Agitated columns

For a given liquid/liquid system, the drop size, and consequently, the specific surface are of the dispersion, are mainly determined by the specific energy dissipation e of the impeller. Calderbank (1958) proposed a correlation that is still widely used (here e is the fraction of the dispersed phase)  

Mass transfer from the continuous phase to the drops and vice versa may be estimated with the relations for rigid spheres, when the drops are very small, such as eq. (4.42). For larger drops, with mobile interfaces, eq. (4.24) combined with eq. (4.30) may be applicable. The Reynolds number may be based on relative velocity, which can be found from eqs. (4.40) and (4.41), or from the modified Reynolds number that is related to e, see eq. (4.44). 

A complication is that also in stirred liquid/liquid dispersions coalescence and redispersion may occur. These phenomena are not only governed by the interfacial tension, but also by concentration gradients. So the surface area may be influenced by the mass transfer process itself. This applies also to the mass transfer coefficients. It has been shown that reversal of e direction of mass transfer may increase or reduce the mass transfer rate by a factor of 3. Consequently, general emprical relations for mass transfer in stirred liquid/liquid dispersions are not very reliable. In the practice of reactor development this is often not a serious problem, since in most situations mass transfer is not a limiting factor in liquidAiquid processes. If mass transfer does limit the reaction rate, one can increase the mass transfer rate by making the dispersion finer. 

Sometimes emulsions or micro-emulsions are used for liquid/liquid processes. Drop sizes are then in the order of 1 pm or smaller, so than an enormous interfacial area is created. This allows rapid reactions to take place without any diffusion limitation. (Sharma, 1988 see also section 13.6). 

There are three wo well known types of column that can be used for continuous countercurrent liquid/liquid contacting rotating disc contactors (RDC) and pulsed packed columns (PPC) (these are not to be confused with three-phase packed coplumn reactors, operated under pulsed flow conditions, see section 4.7.2.3). 

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In many types of contactors, such as stirred tanks, rotary agitated columns, and pulsed columns, mechanical energy is appHed externally in order to reduce the drop si2e far below the values estimated from equations 36 and 37 and thereby increase the rate of mass transfer. The theory of local isotropic turbulence can be appHed to the breakup of a large drop into smaller ones (66), resulting in an expression of the form... 

The earliest large-scale continuous industrial extraction equipment consisted of mixer—settlers and open-spray columns. The vertical stacking of a series of mixer—settlers was a feature of a patented column in 1935 (96) in which countercurrent flow occurred because of density difference between the phases, avoiding the necessity for interstage pumping. This was a precursor of the agitated column contactors which have been developed and commercialized since the late 1940s. There are several texts (1,2,6,97—98) and reviews (99—100) available that describe the various types of extractors. 

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.

Agitated column contactors, 10 767 Agitated drying systems, 18 732 Agitated tanks, heat transfer in,... 

Rotary-agitation columns Reasonable capacity Reasonable HETS Many stages possible Reasonable construction cost Low operating and maintenance cost Petrochemical Metallurgical Pharmaceutical Fertilizer... 

In many lypes of contactors, such as stirred tanks, rotary agitated columns, and pulsed columns, mechanical energy is applied externally in order to reduce the drop size and thereby increase the rate of mass transfer. 

Rotary agitation columns Reasonable capacity, reasonable HETS, many stages possible, reasonable construction cost, low operating and maintenance cost Petrochemical, metallurgical, pharmaceutical, fertilizer... 

Figure 1. Slurry reactors classified by the contacting pattern and mechanical devices (a) slurry (bubble) column (b) countercurrent column (c) co-current upflow (d) co-current downflow (e) stirred vessel (C) draft tube reactor (g) tray column (h) rotating disc or multi-agitated column reactor (i) three-phase spray column — liquid flow —> gas flow.

The liquid-phase mixing in a multistage mechanically agitated reactor is best correlated by Eq. (2.31) in the absence of gas flow and by Eq. (2.32) in the presence of gas flow. The mixing time can be estimated from the study of Paca et al. (1976). Experimental work is needed to estimate gas-phase back-mixing. The use of Eq. (2.36) for the calculation of the gas-liquid volumetric mass transfer coefficient in a multistage mechanically agitated column is recommended. 

Juvekar and Sharma52 also measured the effect of particle loading on the gas-liquid interfacial area in bubble- as well as agitated columns. Their results are shown in Fig. 9-23. The results indicate that the interfacial area was practically independent of the particle loading up to 20 percent. Even for higher particle loadings, the effect on the interfacial area was insignificant. 

B.C. Xu, Interstage Backmixing in Compartmented Agitated Columns, PhD Thesis, University of Arkansas, 1994. 

A = Unagitated Columns B = Mechanically Agitated Columns C = Mixer - Settlers D = Centrifugal Extractors... 

The Karr reciprocating-plate column (Fig. 15-50) is a popular example. It uses dual-flow plates with 50 to 60 percent open area and has no downcomers [Karr, AlChE J., 5(4), pp. 446-452 (1959) Karr and Lo, Chem. Eng. Prog., 72(11), pp. 68-70 (1976) and Karr, AIChE J., 31(4), pp. 690-692 (1985)]. Because of the high open area, a Karr column may be operated with relatively high throughput compared to other types of agitated columns, up to about 1000 gaF(h ft ) [40 m /(h m )] depending upon the application. The plates are mounted... 

Figure 1. Seven-stage agitated column contactor.

Agitated Columns. The size of an extraction column frequently can be estimated from a knowledge of the flow rates and physical properties, combined vdth some empirical generalizations. 

Agitated columns are frequently operated so that the capacity is half what it would be at no agitation (zero rpm or pulsation). Agitation is used to reduce droplet diameter to this equivalent point to increase mass transfer rate and mass transfer area. 

Figure 25. Empirical constant B for determining agitated column diameter.

The countercurrent contact zone height will depend primarily upon the number of stages required ( ) and the column characteristics. The effect of backmixing also increases the column diameter. A reasonable first approximation of extraction height (L) required for agitated columns is ... 

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