Table II.
Typical Results of Fluidized-Bed Contact of Sulfur
Trioxide with Phosphate Rock
Total
Temp.,
0
C.
250-500
310
286-320
297-324
275-331
294-302
327-332
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320
330
325
330
360
315
320
325
320
320
325
335
330
330
330
320-340
325
330
325
325
325
“
Time,
P2O5,
Min.
A.
Wt. %
A.P.Aa,
F,
Wt. %
Wt. %
with SO3 Alone
8.66
3.10
3.84
5.58
8.99
3.10
8.66
3.80
3.60
8.53
9.24
3.60
9.04
3.44
Contact
30.42
30
32.68
30
27.89
30
30.76
30
30.43
30
31.44
15
31.24
30
7 .72
30.3
B. Contact with SO3 and Air
15
28.6
6.9
15
30.2
7.5
30
29.6
8.6
30
31.9
8.4
30
28.8
9.2
30
30.6
6.7
5
29.1
8.5
10
29.6
8.6
C. Contact with SO3, Steam, and Air
15
22.1
10.46
15
24.4
12.74
3
23.1
12.30
3
26.0
11 .47
1
31 .4
9.40
2
30.5
8.94
5
28.8
10.09
10
21.2
9.56
10
29.3
10.60
2.98
15
23.7
13.12
1 .70
15
24.7
1 .70
13.73
15
23.4
12.33
Available phosphoric acid,
P2O5 by AO AC definitions.
sum
5,
Wt. %
4.01
1.65
1.76
3.49
4.14
3.13
3.73
of the vapor-liquid region of the H2O-SO3 system at 1 atm.
Therefore the conversion limit at 340° C. is probably due
to absence of liquid, since the fluidizing phase cannot wet the
rock above this temperature. Stable fluidization was possible
between about 315° and 340° C., and this dictated the choice
of 325° C. as normal operating temperature.
Sulfur dioxide produced only minor conversion of P2O5
to available form at contact times up to 1 hour. Detailed
ture
(6).
results
are reported by Ross (8)
The product of the runs reported in Table IIC was brought
in contact with a series of extractant solutions, in an effort to
discover whether or not the product would yield more P2O5
than the AOAC “availability.” The results may be summarized as follows: Basic and neutral solutions reduce the
P2O5 availability, and acid solutions improve the P2O5 availability slightly (to 16.1% with K2SO4 saturated solution at
65° C.). Both results are to be expected of highly polymerized
P2O5.
Acknowledgment
The authors acknowledge the support of the Tennessee
Corp. for this research. W. O. Land, Jr., gave valuable
assistance with experimental equipment.
!
10.3
5.6
of water-soluble and citrate-soluble
minutes, and all occurred within 15 minutes. The upper limit
of conversion is probably due to a film of reacted material on
the particles of phosphate rock, but this phenomenon was not
studied.
The reaction temperature was usually about 325° C. Observation indicated that conversions dropped sharply above
340° C. This corresponds closely to the maximum tempera-
Literature Cited
(1) Association of Official Agricultural Chemists, Washington,
D. C., “Official Methods of Analysis,” 9th ed., 1966.
(2) Baumgarten, P., Brandenburg, C., Chem. Ber. 72, 555-63
(1939).
(3) Furman, N. H., ed., “Standard Methods of Chemical Analysis,” Vol. I, 6th ed., pp. 442-4, Van Nostrand, Princeton,
N. J., 1962.
(4) Giana, E., German Patent 219,680 (1907).
(5) Hughes, A. E., Cameron, F. K., Ind. Eng. Chem. 23, 1262-71
(1931).
(6) Luchinskii, G. P., Zh. Fiz. Khim. 30, 1207-22 (1956).
(7) Pompowski, T., Zeszty Nauk. Politechn. Gdansk., Chem. 4,
No. 26, 3-28 (1962).
(8) Ross, L. W., Ph.D. thesis, Georgia Institute of Technology,
1966.
(9) Scheel, K., German Patent 966,264 (1957).
(10) Snell, F. D., Biffen, F. M., “Commercial Methods of Analysis,” rev. ed., pp. 216-18, Commercial Publishing Co., New
York, 1964.
Received for review September 23, 1966
Accepted May 1, 1967
FLUIDIZED BED DISPOSAL OF FLUORINE
JOHN
T
.
