Multiphase Flow Regimes

Flow Regimes in Multiphase Reactors. Reactant contacting, product separations, rates of mass and heat transport, and ultimately reaction conversion and product yields are strong functions of the gas and Hquid flow patterns within the reactors. The nomenclature of commonly observed flow patterns or flow regimes reflects observed flow characteristics, ie, armular, bubbly, plug, slug, spray, stratified, and wavy. 

FIG. 6-55 Critical cavitation number vs. diameter ratio (3. Reprinted from Thotpe, Flow regime transitions due to cavitation in the flow through an orifice, " Int. J. Multiphase Flow, i6,1023-1045. Copyright 1990, with kindper-mission from Elsevier Science, Ltd., The Boulevard, Langford Lane, Kldlington 0X5 1GB, United Kingdom. )... 

Akbar MK, Plummer DA, Ghiaasiaan SM (2003) On gas-liquid two-phase flow regimes in microchannels. Int J Multiphase Flow 29 855-865... 

Recently, hydrodynamic aspects of multiphase applications have been studied in detail for the cocurrent and countercurrent flow regimes. Useful correlations were determined and it was found that foams combine high rates and low pressure drop, proving their high potential in multiphase applications [9, 107, 108]. 

Crowley, C. J., G. B. Wallis, and J. J. Barry, 1993, Dimensionless Form of a One-Dimensional Wave Model for the Stratified Flow Regime Transition, lnt. J. Multiphase Flow 7P(2) 369 376. (3) Cumo, M., 1972, Personal communication, Comitato Nazionale Per L energia Nucleare, Milan, Italy. (5)... 

Yan Y, Thorpe RB (1990) Flow regime transitions due to cavitation in flow through an orifice. Int J Multiphase flow 16 1023-1045... 

This review paper is restricted to stirred vessels operated in the turbulent-flow regime and exploited for various physical operations and chemical processes. The developments in the field of computational simulations of stirred vessels, however, are not separated from similar developments in the fields of, e.g., turbulent combustion, flames, jets and sprays, tubular reactors, and multiphase reactors and separators. Fortunately, there is a strong degree of synergy and mutual cross-fertilization between these various fields. This review paper focuses on aspects specific to stirred vessels (such as the revolving impeller, the resulting strong spatial variations in turbulence properties, and the macroinstabilities) and on the processes carried out in them. 

The most reliable methods for fully developed gas/liquid flows use mechanistic models to predict flow pattern, and use different pressure drop and void fraction estimation procedures for each flow pattern. Such methods are too lengthy to include here, and are well suited to incorporation into computer programs commercial codes for gas/liquid pipeline flows are available. Some key references for mechanistic methods for flow pattern transitions and flow regime-specific pressure drop and void fraction methods include Taitel and Dukler (AIChEJ., 22,47-55 [1976]), Barnea, et al. (Int. J. Multiphase Flow, 6, 217-225 [1980]), Barnea (Int. J. Multiphase Flow, 12, 733-744 [1986]), Taitel, Barnea, and Dukler (AIChE J., 26, 345-354 [1980]), Wallis (One-dimensional Two-phase Flow, McGraw-Hill, New York, 1969), and Dukler and Hubbard (Ind. Eng. Chem. Fun-dam., 14, 337-347 [1975]). For preliminary or approximate calculations, flow pattern maps and flow regime-independent empirical correlations, are simpler and faster to use. Such methods for horizontal and vertical flows are provided in the following. 

The flow regimes in capillaries have been investigated extensively, and the reported results provide a good basis for imderstanding the hydrodynamics of multiphase operation of monoliths. Figure 11 shows the well-known two-phase flow patterns observed in tubes (16,17). 

In the previous section, stability criteria were obtained for gas-hquid bubble columns, gas-solid fluidized beds, liquid-sohd fluidized beds, and three-phase fluidized beds. Before we begin the review of previous work, let us summarize the parameters that are important for the fluid mechanical description of multiphase systems. The first and foremost is the dispersion coefficient. During the derivation of equations of continuity and motion for multiphase turbulent dispersions, correlation terms such as esv appeared [Eqs. (3) and (10)]. These terms were modeled according to the Boussinesq hypothesis [Eq. (4)], and thus the dispersion coefficients for the sohd phase and hquid phase appear in the final forms of equation of continuity and motion [Eqs. (5), (6), (14), and (15)]. However, for the creeping flow regime, the dispersion term is obviously not important. 

Many types of multiphase flow exist (i.e., gas-liquid, gas-solid, liquid-liquid, gas-liquid-solid) where within one type of flow several possible flow regimes exist. In Fig. 10 (Ishii, 1975) a classification is given for two-phase flow. 

Especially for multiphase systems flow visualization (Wen-Jei Yang, 1989 Merzkirch, 1987) can provide valuable initial information on the prevailing flow patterns and should at least always be considered as a first step. Of course, in applications that involve extreme conditions such as high temperature and/or pressure it is very difficult if not impossible to apply flow visualization and other techniques should be considered. Here the use of cold flow models which permit visual observation might be considered as an alternative as an important first step to obtain (qualitative) information on the flow regime and associated flow pattern. Of course, multiphase flows exist such as dense gas-solid flows that do not permit visual observation and in such cases the application of idealized flow geometries should be considered. A well-known example in this respect is the application of so-called 2D gas fluidized beds to study gas bubble behavior (Rowe, 1971). 

As is evident from inspection of Table III turbulence modeling of multiphase flow systems requires major attention in the near future. Also the development of closure laws for phenomena taking place in the vicinity of interfaces such as coalescence, breakup, and accumulation of impurities should be considered in more detail. Once these requirements have been met, in principle, it would be possible to predict a.o. flow regime transition and the spatial distribution of the phases with confidence, which is of utmost importance to the chemical engineer dealing with the design of (novel) multiphase reactors. 

Apart from the flow regimes, several other issues control the performance of these multiphase reactors. For example, in a gas-liquid reactor, the rate of mass... 

FIGURE 1.9 Some flow regimes of multiphase systems (from Krishna, 1994). 

The subject of the modeling of multiphase flow processes is quite vast and covers a wide range of sub-topics. It is virtually impossible to treat all the relevant issues in a single book, let alone in a single chapter. Here we attempt to provide a brief review of modeling approaches and cite the key references for further details. An attempt is made to provide sufficient information to develop a baseline model. The first section discusses various flow regimes and their key features. Various approaches to... 

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