Ind. Eng. Chem. Process Des. Dev. 1982, 27, 784-785
784
effective surface per unit of volume leads to substantial cost reductions. Further savings would result if humic acids fixed on these carriers allowed more than 90 recyclings. All considerations concerning adsorption kinetics and their cost consequences are certainly not restricted to humic acids but, in principle, also apply to any suitable adsorbent with a sufficiently short relaxation time independent of the special recovery technique. Acknowledgment We wish to thank Ing. grad. G. Putral, Ing. grad. F. Ringelmann, and Mrs.E. Borchardt for carrying out the experiments. Nomenclature A = uranium accumulation of an adsorbent defined by eq 1 A , = equilibrium accumulation t = time T = relaxation time of the accumulation process c, = uranium concentration in seawater P = uranium production per unit of time
C = elution capacity of an adsorbent Literature Cited Broecker, W. “Chemical Oceanography”; brcourt Brace Jovanovich Inc.: New York, 1974. Campbell, M. H.;Frame, J. M.; Dudey, N. D.; Kid, G. R.; Mesec. V.; Woodfield, F. W.; Bhney, s. E.; Jante, M. R.; Anderson, R. C.; Clark, G. T. Exxon Nuclear Company Report, 1979, XN-RT-15 I. Davles R. V.; Kennedy, J.; McIlroy, R. W.; Spence, R.; Hill, K. M. Nature (London) 1964, 203, 1110. de Luca Rebello, A.; Wagener, K. I n “Envkonmental Biochemistry”; Neriagu, I. O., Ed.; Vol. I; Ann Arbor Scientific Publication Ann Arbor, MI, 1976; p 13. Duret, M. F.; Phillips, G. J.; Veeder, J. I.; Wolfe, W. A.; Williams, R. M. “Nuclear Resources. The Contribution of Nuclear Power to World Energy Supply. 1975 to 2020”; publlshed for the World Energy Conference by 1% Science and Technology Press, 1978. Oeschger, H; Slegenthaler, U Schotterer, U; Qugelmann, A. reelelus 1975, 27, 168.
Institut fur Chemie Nuclear Research Center Julich 0-5170 Julich 1 Federal Republic of Germany Technical University Aachen 0-5100 Aachen Federal Republic of Germany
Dieter Heitkamp*
Klaus Wagener
Received for review May 27, 1980 Revised manuscript received November 9, 1981 Accepted June 25, 1982
M A = amount of adsorbent in contact with seawater to accomplish the uranium production P
CORRESPONDENCE Comments on “Mlnlmum Fluldlzatlon Velocity at Hlgh Temperatures” Sir: From our own measurements (Botterill et al., 1982), we would agree with Pattipati and Wen (1981) that the Wen and Yu (1965) correlation fita the data for minimum fluidization velocity with an accuracy of about *34% over a range of operating temperatures up to -950 “C and at atmospheric pressure. However, we find a tendency for the correlation to underpredict the minimum fluidizing velocity at elevated temperatures. Thus, in Figure 1 a direct comparison is made between prediction and measured values over a range of operating temperatures, the lower values for a particular particle size corresponding to experiments at the higher temperatures. We disagree with them in their conclusion, largely drawn from backfitting values from some of our reported measured minimum fluidizing velocities, that the voidage at minimum fluidization, emf, does not vary with temperature. With particles of Geldart‘s Group “B” (1973), those of intermediate size and density forming beds which tend to bubble as soon as the minimum fluidizing velocity is exceeded, there is a complicated and, as yet, unexplained variation in emf with change in temperature (Batterill et al., 1982). It is this factor which largely accounts for the underprediction of the Wen and Yu correlation and we began our study of bed behavior over a range of operating temperatures because of the accumulating evidence of consistent discrepancies between measured values and predictions based on correlations derived from ambient condition testa. As we point out in our paper (Batterill et al., 1982),the reason the Goroshko equation (1968) gives a reasonable fit over a range of operating temperatures if a fictitious voidage is chosen to fit it to an ambient temperature 0196-4305/82/1121-0784$01.25/0
0.05
0.x)
0.9 1.0 U,f , (measured),m/s
5.0
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Figure 1. Plots of predicted vs. measured values of CJ,.
