Solids feeders

Various devices are available for feeding solids into the fluidized bed. The choice of device depends largely on the nature of the solids feed. Screw conveyors, spray feeders and pneumatic convepng are in common use. 

6 kg of solid particles of density 2590 kg/and surface-volume mean size 748 iim form a packed bed of height 0.475 m in a circular vessel of diameter 0.0757 m. Water of density 1000 kg/m and viscosity 0.001 Pa s is passed upwards through the bed. Calculate (a) the bed pressure drop at incipient fluidization, (b) the superficial liquid velocity at incipient fluidization, (c) the mean bed voidage at a superficial liquid velocity of 1.0 cm/s, (d) the bed height at this velocity and (e) the pressure drop across the bed at this velocity. 

Frictional pressure drop across the bed when fluidized weight of particles upthrust on particles 

Equating this expression for pressure drop across the packed bed to the fluidized bed pressure drop, we determine superficial fluid velocity at incipient fluidization, Umf. 

Using the method given in Chapter 2, we determine the single particle terminal velocity, Uj. 

Allegri, Materials Handling Principles and Practice, Van Nostrand Reinhold, New York, 1984. 

Bain and S.T. Bonnington, The Hydraulic Transport of Solids by Pipeline, Pergamon, New York, 1970. 

Bhatic, and P.N. Cheremisinoff, Solid and Liquid Conveying Systems, Technomic, Lancaster, PA, 1982. 

Bobkowicz and W.G. Gauvin, The effects of turbulence in the flow characteristics of model fiber suspensions, Chem. Eng. Sci., 22, 229-247 (1967). 

Conveyors, hydraulic, Encycl. Chem. Process Des., 11, 262-278 (1980). 

Three main classes of gas compressors are centrifugal and axial, rotary continuous positive displacement, and reciprocating positive 

Throttling of the suction of centrifugal and axial compressors wastes less power than throttling the discharge. Even less power is wasted by adjustment of built-in inlet guide vanes with a servomechanism which is a feedback control system in which the controlled variable is mechanical position. Speed control is a particularly effective control mode, applicable to large units that can utilize turbine or internal combustion drives control is by throttling of the supply of motive fluids, steam or fuel. 

Smaller rotary positive displacement compressors are controlled with external bypass. Such equipment usually has a built-in relief valve that opens at a pressure short of damaging the equipment, but the external bypass still is necessary for smooth control. Large units may be equipped with turbine or gas engine drives which are speed adjustable. Variable speed gear boxes or belt drives are not satisfactory. Variable speed dc motors also are not useful as compressor drives. Magnetic clutches and hydraulic couplings are used. 

Reciprocating compressors may be controlled in the same way as rotary units. The normal turndown with gasoline or diesel engines is 50% of maximum in order that torque remains within 

Valve unloading, a process whereby some of the suction valves remain open during discharge. Solenoid or pneumatic unloaders can be operated from the output of a control instrument. The stepwise controlled flow rate may need to be supplemented with controlled external bypass to smooth out pressure fluctuations. 

Control Mode Centrifugal Rotary Reciprocating and Axial PD PD 

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Standpipes, Solids Feeders, and Solids Flow Control. 17-10... 

The important design parameters for a recirculating fluidized bed with a draft tube were identified by Yang and Keaims (1978a) as the gas bypassing characteristics of the distributor plate, the area ratio between the downcomer and the draft tube, the diameter ratio between the draft tube and the draft tube gas supply, the distance between the distributor plate and the draft tube inlet, and the area ratio of the draft tube gas supply and the concentric solids feeder. The design and operation of a recirculating fluidized bed with a draft tube are discussed below. 

The gas bypassing results obtained from tracer gas injection studies for a flat and a conical distributor plate are shown in Fig. 4. Theflow ratio, FR, is defined as the total gas flow supplied through the draft tube gas supply and the concentric solids feeder divided by the total gas flow supplied through the downcomer gas supply. The A and Y are the actual amounts of gas passing up the draft tube and the downcomer, respectively, determined from the tracer gas injection studies. If FR equals A Y. there is no gas bypassing. If FR is less than A Y. some of the flow supplied through the downcomer gas supply passes into the draft tube. If FR is larger than A/7, the reverse is true. 

Effect of Draft Tube and Downcomer Area Ratio. When a draft tube of 9.55 cm I.D. (downcomer/draft tube area ratio = 7.8) was changed to a draft tube of 5 cm I.D. (downcomer/draft tube area ratio = 30) with other design parameters being the same, the gas bypassing reversed direction, as shown in Fig. 4. With the smaller draft tube (D/dD = 1), the gas bypasses from the draft tube side into the downcomer side for most experimental conditions, except for jet velocities in excess of 76 m/s at the concentric solids feeder with the larger draft tube (D/dD = 1.9), the gas bypasses from the downcomer side into the draft tube side in most experiments. 

Figure 3.7. Solids feeders with variable speed drives, (a) Rotary vane (star) feeder with variable speed drive, (b) Horizontal screw feeder, (c) Belt feeder taking material from a bin with an adjustable underflow weir, (d) Rotary plate feeder Rate of discharge is controlled by the rotation speed, height of the collar, and the position of the plow, (e) Continuously weighing feeder with variable speed belt conveyor.

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