Determination of Mass Transfer Coefficients

The methods to determine mass transfer coefficients can be grouped according to whether the concentration of the transferred compound changes over time  

Which experimental method should be used depends on the type of reactor and how it will be operated, and if clean or process water is to be used for the measurement. Nonsteady state methods are generally simpler and faster to perform if kLa is to be determined in clean water without reaction. For processes that are operated at steady state with a reaction, determination of kLa using steady state methods are preferred, since continuous-flow processes need not be interrupted and operating conditions similar to the normal process conditions can be used. This is especially important for systems with reactions because the reaction rate is usually dependent on the concentration of the reactants present. They are thus often applied for investigations of the mass transfer coefficient under real process conditions with chemical reactions kLa(02) or biological activity kLa(02), e. g. in waste water treatment systems. 

Reactor systems, in which the following general assumptions are valid, are preferable  

There is no net change in gas flow rate in and out of the reactor, so that QG in = Qa out = QG (i. e. when nitrogen is used in kLa(02) measurements to produce oxygen-free water, the volume of nitrogen desorbed approximately equals the volume of oxygen absorbed.) 

The general mass balance for each phase at nonsteady state, considering convection, mass transfer and reaction (e. g. ozone decay), can be written  

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Determination of Mass Transfer Coefficient (kLa) in a Municipal Wastewater Treatment Plant (with PULSAR aerators)... 

Common Problems Inherent to the Determination of Mass Transfer Coefficients with the Nonsteady State Method... 

Table 3-4 Problems inherent to the determination of mass transfer coefficients with the nonsteady state method.

G.B. Dillon, I.J. Harris, The determination of mass transfer coefficients and interfacial areas in gas-liquid contacting systems, Can. J. Chem. Eng. 44 (1966) 307-312. 

The dispersity of the absorbent has a fundamental effect on the reactions involved. As the basis for the determination of mass transfer coefficient, the Sauter mean diameters... 

Ozonation processes can also be used for determination of mass transfer coefficient. In fact, both ozone absorption in organic-free water, which is a slow gas-liquid reaction, and other ozone gas-liquid reactions have been used for this purpose. For example, Roth and Sullivan [59] and Sotelo et al. [60] determined the mass transfer coefficient from ozone absorption in organic-free water, whereas Ridgway et al. [73] and Beltran et al. [67] carried out similar calculations from ozone absorption in water at pH 2 containing indigo and p-nitrophenol, respectively. 

Ridgway D, Sharma RN, Eanlay TR. Determination of mass transfer coefficients in agitated gas liquid reactors by instantaneous reactions. Chem Eng Sci 1989 44 2935-2942. 

Solution of a reactor problem in the mass transfer limit requires an estimation of the appropriate mass transfer coefficient. Fortunately, mass transfer correlations have been developed to aid the determination of mass transfer coefficients. For example, the Sherwood number, Sh, relates the mass transfer coefficient of a species A to its diffusivity and the radius of a catalyst particle, Rp ... 

Figure 7. Determination of mass transfer coefficient. Key , experimental data (9). One and one-half order rate constant k = 85. Conditions pHit 5.0 slurry density, 20 g/L stirring speed, 1800 rpm and 40°C. All three ki s gave the same...

Sobczak, E. (1990) A simple method of determination of mass transfer coefficients and surface reaction constants for crystal growth. Chemical Engineering Science, 45, 561-564. 

Hall, D.W., Scott, K., and Jachuck, R.J.J. (2001), Determination of mass transfer coefficient of a cross-corrugated membrane reactor by the limiting-current technique, International Journal of Heat and Mass Transfer, 44(12) 2201-2207. 

In short, the determination of mass transfer coefficient of two-component system is still relied on experimental measurement although the use of dimensionless group in the data regression can be helpful and reasonable. The collection of published correlations of mass transfer coefficient by Wang et al. [65] and Zhou [64] can be used as reference. 

Several possible reactions for electrochemical determination of mass transfer coefficients are available, but the most frequently used system is potassium ferricyanide to potassium ferrocyanide. Normally the reaction at the working electrode is the reduction of ferricyanide, whereas the oxidation of ferrocyanide takes place at the counter electrode ... 

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