Bottom Clearance

Usually impellers are installed in the lower part of a vessel. When axial impellers are chosen, the power number increases with decreasing bottom clearance due to the flow deflection at the vessel bottom. When radial impellers are used, the power number decreases with decreasing bottom clearance as there is no recirculation below the impeller. 

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A survey of the published literature indicates that the ratio of the maximum to mean energy dissipation rate in the vessel, Smax/ m can vary substantially but typically in the range 10 to 100 [85]. Recent measurements [100] of the turbulent flow properties with a range of impellers and vessel configurations indicate that the differences between the reported ratios of Smax/Cm re partly due to differences in the geometrical variables. For example, detailed factorial designs of experiments showed significant effects of impeller diameter to tank diameter ratio and off-bottom clearance to impeller diameter ratio on the value of emax/Cm-... 

Cheng J, Carreau PJ, Chhabra RP. On the effect of wall and bottom clearance on mixing of viscoelastic fluids. In Tatterson GB, Calabrese RV, Penny WR, eds. Industrial Mixing Fundamentals with Applications. New York Amer Inst Chem Eng, 1995 115-122. 

Fig. 4. Schematic diagram of a concentric-tube airlift bioreactor (with working volume of 1.0 1). 1 Reactor column 2 inner draft tube (with its height of 15 cm) 3 sparger B bottom clearance (2.5 cm) d diameter of draft tube (4.5 cm) D reactor diameter (7.0 cm) H column height (40 cm)...

This is verified by the measuring data obtained in a column of 1.6 m 0, in which a slot injector was installed with a bottom clearance of 1 m and an angle of 25° towards the bottom. The liquid head H above the injector was varied in the range H = 1-7 m. It is shown that the influence of the bubble coalescence in the G/L free jet on the mass transfer - which occurs in a short distance from the nozzle orifice -is equilized only after H = 3 m see Fig. 75. This finding proves that the pi-set, eq. (13.19), is not complete but has to be widened by a pi-number which essentially contains liquid height H. It can be formulated by H = H (g/v2)1 3. 

The above correlation was obtained from the experimental data in a 14.5 cm diameter flat-bottom tank with four baffles over its entire height and a four-blade impeller with d, = dT/2 and with an off-bottom clearance of d,/3. Oguz and Brehm (1988) also proposed the Yagi-Yoshida correlation for both aqueous and organic liquid systems as... 

The effect of off-bottom clearance (C) is pronounced for all impellers, as indicated in Figure 10.7. For a 6BD (Rushton) impeller, the power draw (P) decreases as the impeller is moved closer to the vessel bottom from the typical impeller location of C/D = 1 for a 4BF turbine, P initially decreases as the impeller is moved down from C/D = 1, reaches a minimum at about C/D = 0.7 and then rises again as C/D drops below 0.7 and for a 4BP, the power draw continually increases as the impeller moves down from C/D = 1. 

Figure 10.7. Effect of off-bottom clearance on Np for turbines with full baffling (adapted from Bates, Fondy, and Corpstein, I. ECFunds, 2, p. 310, 1963).

P.M. Armenante and E. Nagamine, Solids Suspension in Agitated Vessels with Impellers Having Small Off-Bottom Clearances, paper presented at the 1996 AIChE Annual Meeting, Chicago, 1996. 

Heng PWS, Wan LSC, Chia CGH. Effect of off-bottom clearance on properties of pellets produced by melt pelleti/aiion. Pharm Dev Tech 1999 4 27-33. 

The main variable is Fr, which represents the influence of gas input rate (and also the energy input rate). Its effect on holdup is much stronger in the downcomer than in the riser, and the balance between the two gives an exponent of almost 1 (1.05) for the total holdup. Interestingly, the behavior of holdup in the gas separator is very close to that of total holdup. As draft-tube bottom clearance ct b increases. Or decreases. ... 

Substituting the definitions of Ei/ and Fr into this expression shows pressure drop to be a function of Wg The bottom clearance ratio, Xt, and the bubble separation group, M, have a strong influence on the pressure drop because of their effect on liquid velocity. 

The Pfaudler impeller stirrer was developed for use in enamel-coated vessels [438] and thus has rounded stirring arms. It is installed with small bottom clearance at a D/d ratio of 1.5 and can be used both with and without baffles. Due to the small bottom clearance it can be used with strongly fluctuating filling levels (e.g. during emptying), since it can efficiently mix even small liquid volumes. 

The power P of a given stirrer type and given installation conditions (vessel diameter D, liquid height H, stirrer bottom clearance h) in a homogeneous liquid depends upon the stirrer diameter d (as the characteristic length), the degree of baffling, the material parameters of the liquid (density p and dynamic viscosity v) and upon the stirrer speed n. 

Fig. 5.2 Normalized distribution of solids (glass beads) over the relative layer height b bs is the bottom clearance of the stirrer. For the experimental conditions see [114]... 

Evaluation of his experimental data together with those from [395] is presented in Fig. 5.17. To eliminate the effect of different geometries (stirrer type, bottom clearance), in the ordinate the particular stirrer speed is related to that n, which applies for d/D = 1/3. 

Turbine and propeller stirrers with a low bottom clearance (h/d = 0.25-0.5) are regarded as optimal stirrers. A perforated ring should be used as gas sparger. The required stirrer power is lower, if the baffles are only installed in the upper half of the liquid height [592]. 

Measurements [364] established, that the lowest stirrer speed for complete dispersion also depended upon the bottom clearance h. In the range d/D = 0.2-0.3 it was found for the 6-blade turbine stirrer that for ... 

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