Solid flow rate effect

Figure 10.11. Effects of gas and solids flow rates on the axial profile of the cross-sectional averaged voidage in a riser with an abrupt exit (from Brereton and Grace, 1993b) (a) Abrupt exit geometry (b) Voidage profiles.

It is too often assumed that because precolumn sample preparation devices, such as solid-phase extraction cartridges, are simple tools, they require relatively little skill or attention to detail for successful use. Cartridges do, however, require attention to detail for successful operation in sample enrichment procedures. Two of the most important parameters to control and understand are flow-rate effects and recovery (or loadability) effects. 

Fig. 22. Effect of solids flow rate on radial voidage profile (after Tung et a/., 1988).

Figure 15 maps the variation of this volume-specific intensity with gas velocity Ut and solids flow rate Gs. The subfigure at the top shows the shadowed cross-section for Gs = 80 kg/(m2s). Maximal Iv corresponds to the most efficient particle-fluid contacting per unit volume, and IFst should be integrated volumetrically to yield the global effectiveness of particle-fluid contacting in a reactor. 

Thorley et al. (T2) studied the effect of column diameter, cone angle, orifice size, bed depth, and gas flow on the particle velocity at the wall for wheat in air. Their data consist of observed particle velocities at the top of the annulus, and the vertical velocity gradients, from which the total solids flow rates at the top and the cross-flow rates can be calculated. A selection from the extensive data reported by these workers is shown in... 

For packed bed of spheres, Barysheva et al. [21] found that flowing particle mean residence time is essentially independent of gas flow rate. Mean solids velocity decreases with an increase of flowing particles diameter, this effect becoming more pronounced at higher solids flow rates. Further, flowing solids velocity decreases with an increase in flowing solids flow rate. This is a consequence of the interaction between individual flowing solids particles. 

D8. In the leaching of sugar from sugar cane, water is used as the solvent. Typically about 11 stages are used in a countercurrent Rotocel or other leaching system. On a volumetric basis liquid flow rate/solid flow rate = 0.95. The effective equilibrium constant is i% = 1.18, where m = (concentration, g/liter, in liquid)/(concentration, liter, in solid) fSchwartzberg. 19801. If pure water is used as the inlet solvent, predict the recovery of sugar in the solvent. 

D12. A countercurrent leaching system is recovering oil from soybeans. The system has five stages. On a volumetric basis, liquid flow rate/solids flow rate = 1.36. 97.5% of the oil entering with the nonsoluble solids is recovered with the solvent. Solvent used is pure. Determine the effective equlibrium constant, m, where m is (kg/m of solute in solvent)/(kg m of solute in solid) and is given by the equation y = m x. 

Knowlton and Hirsan (1978) found that aeration was most effective if it was added at a length-to-dia-meter L/D) ratio of 1.5 or more above the eenterline of the horizontal section of the L-valve. Aeration added at the centerline of the horizontal seetion or at the bottom of the centerline of the standpipe was found to bypass directly to the top of the horizontal section. Thus it was not being efficiently utilized to drag the solids through the constricting bend. The aeration tap locations and the solids flow rate versus aeration rate curves for each aeration tap location investigated in their study are shown in Fig. 22. In summary, L-valve aeration should be added above... 

The effect of physical processes on reactor performance is more complex than for two-phase systems because both gas-liquid and liquid-solid interphase transport effects may be coupled with the intrinsic rate. The most common types of three-phase reactors are the slurry and trickle-bed reactors. These have found wide applications in the petroleum industry. A slurry reactor is a multi-phase flow reactor in which the reactant gas is bubbled through a solution containing solid catalyst particles. The reactor may operate continuously as a steady flow system with respect to both gas and liquid phases. Alternatively, a fixed charge of liquid is initially added to the stirred vessel, and the gas is continuously added such that the reactor is batch with respect to the liquid phase. This method is used in some hydrogenation reactions such as hydrogenation of oils in a slurry of nickel catalyst particles. Figure 4-15 shows a slurry-type reactor used for polymerization of ethylene in a sluiTy of solid catalyst particles in a solvent of cyclohexane. 

If the liquid flow rate is increased above the value required for conveying the solid particles, they will be carried out of the reactor with the liquid. This effect may be of interest in cases where frequent catalyst regeneration is necessary. 

Flynn and Dickens [142] have translated the relaxation methods of fluid kinetics into terms applicable to solid phase thermogravimetry. The rate-determining variables such as temperature, pressure, gas flow rate, gas composition, radiant energy, electrical and magnetic fields are incremented in discrete steps or oscillated between extreme values and the effect on reaction rate determined. 

The distribution of particles velocities in the liquid under a small flowing rate is much closer to that calculated from Eq (31) [85] when the width and depth of channels are big enough (about 2 mm) to ignore the effect of the surface force of the solid wall as shown in Fig. 38. 

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