Turbulent fluidization flow regime

Ammoxidation refers to the catalytic oxidation of a feedstock with ammonia. When propylene is the feedstock, acrylonitrile is produced. Most of the world s acrylonitrile is based on the Sohio (now BP) process in which stoichiometric amounts of propylene and ammonia are reacted with a slight excess of air in a fluidized bed operated in the turbulent fluidization flow regime. The reactor temperature and pressure are 450° C and 1.5 bar, respectively. The reaction usually... 

At gas velocities higher than those used for BFBs we successively enter the turbulent (TB), fast fluidized (FF), and the pneumatic conveying (PC) regimes. In these contacting regimes solids are entrained out of the bed and must be replaced. Thus in continuous operations we have the CFB, shown in Fig. 20.1. Flow models are very sketchy for these flow regimes. Let us see what is known. 

Figure 9.3. Various flow regimes or patterns in dense-phase fluidization (a) Particulate fluidization (b) Bubbling fluidization (c) Turbulent fluidization (d) Slugging (e) Spouting (f) Channeling.

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. 

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. 

It must be noted here that most industrial fluidized bed reactors operate in a turbulent flow regime. Trajectory simulations of individual particles in a turbulent field may become quite complicated and time consuming. Details of models used to account for the influence of turbulence on particle trajectories are discussed in Chapter 4. These complications and constraints on available computational resources may restrict the number of particles considered in DPM simulations. Eulerian-Eulerian approaches based on the kinetic theory of granular flows may be more suitable to model such cases. Application of this approach to simulations of fluidized beds is discussed below. 

Fig. 2 Various flow regimes or patterns in dense-phase fluidization (A) particulate fluidization (B) bubbling fluidization (C) turbulent fluidization (D) slugging (E) spouting (F) channeling (G) fast fluidization. (From Ref...

Most fluidized bed partial oxidation processes are operated in the turbulent flow regime of fluidization. However, DuPont operated a circulating fluidized bed catalytic reactor process for maleic anhydride production in Spain, featuring regeneration of the catalyst (by oxidation) on the downcomer side of the circulating system. 

Flow regime Bubbling, slugging or turbulent, distinct upper interface Fast fluidization Dilute transport... 

It should be mentioned that the combustion technology is not limited to these major designs. In catalytic fluidized bed combustion of low-sulfur natural gas, for example, powder catalysts are operated in the turbulent flow regime where the gas-solid contact is optimal so as to maintain a high combustion efficiency [46]. 

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