Method for measuring the fluidity of fluidized beds

In a method for detecting the fluidity of particles in a vertically extending bed of fluidized particles, an electric charge which is generated on the particles is dissipated through an electrically conductive means in the fluidized bed to give an indication of the fluidity of the particles in the bed.

FIELD OF THE INVENTION 
The present invention relates to fluidized beds where upwardly flowing 
gases are introduced into the bottom portion of a container to fluidize 
particles therein. 
BACKGROUND OF THE INVENTION 
The fluidity of a powder processing bed is an important parameter for 
quality control. There is strong economic incentive to measure the 
fluidity during processing. It allows time for any remedial control 
action, thereby preventing the formation of an off-specification product 
at the end of the run. 
The fluidity of a phosphor fluid bed depends on the inherent cohesiveness 
of the material, the fluidizing gas flow rate, and the spatial 
distribution of gas in the bed volume. The cohesiveness of phosphors is 
well documented, and researchers have found various additives which, when 
blended with the phosphor in optimum amounts, reduce the inherent 
cohesiveness of the phosphor. Techniques of determining bed fluidity, 
however, have deficiencies. Current methods rely on visual observation of 
the bed, measurements of bed expansion, elutriation loss, and normalized 
bed pressure drop. 
While visual observation of the bed is probably the best nonquantitative 
method of estimating the bed fluidity, it requires a transparent wall 
material for the fluid bed. This restricts the wall to materials like 
quartz and high temperature glasses which can stand the elevated 
temperatures needed for gas/solid reactions in the bed. Since these 
materials have the severe handicap of being brittle, a major safety hazard 
is introduced into industrial processes, especially when the chemicals 
being processed in the fluid bed are pyrophoric. An example of such a 
chemical is trimethyl aluminum whose use is discussed in U.S. Pat. No. 
4,678,970. 
It should be noted that bed expansion by itself is not a complete and 
definite measure of bed fluidity. In fluidization of cohesive powders, 
like phosphors for example, the bed may expand simply due to the presence 
of multiple cracks in the bed, without displaying significant powder 
movement. Bed expansion measurements for estimation of bed fluidity should 
be complemented by measurements of elutriation loss and/or normalized bed 
pressure drop. 
One can physically measure the bed expansion using a scale on the external 
wall of the fluid bed. This would require a transparent wall, with its 
associated safety disadvantage. One can also measure bed expansion using 
ultrasonic sensors located in the freeboard. The accuracy of these units 
is often questionable due to interference of the signal by the presence of 
a powder dust cloud in the freeboard. X-ray bed level detection systems 
are sometimes used, but several industries prefer not to adopt radiation 
methods for health reasons. 
Other parameters remaining the same, a higher elutriation loss results from 
a more mobile fluid bed system than from one where the powder movement is 
slight. Measurements of elutriation loss involve weighing the mass of 
powder lost from the bed over a certain period of time. While this is 
feasible in a laboratory process, industrial processes in which flammable 
and/or pyrophoric chemicals are being used are less suited for such 
measurements. It can generally be said that a treatment which increases 
both the bed expansion and the elutriation loss makes the bed more fluid. 
This means that if several different concentrations of an additive were 
being tested for their effect on bed fluidity, that treatment which gave 
the highest bed expansion and the highest elutriation loss could be 
considered as producing the most mobile bed. Elutriation losses are not 
desired, however, in industrial processing, especially when expensive 
powders are being handled. It is possible to minimize bed material loss, 
without sacrificing bed expansion, by suitable design of the freeboard 
section. 
In a gas fluidized bed with no channeling, almost all of the bed weight is 
supported by the pressure drop of the gas. This is typically the case in 
gas fluidization of Geldart type B and A materials. As one moves to finer 
materials, however, gas channeling starts, bubbles disappear and are 
largely replaced by a network of vertical and inclined cracks. Under these 
conditions the ratio of bed pressure drop to bed material weight, often 
referred to as the normalized bed pressure drop, is less than unity. Bed 
fluidity decreases as the normalized bed pressure drop deviates more from 
unity. The bed mass used in the calculation refers to the initial mass of 
material charged to the bed. 
Direct measurement of bed pressure drop is easily accomplished for Geldart 
type A and B materials, by installing one or more pressure transducers 
just above the distributor plate. A similar procedure is problematic for 
Geldart Class C materials (like phosphors) because any screen like device 
used to isolate the sensor from the bed material is easily clogged by the 
fine particles. For these materials, the bed pressure drop is usually 
found by installing a pressure transducer in the plenum section of the 
fluid bed. This provides the total pressure drop during actual operation, 
and the distributor pressure drop when gas is passed through the plate 
with no bed present. Subtraction of the latter data from the former yields 
the bed pressure drop. It is possible to develop a computer based system 
to calculate in real time the bed pressure drop and the normalized bed 
pressure drop. 
