Mass transfer coefficient physical properties

For purely physical absorption, the mass-transfer coefficients depend on trie hydrodynamics and the physical properties of the phases. Many correlations exist for example, that of Dwivedi and Upadhyay (Ind. Eng. Chem. Proc. De.s. izDev.,... 

Dl = diffusivity of transferring solute in liquid, m /sec If the diffusivity, Dl, needed for use in the above equations is not known, it can be estimated from data or methods given in the Perry s Chemical Engineers, Handbook (Section 14 in 4th Edition or Section 3 in 5th Edition). Note that the calculation of the mass transfer coefficients for a given regime involves only physical properties and is independent of agitation conditions. 

Obtain the Taylor-Prandtl modification of the Reynolds Analogy between momentum transfer and mass transfer (equimolecular counterdiffusion) for the turbulent flow of a fluid over a surface. Write down the corresponding analogy for heat transfer. State clearly the assumptions which are made. For turbulent flow over a surface, the film heat transfer coefficient for the fluid is found to be 4 kW/m2 K. What would the corresponding value of the mass transfer coefficient be. given the following physical properties ... 

Correlations for mass transfer coefficients, gas holdup volume, and interfacial area, as functions of system physical properties and agitation rate or flow velocity, etc. 

Since the mass transfer coefficient, k, and the specific interfacial area, a, vary in a similar manner, dependent upon the hydrodynamic conditions and system physical properties, they are frequently combined and referred to as a "ka value" or more properly as a mass transfer coefficient. 

In fact none of these models of the mass-transfer coefficient are of much use for the calculation of Kia values in small scale reactor conditions and we have to obtain them by experiments. However, these models can be used as a guide to estimate the influence of the physical properties of the medium. They also make it possible to consider relative values of Ka for compounds for which in experiments the value of Ka is not measurable as easily as for gases such as oxygen. 

The correlations detailed in Sections 7.6.2.1-7.6.2.5 [17,18] are based on data for the turbulent regime with 4 bubble columns, up to 60 cm in diameter, and for 11 liquid-gas systems with varying physical properties. Unless otherwise stated, the gas holdup, interfacial area, and volumetric mass transfer coefficients in the correlations are defined per unit volume of aerated liquid, that is, for the liquid-gas mixture. 

The value of the liquid phase mass transfer coefficient can be obtained from the experimental data for physical absorption of oxygen into blood saturated with oxygen, or estimated from the data with the same apparatus for physical oxygen absorption into water or a reference liquid or solution with known physical properties. Mass transfer coefficients for liquids flowing through or across tubes or hollow fibers can usually be correlated by equations, such as Equation 6.26a for... 

However, the mass transfer rate can be influenced not only by physical properties, but by chemical reactions as well. Depending on the relative rates of reaction and mass transfer, a chemical reaction can change the ozone concentration gradient that develops in the laminar film, normally increasing the mass transfer coefficient, which in turn increases the mass transfer rate. 

Akita and Yoshida (1974) evaluated the liquid-phase mass-transfer coefficient based on the oxygen absorption into several liquids of different physical properties using bubble columns without mechanical agitation. Their correlation for kL is... 

The sodium sulfite oxidation technique has its limitation in the fact that the solution cannot approximate the physical and chemical properties of a fermentation broth. An additional problem is that this technique requires high ionic concentrations (1 to 2 mol/L), the presence of which can affect the interfacial area and, in a lesser degree, the mass-transfer coefficient (Van t Riet, 1979). However, this technique is helpful in comparing the performance of fermenters and studying the effect of scale-up and operating conditions. 

The aim of this paper is to make measurements with liquids of various physical properties in order to define the effect of the liquid properties and operating conditions on the parameter /q, and the limits of validity of the literature models for the interpretation of mass transfer coefficients in bubble dispersions. The method, which is used for the measurements, was verified in Part I to minimize misinterpretations. 

The film thickness represents a model parameter that can be estimated using mass transfer coefficient correlations. These correlations reflect the mass transport dependence on physical properties and process hydrodynamics and are available from the literature (see, e.g., Refs. 57, 68 and 90). 

Packed height is determined from the relationships in Section III. Application of these relationships requires knowledge of the liquid and gas mass transfer coefficients. It is best to obtain these from experimental data on the system if available, but caution is required when extending such data to column design, because mass transfer coefficients depend on packing geometry, liquid and gas distribution, physical properties, and gas and liquid loads, and these may vary from one contactor to another. 

