Mass transfer falling films

TABLE 5-22 Mass Transfer Correlations for Falling Films with a Free between Gas and Liquid... 

Additionally, the surfactant properties of filmers reduce the potential for stagnant, heat-transfer-resisting films, which typically develop in a filmwise condensation process, by promoting the formation of condensate drops (dropwise condensation process) that reach critical mass and fall away to leave a bare metal surface (see Figure 11.2). This function, together with the well-known scouring effect on unwanted deposits keeps internal surfaces clean and thus improves heat-transfer efficiencies (often by 5-10%). 

A pure gas is absorbed into a liquid with which it reacts. The concentration in the liquid is sufficiently low for the mass transfer to be covered by Fick s Law and the reaction is first-order with respect to the solute gas. It may be assumed that the film theory may be applied to the liquid and that the concentration of solute gas falls from the saturation value to zero across the film. The reaction is initially carried out at 293 K. By what factor will the mass transfer rate across the interface change, if the temperature is raised to 313 K ... 

Ammonia is absorbed in a falling film of water in an absorption apparatus and the film is disrupted and mixed at regular intervals as it flows down the column. The mass transfer rate is calculated from the penetration theory on the assumption that all the relevant conditions apply. It is found from measurements that the muss transfer rate immediately before mixing is only 16 pet cent of that calculated from the theory anil the difference has been attributed to the existence of a surface film which remains intact and unaffected by the mixing process. If the liquid mixing process lakes place every second, what thickness of surface film would account for the discrepancy, ... 

For the case where the ratio of the molar latent heats / is 1.5, what is the ratio of the mass transfer rale m case (b) to that in ease (a) when the mole fraction of the MVC falls front 0.75 to 0.65 across the laminar film ... 

Mass transfer efficiency by conversion anaiysis for the falling film micro reactor... 

GL 22] [R 1] [P 23] The mass transfer efficiency of the falling film micro reactor as a function of the carbon dioxide volume content was compared quantitatively (Figure 5.30) [5]. The molar ratio of carbon dioxide to sodium hydroxide was constant at 0.4 for all experiments, i.e. the liquid reactant was in slight excess. 

It is remarkable that the falling film micro reactor achieved complete conversion for all process variations applied [5]. This is unlike conventional reactor operation reported for this reaction, displaying pronounced mass transfer resistance. 

The Leveque-type correlation, Eq. (26), has been used for mass-transfer from a liquid film falling under gravity (110, W13b), where it holds for Reynolds numbers in the laminar range, in spite of the presence of surface waves on the film. The latter caused small local oscillations of the mass-... 

By substituting the well-known Blasius relation for the friction factor, Eq. (45) in Table VII results. Van Shaw et al. (V2) tested this relation by limiting-current measurements on short pipe sections, and found that the Re and (L/d) dependences were in accord with theory. The mass-transfer rates obtained averaged 7% lower than predicted, but in a later publication this was traced to incorrect flow rate calibration. Iribame et al. (110) showed that the Leveque relation is also valid for turbulent mass transfer in falling films, as long as the developing mass-transfer condition is fulfilled (generally expressed as L+ < 103) while Re > 103. The fundamental importance of the Leveque equation for the interpretation of microelectrode measurements is discussed at an earlier point. 

Falling-film column. A falling-film column (Figure 24.1(d)) is also an empty vessel with liquid, introduced at the top, flowing down the wall as a film to con-tad an upward-flowing gas stream. Ideal flow for each phase is PF. Since neither liquid nor gas is dispersed, the interfacial area developed is relatively small, and gas-liquid contact is relatively inefficient This type is used primarily in the exper-imental determination of mass transfer characteristics, since the interfacial area is w ell defined... 

Fig. 16.12 Principle and advantages of the falling film electrolyser. Advantages include improved mass and heat transfer low gas content in electrolyte low concentration difference constant hydraulic pressure low expected voltage of 2.70 V at 3 kA m-2 leading to power consumption reduction of 70 kWh per tonne of NaOH at 4kA m-2, including circulation pumps and small element depth. 

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. 

When an interfacial film has reduced the circulation within a drop, the wake vortex becomes more marked, while the extraction rate falls to that for a stagnant sphere (74) More detailed studies of the hydrodynamics of naturally moving drops have recently been carried out (75). The mass-transfer rate in 2-component systems should correlate 76) with... 

In the previous section we considered the situation where there was a film of catalyst on the wall of the tube, so that the surface area was the geometric area of the tube wall. Another common situation is where a porous catalyst film is coated on the walls of the tube as shown in Figure 7-20. Finally we consider the case where the gas-phase concentration is uniform across the cross section of the tube and mass transfer rates are large, but C4 falls within the layer of porous catalyst on the wall. The thickness of the film is f, and the surface area of the catalyst per unit volume of catalyst is Sgpc- If we assume that is much less than the tube diameter, then... 

We included the term r = 0 to indicate that there is no reaction in the gas phase. The mass transfer rates obviously have opposite signs, and we have to multiply the mass transfer flux by [areaA olume], where the volume is that occupied by that phase. Note that the mass transfer term after dividing out becomes proportional to R. Since the reactor volume is proportional to R while the surface area for mass transfer is proportional to R, the falling film column obviously becomes less efficient for larger reactor sizes. This is a fundamental problem with the falling film reactor in that small tubes give high mass transfer rates but low total production of product. 

