Fluidization bubbling

The dynamics of bubble formation and growth and of solids movement within the bubbling fluidized beds have been analyzed in great detail, and elaborate computer simulations have been developed for all regimes of flnidization. The reader is referred to the specialized literatnre for details on snch models. Here we describe a fairly simple model that is applicable to the bnbbling regime and that treats a catalytic fluidized bed much like a gas-liquid reactor. 

The bubbles play the role of the gas phase. The role of the liquid is played by an emulsion phase that consists of solid particles and snspending gas in a configuration similar to that at incipient flnidization. The qnasi-phases are in cocurrent flow with mass transfer between the phases and with a solid-catalyzed reaction occurring only in the emnlsion phase. The downward flow of solids that occurs near the walls is not explicitly considered in this simplified model. 

These equations are seen to be special cases of Equations 11.33 and 11.34. The exit concentration is averaged over both phases  

Values for the various parameters in these equations can be estimated from published correlations. See Suggestions for Further Reading. It turns out, however, that bubbling fluidized beds do not perform particularly well as chemical reactors. At or near incipient fluidization, the reactor approximates piston flow. The small catalyst particles give effectiveness factors near 1, and the pressure drop—equal to the weight of the catalyst—is moderate. However, the catalyst particles are essentially quiescent so that heat transfer to the vessel walls is poor. At higher flow rates, the bubbles promote mixing in the emulsion phase and enhance heat transfer, but at the cost of increased axial dispersion. 

The emulsion phase approaches the performance of a CSTR with its inherent lower yield for most reactions. To make matters worse, mass transfer between the emulsion and bubble phases becomes limiting to the point that some of the entering gas completely bypasses the catalytic emulsion phase. The system behaves like the reactor in Example 11.5. 

Considering more parameters, an improved criterion was suggested by Romero and Johanson (1962) as 

Gas flow through emulsion phase = mf Gas flow through bubble phase = 

Expressing the bed expansion in terms of the fraction of the bed occupied by bubbles, 

In practice, the elegant two-phase theory overestimates the volume of gas passing through the bed as bubbles (the visible bubble flow rate) and better estimates of bed expansion may be obtained by replacing (Q Q f) in Equation (7.28) with 

Strictly the equations should be written in terms of Umb rather than U f and Q b rather than Q f, so that they are valid for both Group A and Group B powders. Here they have been written in their original form. In practice, however, it makes little difference, since both Umb and Umf are usually much smaller than the superficial fluidizing velocity, U [so (U — Umt) = (U — Umb)]. In rare cases where the operating velocity is not much greater than Umb/ then Umb should be used in place of Umf in the equations. 

The above analysis requires a knowledge of the bubble rise velocity Ub, which depends on the bubble size dgv and bed diameter D. The bubble diameter at a given height above the distributor depends on the orifice density in the distributor N, the distance above the distributor L and the excess gas velocity (U-Umf). 

---

Fig. 8. (a) A bubbling fluidized bed (b) a circulating fluidized bed. Reproduced by permission of the American Institute of Chemical Engineers, 1990 (42). 

SASOL has pursued the development of alternative reactors to overcome specific operational difficulties encountered with the fixed-bed and entrained-bed reactors. After several years of attempts to overcome the high catalyst circulation rates and consequent abrasion in the Synthol reactors, a bubbling fluidized-bed reactor 1 m (3.3 ft) in diameter was constructed in 1983. Following successflil testing, SASOL designed and construc ted a full-scale commercial reac tor 5 m (16.4 ft) in diameter. The reactor was successfully commissioned in 1989 and remains in operation. 

In a freely bubbling fluidized bed, small bubbles often tend to coalesce into larger... 

Ding, J. and Gidaspaw, D., Bubbling fluidization model using kinetie theory of granular flows, AIChE J, 36, 523, 1990. 

The process essentially involves passing air through a bottom furnace distributor plate and a fixed bed of sand. As air flow rates increase, the fixed bed becomes more unstable and bubbles of air appear (minimum fluidized condition). Above this minimum level, higher air flow rates produce—depending on design—either bubbling fluidized beds or circulating fluidized beds, and the fuel is introduced onto these beds. 

