Mass transfer extractors

Interfacial Mass-Transfer Coefficients. Whereas equiHbrium relationships are important in determining the ultimate degree of extraction attainable, in practice the rate of extraction is of equal importance. EquiHbrium is approached asymptotically with increasing contact time in a batch extraction. In continuous extractors the approach to equiHbrium is determined primarily by the residence time, defined as the volume of the phase contact region divided by the volume flow rate of the phases. 

The behavior of drops in the centrifugal field has been studied (211) and the residence times and mass-transfer rates have been measured (212). PodbieHiiak extractors have been widely used in the pharmaceutical industry, eg, for the extraction of penicillin, and are increasingly used in other fields as weU. Commercial units having throughputs of up to 98 m /h (26,000 gal/h) have been reported. 

TABLE 5-28 Mass Transfer Correlations for Packed Two-Phase Contactors—Absorption, Distillation, Cooling Towers, and Extractors (Packing Is Inert)... 

The mass-transfer coefficients depend on complex functions of diffii-sivity, viscosity, density, interfacial tension, and turbulence. Similarly, the mass-transfer area of the droplets depends on complex functions of viscosity, interfacial tension, density difference, extractor geometry, agitation intensity, agitator design, flow rates, and interfacial rag deposits. Only limited success has been achieved in correlating extractor performance with these basic principles. The lumped parameter deals directly with the ultimate design criterion, which is the height of an extraction tower. 

Heat may be transferred between two insoluble liquids in countercurrent flowthrough an extractor, and the performance can be evaluated in the same general manner as in mass transfer (Fig. 15-20). For a differential contactor the number of overall heat-transfer units based on the hot phase can be derived from the same equations used for the number of mass-transfer units based on the feed (raffinate) phase [Eq. (15-36)]. 

Coalescence The coalescence of droplets can occur whenever two or more droplets collide and remain in contact long enough for the continuous-phase film to become so thin that a hole develops and allows the liquid to become one body. A clean system with a high interfacial tension will generally coalesce quite rapidly. Particulates and polymeric films tend to accumulate at droplet surfaces and reduce the rate of coalescence. This can lead to the ouildup of a rag layer at the liquid-hquid interface in an extractor. Rapid drop breakup and rapid coalescence can significantly enhance the rate of mass transfer between phases. 

In a drop extractor, liquid droplets of approximate uniform size and spherical shape are formed at a series of nozzles and rise eountercurrently through the continuous phase which is flowing downwards at a velocity equal to one half of the terminal rising velocity of the droplets. The flowrates of both phases are then increased by 25 per cent. Because of the greater shear rate at the nozzles, the mean diameter of the droplets is however only 90 per cent of the original value. By what factor will the overall mass transfer rate change ... 

The mass-transfer rate per unit volume of the extractor is given by ... 

Extraction can be performed in stirred tanks if the process proceeds fast and separation of phases is ea.sy, but column extractors are most commonly used. The column can be filled with a particulate material. The liquids flow countercurrently whereby the flow can be uniform or pulsed. Reciprocated and rotary agitators are often used to enhance mass transfer. An example of the latter type is shown in Fig. 7.2-13 (asymmetric rotating disk (ARD) extractor). 

Kl is the mass transfer coefficient for the L phase (m/s), a is the interfacial area per unit volume (m /m ), referred to the total liquid volume of the extractor, V is the total holdup of the tank, and is equal to (Vl+Vq). X is the equilibrium concentration, corresponding to concentration Y, given by... 

An alternative approach to the solution of the system dynamic equations, is by the natural cause and effect mass transfer process as formulated, within the individual phase balance equations. This follows the general approach, favoured by Franks (1967), since the extractor is now no longer constrained to operate at equilibrium conditions, but achieves this eventual state as a natural consequence of the relative effects of solute accumulation, solute flow in, solute flow out and mass transfer dynamics. 

The modelling approach to multistage countercurrent equilibrium extraction cascades, based on a mass transfer rate term as shown in Sec. 1.4, can therefore usefully be applied to such types of extractor column. The magnitude of the... 