A problem with this method of determining bed pressure drop occurs when one 
or more of the precursor chemicals used in the CVD reactions in the bed is 
prone to pyrolysis. Pyrolysis, or thermal decomposition in the absence of 
oxygen, at the distributor plate can lead to partial plugging of the pores 
of the plate. The distributor plate pressure drop is an increasing 
function of the gas flow rate per unit area of plate. As the plate 
gradually plugs up, the flow rate per unit area increases because the flow 
rate of the gases is maintained essentially constant by the mass flow 
controllers. The resulting upward shift in the distributor characteristic, 
pressure drop versus gas flow rate, will result in an error in the 
computation of the bed pressure drop by the subtraction procedure. The 
error will be an over estimate of the bed pressure drop, resulting in a 
rosier picture that shows a lesser extent of channeling than that which 
really exists. 
Heretofore, prior art techniques for monitoring the fluidity of a fluidized 
bed have been deficient. 
SUMMARY OF THE INVENTION 
The present invention provides a method for detecting the fluidity of 
particles in a vertically extending bed of fluidized particles. The 
particles are confined within a vertically oriented container and upwardly 
flowing gases are distributed into a bottom portion of the container for 
fluidizing the particles. An electric charge is generated on the 
particles. The bed is provided with electrically conductive means which 
contact the particles during fluidization and discharge the charge on the 
particles for creating an electrical current in response to particle 
movement. The flow of electrical current from the electrically conductive 
means is detected for indicating the fluidity of particles. 
In a preferred embodiment, several conductive strips are placed in the 
fluid bed in contact with the moving particles. The electrical current 
which passes through the conductive elements to a ground is monitored as a 
measure of bed fluidity. Enhanced safety, ease of operation, and accuracy 
may result from detecting bed fluidity in this manner. 
When a difficult to fluidize Geldart Class C material, such as a phosphor, 
is fluidized with varying amounts of a fluidizing aid, measurement of the 
electrical current results in enhanced monitoring of the fluidity of the 
bed. This is achieved as evidenced by experimental results. 
Disclosed herein is a technique of measuring bed fluidity by monitoring the 
current flowing to ground through a set of conductive strips attached to 
the wall of a fluid bed. The particles are charged due to 
triboelectrification when they rub against the dissimilar material that is 
the wall of the fluid bed. The charge on the particles is discharged when 
they contact the conductive strips on the wall. The flow of this charge to 
ground constitutes the current which is monitored. The more fluid the bed, 
the larger is the downward flux of particles, the higher is the rate of 
charge accumulated by the particles, and the greater is the current to 
ground. This current, also referred to as the wall current in this 
application, can, therefore, be used to track changes in fluidized bed 
mobility as a function of time and processing conditions. In addition, 
this current peaks when the bed fluidity is a maximum. 
In order to correlate the wall current with bed fluidity, the bed expansion 
and elutriation loss were also measured within a bed of phosphor particles 
containing very fine aluminum oxide C (AOC) particles as an additive. It 
is known in the industry that very small, about 20 nm or smaller, aluminum 
oxide C particles can work as a fluidizing aid for cohesive materials, 
like phosphors. The concentration of the additive at which the bed 
expansion reaches a local maximum was noted. The additive loading at which 
the elutriation loss exhibits a peak is found to coincide with the former 
concentration. The bed mobility is, therefore, highest at this 
concentration of the fluidizing aid. It is found that the wall current 
also peaks at tis very same concentration of the fluidizing aid. 
According to the experimental results, the peak wall current correlates 
with the peaks in both the bed expansion and elutriation loss. Since the 
latter parameters are a joint gauge of bed mobility, this proves that the 
current generated is a good measure of bed fluidity. Monitoring the 
current yields valuable information on changes in bed mobility as a 
function of processing condition and time. By overcoming the deficiencies 
described above of existing methods, the present invention provides an 
enhanced method of measuring the fluidity of a fluid bed.

DETAILED DESCRIPTION 
The present invention provides a method for detecting the fluidity of 
particles in a vertically extending bed of fluidized particles. The 
particles are confined within a vertically oriented container and upwardly 
flowing gases are distributed into a bottom portion of the container for 
fluidizing the particles. The pressure of gas in a gas source which 
communicates with the bottom portion for the upward flow of gas is 
preferably adjustable. 