Owing to the high computational load, it is tempting to assume rotational symmetry to reduce to 2D simulations. However, the symmetrical axis is a wall in the simulations that allows slip but no transport across it. The flow in bubble columns or bubbling fluidized beds is never steady, but instead oscillates everywhere, including across the center of the reactor. Consequently, a 2D rotational symmetry representation is never accurate for these reactors. A second problem with axis symmetry is that the bubbles formed in a bubbling fluidized bed are simulated as toroids and the mass balance for the bubble will be problematic when the bubble moves in a radial direction. It is also problematic to calculate the void fraction with these models. 

The fluidized reactor can be a bubbling fluidized type or a fast fluidized type, depending on the gas velocity and the reactivity of the sorbents. We adopted a fast fluidized bed type reactor for carbonation and regeneration reactions in order to identify the chemical characteristics of sorbents in a fast fluidized reactor of 0.025 m i.d. 

Fig. 3 shows the calculated and experimental results of particle fluidization behaviors in a RFB. A high-speed video camera (FASTCAM MAX, Photoron CO., Ltd.) was used for visualization of actual particle fluidization behavior. The bubbling fluidization behaviors, such as the bubble formation, eruption and particle circulation with rotational motion, could be well simulated, and these behaviors were also observed in the experimental results. 

Bio-oil upgrading over Ga modified zeolites in a bubbling fluidized bed reactor... 

Catalytic upgrading of bio-oil was carried out over Ga modified ZSM-5 for the pyrolysis of sawdust in a bubbling fluidized bed reactor. Effect of gas velocity (Uo/U ,f) on the yield of pyrolysis products was investigated. The maximum yield of oil products was found to be about 60% at the Uo/Umf of 4.0. The yield of gas was increased as catalyst added. HZSM-5 shows the larger gas yield than Ga/HZSM-5. When bio-oil was upgraded with HZSM-5 or Ga/HZSM-5, the amount of aromatics in product increased. Product yields over Ga/HZSM-5 shows higher amount of aromatic components such as benzene, toluene, xylene (BTX) than HZSM-5. 

Yerushalmi and Avidan (1985) suggest that the axial dispersion coefficient of solids in slugging and turbulent flow varies approximately linearly with the bed diameter, similar to Thiel and Potter (1978). The data are shown in Fig. 17 although May s results are probably in the bubbling fluidization regime rather than turbulent flow. 

Table 6. Scaling Parameter Values for Bubbling Fluidized Bed Experimental Studies... 

Figure 39. Model of 20 MW bubbling fluidized bed combustor showing tube arrangement. (From Jones and Glicksman, 1986.)...

Figure 40. Deviation from two-phase theory for model of bubbling fluidized bed...

Figure 48. Range of experimental scaling studies for circulating and bubbling fluidized beds.

Glicksman, L. R., and Farrell, P., Verification of Simplified Hydrodynamic Scaling Laws for Pressurized Fluidized Beds Part I Bubbling Fluidized... 

Werther, J., and Schoessler, M., Modeling Catalytic Reactions in Bubbling Fluidized Beds of Fine Particles, Heat and Mass Transfer, (W. P. M. Van Swaay, and H. H. Afgan, eds.), Springer, Berlin (1986)... 

The chapter begins by describing how temperature and pressure affect parameters important for bubbling fluidized beds, and then discusses their effect on regime transitions, circulating fluidized beds, and cyclones. 

One of the basic parameters to be determined when designing bubbling fluidized-bed systems is the minimum fluidization velocity, Ump The effect of temperature and pressure on IJhas been investigated by many researchers (Botterill and Desai, 1972 Botterill and Teoman, 1980 ... 

Entrainment from fluidized beds is also affected by temperature and pressure. Increasing system pressure increases the amount of solids carried over with the exit gas because the drag force on the particles increases at higher gas densities. May and Russell (1953) and Chan and Knowlton (1984) both found that pressure increased the entrainment rate from bubbling fluidized beds significantly. The data of Chan and Knowlton are shown in Fig. 13. 

Figure 1. Average heat transfer coefficients at surface of horizontal tube in bubbling fluidized bed. (From Chandran, Chen and Staub, 1980.)...

---