Absorption film control

Mass-transfer theory indicates that for trays of a given design the factors most hkely to inflnence E in absorption and stripping towers are the physical properties of the flnids and the dimensionless ratio Systems in which the mass transfer is gas-film-controlled may be expected to have plate efficiencies as high as 50 to 100 percent, whereas plate efficiencies as low as 1 percent have been reported for the absorption of gases of low sohibility (large m) into solvents of relatively high viscosity. 

Assuming that the absorption process is gas-film controlled, and that the concentration of the solute is small (i.e., 1 - y = 1), then ... 

Consider a packed tower for the absorption of A from a gas containing inert material (in addition to A) by a liquid containing B (nonvolatile) under continuous, steady-state conditions. Absorption is accompanied by the instantaneous reaction A(g) + bB(() - products. Assume the overall process is liquid-film controlled. 

Find the rate of absorption, the controlling resistance, and what is happening in the liquid film, at a point in the column where Cb = 500 mol/m. ... 

Experiments are carried out at atmospheric pressure on the absorption into water of ammonia from a mixture of hydrogen and nitrogen, both of which may be taken as insoluble in the water. For a constant mole fraction of 0.05 of ammonia, it is found that the absorption rate is 25 per cent higher when the molar ratio of hydrogen to nitrogen is changed from 1 1 to 4 1. Is this result consistent with the assumption of a steady-state gas-film controlled process and, if not, what suggestions have you to make to account for the discrepancy ... 

Figure 2-4 Typical concentration profiles of instantaneous reaction between the gas A and the reactant C, based on film theory, ids Diffusion controlled - slow reaction, (fcl kinetically controlled-slow reaction, (c) gas-film-controlled desorption - fast reaction, 0 liquid-film-controlled desorption-fast reaction, (e) liquid-film-controlled absorption -instantaneous reaction between A and C, (/) gas-film-controlled absorption-instantaneous reaction between A and C, (g) concentration profiles for A, B, and C for instantaneous reaction between A and C-both gas- and liquid-phase resistances are comparable.1 2...

Lime/limestone slurry scrubbing is the dominant commercial technology for flue gas desulfurization 0.). SO2 is absorbed at 50-55°C and pH 5.5-6.0 in an aqueous slurry of excess CaC03 and product solids. The CaS03/CaS04 product is disposed of as solid waste. With greater than 500-1000 ppm SO2 in the flue gas, SO2 absorption is controlled by liquid-film mass transfer resistance because of the limited solubility of SO2 gas and alkaline solids. Additives that buffer between pH 3 and pH 5.5 enhance S02 absorption by providing dissolved alkaline species for reaction with SO2 (8). 

The absorption of H2S in basic solutions is sometimes said to be gas-film controlled, even though H2S has a very low solubility in water... 

When the reactions are in regime 2 or in regime 1-2, the rate of absorption is controlled by diffusion in the film (Figure 14.9a). The corresponding conditions and rate equations are given in Table 14.5. Note that because the reaction of, say, A occurs in the bulk, its presence does not affect the profile of B in the film and hence the rate of absorption of Ai-... 

Chlorine under pressure can be destroyed by sparging it into a tank containing an alkaline solution. The absorption process in this case also is liquid-film controlled [92]. 

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 ... 

In gas-film-controlled absorption systems the value of Koa is a function of both the liquid and the gas flow rates. The value of the exponent b for the effect of liquid flow rate in Equation 3-13 is the same for gas-film-controlled systems as for liquid-film-controlled systems. However, the value of the exponent c for the effect of gas flow rate in Equation 3-13 for a gas-film-controlled system has increased to between 0.67 and 0.80. In the absence of data on a specific system, it is suggested that the value of exponent c be taken as 0.75. In any event, the sum of exponents b and c should be greater than 1.0. Therefore, if the foregoing system were gas-film-controlled and the gas rate was increased from 900 Ib/ft h to 1200 Ib/ft ... 

In gas-film-controlled systems the value of K a is much greater than in liquid-film-controlled systems. This is because the liquid-film resistance reduces the value of the overall coefficient in liquid-film-controlled systems, while it has only a small effect on this value in gas-film-controlled systems. In absorption operations carried out at a constant liquid-to-gas ratio, the number of mols of solute to be absorbed will increase in direct proportion to the gas rate at a fixed gas composition. The K a value for a liquid-film-controlled system, however, will increase only as about the 0.37 power of the flow rates at a constant G/L ratio (b + c = 0.37). Thus, increasing the flow rates in such a system will either reduce the absorption efficiency or increase the packed depth required. For a gas-film-con-... 

