Fluidization standpipe

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. 

This paper covers the history at Exxon where substantially all of the development took place, resulting in the basic patents on the fluid bed (2) fluidized standpipe (3) the integrated system (4) and the downflow design (5). The work at Exxon produced the first designs for commercial plants that were built in 1940-1945 of both the upflow type (4) and the downflow type (5). Some of the business aspects and contributions by others to the development have been reviewed elsewhere (6), as well as some technical aspects (7). 

Let us assume that the voidage 2 is the lowest voidage acceptable for maintaining fluidized standpipe flow. Equation (8.29) allows calculation of the equivalent maximum pressure ratio, and hence the pressure drop between levels 1 and 2. Assuming the solids are fully supported, this pressure difference will be equal to the apparent weight per unit cross-sectional area of the standpipe [Equation (8.26)]. 

Underflow fluidized standpipes in FCC units are operated in a vertical configuration, a completely angled configuration, or a hybrid configuration in... 

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. 

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

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. 

Standard state, for molecules, 24 687—688 Standard state enthalpy change for methanol synthesis, 25 305 Standard-state heat, 24 688 Standard-state heat of reaction, 24 688 Standards-writing organizations, 15 760 Standard Test Conditions (STC), 23 38 Standard test methods, 15 747—748 Standpipe pressure profiles, 11 818 Standpipes, in circulating fluidized beds, 11 817-819 Stand-retting, 11 606 Stannane, 13 613, 24 813... 

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. 

The circulating catalyst physical properties have a direct impact on fluidization and stable standpipe operation. Mechanical problems may cause a loss of catalyst fines, or a change in catalyst density both of which will impact fluidization and may require adjustment to the standpipe aeration. 

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