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Effects of Gas Properties on the Behavior of Fluidized Beds of Small Particles

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Effects of Gas Properties on the Behavior of Fluidized Beds of Small Particles EFFECTS OF GAS PROPERTIES ON THE BEHAVIOR OF FLUIDIZED BEDS OF SMALL PARTICLES Takami KAI and Shintaro FURUSAKI Department of Chemical Engineering, University of Tokyo, Tokyo 113 Key Words: Fluidization, Fluidized Bed, Fluidity, Bubble Frequency, Bubble Size, Pressure Fluctuations, Gas Viscosity, Gas Density The quality of fluidization of FCCparticles (dp =60/im) was studied by using six kinds of fluidizing gases in a 0.081 mdiameter column. As several investigators have described, minimumfluidization velocity was inversely proportional to gas viscosity and was almost independent of gas density. The expansion ratio of the emulsion phase was, however, dependent on gas density and increased with viscosity and density. Both measured bubble frequency and deviation of pressure fluctuations were also influenced considerably by the gas properties. The voidage of the emulsion phase was considered to be dominated by the properties of the gas flowing through the emulsion phase. Thus the apparent viscosity of the emulsion, i.e. the quality of fluidization, is influenced by those properties of gases. properties. Introduction Several investigators have studied the bed behavior The behavior of fluidized beds at elevated tempera- of heterogeneous fluidization at elevated tempera- tures or pressures differs from that under ambient tures or pressures. It was found that bubble size conditions. Hong et al.l2) reported that the particle decreased with increasing temperature9'13'27'29?32) and adhesion forces increased with temperature and af- pressure.19'28) From these results gas density and fected the mean voidage of the bed at minimum viscosity are considered to affect the quality of fluidization. Piepers et al.19) reported that the in- fluidization. creasing elasticity moduluswith pressure explained In the present study, the behavior of fluidized beds the increasing bed expansion with pressure. In addi- of small powders was investigated by changing gas tion to the above causes, one of the main factors properties. For this purpose we measured Umf, the affecting fluidity seems to be a change in properties of expansion ratio of the emulsion phase, bubble fre- fluidizing gases. quency and pressure fluctuations. Six kinds of gases The relationship between minimum fluidization were used to vary the properties of fluidizing gas at velocity and gas properties has been studied exten- ambient pressure. The pressure fluctuation and bub- sively. At low values of the Reynolds number mini- ble signals were measured at ambient and elevated mumfluidization velocity Umf is almost indepen- temperatures. dent of gas density and varies inversely with vis- 1. Experimental cosity.5'7'1^ Therefore, Umf decreases with increasing temperature6'26'29) and is unaffected by the change of 1.1 Materials pressure.3'14'19'29) On the other hand, minimum bub- The solid used in the present study was a cracking bling velocity Umbchanges with gas density and catalyst (FCC) which is characterized as the group A' viscosity.140'14'23* Geldart and Abrahamsen10) have powder in Ikeda's criterion16) for good fluidization. correlated Umb with the properties of fluidizing gas The properties of the dry particles are given in Table and particles. They used the ratio Umb/Umf as a 1. parameter to judge the hydrodynamic behavior of Since fluidized beds of dry particles were affected by small powders. According to their results it is possible electrostatic charges at room temperature, FCCpar- that even powders which behave as group A8) pow- ticles containing a small amountof water in the pores ders when fluidized by air behave as group B powders were provided for the present experiments. The in- whenfluidized by some other gases, such as hydrogen. fluence of electrification was decreased by this treat- Henceit is expected that the characteristics of bub- ment.13) Also, the influence was found to be small at bling fluidized beds are also influenced by gas elevated temperatures.13) 1.2 Experimental apparatus and measuring system Received June 21, 1985. Correspondence concerning this article should be addressed to S. Furusaki. T. Kai is now at Dept. ofChem. Eng., Kagoshima Univ., Kagoshima Minimumfluidization velocities were measured in a 890. fluidized bed of a 0.051 m i.d. acrylic column. The VOL. 19 NO. 1 1986 67 Table 1. Physical properties and size distribution of particles dp 60 ,um pb 0.46 x 103 kg/m3 Fines fraction less . _ 0. than 44/^m " -25/mi 3.1wt% 25- 44 14.8 44- 53 12.6 53- 63 22.3 Size distribution 63- 74 22.5 74- 88 13.3 88-105 6.4 105-149 4.4 149- 0.7 Fig. 1. Viscosity correlation with minimum fluidization settled bed height above the distributor was 0.3m. velocity. Five