91
Ind. Eng. Chem. Process Des. Dev. 1980, 79,91-97
and Motor Octane Numbers of 90.6 and 78.7, respectively, and direct Research and Motor Octane Numbers of 92.3 and 81.5, respectively. These results are similar to previous results from gasoline produced by hydrocracking bituminous coal extract with ZnC1, catalyst (Consolidation Coal Co., 1968a). These results show the ZnC1, gasoline to be an acceptable blending stock and acceptable as an unblended gasoline providing the Research Octane Number is over 91. The chlorine content of the gasoline should not be deleterious, at least by current standards, since chlorine and bromine are normally added to leaded gasolines in much greater amounts to act as lead scavengers. The 200 X 475 "C middle oil and the +475 "C heavy oil would make premium fuel oils that can meet the most stringent current environmental regulations because of their extremely low nitrogen and sulfur contents ( 1, depending on the number of mixing elements and flow rates. The observed phase inversion due to a change in the material of the packing is a very interesting and important phenomenon and should be considered in designing continuous extraction units. I t is apparently related to different wetting properties of the packings and certainly deserves fundamental studies. It has important effects on the nature and characteristics of the dispersion generated from the point of view of the separation stage (Merchuk et al., 1979). In Figure 12 the influence of LIX-64N concentration of the efficiency of copper recovery when Koch motionless mixers with 5 and 10 elements are used is shown. It can also be observed from Figure 12 that a large change in concentration of LIX-64N, viz. from 5% to 1070,results in a small increase in efficiency, especially if compared with the influence of the number of mixing elements. The comparison between the Koch mixer of 10 mixing elements and the agitated vessel is given in Figure 13 and shows that the increase in flow rates increases significantly the efficiency for the Koch mixer while for the mixing vessel the efficiency decreases with increase of flow rates. This is due to the fact that the residence time decreases without an increase in turbulence or interfacial area, which are independent of flow rates in this case. It can also be seen in Figure 13 that the percent of equilibrium attained is higher in the mixed vessel than in the motionless mixer at low flow rates. At high flow rates,
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90
0
/ 80
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0 - 2 0 MIXING E L E M E N T S 3-15 A- I 0 7-
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i
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/
PHASE R A T I O O I A I I ORG P H A S E 10% LlX 64N IN K E R O S E N E
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0
;
40 D
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P h o n r d m O / A 111 I A q coni.) Aq phase 2 I7 G / L coppr R1.22
XI
4
300-
0 L Y
20
3 VI Y
10
200 -
0 100
200
3oc
P, rnrn Hg
Figure 12. Influence of LIX-64N concentration on the percent of equilibrium in Koch motionless mixers. 100-
I
'I"
F L O W R A T E , L/UlH
Figure 14. Pressure drops in Koch motionless mixers.
/
A - Pqlatiar speed 1250 R P M
0-
1)
1,
470
0- IO Mixing element$ F h a o r d i o O / A 1'1 ( A q cmiJ phase: 10% LIX 6 4 N
ctg
0
30 I
2
3
4
5
Flow r a t e , lit. bin.
Figure 13. Comparison between 10 Koch motionless mixer elements and an agitated vessel.
however, the motionless mixer would be superior from the efficiency point of view. The curve of efficiency vs. flow rates is relevant only if we are interested in the throughputs, but one has to keep in mind that there are large differences in the volumes of the two systems. The residence time in the motionless mixer is two orders of magnitude smaller than that in the stirred vessel. Thus in an industrial installation there is great saving in solvent inventory and ground area. Calculations done using standard correlations for the stirred tanks and pressure drops measured in the motionless mixer showed that the energy input in the crossing of the lines in Figure 13 is about the same for both devices. (See Appendix.) Pressure Drops. The various mixers investigated have different pressure drops and thus different energy inputs are required for pumping of the solutions in the liquidliquid extraction process. The pressure drop was measured in all the experiments with an Hg U-type manometer connected to the inlet of the mixing tubes. The pressure drops in Koch motionless mixers of different lengths are shown in Figure 14. One observes how the pressure drop increases both with flow rate and with the number of mixing elements. In Figure 15 are shown the pressure drops in the packed tubes and in the Kenics type motionless mixer. The data for the 10 mixing element Koch motionless mixer are also shown for comparison. In
Figure 15. Pressure drops in packed tubes and Kenics motionless mixers.
