Mass transfer wetted wall

FIG, 14-77 Mass-transfer coefficients versus average gas velocity—HCl absorption, wetted-wall column. To convert pound-moles per hour-square foot-atmosphere to Idlogram-moles per second-square meter-atmosphere, multiply by 0.00136 to convert pounds per hour-square foot to kilograms per second-square meter, multiply by 0.00136 to convert feet to meters, multiply by 0.305 and to convert inches to milhmeters, multiply by 25.4. [Dohratz et at, Chem. Eng. Prog., 49, 611 (1953).]... 

D( = diffusion coefficient of solute in liquid g = gravity-acceleration constant h = length of wetted wall kf = mass-transfer coefficient, liquid phase r = mass rate of flow of hqnid. f = viscosity of liquid = density of hqnid... 

Wetted wall column An experimental apparatus used to determine the mass transfer that takes place through laminar boundary layers. 

In an experimental wetted wall column, pure carbon dioxide, is absorbed in water. The mass transfer rate is calculated using the penetration theory, application of which is limited by the fact that the concentration should not teach more than 1 per cent of the saturation value at a depth below the surface at which the velocity is 95 per cent of the surface velocity. What is the maximum length of column to which the theory can be applied if the flowrate of water is 3 cm3/s per cm of perimeter ... 

Thus either the penetration theory or the film theory (equation 10.144 or 10.145) respectively can be used to describe the mass transfer process. The error will not exceed some 9 per cent provided that the appropriate equation is used, equation 10.144 for L2 jDt > n and equation 10.145 for L2/Dt < n. Equation 10.145 will frequently apply quite closely in a wetted-wall column or in a packed tower with large packings. Equation 10.144 will apply when one of the phases is dispersed in the form of droplets, as in a spray tower, or in a packed tower with small packing elements. 

Figure 10.14. Mass transfer in wetted-wall columns...

Laboratory reactors for studying gas-liquid processes can be classified as (1) reactors for which the hydrodynamics is well known or can easily be determined, i.e. reactors for which the interfacial area, a, and mass-transfer coefficients, ki and kc, are known (e.g. the laminar jet reactor, wetted wall-column, and rotating drum, see Fig. 5.4-21), and (2) those with a well-defined interfacial area and ill-determined hydrodynamics (e.g. the stirred-cell reactor, see Fig. 5.4-22). Reactors of these two types can be successfully used for studying intrinsic kinetics of gas-liquid processes. They can also be used for studying liquid-liquid and liquid-solid processes. 

The proposals made for calculating transfer coefficients from physical data of the system and the liquid and vapour rates are all related to conditions existing in a simpler unit in the form of a wetted-wall column. In the wetted-wall column, discussed in Chapter 12, vapour rising from the boiler passes up the column which is lagged to prevent heat loss. The liquid flows down the walls, and it thus provides the simplest form of equipment giving countercurrent flow. The mass transfer in the unit may be expressed by means of the j-factor of Chilton and Colburn which is discussed in Volume 1, Chapter 10. Thus ... 

Gilliland and Sherwood s data118, expressed by equation 12.23, are shown in Figure 12.4 for a number of systems. To allow for the variation in the physical properties, the Schmidt Group Sc is introduced, and the general equation for mass transfer in a wetted-wall column is then given by ... 

In early work on wetted-wall columns, Morris and Jackson,2I) represented the experimental data for the mass transfer coefficient for the gas film hD in a form similar to equation 12.25, though with slightly different indices, to give ... 

Mass transfer controlled by diffusion in the gas phase (ammonia in water) has been studied by Anderson et al. (A5) for horizontal annular flow. In spite of the obvious analogy of this case with countercurrent wetted-wall towers, gas velocities in the cocurrent case exceed these used in any reported wetted-wall-tower investigations. In cocurrent annular flow, smooth liquid films free of ripples are not attainable, and entrainment and deposition of liquid droplets presents an additional transfer mechanism. By measuring solute concentrations of liquid in the film and in entrained drops, as well as flow rates, and by assuming absorption equilibrium between droplets and gas, Anderson et al. were able to separate the two contributing mechanisms of transfer. The agreement of their entrainment values (based on the assumption of transfer equilibrium in the droplets) with those of Wicks and Dukler (W2) was taken as supporting evidence for this supposition. 

In addition to providing highly selective separations, there are a multitude of other desired characteristics that a gas chromatographic stationary phase should possess. These properties include high viscosity, low surface tension allowing for wetting of the fused silica capillary wall, high thermal stability, and low vapor pressure at elevated temperatures. The stationary phase solvent should also not exhibit unusual mass transfer behavior. 

In many of these experiments, interfacial turbulence was the obvious visible cause of the unusual features of the rate of mass transfer. There are, however, experimental results in which no interfacial activity was observed. Brian et al. [108] have drawn attention to the severe disagreement existing between the penetration theory and data for the absorption of carbon dioxide in monoethanolamine. They have performed experiments on the absorption of C02 with simultaneous desorption of propylene in a short, wetted wall column. The desorption of propylene without absorption of C02 agrees closely with the predictions of the penetration theory. If, however, both processes take place simultaneously, the rate of desorption is greatly increased. This enhancement must be linked to a hydrodynamic effect induced by the absorption of C02 and the only one which can occur appears to be the interfacial turbulence caused by the Marangoni effect. No interfacial activity was observed because of the small scale and small intensity of the induced turbulence. 

