Gas-solid flows in fluidized bed

Lindborg H, Lysberg M and Jakobsen HA (2007) Practical validation of the two-fluid model applied to dense gas-solid flows in fluidized beds. Chem Eng... 

Lindborg H, Lysberg M, Jakobsen HA (2007) Practical Validation of the Two-Fluid Model Applied to Dense Gas-Solid Flows in Fluidized Beds. Submitted to Chem Eng Sci 2007... 

Mueller, P and Reh, L. Particle Drag and Pressure Drop in Accelerated Gas-Solid Flow," in Circulating Fluidized Bed Technology IV (Amos A. Avidan, eds.), pp. 193-198. Somerset, Pennsylvania (1993). 

If the effect of boundaries on gas-solid flow in fast fluidized beds were overlooked, gas/solids interaction might well be misunderstood. 

In a hard sphere approach, particles are assumed to interact through instantaneous binary collisions. This means particle interaction times are much smaller than the free flight time and therefore, hard particle simulations are event (collision) driven. For a comprehensive introduction to this type of simulation, the reader is referred to Allen and Tildesley (1990). Hoomans (2000) used this approach to simulate gas-solid flows in dense as well as fast-fluidized beds. There are three key parameters in such hard sphere models, namely coefficient of restitution, coefficient of dynamic friction and coefficient of tangential restitution. Coefficient of restitution is discussed later in this chapter. Detailed discussion of these three model parameters can be found in Hoomans (2000). 

Xu, B.H. and Yu, A.B. (1997), Numerical simulation of gas solid flow in a fluidized bed by combining discrete particle method with computational fluid dynamics, Chem. Eng. Sci., 52, 2785. 

DPMs can also be used to understand the influence of particle properties on fluidization behavior. It has been demonstrated that ideal particles with restitution coefficient of unity and zero coefficient of friction, lead to entirely different fluidization behavior than that observed with non-ideal particles. Simulation results of gas-solid flow in a riser reactor reported by Hoomans (2000) for ideal and nonideal particles are shown in Fig. 12.8. The well-known core-annulus flow structure can be observed only in the simulation with non-ideal particles. These comments are also applicable to simulations of bubbling beds. With ideal collision parameters, bubbling was not observed, contrary to the experimental evidence. Simulations with soft-sphere models with ideal particles also indicate that no bubbling is observed for fluidization of ideal particles (Hoomans, 2000). Apart from the particle characteristics, particle size distribution may also affect simulation results. For example, results of bubble formation simulations of Hoomans (2000) indicate that accounting... 

Patterns of gas-solid interaction in dryers (a) gas flow across a static bed of solids (6) gas passing through a bed of preformed solids (c) showering action in a rotary dryer d) fluidized solids bed (e) cocurrent gas-solid flow in a pneumatic-conveyor flash dryer. 

As is made clear in other parts of this publication, solid particles in fluidized beds are held in suspension by the upward flow of gas. The velocity at which the particles first become suspended is the minimum fluidization velocity, Ihe gas velocity through... 

Marzocchella A, Zijerveld R, Schouten J, van den Bleek C. Chaotic behavior of gas-solid flow in the riser of a laboratory-scale circulating fluidized bed. AIChE J 43 1458-1468, 1997. 

Of course the spectrum of quantities, which need to be measured in a fluidized bed, is much wider. These include, for example, local solids volume concentrations, solids velocities and solids mass flows, the vertical and the horizontal distribution of solids inside the system or the lateral distribution of the fluidizing gas. In response to these needs a number of more sophisticated measurement techniques were proposed. For example, suction probes were developed to measure local solids and mass flow, heat transfer probes were proposed for detection of de-fluidized zones and solids flow inside fluidized-bed reactors. Other techniques include capacitance probes, optical probes, or y-ray densitometry - a detailed review was given recently by Werther [1]. Cody et al. 2 reported the use of an acoustic probe to measure particle velocity at the wall of fluidized beds. 

This is illustrated in Fig. 4 reproduced from Verloop (1994) and Van den Akker (1998). These authors were able to reproduce this behavior by means of CFD simulations in which they assumed bubbles bursting from the bed surface and entraining particles into the freeboard. Similarly, the overall dilute gas-solid flow in risers (used for catalytic cracking of crude oil) is often dominated by inhomogeneities usually described as strands or clusters (Fligner et al, 1994 Horio and Kuroki, 1994 Yonemura et al, 1995). Harris et al (2002) reviewed experimental literature data on cluster formation in riser flow. Mudde et al (1999) made observations in a turbulent fluidized bed 38.4 cm in diameter by means of a dual gamma-ray densitometry technique, illustrated by the measuring series of Fig. 5, and from such... 

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