Apparatus for contacting particulate material with processing liquid

Apparatus for contacting particles with a fluid includes two tubes having different diameters and each including a straight lower section inclined relative to the horizontal and a straight, vertical upper section, the lower ends of the tubes being joined together. Fluid introduced into the lower ends of the tubes allows particles to drop slowly in the larger diameter tube and fluidizes the same particles in the smaller diameter tube.

BACKGROUND OF THE INVENTION 
This invention, which resulted from a contract with the United States 
Department of Energy, relates to an apparatus for contacting a fluid with 
particulate material and, more particularly, to an apparatus for 
contacting nuclear fuel microspheres with processing liquids. 
The preparation of gel spheres containing fissile uranium requires 
efficient contact of the spheres with process liquids. For various reasons 
it would be advantageous to substitute continuously operating liquid-solid 
contacting equipment for the batch type apparatus which has been used 
heretofore for manufacturing such nuclear fuel. However, systems 
previously designed for aging and washing fuel spheres in a continuous 
type process have not provided satisfactory liquid-solid contact due to 
their failure to provide a controlled, efficient counter-current liquid 
flow which can move the fragile spheres through contact zones without 
damaging them. Recently, a U-shaped column was tested as a means for 
continuously contacting fuel spheres with a liquid flowing therein, but it 
was found that the curved configuration of the lower portion of the column 
allowed excessive accumulation of spheres therein, making it impossible to 
maintain constant and continuous sphere movement by manipulation of liquid 
flow and resulting in damage to the spheres. There is therefore a need for 
an effective means for contacting fuel spheres with process liquids under 
controlled counter-current flow conditions. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide an apparatus in which nuclear 
fuel spheres can be contacted with a process liquid in a continuous flow 
process without being damaged. 
Ahother object of the invention is to provide a countercurrent flow 
liquid-solid contacting apparatus in one section of which nuclear fuel gel 
spheres settle in a processing liquid under the influence of gravity 
without appreciable mixing of the spheres and in another section of which 
the same spheres are fluidized for discharge from the apparatus. 
These objects are achieved by a preferred embodiment of the invention 
comprising a pair of substantially cylindrical tubes having different 
diameters and each including a straight lower section inclined at an angle 
of about 45.degree. relative to a horizontal plane and a straight upper 
section disposed substantially perpendicular to said plane, said tubes 
being communicatively joined to each other at their lower ends, a conduit 
extending into the lower end of the tube having the larger diameter for 
introducing liquid therein, a conduit for introducing a second stream of 
the same liquid into the upper end of the tube having the smaller 
diameter, a conduit extending into the upper end of the tube having the 
larger diameter for removing a stream of said liquid therefrom, a conduit 
feeding a particulate material into the upper end of the tube having the 
larger diameter, and a conduit extending into the upper end of the tube 
having the smaller diameter for removing a stream of said liquid and said 
particulate material therefrom.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
In FIG. 1 reference number 10 generally designates a liquid-solid 
contacting apparatus which is constructed in accordance with the 
principles of this invention and which comprises two cylindrical tubes 
12,14 having different diameters. More particularly, in an embodiment of 
the invention which has been tested, tube 12 has an I.D. of 5.1 cm and 
tube 14 has an I.D. of 7.6 cm. The lower edges of tubes 12,14 are 
coterminous, and tube 12 communicates with tube 14 through an aperture in 
a wall 16 fixed to both tubes. Each tube includes a straight lower section 
that is inclined at an angle of 45.degree. relative to a horizontal plane 
and a straight upper portion which extends vertically from the lower 
section is joined therewith by a curved section. The lower section of tube 
12 has a length of about 3 cm (short side), the upper section of this tube 
has a length of 61 cm, and the curved section between the straight 
sections is a 2 inch IPS schedule 40, 45.degree. pipe ell. The lower 
section of tube 14 has a length of 2 cm (short side), the upper section of 
this tube has a length of 91 cm, and the curved section between the 
straight sections is a 3 inch IPS schedule 40, 45.degree. pipe ell. 
A conduit 18 extends in sealed relation through an aperture in the lower 
end of tube 12 and into the lower end of tube 14 for a distance of 2 cm, 
the longitudinal axis of the conduit being disposed at an angle of 
45.degree. relative to a horizontal plane and the conduit being spaced 0.1 
cm from the wall of conduit 14 and having an I.D. of 0.8 cm. Connected to 
the outer end of conduit 18 is an outlet valve 20. A conduit 22 provided 
with a flow control valve 24 is also connected to conduit 18 and to a 
source of wash liquid. Another conduit 26 is arranged to feed a second 
stream of the same liquid into the upper end of tube 12. Conduits 28,30 
respectively extend into the upper ends of tubes 12,14. Conduit 30 
connects with a conventional pump (not illustrated) and is provided with a 
filter 32. Conduit 28 terminates as an open discharge of adjustable 
elevation. An input conduit 34 extends into the upper end of tube 14 and 
connects with a source of particulate material that will be described 
hereinafter. 
