Bed expansion characteristics

Liquid-solid fluidised systems are generally characterised by the regular expansion of the bed which takes place as the liquid velocity increases from the minimum fluidisation velocity to a value approaching the terminal falling velocity of the particles. The general form of relation between velocity and bed voidage is found to be similar for both Newtonian and inelastic power-law liquids. For fluidisation of uniform spheres by Newtonian liquids, equation (5.21), introduced earlier to represent hindered settling data, is equally applicable  

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Assume that the bed expansion characteristics in gas-solid particulate fluidization can be described by an equation of the Richardson-Zaki form as given by... 

Molerus established the following correlation for Cd on the basis of bed expansion characteristics ... 

Three-phase fluidized beds using small particles display unique bed expansion characteristics. Upon the initial... 

WangJ, van der Hoef MA, Kuipers JAM Why the two-fluid model foils to predict the bed expansion characteristics of Geldart A particles in gas-fluidized beds a tentative answer, Chem Eng Sci 64 622—625, 2009. 

Equation (78) gives an accurate prediction of fluidized bed expansion characteristics for the laminar regime and applies equally well to fixed and suspended particle systems. 

Equation (84) provides accurate quantitative predictions of particulate fluidized bed expansion characteristics in both laminar and turbulent flow regimes. For the intermediate flow regime, only a qualitative trend was observed. Equation (84) is slightly different in dependence of voidage from the drag coefficient suggested by Wen and Yu (1966) as shown here ... 

A Eulerian two phase calculation is performed in two dimensions giving locally averaged velocities and volume fractions for the particle and gas/vapour phases. Furthermore, bed expansion and bubble characteristics are obtained. 

Example. Warm water (40 C) is being used to backwash a bed of resin whose expansion characteristics follow Eq. (67). The superficial velocity is now 15 m hr". A change in the plant utility system will reduce the temperature to 25 5°C. 

Equation (79) well represents the steady-state expansion characteristics of a turbulent regime in fluidized beds. 

Figure 7 Expansion characteristics of a stable liquid fluidized bed. Concentrated suspensions follow bold solid line of slope n, which may at 8 = 8c change to either slope n > n all the way to s = 1 (bold dotted line), or slope n" < to 8 < 1 (bold dashed line). 

Di Felice R, Foscolo PU, Gibilaro LG, Wallis GB, Carta R. Expansion characteristics of tapered fluidized beds. AIChE J 37 1668-1672, 1991a. 

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