Magnetically assisted gas--solid fluidization in a tapered vessel: first report with obeservations and dimensional analysis.
Hristov, Jordan
INTRODUCTION
Fluidization in tapered vessels is a useful fluid-solid contacting
technique with a variety of application in drying (Becker and Salans,
1960; Mathur and Epstein, 1974), combustion (Khoshnoodi and Weinberg,
1978), gasification (Salam and Bhattacharya, 2006), waste water cleaning (Scott and Hancher, 1976), bioreactors (Scott and Hancher, 1976), food
processing (Depypere et al., 2005), plasma (Flamant, 1994) and microwave
(Feng and Tang, 1998) assisted processing, etc.
Originally conceived (Mathur and Epstein, 1974) for
gas-fluidization of Geldart's D particles (Geldart, 1973) this
fluidization technique spreads towards other applications such as
fluidization of cohesive particles (Deiva et al., 1996; Erbil, 1998),
gas-liquid fluidization of highly viscous liquids (Anabtawi, 1995),
drying of agriculture by-products (Wachiraphansakul and Devahastin,
2007), sea foods (Tapaneyasin et al., 2005), pasty materials (Passos et
al., 1997), and pharmaceutical coating processes (Jono et al., 2000) due
to its specific feature to assure cyclic gas-solid contact during the
particle rising through the spout. In most of the cases of practical
applications these devices work with sticky solids due to the high
liquid content. The latter causes particle aggregation and affects the
bed hydrodynamics.
Recently, Bacelos et al. (2007) have published an extensive study
on spouted bed hydrodynamics with controlled effect of interparticle
forces by adding glycerol to glass spheres with large size distribution.
Nowadays, there are a few articles on the effect of interparticle forces
on spouted bed behaviour (Passos et al., 1997; Passos and Mujumdar,
2000; Charbel et al., 2004; Trindade et al., 2004; Bacelos et al.,
2007). In all of them the interparticle forces are mainly generated by
capillary forces and cannot be controlled remotely. The present article
addresses spouted bed behaviour in the case of gas-fluidized magnetic
particles and interparticle forces induced by an external magnetic field
that creates a new branch in the magnetically assisted fluidization
(Hristov, 2002, 2003a,b, 2004, 2006, 2007b).
Magnetically assisted fluidization deals with magnetic solids
fluidized by liquid (Hristov, 2006), gas (Hristov, 2002, 2003a), or
gas-liquid flow (Hristov, 2007b) with two basic magnetization modes:
Magnetization FIRST (magnetization of a fixed bed undergoing
fluidization) and Magnetization LAST (magnetization of preliminarily
fluidized beds). With Magnetization FIRST mode the induced interparticle
forces yield particle aggregation (i.e., magnetic flocculation) that
alters the bed behaviour and creates a Meta-regime commonly known as a
magnetically stabilized bed (MSB). MSB is a fixed bed with induced
interparticle forces of magnetic nature that, to some extent, give it a
mechanical strength enabling bed expansion without fluidization at
velocities beyond the minimum fluidization point in absence of field
(Rosensweig, 1979; Penchev and Hristov, 1990a,b; Hristov, 2002).
Macroscopically, it resembles the homogeneously expanded powders of the
Geldart's Group A (Geldart, 1973). The hydrodynamic behaviour of
such beds is basically reviewed (Hristov, 2002, 2003a,b, 2004, 2006) and
we will avoid more reference information here.
Addressing magnetically assisted tapered bed (MATB), we have to
note that this is the first attempt to perform a magnetically assisted
fluidization in a vessel different from cylindrical columns used in this
field over 40 years (Filippov, 1961; Hristov, 2002, 2006). However, in
order to be exact, we should mention that some attempts to use a tapered
2-D vessel with an axial magnetic system (Helmholtz coils) were
performed by the group of Jovanovic et al. (2004) as a part of
low-gravity experiments with L-S magnetically assisted beds.
Additionally, Jones et al. (1982) have conceived a magnetic field
coupled sponted bed system in view of a magnetic control of the particle
flow by means of a short coil placed at the top of the draft tube of a
classical draft tube-spouted bed device. These inventions and the
experiments thereof are by far away from the subject of the present
work, the common issue is either the vessel shape or the name; an
analysis of them is available elsewhere (Hristov, 2006).
Paper Outline
The article addresses several basic features pertinent to the
magnetic field effects on fluidized solids and the vessel shape effect
with magnetization FIRST mode, among them:
* Bed behaviour, pressure drop histories, bed expansion (porosity)
variations and the physics behind them in presence of an external
transverse magnetic field.
* Effects of solid mass charged to the vessel, particle size, etc.
* Critical velocities, pressure drops at minimum fluidization and
minimum spouting points.
* Pressure drop hysteresis cycles and graphical determination of
critical velocities.
* Dimensional analysis of both the conical vessel and magnetic
field assistance; Data correlations.
Some preliminary thoughts and assumptions elucidating the
background of the experiments reported here are developed next.
Paper Aim and Some Preliminary Thoughts
The introduction generally defines that the present articles
addresses magnetically assisted gas fluidization in a tapered vessel.
Due to the large varieties in both the spouted bed technology and the
magnetic field assisted fluidization some preliminary comments are
needed for better understanding of the explanations further developed in
the article. First of all, we use a single vessel and do not vary its
cone angle unlike the common practice in articles dealing with spouted
beds. We especially avoid this additional process parameter, in this
first article on MATB, and address the field effects and the
fluidization regimes. This approach allows to draw easily a parallelism between the new results and those known from fluidization in cylindrical
vessels (Rosensweig, 1979; Hristov, 2002). To this end, the cone angle
is an important process parameter and its effect has to be studied but
this draws experiments beyond the scope of the present work.
Further, the field is oriented normally to the fluid flow and the
vessel axis. There are several reasons for that, among them: (a)
Applying a transverse field we avoid the field imposed axial channelling
in the bed annulus (Hristov, 2002). (b) The saddle coils system (Penchev
and Hristov, 1990b; Hristov, 2002, 2005) allows a simple extension of
the vessel volume in both axial (increase in height) and lateral
(increase in bed diameter) direction since it ensures a homogeneous
magnetic field over approximately 90 of its internal volume. (c)
Additionally, the bed behaviour could be naturally observed through the
coils "window" that is practically impossible when closely
located short coils are used to create axial fields (Hristov, 2002). To
this end, however, irrespective of these preliminary assumptions, the
axial field assisted fluidization in tapered vessels has not been
investigated yet but this challenging problem is beyond the scope of
this article. At the end, in this text "tapered bed" and
"spout-bed" will be used as equivalent terms albeit under some
circumstances the fluidization cannot reach the spouted bed regime.
EXPERIMENTAL
Experiments with magnetically assisted beds were performed in a
conical vessel (15[degrees], opening angle, 30 mm ID-bottom diameter and
190 mm ID-top diameter). The field was generated by a saddle coils
magnetic system (Penchev and Hristov, 1990b; Hristov, 2002) with 200 mm
ID and 400 mm in height. The field lines were oriented transversely to
the cone axis of symmetry and the fluid flow (see the inset). The field
was steady and the maximum field intensity attained in these experiments
was about 27 kA/m. All magnetic materials used in the experiments are
listed in Table 1. Air was used as a fluidizing agent and the flow
(controlled by a mechanical valve) was measured by a calibrated rotameter. The pressure drop was measured by a U-tube water manometer
connected between the gas inlet and a fine tube (pressure probe) placed
above the bed top surface. Figure 1 presents schematically this
experimental setup.
Due to the impossibility to measure the bed height directly from
the top of the column (the magnetic system hinders the access) a scale
placed at the vessel wall was used. The bed height is calculated by
[h.sub.b] = [h.sub.L] cos a (see the inset in Figure 1), that is in the
present case we have [h.sub.b] = 0.991[h.sub.L].
[FIGURE 1 OMITTED]
RESULTS
Observations 1: Phase Diagrams
Commonly the bed behaviour in conical vessels is illustrated by
pressure drop curves (Jing et al., 2000; Devahastin et al., 2006; Zhong
et al., 2006) and photos or schematic pictures. The magnetically
assisted fluidization provides an additional illustrative tool, that is,
a phase diagram in U-H coordinates (Rosensweig, 1979; Hristov, 2002),
relating two macroscopic process variables controlling the fluidization.
Following the basic approach in the spouted bed mechanics (Mathur and
Epstein, 1974) the gas flow rate is represented here by its volumetric
flow rate Q ([m.sup.3]/s) rather than the superficial fluid velocity U
(m/s) in the gas entrance orifice (Devahastin et al., 2006; Zhong et
al., 2006; Bacelos et al., 2007). To some extent, however, the
superficial gas velocity either at the gas inlet or at the bed top
surface will be used further in this article addressing mainly data
correlations by equations similar to those developed for non-magnetic
beds.
The phase diagrams in Q-H coordinates (Figures 2a-c) represent the
bed behaviour with variations in the amount of the solids as a process
parameter. In general, the solids behave similarly to those in
cylindrical vessels (Hristov, 2002) but different parts of the bed
situated along its axis of symmetry (from the bottom towards the top)
are under different flow conditions, in spite of the fact that the
magnetizations are identical (the field is homogeneous along the radius
of the vessel). The schematic pictures in Figure 3a illustrate
fluidization regimes existing under conditions imposed by the gas flow
and the field applied.
At a given gas flow rate, the bottom part may pass into a fluidized
state, while the top remains completely magnetically stabilized (i.e.,
with a fixed bed structure). The flow range corresponding to MSB is
relatively narrow and bounded by two critical flow rates: (1) [Q.sub.e]
denoting the onset of bed expansion that indicates the pass into a
magnetically stabilized bed-MSB) and (2) [Q.sub.mf-1] at which the
bottom part near the flow entrance becomes fluidized; while the upper
bed layer remains fixed--see B2 in Figure 3a. Increase in the gas flow
rate yields an extension of the fluidized section in height and the bed
exhibits a classical behaviour with bubbling and no fountain
formation--see B3 in Figure 3a. The process of transient bed expansion
continues to its upper bound denoted as [Q.sub.mf-2] when different
structures, dependent on the combination of field intensity and the gas
flow rates, can exist.
The transient bed expansion (or developing fluidization) is complex
in nature and will be discussed separately in the next section. In
brief, the complete bed fluidization beyond the upper bound [Q.sub.mf-2]
depends on the field intensity applied. At low fields (branch B in
Figure 3a) the bed passes into a fluidized state with large bubbles
originating from the cone bottom (B4) and travelling almost along the
vessel axis. With higher field intensities the bed attains a wave-like
structure due to gas slits (B5) travelling from the bottom to the top.
In fact the slits are bubbles deformed in shape by the field. The latter
implies that the field imposes extensional forces to any non-magnetic
voids along the field lines and a compression in direction normal to
them (Hristov, 2002, 2003a) that finally transform any voids into
horizontal slits. The boundary between the bubbling bed and the
"moving slit" regime is not easily detectable either through
pressure drop records or bed height thus only visual observations may
distinguish them: for this reason a dashed line marks the transition
points.
With higher field intensities (branch C in Figure 3a) the bubbling
bed and the travelling slits regimes are replaced by an unstable
fountain (B6 or C4) within a range bounded by [Q.sub.ms-U] and
[Q.sub.ms-S]. The term "unstable fountains" means that the gas
flow rate ([Q.sub.ms-U] < Q < [Q.sub.ms-S]) is not enough to
create a stable central channel; the fountain is practically internal
with respect to the entire bed body and does not reach the bed top which
remains magnetically stabilized (a stabilized Hat). Beyond [Q.sub.ms-S]
the stabilized "Hat" is completely destroyed (C5) and we have
a magnetically controlled spouted bed.
