Mass transfer centrifugation

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. 

The conversion reaches a maximum at 30 Hz. At a higher rate of rotation the increased separatory power of the centrifuge leads to a reduction of the volume of the mixed phase in which the reaction takes place. At reduced rotational speeds of the centrifuge the mixing process becomes less efficient, resulting in larger average drop sizes in the dispersed phase and thus to reduced mass transfer rates and conversion levels. 

The types of equipment used, which range from stirred tanks and mixer-settlers to centrifugal contactors and various types of columns, affect both capital and operating costs [9]. In the decision to build a plant, the choice of the most suitable contactor for the specific situation is most important. In some systems, because of the chemistry and mass transfer rates involved, several alternative designs of contacting equipment are available. In the selection of a contactor, one must consider the capacity and stage requirements solvent type and residence time phase flow ratio physical properties direction of mass transfer phase dispersion and coalescence holdup kinetics equilibrium presence of solids overall performance and maintenance as a function of contactor complexity. This may appear very complicated, but with some experience, the choice is relatively simple. 

The general definition of mass transfer coefficients according to Eqs. (9.31)-(9.33) is also valid for the mass transfer coefficient in a centrifugal field. The... 

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. 

Chen, G.F., Wang, W., Yan, H. and Wang, X.Z., Experimental research on mass transfer in a centrifugal fluidized bed dryer. Drying Tech., 17 (1999) 1845-1857. 

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. 

Meikrantz, D.H., Gam, T., Mann, N., Law, J., Todd, T. 2007. Hydraulic performance and mass transfer efficiency of engineering scale centrifugal contactors. In Advanced Nuclear Fuel Cycles and Systems. Global 2007, Boise, ID, September 9-13. INL/ CONF-07-12167. 

As organic and aqueous phases are macroscopically separated by the membrane, HFM offer several hydrodynamic advantages over other contactors, such as the absence of flooding and entrainment, or the reduction of feed consumption (160, 161). The flowsheets tested in HFM were similar to those developed for centrifugal contactor tests. Computer codes based on equilibrium (162) and kinetics data, diffusion coefficients (in both phases and in the membrane pores), and a hydrodynamic description of the module, were established to calculate transient and steady-state effluent concentrations. It was demonstrated that, by selecting appropriate flow rates (as mass transfer is mainly controlled by diffusion), very high DFs (DI A 11 = 20,000 and DFrm = 830) could be achieved. Am(III) and Cm(III) back-extraction efficiency was up to 99.87%. 

Birdwell, Jr., J. F., and K. K. Anderson. 2001. Evaluation of 5-cm Centrifugal Contactor Hydraulic and Mass Transfer Performance for Caustic-side Solvent Extraction of Cesium. Oak Ridge National Laboratory Report ORNL/TM-2001/137, Oak Ridge, TN. 

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