HOLMES,
LOWELL
B
.
KOPPEL,1
AND
ALBERT
A.
JONKE
Argonne National Laboratory, Argonne, III. 60439
As
volatility processes for the recovery of unspent
fissionable and fertile materials from nuclear reactor fuels
fluoride
grow toward future commercial application, environmental
contamination and waste disposal considerations will require
evaluation of existing methods or development of new methods
for the disposal of the toxic gaseous reagents used in the
process. This paper describes the development of a new
method for the disposal of fluorine. The work is part of a continuing effort at the Argonne National Laboratory to develop
methods for the disposal of gaseous fluoride volatility reagents
and of volatile fission product compounds.
The first requirement is that a fluorine disposal system have
a high efficiency for the removal of fluorine from a gas stream.
1
408
Present address, Purdue University, West Lafayette, Ind.
l&EC
PROCESS DESIGN
AND
DEVELOPMENT
It should also be economic, involve simple equipment and
a minimum amount of chemical
procedures, and consume
If the process is to be used in a nuclear fuel rereactant.
processing plant, it should have a product which is suitable
for packaging and storage as radioactive waste (preferably a
free-flowing solid), and be able to remove fission product compounds associated with the fluorine-containing gas stream.
Existing methods for fluorine disposal include the reaction
of fluorine with liquids, gases, or solids. Gas scrubbers
employing caustic solution are in common use for the disposal
of fluorine (Liimatainen and Levenson, 1953; Slesser and
Schram, 1951; Stainker, 1956). These scrubbers are efficient,
but produce large volumes of liquid wastes, which is considered undesirable for radioactive applications. The reaction
of fluorine with gases such as hydrogen, hydrocarbons (Long,
A fluidized bed process developed for the disposal of fluorine using activated alumina (AA) as the reactive
solid, is over 99.9% effective in the removal of fluorine from a gas stream and utilizes the activated
alumina to near the theoretical conversion. It has capability for high fluorine disposal rates and produces
A factorial experiment was used to determine that ina free-flowing solid product for waste disposal.
to
400°
C.), increasing ratio of bed depth to diameter (3 to 6), and decreasing
creasing temperature (300°
size
I
to
83
(399
microns) significantly increased the capacity (grams of F2 per gram of AA) of actiparticle
vated alumina for fluorine removal. Changing the fluorine concentration from 5 to 75 volume % (v./o.) or the
velocity from 1.25 to 1.65 times the minimum fluidizing velocity (Vm/l had no significant effect. A higher
velocity, 3.0 Vm/, appeared to decrease the capacity slightly. Other solids reactants, which are less
expensive than activated alumina were given preliminary evaluation. Soda ash appears especially
promising.
1955; Turnbull, 1947), S02 (Horton, 1965a), NH3 (Holmes,
1961), and steam (Smiley and Schmidt, 1954) has been used
for fluorine disposal, but these methods produce other gaseous
products which require further treatment.
Many solids have been used
as
reactants
in fluorine
Charcoal (Schmidt, 1957, 1959) has
disposal schemes.
been used to dispose of large quantities of fluorine. The
reaction products are mainly nontoxic carbon-fluorine gases
and some condensables which may plug an absolute filter
system (Horton, 1965b). Packed beds of limestone, soda lime,
and activated alumina have been used for fluorine disposal
(Liimatainen and Levenson, 1953). Packed beds of activated
alumina (AA) have been used routinely at Argonne National
Laboratory for disposal of fluorine from bench-scale and pilot
plant-scale apparatus. Up to 85% of the activated alumina
was consumed with high efficiencies for the removal of fluorine.
Temperatures of over 1000° C. were generated in the packedbed reactors and caused sintering of the bed into a rigid mass
which is difficult to remove from the reaction vessel. Other
experiments at Argonne have shown that packed beds of
activated alumina have limited capabilities for trapping some
of the volatile compounds of fission product tellurium and
ruthenium.
Since the activated alumina-fluorine reaction exhibits a
number of desirable characteristics for a radiochemical process
application, this system was studied in detail. A fluid-bed
reactor was chosen rather than a packed-bed reactor in order
to provide good heat transfer and thus prevent high temperatures which cause bed sintering. A free-flowing product can
thus be achieved. A fluidized bed process can easily be
automated with regard to addition of fresh solid reactants and
withdrawal of the reaction product for waste storage. Determination of the efficiency of this fluidized bed process for fission
product trapping is beyond the scope of the current work.
