Reactors axial flow impeller

For reactors with free turbulent flow without dominant boundary layer flows or gas/hquid interfaces (due to rising gas bubbles) such as stirred reactors with bafQes, all used model particle systems and also many biological systems produce similar results, and it may therefore be assumed that these results are also applicable to other particle systems. For stirred tanks in particular, the stress produced by impellers of various types can be predicted with the aid of a geometrical function (Eq. (20)) derived from the results of the measurements. Impellers with a large blade area in relation to the tank dimensions produce less shear, because of their uniform power input, in contrast to small and especially axial-flow impellers, such as propellers, and all kinds of inclined-blade impellers. 

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... 

Figure 5.7 The impeller reactor (a) general view (A) gas in/outlet (B) pressure sealed leads for thermocouples (C) particle fixed on thermocouple (D) axial flow impeller (E) impeller bearing (F) baffles (G) top/bottom part (H) center part (I) thermowell (J) bronze jacket, (b) axial flow impeller with rectangular openings in the blades in which particles are fixed between gauzes (from Borman et al. [53]).

Mechanically stirred hybrid airlift reactors (see Fig. 6) are well suited for use with shear sensitive fermentations that require better oxygen transfer and mixing than is provided by a conventional airlift reactor. Use of a low-power axial flow impeller in the downcomer of an airlift bioreactor can substantially enhance liquid circulation rates, mixing, and gas-liquid mass transfer relative to operation without the agitator. This enhancement increases power consumption disproportionately and also adds other disadvantages of a mechanical agitation system. 

An aerobic fermentation is to be carried out in a 200-m reactor (4-m dia. X 16 m) with a normal liquid depth of 12 m and atmospheric pressure at the top. A fiat-blade turbine will be used to disperse the air, and two axial-flow impellers will be installed on the same shaft to promote end-to-end mixing. Air will be supplied below the turbine at a superficial velocity of 3 cm/sec (based on 30°C and 1 atm). The Ffenry s law constant for oxygen is 5.2 x 10" atm/m.f. (10% greater than for pure water), and the peak oxygen demand is estimated to be 45 mmol/L-hr. Tests in a small unit show that kj a for this solution is 70% of the value for oxygen absorption in sodium sulfite solution. The solution viscosity is about 1.5 cp. 

A.1.1.2 Impellers Impellers commonly used in stirred tank reactors can be divided into two main categories (1) radial flow and (2) axial flow impellers. Mixed flow impellers, which develop radial as well as axial flows (Fig. 7A.3b), are also available and are used widely for many applications (the upflow version of such mixed flow impellers will be shown later to perform far better than others). High solidity ratio impellers also discussed later in Chapter 7B are gaining acceptance in special applications. Some of the commonly used conventional impellers are shown in Figure 7A.3. 

Where Qa is the volumetric gas flow-rate under the reactor conditions. In the case where two co-axial impellers are used, the installed power should be doubled. 

The principal dimensions of the reactor are shown in Figure 6.1. The numerical values used here are D = 0.48 m //(= 1.02 m H, = 0.34 0 = 0.33 m s = 0.01 m r = 0.044125 m d = 0.0625 m d = 0.04 m and q = 0.07 m. Non-slip boundary conditions are assumed on the vessel wall. Both radial and axial velocities are set to zero on the shaft and impeller disk and the angular velocity is determined by the speed of rotation. On the free surface of the liquid, the axial component of velocity is zero with the other two components of velocity being stress free. Along the central line, below the impeller, the axial component of velocity is stress free and the other two components are zero. The temperature of the jacket at the vessel walls is fixed at 10 °C. Heat is lost by convection and at the free surface and there is an axis of symmetry along the centreline with no flux at the shaft and impeller boundaries. The flow is... 

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