Ind. Eng. Chem. Res. 2005, 44, 7899-7906
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Effects of Equipment and Process Variables on the Suspension of Buoyant Particles in Gas-Sparged Vessels Yuyun Bao, Zhengming Gao,* Zhigang Hao, Jiangang Long, Litian Shi, John M. Smith,† and Norman F. Kirkby† School of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China, and Fluids and Systems Research Centre, School of Engineering (D2), University of Surrey, Guildford GU2 7XH, U.K.
Previous work (Bao et al., 2005) reported the influence of solid concentration and agitator selection on the just-drawdown agitation speed and gas holdup in three-phase reactors containing nonwetting buoyant solids. This paper reports experimental results for the unaerated and aerated aqueous suspension of buoyant (polypropylene and polyethylene) particles in a tall vessel of 0.476 m diameter. The composite agitators used a concave blade dispersing impeller surmounted by one or two up-pumping wide-blade hydrofoils. To find the influence of the density and size of particles on the drawdown of particles, all solids were cleaned up to four times with pure alcohol until they were hydrophilic. The influence on suspension and gas retention of solid density (900955 kg‚m-3) and size (0.5-4 mm), baffle width, and agitator centering have been studied. There is little difference in the gas retention behavior in the presence of similarly sized different particles. Gas holdup is relatively unaffected by particle size, though particles 1 or 2 s. This is analogous to the Zwietering12 criterion for the just-suspended condition for settling particles. In the present three-phase system, the wetted polypropylene powders tended to agglomerate, even after cleaning. The just-drawdown state was considered to be reached when any floating clots of powder had broken up with all undispersed groups remaining at the liquid surface for 0.01 m‚s-1), a relatively low shaft power consumption is needed to pull down the particles for all three baffle arrangements, because then the gas flow dominates the impeller agitation. However, at the same time, more power is needed to disperse gas well, that is to say, the complete dispersion agitation speed is much higher than the just-drawdown agitation speed NJDG at high superficial gas velocities. To study the influence of CBW, the baffle-to-wall clearance, on NJDG and PJDG, the width of the baffles was kept at 0.020 m and the clearance was increased from 0.005 to 0.015 and 0.03 m [the corresponding penetrations (CBW + WB) are 0.05T, 0.07T, and 0.1T). Figure 8 shows the results. It can be seen that the wider the equivalent baffles are, the lower NJDG and PmJDG. Moreover, the values of NJDG and PmJDG for full (“stan-
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Figure 8. (a) Gassed drawdown speeds for various baffle penetration, 6% solids. (b) Power draw at gassed drawdown for various baffle penetration, 6% solids.
dard”) baffles, with WB ) 0.045 m and CBW ) 0.005 m, are the smallest. 2.2. Gas Holdup. Parts a and b of Figure 9 show the influence of partial baffles on gas holdup for two-phase and three-phase systems, respectively. Both these figures show that, for a similar power input and superficial gas rate, the gas holdup is the smallest with narrow baffles. The gas holdup for middle-width baffles is almost as high as with full baffles when the power input is relatively high. However, the drawdown and dispersion performance of solids are poor for these mid-sized baffles. Full baffles are recommended for both gas dispersion and the drawdown of aerated buoyant solids. The influence of baffle-to-wall clearance on gas holdup is shown in Figure 10. When VS is 0.0078 m‚s-1, gas holdup with full baffles is greatest. Gas holdups for the different baffle arrangements are similar as more gas is injected into the system, especially at relatively high power consumption. The present experiments suggest that full baffles are highly satisfactory for both gas dispersion and solids suspension. In a gassed solid-liquid system, the drawdown of buoyant particles is controlled mainly by the turbulence intensity, which is enhanced by the use of full baffles. 3. Off-Centerd Agitators. 3.1. Critical Impeller Speed NJDG and Power Demand PJDG to Draw Down Floating Particles. Hemrajani et al.3 found that off-centered agitators can reduce the power consumption needed to achieve drawdown of buoyant solids in unaerated conditions. The effect on NJDG and PJDG of placing an aerated multi-impeller agitator off-center is shown in Figure 11 and Table 4. It is clear that power can be saved in both aerated and unaerated operation when the agitator shaft is off-centered. Any use of this
Figure 9. (a) Gas holdup and specific power: baffle width and gas rate effects, no suspended solids. (b) Gas holdup and specific power: baffle width and gas rate effects, 6 vol % solids.
approach should, however, be in full awareness of the increased danger of mechanical failure of the agitator shaft resulting from fluctuating nonaxial loading. 3.2. Gas Holdup with an Eccentric Agitator Shaft. Figure 12 shows that gas holdup is nearly same at higher gas rates or power inputs. Significant reductions in gas holdup only result from mounting the multiimpeller off-center at low gas superficial velocity and at power input associated with low agitator speeds. Conclusions The critical just-drawdown agitator speed for different buoyant particles, gas holdup, and shaft power have been measured in a vessel of 0.476 m diameter with four baffles and dished base. The volume is 0.145 m3. Four kinds of buoyant particles have been used in the present work. Particles, with densities in the range from 900 to 955 kg‚m-3, did not lead to much difference in the gas dispersion, although the just-drawdown agitation speed and power consumption decrease with decreasing density difference between liquid and solid. With particles 1 W‚kg-1. Acknowledgment The authors sincerely acknowledge the financial support of Ministry of Education, P. R. China. Nomenclature
Figure 11. Just-drawdown with off-centered agitator (a) NJDG ∼ VS and (b) PmJDG ∼ VS.
lence. It seems that drawdown is related to the surface vortex generated when narrow baffles are used. A
a, b ) constant and exponent on solids loading CBW ) clearance between baffles and the wall of the tank, m Cv ) volumetric solid concentration, m3‚(m3 of suspension)-1 D ) diameter of impeller, m dP ) particle diameter, m Flg ) gas flow number Flg ) Q/ND3 H ) height of liquid in tank (general), m H0 ) height of the liquid without gas input, m HG ) Height of the liquid during dispersion, m NJD ) just-drawdown agitation speed, s-1 NJDG ) just-drawdown agitation speed for aerated system, s-1 NR ) agitator speed needed for gas dispersion, s-1 Pm ) power consumption per unit liquid mass, W‚kg-1 PmJD ) specific power consumption when solids are just drawn down for ungassed system, W‚kg-1 PmJDG ) specific power consumption when solids are just drawn down for aerated system, W‚kg-1 Q ) inlet gas flow rate, m3‚s-1 R′ ) distance of shaft from vessel centerline, m R2 ) correlation coefficients T ) diameter of the tank, m
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VS ) superficial gas velocity, m‚s-1 WB ) width of baffles, m X ) Solid concentration (kg of solid)(kg of liquid)-1 ) Gas holdup R, β, γ, n ) Constant and exponents in eq 3 FL, FS ) densities of liquid and solid, respectively, kg‚m-3 Abbreviations PP ) polypropylene LDPE ) low-density polyethylene HDPE ) high-density polyethylene WHU ) wide-blade, up-pumping hydrofoil WHD ) wide-blade, down-pumping hydrofoil HEDT ) half elliptical hollow-blade disk turbine
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Received for review April 1, 2005 Revised manuscript received July 22, 2005 Accepted July 22, 2005 IE0504093