Higbie (H4) has made use of the foregoing solution to obtain an expression for the liquid-phase mass transfer coefficient for short contact times for absorption in wetted-wall towers. From Eq. (161) one may calculate the total moles of A transferred per unit time per unit cross-sectional transfer area ... 

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). 

Continuous changes in compositions of phases flowing in contact with each other are characteristic of packed towers, spray or wetted wall columns, and some novel equipment such as the FHGEE contactor (Fig. 13.14). The theory of mass transfer between phases and separation of mixtures under such conditions is based on a two-film theory. The concept is illustrated in Figure 13.15(a). 

Wetted-wall columns have been used for many years for determining mass-transfer coefficients on the assumption that the interfacial area across which mass transfer occurs can be obtained accurately from the dimensions of the column and a knowledge of the film thickness. It is therefore of considerable practical interest to determine whether the interfacial waves lead to an appreciable increase in the interfacial area of the film, which would introduce a grave uncertainty into such methods of determining mass-transfer coefficients. 

Zhavoronkov et al. (Z2), 1951 Mass transfer studies (CO into water film) in two diameters of wetted-wall column and on wetted-plate packing (liquid mixed at intervals). Gas velocity had little effect on transfer rates. 

Vivian and Peaceman (V5), 1956 Experimental mass transfer work in short wetted-wall columns (1.9-4.3 cm. long) ripples absent at most flow rates. Rate of desorption of C02 independent of gas velocity up to ArReB = 2200. Width and type of liquid inlet slot had little effect. Acceleration of film important. 

Mirev et al. (Mil), 1961 Experimental studies of rates of absorption of C2H2, SO2 in water films in wetted-wall columns. Experimental results not in agreement with Vyazovov (V8) and penetration theories. Surfactant reduced rippling but appeared to increase interfadal resistance to mass transfer. 

Niebergall (Nl), 1963 Deals with the effects of changes of surface tension during heat and mass transfer in wetted-wall equipment. 

Also, when the wall of the stirring vessel is preferently wetted by the dispersed phase, the wall may be covered with a thin stagnant layer of dispersed phase, which may act, more or less, as a dead corner in so far as the chemical reaction is slowed down by poor mass transfer. However, there still may occur a continuous coalescing of dispersed drops with these stagnant layers and corners, while on the other hand these dead corners will continuously lose new drops, which are taken up again in the living dispersed phase. In this way the dead corners act as a medium of interaction. 

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... 

The rate of mass transfer in the liquid phase in wetted-wall columns is highly dependent on surface conditions. When laminar-flow conditions prevail without the presence of wave formation, the laminar-penetration theory prevails. When, however, ripples form at the surface, and they... 

Fig. 7. Relative decrease of mass transfer coefficient in the presence of surfactant (in comparison to its value in pure sulphite solution) as a function of surface pressure tt. Comparison with literature data measured in bubble columns [12,8] and in wetted-wall column [27].

We studied these phenomena experimentally in a wetted wall column and two stirred cell reactors and evaluated the results with both a penetration and a film model description of simultaneous mass transfer accompanied by complex liquid-phase reactions [5,6], The experimental results agree well with the calculations and the existence of the third regime with its desorption against overall driving force is demonstrated in practice (forced desorption or negative enhancement factor). 

Negligible and medium interaction regimes. Experiments were carried out with an aqueous 2.0 M DIPA solution at 25 °C in a stirred-cell reactor (see ref. [1]) and a 0.010 m diameter wetted wall column (used only in negligible interaction regime, see ref. [4,5]). Gas and liquid were continuously fed to the reactors mass transfer rates were obtained from gas-phase analyses except for CO2 in the wetted wall column where due to low C02 gas-phase conversion, a liquid-phase analysis had to be used [5]. In the negligible interaction regime some 27 experiments were carried out in both reactors. The selectivity factors were calculated from the measured H2S and CO2 mole fluxes and are plotted versus k... 

In small columns, the wetted wall contributes to mass transfer. This problem can be overcome by keeping the ratio of column to packing diameter above 10 (Sec. 9.2.4). For cases where wall effects are significant, Wu and Chen (167) recommend applying the following safety factor. 

Mass-Transfer Correlations Because of the tremendous im-ortance of mass transfer in chemical engineering, a veiy large num-er of studies have determined mass-transfer coefficients both empirically and theoretically. Some of these studies are summarized in Tables 5-17 to 5-24. Each table is for a specific geometry or type of contactor, starting with flat plates, which have the simplest geometry (Table 5-17) then wetted wall columns (Table 5-18) flow in pipes and ducts (Table 5-19) submerged objects (Table 5-20) drops and... 

TABLE 5-18 Mass-Transfer Correlations for Falling Films with a Free Surface in Wetted Wall Columns— Transfer between Gas and Liquid... 

D. Turbulent, vertical wetted wall column with ripples tfsw = = 0.00814ivriv i2e. J1 30 (iSE. ) < 1200 > l / Nsh e = = 0.023tf 8tfs [E] For gas systems with rippling. Fits 5-18-B for ( 2P ) = 1000 / [E] Rounded approximation to include ripples. Includes solid-liquid mass-transfer data to find V6 coefficient on NSc- May use A 83. Use for liquids. See also Table 5-19. [85] [138] p. 213... 

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