Mass transfer liquid-film control

The transfer of mass between phases in a packed tower occurs either as essentially all gas film controlling, all liquid film controlling, or some combination of these mechanisms (see Figure 9-69). To express the ease (low number... 

Overall mass transfer coefficient based on liquid phase, lb mol/ (hr) (ft ) (lb mol/ft ) Overall mass transfer coefficient based on liquid film controlling, lb mol (hr) (ft ) (lb mol/ft3)... 

Two extreme cases of equation 9.2-22 or -22a or -22b arise, corresponding to gas-film control and liquid-film control, similar to those for mass transfer without chemical reaction (Section 9.2.2). The former has implications for the location of the reaction plane (at distance 8 from the interface in Figure 9.6) and the corresponding value of CB. These points are developed further in the following two examples. 

For liquid-film control, there is no gas-phase resistance to mass transfer of A. Thus, in equation 9.2-14, l/kA( l/HAkAg, and KA( = kM, so that equation 9.2-22b may be... 

Flow through the liquid film controls the mass transfer if HA > 250 atm (mole fraction)-1. 

Regimes 2 and 3 - moderate reactions in the bulk (2) or in thefdm (3) and fast reactions in the bulk (3) For higher reaction rates and/or lower mass transfer rates, the ozone concentration decreases considerably inside the film. Both chemical kinetics and mass transfer are rate controlling. The reaction takes place inside and outside the film at a comparatively low rate. The ozone consumption rate within the film is lower than the ozone transfer rate due to convection and diffusion, resulting in the presence of dissolved ozone in the bulk liquid. The enhancement factor E is approximately one. This situation is so intermediate that it may occur in almost any application, except those where the concentration of M is in the micropollutant range. No methods exist to determine kLa or kD in this regime. 

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... 

Catalyst particles are small, so less chance of diffusional resistance to mass transfer. (2) Better control of temperature (because of better heat transfer efficiency and high heat capacity of slurries), attractive for exothermic reactions. (3) No need to shut down for catalyst replacement or reactivation. (4) Partial wetting and need to maintain a coating film of liquid (as needed in the trickle bed) are not issues. (5) The space time yield is usually better in slurry reactors (under comparable conditions). 

First, the overall mass transfer coefRcient k a of the microreactor was estimated to be 3-8 s [43]. For intensified gas liquid contactors, kj a can reach 3 s while bubble columns and agitated tanks do not exceed 0.2 s Reducing the flow rate and, accordingly, the liquid film thickness is a means of further increasing kj a, which is limited, however, by liquid dry-out at very thin films. Despite such large mass transfer coefficients, gas-liquid microreactors such as the falling film device may still operate between mass transfer and kinetic control regimes, as fundamental simulation studies on the carbon dioxide absorption have demonstrated [44]. Distinct concentration profiles in the liquid, and even gas, phase are predicted. 

If the liquid film has the controlling resistance to mass transfer, gas-film coefficients could still be used for design calculations following Eq. (22.45). If liquid-film coefficients are used, and if the factor (1 — x)i is introduced to allow for... 

Kolev, N., and Nakov, S. (1994), Performance characteristics of a packing with boundary layer turbulizers. m. Liquid film controlled mass transfer, Chemical Engineering and Processing, 33(6) 437-442. 

For this test system the K a value was found to increase at slightly less than the 0.10 power of the liquid flow rate. This variation of mass transfer co-efficient with liquid rate is much less than for random dumped packings where K< a varies as the 0.22 to 0.34 power of liquid rate for this liquid-film controlled system (see Chapter 3). This variation of Koa with liquid rate confirms that Intalox structured packings are wetted to a greater extent at low liquid rates than random packings. 

As previously stated in Chapter 3, the mass transfer rate is considered to be determined by the gas-film resistance operating in series with the liquid-film resistance. The film that offers the predominant mass transfer resistance is called the controlling film. Contaminants that have a limited solubility in the liquid phase (a high value of m) usually are considered to be a liquid-film-controlled system. Contaminants that are highly soluble in the liquid phase (a low value of m) usually are considered to be a gas-film-controlled system. 

In a gas-film-controlled system, the effect of liquid rate on the overall Koa value is essentially the same as for a liquid-film-controlled system, such as that shown in Table 5-6. In addition, in such a system the gas rate has a significant effect on the overall mass transfer coefficient. Similarly, Figures 5-4, 5-5, and 5-6 allow a rapid determination of the packed depth required for fume scrubbers in gas-film-controlled systems. In these plots, the overall K a value has been corrected for the effect of the gas flow rate. Because the K a value increases with an increasing gas flow rate, the solute removal efficiency drops only slightly for a fixed packed depth as the gas rate is increased in such a system. 

Air-stripping of water is a liquid-film-controlled mass transfer operation. First, it is necessary to select the air-to-water ratio so that the stripping factor can be calculated. Usually 350 to 750 SCFM of air is used per 100 gpm of water feed. The stripping factor is given by Equation 5-19 using the molar flows of air and water. 

Liquid-film-controlled—Mass transfer operation in which the principal resistance is in the liquid film. 

The gas-phase resistance of mass transfer is neglected, and the diffusion is liquid film control. The liquid phase is pure ethanol ... 

The absorption of carbon dioxide in water has been shown to be almost entirely liquid-film controlled—presumably because of the relatively low solubility of carbon dioxide. Considerable research has, therefore, been conducted on the CO2-H2O system in connection with both absorption and desorption to determine the liquid-frlm resistance to mass transfer when various packings are used. Some of the data obtained are directly applicable to (he design of commercial installations for carbon dioxide absorption and desorption. 

The investigations for determination of the liquid-film controlled mass transfer coefficient are carried out [ISS] with the packings presented in Table 31. The results, some presented in Fig. 76, show that reduction of both the height of the blocks and the distance between the turbulizers leads to inerting... 

To preset the influence of the effect of the turbulizers on the liquid-film controlled mass transfer coefficient, the following equation is proposed [156] ... 

Fig. 103. Dependence between the liquid-film controlled volumetric mass transfer coefficient for Holpack packings and the liquid superficial velocity. The symbols are presented in Table 35. 1-...

Fig. 104. Comparison of the experimental data for the liquid-film controlled mass transfer coefficient of the packings pre t in Table 35 with the line calculated using equation (217).

The transfer process is termed gas film controlling if essentieilly all of the resistance to mass transfer is in the gas film. This means that the gas is usually quite soluble in, or reactive with, the liquid of the system. If the system is liq-... 

Increase in mass-transfer rate per unit area. As stated above, agitated gas-liquid contactors are used, in general, when it is necessary to deal with sparingly soluble gases. According to the terminology of the film theory, absorption is then controlled by the liquid resistance, and agitation of the liquid phase could increase the mass-transfer rate per unit area. As will be... 

In evaluating their results they assumed the film theory, and, because the oxygen is sparingly soluble and the chemical reaction rate high, they also assumed that the liquid film is the controlling resistance. The results were calculated as a volumetric mass-transfer coefficient based, however, on the gas film. They found that the volumetric mass-transfer coefficient increased with power input and superficial gas velocity. Their results can be expressed as follows ... 

Contrary to RPBRs, in SDRs, intensified heat transfer presents the most important advantage. Liquid reactant(s) are fed on the surface of a fast rotating disk near its center and flow outward. Temperature control takes place via a cooling medium fed under the reaction surface. The rotating surface of the disc enables to generate a highly sheared liquid film. The film fiow over the surface is intrinsically unstable and an array of spiral ripples is formed. This provides an additional improvement in the mass and heat transfer performance of the device. 

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