Hydrodynamic Characteristic

As a result of the existence of bubbles/voids in turbulent fluidized beds, the hydrodynamic behavior of turbulent fluidization under relatively low gas velocity conditions is similar, to a certain extent, to that of bubbling fluidization. However, distinct differences exist under relatively high gas velocity conditions, thus rendering many correlations previously developed for the bubbling regime invalid for the turbulent regime. 

As more small bubbles/voids with low rise velocities and long residence times are present in the turbulent bed, they give rise to more significant dense bed expansion in the turbulent 

It is seen that for Geldart types A and B particles, fast fluidization requires superficial gas velocities approximately an order of magnitude greater than that for bubbling dense beds. In many applications of fast fluidization, the particles exiting top of the bed are captured by cyclones and recirculated for makeup injection at the bottom of the bed, hence this regime is also denoted as circulating fluidization, CFB. 

General Characteristics. Energy addition or extraction from fast fluidized beds are commonly accomplished through vertical heat transfer surfaces in the form of membrane walls or submerged vertical tubes. Horizontal tubes or tube bundles are almost never used due to concern with 

The interaction of parametric effects of solid mass flux and axial location is illustrated by the data of Dou et al. (1991), shown in Fig. 19. These authors measured the heat transfer coefficient on the surface of a vertical tube suspended within the fast fluidized bed at different elevations. The data of Fig. 19 show that for a given size particle, at a given superficial gas velocity, the heat transfer coefficient consistently decreases with elevation along the bed for any given solid mass flux Gs. At a given elevation position, the heat transfer coefficient consistently increases with increasing solid mass flux at the highest elevation of 6.5 m, where hydrodynamic conditions are most likely to be fully developed, it is seen that the heat transfer coefficient increases by approximately 50% as Gv increased from 30 to 50 kg/rrfs. 

The experiments of Dou et al. (1991) also indicate that the heat transfer coefficient varied with radial position across the bed, even for a given cross-sectional-averaged suspension density. Their data, as shown in Fig. 20, clearly indicate that the heat transfer coefficient at the bed wall is significantly higher than that for vertical surfaces at the centerline of the bed, over the entire range of suspension densities tested. Almost certainly, this parametric effect can be attributed to radial variations in local solid concentration which tends to be high at the bed wall and low at the bed centerline. 

The data of Fig. 20 also point out an interesting phenomenon—while the heat transfer coefficients at bed wall and bed centerline both correlate with suspension density, their correlations are quantitatively different. This strongly suggests that the cross-sectional solid concentration is an important, but not primary parameter. Dou et al. speculated that the difference may be attributed to variations in the local solid concentration across the diameter of the fast fluidized bed. They show that when the cross-sectional averaged density is modified by an empirical radial distribution to obtain local suspension densities, the heat transfer coefficient indeed than correlates as a single function with local suspension density. This is shown in Fig. 21 where the two sets of data for different radial positions now correlate as a single function with local mixture density. The conclusion is That the convective heat transfer coefficient for surfaces in a fast fluidized bed is determined primarily by the local two-phase mixture density (solid concentration) at the location of that surface, for any given type of particle. The early observed parametric effects of elevation, gas velocity, solid mass flux, and radial position are all secondary to this primary functional dependence. 

---

A more rigorous treatment takes into account the hydrodynamic characteristics of the flowing solution. Expressions for the limiting currents (under steady-state conditions) have been derived for various electrodes geometries by solving the three-dimensional convective diffusion equation ... 

The main aim of the present chapter is to verify the capacity of conventional theory to predict the hydrodynamic characteristics of laminar Newtonian incompressible flows in micro-channels in the hydraulic diameter range from dh = 15 to db = 4,010 pm, Reynolds number from Re = 10 up to Re = Recr, and Knudsen number from Kn = 0.001 to Kn = 0.4. The following conclusions can be drawn from this study ... 

Eulerian two-fluid model coupled with dispersed itequations was applied to predict gas-liquid two-phase flow in cyclohexane oxidation airlift loop reactor. Simulation results have presented typical hydrodynamic characteristics, distribution of liquid velocity and gas hold-up in the riser and downcomer were presented. The draft-tube geometry not only affects the magnitude of liquid superficial velocity and gas hold-up, but also the detailed liquid velocity and gas hold-up distribution in the reactor, the final construction of the reactor lies on the industrial technical requirement. The investigation indicates that CFD of airlift reactors can be used to model, design and scale up airlift loop reactors efficiently. 

Many industrial processes which employ bubble column reactors (BCRs) operate on a continuous liquid flow basis. As a result these BCR s are a substantially more complicated than stationary flow systems. The design and operation of these systems is largely proprietary and there is, indeed a strong reliance upon scale up strategies [1]. With the implementation of Computational Fluid Dynamics (CFD), the associated complex flow phenomena may be anal)rzed to obtain a more comprehensive basis for reactor analysis and optimization. This study has examined the hydrodynamic characteristics of an annular 2-phase (liquid-gas) bubble column reactor operating co-and coimter-current (with respect to the gas flow) continuous modes. 

A realistic description of the dynamic behaviour of an actual mixer-settler plant item should however also involve some consideration of the hydrodynamic characteristics of the separate mixer and settler compartments and the possible flow interactions between mixer and settler along the cascade. 

