Impellers impeller-type stirrer

The oxidation of the PE wax was carried out by babbling air into the polymer melt maintained under stirring at temperatures from 130 C to ITOOC. Due to the low molecular wei t of the PE wax and its low melt viscosity (10-30 cP at 130°C) the oxidation was performed in vessels fitted with a usual impeller-type stirrer. Prom a lab-scale glass vessel of two liters capacity it was scaled up to a stainless-steel reactor of one cubic meter capacity (Figure 1). The main requirement for the oxidation vessel was the need to assure a very intensive mixing of the air bubbles in the wax melt. 

Figure 6.3 One litre suspension polymerisation reactor. Note teflon bearings at the top and bottom of the stainless steel impeller-type stirrer, and stainless steel baffles, to optimise stirring and hence suspension.

At first, the surfactant and thickener were well mixed into the continuous phase (compositions, see Table 23.1). Then the dispersed phase was introduced slowly and dispersed using an impeller type stirrer to prepare the pre-emulsion. Afterwards a rotor-stator device (Polytron PT6000, Kinematica AG, Luzem, CH) was used for further dispersion of the pre-emulsion droplets to produce stable emulsions. Rotational speeds from 1000 to 10,000 rpm were applied within fixed constant time periods, and secondary drop size distributions were measured over time after representative sampling to achieve desired-size secondary droplets of the model emulsions. These model emulsions were stable for several days. However, the model SE was in general used at the same day of preparation for spray characterization employing air-assisted nozzles and rotary atomizers RA. 

Grootseholten etal. (1982) diseuss the influenee of different impeller types on seeondary nueleation rates. There is some evidenee (Serutton etal. 1982) that seeondary nueleation rates are partieularly sensitive to small elearanees between the stirrer and a draft-tube by whieh the stirrer is shrouded. 

Figure 7.7 shows three commonly used types of impellers or stirrers. The six-flat blade turbine, often called the Rushton turbine (Figure 7.7a), is widely used. The standard dimensions of this type of stirrer relative to the tank size are as follows ... 

The exponents were found to be independent of the impeller type, vessel size, impeller clearance and impeller to tank diameter ratio. The dimensionless constant Ci accounted for variations in the system geometry (e.g. on dc/di). This would indicate that the basic mechanism leading to minimum suspension may be the same for rather different stirrer geometries. Table 2, which is an update of the one given by Nienow [19], indicates different exponents found in a few other investigations. Baldi et al. [26] made a relatively successful theoretical approach by assuming that the suspension of particles is mainly due to eddies of a certain critical scale comparable to the particle size. From an energy balance it follows that... 

Naturally, the patterns of liquid movements will vary with the type of impeller used. When marine propeller-type impellers (which often have two or three blades see Figure 7.7c) are used, the liquid in the central part moves upwards along the tank axis and then downwards along the tank wall. Hence, this type of impeller is categorized as an axial flow impeller. This type of stirrer is suitable for suspending particles in a liquid, or for mixing highly viscous liquids. 

Effect of Agitation. Since the reduction process under consideration is a multiphase reaction, it is clear that the best results are obtainable only when the nitro compound, iron, and water-soluble catalyst are in intimate contact. A stirrer that merely pushes the iron around the bottom of the vessel and permits the charge to separate out into layers does not function efli-ciently. It is apparent, therefore, that, a sturdy sleeve-and-propelier or double-impeller type of stirrer will in some cases be superior to the slow-moving plow type, speeding up the reaction considerably. 

In those gas/liquid processes where the gas flow is not sufficient for obtaining an effective contact between the phases, a stirrer can be used for disperging the gas. This applies particularly to small and medium sized reactors. A turbine impeller combined wiA baffles is quite effective (see figure 4.1). The gas can best be introduced via a perforated ring that is placed underneath the impeller. Another well known impeller type has a hollow shaft and hollow blades, through which the gas is sucked from the gas space and introduced into the liquid at the blade tips. When a common flat disc turbine is used, the gas phase is dispersed into small bubbles that circulate with the liquid in upward and downward directions (compare section 4.2.1). Particularly in the zones with lower flow rates, the bubbles will coalesce at a certain rate. A fraction of the larger bubbles will return with the circulating liquid flow to the impeller zone, where they are redispersed, but the... 

