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Ind. Eng. Chem. Res. 1998, 37, 1130-1135
Temperature Effect on the Particle Entrainment Rate in a Gas Fluidized Bed Jeong-Hoo Choi,*,† Ho-Jung Ryu,† Do-Won Shun,‡ Jae-Ek Son,‡ and Sang-Done Kim§ Department of Chemical Engineering, Kon-Kuk University, Seoul 143-701, South Korea, Korea Institute of Energy Research, Taejeon 305-343, South Korea, and Department of Chemical Engineering, KAIST, Taejeon 305-701, South Korea
The qualitative effect of temperature on the particle entrainment rate has been measured in a gas fluidized bed (0.1 m i.d., 1.97 m height). The gas velocity (0.65-2.3 m/s), the bed temperature (12-600 °C), the particle density (2509-6158 kg/m3), and the particle size (0.091-0.363 mm) were considered as experimental variables. The particle entrainment rate increased after an initial decrease with increasing bed temperature. The effect of temperature on particle entrainment rate decreased as either the gas velocity or the particle density increased. Within the experimental range, it could be confirmed that the change of the particle entrainment rate with temperature was very similar to that of the particle size for which the terminal velocity was equal to the gas velocity. Introduction To interpret the effect of gas properties on the particle entrainment rate is an important area relating to analysis of the hydrodynamics and performance of gas fluidized bed reactors. However, few studies in this area have been carried out over limited experimental ranges. Table 1 summarizes previous studies that reported effects of pressure and temperature on the particle entrainment rate. Romanova et al. (1980) and Milne et al. (1993) reported that the particle entrainment rate increased with temperature. However, they did not show the unique effect of temperature because their results included the effect of gas velocity that increased with temperature. Within the temperature range between 27 and 172 °C, the effect of temperature on the entrainment rate could be ignored (George and Grace, 1981). In fluidized bed combustors (Merrick and Highley, 1974; Choi et al., 1989; Lee et al., 1990; Park et al., 1991; Lee et al., 1992), the particle entrainment rate decreased with an increase of temperature. Chan and Knowlton (1984) and May and Russel (1954) found that the entrainment rate was linearly proportional to gas density in a pressurized fluidized bed. Knowlton (1992) and Knowlton et al. (1990) showed particle entrainment to be enhanced by increasing gas viscosity or density because it decreased particle terminal velocity. Chan and Knowlton (1984), Knowlton (1992), and Knowlton et al. (1990) explained the effect of gas properties rather clearly. In addition, the correlation obtained from the data of cold model fluidized beds show that the particle entrainment rate from the bed surface decreases with increasing temperature (Wen and Chen, 1980). However, the validity of extending the correlation to different * Author to whom correspondence should be addressed. Telephone: 82-2-450-3073. Fax: 82-2-454-0428. E-mail: choijhoo@ kkucc.konkuk.ac.kr. † Department of Chemical Engineering, Kon-Kuk University. ‡ Korea Institute of Energy Research. § Department of Chemical Engineering, KAIST.
temperatures and pressures is questionable because the experiments have been carried out at ambient conditions. The purpose of this study was to determine the qualitative effect of temperature on particle entrainment rate in a laboratory scale gas fluidized bed. Temperature, gas velocity, particle density, and size were considered as experimental variables. Consideration on the Qualitative Effect of Temperature The entrainment rate of a particle is proportional to the drag force acting on particle surface. Therefore, using the diameter (dpt) of the particle that has a terminal velocity equal to the gas velocity is convenient to account for the trend of the entrainment rate of the particle. The trend of dpt is able to represent qualitatively the trend of particle carrying capacity of gas for the given particle system. Figure 1 shows the variation of dpt with temperature for three different density particles used in this study, as calculated by the following equations (Kunii and Levenspiel, 1969):
dpt )
3CdFgU2 4g(Fp - Fg)
(1)
Cd ) 24/Rep for Rep e 5.8
(2)
Cd ) 10/Rep1/2 for 5.8 < Rep e 540
(3)
Cd ) 0.43 for 540 < Rep
(4)
Rep ) dptUFg/µ
(5)
where the sphericity of particle was assumed to be unity. The increase of temperature makes the gas density decrease and the gas viscosity increase. Therefore, at a gas velocity it makes the drag force increase after decrease initially; that is, the minimum dpt or the minimum particle entrainment rate occurs with respect to temperature. As can be seen in Figure 1, the
S0888-5885(97)00456-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/12/1998
A.C. A.C. A.C. 101.3-1370 350-3100 510-1970 510-1970 20∼1000 20∼800 27∼172 A.C. A.C. 27∼760 22∼871
0.43∼3.85 0.17∼0.62 0.2∼1.3 0.15∼0.64 0.2∼0.5 0.08∼0.2 0.06∼0.29
Ki* decreased with increasing temperature Et decreased with increasing temperature Et decreased with increasing temperature Et decreased with increasing temperature Et at the bed surface decreased with increasing temperature Et increased with temperature riser solid flux increased with temperature the effect of temperature on Et was negligible Et increased with pressure Et and Ki* increased with pressure Et increased with gas viscosity Et increased with gas viscosity and density A.C. A.C. A.C. A.C. 800 850∼950 750∼950 800∼900
0.61∼2.44 0.9∼1.5 0.038∼0.15 0.36∼0.73
Et decreased with increasing temperature 2.2∼2.3 780∼940
U [m/s] P [kPa] Tb [°C]
2180, 2630
2170∼2750 1400 1400, 2600 1400, 2620
1400 2630 2630 1700 2595 1138 1587, 1138
-0.42
-0.492 0.234 0.300 0.311
-2.0 + 0.025 0.956 0.102 -0.085 + 0.075 -0.370 + 0.038 -0.250 -0.250
air
air air air air
air air air air N2 N2 N2
4.6, 4.0
temperature at which the minimum dpt occurs is shifted to a higher value by an increase in gas velocity and the temperature effect decreases with increasing particle density. On the other hand, terminal velocity of a particle with respect to temperature represents the different trend according to the particle size as can be expected in Figure 1. For example, the terminal velocity of 0.1 mm sand decreases monotonically with increasing temperature over the present experimental range. However, the terminal velocity increases monotonically in 0.256 mm sand and a maximum terminal velocity occurs in 0.2 mm sand.
