt the former denomination of MSFB to indicate that
we are generally speaking about both forms of magnetic beds.
The magnetic field in MSFB is usually fixed (i.e., it does not usually vary
with time) and its magnitude is high enough to strengthen the immobilization
of the particles in the magnetic bed and to avoid the appearance of bubbles
(if the velocity is below a given value). In fact, for these systems, a graphical
depiction of the velocity of the gas (U 0 ) versus the magnitude of the mag-
netic field, like the one presented in Figure 12.2, shows five distinct regions:
(1) Fixed Bed —in this region, for a certain magnitude of the magnetic field
acting externally, the bed behaves as a fixed bed, as long as the velocity is
maintained below a given value—the velocity of fluidization U mf ; (2) Fluidized
Stabilized Bed —this is the region where the system behaves as a fluidized
bed, but stabilized (i.e., with no creation of bubbles), as long as the magnetic
field is maintained below a certain magnitude and for velocities between the
velocity of fluidization, U mf , and the velocity of transition, U t ; (3) Fluidized
Expanded Bed —in this region the system behaves as an expanded stabilized
bed (without formation of bubbles), and expands as a piston for increasing
velocities, as long as the magnetic field is maintained above a certain mag-
nitude and for velocities between the velocity of fluidization, U mf , and the
velocity of transition, U t ; (4) Bubbling Bed —this is the region where the sys-
tem behaves as an ordinary fluidized bed containing bubbles, and exists as
long as the magnetic field is maintained below a certain magnitude and for