the case of empty tubes the pressure drops were negligible. In general, it can be said that the devices that proved to be more efficient for copper recovery are also the ones that give the largest pressure drop. This is to be expected, since the energy necessary to produce turbulence and create new interfacial area must be provided in some way to the system. It is therefore apparent that optimum
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operation conditions will exist, if the objective is maximum copper recovery with minimum energy input. It must be mentioned that in addition to the pressure drops, recoveries, and efficiencies, the different mixers also differ in the dispersion power which in turn affects the separation of phases. This is a very important factor in the liquid-liquid extraction process, since it affect directly the volume of the settlers needed (Merchuk et al., 1979).
(14)2 X 13 =0.555 min = 33.35 s 4 X 3.600 t 2 being the mean residence time in the stirred vessel. These figures are quite representative of the residence times in the range examined. The ratio of the two residence times is t2/tl = 254.9
Conclusion Several motionless mixers were experimentally studied with respect to their efficiencies in copper recovery with solutions of LIX-64N in kerosene. It was found that Koch motionless mixers are much more effective than Kenics type motionless mixers for similar residence times. The high efficiency of Koch motionless mixers can also be obtained using packed tubes as mixers. Plastic and ceramic packing give different efficiencies (even with equal flow rates and phase ratios) and this seems to be related to a different continuous phase obtained. When compared with a stirred vessel, the Koch motionless mixer requires similar power input for an equal amount of copper recovered, but the volume of the equipment is smaller by two orders of magnitude.
which indicates that the residence time in the motionless mixer is smaller than that of the stirred vessel by two orders of magnitude. Power Consumption. The power consumed in the motionless mixer can be determined from the pressure drop and the flow rate. From Figure 14 it can be found that AP = 230 mmHg. Thus
Appendix Comparison of Residence Times and Energy Consumption in the Dynamic and Motionless Mixers. In order to compare the energy consumption and residence times in the two types of mixers investigated, the data reported in Figure 13 are used. The intersect of the lines representing percent of equilibrium vs. flow rate for 10 mixing elements of the Koch mixer and for the stirred vessel a t 470 rpm is taken. At this point the percent of equilibrium is 77.5% and the flow rate is 3.6 L/min, so that we have equal yield and throughput in both devices. Residence Time. The residence time in the motionless mixer, which is 1 cm in diameter and 10 cm in length, is
xD2L tl=-4Q
(1)' X 10 = 2.18 4 X 3.600
R X
X
min = 0.13 s
where t l = mean residence time in the motionless mixer, D = internal diameter of the motionless mixer, L = length of the motionless mixer, and Q = total liquid flow rate. In the stirred vessel, which is 14 cm in diameter with a liquid height of 13 cm, it is
R
t2
X
=
P,being the power consumed in the motionless mixer. In the case of the stirred vessel, the method outlined by Foust et al. (1960) was used. The power is calculated from (-4-3) where Np, = power number, P2= power consumption in the agitated vessel, D = diameter of the agitator, p = mean density of the fluid, and N = agitation speed, rps. Npois obtained from a graph given by Foust et al. (1960) for our mixing condition and mixer geometry. For NRe= 800, the Nporeads 5. In (A-3) we obtain P2= 1.75 X L atm-ls-l, which is quite similar to the value obtained for the motionless mixer. Thus we have obtained similar power consumption for the same extraction performance, but with a residence time smaller by two orders of magnitude in the case of the motionless mixer. Literature Cited Foust, S.A., Wenzel, L. A., Clump, C. W., b u s , L., Andersen, L. B., "Principles of Unit Operations", Wiley, New York, N.Y., 1960. Merchuk, J. C., Wolf, D., Shai, R., Ind. Eng. Chern. Process Des. Dev., submitted for publication. Middleman, S., Ind. Eng. Chern. Process Des. Dev., 13, 78 (1974). Shai, R., M.S. Thesis, Department of Chemical Engineering, Ben Gurion University of the Negev, Israel, 1978. Swanson, R. R., Agers. D. W., 93rd AIME Meeting, New York. N.Y., 1964. Tunison, M. E., Ph.D. Thesis, The University of Wisconsin, 1976.
Received f o r revieu December 4, 1978 Accepted July 7, 1979
