Liquid Fluidized Bed

The calcium bisulfite acid used in the manufacture of sulfite cellulose is the product of reaction between gaseous sulfur dioxide, liquid water, and limestone. The reaction is normally carried out in trickle-bed reactors by the so-called Jenssen tower operation (E3). The use of gas-liquid fluidized beds has been suggested for this purpose (V7). The process is an example of a noncatalytic process involving three phases. 

The absorption rate increased with increasing nominal liquid velocity for all particle sizes and decreased with increasing particle size for all liquid velocities. The absorption rates were lower than those measured in an equivalent gas-liquid system with no solid particles present. The difference is explained as being due to a higher rate of bubble coalescence and, consequently, a lower gas-liquid interfacial area in the gas-liquid fluidized bed. 

The bubble size at formation varied with particle characteristics. It was further observed that the bubble size decreased with increasing fluidization intensity (i.e., with increasing liquid velocity). The rate of coalescence likewise decreased with increasing fluidization intensity the net rate of coalescence had a positive value at distances from 1 to 2 ft above the orifice, whereas at larger distances from the orifice the rate approached zero. The bubble rise-velocity increased steadily with bubble size in a manner similar to that observed for viscous fluids, but different to that observed for water. An attempt was made to explain the dependence of the rate of coalescence on fluidization intensity in terms of a relatively high viscosity of the liquid fluidized bed. 

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. 

These results are, however, only valid for the particle sizes referred to. Lee (L3) has reported measurements of average bubble diameter and gas-liquid interfacial area for gas-liquid fluidized beds of glass beads of 6-mm... 

No work on mass transfer across the liquid-solid interface in gas-liquid fluidized beds has come to the author s attention. 

Viswanathan et al. (V6) measured gas holdup in fluidized beds of quartz particles of 0.649- and 0.928-mm mean diameter and glass beads of 4-mm diameter. The fluid media were air and water. Holdup measurements were also carried out for air-water systems free of solids in order to evaluate the influence of the solid particles. It was found that the gas holdup of a bed of 4-mm particles was higher than that of a solids-free system, whereas the gas holdup in a bed of 0.649- or 0.928-mm particles was lower than that of a solids-free system. An attempt was made to correlate the gas holdup data for gas-liquid fluidized beds using a mathematical model for two-phase gas-liquid systems proposed by Bankoff (B4). 

Based on the same assumption, the results by Viswanathan et al. on gas holdup in beds of larger particles are in agreement with the results of Lee (L3) on bubble breakup in beds of larger particles (05). No work on the residencetime distribution of the gas phase in gas-liquid fluidized beds has come to the author s attention. 

Measurement of the expansion of a gas-liquid fluidized bed provides a measure of the holdup of solids or of the corresponding combined holdup of gas and liquid. From such measurements, the holdup of liquid may be calculated if the gas holdup has been determined independently. 

Fig. 5. Sum of gas and liquid holdup in gas-liquid fluidized bed. Experimental data of Turner (T4) and theoretical curves of 0stergaard (03).

Ostergaard (02) measured the wall-to-bed heat-transfer coefficient in a bed of 3-in. diameter. The media were air, water, and glass ballotini of0.5-mm diameter. It was observed that the heat-transfer coefficient for a liquid fluidized bed near the point of incipient fluidization could be approximately... 

Measurements of heat-transfer coefficients and effective thermal conductivity for gas-liquid fluidized beds have also been carried out by Manchanda (M2). 

In 1962 Jottrand and Grunchard (J7) reported on mass transfer to a small rectangular nickel plate immersed in a liquid fluidized bed of sand particles. Mass-transfer rates were five to ten times higher than those measured in an open pipe flow a maximum rate was measured at a bed porosity of 0.58. Le Goff et al. (Lie) later showed that this maximum is directly related to a maximum in the average kinetic energy of the fluidized particles per unit bed volume. 

Trinet, F., Fleim, R., Amar, D., Chang, H. T., and Rittmann, B. E., Study of Biofilm and Fluidization of Bioparticles in a Three-Phase Liquid-Fluidized-Bed Reactor, Wat. Set. Tech., 23 1347 (1991)... 

Asif et al. (1991) studied distributor effects in liquid-fluidized beds of low-density particles by measuring RTDs of the system by pulse injection of methylene blue. If PF leads into and follows the fluidized bed with a total time delay of 10 s, use the following data to calculate the mean-residence time and variance of a fluid element, and find N for the US model. 

Figure 8.3. Gas-Liquid Fluidized Bed C"Ebullating" Reactor for Hydroliquefaction of Coal CKampiner, in Winnacker-Kuehler, Chemische Technolagie 52, 19723. ...

Lawson, A. and Hassett, N. 1. Proc. Inti. Symp. on Fluidization, Netherlands Univ. Press, Eindhoven, (1967) 113. Discontinuities and flow patterns in liquid-fluidized beds. 

Reuter, H. Chem. Eng. Prog. Symp. Series No. 62 (1966) 92. On the nature of bubbles in gas and liquid fluidized beds. 

Romani, M. N. and Richardson, J. F. Letters in Heat and Mass Transfer 1 (1974) 55. Heat transfer from immersed surfaces to liquid-fluidized beds. 

Three-phase (solid/liquid/gas) fluidized systems are also of some practical importance. There is again a strong analogy between the rise of gas bubbles in normal liquids and in liquid fluidized beds (Dl, R3), although there is evidence of solid/liquid segregation in wakes (R3, S6) which has no parallel for two-phase systems. 

Neglecting in the case of liquid fluidized beds the fraction of liquid which flows as bubbles, the preceding approach can be extended to liquid fluidized beds. In this case,... 

The main mass transport resistance in liquid fluidized beds of relatively small particles lies in the liquid film. Thus, for ion exchange and adsorption on small particles, the mass transfer limitation provides a simple liquid-film diffusion-controlled mass transfer process (Hausmann el al., 2000 Menoud et al., 1998). The same holds for catalysis. 

For 5 < Rep < 100, the following correlation, obtained by Rahman and Streat for mass transfer, is valid for conventional liquid fluidized beds (Rahman and Streat, 1981 Hausmann et al., 2000) ... 

In ebullated (liquid fluidized) beds the particles are much larger (0.2-1 mm) than in gas fluidization (0-0.1 mm). Little... 

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