Experimental
The experimental facility consists of four major parts:
reagent supply, fluid-bed reactor, potassium iodide (KI)
scrubber, and back-up trap, as shown schematically in Figure
1.
The reagent flows are measured with orifice meters and
automatically controlled with pneumatically operated control
valves. Fluorine is fed from cylinders, through traps of NaF
and glass wool to remove
any HF present. Nitrogen is used
as the diluent gas.
The fluid bed reactor is constructed from 2-inch diameter,
25-inch long, nickel pipe and is topped by a 3-inch diameter
disengaging section. Two sintered nickel filters in the disengaging section remove
particulate matter from the off-gas.
Automatically controlled heaters and cooling coils are attached
to the outside walls of the reactor section. A 3/g-inch diameter
tabular alumina sphere acts as a check valve for solids in the
FLUID BED
K
REACTOR
I
SCRUBBER
BACK-UP
TRAP
Figure I. Experimental facility for studying the disposal of gaseous fluoride volatility reagents
cone-shaped gas distributor at the inlet of the column. The
reaction temperature is measured by three thermocouples,
spaced at 1-inch vertical intervals starting 1-inch above the
fluidizing gas inlet.
The potassium iodide (KI) scrubber is a 2-foot long section
of 3-inch i.d. glass pipe, packed with 5-mm. diameter glass
beads to a depth of 12 inches. One liter of 0AM KI solution
is used as the scrub solution. A vacuum
pump and rotameter
are used to bubble a 10-cu. foot per hour sample of the reactor
off-gas continuously through the scrubber.
A packed bed of activated alumina (back-up trap) is used
to remove
fluorine from the reactor off-gas during the breakthrough portion of an experiment.
The procedure for making a run consists of charging a given
quantity of activated alumina (Alcoa, grade F-l) to the reactor,
fluidizing the bed with nitrogen while the reactor is heated to
the desired temperature, and then introducing the fluorine.
The activated alumina is previously dried at the reaction
temperature. The x-ray diffraction pattern of the dried AA
indicates that the compound is 1203· 20. The progress
of the reaction is followed by taking periodic samples of the
scrub solution and determining the iodine present (produced
by F2 + I" — 2F- + V2I2) using a standardized sodium
thiosulfate solution and a starch indicator. The concentration
of fluorine in the off-gas from the reactor can thus be calculated.
Corrections must be made for the change in volume of the
scrub solution due to the periodic removal of samples. Tests
of the scrubber, over the range 5 to 2600 p.p.m. of F2 in N2,
showed the scrubbing efficiency to be essentially constant.
The fluorine disposal experiments are terminated when the F
concentration in the off-gas exceeds 1000 to 2000 p.p.m.
The sintered nickel filters are cleaned of any fines accumulation, using reverse pulses of high pressure nitrogen prior to
removal of the reactor bed for weighing and sampling. The
capacity is determined by chemical determination of fluorine
in the bed. Capacity is defined as the grams of F2 reacting
Since
per gram of activated alumina charged to the reactor.
nearly all of the fluorine is reacting when the experiment is
2
VOL
6
NO. 4
OCTOBER
1967
409
terminated, the capacity at the breakthrough point can be
calculated by linear interpolation from the time of shutdown
to the time at breakthrough. The theoretical maximum
capacity of 0.950 gram of F2 per gram of AA, assuming the
following reaction:
232
4™
3F2
constant temperature in the fluid bed near the gas inlet
where most of the reaction takes place. In the runs with high
concentrations of F2, axial temperature gradients of up to 200°
C. were observed, but these large gradients did not appear to
affect the operation.
The data obtained from the KI scrubber were plotted against
normalized time (2% = actual time per time at shutdown).
