Turbine surface aerators

Turbine surface aerators are designed as liquid-conveying devices (turbines). They are arranged in the liquid surface. Their task consists of sucking up liquid and spraying it over the surface. In this way, on the one hand, the liquid in the basin should be well mixed and a flow velocity should prevail on the bottom, which... 

Fig. 4.35 Turbine surface aerator system CYROX Lurgi/ Frankfurt, Germany. The figure represents a 1 10 scaled-down design with 12 instead of 36 paddles...

Boermaand Lankester [Chem. E/ig. Sci., 23, 799 (1968)] have measured the surface aeration of a six-bladed disk-type turbine (note A well-designed pitched-blade turbine will give equal or better perfor-... 

Fig. 24 Mass transport characteristics ofthe turbine stirrer in surface aeration from [54, 55]...

Surface aeration is usually employed for slow reactions or for batch processes. It can be used in semicontinuous systems when it is desirable to recirculate the gas from the headspace. This is frequently the case in hydrogenation and is referred to as dead-end hydrogenation. In this system, gas is fed continuously to the reactor at the rate at which hydrogen is being consumed no compression costs to overcome the static head of liquid or external recirculation is needed. Feeding gas from the headspace may be preferred when there is a possibility of plugging sparger holes with reaction products. Surface aerators are also extensively used for waste-water treatment. There are two types of surface aerators the brush aerator, and the most commonly used turbine aerator. 

In surface aeration, bubbles are generated at the surface because of the turbulence in the liquid phase. These bubbles are entrained into the bulk liquid by the liquid flow generated by the impeller. For a six-bladed turbine, Van Dierendonck et al. (1968) proposed the following correlation for the minimum impeller speed needed for surface aeration in the absence of gas... 

Surface aeration in the presence of gas sparging was studied by Calderbank (1958) for a six-bladed disk turbine in 0.3, 0.375, and 0.5 m i.d. mechanically agitated contactors. He suggested the correlation... 

The surface aerator has been used for decades in biological waste-water treatment with H < 4 m. For various sizes of turbine stirrers whose disks were positioned exactly in the surface of the liquid, the mass-transfer coefficient can also be related as... 

The power characteristics of a turbine-type surface aerator are given by Ne = P/(pN3d ) = /(Fr), (6.41)... 

In waste-water treatment, the energy-efficient transfer of oxygen in the aqueous phase is very critical. Since reactions are slow, a long contact time is required. Both of these can be achieved by a conventional surface aerator or novel UNOX surface aeration system. The minimum impeller speed needed for surface aeration in the absence of gas sparging can be obtained from Eq. (6.30). Similar calculations in the presence of gas sparging can be carried out using either Eq. (6.31) for a turbine stirrer or Eq. (6.36) for other stirrers. The gas entrainment and the power consumption in the presence of gas entrainment can be obtained using Eqs. (6.37) and (6.38), respectively. 

Zlokarnik and Mann (paper at Mixing Conf., Rindge, New Hampshire, August 1975) have found the opposite of Kalin e, i.e., subsurface diffusers, subsurface sparged turbines, and surface aerators compare approximately as 4 2 1 respectively in terms of O2 transfer efficiency however, Zlokarnik [Adv. Biochem. Eng., 11, 157 (1979)] later indicates that the scale-up correlation used earlier might be somewhat inaccurate. When all available information is considered, it... 

FIGURE 9.3 Aeration units (a) turbine aerator with an air sparger (b) porous ceramic diffuser and (c) surface aerator. 

The power characteristic of turbine stirrers as surface aerators in Fig. 4.36 shows, that they have to be formulated as Ne(Fr) as is to be expected. For a number of formerly conventional types of surface aerators these characteristics are to be found in [625]. They exhibit, in the technologically interesting range of operation of Fr =... 

Fig. 4.36 Power characteristic of the turbine stirrer as surface aerator from (625].

The sorption characteristic of turbine stirrers as surface aerators is shown in Fig. 4.37. This figure contains the data of three researchers, whereby the quite numerous measurements of Schmidtke and Horvath, only approximated by fitting lines, show that for a correlation an additional diameter-containing number is required. If this is incorporated in the form of the Galilei number Go, the four fitting lines can be correlated to a single line and the process characteristic is as follows ... 

If a turbine stirrer, operated as a surface aerator on a laboratory scale, was scaled up to an industrial scale and if a favorable Froude number was selected for the full scale realization, how its efficiency E will be affected by the scale-up could be pre-... 

In a scale-up of = 4 (10) the efficiency of the industrial turbine surfice aerator will amount to 80% (70%) of that of the model the efficiency of a stirrer surface aerator thus decreases slightly with increasing scale [628]. 

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