Solids suspension operating regimes

Figure 3 shows that operating regimes from the onset of turbulent fluidization (beyond the transition velocity, U ) follow different paths depending upon the two general CFB designs defined by Kobro and Brereton [62] and Kunii and Levenspiel [63]. In the first, Fixed Inventory System (FIS), solids inventory in the return leg or standpipe is not controlled. Setting gas velocity and system inventory establishes the riser suspension density profile and solids circulation rate. In the second, Variable Inventory System (VIS), a vessel external to the riser acts as a... 

The comparison between slurry and monolith reactors is summarized in Table 1. Based on the known features of slurry and monolith reactors, it can be concluded that the slurry reactors are preferable for mass-transfer-limited processes as far as the overall process rates are concerned. However, due to the low concentration of solid catalyst in slurry reactors, the productivity per unit volume in these reactors is not necessarily higher than that of monolith reactors. For processes occurring in kinetic regime, the monolith reactors are preferable due to their easier operation. The productivity of slurry reactors might apparently be increased by increasing the catalyst concentration. However, suspensions with a high concentration of fine catalyst particles behave as non-Newtonian liquids, with all the negative consequences in heat and mass transfer. 

Nearly Uniform Suspension Regime This is the state of suspension at which particle concentration and particle size distribution are roughly uniform throughout the vessel. Any further increase in agitation speed or power does not appreciably enhance the solids distribution in the fluid. A coefficient of variation of the solid concentration of about 0.05 (i.e., a uniformity of 95%) is often considered adequate for most process applications. In practice, a concentration gradient as a function of vertical position will always exist. Complete uniformity is theoretically unattainable and impractical to achieve because a thin clear fluid layer always exists at the air-liquid interface, as the axial lift velocity, and, hence, the particle concentration approaches zero near the liquid surface. Nearly uniform suspension is often the desired process result for operations where a representative sample of solids is required or a uniform concentration of solids must... 

However, it is not always easy to distinguish between the flow behavior encountered in the fast fluidization and the transport bed reactors [56]. The transport reactors are sometimes called dilute riser (transport) reactors because they are operated at very low solids mass fluxes. The dense riser transport reactors are operated in the fast fluidization regime with higher solids mass fluxes. The transition between these two flow regimes appears to be gradual rather than abrupt. However, fast fluidization generally applies to a higher overall suspension density (typically 2 to 15% by volume solids) and to a situation wherein the flow continues to develop over virtually the entire... 

Automated samplers function without the need for an operator, giving the possibility of selection of a sampling regime arranged in dependence on time or flow-through quantity. One of these is the sampler shown in Fig. 3.66 [7]. The system has a simple battery-driven peristaltic pump, which can take samples heavily polluted with suspensions of solid material using a suction basket immersed in water. The pump feeds the sample into a 5 1 bottle at the required times determined by the timing unit. 

Fast fluidization. To operate in the fast fluidization regime requires both that U exceed the transport velocity, and that solid particles be fed to the base of the unit with a sufficient flow rate, typically 20-200 kg/m -s, that a relatively dense suspension can be maintained in the reactor. Typical reported values (7) of are 1.5 m/s for 49 pm silica alumina cracking catalyst (pp= 10/0 kg/m ) and 3.8 m/s for hydrated alumina particles (Pp " 2460 kg/m ). There are insufficient data to allow to be correlated in a general way. 

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