Nonmechanical valves

The key to smooth operation of a CFB system is the effective control of the solids recirculation rate to the riser. The solids flow control device serves two major functions, namely, sealing riser gas flow to the downcomer and controlling solids circulation rate. Both mechanical valves or feeders (see Figs. 10.1(a) and (d)) and nonmechanical valves (see Figs. 10.1(b) and (c)) are used to perform these functions. Typical mechanical valves are rotary, screw, butterfly, and sliding valves. Nonmechanical valves include L-valves, J-valves (see Chapter 8), V-valves, seal pots, and their variations. Blowers and compressors are commonly used as the gas suppliers. Operating characteristics of these gas suppliers which are directly associated with the dynamics and instability of the riser operation must be considered (see 10.3.3.2). 

Circulating fluidized beds (CFBs) are high velocity fluidized beds operating well above the terminal velocity of all the particles or clusters of particles. A very large cyclone and seal leg return system are needed to recycle sohds in order to maintain a bed inventory. There is a gradual transition from turbulent fluidization to a truly circulating, or fast-fluidized bed, as the gas velocity is increased (Fig. 6), and the exact transition point is rather arbitrary. The sohds are returned to the bed through a conduit called a standpipe. The return of the sohds can be controUed by either a mechanical or a nonmechanical valve. 

Standpipes are applied to fluidized bed systems in several ways. The following describes industrial applications of standpipes. An application of nonmechanical valves which control the solids flow rates is also given. 

Figure 8.22. Nonmechanical valves and loopseal (a) L-valve (b) J-valve (c) Loopseal.

The L-valve is the most commonly used nonmechanical valve. The pressure drop across the L-valve can be evaluated by Eqs. (10.9) and (10.10), given in Chapter 10. 

Figure 10.1. Various configurations of circulating fluidized bed systems (a) Mechanical valve with reservoir (b) Nonmechanical valve with reservoir (c) Nonmechanical valve without reservoir (d) Mechanical feeder without reservoir.

Typical pressure profiles in a CFB loop with a nonmechanical valve (see Chapter 8) are shown in Fig. 10.6, In this figure, line a-b-c-d represents the pressure drop across the riser,... 

The solids flow rate can be controlled by nonmechanical valves such as the L-valve, as noted in Chapter 8. The L-valve has a long horizontal leg. Thus, it is convenient to characterize the pressure drop across an L-valve by two terms. One term is the pressure drop through the elbow (A/ V). This term can be described by the equations developed for the mechanical valve because the solids flow patterns between the two are similar... 

Entrainment Most fluidized bed reactors employ one or more cyclone, either inside the freeboard region at the top of the vessel or located externally, to capture entrained solids that are then returned continuously to the base of the fluidized bed via a standpipe and a mechanical (e.g., slide) valve or aerated nonmechanical valve (see Ref. ° for details of solid return systems). A flapper gate, acting as a check valve, is commonly employed to prevent backflow of gas up the standpipe. While cyclones are by far the most popular, other gas-solid separators like impingement separators, electrostatic precipitators, filters, and scrubbers are sometimes provided, especially as second- or third stage separators. 

Nonmechanical pumping. Micropumps in this class are usually continuous and include the use of effects such as electrochemical displacement (bubble generation), thermal expansion, electrohydrodynamics, capillarity, and evaporation forces. The most commonly used nonmechanical pumping method is based on electrokinetic flow. In comparison with mechanical micropumps, field-induced flow is advantageous as it acts as both a valve and a pump, enabling both the direction and the magnitude of the flow to be controlled. 

Knowlton TM. Standpipes and nonmechanical valves. In Yang WC, editor. Handbook of fluidization and fluid-particle systems. New York Marcel Dekker 2003. p. 571-597. 

Figure 2A is representative of pressurized bubbling bed air blown partial oxidation of biomass fuel. The sketch indicates that fines recycling might be used, with nonmechanical valves to control the reinjection of fines into the fluidized bed gasifier. The vessel will drain a low-carbon ash product. The sketch also suggests that overbed air injection might be used for fuel gas partial oxidation as a means for fuel gas tar destruction. The... 

Recirculating bed media are fed hot into the primary bed zone at a ratio of 50 to 100 times the coal feed rate. Various types of reinjection, nonmechanical valves and seals are used, such as seal pots, L-valves, J-valves, and others. These are simple refractory lined ducts having solids holding volumes that provide a loop seal, and appropriate aeration nozzles that induce and control solids flow. Cyclones are normally used to separate recirculating solids from the over-... 

This chapter discusses two important elements of a solids transport system standpipes and nonmechanical valves. Although they are very simple in configuration, trying to design and operate these devices without a basic understanding of their principles of operating can lead to much frustration and wasted time. Describing how standpipes and nonmechanical valves operate is the purpose of this chapter. 

In the valve mode of operation, the solids flow rate through the nonmechanical device is controlled by the amount of aeration gas added to it. The most common types of nonmechanical valves are the L-valve and the J-valve. These devices are shown schematically in Fig. 19. The primary difference between these devices are their shapes and the directions in which they discharge solids. Both devices operate on the same principle. It is harder to fabricate a smooth 180-degree bend for a typical J-valve. Therefore the J-valve can be approximated and configured more simply by the geometry shown in Fig. 19C. 

The most common nonmechanical valve is the L-valve, because it is easiest to construct, and also because it is slightly more efficient than the J-valve (Knowlton et al., 1981). Because the principle of operation of nonmechanical valves is the same, nomnechani-cal valve operation is presented here primarily through a discussion of the characteristics of the L-valve. 

Solids flow through a nonmechanical valve because of drag forces on the particles produced by the aeration gas. When aeration gas is added to a nonmechanical valve, gas flows downward through the particles and around the constricting bend. This relative gas solids flow produces a frictional drag force on the particles in the direction of flow. When this drag force exceeds the force required to overcome the resistance... 

When aeration is added to a nonmechanical valve, solids do not begin to flow immediately. The initial aeration gas added is not enough to produce the frictional force required to start solids flow. Above the threshold amount of gas required to initiate solids flow, additional aeration gas added to the valve causes the solids flow rate to increase, and reducing the amount of aeration to the valve causes the solids flow rate to decrease. In general, there is little hysteresis in the aeration-vs.-solids flow rate curve for a nonmechanical valve. 

Nonmechanical valves are used extensively in CFBC systems where Geldart group B solids are used. They are not used in FCC circulating systems where Geldart group A solids are used. 

A nonmechanical device operating in the valve mode is always located at the bottom of an underflow standpipe operating in moving packed bed flow. The standpipe is usually fed by a hopper, which can either be fluidized or nonfluidized. Knowlton and Hirsan (1978) and Knowlton et al. (1978) have shown that the operation of a nonmechanical valve is dependent upon the pressure balance and the geometry of the system. 

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