kinds of gases were used as the fluidizing gases: nitrogen, hydrogen, helium, ethane and carbon could be explained by a dimensionless number Np: dioxide. Pressure fluctuations and bubble frequency were Np = ^gd3p(Pp - P)/fi (1) measured in a stainless steel columnat ambient and The expansion ratio was in inverse proportion to the elevated temperatures. The main part of the column parameter Np. In their experiments, however, the gas was 0.081m i.d. and 1.5m high. The settled bed properties were not changed. height was 0.6m. Fluidizing gases were air and the The relationship between (Le -Lq)/Lq and gas vis- gases noted above. cosity obtained from measurement in this study is Pressure-fluctuation signals were detected by using shown in Fig. 2. The values of the expansion ratio are a differential pressure transducer fitted to a tapping relatively small under conditions using hydrogen and 0.05 m above the distributor. The mean deviation and helium as fluidizing gases. Therefore it is considered dominant frequency of the signal were found from that not only gas viscosity but also gas density calculation. Bubble size was calculated from the influences the expansion of the emulsion phase. bubble frequency measured by an optical probe The expansion ratio in this study was found to be mounted on the central axis of the bed 0.45m above correlated with gas viscosity and density in the form the distributor. Further details on the apparatus of a power-type equation by the method of nonlinear design and measuring system were described in the least squares: previous paper.13) J±lZ^= 58o/V-08 (2) 2. Results 2.1 Minimum fluidization velocity The experimental values of (Le -Lq)/Lq are well cor- Minimum fluidization velocity was determined related by this expression, as can be seen in Fig. 3. from the usual plot of pressure drop against super- Abrahamsen and Geldart2) have also suggested a ficial gas velocity. In a region of low Reynolds relationship to predict the average emulsion-phase number, where viscous forces predominate, Umfis voidage: almost independent of gas density and inversely pro- portional to.gas viscosity.5'7'15* Figure 1 shows a Le _2.54po7 ~^0.10.118/ol6/i° O66exp(0.090F)\0.U8r0.043 ^ correlation between Umfand gas viscosity. The slope ^mf ap 9 \Pp P) ^mf in the figure is equal to -1. It was confirmed from The difference between Lmf and Lq is negligibly small this figure that Umfwas inversely proportional to gas and so the value of LJLmfis almost equal to that of viscosity. LJLq. The average error expressing our data ofLJLq 2.2 Expansion of emulsion phase using Eq. (3) was 6.6%. However, the parameter The equivalent height of the emulsion phase in the (Le-Lq)/Lq is so sensitive to the change ofLe that Eq. aggregative bed, Le, can be determined by extrapolat- (3) was not satisfactory to predict the value of ing the sedimentation curve to time zero.21) At gas (Le-Lq)/Lq. The predicted values from Eq. (3) are velocities above 0.04m/s, Le was almost independent twice as large as the experimental values. of gas velocity2'17'18'20* and bed diameter.17* Geldart and Abrahamsen10) have suggested that the According to the results of Morooka et al.,m the ratio Umb/Umfof group A powders could be expressed expansion ratio of the emulsion phase (Le-Lq)/Lq by 68 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN Fig. 2. Expansion ratio of emulsion phase. Fig. 4. Deviation of amplitude of pressure fluctuations at 280K. Fig. 3. Comparison between experimental and predicted values of the expansion ratio of emulsion phase. Fig. 5. Deviation of amplitude of pressure fluctuations at umf {pP-p)gdp 640K. They showed that the ratio Umb/Umf is a useful criterion for judging the hydrodynamic behavior of small powders. It is interesting that the dependence of (Le -Lq)/Lq on gas properties is just similr to that of Umb/Umf, while the ranges of gas velocities were quite different: Umbwas less than 0.01 m/s and Le was measured at velocities of more than 0.05 m/s. 2.3 Pressure fluctuations and bubble size Figures 4 and 5 show the relationship between the gas velocity and the average deviation of the ampli- tude of the pressure fluctuations for six types of gases at 280 and 640K, respectively. It is seen that the deviation depended not only on temperature but also on the kind of fluidizing gas. In both cases the Fig. 6. Variation of bubble size for six types of fluidizing deviations were very large whenbeds were fluidized gas at 280K. by hydrogen. The deviation for ethylene at 280 K was relatively small, but this did not correspond certainly to the good quality of fluidization. The fluidity might bubble size was largest and the fluidization quality be influenced by the gas adsorption of the particles. was worst when beds were fluidized by hydrogen. The Figures 6 and 7 show the relationship between gas bubble size for ethylene was also relatively large.
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