FIG. 2 illustrates another embodiment of the invention which includes three 
liquid-solid contactors 10a-10c each of which is constructed as described 
above. The additional process apparatus associated with contactors 10a-10c 
will be described hereinafter. 
OPERATION OF PREFERRED EMBODIMENTS OF THE INVENTION 
The operational advantages of the liquid-solid contacting apparatus 
illustrated in FIG. 1 were demonstrated in tests wherein gel spheres 
containing nuclear fuel in the form of UO.sub.3 were washed with 0.5 M 
NH.sub.4 OH solution. Tubes 12,14 of the tested apparatus were constructed 
of glass so that flow of materials therein could be observed. The gel 
spheres, which had a diameter of about 0.35 cm, were fed into the upper 
end of tube 14 at a bulk flow rate of 3.5 liters/hour for about 4 hours. 
During this period of 4 hours a first stream of the above-mentioned 
NH.sub.4 OH solution was introduced into the lower ends of tubes 12,14 
through conduit 18 at a rate of 3.2 liters/min for a first period of 2 
hours and at a rate of 2.9 liters/min for a second period of 2 hours, and 
a second stream of the same solution was introduced into the upper end of 
conduit 12 through conduit 26 at a constant rate of 1.8 liters/min. The 
I.D. of conduit 28 was 1.7 cm and the end of this conduit within tube 12 
was about 13 cm below the level of liquid in tube 14 during the described 
wash operation. During the initial 2 hours of operation of the apparatus, 
NH.sub.4 OH solution was withdrawn from the upper end of tube 14 through 
conduit 30 at a rate of 0.6 liter/min, and during the next 2 hour period 
the wash solution was withdrawn through conduit 30 at a rate of 0.3 
liter/min. The portion of the wash liquid which was introduced into tube 
14 through conduit 18 and which flowed upwardly through tube 12 and 
discharged through conduit 28 was 2.6 liters/min during the entire wash 
period of 4 hours, and all of the wash liquid introduced into the upper 
end of tube 12 through conduit 26 was also discharged through conduit 28. 
The average time that gel spheres were in the countercurrent wash zone was 
1.3 hours. 
The diameter of tube 14 and the flow rate of wash liquid through this 
larger diameter tube were selected so that the gel spheres discharged from 
conduit 34 settled in the wash liquid under the influence of gravity 
without appreciable mixing (i.e., the spheres settled as a moving bed), 
thus avoiding mixing and allowing efficient countercurrent contact of 
spheres and wash liquid in tube 14. However, the flow rate of wash liquid 
in the smaller diameter tube 12 was such that a fluidized mixture of the 
spheres and wash liquid was observed in this tube, which mixture flowed 
out of the upper section of tube 12 through conduit 28 together with the 
additional wash liquid added to tube 12 through conduit 26, this 
additional wash liquid assisting in fluidizing the spheres at the upper 
end of said tube 12. Analyses of the washed spheres showed NO.sub.3 - 
decontamination factors (i.e., the ratio of NO.sub.3 - ions in equilibrium 
with inlet spheres to NO.sub.3 - ions in equilibrium with washed spheres) 
of &gt;400 for the first 0.65 hour wash period and &gt;200 for the second 0.65 
hour wash period. The washed gel spheres were dried and found to be of 
good quality with no signs of damage or inadequate washing. It could be 
observed during the test that the V-shaped configuration of the 
liquid-solid contactor at joined lower ends of tubes 12,14 and the 
arrangement of inlet conduit 18 in tube 14 provided good fluidization of 
gel spheres at the junction of the tubes and consequently good transfer of 
the spheres from tube 14 into tube 12. As mentioned hereinbefore, tests 
conducted with a liquid-solid contactor having a U-shaped configuration 
showed poor transfer characteristics of particulate material at the bottom 
of the apparatus under corresponding flow conditions. 
After the volume rate of solids to be processed is specified for a 
particular application, the important dimensions of a moving bed-fluidized 
bed column are selected as follows. A ratio of liquid volume/solid volume 
is selected to exceed the minimum process requirement without being 
wasteful. This ratio times the volume rate of solids gives the liquid rate 
through the moving bed. The waste liquid exit above the bed will be this 
rate plus any liquids that enter with the solids. The superficial liquid 
velocity which would cause fluidization is calculated by methods 
well-known in chemical engineering technology. The moving bed diameter is 
selected to give a superficial liquid velocity well below the fluidization 
velocity. The moving bed height is selected to give the solids residence 
time (bed volume/volume flow rate of solids) necessary to meet the process 
requirements. The exit flow of slurry from the fluidized bed side must be 
large enough to limit the solids concentration and avoid plugging of the 
exit line. Ten volume percent solids will usually flow well and higher 
concentrations are practical for slurries with good flow properties. The 
exit line diameter must be large enough to avoid bridging by the largest 
solid particle; this diameter should be at least four times the maximum 
particle diameter. For large particles, this line size and the velocity 
required to maintain solids movement in the line size and the velocity 
required to maintain solids movement in the line may result in a larger 
flow than the minimum required to limit the solids concentration. The 
fluidized bed diameter is selected so that the minimum fluidization flow 
would be exceeded if all the exit line flow was through the fluidized bed. 
