Standpipes fluidized flow

Fig. 22. Standpipe and standpipe pressure profiles showiag (—) fluidized flow and (--------) packed bed or defluidized flow.

Let us consider a disturbance in the system such that the gas flow through the fluidized beds increases [Figure 8.17(b)]. If the gas flow through the lower bed increases, although the pressure drops across the lower and upper beds will remain constant, the pressure drop across the upper distributor will increase Pd(new). To match this increase, the pressure across the standpipe must rise to Psp(new) [Figure 8.17(b)]. In the case of an overflow standpipe operating in fluidized flow the increase in standpipe pressure drop results from a rise in the height of solids in the standpipe to Hsp(new)-... 

Since the actual pressure gradient is well below that for fluidized flow, the standpipe is operating in packed bed flow. 

For Geldart group B solids, it is often unnecessary to add aeration at several locations along the standpipe to maintain the standpipe in fluidized flow. Adding aeration at the bottom of the standpipe operating... 

Group B soHds have higher minimum fluidization velocities than Group A soHds. For best results for Group B soHds flowing ia standpipes, standpipe aeration should be added at the bottom of the standpipe, not uniformly along the standpipe. 

The pressure is higher at the bottom of the sohds draw-off pipe due to the relative flow of gas counter to the sohds flow. The gas may either be flowing downward more slowly than the solids or upward. The standpipe may be fluidized, or the solids may be in moving packed bed flow with no expansion. Gas is introduced at the bottom (best for group B) or at about 3-m intervals along the standpipe (best for group A). The increasing pressure causes gas inside and between... 

Seal legs are frequently used in conjunction with solids-flow-control valves to equ ize pressures and to strip trapped or adsorbed gases from the sohds. The operation of a seal leg is shown schemati-caUy in Fig. 17-19. The sohds settle by gravity from the fluidized bed into the seal leg or standpipe. Seal and/or stripping gas is introduced near the bottom of the leg. This gas flows both upward and downward. Pressures indicated in the ihustratiou have no absolute value but are only relative. The legs are designed for either fluidized or settled solids. 

The spent catalyst slide valve is located at the base of the standpipe. It controls the stripper bed level and regulates the flow of spent catalyst into the regenerator. As with the regenerated catalyst slide valve, the catalyst level in the stripper generates pressure as long as it is fluidized. The pressure differential across the slide valve will be at the expense of consuming a pressure differential in the range of 3 psi to 6 psi (20 kp to 40 kp). 

The standpipe provides the necessary head pressure required to achieve proper catalyst circulation. Standpipes are sized to operate in the fluidized region for a wide variation of catalyst flow. Maximum catalyst circulation rates are realized at higher head pressures. The higher head pressures can only be achieved when the catalyst is fluidized. Table 7-5 contains typical process and mechanical design criteria for standpipes. 

It is possible to operate a fluidized bed in either batch or continuous mode. Strictly, most batch applications are in tact operated in semibatch mode where the solids are treated as a batch but the fluidizing medium enters and leaves the bed continuously. In the case ot gas-solid beds used in termentation (see Chapter 6), the fluidizing gas is recirculated although reactants and products flow continuously. In true continuous operation the solids may be ted into a fluidized bed via screw conveyors, weigh teeders or pneumatic conveying lines and can be withdrawn trom the bed via standpipes or by flowing over weirs. 

As the fluidized catalyst descends the standpipe, the increasing pressure compresses the fluidizing gas resulting in a decrease in the gas volume. If allowed to continue without adding aeration, the flowing catalyst will defluidize leading to unstable flow and potential loss of catalyst circulation. This is particularly true... 

Assuming a catalyst density at flowing conditions in the standpipe of about 90% of the catalyst bulk density, the amount of excess gas above minimum fluidization that is entrained with the catalyst into the standpipe may be calculated. Sufficient aeration should be added to sustain minimum fluidization along the length of the standpipe. 

Example 8.3 In an operation of gas-solid circulating flow in a cyclone-standpipe-valve system, the particles are 100 p.m glass beads with a density of 2,500 kg/m3. The particle volume fraction in the standpipe is 0.55. The gas is air with a viscosity of 1.8 x 10-5 kg/m s and a density of 1.2 kg/m3. The particle mass flow is 70 kg/m2 s. The height of solids in the standpipe is 1.4 m. The total pressure head over the standpipe and valve is 4,500 Pa. Estimate the leakage flow of air in the standpipe. If the particle volume fraction at minimum fluidization is 0.5 and the area ratio of valve opening to pipe cross section is 0.6, what is the orifice coefficient of this 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. 

In general, the outlet of an overflow standpipe is immersed into a bed in order to provide an adequate hydrostatic head or seal pressure in the standpipe, as shown in Fig. 8.19(a). The overflow pipe can be operated in either a moving bed or fluidized bed mode. Usually, solids occupy only a fraction of the pipe near the standpipe outlet. The solids flow rate in the pipe depends on the overflow rate from the fluidized bed. Consequently, no valve is needed at the pipe outlet. In the overflow standpipe system where solids transfer from one fluidized bed to the other, the pressure drops across the lower fluidized bed, the grid (distributor), the upper fluidized bed, and the height of solids in the standpipe. For a given standpipe system and given particles, the pressure distribution profile depends on the gas velocity. The difference in the pressure distribution reflects the difference in solids concentration,... 

The pressure drop in the overflow and underflow standpipes can be predicted using Eqs. (8.64) through (8.67) considering either the moving bed flow or the suspension flow. The pressure drops across the fluidized bed and grid or distributor can be evaluated, respectively, by Eqs. (9.7) and (P9.7) given in Chapter 9. 

Figure 8.21. Two applications of angled standpipe for solids flow to a fluidized bed (from Knowlton, 1986) (a) With segregated gas bubble flow (b) Without segregated gas bubble flow.

For a fluidized standpipe, the drag force of particles balances the pressure head as a result of the weight of solids. If the Richardson and Zaki form of equation (Eq. (8.55)) is proposed for the drag force, derive an expression for the leakage flow of gas in this standpipe. Discuss the effect of particle size on the leakage flow assuming all other conditions are maintained constant. 

The book is arranged in two parts Part I deals with basic relationships and phenomena, including particle size and properties, collision mechanics of solids, momentum transfer and charge transfer, heat and mass transfer, basic equations, and intrinsic phenomena in gas-solid flows. Part II discusses the characteristics of selected gas-solid flow systems such as gas-solid separators, hopper and standpipe flows, dense-phase fluidized beds, circulating fluidized beds, pneumatic conveying systems, and heat and mass transfer in fluidization systems. 

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