Contactors, mechanically agitated

Fig. 15. Mechanically agitated columns (a) Scheibel column (b) rotating-disk contactor (RDC) (c) asymmetric rotating-disk (ARD) contactor (d) Oldshue-Rushton multiple-mixer column (e) Kuhni column and (f) reciprocating-plate column.

Mechanically agitated contactors PM M 0.05-0.4 Can he varied over a wide range 0.3-1 1-800 0.3-800... 

The second type of mass-exchange units is the differential (or continuous) contactor. In this category, the two phases flow through the exchanger in continuous contact throughout without intermediate phase separation and recontacting. Examples of differential contactors include packed columns (Fig. 2.6), spray towers (Fig. 2.7), and mechanically agitated units (Fig. 2.8). 

Westerterp et al. (W5) measured interfacial areas in mechanically agitated gas-liquid contactors. The existence of two regions was demonstrated At agitation rates below a certain minimum value, interfacial areas are unaffected by agitation and depend only on nominal gas velocity and the type of gas distributor, whereas at higher agitation rates, the interfacial areas are... 

Kramers et al. (K21) measured gas residence-time distribution in a mechanically agitated gas-liquid contactor of 0.6-m diameter for various gas velocities and agitator speeds. In the region where agitation has an effect on the gas-liquid interfacial area (cf. the study by Westerterp et al. (W5), Section V,D,1), the residence-time distribution was found to resemble closely that of a perfect mixer. 

Table 1 reports a wide spectrum of typologies of biofilm reactor upflow anaerobic sludge bed (UASB), fluidized bed, airlift, fixed bed with and without recycle, mechanically agitated vessel, rotating drum and rotating biological contactor. Each reactor is characterized by positive features and drawbacks. 

Mechanical agitation Group E Rotating disc contactor Oldshue-Rushton column Zeihl column Graesser contractor Group F Scheibel column Mixer-settlers... 

For gas absorption, the equipment possibilities are generally packed columns plate distillation towers, possibly with mechanical agitation on every plate deep-bed contactors (bubble columns or sparged lagoons) and mechanically agitated vessels or lagoons. Packed towers and plate distillation columns are discussed elsewhere. Generally these... 

Some examples of mechanically agitated contactors are the rotating-disk contactor (RDC), Karr, Oldshue-Rushton, Scheibel, and Kiihni columns shown in Figure 11. 

There are several other types of apparata used for reactive absorption, though these are less widespread. In mechanically agitated bubble columns it can be assumed that both phases are ideally mixed, whereas in the jet absorber, the gas stream breaks on a liquid surface and is dispersed in the liquid. In spray towers and venturi scrubbers, liquid is sprayed as fine droplets. Thin-jilm contactors, in which liquid film is scrapped from the walls by impellers, are applied for reactive absorption in viscous liquids. Some units, such as wetted-wall columns, contactor, laminar jet absorber and disc (sphere) column are used mainly in laboratory. 

As mentioned earlier, the gas holdup in mechanically agitated contactors has been found to be, apart from geometric parameters, a function of liquid viscosity, surface tension, the electrolytic nature of the solutions, foaming character, etc. Hence, phenomenologically the reduction in power consumption should also depends on these characteristics of the system. Bruijn et al (1974) concluded that the surface tension does not affect the mechanism of cavity formation and its shape to an appreciable extent. However, liquid viscosity influences the stability of the cavity. Hughmark (1980) presented the... 

A more notable correlation for the heat-transfer coefficient at the wall of mechanically agitated contactors is given by Perry and Chilton (1973) as... 

Novel gas-liquid mechanically agitated reactors such as multistage mechanically agitated reactors, gas-inducing contactors, and cyclone reactors should find an increasing number of applications. 

Philadelphia Gear Co. has recently introduced a mechanically agitated contactor wherein the impeller is surrounded by a draft tube. The company claims higher values of the mass-transfer coefficient in this equipment, presumably as a result of increased gas holdup. The higher gas holdup is, however, expected to decrease the heat-transfer coefficient and the extent of mixing. 

Here we examine commonly used mechanically agitated industrial liquid-liquid contactors. Table XVII describes mixing and mass transfer parameters for mechanically agitated liquid-liquid contactors. The most commonly... 

Typical Values of Mixing and Mass-Transfer Parameters in Mechanically Agitated Contactors (Reprinted from Doraiswamy and Sharma. 1984, courtesy of Marcel Dekker, Inc.)... 

Fig. 26. Mechanically agitated industrial contactors, (a) mixer-settler (b) rotating-disk column (c) mixco column (d) asymmetric rotating-disk column (e) pulsed packed column (f) Podbielniak centrifugal extractor. (Reprinted from Doraiswamy, L. K and Sharma, M. M., Heterogeneous Reactions Analysis, Examples and Reactor Design, Vols. I and 2, 1984, John Wiley and Sons.)...

Spray, packed, and sieve-plate columns give poor mass-transfer rates for consequently require greater height. The mass transfer in such columns can be significantly improved by providing mechanical agitation. Remen (1951) and Oldshue and Rushton (1952) introduced the rotating-disk contactor (see Fig. 26b) and the mixed column (see Fig. 26c). 

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... 

In large-size mechanically-agitated multistage contactors, the speed of the agitator is kept at a relatively low level and horizontal baffles are provided. Here, RTD in the gas and liquid phases may show different behavior compared to a small-scale unit. 

The values of K for bubble columns can probably be calculated from the above correlation if the comparison between bubble columns and mechanically agitated contactors is based on the same value of average liquid circulation velocity, V. ... 

Figure 4. Comparison between experimental and predicted heat transfer coefficients in bubble columns (Equation 24) (analogy with mechanically agitated contactors) for symbol key, see Table II...

In this process, the dried liquid butane feed is used first to recover aluminum chloride and antimony trichloride from spent catalyst. This is accomplished in a scrubber, from which insoluble complex is continuously discarded. The butane stream then picks up recycled hydrogen chloride and enters the reactor, where mechanical agitation causes intimate contacting with about an equal volume of catalyst. Although the bulk of the liquid catalyst is removed by settling in the baffled upper section of the contactor, some is dissolved in the products. This is recovered in the catalyst recovery column where butanes and hydrogen chloride are distilled overhead and antimony trichloride containing a relatively low concentration of aluminum chloride is concentrated as bottoms. The... 

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