Solid holdup

Bubble-column slurry operations are usually characterized by zero net liquid flow, and the particles are held suspended by momentum transferred from the gas phase to the solid phase via the liquid medium. The relationships between solids holdup and gas flow rate is of importance for design of bubble-column slurries, and some studies of this aspect will be reviewed prior to the discussion of transport phenomena. 

Roy et al. (R3) define the critical solids holdup as the maximum quantity of solids that can be held in suspension in an agitated liquid. They present measurements of this factor for various values of gas velocity, gas distribution, solid-particle size, liquid surface tension, liquid viscosity, and a solid-liquid wettability parameter, and they propose the following two correlations in terms of dimensionless groups containing these parameters ... 

While having some advantages over riser reactors, downer reactors also suffer fixrm some serious shortcomings, such as a low solids holdup in the bed, difficulty in even distribution of injected residual on the catalysts, and a high sensitivity to the structure of the inlet [11,12]. Therefore, the development of a new coupled CFB reactor that can fully utilize the advantages of the riser and the downer is of interest. 

Comparing with the conventional three-phase beds, the axial solid holdup distribution is much more uniform and the radial distribution of gas holdup (sg) is much flatter in circulating beds, due to the relatively high Ul and solid circulation. The values of Eg and bed porosity can be predicted by Eqs. (7) and (8) with a correlation coefficient of 0.94 and 0.95, respectively. 

Fig. 3. Radial profile of the gas holdup at different solid holdups...

Figure 3 shows the radial profile of the gas holdup in the riser with increasing superficial gas velocity under different solid holdups. The gas holdup increases with increasing superficial gas velocity at the different solid holdups. At a low superficial gas velocity, the liquid velocity... 

Fig. 4. Effect of solid holdup on gas holdup at Fig. 5. Comparison of calculated and measured different superficial gas velocities gas holdup...

A specially built conductivity probe was used to investigate the gas holdup in a novel internal-loop airlift reactor. The gas holdup generally increases with increasing solid holdup due to increased flow resistance. A model based on energy balance was developed that can be used to predict the average gas holdup in this novel interal-loop airlift reactor. 

However, when the amount of added particles increased(W=2.0 or 3.0wt.%), the effective surface area of cathode plate decreased due to the considerable increase of solid holdup between the two electrodes, thus, the amount of copper recovery decreased. In this experimental conditions, the distance between the two electrodes(LAc) also influenced the recovery of copper, as can be seen in Fig. 7. In this figure, the value of R was maximum when the distance(LAc) was 1.5cm, in all the cases studied. 

Understanding the effect of reactor diameter on the volumetric mass transfer coefficient is critical to successful scale up. In studies of a three-phase fluidized bed bioreactor using soft polyurethane particles, Karamanev et al. (1992) found that for a classical fluidized bed bioreactor, kxa could either increase or decrease with a change in reactor diameter, depending on solids holdup, but for a draft tube fluidized bed bioreactor, kxa always increased with increased reactor diameter. 

The performance of a reactor for a gas-solid reaction (A(g) + bB(s) -> products) is to be analyzed based on the following model solids in BMF, uniform gas composition, and no overhead loss of solid as a result of entrainment. Calculate the fractional conversion of B (fB) based on the following information and assumptions T = 800 K, pA = 2 bar the particles are cylindrical with a radius of 0.5 mm from a batch-reactor study, the time for 100% conversion of 2-mm particles is 40 min at 600 K and pA = 1 bar. Compare results for /b assuming (a) gas-film (mass-transfer) control (b) surface-reaction control and (c) ash-layer diffusion control. The solid flow rate is 1000 kg min-1, and the solid holdup (WB) in the reactor is 20,000 kg. Assume also that the SCM is valid, and the surface reaction is first-order with respect to A. 

A figure such as Figure 22.4 can also be used in general to determine the size of reactor for a given throughput, and specified conversion, /B, in terms of solid holdup, since from equation 22.2-8. The determination of t for a given value of /B... 

Fbo) 1S lOOO kg min-1, and the solid holdup (WB) in the reactor is 15,000 kg Assume also that the SCM is valid, and the reaction is fust-order with respect to A. 

Bisschops et al. have investigated the hydrodynamic capacity and solids holdup in countercurrent two-phase flow in the centrifugal field, as well as the relation between the pressure drop and void fraction. Moreover, the analysis included a check as to whether the two-phase flow in the centrifugal field was homogeneous or heterogeneous. 

Proper t.y Gas holdup Liquid holdup-Solid holdup Liquid distribution... 

Fig. 6. Determination of solid distribution at three axial positions (a) cross-sectional distribution (b) azimuthally averaged solids holdup and (c) overall solids holdup. The horizontal line indicates experimentally determined solids fraction.

The solid circulation rate (Gs, kgm-2s-1) can be controlled by aeration gas in the loop-seal to adjust pressure balance in the CFB loop. Solid circulation rate is a unique operating variable in the CFB system as a function of pressure drop across loop-seal of a CFB photoractor [10] and directly related to solid holdup in the riser where the photodegradation of TCE occurs. 

Solid holdup (es), the volume fraction of solids, and A/JriScr, pressure drop in the riser are defined as... 

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