Before further comments on the complex behaviour of the transient
bed expansion (from [Q.sub.e] to [Q.sub.ms-S]) we have to address the
effect of the initial bed height (represented here by the solids
inventory) on the flow transition. With low solids charged in the vessel
the bed is relatively short (shallow) with [h.sub.b0] = 170 mm and a top
diameter [D.sub.L] = 98 mm that imposes it to high gas superficial
velocities defined by the narrow cross-section of the vessel. These
conditions make the transition region (bounded by dotted lines in Figure
2a) almost unclear. Increase in the bed depth (solids inventory) makes
the transitions more easily detectable since we get a relatively deep
bed with sections exhibiting different fluidization behaviour. For such
deep beds, for instance, a short section near the bottom behaves like
the shallow one mentioned above, while the upper parts exhibit gradual
transition from stabilized into fluidized (or spouted bed) that are
easily detectable both by visual observations and records (pressure
drop, bed height and gas flow rate).
[FIGURE 2 OMITTED]
Observations 2: Transient Bed Expansion
In the regime of transient bed expansion, that is, from the initial
fixed bed into the completely fluidized state, the bed height is almost
constant due to specific stages in the bed behaviour, undergoing
fluidization, namely:
1. The top is stabilized by the field and since the gas flow rate
is not enough either to expand or to fluidize it, this stabilized
"hat" stays at an almost fixed position that provides a
constant bed height (bed porosity [[epsilon].sub.a]) even though the gas
flow rate increases. See C3 and C4 in Figure 3a.
2. At the same time the bottom part is completely fluidized and
increase in the gas flow rate yields a growing fluidized section. The
top particles of the fluidized section reach the bottom of the
"frozen hat" and with increase in the flow rate detach particles from its bottom, thus decreasing its depth. In other words,
the bottom fluidized section expands with almost constant pressure drop
across it (from C3 to C4). When the "frozen hat" reaches a
certain critical thickness it breaks down and the bed becomes completely
fluidized.
3. At moderate field intensities the fluidization manifests itself
by an approximately bubbleless (see the Kwauk's terminology, Kwauk,
1992) fluidization. The regime resembles an upward pseudo-wave flow (B5)
due to almost horizontal thin slits propagating from the bottom to the
top and the absence of a visible solids recirculation typical of tapered
fluidized beds (Mathur and Epstein, 1974; Asenjo et al., 1977; Zhong et
al., 2006; San et al., 2006).
4. Beyond a certain critical velocity this wave-like fluidized
structure (B5) becomes unstable and large pseudo-spherical bubbles grow
upward in the bed--see B4. No central channel exists and a typical
spout-bed structure cannot be reached with increase in the gas velocity.
5. With increase in the field intensity the bed behaviour remains
partially unchanged (upper sections) but beyond a certain critical gas
flow rate a central (axially oriented) channel starts to propagate upwards from the cone. The channel grows in height with increase in the
gas velocity (see C4) but it cannot reach the bed top that remains
magnetically stabilized (a "frozen hat"). With increase in the
gas flow rate, the developing channel reaches the upper stabilized
section, destroys it and a fountain, typical of spouted beds bursts the
bed top surfaces at [Q.sub.ms-U]. This fountain becomes instable and can
easily collapse. The stable fountain at [Q.sub.ms-S] > [Q.sub.ms-U]
and the solids circulations (from the centre towards the wall and then
down to the cone bottom) completely corresponds to a stable bed spouting
regime.
6. Reduction in gas flow rate ([Q.sub.ms-U] < Q <
[Q.sub.ms-S]) or increase in field intensity yields an unstable central
channel and suppression of the bed spouting.
Figure 3b presents schematically observations addressing the height
of parts (sections) of the bed undergoing fluidization. The branch C in
Figure 3a with strong fields is selected to demonstrate the evolution of
the bed internal structure since under these conditions the top section
is fixed that results in an unchanged bed depth even though the gas flow
increases. Besides, in accordance with the phase diagrams in Figure 3
the corresponding field intensities allow to achieve spout formation and
consequent jet spouting regime.
[FIGURE 3 OMITTED]
Pressure Drop
The pressure drop across the bed undergoing fluidization is a
complex response of the particulate system to gas flow and to the
additional conditions imposed by the vessel shape and the external field
applied (Figures 4a-d). Before further comments concerning pressure drop
curves obtained in this study we refer to a parallelism between the bed
behaviour described above and that observed in a non-magnetic spouted
bed represented schematically by the pressure drop curve of San Jose et
al. (1993) (see Figure 3c).
As a first step of the experimental program, pressure drop curves
of non-magnetized beds were measured with variations in the bed weight
as a process parameter. The plots in Figure 4a are typical of spouted
beds but the only new feature is that the shallow bed of 1 kg exhibits
the highest pressure drop. This could be simply explained by the fact
that with a cone of 15[degrees] a bed of 1 kg particles practically is
packed close to the flow entrance. This section of the vessel allows
high superficial gas velocities and higher flow dissipation rates.
Besides, almost the entire bed cross section is subjected to this
"high velocity" flow in contrast to the deeper beds (of 2 and
3 kg) where with increase of the bed depth the greater part of the
annulus is subjected to lower superficial velocities than those in the
central bed section. As a result, high pressure drops represent the bed
reaction to the gas flow.
In the context of the previous comments, we would mention that
these results agree with the non-magnetic experiments of Bacelos et al.
(2007). More precisely, similar changes in bed pressure drop with
variations in bed height and interparticle forces strongly indicate a
redistribution of the gas flow between the central part and the annulus.
That is, the greater part of the gas flow passes through the axial zone
of the bed while the lateral sections (the annulus) are less penetrated
by the flow.
[FIGURE 4 OMITTED]
The above-mentioned behaviour is easily detectable from the
pressure drop curves when different field intensities are applied
(Figure 4d). High field intensities result in stable interparticle
contacts, low bed mobility and low gas flow through the annulus (i.e.,
high gas flow through the central part and the channel). In fact, this
situation resembles the experiments of Bacelos et al. (2007) where the
interparticle contacts of glass spheres are stabilized by addition of
glycerol to the bed.
Referring to the present situation, at low (H < 5 kA/m) and high
field intensities (H > 15 kA/m), the pressure drop curve indirectly
reveals that the flow path is preferentially through the bed central
zone. At moderate fields (7-15 kA/m) the larger particles exhibit lower
pressure drops than the finer ones (Figure 4b). However, the increase in
the field intensity reduces the differences (Figures 4c and d) even
though the material's properties are almost similar (KM-1 is an
artificial magnetite with promoters addressing ammonia synthesis that
yields a reduction in its magnetization--see the last column of Table
1).
With high field intensities, beyond the maximum, corresponding to
[Q.sub.e] and the sharp decrease in the pressure drop curve, all the
beds exhibit almost a flow-independent behaviour. Explicitly, in spite
of the oscillations of the pressure drop it varies around an almost
constant value that is a result of the creation of a stabilized bed
section (MSB) or a stabilized "hat" above the fluidized bottom
section. This effect seems strange, but has a simple explanation,
namely:
(i) The stabilized bed section (denoted as MSB in Figure 3a), or
the "hat," is a fixed bed structure connected in a series with
the bottom fluidized bed. The fixed "hat" structure, for
instance, should exhibit increasing pressure drop if its depth remains
unchanged with increase in the gas flow rate.
(ii) However, the "hat" depth decreases (the MSB depth
too) with increase in the gas flow since the flow swirls and fluidized
particles detach particles from its bottom. The pressure loss in the
"hat" is almost constant due to its decreasing depth with
increasing flow rate.
(iii) The fluidized bed at the bottom exhibits an almost constant
pressure drop too, that finally yields an almost fluid flow-independent
pressure drop across the entire bed.
This simple mechanistic model explains the almost horizontal
section of the pressure drop curves since the pressure drop across the
bed is strongly related to the variations in the bed porosity (the
annular bed porosity) as it is discussed next.
Bed Expansion (Annular Bed Porosity)
The bed porosity [[epsilon].sub.a], known as annular porosity was
calculated through a modification (1b) of the Bacelos et al. (2007)
Equation (1a), that is:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (1a)
where [D.sub.L] = [D.sub.b] + 2[h.sub.b] tan([alpha]/2) with
interparticle forces created by liquid ([v.sub.L]) supplied permanently
to the particle bed.
In absence of a liquid ([v.sub.L] = 0) and with [h.sub.b] =
[h.sub.L] cos [alpha] Equation (1a) reads:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (1b)
with [D.sub.L] = [D.sub.b] + 2[h.sub.L] sin([alpha]/2).
Obviously, this is a formalistic approach trying to represent the
present data by exiting models of non-magnetic spouted beds and to
demonstrate how the magnetic field modifies the bed expansion (porosity
variation) profiles.
The plots in Figure 5 reveal a raise in the bed height with
increase in the field intensity irrespective of the solids inventory in
the vessel and the particle size. The bed height (bed porosities)
attained with increase in the gas flow rate is strongly attributed to
the magnetic interparticle forces and the fled orientations. In general,
the experimental results indicate increase in the maximum attainable bed
porosity from an average 0.65 in absence of a field up to 0.7-0.8 with
field assistance. The bed expansion described in the previous section is
not homogeneous. Hence, the evolution of the bed internal structure,
that is, bottom fluidization, formation of a stabilized "hat,"
internal fountain, etc., affects the porosity curves. In general, the
plateaux in the [epsilon] - f (Q) curves (see inset in Figure 5)
correspond to the formation of stabilized "hats" and
developing fluidization with internal fountain, as it is commented next.
A comparison of a pressure drop curve and its corresponding
porosity evolution with increase in gas flow rate (Figure 6) points to
the link between the bed structure and its response to the fluid flow,
that is, the pressure drop. It is obvious that the pressure drops attain
maximum within the flow range corresponding to the first plateaux of the
porosity curve. The second plateaux match the region of completely
developed spouted beds at high gas flow rates with almost independent
pressure drop--see, for example, Figures 4d and 5d. In this context, the
common articles on spouted beds (Asenjo et al., 1977; Deiva et al.,
1996; Jing et al., 2000) following the ideas of Kwauk (1992), try to
calculate the pressure drop by means of modifications of the
Ergun's equation with assumption of homogeneous flow distribution
across the bed cross-sections at all levels: from the bottom to the top
surface. However, we suggest that such equations are inadequate to the
physical situation in MATB since the bed structure is generally
heterogeneous. Nevertheless, this calls for creation of adequate models
that is beyond the scope of the present article.
Critical Values at Transition Points
Onset of stabilized bed or fluidization at the vessel bottom
The phase diagrams are useful tools enabling the presentation of
complex phenomena by macroscopic variables such as Q and H. However,
from purely fluidization standpoint we are interested in critical values
of both the gas flow and the pressure drop that mark the transitions
between the regimes. Following the basic rules in the area of spouted
beds (Mathur and Epstein, 1974) we are interested in the maximum
pressure attainable as a function of the operating conditions. Figure 7
presents critical gas flow rates [Q.sub.e] corresponding to the maximum
pressure drop [DELTA][P.sub.e]. In summary, the increase in bed weight
and particle size yields higher values of [Q.sub.e] that are almost
independent of the field intensity.
The pressure drop [DELTA][[P.sub.e] corresponding to [Q.sub.e] is
affected by the field intensity and the solids inventory (Figures 8a and
b). At low field intensities the field enables a decrease in
[DELTA][[P.sub.e], while at moderate fields the data do not indicate a
clear tendency. With increase in the field strength and reaching
condition allowing formation of a fountain the value of
[DELTA][[P.sub.e] increases but some data obtained with KM-1 (500-613)
and Magnetite (200-315) show a field-independent behaviour.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
The critical gas flow [Q.sub.e] and the drop [DELTA][[P.sub.e] may
give more adequate information through the gas power dissipation rate
[N.sub.e] = [Q.sub.e] [DELTA][[P.sub.e] (see Figures 9a and b). The
energy required to start the bed deformation decreases as the field
intensity is increased that contrasts with the data obtained with
cylindrical vessels (Hristoy, 2002)--see the inset in Figure 9. This
could be attributed to the fact that field action orients the particles
along the field lines and makes the central zone of the bed (along its
axis of symmetry) more fragile (weaker) that enables easily fluidization
and channel formation. This tendency corresponds to field intensities
below 15 kA/m, while beyond the range of 12-15 kA/m the power
dissipation required to deform the bed increases parallel to field.