Further work is needed to determine in which regimes, if any, fluid bed behave as chaotic systems. Additional testing is needed to determine the sensitivity of important bed hydrodynamic characteristics to the Kolmogorov entropy, to quantitatively relate changes of entropy to... 

The solids circulation pattern and solids circulation rate are important hydrodynamic characteristics of an operating jetting fluidized bed. They dictate directly the solids mixing and the heat and mass transfer between different regions of the bed. 

Tarmy, B. L., Chang, M., Coulaloglou, C. A., and Ponzi, P. R., The Three-Phase Hydrodynamic Characteristics of the EDS Liquefaction Reactors Their Development and use in Reactor Scaleup, Proc. 8th Int. Symp. Chem. Reaction Eng., 30 239 (1984)... 

Typical flow rates in FTA vary between 0.5 and 5.0 ml/min per channel, although higher values have also been used. Most of the published work on CL with HA is based on equal flow rates for every stream entering the manifold. Nevertheless, if different rates must be used, deterioration of repeatability and reproducibility might appear due to incomplete mixing and anomalous hydrodynamic characteristics. These problems can be avoided if the general rule that the ratio of the fastest to the slowest flow rate should not exceed the value of 3 or 4 is followed. 

In the last decade there were many papers published on the study of enzyme catalyzed reactions performed in so-called chromatographic reactors. The attractive feature of such systems is that during the course of the reaction the compounds are already separated, which can drive the reaction beyond the thermodynamic equilibrium as well as remove putative inhibitors. In this chapter, an overview of such chromatographic bioreactor systems is given. Besides, some immobilization techniques to improve enzyme activity are discussed together with modern chromatographic supports with improved hydrodynamic characteristics to be used in this context. 

The rotating disc electrode is constructed from a solid material, usually glassy carbon, platinum or gold. It is rotated at constant speed to maintain the hydrodynamic characteristics of the electrode-solution interface. The counter electrode and reference electrode are both stationary. A slow linear potential sweep is applied and the current response registered. Both oxidation and reduction processes can be examined. The curve of current response versus electrode potential is equivalent to a polarographic wave. The plateau current is proportional to substrate concentration and also depends on the rotation speed, which governs the substrate mass transport coefficient. The current-voltage response for a reversible process follows Equation 1.17. For an irreversible process this follows Equation 1.18 where the mass transfer coefficient is proportional to the square root of the disc rotation speed. 

The design equations previously described are only valid when there are no factors which modify the kinetics of the immobilized biocatalyst (partition effects, heat and mass transfer effects and decay of biological activity) and the hydrodynamic characteristics of the reactor (back-mixing). Thus, the kinetic constants and used in those equations are intrinsic values obtained in the absence of those factors, being only dependent on the conformational and stereochemical effects inherent in the immobilization procedure used. 

G. Wild, F. Larachi and A. Laurent, The hydrodynamics characteristics of cocurrent downflow and cocurrent upflow gas-liquid-solid catalytic fixed bed reactors the effect of pressure, Revue de l lnstitut Franfais du Petrole, 46 (1991) 467-490. 

Hydrodynamic characteristics of BSCR Effective properties of BSCR... 

Because of the external diffusional character of kinetics, the degree of ammonia conversion can be predicted theoretically as a function of geometric parameters of the catalyst bed and hydrodynamic characteristics of the gas stream. 

A.B. Russell, C.R. Thomas, M.D. Lilly, The influence of vessel height and top-section size on the hydrodynamic characteristics of airlift fermentors, Biotechnol. Bioeng. 43 (1994) 69-76. 

C. Bentifraouine, C. Xuereb, J.R Riba, An experimental study of the hydrodynamic characteristics of external-loop airlift contactors, J. Chem. Technol. Biotechnol. 69 (1997) 345-349. 

Benincasa et al. (2003) also studied the effect of ionic strength and electrolyte composition on hydrodynamic characteristics of HS. The author s reported that components of different HS fractions behave like organic acids, but that the retention level of fractions with larger components may not be accurately modulated by varying mobile phase properties as these species are either totally retained in acidic phases or released before the void peak at pH 4.2. Authors concluded that pronounced differences exist in the physicochemical properties of some HS components even when particle sizes were similar. 

Phytoplankton is at one of the initial levels of the trophic hierarchy of the ocean system. As field observations have shown, the World Ocean has a patchy structure formed by a combination of non-uniform spatial distributions of insolation, temperature, salinity, concentration of nutrient elements, hydrodynamic characteristics, etc. The vertical structure of phytoplankton distribution is less diverse and possesses rather universal properties. These properties are manifested by the existence of one to four vertical maxima of phytoplankton biomass. 

FIGURE 3.33 Geometry and hydrodynamic characteristics of the microfluidic capillary system (CS). (a) top view of a CS. (b) The flow of liquid (arrows) is superimposed on the cross section (not to scale) of the CS. CRV capillary retention valve CP, capillary pump [459]. Reprinted with permission from the American Chemical Society. 

Kjerfve, B.J., and Magill, K.E. (1989) Geographic and hydrodynamic characteristics of shallow coastal lagoons. Mar. Geol. 88, 187-199. 

---