A. C., ambient condition. a
0.9 1.15 3.0
3.92 2.13 2.13
0.1 0.152 0.25 × 0.43
0.3 0.2 0.2
Romanova et al. (1980) Milne et al. (1993) George and Grace (1981) May and Russel (1954) Chan and Knowlton (1984) Knowlton (1992) Knowlton et al. (1990)
3.96 2.10 0.9 0.6
Experimental Section
Merrik and Highley (1974) Lee et al. (1990) Park et al. (1990) Lee et al. (1992) Wen and Chen (1980)
Choi et al. (1989)
0.3 × 0.3 1.01 × 0.83 0.91 × 0.91 0.11 0.4 × 0.2 0.3
Fp [kg/m3] gas Ht [m] Dt [m]
dp or d h p [mm]
Figure 1. Effects of temperature and solid density on dpt.
investigators
Table 1. Summary of Previous Studies on Effects of Pressure and Temperature on the Particle Entrainment Rate
A.C.a
remarks
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 1131
The schematic diagram of the experimental setup is shown in Figure 2. The fluidized bed column made of stainless steel (SUS 316) was 0.1 m in diameter and 1.97 m in height from the distributor plate to the gas exit level. An electric heater of 6 kwe capacity was installed on the outside wall of the fluidized bed, which was also insulated with rockwool to minimize heat loss. The fluidizing air was injected into the bed through a perforated plate distributor that had 37 holes, 3 mm in diameter. The axial pressure profile of the fluidized bed was measured at seven pressure taps with water manometers and a micromanometer, and the axial temperature profile with nine thermocouples (K-type) and a datalogger (Molytek model 2702). The axial temperature profile of the fluidized bed was maintained as uniform because the temperature deviation of the gas exit from
1132 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998
∼0.35 m from the axial pressure profile over the present experimental condition. According to the correlation of Baeyens and Geldart (1974) on height in the bed at which complete slugging sets, the expanded height was too low to expect the slugging effect. The flux of entrained solids was measured by diverting and collecting the solids recovered by the primary cyclone. The elutriation rate constant of each particle size was defined by eq 6:
Ki* )
entrainment flux of particles of size i weight fraction of bed particles of size i
(6)
which used the arithmetic mean value of weight fractions of bed particles before and after sampling the entrained particles as the denominator. The elutriation rate constant was not considered for particles 0.075 mm in screen size. Bed temperature (12-600 °C), superficial gas velocity (0.65-2.3 m/s), particle density (2509-6158 kg/m3), and size (0.091-0.363 mm) were considered as experimental variables. Results and Discussion Figure 2. Schematic diagram of experimental apparatus. (1) Air compressor; (2) filter; (3) pressure regulator; (4) flowmeter; (5) electrically heated fluidized bed; (6) cyclone(I); (7) cyclone(II); (8) bag filter; (9) diverter; (10) sample pot; (11) rotary valve; (12) micro-manometer; (13) water manometer; (14) data logger. Table 2. Physical Properties of Bed Materials bed material property size distribution sieve size [µm] -425-+300 -300-+212 -212-+150 -150-+106 -106-+75 -75-+0 mean particle diameter [mm] apparent density [kg/m3] bulk density [kg/m3]
sand
emery
cast iron
weight fraction 0.064 0.010 0.012 0.329 0.042 0.108 0.383 0.428 0.276 0.168 0.448 0.320 0.052 0.069 0.190 0.005 0.004 0.094 0.178 0.145 0.110 2509 3981 6158 1298 1697 1981
the bed temperature was
