Volumetric gas-liquid mass transfer

The volumetric gas-liquid mass transfer coefficient ki a) has been obtained by fitting the concentration profile of dissolved oxygen to the axial dispersion model [8, 18]. The value of... 

The proposed catalyst loading, that is the ratio by volume of catalyst to aniline, is to be 0.03. Under the conditions of agitation to be used, it is estimated that the gas volume fraction in the three-phase system will be 0.15 and that the volumetric gas-liquid mass transfer coefficient (also with respect to unit volume of the whole three-phase system) kLa, 0.20 s-1. The liquid-solid mass transfer coefficient is estimated to be 2.2 x 10-3 m/s and the Henry s law coefficient M = PA/CA for hydrogen in aniline at 403 K (130°C) = 2240 barm3/kmol where PA is the partial pressure in the gas phase and CA is the equilibrium concentration in the liquid. 

The hydrodynamic parameters that are required for stirred tank design and analysis include phase holdups (gas, liquid, and solid) volumetric gas-liquid mass-transfer coefficient liquid-solid mass-transfer coefficient liquid, gas, and solid mixing and heat-transfer coefficients. The hydrodynamics are driven primarily by the stirrer power input and the stirrer geometry/type, and not by the gas flow. Hence, additional parameters include the power input of the stirrer and the pumping flow rate of the stirrer. 

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 ... 

Correlations for gas holdup and the volumetric gas-liquid mass-transfer coefficient can have the general form... 

Increasing the catalyst loading decreases the gas holdup and the volumetric gas-liquid mass transfer coefficient [see, e.g., Maretto and Krishna, Catalysis Today, 52 279 (1999)]. 

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. 

For baffled agitators, sparged with submerged impellers, some of the useful correlations for the gas-liquid interfacial area aL and the volumetric gas-liquid mass-transfer coefficient are outlined in Table XXL The correlations are liquid mass-transfer coefficient are outlined in Table XXI. The correlations are valid under nonflooding conditions (i.e., low gas flow rate). 

Determination of the Volumetric Gas-Liquid Mass Transfer Coefficients at Pressures up to 5 MPa... 

The reported study on gas-liquid interphase mass transfer for upward cocurrent gas-liquid flow is fairly extensive. Mashelkar and Sharma19 examined the gas-liquid mass-transfer coefficient (both gas side and liquid side) and effective interfacial area for cocurrent upflow through 6.6-, 10-, and 20-cm columns packed with a variety of packings. The absorption of carbon dioxide in a variety of electrolytic and ronelectrolytic solutions was measured. The results showed that the introduction of gas at high nozzle velocities (>20,000 cm s ) resulted in a substantial increase in the overall mass-transfer coefficient. Packed bubble-columns gave some improvement in the mass-transfer characteristics over those in an unpacked bubble-column, particularly at lower superficial gas velocities. The value of the effective interfacial area decreased very significantly when there was a substantial decrease in the superficial gas velocity as the gas traversed the column. The volumetric gas-liquid mass-transfer coefficient increased with the superficial gas velocity. 

Lemcoff and Jameson71 measured the volumetric gas-liquid mass-transfer coefficient during hydrogenation of acetone in a vibrating slurry reactor. They correlated the data obtained with Raney nickel Nicat 102 catalyst (92 percent nickel) to the temperature (in the range 7 through 21 °C) and the frequency of oscillation /. The correlation is graphically illustrated in Fig. 9-25 and analytically-represented by the equation... 

Just as with the gas holdup, gas-liquid interfacial area should also be divided into two parts. The literature, however, gives a unified correlation. The same is true for volumetric gas-liquid mass transfer coefficients and mixing parameters for both gas and liquid phases. The fundamental r.echanism for inter-phase mass transfer and mixing for large bubbles is expected to be different from the one for small bubbles. Future work should develop a two phase model for the bubble column analogous to the two phase model for fluidized beds. 

Figure 5 Effects of superficial (a) gas and (b) liquid velocities on volumetric gas-liquid mass transfer coefHcients, in monolith reactors with different channel sizes. (From Ref. 18.)...

The volumetric gas-liquid mass transfer coefficient in an airlift reactor increases with gas velocity and its holdup. Based on the penetration theory and the isotropic turbulence theory, the following theoretical... 

However, for a given gas velocity, any change in gas or liquid properties, downcomer and riser geometry, phase separation conditions, liquid volume, reactor height, or gas distribution causes changes in liquid velocity and gas holdup. Therefore, no generalized model or correlation for the volumetric gas-liquid mass transfer coefficient in airlift reactors exists. 

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