Types of multiphase flows

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. 

Our treatment of Chemical Reaction Engineering begins in Chapters 1 and 2 and continues in Chapters 11-24. After an introduction (Chapter 11) surveying the field, the next five Chapters (12-16) are devoted to performance and design characteristics of four ideal reactor models (batch, CSTR, plug-flow, and laminar-flow), and to the characteristics of various types of ideal flow involved in continuous-flow reactors. Chapter 17 deals with comparisons and combinations of ideal reactors. Chapter 18 deals with ideal reactors for complex (multireaction) systems. Chapters 19 and 20 treat nonideal flow and reactor considerations taking this into account. Chapters 21-24 provide an introduction to reactors for multiphase systems, including fixed-bed catalytic reactors, fluidized-bed reactors, and reactors for gas-solid and gas-liquid reactions. 

Discussions on flow modeling so far have been more or less restricted to singlephase reactors. However, in a broad range of application areas, multiple phases are involved in chemical reactions (see examples cited by Ramachandran and Choudhari, 1983 Doraiswamy and Sharma, 1984 Kunii and Levenspiel, 1991 Shah, 1991 Dudukovic et al, 1999). Reactors carrying out such reactions are generically termed multiphase reactors. There are several types of multiphase reactors and several methods are available to classify these reactors. One of the simplest methods of... 

The list is merely suggestive. Complexity of reactive flows may greatly expand the list of issues on which further research is required. Another area which deserves mention here is modeling of inherently unsteady flows. Most flows in engineering equipment are unsteady (gas-liquid flow in a bubble column reactor, gas-solid flow in a riser reactor and so on). However, for most engineering purposes, all the details of these unsteady flows are not required to be known. Further work is necessary to evolve adequate representation of such flows within the CFD framework without resorting to full, unsteady simulations. This development is especially necessary to simulate inherently unsteady flows in large industrial reactors where full, unsteady simulations may require unaffordable resources (and therefore, may not be cost effective). Different reactor types and different classes of multiphase flows will have different research requirements based on current and future applications under consideration. 

Other types of multiphase fiows are free surface fiows, where there is a well-defined interface between two continuous phases. Examples of such flows can be found in liquid separators, unbaffled mixing vessels where surface deformation occurs when a central vortex forms, mold filling applications, and blow... 

In this section the application of multiphase flow theory to model the performance of fluidized bed reactors is outlined. A number of models for fluidized bed reactor flows have been established based on solving the average fundamental continuity, momentum and turbulent kinetic energy equations. The conventional granular flow theory for dense beds has been reviewed in chap 4. However, the majority of the papers published on this topic still focus on pure gas-particle flows, intending to develop closures that are able to predict the important flow phenomena observed analyzing experimental data. Very few attempts have been made to predict the performance of chemical reactive processes using this type of model. 

Incompressibility of the fluid has generally been assumed throughout the book, albeit this is not always stated explicitly. This is a satisfactory approximation for most non-Newtonian substances, notable exceptions being the cases of foams and froths. Likewise, the assumption of isotropy is also reasonable in most cases except perhaps for liquid crystals and for fibre filled polymer matrices. Finally, although the slip effects are known to be important in some multiphase systems (suspensions, emulsions, etc.) and in narrow channels, the usual no-slip boundary condition is regarded as a good approximation in the type of engineering flow situations dealt with in this book. 

The analysis of experimental data revealed a correlation between the hydrodynamic mode of a tubular turbulent device and the interphase tension in the flow of the two-phase liquid-gas reaction system (Figure 2.52). This correlation confirms that the addition of surfactants is a reasonable solution for a reaction system with an interphase boundary. It leads to a decrease of bubble size and mass exchange intensification in the gas-liquid flow of fast chemical processes. In addition, the liquid-phase longitudinal mixing rate increases and the hydrodynamic mode of a process approaches perfect mixing conditions. Fast chemical processes, in two-phase systems, require consideration of the selective adsorption of feedstock reactants and reaction products on to the interphase boundary, and a change of the hydrodynamic motion structure of the continuous phase. A change in the work required to form the new surface is a typical phenomenon for all types of multiphase systems and depends on... 

A statistical description of multiphase flow might be developed based on an analogy to the Boltzmann theory of gases [11, 39, 60,63, 66, 91, 125,135]. The fundamental variable is the particle distribution function with an appropriate choice of internal coordinates relevant for the particular problem in question. Most of the multiphase flow modeling work performed so far has focused on isothermal, non-reactive mono-disperse mixtures. However, in chemical reactor engineering the industrial interest lies in multiphase reactive flow systems that include poly-dispersed mixtures of multiple particle types, with their associated effects of mixing, segregation and heat and mass transfer. 

In addition to the reduction in performance, flow maldistribution may result in increased corrosion, erosion, wear, fouling, fatigue, and material failure, particularly for Hquid flows. This problem is even more pronounced for multiphase or phase change flows as compared to single-phase flows. Flow distribution problems exist for almost all types of exchangers and can have a significant impact on energy, environment, material, and cost in most industries. 

A useful classification of lands of reaclors is in terms of their concentration distributions. The concentration profiles of certain limiting cases are illustrated in Fig. 7-3 namely, of batch reactors, continuously stirred tanks, and tubular flow reactors. Basic types of flow reactors are illustrated in Fig. 7-4. Many others, employing granular catalysts and for multiphase reactions, are illustratea throughout Sec. 23. The present material deals with the sizes, performances and heat effects of these ideal types. They afford standards of comparison. 

Knowledge of these types of reaetors is important beeause some industrial reaetors approaeh the idealized types or may be simulated by a number of ideal reaetors. In this ehapter, we will review the above reaetors and their applieations in the ehemieal proeess industries. Additionally, multiphase reaetors sueh as the fixed and fluidized beds are reviewed. In Chapter 5, the numerieal method of analysis will be used to model the eoneentration-time profiles of various reaetions in a bateh reaetor, and provide sizing of the bateh, semi-bateh, eontinuous flow stirred tank, and plug flow reaetors for both isothermal and adiabatie eonditions. 

Balanced bellows type valves are normally used where the relief valves are piped to a closed flare system and the back-pressure exceeds 10% of the set pressure, where conventional valves can t be used because back-pressure is too high. They are also used in flow lines, multiphase lines, or for ptu affinic or asphaltic crude, where pilot-operated valves can t be used due to possible plugging of the pilot line. An advantage of this type of relief valve is, for corrosive or dirty service, the bellows protects the spring from process fluid. A disadvantage is that the bellows can fatigue, which will allow process fluid to escape through the bonnet. For HjS service, the bonnet vent must be piped to a safe area. 

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