Overall gas—liquid mass transfer

An important relatively recent achievement is the development of the so-called liquid entrained reactor (LER) for liquid phase methanol synthesis. To assess the performance of these reactors, much simultaneous work has been done on MARs, comparing the performance of the two types of reactors for this reaction. A good discussion of this can be found in a paper by Vijayaraghavan et a. (1993). An equation developed for the overall gas-liquid mass transfer coefficient (see Ko, 1987 Lee et al., 1988 Parameswaran et al., 1991) is... 

Gas-Liquid Mass-Transfer and Reaction Rates. Experimentally, one of the easiest ways to determine the overall gas-liquid mass-transfer coefficient in unsparged agitated tanks is measurement of pressure drop, where Ki a is the mass-transfer coefficient (s ), Cg is the instantaneous gas concentration in the liquid, and Cg is the equilibrium gas concentration in the liquid. This is also commonly known as the dynamic method (62), and it has been used in catalytic work (63). 

In sparged agitated tanks, a considerable increase in interfacial area occurs and overall gas-liquid mass transfer is considerably higher. Many correlations indicate that the overall mass transfer increases approximately as volumetric rate to the 1/2 power. Studies of nonelectroljdic solutions as well as electrolytic solutions have been performed (78). Correlations are published for the critical agitation speed for impeller flooding (79). Correlations for multistage stirred tanks are also available. 

In two-phase downflow and upflow fixed-bed reactors, gas-liquid mass transfer resistance can be detrimental to the overall reactor performance [14, 32]. Therefore, accurate estimation of gas-liquid mass transfer parameters is important for achieving successful reactor design or scale-up. The overall gas-liquid mass transfer coefficient may be expressed, according to the two-film concept, in terms of the liquid-side and the gas-side mass transfer coefficients ... 

Liquid-solid mass-transfer coefficient of compound i, cm / (cm s) Overall gas-liquid mass-transfer coefficient of compound i in the liquid... 

Gas-Liquid Mass Transfer. Gas-liquid mass transfer within the three-phase fluidized bed bioreactor is dependent on the interfacial area available for mass transfer, a the gas-liquid mass transfer coefficient, kx, and the driving force that results from the concentration difference between the bulk liquid and the bulk gas. The latter can be easily controlled by varying the inlet gas concentration. Because estimations of the interfacial area available for mass transfer depends on somewhat challenging measurements of bubble size and bubble size distribution, much of the research on increasing mass transfer rates has concentrated on increasing the overall mass transfer coefficient, kxa, though several studies look at the influence of various process conditions on the individual parameters. Typical values of kxa reported in the literature are listed in Table 19. 

Kl = the overall gas-phase mass transfer coefficient as defined in equation (3.59) kf = the liquid-solid mass transfer coefficient... 

The gas-liquid mass transfer for organic solutions and the liquid-solid mass transfer are evaluated using the appropriate correlations (eqs. (3.427) and (3.435), respectively), while the Fogler s overall coefficient (K°A) is (eq. (3.379))... 

Furthermore, a liquid-phase distributor is used on the top of the bed and the overall gas-phase mass transfer coefficient was experimentally measured as 0.153 s 1 for liquid flow rate equal to 14 X 10 5 m3/s. Under these conditions, the experimental value of sulfur dioxide conversion was approximately 18%. 

Now, we shall define the overall coefficients. Even in the case when the concentrations at the interface are unknown (cf. Figure 6.1), we can define the overall driving potentials. Consider the case of gas absorption the overall coefficient of gas-liquid mass transfer based on the liquid phase concentrations (mh ) is defined by... 

Similarly, the overall coefficient of gas-liquid mass transfer based on the gas concentrations Kq (m h ) can be defined by... 

The overall coefficients of liquid-liquid mass transfer are important in the calculations for extraction equipment, and can be defined in the same way as the overall coefficients of gas-liquid mass transfer. In liquid-liquid mass transfer, one component dissolved in one liquid phase (phase 1) will diffuse into another liquid phase (phase 2). We can define the film coefficients /C i (nr h" ) and k (m h ) for phases 1 and 2, respectively, and whichever of the overall coefficients A (m h ), defined with respect to phase 1, or Al2 (mh ) based on phase 2, is convenient can be used. Relationships between the two film coefficients and two overall coefficients are analogous to those for gas-liquid mass transfer that is,... 