Generally, the vertically oriented container comprises a vertically 
oriented wall with a gas permeable bottom portion extending transverse to 
the vertical walls for the upward flow of gas therethrough. The gas 
permeable member is connected to a source of gas under pressure which is 
delivered to the bottom portion which is in the form of a plenum. 
During the process of fluidization, particles of powder come in contact 
with the wall. This contact is characterized by a sliding action along the 
wall, of the downward moving particles. These descending particles along 
the wall form a part of the downward flux of material in a fluidized bed. 
The downward flux as well as the inner ascending flux of particles are a 
consequence of the hydrodynamics of fluidized beds (see Kunii and 
Levenspiel, Fluidization Engineering, Krieger Publishing, 1977), and 
increase with the superficial gas velocity. The latter is defined as the 
ratio of the volumetric flow rate of fluidizing gas to the open cross 
sectional area of the fluid bed. 
When two dissimilar insulating materials rub against each other, 
triboelectrification results. Triboelectrification refers to the transfer 
of electronic charge between these materials. In the particular case 
presented in this invention, the glass wall and the descending phosphor 
particles along the wall constitute the two dissimilar entities. Due to 
the frictional contact between these materials, the glass and the phosphor 
particles become charged. In order to maintain charge neutrality, the 
number of electrons lost by one entity is numerically equal to that gained 
by the other. In other words, triboelectrification results in both a 
positively and a negatively charged entity. While a particular phosphor 
may be negatively charged, and the glass therefore positively charged, 
when fluidized in a glass walled fluid bed, it is also possible to have a 
different phosphor which when fluidized will be positively charged with a 
negatively charged glass wall. The concept presented in this invention is 
NOT limited to a particular polarity of the phosphor when fluidized. 
While a glass walled vessel has been used as the vertically oriented 
container for the fluidized bed in this invention, any suitable insulating 
material with a different triboelectric characteristic than the powder 
being fluidized may be used. This will serve to charge the powder 
particles. Preferably the insulating material comprises the vertically 
oriented container, although it is contemplated that other insulating 
structures may be introduced into the container for contacting the powder. 
Preferably the insulating material is an abrasion resistant insulating 
material. Preferred materials are ceramic materials. In a preferred 
embodiment, the particles comprise phosphor particles. The relative 
triboelectric characteristics of the particles and the insulating 
structure may be determined by methods known in the art. Generally, the 
greater the difference in triboelectric properties, the larger will be the 
charge on the particles, resulting in an increased current flow when the 
particles are subsequently discharged. 
For the particular phosphor investigated in this invention, the material 
develops a negative surface charge when fluidized in a glass walled 
fluidized bed. For a given number of electrons transferred from the glass 
wall to a single phosphor particle, it follows that the larger the 
descending flux of particles, the higher is the rate of charge accumulated 
on the phosphor particles. Anything which increases the fluidity of the 
fluidized bed, will increase the descending (as well as the ascending) 
flux. When these charged particles, during subsequent descents and/or 
ascents, contact the grounded electrically conductive means, the charge on 
the particles is bled to ground. 
The flow of this charge constitutes a current which is detected for 
indicating the fluidity of the fluidized bed. The detecting and indicating 
means may comprise an ammeter. Such an ammeter may include a visual 
display of current flow or include another device, such as an 
oscilloscope, to give an indication of current flow. Higher current 
readings are indicative of greater fluidity. The pressure of the source of 
the gas may be adjusted in response to said detecting means indicating the 
fluidity of the fluidized bed. For example, the gas pressure may be 
increased to compensate for an observed drop in bed fluidity. 
It follows from the discussion in the previous paragraphs that anything 
which increases the bed fluidity will increase the wall current. In 
addition, the wall current is a quantitative measure of the bed fluidity. 
Bed fluidity may be increased by an intrinsic decrease in powder 
cohesiveness (as for example by addition of appropriate amounts of fine 
Aluminum Oxide C), by increasing the gas pressure, hence the superficial 
gas velocity, or by changing the spatial distribution of fluidizing gas in 
the fluid bed. Phenomena like agglomeration and loss of fluidizing aid 
will reduce the bed mobility and reduce the wall current. 
The grounded electrically conductive means, referred to earlier, preferably 
comprises a plurality of metal elements. Preferred metal elements include 
conductive elements which do not contaminate phosphor particles. Such 
elements comprise aluminum, nickel, copper or alloys thereof which may be 
in the form of strips, so as to increase the area of contact with the 
phosphor particles. Although an increased area enhances the contact with 
the particles, the metal elements are preferably arranged so as not to 
interfere with the movement of the powder. A preferred location for the 
conductive elements is an arrangement closely adjacent the vertically 
oriented walls of the container, so as not to interfere with powder 
movement. 
