Aeration mass transfer coefficient

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... 

Calculate mass transfer coefficient in a 60 m3 fermenter with a gas and liquid interfacial area of a = 0.3 m2-m 3, given pbroth = 1200kg m-3. The small reactor has working volume of 0.18m3, 1 vvm aeration rate. Oxygen transfer rate (OTR) is 0.25kmol in 3 h 3. There are two sets of impellers, and flat-blade turbine types of impeller were used, HL= 1.2/),. Find the exact specifications of a large fermenter. 

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

The PULSAR units are high efficiency static aerators that have been developed for municipal wastewater treatment plants and have successfully been used over extended periods of time without any operational problems such as unstable operation or plugging up during intermittent operation of the air pumps (Chourda-kis, 1999). Data have been collected from a pilot plant unit at the Wastewater Treatment plant of the Industrial Park (Herakleion, Crete). A series of experiments were conducted for the determination of the mass transfer coefficient (kLa) and are shown in Figure 17.4. The data are also available in tabular form as part of the parameter estimation input files provided with the enclosed CD. 

The solid-liquid mass transfer coefficient without aeration is a function of power consumption per unit volume of the liquid. One typical case is the Levins and Glastonbury correlation for small particles (<2 mm), fully suspended and moderate density differences (Treybal, 1980) ... 

One more correlation is that of Calderbank-Moo-Young for the solid-liquid mass transfer coefficient in stirred tanks without aeration (Kato et al., 2001)... 

As has been noted elsewhere, the aeration of liquid is generally not desirable. This is why the gas phase that is employed for the reaction is mixed with the air dissolved due to surface aeration, and thus the mass-transfer coefficient decreases due to the reduction of the partial pressure of the reacting gas in this gas-air mixture. Calderbank proposed an equation for the minimum stilling rate for surface aeration for gas-liquid systems (Panja and Phaneswara Rao, 1993) ... 

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. 

An aerated stirred-tank fermenter equipped with a standard Rushton turbine of the following dimensions contains a liquid with density p = 1010kgm and viscosity n = 9.8 X 10 Pa s. The tank diameter D is 0.90 m, liquid depth Hl = 0.90 m, impeller diameter d = 0.30 m. The oxygen diffusivity in the liquid Dl is 2.10 X 10 5 cm- s T Estimate the stirrer power required and the volumetric mass transfer coefficient of oxygen (use Equation 7.36b), when air is supplied from the tank bottom at a rate of 0.60 m min at a rotational stirrer speed of 120 rpm, that is 2.0 s T... 

Historically, the alpha factor, a, was developed from oxygen mass transfer studies in the aerated basins of municipal waste water treatment plants. It thus denotes the ratio of the mass transfer coefficient for oxygen measured in the waste water (WW) to that measured in tap water (TP). 

For aerated mixing vessels in an aqueous solution, the mass-transfer coefficient is proportional to the power consumption (Lopes De Figueiredo and Calderbank, 1978) as... 

A. Schumpe, W.D. Deckwer, Gas holdups, specific interfacial areas, and mass transfer coefficients of aerated carboxymethyl cellulose solutions in a bubble column, I EC Process Des. Develop. 21 (1982) 706-711. 

Gas holdup and volumetric gas-liquid mass-transfer coefficients are correlated with the gassed power input/volume and with the aeration rate (actual gas superficial velocity), e.g., the correlation of van t Riet [Ind. Eng. Chem. Proc. Des. Dev. 18 357 (1979)] for the volumetric mass-transfer coefficient of coalescing and noncoalescing systems ... 

The determination of the overall mass transfer coefficient, kLa, using 02 electrodes has an accuracy of 5 %. This suffices to determine the process characteristic in the so-called volume aeration in stirring vessels, as has been demonstrated by a comparative evaluation of measurements in vessel sizes covering liquid volumes from V = 2.5 liters to 906 m3 [53]. 

In surface aeration, the absorption rate is also measured with 02 electrodes in the liquid volume. By this method, the liquid-side overall mass transfer coefficient, kLa, is determined (a - volume-related mass transfer area = surface of all gas bubbles in the liquid volume). Due to the fact that the mass transfer in surface aeration occurs almost solely in the liquid surface, A, and by no means in the liquid volume, V, the measured kLa has to be multiplied by V to obtain the target quantity kLA = kLa V. 

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 surface aerator has been used for decades in biological waste-water treatment with H < 4 m. For various sizes of turbine stirrers whose disks were positioned exactly in the surface of the liquid, the mass-transfer coefficient can also be related 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. 

The experimental technique involves batch gas absorption (by surface aeration) in a liquid. The pressure of the enclosed gas phase in the reactor decreases with time because of the absorption. This decrease in pressure with time allows the estimation of the mass-transfer rate and the volumetric mass-transfer coefficient, kLaL. The total pressure decrease until equilibrium is reached gives the equilibrium solubility C. The relevant equations for the calculations of C and kLaL are derived by Albal et al. (1983), Deimling et al. (1985), and Karandikar et al. (1986). These can be expressed as... 

An experiment to determine the overall mass transfer coefficient of a tap water is performed using a settling column 4 m in height. The result of the unsteady state aeration test is shown below. The experiment is performed in Allegheny County, MD. For practical purposes, assume mass density of water = 1000 kg/m Assume an ambient temperature of 28°C. Calculate KiU. 

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