Mass transfer coefficients agitated vessels

Stenberg, O., and Andersson, B. (1988a), Gas-liquid mass transfer in agitated vessels-I. Evaluation of the gas-liquid mass transfer coefficient from transient-response measurements, Chemical Engineering Science, 43(3) 719-724. 

Interfacial Area This consideration in agitated vessels has been reviewed and summarized by Tatterson (op. cit.). Predictive methods for interfacial area are not presented here because correlations are given for the overall volumetric mass transfer coefficient liquid phase controlhng mass transfer. 

Measurements simply of the extent of extrac tion in an agitated vessel lead to the overall Volumetric mass-transfer coefficients, Kca, or... 

The mass transfer coefficient is expected to relate gas power per unit volume and gas terminal velocity. Measurement of gas bubble velocity is troublesome in the experimental stage of aeration. Extensive research has been conducted for an explanation of the above correlation. Gas-liquid mass transfer in low viscosity fluids in agitated vessels has been reviewed and summarised as stated in (3.5.1.7)—(3.6.2) 3... 

Acording to Fishwick et al. (2003), the injection of gas in a baffled vessel leads to a decrease in the mass transfer coefficients and this effect becomes more intense at higher gas rates. The significance of gas dispersion is, however, less pronounced at higher agitation speeds. It is also observed that under high agitation speeds in baffled vessels, a considerable amount of ah is dispersed inside the vessel even in the absence of an injected gas. 

The double lines in Figure 3.44 represent the Sh number based on the mass transfer coefficient, in the case of a single-particle fall in water, for three different particle densities (Harriot, 1962). This value is considered to be the minimum mass-transfer coefficient in liquid-solid films in agitated vessels. Taking into account the fact that the actual Sh value in an agitated vessel is 1.5 -8 times its minimum value, it is apparent that the mass transfer coefficients are much higher in the case of agitated vessels. 

Agitated vessels (liquid-solid systems) Below the off-bottom particle suspension state, the total solid-liquid interfacial area is not completely or efficiently utilized. Thus, the mass transfer coefficient strongly depends on the rotational speed below the critical rotational speed needed for complete suspension, and weakly depends on rotational speed above the critical value. With respect to solid-liquid reactions, the rate of the reaction increases only slowly for rotational speed above the critical value for two-phase systems where the sohd-liquid mass transfer controls the whole rate. When the reaction is the ratecontrolling step, the overall rate does not increase at all beyond this critical speed, i.e. when all the surface area is available to reaction. The same holds for gas-liquid-solid systems and the corresponding critical rotational speed. 

Keywords Gas-liquid contactor Bubble column Agitated vessel Mass transfer coefficient Viscosity Surfactants... 

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. 

Volumetric mass-transfer coefficients for mechanically agitated fermenters with air sparged into the vessel below the agitator are also reviewed by Joshi et al. (1982) and Schugerl (1981). The effects of physical properties of fluids on the volumetric mass-transfer coefficient have been investigated by Miller (1974), Zlokarnik (1978), Yagi and Yoshida (1975), and Henzler (1980). Yagi and Yoshida (1975) developed the correlation... 

Zlokarnik (1978) examined the effect of coalescence on the volumetric mass-transfer coefficient in agitated vessels. 

For mold pellets and other suspended particles with densities close to that of the continuous phase, the agitation in a stirred mixing vessel creates the dominant force for relative fluid motion between the two phases. The intrinsic gas-liquid mass-transfer coefficient under these conditions is given by Calderbank (1967) as... 

Perez and Sandall (1974) studied the absorption of carbon dioxide in aqueous carbopol solution. The rheological behavior of the solution was described by the power law model with flow behavior indices varying from 0.91 to 0.59. For an agitated vessel with a turbine impeller, the mass-transfer coefficient across the unbroken interface was correlated as... 

The gas-liquid volumetric mass-transfer coefficient for the agitation of power-law fluid in an aerated vessel can be expressed in the form kLaL = f PJV, ug) (Hocker et al, 1981). For the mass transfer in a vessel with an unbroken interface, the relationship Sh = /(Re, Sc) given by Eq. (7.4) is recommended. 

Oyama and Endoh (012) studied the solution of sugar in water in 6.7- and 10.8-in. baffled vessels using paddles and flat-blade turbines. They report a mass-transfer coefficient which was proportional to the cube root of the particle diameter and to the cube root of the impeller power consumption per unit mass of agitated liquid. 

To integrate d with t, we must relate the mass transfer coefficient (k) to the independent parameters of the system. Levins and Glastonbury (1972) have developed an accurate correlation to predict mass transfer coefficients for suspended particles in agitated vessels. 

Agitation mainly affects the a in kifl, since it represents the total interfacial area per unit volume of gas plus liquid. It therefore has units of length. The units of kifl are usually expressed in s or min. Typical ki a values for agitated vessels lie between 0.05 to 0.4 s. The mass-transfer coefficient is one of the resistances to transport of species (i) from the gas phase inside the bubble to the bulk fluid outside the bubble. The overall resistance 1/A l is the sum of the inside and outside resistances shown in Equation (9.39), where E is the equilibrium constant. In the great majority of cases, l- This implies that kQ is small compared with k and means that the liquid film resisfance outside the bubble is controlling ... 

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