746
Ind. Eng. Chem. Res. 2000, 39, 746-751
GENERAL RESEARCH Flow Characteristics in a Large Jetting Fluidized Bed with Two Nozzles Qingjie Guo,* Zhenyu Liu, and Jiyu Zhang State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China
The penetration depth for millet (1.64 mm) as fluidization particles (Geldart group D) was determined in a large jetting fluidized bed of 0.5 m inner diameter and 8 m height and with two nozzles is recorded by a video camera and analyzed frame by frame. Based on experimental data, an empirical correlation for the penetration depth is proposed. Experimental results indicate that the penetration depth increases with an increase in the jetting velocity. Under the same jet gas velocity, the penetration depth decreases as the nozzle distance decreases. Meanwhile, a simple correlation for jet coalescence height is found. A radial voidage profile in the jetting fluidized bed is investigated using a PC-4 fiber optic concentration probe, the effects of jet gas velocity, and the distance between two nozzles on the radial voidage distribution, and a radial profile with unequal jet gas velocity in a jetting fluidized bed is studied. Introduction Jetting fluidized beds are widely used in a number of industrial processes because of various advantages, such as high rates of heat and mass transfer and chemical reaction. These processes include catalytic and flame processes, combustion and gasification of coal, treatment of wastes, cleaning of dusty gases, coating, and granulation. Such processes are mainly controlled by jet phenomena. Moreover, gas jets often cause wear on fluidizedbed walls and internals by solid impingement. Many investigators have studied the hydrodynamics of jetting fluidized beds. The jet penetration depth has been investigated in the literature and can be determined by measuring local bed voidage using optical probes (Wen et al., 1982), analyzing pressure signals (Vaccaro et al., 1997), determining the momentum profile by Pitot tubes (Behie et al., 1970; Raghunathan et al., 1989), and analyzing motion pictures of bubble frame by frame (Yang et al., 1978). Studies on voidage distribution in jetting fluidized beds are relatively scarce. Gidaspow et al. (1983) reported the average voidage profiles in a twodimensional jetting fluidized bed and found the area of maximum porosity above the jet inlet in the case of the circular nozzle. Kuipers et al. (1992) employed IR-LED light transmission techniques in a two-dimensional bed with a jet to investigate the porosity distribution and found that there were elliptic concentration contours near the lean jet region. Fiber optic probes were used by Guo et al. (1996) and Luo et al. (1997) to measure the particles’ volume fraction in the jetting fluidized bed with a nozzle. A fiber optic technique has the advantages of simplicity, high accuracy, and low cost. * Corresponding author. Present address: Thermal Engineering Department, Tsinghua University, Beijing 100084, P. R. China. Tel: 86-10-62781743. Fax: 86-10-62781743. Email:
[email protected].
Previous research work is mainly focused on the jetting fluidized bed with one nozzle; little work has been carried out in the jetting fluidized bed with double nozzles. On the basis of X-ray techniques, Yates et al. (1995) studied the gas discharge behavior from two separate nozzles at variable nozzle distance using FCC powders (Geldart group A). A simple correlation was found for the coalescence height of bubbles formed at the nozzles. The maximum jet velocity used in the study was only 8 ms-1. Luo et al. (1997) investigated jet momentum dissipation in a 300 × 50 mm two-dimensional fluidized bed with two nozzles using a multichannel pitot tube system. In this paper, correlations of jet coalescence height and jet penetration depth in a large jetting fluidized bed with double nozzles are proposed. Meanwhile, the radial voidage distribution, effects of jet gas velocity, and distances between two nozzles on the radial porosity distribution are studied using a fiber optic probe, and radial porosity profiles with unequal jet gas velocity in the jetting fluidized bed are reported. Experimental Section A schematic diagram of the jetting fluidized bed with two nozzles used in the experiments is shown in Figure 1. This system basically includes a jetting fluidized-bed body, a semiconical gas distributor, a segregating column, a plenum chamber, conveying pipelines, a cyclone, two semicircular nozzles, and a storage tank. The semicircular fluidized bed has an i.d. of 0.5 m and is 8.0 m in height. Its front plate is made of 7.0 mm thick Plexiglass, and its other parts are made of 10 mm thick cast steel. The semiconical gas distributor of 70° cone angle is 0.68 m in height. The segregating column has a diameter of 0.27 m and is 0.61 m in height. The two semicircular nozzles located vertically 11 mm from the front plate are 0.042 m in inside diameter, with an exit 0.19 m above the top of the semiconical gas
10.1021/ie990210o CCC: $19.00 © 2000 American Chemical Society Published on Web 02/11/2000
Ind. Eng. Chem. Res., Vol. 39, No. 3, 2000 747
a
b Figure 1. Schematic diagram of the experimental apparatus: (1) jetting fluidized bed; (2) jet nozzle; (3) semiconical distribution; (4) segregating column; (5) plenum chamber; (6) tank; (7) conveying pipeline; (8) cyclone; (9) flowmeter; (10) storage tank; (11) coordinate origin. Table 1. Physical Properties of Experimental Particles particulate
dp, mm
Fp, kg (m3)-1
umf, ms-1
mf
millet
1.64
1335
0.58
0.404