These tendencies correspond to different bed behaviours and regimes
beyond [Q.sub.e]. The declining branches of the [N.sub.e] = f (H) plots,
for instance, correspond to easily developing wave-like bubbling regimes
and to the absence of stable fountains. The rising branches correspond
to strong bed stabilization, formation of "hats" and
development of internal fountain. The internal fountain breaks the
stabilized structures located above it and certainly needs more power to
be dissipated in the bed.
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
Minimum spouting conditions
The onset of the spouting was commented with the analysis of the
bed behaviour and some information is provided by the phase diagrams.
Now, we address the velocity range bonded by [Q.sub.ms-U] and
[Q.sub.ms-S] and the conditions affecting it. With low solids in the
vessel and a shallow bed of l kg the range [Q.sub.ms-U] < [Q.sub.ms]
< [Q.sub.ms-S] is relatively wider (Figure l0a) than those exhibited
by the thicker beds of 2 and 3 kg (Figure 10b).
Irrespective of the amount of the solids in the beds the critical
flows ([Q.sub.ms-U] and [Q.sub.ms-S]) bounding the fountain development
increase parallel to the field intensity that is a logical effect of the
stabilizing mechanism of the induced interparticle forces. The increase
in [DELTA][Q.sub.ms] = [Q.sub.ms-U] - [Q.sub.ms-S] with increase in the
amount of solids in the vessel could be attributed to redistribution of
the gas flow between the central channel and the annulus with increase
in the bed depth; that is, with increase in the solids inventory (depth)
the gas passes preferentially through the central bed zone that
predetermines the spout formation as in the non-magnetic beds (Mathur
and Epstein, 1974). With a short bed (1 kg solids) the transition from
[Q.sub.ms-U] to [Q.sub.ms-s] is accompanied by large bubbles, plugging
and difficult to define flow structures.
[FIGURE 10 OMITTED]
[FIGURE 11 OMITTED]
The above comments address the behaviour of the magnetite particles
that are almost spherical due to their nature (derived from natural
magnetite sands). The KM-1 catalyst particles were obtained by crushing
and by a consequent attrition to get almost spherical shapes. The KM-1
catalyst particles exhibit behaviour similar to the shallow (1 kg)
magnetite bed. The previous logical explanation does not work here since
the initial bed depth does not vary. Nevertheless, we refer to the fact
that KM-1 has lower magnetic properties than the magnetite. In this
context, the creation of a flow structure corresponding to stable
spouting requires stronger fields and higher flow rates to be applied.
No further information could be extracted from the present results and
this definitively calls more precise future experiments on the minimum
spouting conditions in MATB.
Further, addressing the stable spouting flow rate [Q.sub.ms-S] it
is practically equal for beds of 2 and 3 kg, while the shallow bed (1
kg) exhibits low values (Figure 11). The difference practically
disappears at high field intensities (H > 171kA) when it might
suggest that the magnetic interparticle forces dominate those generated
by the gravity.
In the context of the boundaries of the unstable fountain
formation, the data representing the pressure drop at the [Q.sub.ms-U]
and [Q.sub.ms-S] are quite illustrative of the simultaneous effect of
both the field intensity and the solids inventory. In general, the
higher fields, the lower pressure drops at the minimum spouting points.
The differences in the pressure drop across the central channel (it
might suggest that the almost entire flow passes through it) that
characterize the low field range (H < 10 kA/m) practically disappear
with increase in the field intensity irrespective of the solids
inventory (Figure 12a). The KM-1 particles (Figure 12b) exhibit sharper
decrease in the pressure drop at [Q.sub.ms-U] and [Q.sub.ms-S] at H <
5 kA/m. The almost smooth plots at H > 10 kA/m, in contrast to the
plots in Figure 12a, have no logical explanations at this moment that
also calls for future experiments focused on the minimum spouting
conditions.
[FIGURE 12 OMITTED]
Pressure Drop Hysteresis and Results Thereof
Normal and abnormal cycles
The fluidization experiments provide not only visual observations
of the bed behaviour but also records of pressure drop curves that are
commonly used to analyze the bed behaviour as it has been done earlier
in this work. Further, the common pressure drop curve of a spouted bed,
irrespective of the bed geometry has several characteristic points that
distinguish the regimes and the critical gas velocities. The pressure
drop is a response of the bed to the fluid flow and every physical
phenomenon occurring in the bed will affect the shape of the [DELTA]P-U
curve. Two types of pressure drop hysteresis cycles were obtained: (1)
"Normal" cycles with de-fluidization curves located below the
branches corresponding to the increasing gas flows (see Figure 13). This
is the common type of pressure drop curves known from the nonmagnetic
spouted beds. The intersection of the branches defines the points
denoted as [U.sub.H] (the subscript H means "hysteresis") that
might be the minimum fluidization point or the minimum spouting point
depending on the field intensity applied. (2) "abnormal"
cycles with de-fluidization branches located above than those
corresponding to increasing gas flow (see Figure 14). These cycles were
observed with Magnetite (200-315) only. The braches of the abnormal
cycles have common points (intersections) close to the minima of the
pressure drop curves corresponding to the increasing gas flow.
Characteristic points of the hysteresis cycles
Commonly the minimum spouting velocity is detected graphically from
the pressure drop curves (Wang et al., 2004) either from the minimum in
the branch corresponding to increasing gas flow or from the hysteresis
cycle, that is, the intersection of the lines approximating the upward
and downward fluidization curves. Hence, it is of primary interest to
test this approach with pressure drop hysteresis cycles of magnetically
assisted tapered beds.
The common pressure drop curve (increasing flow) of a spouted bed
exhibits a minimum that in absence of a magnetic field or other
interparticle forces defines the minimum spouting point ([Q.sub.ms]). In
magnetically assisted beds such minima define sudden changes of the bed
hydraulic resistance that occur at the onset of fluidization (low field
intensities) or MSB at the top and fluidization at the vessel bottom.
Due to the delayed fluidization caused by magnetic field assistance the
system attains complete fluidization conditions at greater than
velocities than those defined by the minima. With the "normal
cycles" the intersections of the fluidization and de-fluidization
branches define UH close to the minimum fluidization or the minimum
spouting points. As a rule the values of [U.sub.H] are greater than the
critical velocities determined visually. Only two cycles in Figure 13
shows points of [U.sub.H] close to [Q.sub.mf] (Figure 13b) and
[Q.sub.ms] (Figure 13g).
The "abnormal" cycles have more than one intersection
points of the fluidization and de-fluidization branches. As mentioned
above the first one is close to the minimum of the fluidization branch
and as rule defines [U.sub.H] close to the onset of MSB with fluidized
bottom section (Figures 1d-f). The second intersection point is close to
the onset of fluidization (Figure 14b and f). The attempt to approximate
the sections of the hysteresis cycle by straight lines (see the dashed
lines in Figure 14) provides points denoted as [U.sub.H]-? since there
is no unique characteristic point defined by the straight lines. As a
rule the lines approximating the descending sections of the fluidization
curves and the fixed bed-sections of the de-fluidization branches define
[U.sub.H]-? points close to the minima. The plots in Figure 14a, c, e,
and f reveal that these are the minimum fluidization points. In two
other cases (Figure 14b and d) the intersections define [U.sub.H]-?
close to the onset of MSB.
The second idea, applied to the abnormal cycles, is to approximate
by straight lines the fluidization branch beyond the minimum and
corresponding to the de-fluidization curve (within the same velocity
range). The intersections of these straight lines are close to the
minimum fluidization point (Figures 14a and b) or the minimum spouting
point (Figure 14f). Only in the case of the cycle shown in Figure 14e
the both approaches define unique point [U.sub.H]. Further, this method
yields some strange results (Figures 14c and d) with intersections
located in the sections corresponding to the initial fixed beds that is
unrealistic. However, bearing in mind the inherent inexactness of the
graphical method it might suggest that these intersections are close to
the minima that, in fact, correspond to the results of the first method.
[FIGURE 13 OMITTED]
[FIGURE 14 OMITTED]
The comments in the previous sections concerning the results of the
graphical methods tested reveal that the magnetic field assistance
alternate the bed behaviour with respect to the nonmagnetic counterparts
and the approaches based on the treatment of pressure drop hysteresis
cycle should be re-evaluated. To this end, this conclusion envisages
more detailed studies within broad range magnetic particulate materials,
cones of different angles and field intensities that should provide
enough experimental data enabling application of graphical methods to
pressure drop hysteresis cycles. The graphical methods demonstrated
above only mark what happens but the thorough analysis is beyond the
scope of this work.
Dimensional Analysis and Data Correlation
Dimensional analysis--preliminarily thoughts
Non-magnetic background. Tapered fluidization involves more
geometrical characteristics of the bed in the group of variables that
certainly increase the number of dimensionless ratios, since all these
geometric characteristics have dimension of length L (m). Such ratios
exist practically in every article on spouted bed, and good examples
could be found elsewhere (Kmiec, 1980; Olazar et al., 1993; Jing et al.,
2000; Zhong et al., 2006). Most of them, with minor variations, repeat
the Mathur and Gishler (1955):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (2a)
The common approach is to correlate the bed geometry through
dimensionless ratios such as ([D.sub.B]/[D.sub.c] and
([h.sub.b]/[D.sub.c]) (Shirvanian and Calo, 2004) while the phase
properties are commonly expressed by the density ratio either as
([[rho].sub.s] - [[rho].sub.f])/[[rho].sub.f] or
[[rho].sub.s]/[[rho].sub.f] (Shirvanian and Calo, 2004; San et al.,
2006). This yields relationships as the following, for example (Wu et
al., 1987):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (2a)
Commonly the critical gas velocities ([U.sub.ms]) are normalized
(scaled) by [square root of (2g[h.sub.b])] (Wu et al., 1987; San et al.,
2006) as in (1b) or through the particle diameter based Reynolds number [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] =
[U.sub.ms][d.sub.p][[rho].sub.f]/[[eta].sub.f] (Aravinth and Murugesan,
1997; Mgalhaes and Pinho, 2006) with [U.sub.ms] defined by the
cross-sectional area of the gas inlet orifice. In other cases
dimensional correlations are used such as (Costa and Taranto, 2003):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (1c)
In this context the pressure drop correlations are either in
dimensional [DELTA][P.sub.max] (San et al., 2006) or dimensionless form
[DELTA][P.sub.max]/[[rho].sub.b]g[h.sub.b] (Costa and Taranto, 2003)
with bed height used as a length scale.
The common dimensionless groups (see (1a)-(1c)) are the
particle-diameter-based Reynolds number [MATHEMATICAL EXPRESSION NOT
REPRODUCIBLE IN ASCII.] = [U.sub.ms][d.sub.p][[rho].sub.f]/[[eta].sub.f]
(Aravinth and Murugesan, 1997; Mgalhaes and Pinho, 2006) and the
Archimedes number (Costa and Taranto, 2003; San et al., 2006). In some
correlations (He et al., 1997), additional Froude number g[d.sub.p]/[U.sup.2.sub.0] is used.
