Dispersants flow/processability

J. Gotz, K. Zick, C. Heinen, T. Konig 2002, (Visualisation of flow processes in packed beds with NMR imaging Determination of the local porosity, velocity vector and local dispersion coefficients), Chem. Eng. Process. 41 (7), 611-630. 

Three main flow patterns exist at various points within the tube bubble, annular, and dispersed flow. In Section I, the importance of knowing the flow pattern and the difficulties involved in predicting the proper flow pattern for a given system were described for isothermal processes. Nonisother-mal systems may have the added complication that the same flow pattern does not exist over the entire tube length. The point of transition from one flow pattern to another must be known if the pressure drop, the holdups, and the interfacial area are to be predicted. In nonisothermal systems, the heat-transfer mechanism is dependent on the flow pattern. Further research on predicting flow patterns in isothermal systems needs to be undertaken... 

Sorption/desorption is the key property for estimating the mobility of organic pollutants in solid phases. There is a real need to predict such mobility at different aqueous-solid phase interfaces. Solid phase sorption influences the extent of pollutant volatilization from the solid phase surface, its lateral or vertical transport, and biotic or abiotic processes (e.g., biodegradation, bioavailability, hydrolysis, and photolysis). For instance, transport through a soil phase includes several processes such as bulk flow, dispersive flow, diffusion through macropores, and molecular diffusion. The transport rate of an organic pollutant depends mainly on the partitioning between the vapor, liquid, and solid phase of an aqueous-solid phase system. 

Acknowledgements Many people contributed to sampling and initial processing of a vast amount of dispersion flow data Lazareva Z.A., Meretskov S.G., Shavurin S.A., Kotov... 

An outstanding feature of two-phase cocurrent flow is the variety of possible flow patterns, ranging all the way from a small quantity of gas dispersed as bubbles in a continuous liquid medium, to the opposite extreme of a small amount of liquid dispersed as droplets in a continuous gas stream. The importance of these flow patterns can be shown when one plots the rate of a transport process as a function of a flow rate of one phase while the flow rate of the second phase is maintained constant. The flux will be found not merely to increase or decrease in a smooth fashion, but rather to show in different flow ranges minima or maxima demonstrating the presence of fundamentally different transfer processes. A comprehensive understanding of the flow processes is necessary before the nature of the flow pattern can be predicted for any given set of flow conditions. 

In porous media the flow of water and the transport of solutes is complex and three-dimensional on all scales (Fig. 25.1). A one-dimensional description needs an empirical correction that takes account of the three-dimensional structure of the flow. Due to the different length and irregular shape of the individual pore channels, the flow time between two (macroscopically separated) locations varies from one channel to another. As discussed for rivers (Section 24.2), this causes dispersion, the so-called interpore dispersion. In addition, the nonuniform velocity distribution within individual channels is responsible for intrapore dispersion. Finally, molecular diffusion along the direction of the main flow also contributes to the longitudinal dispersion/ diffusion process. For simplicity, transversal diffusion (as discussed for rivers) is not considered here. The discussion is limited to the one-dimensional linear case for which simple calculations without sophisticated computer programs are possible. 

If one could use heterogeneous catalysts such as dispersed metals to promote this type of reaction, product separation would be facilitated and the more efficient flow processes could be used instead of the commonly employed batch mode. There are, however, a number of problems which must be overcome before such systems can be used practically. In the first place it must be shown that dispersed metals can promote these reactions and, secondly, a more detailed knowledge must be acquired of substrate adsorption on the catalyst and the interaction of the adsorbed species to give the product. 

A flow-pattern map comprises dispersed flow, annular flow, slug-dispersed flow and slug-annular flow [278]. The highest specific interface measured amounts to 16 000 m2/m3. A porous surface structure (100 cm2) in the reaction channel can be generated by a sulfurhexafluoride plasma etch process with silicon nitride masking [278],... 

Flow models show potential or velocity fields resulting from the groundwater flow, unsaturated flow, or in the soil. These potential fields adequately describe the flow process together with further boundary conditions, such as pore volume, dispersivity, etc., in order to calculate the transport behavior (Table 16). 

It appears, at present, that although crossflow and other macromixing models described earlier give a more correct description of the trickle flow process, the dispersion model predicts the conversion data satisfactorily, at least for simple reactions. 

The general problem of building a model for an actual process begins with a flow description where we qualitatively appreciate the number of flow regions, the zones of interconnection and the different volumes which compose the total volume of the device. We frequently obtain a relatively simple CFM, consequently, before beginning any computing, it is recommended to look for an equivalent model in Table 3.4. If the result of the identification is not satisfactory then we can try to assimilate the case with one of the examples shown in Figs. 3.26-3.28. If any of these previous steps is not satisfactory, we have three other possibilities (i) we can compute the transfer function of the created flow model as explained above (ii) if a new case of combination is not identified, then we seek where the slip flow can be coupled with the CFM example, (iii) we can compare the created model with the different dispersion flow models. 

For multiphase flow processes, turbulent effects will be much larger. Even operability will be controlled by the generated turbulence in some cases. For dispersed fluid-fluid flows (as in gas-liquid or liquid-liquid reactors), the local sizes of dispersed phase particles and local transport rates will be controlled by the turbulence energy dissipation rates and turbulence kinetic energy. The modeling of turbulent multiphase flows is discussed in the next chapter. 

Using the VOF approach, flow processes around individual dispersed phase particles are resolved unlike with EL or EE approaches. In this approach, the participating fluids share a single set of conservation equations. The governing equations can, therefore, be written ... 

The other situation which may require special treatment is a boundary of multiphase dispersion through which dispersed phase particles are allowed to escape, but not the continuous phase (for example, the top surface of gas-liquid dispersion in a bubble column reactor). The standard outlet boundary conditions need to be suitably modified to represent the observed flow processes. It is possible to simulate the actual behavior by specifying appropriate sink near the top surface (see Ranade, 1998 and Chapter 11). 

The recent progress in experimental techniques and applications of DNS and LES for turbulent multiphase flows may lead to new insights necessary to develop better computational models to simulate dispersed multiphase flows with wide particle size distribution in turbulent regimes. Until then, the simulations of such complex turbulent multiphase flow processes have to be accompanied by careful validation (to assess errors due to modeling) and error estimation (due to numerical issues) exercise. Applications of these models to simulate multiphase stirred reactors, bubble column reactors and fluidized bed reactors, are discussed in Part IV of this book. 

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