Examples of the data from two typical runs are presented in
Figure 2. All of the runs were characterized by a long initial
period where the concentration of fluorine was less than 50
p.p.m. and then a short period where there was a rapid inSince the data from
crease in concentration (breakthrough).
the scrubber give the average fluorine concentration from the
time of the previous sample to the time of the present sample
a
2AIF3 4* H2O 4- 1-5 O2
—*
After a series of shakedown runs to check out the equipment
and procedure, a factorial experiment was used to determine
the effect of the five most likely important independent variables on the capactiy of activated alumina for fluorine. The
variables were:
temperature ( '), particle size (DT), ratio of
bed depth to diameter (L/D), fluorine concentration (volume
per cent, v./o.) and gas velocity (V). For the ratio of bed
depth to diameter, only the bed depth was varied, since the
diameter was fixed at 2.0 inches. For velocity, a multiple
of the minimum fluidizing velocity (Vm/) was used. Each
variable was studied at the two levels shown in Table I. A
complete study of the effects and interactions of all five variables
at two levels would require a factorial experiment (Davies, 1954;
A fractional factorial
32 runs.
Hicks, 1964) consisting of (2)5
experiment was actually used in which only 16 runs were
made (half replicate). This technique determines the effects
of all the variables, the first-order interactions—e.g., T, Dp
interaction—and experimental error, but does not give any
information on the effects of higher order interactions—e.g.,
T, Dp, L/D interaction. Higher than first-order interactions
are rather uncommon
in physical situations.
(dark line), a linear interpolation (fine line) was used to
approximate the continuous concentration curve.
The predried activated alumina is the monohydrate of
aluminum oxide ( 1203· 20) and, therefore, some HF is
produced by the reaction of the H20 and F2. The HF is not
completely sorbed by unreacted activated alumina and small
concentrations of HF were observed in the off-gas from the
fluid-bed reactor. The HF concentration was highest during
early portions of the run but never exceeded the concentration
of fluorine in the off-gas and was usually considerably less.
Results of Factorial Experiment. The half replicate
factorial experiment is shown in Table II along with the
values of capacity at breakthrough as defined at the point at
which the off-gas concentration reached 200 p.p.m. and at the
point at which less than 99.9% of the fluorine was removed
by the activated alumina. The experiments were made in a
random sequence, as shown by the run numbers. The sequence was not randomized with respect to Dv, since recalibration of the flowmeters was required whenever Dp changed
because of the difference in minimum fluidizing velocity.
The capacity data in Table II were subjected to an analysis
of variance test with the aid of a digital computer. The
results of this analysis using the capacity data at either breakthrough point (200 p.p.m. or 99.9% removal) showed that
for the range of the variables studied, temperature, particle
size, and ratio of bed depth to diameter were the only variables
which had significant effects on the capacity. Furthermore,
these variables were significant at the 99% confidence level,
=
Results and Discussion
All of the runs were operationally very smooth. The
product beds were always free-flowing and no significant
pressure buildups were noted due to the deposition of fines
on the sintered metal filters.
The automatic filter blowback
was
not
used
of
system
during any the runs.
All of the experiments at 5 and 10 volume % F2 in N2 were
made without the addition of coolant to the reactor walls.
Runs at 30 to 75 volume % F2 required coolant to maintain
Table I.
Independent Variables
Low Level
Temperature,
Bed size,
L/D
°
300
C.
400
6
3
Particle size, mesh (microns)
Velocity, multiple of Vmf
Concentration, volume %
High Level
48 to 100 (183)
1.25
28 to 48 (399)
1.65
10
5
Tn
Figure 2.
410
l&EC
PROCESS DESIGN
Fluorine concentration
AND
DEVELOPMENT
Tn
ii
fluid-bed off-gas for typical
runs
Table II.
Fractional Factorial Experiment for Fluid-Bed Disposal of Fluorine with Activated Alumina
Capacity, Gram of 72 per Gram of AA
Run
L/D
399
399
399
399
399
399
399
399
183
183
183
183
183
183
183
183
5
2
1
8
4
7
3
6
12
11
16
13
10
15
14
9
which
Volume
Dp,
Microns
V,
6
6
6
6
X Vmf
1.65
1.65
eT
400
300
400
300
400
300
400
300
400
300
400
300
400
300
400
300
1.25
1.25
1.65
3
3
3
1.65
3
6
1.25
1.25
1.65
1.65
1.25
1.25
1.65
1.65
1.25
1.25
6
6
6
3
3
3
3
there is less than one chance in 100 that the obwere due to experimental error.
The effects of
changing velocity, changing concentration, and all first-order
interactions of the variables were not significant.
