Stirred reactors, with pitched

Double-impeller combinations Bouaifi et al. (2001) derived the following correlations for stirred gas-liquid reactors with various combinations of double impellers. The impellers used were the lightning axial flow impeller (A-310), the four 45° pitched blade turbine pumping down (PBTD) and the Rushton disk turbine (RDT). Furthermore, the tank was a dish-bottom cylindrical tank equipped with four baffles, while the gas was introduced by a ring sprager. The gas-flow rate ranged from 0.54 to 2.62 L/s, whereas the rotational speed was from 1.66 to 11.67 s. The gas holdup is... 

Pitch-blade turbine (paddle stirrer with pitched blades) and propeller stirrers provide high mixing with an axial flow pattern. Both of these stirrers are normally used for low-viscosity liquids and in vessels with baffles. They are well suited for providing liquid homogenization and suspension of solids in slurry reactors. The stirrers can also be used in viscous fluids and for vessels with H/dT > 1, which are generally encountered in fermentation processes. For these situations, axial flow is increased with the use of multistage stirrers with pitched stirring surfaces. 

This concept for the determination of the effective viscosity for mixing processes is also transferable to stirred vessels, because in the laminar flow range the above circumstances also apply for propeller loop reactors [351]. This was proved with tanks with pitched-blade stirrers [540]. The nO values could be better correlated with RSefr numbers, which were produced according to expression (3.25), than those which contained ft g according to expression (1.45) after Metzner-Otto. 

FIGURE 7A.3 Some commonly used impellers for two-/three-phase stirred reactors, (a) Standard six-blade Rushton turbine, (b) Six-blade 45° pitched turbine, (c) Lightnin A315 . (d) SC ABA 6SRGT. (Reproduced from Middleton 2000 with permission from Elsevier. 1992.)... 

FIGURE 7A.9 Hydrodynamic regimes in three-phase (gas-Iiquid-soHd) stirred tank reactors downflow pitched turbine. A, no dispersion of gas solid settled on bottom B, gas dispersed beginning of solid suspension C, gas dispersed off-bottom suspension of solids D, recirculation of mixture and possible surface aeration. (Reproduced from Rewatkaret al. 1991 with permission from American Chemical Society. 1991, American Chemical Society.)... 

Duty It is required to design a stirred reactor using microcarrier-supported animal cells. The working capacity of the reactor (volume of broth) is 20 m The dispersion height to diameter ratio = 1. Disengagement volume equivalent to 1/3 of the total volume is to be provided above the broth level. Therefore, total reactor volume is 30 m and vessel diameter is 3 m. Impeller to be used is six-blade upflow pitched turbine with DIT = CIT = 0.33. 

A typical stirred-tank reactor is shown in Fig. 5.4-3. It is a cylindrical vessel with elliptical or torospherical bottom and cover. It is equipped with an axially mounted stirrer rotating with a speed from 25 rpm (large scale) to 2000 rpm (laboratory). Fig. 5.4-4 shows the stirrers that are mostly used in fine chemicals manufacture, viz. the marine propeller, turbine, flat- or pitched-blade agitator, and anchor. Agitators move the fluid into axial and radial direction. Marine propellers and pitched-blade stirrers predominantly impose axial motion. 

Reactor and Reactor Conditions. A 5-litre glass reactor (15 cm diameter) fitted with four stainless steel baffles (10 cm x 1.5 cm) immersed in a thermostatted oil bath at 80 °C (reflux temperature of methyl acrylate) was used for polymerisation. Stirring was by means of a marine type impeller (6 cm diameter and pitch 45°). The overall reaction rate was sufficiently slow to ensure isothermal conditions. Additions of solutions of the more reactive monomer (styrene, of molar concentration 0.8) to the reactor were made using a computer controlled positive displacement pump (Precision Metering Ltd.) with four long-stroke pump heads, 90 out of phase to minimise pulsation of the flow. 

Stirred tank paddles power input suspend solids, 0.2 to 1.6 kW/m UD = 0.7 to 1.05/1. Baffle, four 90° baffle width = 0.08 x tank diameter off-the-wall distance = 0.015 x tank diameter. Minimum level of liquid = 0.15 x tank diameter for impeller tank diameter 0.28 1 and minimum level = 0.25 x tank diameter for impeller tank diameter = 0.4 1. Use a foot bearing plus a single, main axial hydrofoil impeller diameter = 0.28 x tank diameter located 0.2 x tank diameter from the bottom plus a pitched blade impeller diameter = 0.19 x tank diameter located 0.5 x tank diameter from the bottom. Liquid fluidized bed in general, particle diameter 0.5 to 5 mm with density and diameter of the particle dependent on the application. The superficial liquid velocity to fluidize the bed depends on both the diameter and the density difference between the liquid and the particle. Usually, the operation is particulate fluidization. Particle diameter 0.2 to 1 mm reactors superficial liquid velocity 2 to 200 mm/s. Fluidized adsorption bed expands 20 to 30% superficial liquid velocity for usual carbon adsorbent = 8 to 14 mm/s. Fluidized ion exchange bed expands 50 to 200% superficial liquid velocity for usual ion exchange resin = 40 mm/s. Backwash operations fixed-bed adsorption superficial liquid velocity = 8 to 14 mm/s fixed-bed ion exchange superficial backwash velocity = 3 mm/s. 

Wheat bran and sodium hydroxide were blended at room temperature in a separate reactor one hour before each experiment. Tlie initial L/S ratio was seven and the mixture was stirred for five minutes. The L/S ratio was increased to ten just before the introduction of the mixture into the twin screw extruder with a Nemo excentric-screw pump. Straw was introduced in the extruder s first section with a screw feeder. Straw was mixed with the alkaline dough in the first zone of the barrel through the neutral pitch element and the reverse-pitch screw element successively. The washing water was injected downstream from this zone, and the mixture was conveyed through the second reverse pitch located just downstream from the filtration module. The filtrate was collected and kept in a cold room before further processing, while the refined cellulosic fibres were gathered at the barrel outlet. 

The data used to illustrate emulsion polymerization at low Reynolds numbers were obtained by emulsion polymerization of styrene for 5 hours at 70 in a 500-ml reactor equipped with a 1.5-inch pitched-blade impeller under conditions where 500stirring rate. Although there are only three points, they form a straight line that extrapolates to a point close to the origin. 

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