The two inlet lines for liquid are sized so that the exit flow can be 
supplied completely by one inlet flow or by any split between them as 
required for good control of the solids movement. The height of the exit 
discharge is made adjuatable and is set by observation during initial 
operation to give a reasonable liquid level above the moving bed. 
An example of specific column size calculations is as follows. The solids 
are UO.sub.3 gel spheres of 1.5 g/cm.sup.3 particle density and 5.6 liters 
per hour bulk volume flow rate. The wash liquid used in the tests that 
have been conducted was an aqueous solution consisting of 1.0 M (NH.sub.4 
OH+NH.sub.4 NO.sub.3) and other solutes with a density near 1.05 
g/cm.sup.3. The two gel sphere diameters of concern are 0.07 cm and 0.35 
cm. A liquid volume/solid volume ratio of four is selected to assure good 
washing even if some uneven flows or channeling occurs. The liquid rate is 
then (4) (5.6)/60=0.37 liter per minute and this will be the waste liquid 
exit flow from above the moving bed. Using the methods of calculation in 
Perry's Chemical Engineers' Handbook, pages 5-52 to 5-54, the minimum 
fluidization velocities are about 0.28 cm/sec for 0.07 cm diameter and 2.8 
cm/sec for 0.35 cm diameter. The minimum countercurrent wash time is about 
two hours for the 0.35 cm diameter, but could be much shorter for the 
small diameter. The minimum column cross section for the 0.07 cm diameter 
and 0.37 liter per minute is then 22 cm.sup.2 area or 5.3 cm diameter. A 
conservative design is then one fourth the fluidization velocity or about 
10 cm or 4 in diameter. The moving bed length to give a two hour residence 
time or (2) (5.6) volume is 40 cm or 4.7 ft. The bed length could be much 
shorter for the 0.07 cm diameter spheres. The slurry exit rates should be 
(10)(5.6)/60=1.0 liter per minute as a minimum. The line sizes should 
allow for larger particles than the average and should be about 1-cm-ID 
and 2-cm-ID for the 0.07 and 0.35 cm average particle sizes. The 1.0 liter 
per minute minimum flow for the 0.07 cm spheres and 1-cm-ID will provide 
good hydraulic flow, but the 2-cm-diameter line and 0.35 spheres will 
require about 5 liters per minute. The maximum column diameters for 
fluidization are then about 9 cm and 6 cm for the 0.07 and 0.35 cm 
spheres. A 5-cm diameter fluidized bed would allow fluidization. 
FIG. 2 illustrates the use of a plurality of liquid-solid contactors of the 
disclosed type in a system for manufacturing nuclear fuel spheres. Drops 
of a broth containing UO.sub.3 are discharged from line 36 into a gelation 
vessel 38, and the gelled spheres thus formed are transferred by means of 
a line 40 to the large diameter tube of contactor 10a. Silicone oil is 
pumped from tank 42 by pump 44 through line 46 to the lower end of 
contactor 10a, the oil being heated by heater 48 before entering the 
contactor. Silicone oil is also pumped by pump 44 through line 50 to the 
upper end of the smaller diameter tube of the contactor. Gelled spheres 
are hardened (or "aged") by contact with silicone oil in contactor 10a and 
then pass through line 52 to the upper end of the larger diameter tube of 
second contactor 10b, tri-chloroethylene (TCE) being introduced through 
line 54 and into the lower end of the second contactor by means of line 56 
connected to pump 58 and to a storage tank 60 containing the TCE. A branch 
line 62 also conducts TCE into the upper end of the smaller diameter tube 
of contactor 10b. TCE which passes through the larger diameter tube of 
contactor 10b is conducted from the upper end of said tube through a 
conduit 63 to an organic recovery unit 64 which separates silicone oil 
from TCE and returns the latter to tank 60. Gel spheres and TCE wash 
liquid flow from the upper end of the smaller diameter tube of contactor 
10b through line 66 to a porous conveyor belt 68 where spheres are 
separated from the wash liquid, the spheres then being carried by the 
conveyor belt to the upper end of the larger diameter tube of third 
contactor 10c and the wash liqud being recycled through line 70 to tank 
60. An aqueous solution of NH.sub.4 OH is pumped by pump 72 from tank 74 
through line 76 into the lower end of contactor 10c and also through line 
78 into the upper end of the samller diameter tube of that contactor. 
Spheres washed in contactor 10c flow through line 80 to a second porous 
conveyor belt 82 where NH.sub.4 OH wash liquid is separated from the 
spheres and recycled to tank 74 via line 84, the spheres passing through a 
dryer 86 before being discharged from the conveyor belt. Many other uses 
for liquid-solid contactors constructed in accordance with the principles 
of this invention will be apparent to persons familiar with chemical 
process equipment.