Wetted wall

In wetted-wall units, the walls of a tall circular, slightly tapered combustion chamber are protected by a high volume curtain of cooled acid flowing down inside the wall. Phosphoms is atomized by compressed air or steam into the top of the chamber and burned in additional combustion air suppHed by a forced or induced draft fan. Wetted-waU. plants use 25—50% excess combustion air to reduce the tail-gas volume, resulting in flame temperatures in excess of 2000°C. The combustion chamber maybe refractory lined or made of stainless steel. Acid sprays at the bottom of the chamber or in a subsequent, separate spraying chamber complete the hydration of phosphoms pentoxide. The sprays also cool the gas stream to below 100°C, thereby minimising corrosion to the mist-collecting equipment (typically type 316 stainless steel). 

A. Laminar, vertical wetted wall column Ws/, 3.41 — D 5fa (first term of infinite series) [T] Low rates M.T Use with log mean concentration difference. Parabolic velocity distribution in films. 

D. Rectification in vertical wetted wall column with turbulent vapor flow, Johnstone and Pigford correlation =0.0.328(Wi) Wi P>vP 3000 < NL < 40,000, 0.5 < Ns. < 3 N=, v,.gi = gas velocity relative to R. liquid film = — in film -1 2 " [E] Use logarithmic mean driving force at two ends of column. Based on four systems with gas-side resistance only, = logarithmic mean partial pressure of nondiffusing species B in binary mixture. p = total pressure Modified form is used for structured packings (See Table 5-28-H). 

From wetted wall column and dissolution data— see Table 5-22-B. 

Wetted-wall (falling-film) column Countercurrent, cocurrent Differential Liquid and/or gas Absorption, rectification, stripping, evaporation... 

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... 

FIG. 14-78 Liqnid-film resistance in absorption of gases in wetted-wall columns. Theoretical lines are calculated for oxygen absorption in water at 55 F. To convert feet to meters, multiply by 0.3048 C = /9 ( F — 32). (Sheixuood and Vigfoid, Absorption and Extraction, 2d ed., McGraw-Hill, New York, 1952. )... 

ESPs for liquid droplets and mists are essenhally of the wetted wall type. Figure 29-9 shows a wet wall precipitator with tubular collechon electrodes (1). The upper ends of the tubes form weirs, and water flows over the tube ends to irrigate the collechon surface. 

Fig. 29-9. Wet-wall ESP with tubular collection electrodes. Source Oglesby, S., Jr, and Nichols, G. B., Electrostatic precipitators, in "Air Pollution," 3rd ed., Vol. IV (A. C. Stern, ed.), p. 238, Academic Press, New York, 1977.

Since static discharges frequently occur at the liquid interface as liquid drains from the wetted wall, a vapor ignition hazard may also exist [ 157,159]. These discharges may be either brush types or PBDs (2-6.2 and 2-6.5). In 1997 it was concluded that Manufacturers have so far not paid attention to requests by the operators of enamel apparatus for a coating which is completely or partially capable of dissipation [159]. However, this situation is believed to be changing and it might now be possible to specify special static dissipative coatings for the vessel and stirrer. 

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

Their measured rate constant was less than that estimated from wetted-wall column absorption experiments (S2). 

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. 

Controlled contact between two immiscible liquids has also been achieved by flowing one liquid along a solid support submerged in the second phase [28,29]. Several different arrangements have been used, although all are based on similar principles. For example, a wetted wall column which offered liquid-liquid contact times of 0.5-10 s was used to measure solute transfer rates [29]. 

Chen, N.H. New Equation Gives Tower Diameter, Chemical Engineering, Feb. 5, 1962, p. 109. Chart for Height of Transfer Unit in Wetted Wall Columns, Chemical Engineering, May 10, 1965, p. 193. 

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