This brief presentation is by far of completeness but it focuses
the attention on the common manners to correlate the spouted bed
characteristics irrespective to the vessel geometry. This figures the
existing non-magnetic background (correlations derived for non-magnetic
beds) that would enable easily to understand the dimensionless analysis
developed further in this work. Only two works (Hristov, 2006, 2007a)
have been devoted so far to dimensionless analysis of magnetic field
assisted fluidization. They deal mainly with process physics and
development of dimensionless groups rather then with vessel geometry
effects that inherently relate them to cylindrical fluidized beds. To
this end, in the case of non-magnetic conical beds, we especially refer
to the work of Bi (2004) where comprehensive collections of
dimensionless scaling relationships pertinent to [U.sub.ms] through
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] =
[U.sub.ms][d.sub.p][[rho].sub.f]/[[eta].sub.f] are thoroughly analyzed.
Problems with the basic units of the mechanical and magnetic
systems of units and the origin of the "pressure transform".
In the course of the development of our analysis we will avoid the
formalism of the classical dimensional analysis and will address a
simple approach referring to the physical basis (Hristov, 2006, 2007a).
This approach employs a preliminary transformation of the variables
provoked mainly by the fact that the magnetic system is based on four
units (M, L, T, I), while the mechanical counterpart has only three
units, namely (M, L, T). Besides, the dimension of the field intensity H
is (A/m) that does not match the dimensions of mechanical variables.
This problem could be easily avoided by formation of a new variable
representing the magnetic field action on the granular media, namely the
magnetic pressure, [P.sub.m] = [[micro].sub.0] MH (see Hristov, 2006,
2007a). In accordance with this approach, termed "pressure
transform" all the variables involved in the process of
fluidization can be grouped into a new set of variables with a unique
dimension of pressure (Pa). The text developed in Appendix demonstrates
the approach and gives the basic rules of building-up dimensionless
correlations addressing the physical basis of the phenomena.
Power-law: data correlations
Two basic characteristics of fluidized system under consideration
were correlated to experimental data: pressure drop and fluid velocity
at the minimum spouting point. In accordance with the general
expressions developed in the Appendix, the following relationships were
used:
Pressure drop:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (3a)
or in a dimensional form as:
[DELTA] [equivalent to]
[[rho].sub.s]g[d.sub.p][f.sub.1](Ga;[Bo.sub.g-c];[DELTA][D.sub.bL]) (3b)
Fluid velocity (superficial velocity at the gas inlet orifice):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (4a)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (4b)
where [U.sub.f] = [U.sub.ms-S] = [Q.sub.ms-S]/[S.sub.b].
Examples of correlations developed are summarized in Tables 2, 3a
and 3b. For seek of clarity some additional comments addressing the
correlation equations will be developed next.
Data correlations--brief comments on the equations chosen
(1) Simple multiplication of two terms representing the
non-magnetic (Ga [DELTA][D.sub.bL]) and magnetic conditions
([A.sub.dp]-[B.sub.dp] [Bo.sub.g-m]) imposed to the bed is chosen to
create the data correlations, namely:
[DELTA][P.sub.ms]/[[rho].sub.s]g[d.sub.p] =
(Ga[DELTA][D.sub.bL])([A.sub.dp]-[B.sub.dp][Bo.sub.g-m] (5a)
and
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (5b)
The form (5b) is developed in Example 3 of Appendix and tries to
convert (5a) into an expression more familiar to the people dealing with
non-magnetic spouted beds (see Equations (1a)-(1c)).
(2) The form of Equations (5a) and (5b) chosen in this work do not
satisfy exactly the rules of the dimensionless analysis (Kline, 1965;
Barrenblatt, 1996) teaching that, for example, the correct form is a
product of power-law terms, namely:
[DELTA][P.sub.ms]/[[rho].sub.s]g[d.sub.p] =
m[(Ga).sup.[alpha]][([DELTA][D.sub.bL]).sup.[beta]][([Bo.sub.g-m]).sup.y] (6)
with m, [alpha], [beta], [gamma] determined through scaling to
experimental data.
Misinterpretations, however, might appear with the extremes
pertinent to the Bond number rather mathematically than physically,
namely: (1) with [Bo.sub.g-m] [right arrow] 0 (cohesionless particles)
that gives ([DELTA][P.sub.ms]/[[rho].sub.s]g[d.sub.p]) [right arrow] 0
as well as with (2) with [Bo.sub.g-m] [right arro] [infinity] (too
sticky particles that is impossible to fluidize) yielding
([DELTA][P.sub.ms]/[[rho].sub.s]g[d.sub.p]) [right arrow] [infinity]. In
order to avoid such misreading of (6) the form of Equations (5a) and
(5b) was chosen. Some reasonable standpoints supporting this approach
are:
* First of all, the Bond number [Bo.sub.g-m] never goes to zero
since even though in absence of magnetic fields there is a residual
particle magnetization. The extreme [Bo.sub.g-m] [right arrow] 0, in
fact, means that the induced magnetic cohesion is negligible with
respect to the gravity effects on the interparticle contacts.
* Further, the extreme [Bo.sub.g-m] [right arrow] [infinity] is a
mathematical boundary than a physical one because the upper field
intensity limit is imposed by the magnetization at saturation [M.sub.s]
of the particle material.
* Last, for real materials employed by magnetically assisted
fluidization (Hristov, 2002) the Bond number (Hristov, 2006; Valverde
and Castellanos, 2007) gets finite values. In this context, the classic
form of data correlation (6) will never attain these extremes and the
data can easily be correlated through it providing numerous values of
exponents. However, for seek of clarity of the physical explanations in
this first appearing article on MFATB we present the data in the forms
((5a) and (5b)) which clearly separate the magnetic and the nonmagnetic
effects on the bed behaviour. Besides, we especially accept the
exponents [alpha], [beta] and the pre-factor m equal to 1 that enable to
concentrates all magnetic effects in the term [([Bo.sub.g-m]).sup.y]
represented as linear approximation, namely [([Bo.sub.g-m]).sup.y]
[approximately equal to] ([A.sub.dp] - [B.sub.dp][Bo.sub.g-m]).
* An alternative scaling was performed (see Table 3b) in the form
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (7a)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (7b)
This form avoids misinterpretations occurring with the linear
approximation at [Bo.sub.g-m] [right arrow] 0 where unreasonable
negative terms appear as result of the regression analysis performed.
Data processing performed with a part of the experimental results
yields equations summarized in Tables 2, 3a and 3b. The exponential approximation, for instance, of the minimum spouting velocity
[U.sub.ms-s] is more realistic then the linear equation with negative
terms that enables only to fit the numerical data but is illogical.
Further analysis of dimensional analysis and data approximation problems
are available elsewhere (Hristov, in preparation).
DISCUSSIONS
This work draws a new idea to perform magnetically assisted
fluidization in conical vessels unlike the common practice (over more
than 45 years after the first work of Filippov (1961)) to employ
cylindrical vessels (Hristov, 2002, 2003a,b, 2004, 2006). Analysis was
drawn through the entire text but in this brief discussion we address
some key points, among them:
(1) Parallelism with non-magnetic beds.
(2) Dimensional analysis applied.
(3) Trends and new problems.
(4) Reasonable application of magnetically assisted fluidization in
tapered vessels.
Parallelism in Bed Behaviour with Non-Magnetic Counterparts
Addressing the stage in the fluidization development, we have to
mention that similar flow structures have been observed with scrap-wood
particles of different size and humidity (Leslous et al., 2004), for
example. First, the particle humidity (causing interparticle forces) has
an effect similar to that one imposed by the increasing field intensity
and leads to increasing pressure drop. Besides, the slugging observed
fine particles with low humidity (low degree of interparticle forces)
exactly corresponds to the wave-like regime observed with low field
intensities creating unstable particle aggregates. In this context, the
increase in particle size of rather dry particles (Leslous et al., 2004)
results in a spouted bed tapped by a fixed bed. That is, the increase in
the interparticle dry friction and particle size delays the development
of the spout towards the bed surface. These effects are similar to those
observed with increased interparticle magnetic forces and formation of
aggregates.
In the context of interparticle forces effects on the bed behaviour
we have to mention too that progressive development of internal spouts
and top located "caps" (the term "hats" is used
here) have been observed in liquid-fluidized (Peng and Fan, 1997) and
gas-fluidized (Wang et al., 2005) non-magnetic beds due to strong
particle interlocking (term used by Wang et al. (2005)) that in general
is equivalent to performance of interparticle forces.
These short comments on the parallelism draw only some basic lines
of similarity but further experimental work has to be done in that
direction. This is a problem of the future and depends mainly on the way
how the idea of MFATB will be accepted by the people working on
fluidization in interdisciplinary areas. Some ideas are drawn next but
the main development of MFATB fluidization needs a lot of intuition,
imagination and a real physical analysis of problems that might be
solved through this technique.
Dimensional Analysis Applied
The dimensional analysis applied to the MFATB data does not follow
the classical rules and originates in two recent works (Hristov, 2006,
2007a) but has a starting point in the scaling of differential equations
similar to those used by He et al. (1997), for example. Scaling terms of
a certain differential equation and consequent non-dimensionalization
procedures generate dimensionless groups as pre-factors of dimensional
terms of order of unity that is the mathematical side of the coin. But
physically, in fact, these operations simply mean the comparison of
effects of physical fluxes entering an elementary volume of the medium.
In cases where no developed mathematical models exist, but with a clear
standpoint about the main factors affecting the system of interest, the
definition of the physical fields contributing the process is the
primary step. Then, by simple definition of the surface forces acting on
it, in fact, we define the scales of these fluxes that exactly
correspond to the initial step of the scaling procedure of differential
equations. This idea is sketched in Example 4 of Appendix. The idea of
the "pressure transform" is simple and addresses the useful
fact that applying the classical dimensional analysis we might forget
some variables or to use some inadequate ones. However, we can never
forget the physical fields (gravity, fluid flow, cohesion, magnetic, or
electric) and their fluxes expressed by the surface forces caused by
their action on the system if the preliminary analysis is correct. The
approach is quite fruitful in interdisciplinary areas of research with
complicated cross-field effects and undeveloped models, as it was
demonstrated by Hristov (2006, 2007a) and the present work.
Graphical Methods Applied
The results on magnetic field assisted tapered fluidization raised
too many questions (Bi, 2008) concerning the application of the
well-known methods from non-magnetic beds to the new system. In this
context, the present report defines that with "normal"
hysteresis cycles the classical graphical method to determine the
minimum spouting velocity (Wang et al., 2004) provides results that
match the onset of MSB, the minimum fluidization point and, to some
extent, the minimum spouting point. These deviations are mainly
attributed to the altered bed behaviour imposed by the field induced
anisotropy in particle arrangement and the strong particle aggregation
with increase in the field intensity. The graph ical method applied to
the "abnormal" hysteresis cycles provides points close to the
minima of the fluidization branches of the fluidization curves.
The test to apply the classical graphical method to the hysteresis
cycles of MFATB is only a "shoot" to the new system with an
old weapon that clearly indicates that this could be done with caution
since the results might be strange or inspected. However, these
challenging problems call new experiments and further deep analyses.
Trends and Unsolved Problems
1. Fluidization with magnetization LAST mode.
2. Axial field application.
3. Fluidization of admixture both for stabilization and segregation
studies with different magnetization modes (FIRST, LAST, or ON-OFF)
(Hristov, 2002, 2003b).
4. Drying of wet non-magnetic materials since the magnetic
materials may absorb electromagnetic energy supplied by external
electromagnetic fields superimposed to the stabilizing DC field.
5. Liquid-solid and three-phase fluidization as a counterpart
version of the existing practice to amply cylindrical columns only
(Hristov, 2002, 2003b, 2006).