A gas component A in air is absorbed into water at latm and 20 °C. The Henry s law constant of A for this system is 1.67 X 10 Pa m kmol h The liquid film mass transfer coefficient and gas film coefficient I(q are 2.50x10 and 3.00 X10" ms respectively, (i) Determine the overall coefficient of gas-liquid mass transfer (ms ). (ii) When the bulk concentrations of A in the gas phase and liquid phase are 1.013 X 10 Pa and 2.00 kmol m , respectively, calculate the molar flux of A. 

Finally, in Fig. 3.4-12 [24], a comparison is given for the overall, gas-based, mass transfer coefficient for several liquid-to-gas and solid-to-gas packed beds and column systems. In Fig. 3.4-12, for a given data point, the vertical distance up to the Tan et al. [27] correlation (which is for a solid-to-fluid boundary layer) would provide a measure of the liquid-side mass-transfer resistance associated with the liquid. This is so because amount of the large gas... 

Fig. 3.4-12. Comparison of the overall, gas-based, mass-transfer coefficient for several liquid-to-gas and solid-to-gas packed beds and column systems [24] with permission of Pergamon Press...

The approach followed by Prugh (1986) is useful in estimating the overall effectiveness of a specific water-spray curtain, but important parameters such as drop size and gas-liquid mass transfer are not accounted for. 

In many practical applications, gas-liquid mass transfer plays a significant role in the overall chemical reaction rate. It is, therefore, necessary to know the values of effective interfacial area (aL) and the volumetric or intrinsic gas-liquid mass transfer coefficients such as kLah, kL, ktaL, kg, etc. As shown in Section IX, the effective interfacial area is measured by either physical e.g., photography, light reflection, or light scattering) or chemical methods. The liquid-side or gas-side mass-transfer coefficients are also measured by either physical (e.g., absorption or desorption of gas under unsteady-state conditions) or chemical methods. A summary of some of the experimental details and the correlations for aL and kLaL reported in the literature are given by Joshi et al. (1982). In most practical situations, kgaL does not play an important role. 

Gas-liquid mass transfer can have a strong effect on TBR overall performance therefore its accurate evaluation is essential for achieving successful design and scale-up. In spite of the vast information available on gas-liquid mass transfer characteristics of atmospheric TBRs [1,2] only a few researchers have studied how interfacial areas, a, and volumetric liquid-side mass transfer coefficients, kLa, evolve at elevated pressures. For example, it has been reported that both a and kLa increase as gas density is rised while the gas superficial velocity is kept constant [3-5], Similar observations regarding gas hold-up and two-phase pressure drop, as well as the delay in the onset of pulsing have also been reported [6],... 

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. 

Gas-liquid mass transfer is commonly modeled in terms of a gas film (between the bulk gas and interface) and a liquid film (between the interface and bulk liquid). Hindrance to mass transfer causes soluble gas (e.g., O2) concentrations to decrease across these films. The highest mass transfer resistance usually exists in the liquid film therefore, it controls the overall oxygen transfer rate (OTR). In aerobic fermentation, an effective fermenter design achieves an efficient OTR through intimate gas-liquid contact. OTR is described in terms of oxygen concentration and characteristics of the gas-liquid interface, as follows ... 

The film mass transfer coefficient can be extracted from the overall mass transfer coefficient when the experiments are carefully designed, so that the gas-liquid mass transfer is predominantly controlled by... 

Calculate the number of overall gas phase mass transfer units for acetone, equivalent to one tray in the absorber column of Example 14.1. The K-value of acetone and its diffusion coefficients in the liquid and vapor are given ... 

A model has been developed for oxidation of calcium sulfite in a three-phase, semibatch reactor, The overall rate of conversion to sulfate depends on the rates of solid dissolution and liquid phase chemical reaction. In this first treatment of the problem, gas-liquid mass transfer resistance did not affect the overall rate of oxidation. 

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