Mass transfer coefficients factors influencing

In addition, it was concluded that the liquid-phase diffusion coefficient is the major factor influencing the value of the mass-transfer coefficient per unit area. Inasmuch as agitators operate poorly in gas-liquid dispersions, it is impractical to induce turbulence by mechanical means that exceeds gravitational forces. They conclude, therefore, that heat- and mass-transfer coefficients per unit area in gas dispersions are almost completely unaffected by the mechanical power dissipated in the system. Consequently, the total mass-transfer rate in agitated gas-liquid contacting is changed almost entirely in accordance with the interfacial area—a function of the power input. 

From the above it can be concluded that only the reaction with component B may enhance mass transfer of ozone substantially. And only if the Hatta number HaB is much higher than 1. Therefore it can be expected that whenever we have to deal with an enhancement of mass transfer due to chemical reactions, this influences the selectivity of the oxidation process in a negative way. The factor which has to be considered in this respect is the Hatta number for the reaction of ozone with component B (equation 29). HaB increases with increasing value of kB and CBb and with decreasing value of the mass transfer coefficient for ozone, kHq,... 

This chapter will first provide some basics on ozone mass transfer, including theoretical background on the (two-) film theory of gas absorption and the definition of over-all mass transfer coefficients KLa (Section B 3.1) as well as an overview of the main parameters of influence (Section B 3.2). Empirical correction factors for mass transfer coefficients will also be presented in Section B 3.2. These basics will be followed by a description of the common methods for the determination of ozone mass transfer coefficients (Section B 3.3) including practical advice for the performance of the appropriate experiments. Emphasis is laid on the design of the experiments so that true mass transfer coefficients are obtained. 

It is obvious that re-atomization yields decrease the mean diameter of the liquid droplets and thus an increased interface area at the same time, it results in reduced average transfer coefficients, because heat and mass transfer coefficients between gas flow and particle or droplet are in positive correlation with the diameter of the particle or droplet, while coalescence of droplets yields influences opposite to those described above. In their investigation on the absorption of C02 into NaOH solution, Herskowits et al. [59, 60] determined theoretically the total interface areas and the mass transfer coefficients by comparing the absorption rates with and without reaction in liquid, employing the expression for the enhancement factor due to chemical reaction of second-order kinetics presented by Danckwerts [70],... 

Transfer properties, the heat and mass transfer coefficient and friction factor, depend not only on transport and thermodynamic properties but also on the hydro-dynamic behavior of a fluid. The geometry of the system will influence the hydro-dynamic behavior. By reducing the parameters by arranging them into dimensionless groups, we can reduce the number of parameters that have to be varied to correlate any of the transfer properties. For example, the ffiction factor equation. 

The above calculation is quite tedious and gets complicated by the fact that the properties which ultimately control the magnitude of these fourteen unknown quantities further depend on the physical and chemical parameters of the system such as reaction rate constants, initial size distribution of the feed, bed temperature, elutriation constants, heat and mass transfer coefficients, particle growth factors for char and limestone particles, flow rates of solid and gaseous reactants. In a complete analysis of a fluidized bed combustor with sulfur absorption by limestone, the influence of all the above parameters must be evaluated to enable us to optimize the system. In the present report we have limited the scope of our calculations by considering only the initial size of the limestone particles and the reaction rate constant for the sulfation reaction. 

The composition of the mobile phase also has an influence on the partition coefficients of inorganic substances and the separation efficiency. Concentrations of the mobile-phase constituents should provide partition coefficient values needed for the enrichment or separation of components under investigation. If a step-elution mode is used, partition coefficients higher than 10 and less than 0.1 are favorable for the enrichment of components into the stationary phase and their recovery into the mobile phase, respectively. Chemical kinetics factors may also play an important role in the separation of inorganic species by CCC [9]. It has been shown that the values of mass-transfer coefficients determine the... 

Mass-Transfer Models Because the mass-transfer coefficient and interfacial area for mass transfer of solute are complex functions of fluid properties and the operational and geometric variables of a stirred-tank extractor or mixer, the approach to design normally involves scale-up of miniplant data. The mass-transfer coefficient and interfacial area are influenced by numerous factors that are difficult to precisely quantify. These include drop coalescence and breakage rates as well as complex flow patterns that exist within the vessel (a function of impeller type, vessel geometry, and power input). Nevertheless, it is instructive to review available mass-transfer coefficient and interfacial area models for the insights they can offer. 

In Chapter 7 we discussed the basics of the theory concerned with the influence of diffusion on gas-liquid reactions via the Hatta theory for flrst-order irreversible reactions, the case for rapid second-order reactions, and the generalization of the second-order theory by Van Krevelen and Hofitjzer. Those results were presented in terms of classical two-film theory, employing an enhancement factor to account for reaction effects on diffusion via a simple multiple of the mass-transfer coefficient in the absence of reaction. By and large this approach will be continued here however, alternative and more descriptive mass transfer theories such as the penetration model of Higbie and the surface-renewal theory of Danckwerts merit some attention as was done in Chapter 7. 

The performance of these reactors is greatly influenced by (1) axial, radial, and global distribution of liquid and solids in the bed and (2) changes in bubble size, velocity, breakup, and coalescence. The second set of factors leads to an enhancement in the rates of heat and mass transfer. This happens because each particle (assumed to be spherical) is surrounded by a gas-liquid mixture of low pseudohomogheneous density. Consequently, the particle terminal velocity increases, which in turn has a positive effect on the mass transfer coefficient. A number of papers have been published (e.g., Arters and Fan, 1986, 1990 Fan, 1989 Nikov and Delmas, 1992 Boskovic et al., 1994 Kikuchi et al., 1995) on mass transfer in these reactors. 

Co(ll) concentration in the feed solution decreased, metal permeation was independent of the mass transfer coefficient and the aqueous diffusion film controlled the permeation process (aqueous mass transfer coefficient = 4.8 x 10 cm/s). On the other hand, separation of Co(II)/Li(I) with the leqnired purity (separation factor of Co/Li 25) is possible by this process under optimized conditions. The performance of the system is better when Acorga PT5050 or Cyanex 272 was used as carriers. Also, membrane diluents chosen in any liquid membrane process influences the membrane performance. In Figure 32.6, the effect of the diluent on Co(II) transport is represented, and... 

The expected mass transfer coefficient can be predicted from the critical value by multiplying by an enhancement factor ranging from about 1.1 for particles 200pm to about 1.4 for particles 5mm. Particle density also influences the rate of mass transfer. The reason for this enhancement is the increased level of turbulence at which larger and denser particles become suspended in the liquid. 

The film theory has an important drawback. Although, the value of 6 is not known, one should regard it as uniquely dete mined by the hydrodynamics of the liquid phase. On the basis, Eq.l2 would predict k to be proportional to the diffusivity D. Empiri cal mass transfer coefficient correlations available in the lit rature for a liquid in contact with a gas consistently indicate that in fact k is proportional to the square root of D. Therefore, analyses based on the film theory model are not expected to predict correctly the influence of diffusivity values on the enhancement factor I. Therefore, one is lead to a more complex model of the fluid mechanics involved, the penetration theory model. This model leads, in its several variations, to the correct prediction of the... 

The aim of this research is to show the influence of cross-flow model and ADPF model on the estimation of mass transfer coefficient, liquid hold up and adsorption factor by dynamic analysis of a TBR. 

High mass transfer rates will influence not only the mass transfer coefficient but also the heat transfer coefficients and friction factor. Analysis of film theory penetration theory and boundary layer theory (21) show that the relation of the various coefficients at high (k ) and low mass transfer (kj ) can be given by 0 s ... 

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. 

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