Yang Fluidization, Solids Handling, and Processing

Recirculating and Jetting Fluidized Beds 257

Effect of Distance Between Distributor Plate and Draft Tube Inlet.

As expected, the closer the distance between the distributor plate and the draft tube inlet the lower the solids circulation rate as shown in Figs. 8 and 9. This is not only because of the physical constriction created by locating the distributor plate too close to the draft tube inlet but also because of the different gas bypassing characteristics observed at different distributor plate locations as discussed earlier. When the distance between the distributor plate and the draft tube inlet becomes large, it can create start-up problems discussed in Yang et al. (1978).

2.3Design Example for a Recirculating Fluidized Bed with a Draft Tube

From the experimental evidence, the design of a recirculating fluidized bed with a draft tube involves the specification of a number of design parameters and an understanding of the coupling effects between the design and the operating variables. A procedure is presented here for the design of a bed to give a specified solids circulation rate. This design procedure assumes that the solids and gas characteristics, feed rates, and operating temperature and pressure are given. The design parameters to be specified include the vessel diameter, draft tube diameter, draft tube height, gas distributor, and distributor position. These parameters can be specified using the solids circulation rate model, experimental data on gas bypassing, and process requirements (e.g., selection of gas velocity in the bed above the draft tube).

Determination of the Gas Bypassing Characteristics of the Distributor Plate Experience indicates that a simple theoretical model to predict gas bypassing accurately that takes into account all the design and operating variables cannot be developed. Empirical correlations, however, can be obtained by conducting experiments with tracer gas injection for a given distributor plate design at different operating conditions and at different distances from the draft tube inlet. Distributor plate designs can be studied and an optimum design selected to provide the desired solids circulation rate.

Design for Desired Solids Circulation Rate It is assumed that the total gas flow into the bed is known. When the operating fluidizing velocity is selected for the fluidized bed above the draft tube, the diameter of the vessel is determined. The final design decisions include selection of the draft tube diameter, the distributor plate design, the separation between the draft

Recirculating and Jetting Fluidized Beds 259

1. Assume a solid circulation rate per unit draft tube area, Wsr , and calculate the particle velocity in the downcomer, Upd , from the following equation

2.Calculate Ufd from Eq. (17) and the pressure drop across the downcomer, P1-4, from Eq. (9) assuming ε d = ε mf.

3.Use trial and error between Eqs. (12)–(14) to evaluate Upr (the equilibrium particle velocity in the draft tube after acceleration), ε r (the equilibrium voidage in the draft tube after acceleration), and fp (the solid friction factor).

4.Numerically integrate Eq. (11) to obtain the particle acceleration

length and Eq. (15) to obtain the pressure drop across the draft tube, P2-3. 5. Compare the pressure drop across the downcomer, P1-4, and that across the draft tube, P2-3. If they are not equal, repeat procedures 1

through 5 until P1-4 P2-3.

The results in Fig. 11 show that the solids circulation rate increases with a decrease in downcomer/draft tube area ratio; however, the operating condition inside the draft tube will eventually approach the choking condition where a slugging-type transport prevails. In the example, this may occur at a downcomer/draft tube area ratio of about 10. Correlations for predicting the choking point in a vertical pneumatic conveying line are available (Punwani et al., 1976; Yang, 1983). At an even lower downcomer/ draft tube area ratio, the solids circulation will approach the conditions of the low-velocity circulating fluidized bed described by LaNauze (1976). His model can be used for solid circulation calculations in the low draft tube velocity regime.

Start-up and Shutdown Considerations. Both cold flow experiments and actual pilot-plant experience show that, if operating conditions and design parameter are not selected carefully, start-up (initiate solid circulation) might be a problem (Yang et al., 1978). The primary design parameters that will affect the start-up are the distance between the grid and the draft tube inlet (L) and the diameter ratio between the draft tube and the draft tube gas supply nozzle and the concentric solid feeder (D/dD and D/ds). The maximum allowable distance, L, can be determined by applying the jet penetration equation suggested by Yang and Keairns (1978b)

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observed by Peterson (1966) who found that vertical baffles covering at least one half of the bed height were necessary to stabilize the operation. In an industrial environment where solids are processed in large vessels, multiple draft tubes may be both necessary and beneficial. Exploratory tests in a two-dimensionalbed with three draft tubes were reported by Yang and Keairns (1989).

A schematic of the two-dimensional test apparatus with three draft tubes is shown in Fig. 12. The two-dimensional bed is constructed with transparent Plexiglas plates in the front and aluminum plates at the back with a cross-section of 50.8 cm by 2.54 cm and 244 cm high. The three draft tubes have a cross-sectional area of 2.54 cm by 2.54 cm each and 91 cm high. The three draft tubes divide the bed into four separate downcomers. The two downcomers next to the side walls have a cross-section of 5.9 cm by 2.54 cm while the remaining two downcomers have a cross-section exactly two times, i.e., 11.8 cm by 2.54 cm. If all three draft tubes operate similarly, the bed should have three identical cells, each with a single draft tube. The distance between the draft tube inlet and the air distributor plate was maintained at a constant spacing of 5.1 cm throughout the experiments.