The electrically conductive means is preferably electrically isolated from 
the insulating material. When the electrically conductive elements are 
closely adjacent the vertically oriented walls of the container, the 
strips are preferably isolated from the wall by an insulating material. 
Such electrical isolation may be provided by backing the strips with an 
electrically insulating layer containing adhesive. If the electrically 
conductive means is in the form of self-supporting rods or strips, the 
rods or strips may be conveniently spaced from the insulating wall. 
It is worth noting, that during the fluidization of phosphors in a glass 
container, if the positive charge on the insulated material viz. the glass 
wall were to survive for a time scale significantly greater than the 
average circulation time of a phosphor particle, then the wall current 
would rapidly drop to zero. This is because the charge on the particles is 
due to fresh charge transferred from the glass wall during every descent. 
The physics of electrostatics does not favor transfer of additional 
electrons to the phosphor particles from an already positively charged 
wall. Several mechanisms may be responsible for bleeding off the charge 
accumulated on the glass wall. As an example, small amounts of moisture in 
the fluidizing gas may neutralize the charge on the glass. This mechanism 
will not destroy the charge on the particles because the surface area of 
the particles is orders of magnitude greater than that of the glass. While 
the inventors do not wish to be bound by an explanation of one or more of 
these mechanisms, the fact remains that conditions exist which maintain 
the wall current, as evidenced by the information presented in this 
application. 
Disclosed herein is a technique of measuring bed fluidity by monitoring the 
current flowing to ground through a set of conductive strips attached to 
the wall of a fluid bed. The particles are charged due to 
triboelectrification when they rub against the dissimilar material that is 
the wall of the fluid bed. The charge on the particles is discharged when 
they contact the conductive strips on the wall. The flow of this charge to 
ground constitutes the current which is monitored. The more fluid the bed, 
the larger is the downward flux of particles, the higher is the rate of 
charge accumulated by the particles, and the greater is the current to 
ground. This current can, therefore, be used to track changes in fluidized 
bed mobility as a function of time and processing conditions. In addition, 
this current peaks when the bed fluidity is a maximum. 
EXAMPLE 
Bed fluidity was studied using a zinc orthosilicate phosphor, GTE type 
2285, with a particle density of 4.107 g/cc as measured by a pycnometric 
technique. The specific surface area based mean diameter of the powder 5 
was 4.67 microns, while the volume based mean diameter was 11.55 microns. 
The fluidizing aid was AOC with a BET surface area of 82 m.sup.2 /g AOC is 
an aluminum oxide powder which functions as a fluidizing aid for phosphors 
in the lighting industry. The dominant phase in AOC is gamma alumina, and 
the primary particle size of the material is about 20 nm. 
Ten samples, SRI through SR10, were prepared by V-blending 3 kg lots of the 
phosphor with varying amounts of AOC. The following concentrations of AOC, 
expressed as a percentage of the phosphor mass, were used in these 
samples: 0.0, 0.025, 0.05, 0.1, 0.2, 0.35, 0.55, 0.8, 1.0 and 1.1. The 
mixing of the two materials was conducted for 30 minutes with the 
activation of an exciter bar in the blender. 
A schematic of the invention is shown in FIG. 1. As conductive elements, 
eight 0.56 cm wide copper strips 2 with adhesive backing were attached 
symmetrically on the inside of a 10.16 cm ID and 86.4 cm high Pyrex tube 
1. The backing electrically isolated the strips 2 from the tube 1. The 
strips 2 did not contact the 5 micron porosity and 0.157 cm thick 
stainless steel distributor plate 7 which was gas permeable. At the top of 
the tube 1, the copper strips 2 were electrically connected to a Keithley 
picoammeter 3 which delivered a signal to a Tektronix 7854 oscilloscope 4 
which served as the indicator means for measuring the current in amperes. 
The plenum chamber was constructed of PVC and silicone rubber was used to 
make the necessary seals. The chamber consisted of a 2.54 cm high straight 
section above a conical section with a cone angle of 45. and a frustum 
height of 3.175 cm. A one meter metal ruler 6 was taped to the outer wall 
of the tube 1 to provide a reading of the bed height. Air was fed to the 
bottom of the plenum chamber at inlet 9 through a 0.95 cm ID Tygon tube, 
the flow rate being read off a rotameter 10. A manometer is shown at 11. 
The observed flow rate was corrected for the small deviation of the 
rotameter downstream pressure from atmospheric conditions. A pressure tap 
is provided at 8. 
