Wetted-wall tower

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. 

Transfer coefficients for the liquid phase can be described by the kind of dimensionless relationship commonly obtained from wetted-wall tower experiments. 

Since the experiments of Tung and Drickamer, the resistance to diffusion through an interface has been further studied in gas-liquid systems by Emmert and Pigford (E2), who studied the absorption and desorption of CO 2 and 02 in water in a wetted-wall tower and interpreted their results in terms of accommodation coefficients. They... 

The problems discussed here are basic in the description of absorption in falling films, performance of wetted-wall towers, operation of tubular reactors, and fluid blending. 

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). 

Fig. 5. Gas flow by forced convection through a wetted-wall tower, (a) with flat velocity profile, and (b) with parabolic velocity profile.

Heat and vapor transfer in a wetted-wall tower. Ind. Eng. Chem., 33 436-442. 

The coefficient k has been studied in experimental devices in which the area of contact between phases is known and where boundary-layer separation does not take place. The wetted-wall tower shown in Fig. 21.3, which is sometimes used in practice, is one device of this type. It has given valuable information on mass transfer to and from fluids in turbulent flow. A wetted-wall tower is essentially a vertical tube with means for admitting liquid at the top and causing... 

Slightly more accurate correlations for pipe flow have been presented for different ranges of the Schmidt number. Data for evaporation of several liquids in wetted-wall towers (Fig. 21.3) were correlated with slightly higher exponents for both the Reynolds and Schmidt numbers... 

Mass transfer from the inner wall of a tube to a moving fluid has been studied extensively, and most experimental data come from wetted-wall towers. In Figure 2.4, a volatile pure liquid is allowed to flow down the inside surface of a circular pipe while a gas is blown upward or downward through the central core. Measurement of the rate of evaporation of the liquid into the gas stream over the known surface per-... 

To illustrate the concept of simultaneous mass and heat transfer, consider the following air humidification problem. Water flows down the inside wall of a 25.4-mm-diameter wetted-wall tower of the design of Figure 2.4, while air flows upward through the core. At a point in the tower, humid air flows at a mass velocity of 10.0 kg/m2-s. The temperature of the gaseous mixture is 308 K, the pressure is 1 atm, and the partial pressure of water vapor in the mixture is 1.95 kPa. Assuming that the process is virtually adiabatic, estimate the temperature of the liquid water at that point, and the rate of water evaporation. 

For this purpose, let us consider the absorption of a soluble gas such as ammonia (substance A) from a mixture with air, by liquid water, in a wetted-wall tower. The ammonia-air mixture may enter at the bottom of the tower and flow upward, while the water flows downward around the inside of the pipe. The ammonia concentration in the gas mixture diminishes as it flows upward, while the water absorbs the ammonia as it flows downward and leaves at the bottom as an aqueous ammonia solution. Under steady-state conditions, the concentrations at any point in the apparatus do not change with the passage of time. 

As a design engineer, you are asked by your boss to design a wetted wall tower to reduce a toxic gas in an air stream down to some acceptable level. At your disposal are two solvents, which you can use in the tower one is nonreactive with the toxic gas but is cheap, whereas the other is reactive and quite expensive. In order to choose which solvent to use, you will need to analyze a model to describe the absorption of the toxic gas into the flowing solvent (see Fig. 10.16). 

Wetted-wall Towers. As indicated previously, these have been used in laboratory investigations where it was desired to exert some control over the interfacial area, but it is not likely that they will be useful in industrial work. The significant results will be summarized here only briefly. 

Fallah, Hunter, Nash, and Strang (27, 77) studied the hydraulics of wetted-wall towers operated with stationary liquid cores composed of hydrocarbons and a moving wall-liquid of water. It was clearly shown that the interface and a portion of the core liquid immediately adjacent to the interface moved downward in the direction of the flow of wall-liquid. 

Patience and Chaouki [98] adopted the two-phase model of Brereton et al. [96] to interpret their gas RTD data obtained with a radioactive tracer gas. The two model parameters, crossflow coefficient, k, and (ratio between core and riser cross-sections), were evaluated by fitting the model to the experimental data. They found that the crossflow coefficients varied between 0.03 to 0.1 m/s, and varied from 0.98, at high gas velocities, to 0.5, at low velocities. They attributed gas crossflow between core and annulus by supposing that solids drag gas to the annulus as they condense along the wall and then carry it downward for a certain distance. Solids are reintroduced into the core as they are stripped off the wall and re-entrained into the core gas flow. They developed a correlation for describing this gas mass transfer based on the analogy with wetted wall towers, as ... 

Film of Water on Wetted-Wall Tower. Pure water at 20°C is flowing down a... 

Velocity Profile in Wetted-Wall Tower. In a vertical wetted-wall tower, the fluid flows down the inside as a thin film 5 m thick in laminar flow in the vertical z direction. Derive the equation for the velocity profile as a function of x, the distance from the liquid surface toward the wall. The fluid is at a large distance from the entrance. Also, derive expressions for av and max- Hint At. X = 6, which is at the wall, = 0. Atx = 0, the surface of the flowing liquid. 

Mass transfer for flow inside wetted-wall towers. When a gas is flowing inside the core of a wetted-wall tower the same correlations that are used for mass transfer of a gas in laminar or turbulent flow in a pipe are applicable. This means that Eqs. (7.3-24) and (7.3-25) can be used to predict mass transfer for the gas. For the mass transfer in the liquid film flowing down the wetted-wall tower, Eqs. (7.3-22) and (7.3-23) can be used for Reynolds numbers of 4T/p as defined by Eq. (2.9-29) up to about 1200, and the theoretically predicted values should be multiplied by about 1.5 because of ripples and other factors. These equations hold for short contact times or Reynolds numbers above about 100 (SI). 

EXAMPLE 10.4-1. Interface Compositions in Interphase Mass Transfer The solute A is being absorbed from a gas mixture of A and B in a wetted-wall tower with the liquid flowing as a film downward along the wall. 

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