concmtration appeared to have little effect on the transfer rate. it \vould be expected to have a marked effect if the concentration \vere reduced to well below 1jyc. T h e transport of magnesium for all three extractions is shown in Figure 9 . No significant difference in rate is indicated for the extraction in which the 3lgC12 concentration in the salt is 2SY0 and the t\vo extractions in Lvhich the concentration is 15yc, In the case of magnesium no transfer is observed if n o electrical path is provided bet\veen the t\vo alloys. LYith the tantalum stirring fins making contact between the t\vo alloys the extractor behaves as a short-circuited concentration cell a n d the transfer of magnesium a t the interfaces occurs by the electrode Reactions 4 and 5. T h e extent to which these reactions or the short-circuit between the t\vo alloys affects the transfer of the other solutes is not known. Although this separation procedure has been applied to a specific problem. the method c a n be applied to many other separation problems by the appropriate choice of metal a n d salt solvents. Since a high degree of decontamination c a n be achieved. such a procedure, became of its simplicity, m a y be more advantageous than complicated countercurrent methods. Ac knowledgmenl
T h e authors are particularly indebted to K. Malaby of the radiochemistry group under the direction of A . F. Voigt for performing the large number of analyses that \yere required. T h e constriiction of various parts of the apparatus by A. Johnson. D. Howell? a n d R . Seliger is gratefully acknowledged. Literature Cited
(1) Chiotti. P.. “Proceedings of Thorium Fuel Cycle Symposium,” U . S. .\t. F.nergy Comm.. Kept. TID-7650, Book 1, 436-53 (19 62) .
(2) Chiotti, P., Mason, J. T., Gill, K. J., 7’rans. .l-Irt. Soc. AI,ME 227, 910-16 (1963). (3) Chiotti, P., Parry, S. J. S., J . LPss-Common .tfetals 4, 315-~37 (1962). (4) Chiotti, P., Shoemaker, H . E.. Ind. En?. Chrm. 50, 137-40 (1958). (5) Chiotti. P., Stevens. E. R., Trans. . M P ~Sac. . AI.ME. in prpss. (6) Chiotti. P., Voigt, A. F., “Pyrometallurgical Processing,” in “Progress in Nuclear Energy,” Series 111, Vol. 3. “Process Chemistry,” pp. 340-54, Perganion Press. S e w York, 1961. (7) Chiotti. P.. Woerner, P. F., J . Less-Common M r t a l s 7. 11 1-19 (1 964). (8) Chiotti, P., Woerner, P. F., Parry, S. J. S., in ‘.I1 Ciclo V-Th,” pp. 405-31. Comitato Nazionale Energia Nucleare. Rome, 1963. (9) Hamer, \.V.J., Malmberg, M. S.. Rubin, R. .J.. J . Electrochem. SOC.103, 8-16 (1956). (10) Johnson, I.. Anderson, K. E., “Solubility of Neodymium in Liquid Zinc,“ Chem. Engr. Div., Argonne National Laboratory, Xrgonne. Ill.. Summary Kept. ANL 6687 119631. (11) Knighton, J. K., Burris,’E., Jr.; Feder,‘H. M., “Purification of Reactor Fuels Using Liquid Zinc,” U. S. At. Energy Comm., Argonne National Laboratory, Argonne, Ill.. Kept. ANL 6223, (1961). (12) Laitinen, H. .A, Ferguson, LV. S., Osteryoung. K. A . , J . Electrochem. Soc. 104, 516-20 (1957). (13) \Viswall. R. H., Egan, J. J., Ginell, LV. S.? Miles, F. T., Powell. J. K., “Recent Advances in the Chemistry of Liquid Metal Fuel Reactors,” in Proceedings of Second International Conference on Peaceful Uses of Atomic Energy, Vol. 17, p. 424, United Nations, Geneva. 1958. (14) Yamamoto, A. S., Klimek, E. J., Rostoker, FV., Xrmour Research Foundation of Illinois Institute of Technology, LVADC Tech. Rept. 56-411 (1957). (15) Yang, L., Hudson. R. C.. Trans. M e t . Soc. A I M E 215, 589-600 (1959). i
,
RECEIVED for review July 17, 1964 ACCEPTED NOVEMBER 20, 1964 Division of Nuclear Chemistry and Technology, 148th Meeting, ACS, Chicago, Ill., September 1964. Contribution 1552. \.l’ork performed in the Ames Laboratory of the U. S. ’Atomic Energy Commission.
COM M UNICATION
HEAT TRANSFER IN PACKED-FLUIDIZED BEDS Further work in packed-fluidized beds has provided a better understanding of the effect of particle shape and thermal properties upon heat transfer in this system. Spherical packing of the same diameter but widely varying thermal properties yielded no significant difference in effective thermal conductivity, k,,,, over a full range of gas velocities. However, comparably sized cylindrical packing gave kerf values as much as four times greater than those for the spheres. This suggests that heat transfer in this system i s almost completely via the fluidized particles.
RECENTLY described
studies (3) verified the improvement of heat transfer in systems of fine solid particles fluidized in the interstices of stationary packing as compared to transfer in systems of packing alone. These packed-fluidized bed systems \\‘ere investigated to determine the effects of some of the more important operating variables a n d to provide a basis for proposals of mechanisms. Although the effect of variables such as gas velocity a n d the size of packing a n d fluidized particles was reasonably clear-cut, the results were inconclusive in differentiating between a n effect of the packing shape and thermal properties. I t was determined only that for systems of the same fluidiied material. those with packing of brass cylinders gave higher values of effective thermal conductivity, keff, than those with steel sphere packing of the same size. Subsequent experimentation ( 7 ) has provided evidence that allo\vs this question to be resolved a n d is the subject of this communication.
