Fast fluidized bed reactors

The reactivities of spray-dried sorbents were examined in a fast fluidized bed. The reactor was operated at a carbonation temperature of 50 °C, and a gas velocity of 2 m/s with an initial sorbent inventory of 7 kg to compare CO2 concentration profiles in effluent gas for spray-dried Sorb NH series and NX30 sorbent. Figure 5 shows the comparison of CO2 concentration profiles in effluent gas of Sorb NHR, NHR5, and NX30 in a fast fluidized-bed reactor. The CO2 removals of Sorb NHR and NHR5 were initially maintained at a level of 100 % for a short period of time and quickly dropped to a 10 to 20 % removal level. 

Fig. 3, Effect of carbonation temperature on CO2 removal in a fast fluidized-bed reactor.

When a chemical reaction occurs in the system, each of these types of behavior gives rise to a corresponding type of reactor. These range from a fixed-bed reactor (Chapter 21-not a moving-particle reactor), to a fluidized-bed reactor without significant carryover of solid particles, to a fast-fluidized-bed reactor with significant carryover of particles, and ultimately a pneumatic-transport or transport-riser reactor in which solid particles are completely entrained in the rising fluid. The reactors are usually operated commercially with continuous flow of both fluid and solid phases. Kunii and Levenspiel (1991, Chapter 2) illustrate many industrial applications of fluidized beds. 

Figure 23.2(b) shows a fast-fluidized-bed reactor, together with external equipment, such as cyclones, for separation of fluid and solid particles carried out of the reactor, and subsequent recirculation to the reactor. In a fast-fluidized bed, the fluidization velocity... 

Figure 23.2 Some features of (a) a fluidized-bed reactor (b) a fast-fluidized-bed reactor and (c) a pneumatic-transport reactor...

Fast fluidized bed reactors have been widely used in the petrochemical industry, coal combustion and calcining processes. In order to formulate and explore potential uses of FFB, features of present-day applications will be critically examined and then compared in a collocation chart for both commercial-scale processes and laboratory/pilot-scale studies on new processes. 

Probably one of the least stressed advantages of fast fluidized bed reactors is the ability to independently adjust the solids and gas retention times. This feature can help to increase the turndown ratio of the reactor or to manage those processes with varying feed quality or product requirements. 

Results obtained in the fast fluidized bed reactor show that both the conversion of butene and the yield of butadiene are increased by about 15 to 25% as compared to the turbulent fluidized bed reactor. It is reported that this process is being commercialized in Liaoning Province, in northeast China. 

The fast fluidized bed reactor can offer several considerable advantages over alternative reactors for many catalytic and non-catalytic reactions, especially for very fast exothermic/endothermic reactions. With the mushrooming of high activity catalysts and the ever increasing pressure for energy conservation, environmental controls, etc., FFB can play more and more important roles in these areas. More potential commercial applications of FFB in the near future include hydrocarbon oxidations, ammoxidation, gasoline and olefines production by concurrent downflow FFB and basic operation for organic chemical productions. 

Obviously, more research is needed on the turbulent and fast fluidized-bed reactors. At present, tests on a series of pilot plant reactors ending with a semicommercial unit are unavoidable. 

Figure 9.3 Classification of fluidized-bed reactors, (a) Bubbling-bed reactor (b) turbulent- (or fluid-) bed reactor (c) Fast fluidized-bed reactor (d) transport reactor. (From Joshi, J.B. and Doraiswamy, L.K. In Albright s Chemical Engineering Handbook [Ed. Lyle, F.), CRC Press, Boca Raton, FL, 2009.)...

Examples illustrating the model and additional discussions on the model are found in Levenspiel (1972) and Kunii and Levenspiel (1991). Applications of the model were reported in several recent studies, including scale-up studies of catalytic reactors by Botton (1983) and Dutta and Suciu (1989), a gasification study of coal carried out by Matusi et al. (1983), and a study of fast fluidized bed reactors by Kunii and Levenspiel (1998). 

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