As with the falling film reactor, the rate of mass transfer to the catalyst goes as R, while the size of the reactor goes as R, so this reactor becomes very inefficient except for very small-diameter tubes. However, we can overcome this problem, not by using many small tubes in parallel, but by allowing the gas and liquid to flow (trickle) over porous catalyst pellets in a trickle bed reactor rather than down a vertical wall, as in the catalytic wall reactor. 

The double lines in Figure 3.44 represent the Sh number based on the mass transfer coefficient, in the case of a single-particle fall in water, for three different particle densities (Harriot, 1962). This value is considered to be the minimum mass-transfer coefficient in liquid-solid films in agitated vessels. Taking into account the fact that the actual Sh value in an agitated vessel is 1.5 -8 times its minimum value, it is apparent that the mass transfer coefficients are much higher in the case of agitated vessels. 

Next we consider a fluid flowing through a circular tube with material at the wall diffusing into the moving fluid. This situation is met with in the analysis of the mass transfer to the upward-moving gas stream in wetted-wall-tower experiments. Just as in the discussion of absorption in falling films, we consider mass transfer to a fluid moving with a constant velocity profile and also flow with a parabolic (Poiseuille) profile (see Fig. 5). 

A knowledge of the velocity profiles within falling films under various flow conditions would be of very great value, making it possible to calculate the rates of convective heat and mass transfer processes in flowing films without the need for the simplified models which must be used at present. For instance, the analyses of Hatta (H3, H4) and Vyazovov (V8, V9) indicate clearly the differences in the theoretical mass-transfer rates due to the assumption of linear or semiparabolic velocity profiles in smooth... 

Fulford (F6), 1960 Brief review of heat and mass transfer to falling liquid films. 

Wetted-wall or falling-film columns have found application in mass-transfer problems when high-heat-transfer-rate requirements are concomitant with the absorption process. Large areas of open surface... 

Wasden FK, Dukler AE. A numerical study of mass transfer in free-falling wavy films. AIChE 1990 36(9) 1379-1390. 

Mass transfer in falling-film absorbers is strongly dependent on the gas velocity in the tubes, the liquid and gas distribution, and the tube surface conditions. The maximum capacity of falling-film absorbers is normally restricted either by flooding or by pressure drop. Another important limit in these absorbers is film breakup. If heat flux is excessive, dry areas may form at the tube wall and reduce mass transfer. 

A pure gas is absorbed into a liquid with which it reacts. The concentration in the liquid is sufficiently low for the mass transfer to be governed by Fick s law and the reaction is first order with respect to the solute gas. It may be assumed that the film theory may be applied to the liquid and that the concentration of solute gas falls from the saturation value to zero across the film. Obtain an expression for the mass transfer rate across the gas-liquid interface in terms of the molecular diffusivity, D, the first-order reaction rate constant ft, the film thickness L and the concentration Cas of solute in a saturated solution. The reaction is initially carried out at 293 K. By what factor will the mass transfer rate across the interface change, if the temperature is raised to 313 K Reaction rate constant at 293 K = 2.5 x 10 6 s 1. Energy of activation for reaction (in Arrhenius equation) = 26430 kJ/kmol. Universal gas constant R = 8.314 kJ/kmol K. Molecular diffusivity D = 10-9 m2/s. Film thickness, L = 10 mm. Solubility of gas at 313 K is 80% of solubility at 293 K. 

Jahnisch et al. used an IMM falling-film microreactor for photochlorination of toluene-2,4-diisocyanate [38] (see also Chapter 4.4.3.3, page 161). As a result of efficient mass transfer and photon penetration, chlorine radicals were well distributed throughout the entire film volume, improving selectivity (side chain versus aromatic ring chlorination by radical versus electrophilic mechanism) and spacetime-based yields of l-chloromethyl-2,4-diisocyanatobenzene compared to those obtained using a conventional batch reactor. 

The mass transfer efficiency of the falling-film microreactor and the microbubble column was compared quantitatively according to the literature reports on conventional packed columns (see Table 4.3) [318]. The process conditions were chosen as similar as possible for the different devices. The conversion of the packed columns was 87-93% the microdevices had conversions of 45-100%. Furthermore, the space-time yield was compared. Flere, the microdevices resulted in larger values by orders of magnitude. The best results for falling-film microreactors and the microbubble columns were 84 and 816 mol/(m3 s), respectively, and are higher than conventional packed-bed reactors by about 0.8 mol/(m3 s). 

A more detailed mass transfer study on the carbon dioxide absorption in sodium hydroxide solution was performed using a falling-film microreactor [319]. Experimental investigations were made at a liquid flow of 50ml/h, with three NaOH concentrations (0.1,1 and 2 M), at a fixed inlet molar ratio C02 NaOH of 0.4, and for a range of C02 concentration of 0.8-100%. A two-dimensional reactor model was developed, and the results are similar to the experimental data at low NaOH concentrations (0.1 and 1 M). The agreement is less pronounced for higher concentrations such as 2 M NaOH, which could be explained by either maldistribution of... 

Enantioselectivity was roughly the same for the three reactors, being 80-90 and 62-65% for the Rh/Josiphos and Rh/Diop catalysts, respectively [266]. Conversion was very different. For fixed reaction time, the batch reactor and the falling-film microreactor had higher conversions than the Caroussel reactor. This was indicative of operation under mass transfer regime in the latter. On the basis of these data, it was concluded that the mass transfer coefficients kya of the helical falling-film microreactor are in between the boundaries given by the known kta values of 1-2 s 1 for small batch reactors and about 0.01 s-1 for the Caroussel reactor. 

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