6. Dimensional analysis, modelling and scale-up.
Reasonable Applications of MFATBs--Some Suggestions
The basic data concerning the bed behaviour with magnetization
FIRST mode allow envisaging some applications of MFATB, among them:
1. Separation of magnetic and non-magnetic particles in both batch
and continuous modes utilizing the high velocity range with well formed
central channel and stable particle circulation: the magnetic grains
remain in the vessel while the non-magnetic ones are entrained by the
air flow.
2. Deep bed filters for particle capture from dusty gases employing
various regimes emerging with increase in gas velocity, among them:
* Homogeneous bed expansion with MSB regime and gas velocities
greater that those existing in cylindrical beds under the same condition
imposed by the field.
* Regime with a magnetically stabilized HAT and a bottom fluidized
section as a promising combination of two filter sections connected in a
series.
3. Heat transfer devices for low temperature gases operating below
the Currie point of the magnetic material employed. In this context, the
heat transfer might be combined with deep bed filter application
mentioned above.
4. Bioreactors (L-S or G-L-S) with cells or enzymes immobilized on
magnetic supports since both the variable vessel cross-section and the
induced magnetic cohesion might be a suitable combination avoiding many
problems in such devices (Hristov and Ivanova, 1999; Hristov, 2006).
These are only ideas based on the current status of the magnetic
field assisted fluidization (Hristov, 2002, 2003a,b, 2004, 2006, 2007b)
and might be expected that articles with new results will appear soon.
CONCLUSIONS
The article presents first experimental results on magnetically
assisted gas-solid fluidization in a tapered bed in presence of an
external transverse magnetic field. This is a novel branch in the
magnetically assisted fluidization and some principle results will be
outlined, among them:
* The bed behaviour is controlled by the intensity of the external
magnetic field. The principle process variables such as bed depth, field
intensity, particle size, cone angle were detected. Phase diagrams
similar to those used to describe cylindrical beds were created with
definition of new critical velocities separating the regimes.
* The field intensity increase yields increase in all critical
velocities such as: minimum velocity of bed expansion, minimum spouting
velocity known from the non-magnetic fluidization.
* The pressure drop hysteresis cycles of magnetically assisted
tapered beds might "normal" as the classic one observed with
non-magnetic beds or "abnormal" with inverted location of the
fluidization and de-fluidization branches.
* The graphical methods commonly applied to pressure drop
hysteresis cycles of non-magnetic beds are valid for "normal"
cycles of MATB too, but defines either the onset of MSB or points that
do not match the visual detected minimum fluidization or minimum
spouting points. Additional graphical approach applied to
"abnormal" pressure drop cycles provides points close to the
minima of the fluidization branches of the fluidization curves.
* A detailed analysis and parallelism to behaviour exhibited by
non-magnetic spouted beds of cohesive particles were performed.
Similarly to the non-magnetic counterparts, the increases in bed weight
and particle size yields increase in the maximum pressure drop. The
external magnetic field augments the maximum pressure drop exhibited by
the bed before the fluidization onset but the tendency with respect to
the minimum spouting pressure drop is just the opposite.
* Dimensional analysis utilizing a "pressure transform"
of the initial set of process variables is applied to develop the
principle scaling equations. Samples providing scaling equations
concerning both the pressure drop and gas velocity at the minimum
spouting point are developed.
NOMENCLATURE
[A.sub.dp], [A.sub.hb] dimensionless coefficients
Ar = [d.sup.3.sub.p]
[[rho].sub.f]
([[rho].sub.s]-
[[rho].sub.f])g/
[[eta].sup.2.sub.f] Archimedes number
[B.sub.dp], [B.sub.hb] dimensionless coefficients
[Bo.sub.g-c] =
[P.sub.c]/[P.sub.g]
= [P.sub.c]/
[[rho].sub.s] Bond number of granular materials with a
g[d.sub.p] natural cohesion
[Bo.sub.g-m] =
([[micro].sub.0]MH)/
[[rho].sub.s]
g[d.sub.p] magnetic Bond number for granular materials
[D.sub.b] diameter of the flow entrance (denoted also
as [D.sub.i] in the equations summarized in
Table 2) (m)
[D.sub.c] column diameter (diameter of the cylindrical
section above the cone) (m)
[D.sub.t] tube diameter (diameter of the cylindrical
section above the cone)--a symbol used in
the equations summarized in Table 2 (m)
[DELTA][D.sub.bL] dimensionless ratio of bed geometric
characteristics defined by Equation (A-2c)
[d.sub.p] particle diameter (m)
[MATHEMATICAL
EXPRESSION NOT
REPRORUCIBLE IN dimensionless pre-factor in exponential
ASCCII.] data correlation (Equations (7a) and (7b))
Fr = [U.sup.2.sub.f]/
g[d.sub.p] particle diameter defined Froude number
Ga = ([d.sup.3.sub.p]
g[[rho].sub.f])/
[[eta].sup.2.sub.f] Galileo number
[G.sub.SCR] solids circulation rate (kg/[m.sup.2]s)
g gravity acceleration (9.81 [m.sup.2]/s)
H magnetic field intensity (A/m)
[g.sub.b] bed height (m)
[h.sub.b0] initial bed height (m)
[h.sub.L] bed length at the wall (see Figure 1) (m)
K dimensionless coefficient (see the equation
of Wu et al. (1987) in Table 2)
[K.sub.sf] exchange coefficient defined by Gidaspow
(1994) and related to the Reynolds number
[K.sub.dp] dimensionless exponent in data correlations
(Equations (7a) and (7b))
M magnetization (A/m)
[M.sub.s] magnetization at saturation (A/m)
[M.sub.p] mass of particles charged into the vessel(kg)
N = [DELTA]PQ gas flow power dissipated in the bed (W)
[N.sub.e] = [DELTA] gas flow power dissipation in the bed
[P.sub.e][Q.sub.e] required to deform the bed body (W)
[P.sub.c] cohesion (Pa)
[P.sub.g] =
[[rho].sub.s] gravity pressure peer unit surface of
g[d.sub.p] interparticle contacts (Pa)
[DELTA]P pressure drop (Pa)
[DELTA][P.sub.e] pressure drop at the onset of bed expansion,
that is, the maximum pressure drop attainable
before the bed expansion onset (Pa)
Q volumetric gas flow rate ([m.sup.3]/s)
[Q.sub.e] volumetric gas flow rate at the onset of
initial bed expansion ([m.sup.3]/s)
[Q.sub.mf-1] volumetric gas flow rate at the fluidization
onset in the bottom part of the bed
([m.sup.3]/s)
[R.sub.ep] =
[[rho].sub.f]
[d.sub.p][U.sub.f]
/[[eta].sub.f] particle Reynolds number
[S.sub.b] = [pi] cross-section area of the gas inlet
[D.sup.2.sub.b]/4 orifice ([m.sup.2])
U superficial gas velocity (denoted also as
[U.sub.f] or [U.sub.o]--see the text) (m/s)
[U.sub.mf] minimum fluidization velocity (depends on
the cross-section of the bed specified) (m/s)
[U.sub.ms] minimum spouting velocity (depends on the
cross-section of the bed specified) (m/s)
[Q.sub.mf-2] volumetric gas flow rate at the fluidization
onset over the entire bed ([m.sup.3]/s)
[Q.sub.ms-U] volumetric gas flow rate at the minimum
spouting (unstable) point ([m.sup.3]/s)
[Q.sub.ms-S] volumetric gas flow rate at the minimum
spouting (stable) points ([m.sup.3]/s)
[v.sub.L] liquid volume added to the bed in the
experiments of Bacelos et al. (2007)
([m.sup.3])
[v.sub.p] =
[M.sub.p]/
[[rho].sub.s] volume occupied by the solids ([m.sup.3])
[V.sub.bed] = ([pi]
[h.sub.bo]/12)
([D.sup.2.sub.L]
+ [D.sub.L][D.sub.b]
+ [D.sup.2.sub.b])
-([m.sup.3]) particle bed volume
Greek Symbols
[alpha] cone angle ([degrees])
[epsilon] porosity
[[epsilon].sub.0] initial bed porosity
[[epsilon].sub.a] annular bed porosity
[[theta].sub.s] particle sphericity
[micro] magnetic permeability (Wb/A m) or (H/m)
[[micro].sub.0] magnetic permeability of the space (Wb/Am)
[phi] internal friction angle of particle phase (Equation
(A-6)) ([degrees])
[[eta].sub.f] fluid dynamic viscosity (denoted also as
[eta] for simplicity of the expressions)
(Pa s)
[upsilon] fluid kinematic viscosity ([m.sup.2]/s)
[[rho].sub.f] fluid density (kg/[m.sup.3])
[[rho].sub.g] gas density (kg/[m.sup.3])
[[rho].sub.s] solid particle density (kg/[m.sup.3])
[tau] shear-stress in a liquid (Pa)
Subscripts
f fluid
G gas
max maximum
ms minimum spouting
p particle
p-ms a particle diameter related value at
the minimum spouting conditions
s solids
Abbreviations
MFATB magnetic field assisted spouted bed
MASB magnetically assisted spouted bed
APPENDIX A: DIMENSIONAL ANALYSIS--A RATIONALE OF THE PRESSURE
TRANSFORM APPROACH
Tapered fluidization involves more geometrical characteristics of
the bed in the group of variables than cylindrical bed. This certainly
increases the number of dimensionless ratios, since all geometric
characteristics of both the vessel and the bed have dimension of length
L (m). Such ratios exist practically in every article on spouted bed,
and good examples could be found elsewhere (Kmiec, 1980; Olazar et al.,
1993; Jing et al., 2000, Zhong et al., 2006). Most of them, with minor
variations, repeat the Mathur-Gishler (MG) correlation (see (la) and
(lc)) for the minimum spouting velocity and others concerning the
maximum pressure drop.
In the course of the development of our analysis we will avoid the
formalism of the classical dimensional analysis and will treat the
physical basis (Hristov, 2006, 2007a) of the phenomena. This approach
employs a preliminary transform of the variables involved in the
dimensional analysis provoked mainly by the fact that the magnetic
systems is based on four units (M, L, T, I), while the mechanical
counterpart has only three units, namely (M, L, T). Besides, the
dimension of the field intensity H is (A/m) that does not match the
dimensions of mechanical variables. This problem could easily be avoided
by the formation of a new variable representing the magnetic field
action on the granular media, namely the magnetic pressure, [P.sub.m] =
[[micro].sub.0] MH (see Hristov, 2006, 2007a). In accordance with this
approach, termed "pressure transform," all the variables
involved in the process of fluidization can be grouped into a set of new
variables with a unique dimension of pressure (Pa) (Hristov, 2006,
2007a).
Let us see how dimensionless groups emerge through a transformation
of the initial set of variables into a new set having homogeneous
dimensions of pressure, that is, N/[m.sup.2] = Pa.
The initial variables pertinent to the magnetically assisted
tapered fluidization and provided by the principle sub-systems forming
the fluidized bed are as it follows:
The fluid (gas) participates with: [[rho].sub.g], [[eta].sub.f],
[U.sub.f], g, [DELTA]P (A-1a)
The solids participate with: [[rho].sub.s], [d.sub.p], g,
[h.sub.b0], [P.sub.c] (cohesion) (A-1b)
The vessel geometry provides: [D.sub.B], [D.sub.L], tg ([alpha]/2)
(A-1c)
The "pressure transform" means that we can read the set
of variables as:
Fluid: [[rho].sub.f] [U.sup.2.sub.f] = [P.sub.U], [[eta].sub.f],g,
[DELTA]P (A-2a)
Solids: [P.sub.g] = [[rho].sub.s]g[d.sub.p] (gravity pressure),
[P.sub.c] (cohesion) (A-2b)
Vessel: [DELTA][D.sub.bL] = [D.sub.L] - [D.sub.b]/2[h.sub.b0] = tg
([alpha]/2) (A-2c)
Besides, since the tapered bed hydrodynamics has neither a
prescribed velocity scale nor a length scale due to its specific
geometry, the use of [U.sub.f] in a dimensionless group is a matter of
discussion. Using the particle diameter [d.sub.p] as a length scale the
fluid flow group provides [Re.sub.p] =
([[rho].sub.f][d.sub.p]/[[eta].sub.f]) [U.sub.f] and Fr =
[U.sup.2.sub.f]/g[d.sub.p] that are the Reynolds and Froude number,
respectively--pertinent to particle dynamics in the fluid flow. However,
both dimensionless groups employ the velocity [U.sub.f] that is hard to
be defined properly (see bellow) and we cannot use correctly these
dimensionless groups. To be precise, in cylindrical fluidized beds, for
instance, the fluid superficial velocity based on the tube diameter
which does not vary along the bed axis and can be used as a reliable
velocity scale; that allows to use both Fr and Re numbers based on it.
