Solid-liquid fluidized beds particle density effect

Fig. 11. Effect of density difference at various liquid viscosities on particle Reynolds number evaluation at lower critical particle diameter, (a) Solid-liquid fluidized beds [a = 3.0, Cv = f(s), pi = 1000 kg/m ]. (b) Gas-solid fluidized beds [a = 3.0, Cy = /(e), po = 1 kg/m ]. (c) Unified stability map of particle Reynolds number vs density difference for different values of transition hold-up solid-liquid fluidized beds [a = 3.0, Cy = f(s), p-l = 1 mPas, pi = 1000 kg/m ].

Fig. 13. Effect of liquid density on lower particle critical diameter solid-liquid fluidized beds [a = 3.0, Cy = /(e), Ml = 1 mPas].

In the case of solid-liquid fluidized beds, the effect of virtual mass is to make the bed more unstable as shown in Fig. 21. This can be explained as follows The effect of virtual mass is to increase the apparent density of the particle. As discussed in this section earlier, an increase in the particle density makes the system more unstable. This observation is consistent with Fig. 21. As the particle density increases, say, ps - 9000 kg/m and Pl = 1000 kg/m, the effect of virtual mass is negligible and the curves are seen to merge irrespective of the formulation of virtual mass coefficient. 

EFFECTIVE (OR AERODYNAMIC) PARTICLE DENSITY is when the measured volume includes both the closed and the open pores. This volume is within an aerodynamic envelope as seen by the gas flowing past the particle the value of density measured is therefore a weighted average of the solid and immobilised gas (or liquid) densities present within the envelope volume. The effective density is clearly of primary importance in applications involving flow round particles like in fluidization, sedimentation or flow through packed beds. 

Sorting of particles, particularly of bidisperse mineral mixtures, is more cleanly effected by sink-and-float separation. This is most simply done by using a liquid nonsolvent intermediate in density between that of the two particle species. With overflow and underflow discharge streams, and a continuous feed suspension introduced at an intermediate position, the sharpness of separation decreases as the feed rate, the feed solids concentration, and the under-flow/overflow ratio are increased (Nasr-El-Din et al., 1988, 1990). In the absence of an acceptable liquid, a homogeneous suspension, viz., a water-fluidized bed of narrowly cut fine sand (e.g., —325 - - 400 mesh), could... 

Bubble formation in liquids with the presence of particles, as in slurry bubble columns and three-phase fluidized bed systems, is different from that in pure liquids. The experimental data of Massimilla et al. (1961) in an air-water ass beads three-phase fluidized bed revealed that the bubbles formed from a single nozzle in the fluidized bed are larger than those in water, and the initial bubble size inereases with the solids concentration. Yoo et al. (1997) investigated bubble formation in pressurized liquid solid suspensions. They used aqueous glyeerol solution and 0.1 mm polystyrene beads as the liquid and solid phases, respectively. The densities of the liquid and the particles were identical, and thus the partieles were neutrally buoyant in the liquid. The results indicated that initial bubble size deereases inversely with pressure under otherwise eonstant eonditions, that is, gas flow rate, temperature, solids eoneentration, orifiee diameter, and gas chamber volume. Their results also showed that the particle effect on the initial bubble size is insignificant. The difference in the finding regarding the particle effect on the initial bubble size between Massimilla et al. (1961) and Yoo et al. (1997) is possibly due to the difference in particle density. 

The simulation also provides some information about the bubble particle interaction as shown in Fig. 25. As the bubble rises in the liquid-solid fluidized bed, an interaction between the bubble and particles takes place. In the simulation model, the bubble-particle interaction is accounted for by adding a surface-tension-induced force to the particle motion equation. This force is also added to the source term of the liquid momentum equation for the liquid elements in the interfacial area to account for the particle effect on the interface. The particle movement is determined from the resulting total force acting on the particle. From the simulation results, it is seen that most particles contacting the bubble do not penetrate the bubble only one or two particles penetrate. Instead, they pass around the bubble surface. When the particles penetrate the bubble, they fall through quickly to the bubble base because of the low viscosity and density of the gas phase. 

There are several possible ways of inducing the necessary incoherent motion of particles to facilitate their resonance line narrowing. An obvious way is to produce a fluidized bed in the NMR sample tube, but this has been tried without success. An alternative is to utilize the effects of Brownian motion where molecular bombardment of fine particles in suspension can cause their incoherent motion. The latter approach (ultrafine particle NMR) has been demonstrated using very small particles (nm size) that were perceived to be necessary to respond appropriately to Brownian motion. The necessity to use extremely small particles for this type of experiment is open to question because many experiments in the author s laboratory have shown that micrometre sized particles, suspended in density matched liquids, respond to Brownian motion and yield resonances that are significantly reduced relative to those from static solids. 

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