The magnitude of the effects (grams of F2 per gram of AA)
of each variable can be calculated by taking the average
capacity for those runs where the variable of interest was at its
high level and subtracting the average capacity for those
runs
where the variable was at its low level. Inspection of
Table II shows that the effects of the other four variables are
cancelled and only the effect of the variable of interest is
obtained in this manner.
The magnitudes of the effects of the
variables are given in Table III.
Temperature has the
largest effect and ratio of bed depth to diameter and particle
size have smaller effects. Changes in velocity and concentration over the range studied do not produce significant effects.
The standard deviation of the capacity for breakthrough
at 200 p.p.m. was „
0.061 gram of F2 per gram of A A and
at 99.9% removal was 0 = 0.058 gram of F2 per gram of A A.
This means that the observed effects on Table III have 95%
confidence limits of (~2) 0/2 = ±0.06 gram of F2 per gram
of A A and that the capacity data of Table II have 95%
confidence limits of (~2) 0 = ±0.12 gram of F2 per gram of
72 in
O
10
10
5
10
5
5
10
10
5
5
10
5
10
10
5
At 200
At 99.9%
p.p.m.
removal
0.627
0.294
0.660
0.344
0.206
0.153
0.553
0.283
0.630
0.294
0.183
0.141
0.429
0.187
0.740
0.388
0.797
0.412
0.642
0.251
0.676
0.293
0.258
0.174
0.737
0.357
0.797
0.410
0.583
0.238
0.658
0.257
means
=
AA.
The results
1
Ni
5
are
presented in
a more
useful
manner
in Equa-
and 2.
)
Capacity at
200 p.p.m.
>=
breakthrough )
Capacity at
)
99.9% removal >
breakthrough
1
+ 0.00307(7-623) + 0.0593
(D/D-4.5)
0.000750(Dp-291) in (1)
grams of F per gram of AA
0.443
-
2
0.410
==
+ 0.00280(7-623) -ff
Effect of Independent Variables
Table III.
served effects
tions
%
Effect on Capacity, Gram
of 72 per Gram of AA
At 99.9%
0.280
0.196
-0.190
Not significant
0.410
0.443
Mean capacity
Mechanism. It is likely that the reaction of activated
alumina with fluorine can be characterized by a continuous
reaction model as described by Levenspiel (1962), since AA has
a porous structure
and high surface area (260 to 290 meters
per gram). The model is shown schematically in Figure 3.
It is suspected that the reaction rate becomes pore diffusionlimited because of partial plugging of the pores by the buildup
of reaction product. The product, A1FS, is stoichiometrically
less dense than the reactant, A A, and would, therefore tend to
fill the pores. There is an indication that diffusion through the
gas film surrounding the particle does not limit the reaction
rate, since gas velocity does not appear to have a significant
effect on capacity.
Figure 4 shows the rate of a solid-gas reaction vs. time with
temperature as a parameter and L/D, Dv, V, and volume per
cent constant.
These curves represent the maximum possible
rate of reaction vs. time for a reaction which becomes pore
diffusion-controlled. The curve at the higher temperature,
0.0653
(L/D-4.5)
0.00088(Dp-291) in
grams of F2 per gram of AA
At 200
p.p.m.
0.307
0.178
-0.162
Increasing Variable
Temperature, 300° to 400° C.
Bed size, L/D, 3 to 6
Particle size, 48-100 to 28-48-mesh
)
Velocity, 1.25 to 1.65 Vm¡
Concentration, 5 to 10 vol. % )
TIME
-
( \
(2)
l
r:
°
where 7 is in K., and Dp is in microns.
These equations represent the best (least square) linear
interpolation over the range of variables studied (Table I).
The equations should, not be used to extrapolate outside of the
ranges, and the user should not assume that the effects of
velocity and concentration are not significant outside of the
ranges studied. Within the range of variables studied, the
maximum capacity can be obtained using 7 = 400° C., Dv =
183 microns, L/D = 6, any concentration from 5 to 10 v./o.,
and any velocity from 1.25 to 1.65 Vmf (compare run 16 to
other runs of Table II).
I
LOW
CONVERSION
1
u.
oL
Es
J-1-L)
R
O
R
RADIAL POSITION
Continuous reaction model
Figure 3.
VOL.
6
NO.