Polyethylene beads of relatively narrow size distribution with a harmonic mean diameter of 2800 mm and a particle density of 910 kg/m3 were used as the bed material. A static bed height of 1.4 m was employed.

Two different series of experiments were carried out. In one series, the three draft tube velocities were maintained essentially constant while the aeration to downcomers was varied. One of the three draft tube velocities was purposely increased to simulate possible unbalanced operation conditions in an actual industrial plant in the second series of experiments. Each experiment was characterized by solid particle velocity in each downcomer, the pressure drop across each draft tube, and the pressure drop across each downcomer.

When all three draft tubes were operated at similar velocities, the pressure drops across all draft tubes and downcomers were comparable. However, solid particle velocities in outside downcomers close to the walls were substantially less due to wall effect and redistribution of downcomer aeration flow. Smooth operations under these conditions were possible. The solid particle velocities in outside downcomers can be increased by enlarging the downcomer cross-section or by increasing downcomer aeration through separate plenums to minimize wall effects.

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When one of the three draft tube velocities was increased to simulate upset conditions, stable operations were still possible. These upset conditions could also be detected by pressure drop differences among various draft tubes and downcomers when differences in draft tube velocities were large. For severe upset conditions, where some of the draft tubes become downcomers, pressure drop measurement alone could not distinguish the solids flow pattern inside the draft tubes.

The design methodology proposed earlier for beds with a single draft tube is still applicable here for beds with multiple draft tubes.

2.4Industrial Applications

The application of the recirculating fluidized bed with a draft tube was probably first described by Taskaev and Kozhina (1956). They utilized the bed for low temperature carbonization of coals in a 15 cm diameter column with a 2.5 cm diameter draft tube. A “seeded coal process” was later developed by Curran et al. (1973) using the same concept to smear the “liquid” raw coal undergoing the plastic transition onto the seed char and the recirculating char during low temperature pyrolysis. Westinghouse successfully demonstrated a first stage coal devolatilizer with caking coals in a pilot scale Process Development Unit employing a similar concept where the downcomer was fluidized and the jet issuing from the draft tube was immersed in a fluidized bed above the draft tube (Westinghouse, 1977). The same concept was also proposed for extending fluidized bed combustion technology for steam and power generation (Keairns, et al., 1978). The British Gas Council has also developed the concept for oil and coal gasification (Horsler and Thompson, 1968; Horsler et al., 1969). The development eventually resulted in a large-scale recirculating fluidized bed hydrogenator gasifying heavy hydrocarbon oils (Ohoka and Conway, 1973). McMahon (1972) also described a reactor design for oil gasification using a multiplicity of draft tubes. The Dynacracking process developed by Hydrocarbon Research Inc. in the 1950’s (Rakow and Calderon, 1981) for processing heavy crude oil also utilized an internal draft tube. More recently, gasification in a recirculating fluidized bed with a draft tube was described by Judd et al. (1984) and coalwater mixture combustion, by Lee and Kim (1992).

Other industrial applications of the concept include that for coating tablets in the pharmaceutical industry (Wurster et al. 1965), for drying of

264 Fluidization, Solids Handling, and Processing

dilute solutions containing solids (Hadzismajlovic, 1989), and for mixing and blending (Decamps et al. 1971/1972; Matweecha, 1973; Solt, 1972; Krambrock, 1976). Both Conair Waeschle Systems and Fuller Company supply commercial blenders based on the concept. The concept was also proposed as a controllable solids feeder to a pneumatic transport tube (Decamps et al., 1971/1972; Silva et al., 1996).

Although most of the experimental data reported here were obtained with large particles, Geldart Class B and D powders, it is believed that the concept can equally be applied for any fine aeratable and free-flowing solids, Geldart's Class A powders.

A similar concept has also been used for liquid-solids and liquid-gas- solids contacting devices (Oguchi and Kubo, 1973; Fan et al., 1984) and bioreactors (Chisti, 1989). Bubble columns fitted with draft tubes have also been employed in the chemical process industries as airlift reactors for gas-liquid contacting operations. Examples are the low-waste conversion of ethylene and chlorine to dichloroethane, biological treatment of high strength municipal and industrial effluent and bioreactors. Critical aspects of the design and operation of bubble columns with draft tubes have recently been reviewed by Chisti and Moo-Young (1993). Freedman and Davidson (1964) also carried out a fundamental analysis for gas holdup and liquid circulation in a bubble column with a draft tube. Extensive experimentation in a bubble column with a draft tube was conducted by Miyahara et al. (1986) and in-depth analysis by Siegel et al. (1986). The effects of geometrical design on performance for concentric-tube airlift reactors were studied by Merchuk et al. (1994).

3.0JETTING FLUIDIZED BEDS

In a gas fluidized bed, the introduction of gas is usually accomplished through distributors of various designs. Any time the gas is distributed through orifices or nozzles, a jetting region appears immediately above the grid. A large fluctuation of bed density occurs in this zone, indicating extensive mixing and contacting of solids and gas. If the chemical reactions between gas and solids are fast, much of the conversion may occur in this jetting region.

Another type of fluidized bed, where the jetting phenomena is an important consideration, is the spouted fluid bed, where a large portion of