Note that the above formulation includes allowance for the fractional phase holdup volumes, hL and ho, the phase flow rates, L and G, the diffusion coefficients Dl and Dq, and the overall mass transfer capacity coefficient Klx a, all to vary with position along the extractor. 

The simplest form of extractor is a spray column. The column is empty one liquid forms a continuous phase and the other liquid flows up, or down, the column in the form of droplets. Mass transfer takes places to, or from, the droplets to the continuous phase. The efficiency of a spray tower will be low, particularly with large diameter columns, due to back mixing. The efficiency of the basic, empty, spray column can be improved by installing plates or packing. 

Laboratory devices, gas-liquid mass transfer, 15 690-692 Laboratory extractors, 10 768 Laboratory flocculant testing, 11 638-639... 

The modelling approach to multistage countercurrent equilibrium extraction cascades, based on a mass transfer rate term as shown in Section 1.4, can therefore usefully be applied to such types of extractor column. The magnitude of the mass transfer capacity coefficient term, now used in the model equations, must however be a realistic value corresponding to the hydrodynamic conditions, actually existing within the column and, of course, will be substantially less than that leading to an equilibrium condition. 

However, the column can also be operated in reverse by filling it with toluene and adjusting the principal interface at the bottom of the column. Then water is the dispersed phase and would break into drops at the feed point at the top of the column. These drops descend in the toluene phase and, at the bottom of the column, coalesce to a water homophase that is below the toluene phase. When needed, the principal interface can be adjusted somewhere between top and bottom of the column, whereby the heavier liquid is dispersed above and the lighter liquid below the principal interface. How is it decided which of the two liquids should be dispersed Understanding the flow and mass transfer processes in the extractor, which are analysed in this chapter, provides the answer. At this point, only the important factors are listed thus, the dispersed phase should be ... 

Flow and mass transfer the transport of the two phases through an extractor and the production of intensive phase contact are complex hydrodynamic problems. Mass transfer provides the main dimensions of the extractor. This chapter is chiefly interested in a suitable extractor design, but also in the restrictions of the calculation. 

The design engineer can use the dispersion coefficients determined in this way for the calculation of the real course of concentrations, c, of any component in the dispersed d) and continuous (c) phases along the countercurrent column. If the mass transfer between the two phases, the actual task of an extractor, is included in the balance, the balance equations for an element of height dh of the extractor for stationary conditions is ... 

For simplicity, this section discusses only the mass transfer of one component in a liquid-liquid system with negligible miscibility of both liquids and with one transitional component. On the other hand, calculations must consider mass transfer rates of several components and more or less strong variation in the mass flows along the column, where both complicate the equation considerably [21-23]. Chemical reactions may cause further complications. Their kinetics can enhance the mass transfer coefficients and, therefore, the reaction equations have to be part of the mathematical model of the extractor [24,25]. 

Fig. 9.21 The mass transfer performance of a centrifugal extractor (see Fig. 9.9) with four concentric sheets (stages). This is dependent on the radial path with the field force as parameter n, number of stages h, height of a stage r , radial drop path in a stage r,-, inactive radial distance (thickness of perforated sheet and of coalesced layer in a stage). (From Ref. 9.)...

These phenomena of surface energy all are time dependent. The shorter the contact time of the phases, the more difficult it is for them to develop. Therefore, flow and mass transfer in centrifugal extractors are hardly affected. 

Only a careful analysis of the differences that appear in construction, in the processes of flow and mass transfer and measuring and observing the behavior and the performance of large-scale extractors, can answer these questions. This provides formula of experience for the factors of scale-up that have been listed [4] for the countercurrent extractors described here. 

Dekker et al. [170] have also shown that the steady state experimental data of the extraction and the observed dynamic behavior of the extraction are in good agreement with the model predictions. This model offers the opportunity to predict the effect of changes, both in the process conditions (effect of residence time and mass transfer coefficient) and in the composition of the aqueous and reverse micellar phase (effect of inactivation rate constant and distribution coefficient) on the extraction efficiency. A shorter residence time in the extractors, in combination with an increase in mass transfer rate, will give improvement in the yield of active enzyme in the second aqueous phase and will further reduce the surfactant loss. They have suggested that the use of centrifugal separators or extractors might be valuable in this respect. 

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