4
OCTOBER
1
967
411
Figure 4. Maximum reaction rate for
gas reaction
a
solid-
7h¡, gives a higher reaction rate due to the normal effects of
temperature on chemical reaction rates and diffusion rates.
For the F2-AA reaction, the rate of consumption of F2 prior to
breakthrough (BT) is essentially the feed rate of fluorine, since
over 99.9% of the fluorine reacts.
This being the case, it is
not surprising that velocity and concentration had little effect
on the capacity.
The net consumption rate continues to be
the feed rate of fluorine until that time when the maximum
reaction rate profile is reached for the temperature of the
reaction. The rate then rapidly decreases along the maximum
reaction rate curve and a breakthrough of fluorine is observed.
At the higher temperature, the breakthrough is postponed
and a higher capacity is reached. The capacity is proportional to the feed rate multiplied by the time at breakthrough
or the area under the curve.
If partial plugging of the pores is caused by stoichiometrically
less dense reaction product such that pore diffusion resistance
increases, it is not surprising that the smaller particles attain a
greater capacity for fluorine. It is suspected that the larger
particles have only partially reacted cores (see Figure 3) at
the time of breakthrough.
High values of L/D gave greater capacities. This observed
effect is possibly due to poor solids mixing (Nicholson and
Smith, 1966) in the deep beds. In deep beds, some of the
particles may remain in the bottom of the reactor for longer
times and therefore will react to near the maximum theoretical
capacity, since the concentration of fluorine may be high
mass transfer resistance in the pores.
enough to overcome
The breakthrough of fluorine is postponed, since, in deeper
beds, the particles at the top have not yet fully reacted and
are able to remove
small amounts of fluorine coming from
the lower portions of the bed. The net effect is that the
capacity is higher in deep beds. There may be practical
limits to the observed positive effect of increasing L/D when
gas-solids contacting becomes poor because of slugging.
Extreme Conditions. A series of five additional runs was
made to determine the effect on capacity of operating with
conditions outside the range of those used in the factorial
experiment. Since there are practical limitations on using
smaller particles of activated alumina and deeper beds (because
of the size of the reactor), these variables were not investigated
further. One of the five runs was made to determine the effect
of predrying the activated alumina.
One run of this series was made under the same conditions
as in run
16 (which gave the highest capacity in the factorial
412
l&EC
PROCESS
DESIGN
AND
DEVELOPMENT
experiment), except that the temperature was 450° C. compared to 400° C. in run 16. The resulting capacity was 0.803
gram of F2 per gram of AA, which is only slightly higher than
0.797 gram of F2 per gram of AA for run 16. This difference
was too small to be significant.
It is not surprising that there
was only a small increase in capacity, since the capacity values
are near the theoretical limiting capacity of 0.950 gram of F2
per gram of AA.
A run was made using a higher velocity in an attempt to
increase the fluorine throughput capabilities of the system.
The run was similar to run 12, except that the velocity was
3.0 Vmf. The capacity obtained was 0.637 gram of F2 per
gram of AA, which can be compared to 0.740 gram of F2 per
gram of AA in run 12. The capacity at the high velocity is
lower than that of run 12 by an amount which is about 1.7
times the standard deviation. This means
there is less
than one chance out of 10 that the observed effect is due to
The decrease in capacity at higher
experimental error.
velocities is probably due to poorer gas-solids contact (Chakravarty et ah, 1963).
Two runs were made to determine the effect of high fluorine
concentrations on capacity. Both runs were similar to run 12
except that 30 and 75 volume % fluorine were used. These
experiments gave capacities of 0.757 and 0.739 gram of F2
per gram of AA compared to 0.740 for run 12. It is therefore
concluded that F2 concentration has no significant effect of
capacity over the range 5 to 75 volume %. The runs at the
higher concentrations required external cooling and produced
axial temperature gradients as high as 200° C. in the fluidized
bed but did not adversely affect the operation.
A single run was made which demonstrated that predrying
the activated alumina had no significant effect on the capacity.
Run 14 was repeated, except that the bed was charged “as
received” rather than predried as in all of the other runs.
The normal moisture loss on drying was about 3.2% at 300° C.
and 4.3% at 400° C. The capacity obtained was 0.695 gram
of F2 per gram of AA compared with 0.676 gram of F2 per
gram of AA for run 14. These results differ by only 0.3 times
the standard deviation, so the effect of predrying was considered too small to be significant.