distributor; the distance between the two nozzles can be adjusted according to the requirements of the experiments. The fluidization air, measured by four rotameters, is provided to the two nozzles, semiconical gas distributor, and plenum chamber. The cyclone is installed at the exit of the fluidized bed to recover most of the entrained particles. In all cases, the background air from the semiconical distributor and the plenum chamber was maintained at minimum fluidization. With the background air at minimum fluidization and no jet flow, the bed depth is defined as the height from the exit of the jet nozzles to the surface of the dense phase bed; the bed may be operated with depths of around 370, 400, 450, and 660 mm. The nozzle gas velocity ranged from 16.1 to 50.9 ms-1. The fluidized particles are millet. Their physical properties are summarized in Table 1. The PC-4 fiber optic probe system, made by the Institute of Chemical Metallurgy, Chinese Academy of Sciences, is composed of a light source, optic fibers, a photomultiplier, a voltage integrator, a data recorder, an A/D converter, and a computer. The probe (4 mm inside diameter) consists of a bundle of thousands of 16 µm fine fibers. Half of the fibers are light projectors, and the other half are light receivers. The fibers of the light projectors and light receivers are arranged in an alternating array. The light projector fibers carry light from the light source and project it onto particles. The light receiver fibers transmit light reflected by the particulates to a photomultiplier where light signals are converted to electrical signals, with the voltage output proportional to the intensity of the reflected light. The probe calibration used the same method as Zhou et al. (1994). Before each experiment, the same procedure is followed to calibrate the probe for the particles. The optic fiber probe was inserted horizontally into the
Figure 2. (a) Influence of the jet gas velocity on the penetration depth. Conditions: p ) 0.36 m, H0 ) 0.66 m. (b) Effects of the nozzle distance on the penetration depth. Conditions: p ) 0.230.36 m, H0 ) 0.66 m.
jetting fluidized bed through ports at various axial locations. Meanwhile, the probe was traversed across the radial direction to measure the solid volume concentration at 13 radial positions. The duration of each measurement was 30 s. Flow behavior of the jetting fluidized bed was recorded for 180 s by a National M-7 video camera and then analyzed frame by frame on a Panasonic HD-100 player. The origin is in the center of the exit of the front plate, the y direction is along the axis of the front plate, and the x direction is along the front plate, as shown in Figure 1. Jet coalescence height and jet penetration depth were the average of 60 jet flows measured by the jog/shuttle function of a Panasonic HD-100 player. Results and Discussion 1. Penetration Depth and Jet Coalescence Height. The effect of nozzle distance on the penetration depth is studied. Figure 2a shows that the penetration depth varies with jet gas velocity. As many researchers have pointed out, the penetration depth increases with increasing jet gas velocity. Obviously, the larger the jet
748
Ind. Eng. Chem. Res., Vol. 39, No. 3, 2000
positions of the two jets coalescing are often on the axis of the bed. The vertical distance from the position of coalescence to the exit of the nozzle is then defined as the jet coalescence height. Generally speaking, the jet coalescence height is smaller than the jet penetration depth, as shown in Figure 3. This implies that steam and air jets will dissipate into emulsion in an ashagglomerating coal gasifier, resulting in the loss of steam and air. All of the experimental data on the jet coalescence height, Hc, was found to be a function of the ratio of the orifice separation distance, p, to the orifice diameter, d0, and of the dimensional superficial gas velocity, us/umf. A correlation is proposed as follows.
( ) ()
Figure 3. Schematic diagram of separate jet and jet coalescence for two jets with equal jet velocity.
gas velocity, the larger the gas jet momentum. Therefore, the penetration depth increases with increasing jet gas velocity. In particular, when the jet gas velocity is fixed, the decrease in the nozzle distance decreases the penetration depth, as shown in Figure 2b. There are two types of flow behavior for two jets. One is separate jet flow, i.e., jets collapse and bubbles form as for a single jet; the other involves jet coalescence within the penetration depth, as shown in Figure 3. Because the small nozzle distance makes the two jets combine in the jet region, the jet gas dissipates quickly into emulsion and the momentum of the jet gas is lost. Table 2 shows that studies of the penetration depth differ not only in experimental conditions, such as bed and nozzle configurations, bed material, d0, and dp, but also in measuring techniques. Basov et al. (1969) and Merry (1975) examined the penetration depth of multiple jets of the gas distributor in small-scale facilities. Yang et al. (1979) obtained a penetration depth from multiple jet data of the literature. On the basis of literature data, Blake et al. (1990) concluded that adjacent jets appear to increase the jet height relative to that associated with an isolated jet, but this conclusion was not proved by experiments. Few researchers have investigated the jet penetration depth in a large-scale jetting fluidized bed with two or more separate nozzles; the correlation proposed by other researchers showed a large error in the fitting of the experimental data obtained in this work. Therefore, it might be difficult to find a generalized expression for various working conditions encountered in practice. A correlation which can be used in a large jetting fluidized bed with two nozzles is proposed, which includes the Reynolds number, the two-phase Froude number, and p/d0. This correlation indicates the effect of separation on the penetration depth.
()
Lj p ) 0.02857 d0 d0 Fr* )
0.224
Ff u02 Fp - Ff gd0
(Fr*)0.024(Rep)0.711 Rep )
(1)
Ffdpu0 µ
where u0 is calculated using the equivalent diameter of the nozzle. The application range of eq 1 is as follows.
4.3