Thermodynamic non-idealities are taken into account while calculating necessary physical properties such as densities, viscosities, and diffusion coefficients. In addition, non-ideal phase equilibrium behavior is accounted for. In this respect, the Elec-trolyte-NRTL model (see Section 9.4.1) is used and supplied with the relevant parameters from Ref. [50]. The mass transport properties of the packing are described via the correlations from Refs. [59, 61]. This allows the mass transfer coefficients, specific contact area, hold-up and pressure drop as functions of physical properties and hydrodynamic conditions inside the column to be determined. 

As indicated above, for purely physical absorption, the mass-transfer coefficients depend on the hydrodynamics and the physical properties of the phases. The literature contains measured values of mass-transfer coefficients and correlations (see discussion on agitated tanks and bubble columns below). Tables 19-7 and 19-8 present experimental information on apparent mass-transfer coefficients for absorption of select gases. On this basis, a tower for absorption of S02 with NaOH is smaller than that with pure water by a factor of roughly 0.317/7.0 = 0.045. Table 19-9 lists the main factors that are needed for... 

The volumetric mass transfer coefficient k-a has been used by most Investigators to characterize mass transfer capability of stirred tank reactors. It would seem preferable to be able to predict k a from separate correlations for its constituent parameters kL and a, since their values are predominantly dependent on different physical properties of the system. 

In applying the model, some mineral parameters, such as numbers, n, and mean radii, Rq of various mineral particles may be estimated by mineralogical techniques. For physical properties such as phase equilibrium constants, K, published ternary and binary data may be used on an approximate basis. Kinetic parameters such as reaction rate constants, k, or mass transfer coefficients can be very roughly estimated based on laboratory experiments. Their values may then be varied in a series of computer runs until the results match pilot plant data. A reasonably good match will, at the same time, confirm the remaining variables, rate equations and other assumptions. 

The physical properties of the gas and the mass transfer coefficient are assumed to be independent of position in the reactor. 

For development of nonequilibrium methods to continue, the calculations for mass transfer coefficients and interfacial areas required by these models will have to be added to physical property packages. Krishnamurthy and Taylor (89) present methods and recommendations for calculating the mass and energy transfer coefficients and rates. Help may be available from published manuals or supplier literature. 

This form is particularly appropriate when the gas is of low solubility in the liquid and "liquid film resistance" controls the rate of transfer. More complex forms which use an overall mass transfer coefficient which includes the effects of gas film resistance must be used otherwise. Also, if chemical reactions are involved, they are not rate limiting. The approach given here, however, illustrates the required calculation steps. The nature of the mixing or agitation primarily affects the interfacial area per unit volume, a. The liquid phase mass transfer coefficient, kL, is primarily a function of the physical properties of the fluid. The interfacial area is determined by the size of the gas bubbles formed and how long they remain in the mixing vessel. The size of the bubbles is normally expressed in terms of their Sauter mean diameter, dSM, which is defined below. How long the bubbles remain is expressed in terms of gas hold-up, H, the fraction of the total fluid volume (gas plus liquid) which is occupied by gas bubbles. 

The volumetric gas-liquid mass transfer coefficient, khaL, largely depends on power per unit volume, gas velocity (for a gassed system), and the physical properties of the fluids. For high-viscosity fluids, kLaL is a strong function of liquid viscosity, and for low-viscosity fluids (fi < 50 mPa s), kLaL depends on the coalescence nature of the bubbles. In the aeration of low-viscosity, pure liquids such as water, methanol, or acetone, a stable bubble diameter of 3-5 mm results, irrespective of the type of the gas distributor. This state is reached immediately after the tiny primary bubbles leave the area of high shear forces. The generation of fine primary gas bubbles in pure liquids is therefore uneconomical. 

The volumetric gas-liquid mass transfer coefficient, kLaL, depends upon physical properties such as viscosity, density, and surface tension of liquid. In general, aL oc Pl2/< l6- The coalescence characteristics of the vessel have a pronounced effect on aL and kLaL. The correlation presented by Judat (1982) is recommended for this purpose. Foaming characteristics can also influence kLaL. In general, the use of kLaL = f(P/V, ug) relationship is recommended for a given aerated vessel. The diameters of stirrer and vessel and the heights of stirrer and liquid level also affect kLaL. The work of Calderbank and coworkers in this area is most worth noting. 

The liquid-solid mass-transfer coefficient depends mainly on the agitation speed, the particle size, and the physical properties of the system. While ks oc N°-2 this relationship may depend on the particle size (Sano et al., 1974). In a dimensionless form, Sh oc RemSc0 5 however, the value of m changes at some critical Reynolds number when all particles are suspended. The most generalized relationship is given by Eq. (3.34), and its use is recommended. 

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