Standpipe Flows

The overall gas flow rate in the hopper is thus obtained by 

The model given here for flows in a vertical standpipe follows closely the approach of Ginestra et al. (1980), Chen et al. (1984), and Jackson (1993). In this model, it is assumed that 

The cylindrical coordinate system is selected for the standpipe, as shown in Fig. 8.14. For a one-dimensional steady motion of solids, the momentum equation of the particle phase can be written as 

For uniform gas-solid flows, the well-known Richardson and Zaki correlation [Richardson and Zaki, 1954 Growther and Whitehead, 1978] given next can be applied 

Upt is the particle terminal velocity, and n is the modified Richardson-Zaki index for gas-solid systems. Thus, on the basis of Eqs. (8.52) and (8.53), a general momentum equation for the solid phase becomes 

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Li (1994) has also studied the mechanics of arching in moving-bed standpipe flow. He was able, for this downflow situation, to obtain the critical arching span which agrees with reported data. The critical or minimum radius R, for no arching is given as... 

In this chapter, the mechanics of hopper flow and standpipe flow along with their operational characteristics are described. Problems such as segregation, inconsistent flow rate, arching, and piping that disrupt and obstruct the flow of bulk solids in hoppers are discussed. Remedies with respect to the use of flow-promoting devices such as vibrators and aerating jets to reinitiate the flow are presented. The importance of the flowability of the solids to be handled in relation to the hopper design is emphasized. 

The bulk flux of solids in a standpipe depends not only on the geometry of the standpipe but also on its feed devices and flow control valve, located at the top and bottom of the standpipe, respectively. The flow pattern of solids in the pipe may vary with the pipe distance, depending on the variation of the slip velocity in the pipe, uzp — uz. The effect of gas compression due to pressure increase may also be very significant. As a result, p varies along the standpipe, which in turn changes a and, consequently, the slip velocity. Thus, for a standpipe flow, it is possible that several flow patterns exist in various axial locations as solids travel through the standpipe. 

Figure 8.17. Schematic illustration of multiplicity of steady standpipe flows (after Chen et al., 1984).

In standpipe systems, aeration from the side is often implemented to provide a lubricating effect for the solids flow. Aeration is also used in process applications to provide a stripping gas to remove undesirable gas entrained by the solid particles in the standpipe flow. With side aeration, the multiplicity of flows in the standpipe becomes much more complicated. The flow patterns of solids and gas depend not only on Ap and y but also on the aeration locations and aeration rates. For a single aeration point, the possible number of flow regimes increases to 12 [Mountziaris and Jackson, 1991]. 

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. 

During its operations, Sasol experienced deficiencies in standpipe flow whenever fines were lost—deficiencies that paralleled Kellogg s experience in early 1948, cited earlier. 

Smolders K. Baeyens J. (1995) The operation of L-valves to control standpipe flow, Adv. Powder Technology, 6, 163-176. 

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