In a conical bed (and in all types of tapered vessels too), there is no
unique superficial velocity that might be used as a velocity scale since
the vessel cross-section varies along the bed axis. In most of the works
on spouted beds the superficial gas velocity at the gas inlet orifice is
used for correlations. However, we have to be aware that in this way we
refer to the fluid dynamics of the flow entrance orifice but not to
fluid-particle dynamics. The velocity in Fr and Re numbers is the
fluid-particle relative velocity, while the superficial velocity based
on the inlet vessel diameter (and Fr and Re based on it) is relevant to
the hydrodynamics of the entering fluid jet but not to fluid-particle
dynamics. The present work does not address this crucial point in the
modelling of spouted beds and the discussion is beyond the scope of its
topic. The comments just done only refer to the fact that Fr and Re
numbers are missing in the set of developed dimensionless groups and try
to explain what the physical reasoning leading to this standpoint is. If
physics is ignored and the superficial velocity is based mechanistically on the gas entrance diameter, the result is a huge amount of equations
pertinent to particular devices. In fact, this is a result of
formalistic creations (or of tradition, inertia or conventionalism, or
something similar) of correlations not based on the process physics. The
next paragraph explains how to avoid the problem through merging Fr and
Re numbers into one dimensionless group without [U.sub.f].
The problem with the unknown velocity scale, just commented above,
could be avoided in a classical manner by the definition of the Galileo
number [(Re).sup.2]/Fr = Ga =
[d.sup.3.sub.p]g[[rho].sub.f]/[[eta].sup.2.sub.f] immobilizing all
variables provided by the fluid flow field. This classical approach, for
instance, exists in many textbooks dealing with sedimentation or with
fluidization.
Therefore, now the new set of variables becomes:
Fluid: Ga = [d.sup.3.sub.p]g[[rho].sub.f]/[[eta].sup.2.sub.f] and
[DELTA]P (A-3a)
Solids: [P.sub.g] = [[rho].sub.s]g[d.sub.p] (gravity pressure),
[P.sub.c] (cohesion) (A-3b)
Vessel: [DELTA][D.sub.bL] = [D.sub.L] - [D.sub.b]/2[h.sub.b0] = tg
([alpha]/2) (A-3c)
Using [P.sub.g] = [[rho].sub.s]g[d.sub.p] as a natural pressure
scale at the particle level scale (Hristov, 2007a) we get two
dimensionless ratios, namely [Bo.sub.g-c] = [P.sub.c]/[P.sub.g] that is
the granular Bond number (Hristov, 2006, 2007a) as a measure of the
stability of the interparticle contacts and the ratio [DELTA]P/[P.sub.g]
commonly used in fluidization to make the pressure drop dimensionless.
Therefore, the final set of variables becomes:
Fluid: Ga = [d.sup.3.sub.p]g[[rho].sub.f]/[[eta].sup.2.sub.f] and
[DELTA]P/[[rho].sub.s]g[d.sub.p] (A-4a)
Solids: [Bo.sub.g-c] = [P.sub.c]/[[rho].sub.s]g[d.sub.p] (A-4b)
Vessel: [DELTA][D.sub.bL] = [D.sub.L]-[D.sub.b]/2[h.sub.b0] = tg
([alpha]/2]) (A-4c)
The solids and the vessel "provide" the independent
variables while the fluid flow "generates," depended
variables, namely:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (A-5)
This is the basic dimensionless relationship in the case of
gas-fluidized beds with negligible buoyancy and significant
interparticle forces whatever is their nature--cohesion, capillarity,
electric, or magnetic. If the buoyancy takes place (liquid-solid
fluidization), the Archimedes force has to be accounted for by a simple
multiplication, that is:
Ga ([[rho].sub.s] - [[rho].sub.f]/[[rho].sub.f] =
[d.sup.3.sub.p][[rho].sub.f]([[rho].sub.s]-[[rho].sub.f])g/[[eta].sup.2.sub.s] = Ar
providing the Archimedes number. To this end, we have to recall
that although Ga and Ar are similar in their mathematical derivations
they have different physical meanings. Besides, in order to avoid
misunderstanding, we have to mention that the use of the Archimedes
number in the case of gas-fluidized beds is physically incorrect since
the buoyancy is negligible, albeit there is an astonishing plethora of
correlations doing that mechanistically. These comments address problems
beyond the scope of the present work but clarify why Ar does not appear
in the group of independent variables used in the correlations developed
in this analysis. To those of the readers who take care about the effect
of the density difference effect on the bed behaviour let see the
correlations ((A-8a)-(A8c)-Example 2) bellow where the ratio
[[rho].sub.s][[rho].sub.f] appears automatically in the left-side part
of the relationships. Brief comments complementing the above remarks are
available in Shirvanian and Calo (2004), where at
[[rho].sub.s][[rho].sub.f] the Archimedes number is presented as
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.]
To this end, the ratios such as [[rho].sub.s]/[[rho].sub.F] and
[h.sub.b]/[d.sub.p] or [D.sub.c]/[d.sub.p] frequently appearing in
literature (see Table 2 for examples) as results of dimensional analyses
performed clearly indicate that initial choices of scales were not well
designed. The use of the particle diameter as a length scale, for
example, is valid at the particle level only and cannot be used to make
dimensionless either the bed height [h.sub.b] or the column diameter
[D.sub.c], since at this macroscopic level it is insignificant.
The effect of the interparticle forces expressed by the Bond number
has to disappear from the group of independent variables if their
origins are cohesion, capillarity or electrostatic forces and the bed is
fluidized by liquid. It persists as an independent variable in the case
of magnetic interparticle forces only (in liquid-solid magnetically
assisted beds) through the magnetic bond number, [Bo.sub.g-m] =
[[micro].sub.0] MH/[[rho].sub.s]g[d.sub.p] (Hristov, 2006, 2007a;
Valverde and Castellanos, 2007) since the liquids do not affect the
magnetic interaction of the particles.
With dominating buoyancy the basic dimensionless relationship
pertinent to MFATB is:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (A-6)
with magnetization at saturation Ms is used in the nominator of
[Bo.sub.g-m], (Hristov, 2006, 2007a).
This general expression provides well-known equations concerning
spouted bed characteristics as it is exemplified next.
Example 1
Pressure drop at a critical point (minimum spouting or maximum
attainable pressure drop) in a dimensionless form is:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (A-7)
of coarse particles and negligible interparticle forces
Example 2
Since the bed is an obstacle to the fluid flow, the pressure drop
across it is proportional to [[rho].sub.f][U.sup.2.sub.f], that is
[DELTA]P [equivalent to] [[rho].sub.f] [U.sup.2.sub.f], we obtain from
(A7)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (A-8a)
This relationship can be read as:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (A-8b)
That with some algebraic manipulations provides the famous
Mathur-Gishler formula, namely:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (A-8c)
The second term in (A8c) can be expressed in various ways as a
function of the process parameters and bed geometry as it is illustrated
briefly by Equations (1a)-(1c).
Example 3 (the bed weight effects)
The analysis performed above omits the bed weight contribution to
the set of initial variables pertinent to the effect of the gravity (see
(A-1b)). This is a macroscopic effect in contrast to the microscopic gravity pressure [[rho].sub.s]g[d.sub.P]. The data reported in this work
clearly reveal that the pressure drop depends on the solids inventory in
the bed similar to non-magnetic spouted bed. If the bed weight G will be
included in the set of gravity-related variables, then the question is:
what is the surface S that might provide the macroscopic gravity
pressure, that is, how to define [P.sub.G] = (G/S)? The bed volume is
(Finlayson et al., 1997) [V.sub.bed] = ([pi] [h.sub.b0]/12)
([D.sup.2.sub.L] + [D.sub.L][D.sub.b] + [D.sup.2.sub.b]) that gives G =
[[rho].sub.s] (1-[[epsilon].sub.0])g[V.sub.bed] = [[rho].sub.s]
(1-[[epsilon].sub.0])g[h.sub.b0] ([pi]/12) ([D.sup.2.sub.L] +
[D.sub.L][D.sub.B] + [D.sup.2.sub.B]). Defining an effective surface as
S = ([pi]/12) ([D.sup.2.sub.L] + [D.sub.L][D.sub.B] + [D.sup.2.sub.B])
the bed weight per unit area is [P.sub.G] = G/S = [[rho].sub.s]
(1-[[epsilon].sub.0])g[h.sub.b0]. Then, the ratio of both gravity
pressure scales, that is, [P.sub.G] and [P.sub.g] yields:
[P.sub.G]/[P.sub.g] = [[rho].sub.s]
(1-[[epsilon].sub.0])g[h.sub.b0]/[[rho].sub.s]g[d.sub.p] =
(1-[[epsilon].sub.0]) ([h.sub.b0]/[d.sub.p]
This operation means that the gravity pressure at the microscopic
level is the basic pressure scale used for non-dimensionalization as it
was done earlier in this text. Finally, the scaling of the pressure drop
across the bed becomes (see (A5a)):
[DELTA]P/[P.sub.g] = [DELTA]P/[[rho].sub.s]G[d.sub.p]
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.]
Therefore, the scaling [DELTA]P/[[rho].sub.s]g[d.sub.p] is
determined by Ga, [DELTA][D.sub.BL], [h.sub.b0]/[d.sub.p] defining the
initial bed geometry and conditions, while [B.sub.og-m] is the unique
independent variable controlled by the external magnetic field. This
example demonstrates how the ratio [h.sub.b0]/[d.sub.p] emerges in the
group of dimensionless process variable through the accepted
"pressure transform approach." To this end, the ratio
([DELTA]P/[P.sub.g]) [([P.sub.G]/[P.sub.g]).sup.-1] simply gives
[DELTA]P/[[rho].sub.s]g[h.sub.b0] and (A-9a) takes the form:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (A-9b)
as in most of the non-magnetic correlations (see Equations
(1a)-(1c)) since (1-[[epsilon].sub.0]) is almost constant. Moreover, the
initial bed conditions expressed bed (1-[[epsilon].sub.0]) has to appear
in the prefactor of the power law in accordance with the rules of the
dimensional analysis (Kline, 1965; Barrenblatt, 1996), so the correct
form of the RHS of the second expression in (A-9b) is
(1-[[epsilon].sub.0])f(Ga;[Bo.sub.g-c]; [DELTA][D.sub.bL]).
Example 4: the physical meaning of the "pressure
transform"
Last but not least, the principle advantage of the pressure
transform approach (Hristov, 2006, 2007a) is its physical adequacy and
avoidance of formal algebraic calculations. By expression of the final
set of variables as surface forces (dimension of pressure Pa) we in fact
compare the significance of the fluxes of the physical fields acting on
an elementary volume of the system. Looking deeply at the physics, in
the case of fluid flow for example, the surface forces are proportional
to the fluxes of the convection and the diffusion momentum transfer.
