Packings liquid-film controlled mass

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. 

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... 

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 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... 

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. 

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. 

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 gas-film controlled mass transfer coefficient is investigated [1S6] for all packings presented in Table 31. The investigations are carried out in the system sulphuric acid water solution for the liquid phase and ammonia-air far the gas. In all cases the influence of the end effect is taken into ffixounL... 

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-...

The investigations of the gas-film controlled mass transfer of Holpack are carried out by Daraktschiev, Kolev and Tschapkanova [185] for the packings presented in Table 35. As a model process the absorption of KH3 is used. The values of ka for the packing Nr 2 obtained for two different liquid superficial velocities, 0.0039 and 0.017 m /(m s), are presraited in Fig. 105. They show that the influence of I is to be neglect. ... 

The comparison of fte liquid-side mass transfer coefficients of Holpack and of the packing of Kolar at equal liquid superficial velocities shows that they are significantly higher for the Holpack. The maximal values of these coefficients are approximately fte same. The comparison of fte gas-film controlled mass transfer coefficients shows also that fte values for fte Holpack at fte same gas velocity are quite higher. The comparison of fte maximal values for both types of packings shows that they are practically fte same. 

Well mixing of gas and liquid phases is a dear issue. Smaller, finer packing allows us to increase the surface area between the gas and liquid contact, for example, the mass transfer could be increased in a column with very fine packing with counter-current gas flow. However, a liquid film miming through a bed of fine material is problematic when the liquid film thickness is around the same as the clearance between the bits of packing. Liquid flow essentially stops and the column floods. The key is the thickness of the liquid film and the factors that control that. The equations describing the problem indicate that most factors relate to the physical properties of... 

Because Intalox structured packing 2T has an absorption efficiency greater than 1-in. metal Pall rings, this packing will be evaluated. At a fixed liquid rate, the mass transfer coefficient will increase at the 0.75 power of the gas rate for this gas-film-controlled absorption (see Chapter 3). At 10,800 CFM air flow, the mass transfer coefficient for Intalox structured packing 2T will be more than sufficient to handle the absorption of an additional 50% of acetic acid vapor with the same mass transfer driving force and packed depth. The inlet air has a density of 0.0728 Ib/ft, while the inlet liquid has a density of 62.2 Ib/ft and a viscosity of 0.81 cps. Because only 232 Ib/h of acetic acid vapor for three trains must be removed by the scrubber, the physical properties of the gas and liquid streams do not change from top to bottom of this tower. The flow parameter at the bottom of the scrubber will be ... 

The three towers usually are sized to a common diameter that will give a pressure drop of 0.10 in. to 0.15 in. H20/ft of packed depth, because the overall pressure drop desired for the entire drying system is only 4 in. to 5 in. H2O. This process largely is gas-film-controlled, as expected. It is desirable to keep the packed depth in each of the three columns the same. To accomplish this configuration, an iterative design procedure is necessary. The driving force for mass transfer is the difference between the partial pressure of water vapor in the gas stream and the vapor pressure of water above the liquid phase. Because of the high liquid flow rate, as a first approximation, the acid concentration and temperature in eath tower can be assumed constant in order to establish the amount of water vapor removed in each column, and thereby the acid concentration. 

Relative Kga valid for all systems controlled by mass transfer coefficient (Kg) and wetted area (a) per unit volume of column. Some variation should be expected when liquid reaction rate is controlling (not liquid diffusion rate). In these cases liquid hold-up becomes more important. In general a packing having high liquid hold-up which is clearly greater than that in the falling film has poor capacity. 

Packed distillation towers can often be operated over a moderate range of flow rates at nearly constant separation efficiency. Data for isooctane-toluene separation at total reflux are shown in Fig. 22.25. The three Intalox metal (IMTP) packings numbered 25, 40, and 50 correspond to nominal sizes of 1,1.5, and 2 in., respectively. As the capacity parameter increases, both the liquid rate and the vapor rate increase, which explains why HETP is nearly constant. The gas film has the controlling resistance to mass transfer, and Hoy increases with the 0.3 to... 

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