Bubbles high solids feed

In membrane processing the application of surface shear is required to control concentration polarization and fouling for high solids feeds or to assist cake removal for batch membrane filtration of low solids feeds. For submerged membranes the common practice is to use two-phase bubbly flow to induce surface shear. This section deals with the role of bubbles as well as other hydrodynamic aspects of submerged membranes. 

High Solids Feed by (Bubbled) Cross-Flow Filtration For applications, such as MBRs, the particulate solids content is relatively high [suspended solids (SS) up to 10-15 g/L] with additional colloids and macrosolutes. Under these conditions bubbled cross flow is essential to limit concentration polarization and subsequent fouling. 

As the velocity is further increased, a point is reached when the bed density becomes a strong function of the rate of solids feed. This corresponds to the fast fluidization regime and is characterized by the significant fact that fluid-bed densities as high as those in a bubbling bed can be maintained by adjnsting the solids flow rate. 

X 10 N/m. Choose room-temperature operation, T = 30°C = 303.16 K. Initial gas volume = nRgT/P = (267.75 x 8314 x 303.16)/ .8 x 10 = 3749 m. Since this volume is very high compared to the reactor volume of 10 m, choose semibatch mode of operation with continuous supply of feed gas and removal of residual gas to maintain the reactor pressure. In view of this, a slurry of Ca(OH)2 in water is prepared and CO2 is bubbled through it for 4 h. The mother liquid phase will be initially saturated with Ca(OH)2. As the reaction proceeds, CaCOj precipitates out. The mother liquor can be separated from the solid CaC03 precipitate by filtration. For ease of operation and control, assume constant gas flow rate. 

The CSTR is used extensively in situations where intense agitation is required, such as the addition of a gaseous reactant to a liquid by transfer between the bubbles and the continuous liquid, and the suspension of a solid or second liquid within a continuous liquid phase. Polymerization reactions are sometimes conducted in CSTRs. It is common to employ a cascade or series of CSTRs in which the effluent from the first reactor is used as feed to the second and so forth down the cascade (Figure 1.4). The cascade permits one to realize high conversion of reactant, while minimizing total reactor volume. 

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