That is, the fluid momentum flux transferred by convection is U
([[rho].sub.f]U) = [[rho].sub.f] [U.sup.2] with a pre-factor U of the
fluid dynamic pressure [[rho].sub.f] [U.sup.2], while its diffusion
counterpart is [tau] = - ([upsilon]/[d.sub.p]) ([[rho].sub.f]U)
(Newton's law), with a pre-factor [upsilon]/[d.sub.p] and [d.sub.p]
as a length scale at the particle level. The ratio of these fluxes, that
is the comparison of their significance, is the Reynolds number
[[rho].sub.f][U.sup.2]/[tau] = Re. Similarly the ratio of the convection
momentum flux U([[rho].sub.f]U) = [[rho].sub.f][U.sup.2] to that
generated by gravity related body forces [[rho].sub.f]g[d.sub.p] yields
the Froude number Fr = [U.sup.2]/g[d.sub.p].
REFERENCES
Anabtawi, M. Z., "Gas Holdup in Highly Viscous Liquids in
Gas-Liquid Spouted Beds," J. Chem. Eng. Jpn. 28, 684-688 (1995).
Aravinth, S. and T Murugesan, "A General Correlation for the
Minimum Spouting Velocity," Bioprocess Eng. 16, 289-293 (1997).
Asenjo, J. A., R. Munoz and D. L. Pyle, "On the Transition
from a Fixed to a Spouted Bed," Chem. Eng. Sci. 32, 109-117 (1977).
Bacelos, M. S., M. L. Passos and J. T. Freire, "Effect of
Interparticle Forces on the Conical Spouted Bed Behaviour of Wet
Particles with Size Distribution," Powder Technol. 174, 114-126
(2007).
Barrenblatt, G. L, "Self-Similarity and Intermediate
Asymptotics: Dimensional Analysis and Intermediate Asymptotics,"
Cambridge University Press, UK (1996).
Becker, H. A. and H. R. Salans, "Drying Wheat in a Spouted
Bed," Chem. Eng. Sci. 13, 97-112 (1960).
Bi, X., "A Discussion on Minimum Spout Velocity and Jet
Penetration Length," Can. J. Chem. Eng. 82, 4-10 (2004).
Bi, X., "Concerning the Pressure Drop Hysteresis Cycles of
MFATB," Private Communication (2008).
Charbel, A. L. T., G. Massarani and M. L. S. Passos, "Analysis
of Effective Solid Stresses in a Conical Spouted Bed," Braz. J.
Chem. Eng. 16, 433-449 (2004).
Costa, M. A. and O. P. Taranto, "Scale-Up and Spouting of
Two-Dimensional Beds," Can. J. Chem. Eng. 81, 264-267 (2003).
Deiva, V R., J. Chaouki and D. Klvana, "Fluidization of
Cryogels in a Conical Column," Powder Technol. 89, 179-186 (1996).
Depypere, F., J. G. Pieters and K. Dewettinck, "Expanded Bed
Height Determination in a Tapered Fluidized Bed Reactor," J. Food
Eng. 67, 353-359 (2005).
Devahastin, S., R. Tapaneyasin and A. Tansakul, "Hydrodynamic
Behaviour of a Jet Spouted Bed of Shrimp," J. Food Eng. 74, 345-353
(2006).
Erbil, A. C., "Prediction of the Fountain Heights in Fine
Particle Spouted Bed Systems," Turkish. J. Eng. Environ. Sci. 22,
47-55 (1998).
Feng, H. and J. Tang, "Microwave and Spouted Bed Drying of
Frozen Blueberries: The Effect of Drying and Pre-Treatment Methods on
Physical Properties and Retention of Flavour Volatiles," J. Food
Process. Preserv. 23, 463-479 (1998).
Filippov, M. V., "A Fluidized Bed of Ferromagnetic Particles
and the Action of a Magnetic Field on it," Troudii Inst. Fiziki
Latv. SSR 12, 215-236 (1961).
Finlayson, B. A., J. F. Davis, A. W Westerberg and Y. Yamashida,
"Chapter 3-Mathematics," In "Perry's Chemical
Engineering Handbook," 7th ed., R. H. Perry, D. W Green and J. O.
Maloney, Eds., McGaw-Hill, New York (1997), pp. 3-11.
Flamant, G., "Plasma Fluidized and Spouted Bed Reactors: An
Overview," Pure Appl. Chem. 66, 1231-1238 (1994).
Geldart, D., "Types of Gas Fluidization," Powder Technol.
7, 285-292 (1973).
Gidaspow, D., "Multipahse Flow and Fluidization,"
Academic Press, New York (1994).
He, Y. L., C. J. Lim and J. R. Grace, "Scale-up Studies of
Spouted Beds," Chem. Eng. Sci. 52, 329-339 (1997).
Hristov, J. Y., "Gas Fluidization of Ferromagnetic Granular
Materials in an External Magnetic Field," PhD Thesis, UCTM, Sofia,
Bulgaria (1994).
Hristov, J. Y., "Magnetic Field Assisted Fluidization--A
Unified Approach. Part 1. Fundamentals and Relevant Hydrodynamics,"
Rev. Chem. Eng. 18, 295-509 (2002).
Hristov, J. Y., "Magnetic Field Assisted Fluidization--A
Unified Approach. Part 2. Solids Batch Gas-Fluidized Beds: Versions and
Rheology" Rev. Chem. Eng. 19, 1-132 (2003a).
Hristov, J. Y., "Magnetic Field Assisted Fluidization--A
Unified Approach. Part 3. Heat Transfer-A Critical Re-Evaluation of the
Results," Rev. Chem. Eng. 19, 229-355 (2003b).
Hristov, J. Y., "Magnetic Field Assisted Fluidization--A
Unified Approach. Part 4. Moving Gas-Fluidized Beds," Rev. Chem.
Eng. 20, 377-550 (2004).
Hristov, J. Y., "External Loop Airlift with Magnetically
Controlled Liquid Circulation," Powder Technol. 149, 180-194
(2005).
Hristov, J. Y., "Magnetic Field Assisted Fluidization--A
Unified Approach. Part 5. A Hydrodynamic Treatise on Liquid-Solid
Fluidized Bed," Rev. Chem. Eng. 22, 195-377 (2006).
Hristov, J. Y., "Magnetic Field Assisted Fluidization:
Dimensional Analysis Addressing the Physical Basis," China
Particuol. 5, 103-110 (2007a).
Hristov, J. Y., "Magnetic Field Assisted Fluidization--A
Unified Approach. Part 6. Topics of Gas-Liquid-Solid Fluidized Bed
Hydrodynamics," Rev. Chem. Eng. 23, 373-526 (2007b).
Hristov, J. Y., "Magnetic Field Assisted Tapered G-S
Fluidization: Magnetically Assisted Gas-Solid Fluidization in a Tapered
Vessel: Further Results Concerning the Magnetization Last Mode with a
Pertinent Dimensional Analysis," (in preparation).
Hristov, J. Y. and V N. Ivanova, "Magnetic Field Assisted
Bioreactors," In "Recent Research Developments in Fermentation and Bio-Engineering," Vol. 2, SignPost Research, Trivandrum, India
(1999), pp. 41-95.
Jing, S., Q. Hu, J. Wang and Y. Jin, "Fluidization of Coarse
Particles in Gas-Solid Conical Beds," Chem. Eng. Process. 39,
379-387 (2000).
Jones, T. B., M. H. Morgan and P. W Dietz, "Magnetic Field
Coupled Spouted Bed System," US Patent 4 349 967 (1982).
Jono, K., H. Ichikawa, M. Miyamoto and Y. Fukumori, "A Review
of Particulate Design for Pharmaceutical Powders and Their Production by
Spouted Bed Coating," Powder Technol. 113, 269-277 (2000).
Jovanovic, G. N., T. Sornchamni, J. E. Atwater, J. R. Akse and R.
R. Wheeler Jr., "Magnetically Assisted Liquid-Solid Fluidization in
Normal and Microgravity Conditions: Experiment and Theory," Powder
Technol. 148, 80-91 (2004).
Khoshnoodi, M. and F. J. Weinberg, "Combustion in Spouted
Beds," Combust. Flame 33, 11-21 (1978).
Kline, S. J., "Similitude and Approximation Theory,"
McGraw-Hill, New York (1965).
Kmiec, A., "Hydrodynamics of Flow and Heat Transfer in Spouted
Beds," Chem. Eng. J. 51, 189-200 (1980).
Kwauk, M., "Fluidization. Idealized and Bubbleless with
Applications," Science Press/Ellis Horwood Beijing/New York (1992).
Leslous, A., A. Delebarre, P. Pre, S. Warlus and N. Zhang,
"Characterization and Selection of Materials for Air Biofiltration
in Fluidized Beds," Int. J. Chem. React. Eng. 2, Article A20
(2004).
Mathur, K. B. and N. Epstein, "Spouted Bed," Academic
Press, New York (1974).
Mathur, K. B. and P. E. Gishler, "A Technique for Contacting
Gases with Coarse Solid Particles," AIChE J. 1, 157-164 (1955).
Mgalhaes, A. and C. Pinho, "Pressure Drop in a Spouted Bed of
Cork Stoppers," Engenharia Termica (Theom. Eng.) 5, 30-35 (2006).
Olazar, M., M. J. San Jose, A. T. Aguayo, J. M. Arandes and J.
Bilbao, "Pressure Drop in Conical Spouted Bed," Chem. Eng. J.
51, 53-60 (1993).
Passos, M. L., G. Massarani, J. T. Freire and A. S. Mujumdar,
"Drying of Pastes in Spouted Beds of Inert Particles: Design
Criteria and Modeling," Drying Technol. 15, 605-624 (1997).
Passos, M. L. and A. S. Mujumdar, "Effect of Cohesive Forces
on Fluidized and Spouted Beds of Wet Particles," Powder Technol.
110, 222-238 (2000).
Penchev, I. P. and J. Y. Hristov, "Behaviour of Fluidized Beds
of Ferromagnetic Particles in an Axial Magnetic Field," Powder
Technol. 61, 103-118 (1990a).
Penchev, I. P. and J. Y. Hristov, "Fluidization of
Ferromagnetic Particles in a Transverse Magnetic Field," Powder
Technol. 62, 1-11 (1990b).
Peng, Y. and L. T. Fan, "Hydrodynamic Characteristics of
Fluidization in Liquid-Solid Tapered Beds," Chem. Eng. Sci. 52,
2277-2290 (1997).
Rosensweig, R. E., "Fluidization: Hydrodynamics Stabilization
with a Magnetic Field," Science 204, 57-60 (1979).
Salam, P. A. and S. C. Bhattacharya, "A Comparative Study of
Charcoal Gasification in Two Types of Spouted Bed Reactors," Energy
31, 228-243 (2006).
San Jose, M. J., M. Olazar, A. T. Aguayo, J. M. Arandes and X. X.
Bilbao, "Expansion of Spouted Bed in Conical Contactors,"
Chem. Eng. J. 51, 45-52 (1993).
Sau, D. C., S. Mohanty and K. C. Biswal, "Minimum Fluidization
Velocities and Maximum Bed Pressure Drops for Gas-Solid Tapered
Fluidized Beds," Chem. Eng. J. 118, 151-157 (2006).
Scott, C. D. and C. W Hancher, "Use of a Tapered Fluidized Bed
as a Continuous Bioreactor," Biotechnol. Bioeng. 18, 1393-1403
(1976).
Shirvanian, P. A. and J. M. Calo, "Hydrodynamic Scaling of
Rectangular Spouted Bed Vessel with a Draft Duct," Chem. Eng. J.
103, 29-34 (2004).
Tapaneyasin, R., S. Devahastin and A. Tansakul, "Drying
Methods and Quality of Shrimp Dried in a Jet-Spouted Bed Dryer," J.
Food Process Eng. 28, 35-52 (2005).
Trindade, A. L. G., M. L. Passos, E. F. Costa Jr. and E. C.