The Rushton impeller is still the most commonly used radial mixing device for standard applications. New, more efficient impeller types were developed in the 1980s and 1990s (see Table 1.6). These developments improved mixing at significantly smaller power numbers. They are also favorable in the context of prevention of stirrer flooding. 

A comparison between the two last correlations and experimental data for small laboratory reactors (25 to 300 cm3) equipped with a magnetic stirrer or a traditional six-blade impeller (not Rushton-type) and using a catalytic hydrogenation in organic solvent has been published (Fig. 45.5) [55]. 

Estimate the stirrer power requirement P for a tank fermentor, 1.8 m in diameter, containing a viscous non-Newtonian broth, of which the consistency index A = 124, flow behavior index n = 0.537, density p = 1050 kg m", stirred by a pitched-blade, turbine-type impeller of diameter d = 0.6 m, with a rotational speed AT of 1 s . ... 

Although low shear emulsification equipment (mechanical stirrers or impellers) can differ in the type of fluid flow imparted to the mixture (axial-flow propeller or radial-flow turbines), no subclasses have been defined. 

Extractor 10 Back pressure regulator 15 Stirrer (Turbine type impeller)... 

Until now, bioreactors of various types have been developed. These include loop-fluidized bed [14], spin filter, continuously stirred turbine, hollow fiber, stirred tank, airlift, rotating drum, and photo bioreactors [1]. Bioreactor modifications include the substitution of a marine impeller in place of a flat-bladed turbine, and the use of a single, large, flat paddle or blade, and a newly designed membrane stirrer for bubble-free aeration [13, 15-18]. Kim et al. [19] developed a hybrid reactor with a cell-lift impeller and a sintered stainless steel sparger for Thalictrum rugosum cell cultures, and cell densities of up to 31 g L1 were obtained by perfusion without any problems with mixing or loss of cell viability the specific berberine productivity was comparable to that in shake flasks. Su and Humphrey [20] conducted a perfusion cultivation in a stirred tank bio-... 

Loiseau et al. (1977) found that their data for nonfoaming systems agreed well with Eq. (3.3). Calderbank (1958), Hassan and Robinson (1977), and Luong and Volesky (1979) have also proposed correlations for power consumption in gas-liquid systems. Nagata (1975) suggested that power consumption for agitated slurries can be reasonably predicted from these correlations by the correction factor psi/pL, where psl is the density of the slurry. Power consumption for a gas-liquid-solid system has also been studied by Wiedmann et al. (1980). They examined the influence of gas velocity, solid loading, type of stirrer, and position of the stirrer blades on power consumption plots of power numbers vs. Reynolds numbers for propeller and turbine type impellers proposed by them are shown in Fig. 13. 

Numerous types of stirrers are used in practice, and Fig. 3.2 shows the most commonly used ones. The stirrers for low-viscosity media are typically marine propeller and pitched-blade turbine stirrers, which cause axial fluid motion, and flat-blade turbines and impellers, which generate radial fluid motion. The former stirrers are suitable for uniformly suspending solids. The latter type are the preferred ones for carrying out exothermic reactions, like autooxidation reactions (see Section 8.3), where the heat generated dining the process has to be removed effectively through the reactor walls. 

R. Gas-inducing impeller with dense solids 1c nr 2 ShGL = = (1.26 x 10- ) N XX1 NRe = pmil/p,Nsc = W(pD), NWe = pbPdl/c [E] Hydrogenation with Raney-type nickel catalyst in stirred autoclave. Used varying T,p, solvents. dst = stirrer diameter. [78]... 

Kipke [276] investigated the effect of different stirrer types and their d/D ratios on the droplet size and its distribution. This study utilized the already mentioned coalescence-prone camauba wax/water system (type 2442 p = 825 kg/m, pj = 2 mPa s at 95°C). The wax/water dispersion could be frozen in with ice water and the droplet size distribution determined by sieving. The stirrer types investigated were 2-stage Intermig d/D — 0.5 0.6 0.7) propeller stirrer (d/D = 0.31 0.37) pitched-blade turbine d/D = 0.31) 6-blade turbine stirrer, Pfaudler impeller stirrer (d/D = 0.575). The experimental data are presented in Fig. 6.7. It is evident that in this material system the droplet size distribution extended to dp/d32 = 0.4-1.5. 

For constant values of P/V (11-13 kW/m ) and d/D = 0.31, it was found, that all stirrer types with the exception of the Pfaudler impeller stirrer performed simi-... 

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