Several runs were conducted for a particular loading of AOC in the 
phosphor. Prior to the first run for a sample, the mass of powder 5 being 
fed to the Pyrex tube 1 was weighed. The mass of material in the bed at 
the end of the last run was also measured to determined the loss due to 
elutriation. The expanded bed height and time averaged wall current were 
also recorded. 
Detailed statistical analysis of the data was performed using ANOVA and the 
Tukey's Post Hoc Test. The former indicates whether detectable differences 
exist between the various AOC treatments. It does not, however, tell which 
of the AOC treatments are different from each other. The Tukey's Post Hoc 
Test makes that possible by conducting all possible pairwise comparisons 
between the subgroups. 
Bed Expansion 
The expanded bed height at the maximum gas superficial velocity was divided 
by the static bed height to yield a normalized bed expansion. The mean bed 
expansion is shown in FIG. 2 as a function of the AOC level in the 
phosphor fed to the Pyrex tube 1. 
Statistical hypothesis testing indicates that there is insufficient 
evidence, at the 5% significance level, to reject the null hypothesis that 
the mean bed expansion is the same over an AOC concentration range of 
0.55% to 1.1%. The analysis also indicates that there is no detectable 
change in bed expansion over an additive range of 0 to 0.1%. Detectable 
differences in bed expansion are manifested, as the AOC loading is varied 
from 0.1% to 0.55%. The highest mean bed expansion is realized at an AOC 
loading of 0.35%, indicating that maximum bed fluidity probably occurs at 
this concentration of the fluidizing aid. 
Elutriation Loss 
The initial charge of powder 5 fed to the fluid bed, for a given AOC level, 
experiences a gradual loss in mass over the course of the runs. This is 
attributed to elutriation. The highest superficial velocity in the 
experiments was about 8 cm/s. Parameters like minimum fluidization 
velocity and terminal velocity cannot be applied to fluidization of 
Geldart type C materials, like this phosphor. This is because 
inter-particle forces, which are practically absent in Group B and A 
materials, play a major role in the hydrodynamics of these fine particles. 
The percentage of initial bed mass lost due to entrainment, expressed on a 
per run basis, is shown in FIG. 3. It is important to note that this loss, 
for a particular concentration of AOC, is a cumulative one resulting from 
the imposition of gradually increasing superficial velocities over every 
run. Since the gas flow rate settings were practically identical for all 
the runs and the duration of a particular flow setting varied very little 
from run to run, it may be concluded that the elutriation loss shown in 
FIG. 3 reflects primarily the effect of AOC. 
It is observed that the elutriation loss peaks at an AOC level of 0.35%. 
This knowledge, when coupled with the bed expansion information, leads to 
the conclusion that the peak bed mobility occurs at a fluidizing aid 
concentration of 0.35%. 
SEM micrographs indicate that fluidization does not preferentially strip 
the much smaller AOC particles from the larger phosphor particles in the 
bed. Considerable AOC still remains attached to the bed phosphor at the 
end of the last run. The presence of AOC on the surface of the elutriate 
is also evident. 
Generated Current 
The current generated by the particles 5 contacting the strips 2 is 
measured by the detecting means in the form of a picoammeter 3. The 
picoammeter 3 has an indicator that may be visually read. The effect of 
AOC on the wall current is shown in FIG. 4. No current is detected below 
an AOC level of 0.11%. The mean wall current for an AOC level of 0.35% is 
the highest and is detectably different from the mean currents for all the 
other treatments. 
This AOC level is identical to that at which the bed expansion and the 
elutriation loss also exhibit local maximum. In other words, the wall 
current peaks when the bed has the highest degree of fluidity. 
This invention has found that the peak wall or generated current correlates 
with the peaks in both the bed expansion and elutriation loss. It has been 
stated earlier that the latter parameters are a joint gauge of bed 
mobility (used interchangeably with fluidity). The wall current can, 
therefore, rightly be used as a measure of bed fluidity. The correlation 
of the wall current with bed mobility is further substantiated by the 
increase in the current with gas superficial velocity. FIG. 5 shows this 
effect very clearly. A higher gas velocity results in an increase in the 
solids flux, and as has been described in the "Detailed Description" 
section, this in turn should produce a higher wall current. 
Monitoring of the wall current can yield valuable information on changes in 
bed fluidity as a function of processing condition and time. By overcoming 
the deficiencies described in the "Background" section of this application 
this invention makes a positive contribution to the general field of fluid 
bed technology and allied fields including, but not necessarily limited 
to, powder CVD.