Mass transfer, enhancement

This observation is supported by experiments carried out with a phosphate buffer and a fluorescein solution for visualization of the mixing process. However, in the experiments there are indications that the critical Reynolds number where the mass transfer enhancement sets in is lower (-7) than predicted by the simulations, a fact which is not well understood. 

In chemical micro process technology there is a clear dominance of pressure-driven flows over alternative mechanisms for fluid transport However, any kind of supplementary mechanism allowing promotion of mixing is a useful addition to the toolbox of chemical engineering. Also in conventional process technology, actuation of the fluids by external sources has proven successful for process intensification. An example is mass transfer enhancement by ultrasonic fields which is utilized in sonochemical reactors [143], There exist a number of microfluidic principles to promote mixing which rely on input of various forms of energy into the fluid. 

Mass transfer effects, in membrane processes, 75 723-725 Mass transfer enhancement, by static mixers, 75 708-709... 

Multiphase reactors can also look very similar to separation units, but there are important differences or additions. First, reactions are frequently catalyzed by sohds so the sohd walls or stmctures in the reactor are often catalysts whose activity and stabihty are important as well as the mass transfer enhancements they provide. 

Use of reaction kinetic data from the literature Are kinetic data (e. g. kn(M)) from the literature useful for comparisons with my own data and may they be used for predictions of my own results Yes, if all data necessary to describe the entire process have been assessed and reported comparisons are only possible on this basis. Consider that mass transfer might influence the (apparent) disappearance kinetics of M in the specific system. Keep in mind, that the kD (and kR) values do not take into account the effect of mass transfer enhancement on the removal rate of M (r(M)). B 1 B 4 B 3.2... 

The enhancement factor E was defined to assess mass transfer enhancement due to (ozone) gas absorption into a liquid accompanied by simultaneous reaction B 3.2 Fig. B 3-5... 

In cases where the major resistance is in the liquid phase, the ratio RL/ RT= 1 and the simplification can be made that the over-all coefficient is equal to the liquid film coefficient. Which resistance dominates has to be determined from the ratio kLa / (kGa Hc) (Table 3-3). For compounds with a low Hc such as semi-volatile organic compounds, both resistances can be important (Libra, 1993). In oxygen transfer the liquid-side resistance dominates and KLa = kLa. This is also true for most of the cases in ozone mass transfer, unless there is strong mass transfer enhancement by very fast or instantaneous reactions of... 

The most common and appropriate methods used to determine the mass transfer coefficient and the problems inherent in each are presented in the following sections. The methods are discussed from a practical viewpoint for the direct determination of the ozone mass transfer coefficient. However, it may be impractical, even impossible to use ozone as the transferred species, because of fast reactions which cause mass transfer enhancement etc. Then the oxygen mass transfer coefficient can be used to indirectly determine the ozone mass transfer coefficient. The procedure is described below and special aspects of oxygen mass transfer experiments are referred to in the following sections whenever necessary or of general importance. 

Nonsteady State Methods without Mass Transfer Enhancement... 

Fortunately changes in k,a due to mass transfer enhancement from ozone decay can be neglected, as Huang et al. (1998) showed by example of cyanide ozonation in strongly basic solutions (pH = 12-14) in a system where the value of the purely physical liquid side mass transfer coefficient was not too low (kL° > 0.03 cm s l). This is supported further by the results from several ozonation experiments, which showed that no ozone decay occurs in the liquid film at lower pH values (phenol, pH = 10 (Metha et ah, 1989) 4-nitrophenol, pH = 8.5 (Beltran et ah, 1992 a)). 

Since we know the mass of ozone transferred has to have reacted or left the system, it is relatively easy to determine the reaction rate for slow reactions, which are controlled by chemical kinetics with this method. For kinetic regimes with mass transfer enhancement, the two rates, mass transfer and reaction rate are interdependent. Whether kLa or kD can be determined in such a system and how depends on the regime. Possible methods are similar to those described below in Section B 3.3.3 (see Levenspiel and Godfrey, 1974). 

The semi-batch set-up uses a chemical reaction to immediately remove the absorbed gas from the liquid. In oxygen transfer measurements, sulfite (S032-) or hydrazine (N2H4) have been used to remove the oxygen transferred (Charpentier, pp. 42-49, 1981). For example, the addition rate of the reactant (S032- or N2H4) is adjusted until the system comes to a steady state dissolved oxygen concentration of about 2 mg/L. Then the sulfite addition rate equals the transfer rate. Mass transfer enhancement must be avoided, which is difficult with hydrazine, so that it is rarely used. 

Furthermore, it is almost impossible to use ozone for fc, -measurements when organic substances are present that are (easily) oxidized by molecular ozone. Mass transfer enhancement will occur during such measurements, so that the mass transfer coefficient based on only the physical process cannot be determined. In this case, the oxygen mass transfer coefficient kLa 02) should be determined to assess the mass transfer rate without reaction. The enhanced mass transfer due to reaction should be considered separately, because it is not only dependent on the parameters listed above in equation 3-10, but also dependent on the concentration of the reactants. 

The pseudo-first order rate constants (resp. coefficients) for the direct reaction of some compounds may almost be in the order of typical hydroxyl rate constants (kR > 10 M s ), due to high concentrations of the pollutants as well as mass transfer enhancement. For example, Sotelo et al. (1991) measured values of 6.35 106 and 2.88 106 M l s"1 for the dissociating hydroxylated phenols, resorchinol (1,3-dihydroxybenzene) and phlorogluci-nol (1,3,5-tn hydroxybenzene) respectively (pH = 8.5 and T= 20 °C). 

Due to the complexity of most waste waters and unknown oxidation products, differences in lumped parameters such as COD or preferably DOC are used to quantify treatment success. A model to describe the oxidation process, including physical and chemical processes, based on a lumped parameter has been tried (Beltran et al., 1995). COD was used as a global parameter for all reactions of ozone with organic compounds in the chemical model. The physical model included the Henry s law constant, the kLa, mass transfer enhancement (i. e. the determination of the kinetic regime of ozone absorption) as well as the... 

Henschke M, Pfennig A. Mass transfer enhancement in single-drop extraction experiments. AIChE J 1999 45 2079-2086. 

M. Henschke and A. Pfennig, Mass Transfer Enhancement in Single-Drop Extraction Experiments. AIChE J., 1999, 45 (10), 2079-2086. 

Electrical fields have been applied to solvent extraction. The following mass transfer enhancements were mentioned by Weatherley et al.52 ... 

Figure 7 The stationary-phase mass transfer enhancement for nonsorbed tracers. Diffusion coefficients (a) 5x 1CT9 m2/s (b) 1CT9 m2/s and (c) 10 ° m2/s. Other values used in calculations dp= 7.0 pm ua = 2.0 mm/s.

When a process is produced under the divergent or convergent action of two different forces, the ratio between them represents a dimensionless number. The heat and mass transfer enhanced by the supplementary action of a pulsating field (vibration of apparatus, pulsation of one (or two) phase flow(s), ultrasound action etc.) has been experimented and applied in some cases [6.25-6.27]. Then, the new... 

Mass Transfer Enhancement Low viscosity, high diffusivity, and very low surface tension of SCFs can improve the rates of diffusion-limited reactions (such as enzymatic reactions) by reducing mass transfer resistances (170). Mass... 

Computational fluid dynamics simulations were used by Li et al. [68] to determine mass transfer coefficients and power consumption in channels filled with non woven net spacers. The geometric parameters of a non woven spacer were found to have a great influence on the performance of a spacer in terms of mass transfer enhancement and power consumption. The results from the CFD simulations indicated that an optimal spacer geometry exists. 

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