Minimum solids circulation rate

Similar relations have been presented by other authors (Horio, 1991 Perales et ai, 1991). The minimum solids circulation rate can be estimated from the following empirical formula ... 

Aeration of the downcomer can also be provided with a conical distributor plate (No. 3 flow) with greatly increased solids circulation rate as shown in Fig. 8. At lower downcomer aeration, the solids circulation rate is essentially similar to that without downcomer aeration at a distributor plate location ofL = 21.7 cm. At higher downcomer aeration, however, a substantial increase in solids circulation rate is realized with the same total gas flow rate. Apparently, a minimum aeration in the downcomer is required in order to increase substantially the solids circulation rate. For polyethylene beads, this critical aeration rate is at a downcomer superficial... 

In practice, unstable operation may occur at a higher gas velocity than that at choking or at nonchoking transition to dense-phase fluidization. Thus, the minimum operable gas velocity for a given solids circulation rate can be higher than t/tf for fast fluidization operation in some CFBs. The factors contributing to this unstable situation are... 

Solution The maximum solids circulation rate through an L-valve is limited by the pressure drop across the downcomer under minimum fluidizing conditions. Thus, the pressure balance can be written in terms of voidage and solids inventory in the downcomer as (see Fig. 10.6)... 

Equations (El 0.13) and (E10.17) can be solved simultaneously to obtain the relationship between the minimum operable gas velocity and solids circulation rate, as given in Fig. E10.2. To illustrate the dependence of operable velocities on system designs, the results for three solids inventory heights in the downcomer are given as shown in Fig. E10.2. It is seen in the figure that for a given solids circulation rate, a higher inventory level yields a lower minimum operable velocity. 

Above a certain liquid circulation rate (or power input), the venturi loop reactor maintains the solid catalyst in complete suspension without any sedimentation problems. Further, as shown by Bhutada and Pangarkar (1989), above this power input, the solid concentration is uniform in both the axial and radial directions. This clearly shows that as long as a certain minimum power input is maintained, the catalyst concentration is independent of the location in the venturi loop reactor. In sharp contrast to this, there is substantial variation of soUd concentration in conventional stirred (or sparged) reactors operated at the just-suspended condition (Section 7 A.7.1). This characteristic behavior helps in attaining uniform rates throughout the reactor volume, avoids hot spots, and enhances selectivity. 

The rheological properties of the drilling fluid have a marked influence on the performance of solids control equipment. Froment et al. (163) have pointed out that an increase in the viscosity of the drilling fluid will decrease the flow rate capacity of the shale shaker and will increase the minimum particle size of the solids in the separated stream from a hydrocyclone that is returned to the circulating drilling fluid. For example, Figure 59 shows the particle size distribution of the solids in the under flow from a hydrocyclone. The density and viscosity of the drilling fluid are observed to have a marked effect on the separation characteristics of the hydrocyclone. 

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