Fluidized beds particle residence time

Features - particles of growing polymer form in fluidized bed - catalyst residence time 2-4 hours - morphology and psd of catalyst are important - previously (pre-1990s) restricted in range of comonomer that could be used because of emergence of "condensed" mode operation, a wide range of comonomers may now be used... 

Yang, W. C., and Keaims, D. L., A Study of Fine Particles Residence Time in a Jetting Fluidized Bed, Powder Technol., 53 169 (1987)... 

Hanni et al. (1976) described a continuous fluidized bed drier using the centrifugal principle. It consisted of a perforated horizontal cylinder rotating inside a plenum with hot air blown across the outside of the cylinder and perpendicular to the axis of rotation (Figure 1.20). Vanes placed in the air inlet allowed the incident angle of the air flow to be varied from 0° to 45° to the perpendicular. Particles were fluidized inside the Teflon-coated stainless steel cylinder which could be tilted by up to 6° in order to control particle residence time. The cylinder, of... 

In these devices classification occurs in the space over a fluidized bed from which fine particles are carried away with the air stream. The air velocity must essentially exceed the maximal fluidization velocity of fine fraction. These classifiers operate on the gravitational counterflow separation principle with fluidized bed used to increase particles residence time in the separator. Coarse particles can move horizontally across the bed to the exit, which is another advantage of this device. 

Fluid-Bed Process Union Carbide and Shell technologies are combined in this process Two fluidized-bed reactors are used in series to produce impact-resistant polypropylene. As with other processes, reaction conditions are 50-100°C and 10-40 bar, with particle residence times between 1 and 4 hours. 

Sizing of the bed is based on simple holdup mass balance. Cross-sectional area of the fluidized bed can be determined from the following equation after the solids flow rate (dry basis), F , bed density, and bed height, are specified and particle residence time, tg, is determined ... 

In reality, particles spend vastly different amounts of time in the bed, depending very much on the locations of feed and withdrawal nozzles, on flow and mixing patterns in the bed, and on many other factors. The particles residence time is an even more important consideration in fluidized bed design. 

Particle residence time distribution in a fluidized bed is more close to that of a stirred tank reactor (CSTR) or ideal backmix reactor than that of a plug flow reactor (Yagi and Kunii, 1961). In a perfect plug flow reactor, all particles have the same residence time, which is equivalent to the mean residence time of particles and can be calculated by... 

R(t) dt is the fraction of solids staying in the bed for the time period between t and t + dt. Yagi and Kunii (1961) found experimentally that Eq. (187) represented the particle residence time in a fluidized bed quite well. The fraction of solids spending less than time t can then be calculated from... 

Harris AT, Thorpe RB, Davidson JF. The measurement of particle residence time distributions in circulating fluidized beds. In Grace JR, Zhn J, de Lasa HI, eds. Circulating Fluidized Bed Technology VII. Ottawa CSChE, 2002, pp. 145-152. 

The well-mixed continuous fluidized bed dryer (Figure 8.8) is one of the most common fluidized bed dryers used in the industry. As the bed of particles is perfectly mixed, the bed temperature is uniform and is equal to the product and exhaust gas temperatures. However, particle residence time distribution is necessarily wide, thus resulting in wide range of product moisture content. On the other hand, as the feed material is continuously charged into the fluidized bed of relatively dry particles, this gives the added advantage of enhanced fluidizability and better fluidization quality. In some cases, a series of well-mixed continuous dryers may be used with variable operating parameters. In addition, a well-mixed continuous... 

Wang, X., Rhodes, M., 2003. Determination of particles residence time at the wall of gas fluidized beds by discrete element method simulation. Chem. Eng. Sci. 58 387-395. 

The hydrolysis of PET in a fluidized bed at 450°C with steam as fluidizing gas and quartz sand as bed material shows similar results [39]. In fluidized bed reactors, a material of fine particles is fluidized by a gas stream from the bottom of the reactor. The behavior of a fluidized bed is comparable with a liquid and shows excellent properties for heat and mass transport. Products are removed immediately from the hot reactor zone by the fluidizing gas. Typical residence times ranges from seconds to Imin. 

The results of Massimilla et al., 0stergaard, and Adlington and Thompson are in substantial agreement on the fact that gas-liquid fluidized beds are characterized by higher rates of bubble coalescence and, as a consequence, lower gas-liquid interfacial areas than those observed in equivalent gas-liquid systems with no solid particles present. This supports the observations of gas absorption rate by Massimilla et al. It may be assumed that the absorption rate depends upon the interfacial area, the gas residence-time, and a mass-transfer coefficient. The last of these factors is probably higher in a gas-liquid fluidized bed because the bubble Reynolds number is higher, but the interfacial area is lower and the gas residence-time is also lower, as will be further discussed in Section V,E,3. 

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