Comparison with Packed Beds. It is interesting to compare the throughput capabilities for the fluid-bed reaction
system with that for a packed-bed reactor.
Throughput is
the feed rate of fluorine in pounds of F2 per hour and square
feet of reactor cross section. Using 183-micron (48- to 100mesh) activated alumina and a concentration of 75 volume %,
the fluorine throughput rate is about 40 lb./hr. sq. ft. Rates
up to 140 lb./hr. sq. ft., can be obtained with 28- to 48-mesh
alumina but result in lower capacities. Data obtained earlier
for packed beds of activated alumina gave maximum throughputs of about 3 lb./hr. sq. ft. Above this throughput rate
It is obvious
sintering of the packed bed started to occur.
that if the user requires a free-flowing reaction product, the
fluid-bed technique will allow much higher fluorine throughput
rates.
The experiments with packed beds of AA gave maximum
capacities at breakthrough of about 0.85 gram of F2
per gram of AA for runs where bed temperatures were over
1000° C. and sintering occurred. The capacities achieved in
fluidized beds (up to 0.8 gram of F2 per gram of AA) were only
slightly lower than the packed bed capacities.
Nature of Solid Reaction Products. The solid reaction
products appear to be suitable for waste disposal. The beds
No
were
free flowing at the conclusion of all of the runs.
significant change in particle size is caused by the stoichiometrically less dense reaction product or by attrition due to the
Table IV.
Experimental Conditions for the Disposal of
Fluorine Using Na2C03 and CaC03
°
400
Temperature, C.
Bed size,
L/D
Particle size, mesh
Concentration, vol. % F2
Velocity, multiple of Vm/
6
—60, +100
10
1.65
turbulence of the fluid bed. The bulk density of the product
from run 16 was 1.22 grams per cc. untapped and 1.36 grams
per cc. tapped.
Other Solid Reagents. Two other solid reagents, limestone
(CaC03) and soda ash (Na2C03), both of which are less
expensive than activated alumina, have been tested for fluorine
disposal using the conditions shown in Table IV. Qualitatively, the results of single experiments on each of the solids
were similar to the results for activated alumina.
The curves
of fluorine concentration vs. time were like those in Figure 2.
The runs were characterized by a long period of high removal
efficiency, followed by an abrupt breakthrough period. The
capacity of soda ash was 0.32 gram of F2 per gram and the
capacity of the limestone was 0.045 gram of F2 per gram.
These capacities correspond to about 90 and 12% of the
theoretical maximum capacities for the soda ash and limestone,
respectively. It appears that the reaction rate with limestone
becomes diffusion-cont rolled and breakthrough occurs.
The
product of the soda ash reaction, sodium fluoride (NaF),
apparently does not hinder the reaction, since the reaction
product is stoichiometrically denser than the reactant and
thus would not tend to produce an increased resistance to mass
transfer. Since NaF is known to be an effective sorber for
certain volatile fluoride compounds, the NaF product from the
soda ash reaction may effectively remove
volatile fission
producís associated with the fluorine stream.
Conclusions
A fluidized bed process can be used effectively to dispose ot
fluorine using activated alumina as the reactive solid. The
process is capable of high fluorine disposal rates and efficiencies
(over 99.9%) over a wide range of the independent process
variables.
A factorial experiment determined that increasing temperature (300° to 400° C.), increasing ratio of bed depth to diameter (3 to 6), and decreasing particle size (399 to 183 microns)
significantly increase the capacity of the activated alumina for
fluorine removal to near the theoretical maximum value.
There were no significant effects of changing the fluorine
concentration from 5 to 75 volume % or of changing the
velocity from 1.25 to 1.65 Vm/. Higher values of velocity
(3.0 Vm/·) may slightly decrease the capacity of activated
alumina for fluorine.
Other solid reactants, which are less expensive than activated alumina, are also being evaluated. Soda ash is especially promising.
Acknowledgment
The authors thank C. B. Schoffstoll for his help in construction and operation of the equipment.
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Received for review November 16, 1966
Accepted May 15, 1967
Division of Nuclear Chemistry and Technology, 152nd Meeting,
ACS, New York, N. Y., September 1966. Work performed under
the auspices of the U. S. Atomic Energy Commission, Contract
No. W-31-109-eng-38.
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