Although acid gases may have a high solubility in Selexol solvent, this system is considered liquid-film controlled. Absorption is limited by diffusion in the liquid phase due to the high viscosity of Selexol solvent therefore, liquid properties are important in determining the mass... 

When super-dry chlorine (having a maximum water content of 10 mol ppm) is required, a fourth drying tower is used. This column removes very little water vapor, so that the liquid recirculated will be about 96 wt % sulfuric acid. Because of the high liquid-phase viscosity (greater than 15 cps), the absorption in this tower will be substantially liquid-film controlled. 

This system is almost a pure gas-film-controlled absorption. The sum of the exponents b and c in Equation 3-13 is greater than unity. Thus, an increase in gas flow rate at constant gas composition will increase the mols of SO3 to be absorbed in direct proportion. By holding a constant liquid-... 

By material balance, 114.1 Ib-mols/h of H2S and at least 357.8 11> mols/h of CO2 must be absorbed. Thus, the lean solution feed rate will be 549,000 Ib/h. The basic Kca values for 2.5 normal DEA should be 2(0.55) times the values given in Table 3-3 thus, for 40IMTP packing, the basic KqB, would be 3.15 Ib-mol/h ft -atm for CO2 absorption. The K< a value for H2S would be 2.5 times that for CO2 or a basic a of 7.87 Ib-mol/hr ft atm. These basic Kca values would be increased by a factor of 1.49 to account for the effect of 37.7 gpm/ft liquid rate. There is negligible correction needed for gas rate because both H2S and CO2 represent liquid-film-controlled absorptions. 

For this gas-film-controlled system, the rate of mass transfer is proportional to gas rate to approximately the 0.8 power. This is similar to data on the isothermal evaporation of pure liquids from a pipe wall into a turbulent air stream. There, Gilliland and Sherwood showed that the mass transfer rate is proportional to the gas mass velocity to the 0.83 power [4]. For systems evaporating water into an air stream, the effect of the liquid rate is similar to that for absorption. Because a large percentage of the heat is transferred by the vaporization of water, it is reasonable to find that the effect of gas and liquid rates is similar to that for other gas-film-controlled mass transfer operations. 

Packed columns maintain a high separation efficiency in both liquid-film-controlled and gas-film-controlled systems. For many years, packed columns have been used for absorption systems that characteristically are liquid-film-controlled. As an example, the HETP value for a trayed column might be twice as great in an absorption operation as compared to a distillation operation. For the same systems, the HETP value for IMTP packing in the absorption operation typically would be only one-third greater than for the distillation operation. 

Typical operating data from a commercial absorber are presented in Table 5-6. Contact in this absorber is by means of perforated trays. Unfortunately, the number of trays is not specified so that tray efficiencies cannot be calculated from the data. However, since this has been shown to be a liquid-film controlled system, tray efficiencies would be expected to be quite low. Data reported from other installations have indicated tray efficiencies (Murphree vapor) to be on the order of 5% for absorption of CO2 in hot potassium carbonate solutions. 

Tepe and Dodge found Kca to be essentially independent of gas rate, a condition which would normally indicate that the liquid film was controlling absorption. However, the exponent relating the effect of liquid rate is not as high as would be expected in a simple liquid-film-controlled absorption. As shown in the figure, the absorption coefficient increases with increased sodium hydroxide concentrations up to about 2 N and then decreases. The decrease is presumably due to the higher viscosity of more concentrated solutions—a phenomenon also observed for alkanolanune solutions. 

The absorption of carbon dioxide in water at elevated pressure was formerly an important industrial process, particularly for the purification of synthesis gas for ammonia production. The process has now generally been replaced by more efficient systems which employ chemical or physical solvents with much higher capacities for carbon dioxide than water. Such systems are described in Chapters 2, 3, 3, and 14. A description of the water wash process for carbon dioxide removal is included in this chapter because of its historical interest, its technical value as a classical liquid film-controlled operation, and the hope that the extensive work done on the process will prove usefril in the development of new processes or applications. 

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. 

Because of the very high solubility of HCl in water and the rapidity with which the reaction with water occurs, the absorption is completely gas-film controlled. With concentrated gas streams, in which the low concentration of inert gas permits rapid diffusion through the... 

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. ... 

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