Solid-liquid, suspension fluid motion

Each stage of particle formation is controlled variously by the type of reactor, i.e. gas-liquid contacting apparatus. Gas-liquid mass transfer phenomena determine the level of solute supersaturation and its spatial distribution in the liquid phase the counterpart role in liquid-liquid reaction systems may be played by micromixing phenomena. The agglomeration and subsequent ageing processes are likely to be affected by the flow dynamics such as motion of the suspension of solids and the fluid shear stress distribution. Thus, the choice of reactor is of substantial importance for the tailoring of product quality as well as for production efficiency. 

Baffles are essential for solids suspension operations involving solids that are heavier than the liquid. They convert the swirling motion into top-down or axial fluid motion, which helps to lift and suspend the solids. 

For multi-phase fluid flow (gas-solid, gas-liquid, gas-solid-liquid, etc.) through vertical pipes and vessels, the density often decreases with height. As a first approximation, the pressure drop is assumed to be due entirely to the hydrostatic head of the fluid and the solids (Motional and inertial conMbutions to the pressure drop are assumed to be negligible—which can introduce an error of up to 10%). The suspension density for a gas-soUds system is ... 

What this shows is that, from the definition of off-bottom motion to complete uniformity, the effect of mixer power is much less than from going to on-bottom motion to off-bottom suspension. The initial increase in power causes more and more solids to be in active communication with the liquid and has a much greater mass-transfer rate than that occurring above the power level for off-bottom suspension, in which slip velocity between the particles of fluid is the major contributor (Fig. 18-23). 

If the fermentation contains sohds, then special precautions may be required to prevent sedimentation. One is a fluidized bed, where a dense slurry phase in the bottom of the tank is kept in gentle motion via upflow of the hquid or some induced gas/liquid momentum. The upflow anaerobic sludge bed (UASB) reactor is an example. In biogas installations, sohds may sediment and bacterial consortia may take several months to convert ah the material into methane and CO2. Because of the slow reaction rates, these instahations can be extremely large, up to 20 000 m. Other devices are packed-bed bioreactors, where ceUs are immobihzed on a solid support and the liquid is percolated or trickled down to supply nutrients and remove products. These can be found in specific antibiotic or food fermentations. Because of the lack of free fluid flow, the performance characteristics of such vessels are usually poor compared to suspensions. 

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