Solids mixing rate

If the bubble frequency, bubble diameter, and bubble velocity are known, the solids mixing rate can be calculated. 

Applicability over a wide range of particle properties and high solids mixing rates... 

Solids mixing rate theory has been discussed in papers by Brothman et al. (B5), Coulson and Maitra (C3, M2), Visman and Van Krevelen (VI), Weidenbaum and Bonilla (W2), Weidenbaum (Wl), Lacey (L2), and Oyama and Ayaki (09). In all of these papers, some sort of a rate equation has been postulated. These are tabulated in Table III, which is an expanded version of a similar one from Weidenbaum and Bonilla (W2). 

The solids circulation pattern and solids circulation rate are important hydrodynamic characteristics of an operating jetting fluidized bed. They dictate directly the solids mixing and the heat and mass transfer between different regions of the bed. 

A turbine type agitator is commonly used for liquid-solid systems. Mixing rates depend on the forces required to suspend all solid particles. Minimum levels can be determined for (1) lifting the particles, and (2) for suspending them in an homogeneous manner [200]. Similar requirements apply to liquid-liquid systems. For cases where two poorly miscible fluids of about equal volume are used in the reaction, the mixer is placed at the interface. For a bench-scale experimental system of about 2 liters capacity, the minimum rotational speed to obtain well-dispersed system is 300 to 400 rpm [201], depending on the type of mixer. This rotational value decreases as the vessel volume increases. 

Batch-Solids, Mixed Constant Flow of Fluid. Inserting the rate of Eq. lAa into the performance expression for mixed flow gives... 

The hydrodynamic parameters that are required for stirred tank design and analysis include phase holdups (gas, liquid, and solid) volumetric gas-liquid mass-transfer coefficient liquid-solid mass-transfer coefficient liquid, gas, and solid mixing and heat-transfer coefficients. The hydrodynamics are driven primarily by the stirrer power input and the stirrer geometry/type, and not by the gas flow. Hence, additional parameters include the power input of the stirrer and the pumping flow rate of the stirrer. 

Voidage profiles represent one of the most important aspects of the flow structure of fast fluidization, which play an important role in gas and solids mixing, mass and heat transfer, and conversion in a chemical reactor. Considerable efforts have been given to studying the axial and radial variation of solids concentration axially, dilute at the top and dense at the bottom, and radially, dilute in the center and dense in the vicinity of the wall. As already mentioned in Section II, these variations depend mainly on gas velocity and solids circulation rate and are also influenced by the configuration of the apparatus. 

Fig. 33. Gas mixing extent as a function of voidage determined by gas velocity and solids circulation rate (after Li and Wu, 1991).

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