Experiments were performed utilizing stationary packing materials of widely different thermal properties but having the same shape, thus eliminating a n y effect of shape o n k e f f . These results were then compared mith those obtained for a change in shape, utilizing the same material. T h e experimental procedure was similar to that described earlier (,3). T h e packing materials were copper, glass, a n d steel spheres a n d steel cylinders. T h e significant dimension was 0 5 inch for each shape, the sphere being 0.5 inch in diameter and the cylinders being 0.5 inch long and 0.5 inch in diameter. T h e fluidized material used was glass beads of 0 0058-inch average diameter in all cases. Air was used as the fluidizing medium. A wire screen above the bed of glass packing prevented expansion a n d motion of the fixed packinq. This w a s particularlv necessary because with packing and particles of the same density, the packing tended to float and provide a secondary fluidized phase. I n fact. for this case if the motion of the packing was not arrested, the heat transfer was considerably greater than for static packing. VOL. 4
NO. 2
APRIL 1 9 6 5
239
60
-
w;
50-
/
40 -
/
,-STATIONARY PACKING
\
I
STATIONARY PACKING
I
-FLUIDIZED
FLOWING
PAR T I C L E S
GAS
'FLOWING GAS
I 1
I PACKED BED
PACKED-FLUIDIZED BED
a
Figure 2.
I/ I
0
FLUIDIZED GLASS B E A D S - 0 0 0 5 8 INCH
I
20
1
I
40
~
60
'
'
80
I
100
AIR MASS VELOCITY, G ( l b / h r - s q
~
I ~
120
~
140
'
o,9k
'
I n Figure 1. k,ff values for the four packing conditions are presented as a function of air mass velocity. No significant difference in keff is noticeable for the spheres, regardless of the fact that three different materials of widely varying thermal properties were used and are represented in this plot. The effect of packing thermal properties on kerf is considered, therefore, to be negligible. 'The system with cylindrical packing has higher kerf values over the range of gas velocities used than those with spherical packing of the same size. This is clearly illustrated in Figure 1, In a packed bed, heat transport may be thought to occur via three parallel paths, as shown in Figure 2 A . First: heat may flow continuously through the network of solid packing. Second, transfer may occur by a path through the continuous gas phase. Finally, heat transport may proceed by the path solid-gas-solid. In the packed-fluidized system, the fluidized particles provide a n additional heat-carrying capacity (Figure 2 B ) . I n fact, it has been demonstrated ( 2 ) that for a variety of fluidized particle systems, heat transfer from a surface to a fluidized bed is predominantly the result of heat absorption by particles arriving a t the surface from the bulk of the bed and subsequently returning to that position and condition. I n Figure 3, the ratio of the heat flux by particle transport is compared to the total heat flux for the various systems of particles. It is apparent that almost the entire heat transport is via the particles. T h e fact that heat transfer is independent of packing thermal properties in a packed-fluidized bed indicates that paths which include the solid packing material-Le., the parallel paths, packing alone, and packing-fluidized particlespacking, etc., as shown in Figure 2B-are of relatively little importance. Therefore, the path of consequence in a packedfluidized bed would be that of fluidized particles alone. I t follows that any means of providing greater particle circulation should improve packed-fluidized bed heat transfer. T h e shape of the packing determines the make-up of void spaces and hence the channels for flow of gas and particles in l & E C PROCESS D E S I G N A N D D E V E L O P M E N T
I
-A -.. -
x~x-x-x---Bc
ft)
Figure 1. Effective thermal conductivity for various stationary packing materials
240
!
B Heat transport paths
--g
- -- - -
-----_
-0
FLUIDIZED SOLID PARTICLES X FUSED ALUMINA, LARGE B - FUSED ALUMINA, SMALL 0 - COPPER A - GLASS
4;
e*'*
".,
-
0.5
1
Figure 3. flux
2
3 4 5 6 7 G/Gmf (REDUCED MASS VELOCITY OF GAS)
8
9
Ratio of heat flux via particles to total heat
the packed or packed-fluidized system. Some packings, no doubt, allow better particle circulation than others. O n the basis of improved radial heat transfer and visual observation, it \\.odd seem that the cylindrical packing allo\vs greater overall particle movement than the spherical packing. Other means of enhancing particle motion. maintaining a high ratio of packing to particle size by either increasing packing size or decreasing particle size and increasing gas velocity, at least to certain limits, have been shown to improve the effective conductivity in the packed-fluidized bvd ( 3 ) . Nomenclature
G = average gas velocity, lb./hr.-sq. ft. G,v, = average gas velocity a t incipient fluidization, lb./hr,/ sq. ft. kef! = effective thermal conductivity of a packed-fluidized bed, B.t.u./hr.-ft,-'F. = total heat flux, B.t.u./hr.-sq. ft. q. q p = heat flux via particle transport, B.t.u./hr.-sq. ft. literature Cited
(1) Frischmuth, R. \V., Jr., M.S. thesis, Northwestern University. August 1963. (2) Ziegler, E. N., Brazelton. LY. T., IND. ENG.CHEM.FUNDAM E N T A L S 3, 94 (1964). (3) Ziegler, E. N., Brazelton, \V. T., IUD. Ezc. CHEM.PROCESS DESIGN DEVELOP. 2, 276 (1963).
Northwestern University Evanston, Ill.
EDWARD N. Z I E G L E R R O B E R T W. F R I S C H M L TH, J R . W I L L I A M T. B R A Z E L T O N RECEIVED for review August 6, 1964 ACCEPTED October 29. 1964
