Scale mass transfer rate

Flow Reactors Fast reactions and those in the gas phase are generally done in tubular flow reaclors, just as they are often done on the commercial scale. Some heterogeneous reactors are shown in Fig. 23-29 the item in Fig. 23-29g is suited to liquid/liquid as well as gas/liquid. Stirred tanks, bubble and packed towers, and other commercial types are also used. The operadon of such units can sometimes be predicted from independent data of chemical and mass transfer rates, correlations of interfacial areas, droplet sizes, and other data. 

Asymptotic Solution Rate equations for the various mass-transfer mechanisms are written in dimensionless form in Table 16-13 in terms of a number of transfer units, N = L/HTU, for particle-scale mass-transfer resistances, a number of reaction units for the reaction kinetics mechanism, and a number of dispersion units, Np, for axial dispersion. For pore and sohd diffusion, q = / // p is a dimensionless radial coordinate, where / p is the radius of the particle, if a particle is bidisperse, then / p can be replaced by the radius of a suoparticle. For prehminary calculations. Fig. 16-13 can be used to estimate N for use with the LDF approximation when more than one resistance is important. 

The ROTOBERTY internal recycle laboratory reactor was designed to produce experimental results that can be used for developing reaction kinetics and to test catalysts. These results are valid at the conditions of large-scale plant operations. Since internal flow rates contacting the catalyst are known, heat and mass transfer rates can be calculated between the catalyst and the recycling fluid. With these known, their influence on catalyst performance can be evaluated in the experiments as well as in production units. Operating conditions, some construction features, and performance characteristics are given next. 

When electrically insulated strip or spot electrodes are embedded in a large electrode, and turbulent flow is fully developed, the steady mass-transfer rate gives information about the eddy diffusivity in the viscous sublayer very close to the electrode (see Section VI,C below). The fluctuating rate does not give information about velocity variations, and is markedly affected by the size of the electrode. The longitudinal, circumferential, and time scales of the mass-transfer fluctuations led Hanratty (H2) to postulate a surface renewal model with fixed time intervals based on the median energy frequency. 

Finally, Bakker and Van den Akker calculated local values for the specific mass transfer rate kfl, by estimating local Ay-values from local values of the Kolmogorov time scale /(v/e) and by deriving local values of the specific interfacial area a from local values for bubble size and bubble hold-up. 

Although the application of fluidisation techniques to sublimation-desublimation processes was first proposed by Matz" 11, the technique has not yet been widely adopted for large-scale commercial use, despite its obvious advantage of improving both heat and mass transfer rates. G aiko 112 1 has, however, reported on a fluidised-bed de-sublimation unit operating in the United States for the production of aluminum chloride at the rate of 3 kg/s (11 tonne/h). 

For the scale-up of reverse micelle extractions, it is important to know which factors determine the mass transfer rate to or from the reverse micelle phase. So far most work has concentrated on the kinetics of solubilization of water molecules [34,35], protons [36], metal ions [20,35,37,38 0], amino acids [41], and proteins [8,35,42,43]. There are two separate processes forward transfer, which is transfer of solute from the aqueous to the reverse micelle phase, and back transfer, which is the antithesis of the first one. 

There is conflicting evidence regarding the extent to which imposed vibrations increase particle to fluid heat and mass transfer rates (G2), with some authors even claiming that transfer rates are decreased. For sinusoidal velocity variations superimposed on steady relative motion, enhancement of transfer depends on a scale ratio A/d and a velocity ratio Af /Uj (G3). These quantities are rather like the scale and intensity of turbulence (see Chapter 10). For Af /Uj < l/2n, the vibrations do not cause reversal in the relative motion and the enhancement of mass transfer has been correlated (G3) by... 

Rate of protein transfer to or from a reverse micellar phase and factors affecting the rate are important for the practical applications of RME for the extraction and purification of proteins/enzymes and for scale-up. The mechanism of protein exchange between two immiscible phases (Fig. 2) can be divided into three steps [36] the diffusion of protein from bulk aqueous solution to the interface, the formation of a protein-containing micelle at the interface, and the diffusion of a protein-containing micelle in to the organic phase. The reverse steps are applicable for back transfer with the coalescence of protein-filled RM with the interface to release the protein. The overall mass transfer rate during an extraction processes will depend on which of these steps is rate limiting. 

Scale-up based on the mass-transfer rate between phases is directly related to liquid turbulence and motion at the interface. Scale-up of solids dissolution rate or mass transfer between liquid phases is adequately handled utilizing 2/3 as an exponent... 

The most frequently used contactors in full-scale waste water ozonation systems are bubble column reactors equipped with diffusers or venturi injectors, mostly operated in a reactor-in-series counter-current continuous mode. Many full-scale ozone reactors are operated at elevated pressure (2-6 barabs) in order to achieve a high ozone mass transfer rate, which in turn increases the process efficiency. 

Two-phase gaslwater injector nozzles are mostly used in pilot- or full-scale bubble column applications (Krost, 1995) or in specialized, newly developed reactor types. An example is the Submerged Impinging Zone Reactor (IZR) (Gaddis and Vogelpohl, 1992 Air Products, 1998), which is constructed for very high mass-transfer rates. 

Normally, it makes little sense to apply such systems in lab-scale ozonation experiments, since the high mass transfer rates are only achieved at high gas flow rates which because of the typical operation characteristics of EDOGs accordingly means low ozone gas concentrations. An appropriate field of application was, however, presented in the study of Sunder and Hem pel (1996) who operated a tube-reactor for the ozonation of small concentrations of perchloroethylene. An injector nozzle coupled with the highly efficient Aquatector ozone-absorption unit was installed in front of the tube-reactor. Both the gas and liquid were partially recycled in this system. According to the authors more than 90 % of the ozone produced was absorbed in demineralized water and dissolved ozone concentrations ranged up to 100 pmol L-1 (cL = 5 mg L-1, T= 20 °C). 

Correlations based on dimensional analysis with the above variables in equation 3-10 would allow mass transfer rates to be easily predicted, e. g. in scaling-up lab results to full-scale or for changes in the liquid properties. However, no correlations have been developed with this complexity. 

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