Biscaia, Jr., "The Effect of Interparticle Cohesive Forces on the
Simulation of Fluid Flow in Spout-Fluid Beds," Braz. J. Chem. Eng.
21, 113-125 (2004).
Valverde, J. M. and A. Castellanos, "Magnetic Field Assisted
Fluidization: A Modified Richardson-Zaki Equation," China
Particuol. 5, 61-70 (2007).
Wachiraphansakul, S. and S. Devahastin, "Drying Kinetics and
Quality of Okara Dried in a Jet Spouted Bed of Sorbent Particles,"
LWTFood Sci. Technol. 40, 207-219 (2007).
Wang, Z., X. Bi, C. J. Lim and P. Su, "Determination of
Minimum Spouting Velocities in Conical Spouted Beds," Can. J. Chem.
Eng. 82, 11-19 (2004).
Wang, Z., X. Bi and C. J. Lim, "Particle Interlocking in
Conical Spouted Beds," Chem. Eng. Sci. 60, 5276-5283 (2005).
Wu, S. W M., C. J. Lim and N. Epstein, "Hydrodynamics of
Spouted Beds at Elevated Temperatures," Chem. Eng. Commun. 62,
251-268 (1987).
Zhong, W, X. Chen and M. Zhang, "Hydrodynamics Characteristics
of Spout-Fluid Bed: Pressure Drop and Minimum Spouting/Spout-Fluidizing
Velocity," Chem. Eng. J. 118, 37-46 (2006).
Manuscript received December 17, 2007; revised manuscript received
February 14, 2008; accepted for publication February 22, 2008.
Jordan Hristov *
Department of Chemical Engineering, University of Chemical
Technology and Metallurgy, 1756 Sofia, 8 Kl. Ohridsky Blvd., Bulgaria
* Author to whom correspondence may be addressed. E-mail address:
jyhoa actm.eda; jordan.hristovaa mail.bg; http://hristov.com/jordan
Can. J. Chem. Eng. 86:470-492, 2008
[c] 2008 Canadian Society for Chemical Engineering DOI 10.1002/cjce.20046
Table 1. Materials used in the experiments.
Material Fraction Density Magnetization
([micro]m) (kg/[m.sup.3]) of saturation
Ms(kA/m) *
Magnetite ([Fe.sub.3] 200-315 5140 477.36
[O.sub.4]) sand 315-400
Ammonia catalyst 500-613 5100 236.34
KM-1, H. Topsoe 613-800
* See Penchev and Hristov (1990a) and Hristov (2002).
Table 2. Examples of data correlations through the scaling rules
developed in the present work and addressing the magnetic field
effect through the magnetic bond number.
Materials and Equation (Minimum Comments
bed conditions spouting pressure drop,
[DELTA][P.sub.ms])
[Fe.sub.3] [O.sub.4] P-a [DELTA][P.sub.ms]/ Particle
(315-400 [micro]m) [[rho].sub.s]g[d.sub.p] = diameter as
(Ga.[DELTA][D.sub.bL]) a length
([A.sub.dp] - [B.sub.dp] scale
[Bo.sub.g-m])
P-b [DELTA][P.sub.ms]/ Bed depth
[[rho].sub.s]g[h.sub.b0] = as a length
[(Ga.[DELTA][D.sub.bL]) scale
([A.sub.dp] - [B.sub.dp]
[Bo.sub.g-m])] x
([d.sub.p]/[h.sub.b0])
(1 - [[epsilon].sub.0])
G = 1 kg P 1-a [DELTA][P.sub.ms]/
[[rho].sub.s]g[d.sub.p] =
(Ga.[DELTA][D.sub.bL])
(18.176 - 0.0105
[Bo.sub.g-m])
R = 0.975; N = 7 data
points; SD = 1.584;
P = 2.347 x [10.sup.-4]
P 1-b [DELTA][P.sub.ms]/
[[rho].sub.s]g[h.sub.b0] =
[(Ga.[DELTA][D.sub.bL])
(18.176 - 0.0105
[Bo.sub.g-m])] x
(1.255.[10.sup.-3])
G = 2 kg P 2-a [DELTA][P.sub.ms]/
[[rho].sub.s]g[d.sub.p] =
(Ga.[DELTA][D.sub.bL])
(7.140 - 0.00381
[Bo.sub.g-m])
R = 0.7454; N = 6 data
points; SD = 1.881;
P = 0.089
P 2-b [DELTA][P.sub.ms]/
[[rho].sub.s]g[h.sub.b0] =
[(Ga.[DELTA][D.sub.bL])
(7.140 - 0.00381
[Bo.sub.g-m])] x
(0.865.[10.sup.-3])
G = 3 kg P 3-a [DELTA][P.sub.ms]/
[[rho].sub.s]g[d.sub.p] =
(Ga.[DELTA][D.sub.bL])
(3.611 - 0.00309
[Bo.sub.g-m])
R = 0.915; N = 5 data
points; SD = 0.4624;
P = 0.01128
P 3-b [DELTA][P.sub.ms]/
[[rho].sub.s]g[h.sub.b0] =
[(Ga.[DELTA][D.sub.bL])
(3.611 - 0.00309
[Bo.sub.g-m])] x
(0.721.[10.sup.-3])
Notes: (1) For sake of clarity of presentation the numerical values
of the term ([d.sub.p]/[h.sub.b0]) (1 - [[epsilon].sub.0]) in the
equations denoted as "b" are especially located at the end of the
numerical expressions, thus repeating their analytical forms. (2) The
data correlations were performed by Origin 6.0. Information (common
nomenclature is used) pertinent to accuracy of approximation is
available close to each equation.
Table 3a. Examples of data correlations through the scaling rules
developed in the present work and addressing the magnetic field
effect through the magnetic bond number. Linear fits.
Bed Equation Equation: Linear fits Comments
weight Code (Minimum Spouting Velocity,
[U.sub.ms-S] = [Q.sub.ms-S/
[S.sub.b]), [Fe.sub.3] [O.sub.4]
(315 - 400 [micro]m)
UL-a [[rho].sub.f][U.sup.2.sub.ms-S]/ Particle
[[rho].sub.s]g[d.sub.p] = diameter as
(Ga.[DELTA][D.sub.bL]) a length
([A.sub.Udp] - [B.sub.Udp] scale
[Bo.sub.g-m]) [right arrow]
[U.sup.2.sub.ms-s]/g[d.sub.p] =
[(Ga.[DELTA][D.sub.bL])
([A.sub.Udp] - [B.sub.Udp]
[Bo.sub.g-m])] ([[rho].sub.s]/
[[rho].sub.f])
UL-b [U.sup.2.sub.ms-S]/g[h.sub.b0] = Bed depth
[(Ga.[DELTA][D.sub.bL]) as a length
([A.sub.Udp] - [B.sub.Udp] scale
[Bo.sub.g-m])] x
[([[rho].sub.s]/[[rho].sub.f]) x
([d.sub.p]/[h.sub.b0])
(1 - [[epsilon].sub.0])]
G = 1 kg UL-1-a [U.sup.2.sub.ms-S]/g[d.sub.p] =
(Ga.[DELTA][D.sub.bL])
(-1.386 + 0.0628[Bo.sub.g-m]) x
5745.2;
R = 0.9607; N = 7 data points;
SD = 1.13; P = 5.732 x [10.sup.-4]
UL-1-b [U.sup.2.sub.ms-S]/g[h.sub.b0] =
(Ga.[DELTA][D.sub.bL])
(-1.386 + 0.0628 [Bo.sub.g-m]) x
5.24
G = 2 kg UL-2-a [U.sup.2.sub.ms-S]/g[d.sub.p] =
(Ga.[DELTA][D.sub.bL])
(-12.29 + 1.01 [Bo.sub.g-m]) x
5745.2;
R = 0.9274; N = 6 data points;
SD = 22.38; P = 0.00771
UL-2-b [U.sup.2.sub.ms-S]/g[h.sub.b0] =
(Ga.[DELTA][D.sub.bL])
(-12.29 + 1.01 [Bo.sub.g-m]) x
3.358
G = 3 kg UL-3-a [U.sup.2.sub.ms-S]/g[d.sub.p] =
(Ga.[DELTA][D.sub.bL])
(-6.021 + 0.7506 [Bo.sub.g-m]) x
5745.2;
R = 0.9827; N = 5 data points;
SD = 0.784; P = 4.463.[10.sup.-4]
UL-3-b [U.sup.2.sub.ms-S]/g[h.sub.b0] =
(Ga.[DELTA][D.sub.bL])
(-6.021 + 0.7506 [Bo.sub.g-m]) x
2.966
Notes: For sake of clarity and coherence with the dimensional analysis
developed the correlations are expressed through the ratios
[U.sup.2.sub.ms-U]/g[d.sub.p] and [U.sup.2.sub.ms-U]/g[h.sub.b0].
Table 3b. Examples of data correlations through the scaling rules
developed in the present work and addressing the magnetic field effect
through the magnetic Bond number. Exponential fits.
Bed Equation Equation: Exponential fits Comments
weight Code (Minimum Spouting Velocity,
[U.sub.ms-S] = [Q.sub.ms-S]/
[S.sub.b]), [Fe.sub.3][O.sub.4]
(315 - 400 [micro]m)
UE-a [[rho].sub.f][U.sup.2.sub.ms-S] = Particle
(Ga.[DELTA][D.sub.bL]) [E.sub.Udp] diameter as
exp ([k.sub.dp] [Bo.sub.g-m]) a length
[right arrow] [U.sup.2.sub.ms-s]/ scale
g[d.sub.p] =
[(Ga.[DELTA][D.sub.bL]) [E.sub.Udp]
exp ([k.sub.dp] [Bo.sub.g-m])]
([[rho].sub.s]/[[rho].sub.f])
UE-b [U.sup.2.sub.ms-S]/g[h.sub.b0] = Bed depth
[(Ga.[DELTA][D.sub.bL]) [E.sub.Udp] as a length
exp ([k.sub.dp] [Bo.sub.g-m])] x scale
[([[rho].sub.s]/[[rho].sub.f]) x
([d.sub.p]/[h.sub.b0])
(1 - [[epsilon].sub.0])]
G = 1 kg UE-1-a [U.sup.2.sub.ms-S]/g[d.sub.p] =
[(Ga.[DELTA][D.sub.bL]) 0.578 exp
(0.0181 [Bo.sub.g-m])] x 5745.2
UE 1-b [U.sup.2.sub.ms-S]/g[h.sub.b0] =
[(Ga.[DELTA][D.sub.bL]) 0.5 78 exp
(0.0181 [Bo.sub.g-m])] x 5.24
G = 2 kg UE-2-a [U.sup.2.sub.ms-S]/g[d.sub.p] =
[(Ga.[DELTA][D.sub.bL]) 7.89 exp
(0.022[Bo.sub.gm])] x 5745.2
UE-2-b [U.sup.2.sub.ms-S]/g[h.sub.b0] =
[(Ga.[DELTA][D.sub.bL]) 7.89 exp
(0.022 [Bo.sub.gm])] x 3.358
G = 3 kg UE-3-a [U.sup.2.sub.ms-S]/g[d.sub.p] =
[(Ga.[DELTA][D.sub.bL]) 11.238 exp
(0.0168 [Bo.sub.g-m])] x 5745.2
UE-3-b [U.sup.2.sub.ms-S]/g[h.sub.b0] =
[(Ga.[DELTA][D.sub.bL]) 11.238 exp
(0.0168 [Bo.sub.g-m])] x 2.966
Notes: For sake of clarity and coherence with the dimensional
analysis developed the correlations are expressed through the ratios
[U.sup.2.sub.ms-U]/g[d.sub.p] and [U.sup.2.sub.ms-U]/g[h.sub.b0].