Fluidization flow regimes

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... 

The net upward flux of particles averaged over the column cross section is typically 20-200 kg/m s in this fast fluidization flow regime. 

The factors considered for choosing a fluidization/flow regime include... 

Studies that report local particle flux data are summarized in Table 4. For the fast fluidization flow regime, radial profiles of local particle flux are generally similar in shape to the radial profiles of particle velocity described above upward near the center line, dropping off with increasing r, and negative near the wall. One such experimental profile, shown in Figure 11, is seen to be symmetrical around the axis of the column. Note that the flux is downward in this case only in a thin region adjacent to the outer wall of the column. 

For the fast fluidization flow regime, particles travel downward near the wall, engendering axial dispersion of both soUds and gas. 

As fluidized beds are scaled up from bench scale to commercial plant size the hydrodynamic behavior of the bed changes, resulting, in many cases, in a loss of performance. Although there have been some studies of the influence of bed diameter on overall performance as well as detailed behavior such as solids mixing and bubble characteristics, generalized rules to guide scale-up are not available. The influence of bed diameter on performance will differ for different flow regimes of fluidization. 

Gas jets in fluidized beds were reviewed by Massimilla (1985). A more recent review is by Roach (1993) who also developed models to differentiate three jet flow regimes jetting, bubbling and the transition. However, most of the data were from jets smaller than 25 mm. The discussion here will emphasize primarily large jets, up to 0.4 m in diameter, and operation at high temperatures and high pressures. The gas jets can also carry solids and are referred to as gas-solid two-phase jets in this discussion. 

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. 

For the fluidized bed process the bed expansion as a consequence of an increase in linear flow rate has to be considered. In a simplified picture diffusive transport takes place in a boundary layer around the matrix particle which is frequently renewed, this frequency being dependent on velocity and voidage, as long as convective effects, e.g. the movement of particles are neglected. Rowe [74] has included these considerations into his correlation for kf in fluidized beds, which is applicable for a wide range of Reynolds numbers, including the laminar flow regime where fluidized bed adsorption of proteins takes place (Eq. 19). The exponent m is set to 1 for a liquid fluidized bed, a represents the proportionality factor in the correlation for packed beds (Eq. 18) and is assumed as 1.45. 

Figure 20 Predicted flow regime diagram of the industrial MIP reactor, with solids flux as a function of the imposed total pressure drop at fixed gas flow rate. The snapshots of voidage profile refer to the transition, from left to right, the dilute transport, choking transition in between with different solids inventory, to the dense fluidization (Lu et al., 2007).

The transport behavior of fluidized bed systems can be described in light of the properties of the fluidized particles and the flow regimes. In the following, particle and regime classifications along with general components in a fluidized bed are given. 

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.

Cai, P. (1989). Flow Regime Transition in Dense-Phase Fluidized Beds. Ph.D. Dissertation. 

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