measurement is fortuitous. It comes about because a term was omitted to simplify the solution of a quadratic equation. This has a very pronounced effect through the important transitional flow regime: a region covered by the operating conditions of many commercial beds. With larger and/or denser particles falling within Geldart’s Group “Dn, we find no change in emf with operating temperature. The transition between the two typea of behavior is associated with change in operating conditions through Remf 12.5 at Ar 26000 and there is then a marked
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0 1982 American Chemical Society
Ind. Eng. Chem. Process Des. Dev. 1982, 21, 785-780
change in bed-to-surface heat transfer coefficient too (Botterill et al., 1981). If the measured values for emf are used in the Ergun equation (1952)and a value of r#~ chosen to fit a given result, then predictions for minimum fluidization velocity based on that equation agree closely with the measured values over the full range of temperatures tested (Botterill et al., 1982).
Literature Cited Botterill, J. S. M.;Teomn, Y.; YCreglr, K. R.. AICMSymp. Ser. 1981, No. 208, 77, 330.
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Botterlll, J. 5. M.; Teomen, Y.; Yhegir, K. R. Powder Techno/. 1982, 31, 101. Ergun, S. Chem. Eng. Prog. 1952, 48, 89. caldart, D. h w d w Tschnd. 1979, 7, 285. Qoroshko, V. D.; Rozenbaum, R. 6.; Todes, 0. H. I z v . VUIOV. Neft’i Qaz, 1958, 1 , 125. Pafflpeti, R. R.; Wen, C. Y. Ind. €ng. Chem. Procsss Des. Dev. 1981. 20, 705. Wen, C. Y.; Yu, Y. H. A I C E J . 1965, 72. 610.
J. S. M. Botterills
Department of Chemical Engineering The University of Birmingham Birmingham B15 2TT United Kingdom
Y. Teoman I(.R. Yuregir
Response to Comments on “Mlnimum Fluldlration Veloclty at High Temperatures”
Sir: Botterill et al. (1982)reported that based on their experiments the voidage at minimum fluidization, emf, varies with temperature and therefore the minimum fluidization velocity at high temperature cannot be accurately estimated unless emf at elevated temperatures is known. They showed that emf increased with increasing temperature for small particles (less than 530 pm) but emf did not increase with temperature for large particles (larger than 1230 pm). For the intermediate size, emf changes erratically, decreasing fiist and then increasing with temperature. We feel that there is no reason why emf should vary with temperature and that we can explain why their experimental results show variation with temperature. Botterill et al. (1982)performed their experiments using air at room temperature as the fluidizing medium. Their fluidized bed was heated by an electric heater or by burning propane gas to reach the desired temperature. Although it was stated in their paper that the bed temperature was maintained within 10 “C throughout the bed, air entering the fluidized bed at room temperature must be raised to the bed temperature somewhere in the bed. The thermocouple which measured the average of the solid temperature and the gas temperature could not indicate the temperature change of the gas. Consequently, the expansion of the air or the increasing gas velocity in the bed has been ignored. In order to prove this effect we conducted two sets of experiments to measure -, one set with room temperature air as the fluidizing medium with the bed heated electrically to reach the desired temperature and the other set with air heated to the bed temperature before it is introduced to the bed. We have built a system in which a 2 in. diameter fluidized bed is used as a preheater which is heated with powerfull electric heaters so that air can be heated to the bed temperature prior to being introduced into the main 6 in. diameter fluidized bed which is heated electrically. The whole installation is well insulated. No propane is 0196-4305/82/1121-0785$01.25/0
I
I
,
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1
0.35 0.4 0
200
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600
BED T E M P E R A T U R E
800,
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Figure 1. Variation of td with bed temperature.
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used to heat the system. Excellent uniformity of gas and solid temperature can be maintained in the &in. bed with this arrangement. Using this equipment, we performed experiments with and without preheating the air and obtained according to the procedure described by Botterill et al. (1982). The results are shown in Figure 1. It can be observed that there is an increase in emf with temperature when the air is not preheated, but there is no significant variation in emf with temperature when the air is preheated. It is important to make sure that the air property entering into the bed is the same as that in the bed. The minimum fluidization velocity of 250-pm sand changes from 5.28 cm/s at 20 “C to 2.16 cm/s at 850 “C. If air is not preheated, it is possible that different portions of the bed reach minimum fluidizing conditions at different velocities and the Ud measured under this condition can be greater than Vmfmeasured with preheated air. We feel that this is the reason why the experimental data of 0 1982 American Chemical Society
