Absorption columns mass-transfer coefficients

As indicated above, for purely physical absorption, the mass-transfer coefficients depend on the hydrodynamics and the physical properties of the phases. The literature contains measured values of mass-transfer coefficients and correlations (see discussion on agitated tanks and bubble columns below). Tables 19-7 and 19-8 present experimental information on apparent mass-transfer coefficients for absorption of select gases. On this basis, a tower for absorption of S02 with NaOH is smaller than that with pure water by a factor of roughly 0.317/7.0 = 0.045. Table 19-9 lists the main factors that are needed for... 

In 1966, in a paper that now is considered a classic, Danckwerts and Gillham [Tmns. Inst. Chem. Eng., 44, T42 (1966)] showed that data taken in a small stirred-ceU laboratoiy apparatus could be used in the design of a packed-tower absorber when chemical reactions are involved. They showed that if the packed-tower mass-transfer coefficient in the absence of reaction (/cf) can be reproduced in the laboratory unit, then the rate of absorption in the l oratoiy apparatus will respond to chemical reactions in the same way as in the packed column even though the means of agitating the hquid in the two systems might be quite different. 

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).]... 

Yoshida and Akita (Yl) determined volumetric mass-transfer coefficients for the absorption of oxygen by aqueous sodium sulfite solutions in counter-current-ffow bubble-columns. Columns of various diameters (from 7.7 to 60.0 cm) and liquid heights (from 90 to 350 cm) were used in order to examine the effects of equipment size. The volumetric absorption coefficient reportedly increases with increasing gas velocity over the entire range investigated (up to approximately 30 cm/sec nominal velocity), and with increasing column diameter, but is independent of liquid height. These observations are somewhat at variance with those of other workers. 

The mass transfer coefficients considered so far - namely, kQ,kj, KQ,andKj - are defined with respect to known interfacial areas. However, the interfacial areas in equipment such as the packed column and bubble column are indefinite, and vary with operating conditions such as fluid velocities. It is for this reason that the volumetric coefficients defined with respect to the unit volume of the equipment are used, or more strictly, the unit packed volume in the packed column or the unit volume of liquid containing bubbles in the bubble column. Corresponding to /cg, Kq, and we define k a, k, a, K, /i, and K a, all of which have units of (kmol h m )/(kmol m ) - that is, (h ). Although the volumetric coefficients are often regarded as single coefficients, it is more reasonable to consider a separately from the Ar-terms, because the effective interfacial area per unit packed volume or unit volume of liquid-gas mixture a (m m ) varies not only with operating conditions such as fluid velocities but also with the types of operation, such as physical absorption, chemical absorption, and vaporization. 

The influence of pressure on the mass transfer in a countercurrent packed column has been scarcely investigated to date. The only systematic experimental work has been made by the Research Group of the INSA Lyon (F) with Professor M. Otterbein el al. These authors [8, 9] studied the influence of the total pressure (up to 15 bar) on the gas-liquid interfacial area, a, and on the volumetric mass-transfer coefficient in the liquid phase, kia, in a countercurrent packed column. The method of gas-liquid absorption with chemical reaction was applied with different chemical systems. The results showed the increase of the interfacial area with increasing pressure, at constant gas-and liquid velocities. The same trend was observed for the variation of the volumetric liquid mass-transfer coefficient. The effect of pressure on kia was probably due to the influence of pressure on the interfacial area, a. In fact, by observing the ratio, kia/a, it can be seen that the liquid-side mass-transfer coefficient, kL, is independent of pressure. 

Most of the properties change somewhat from one end to the other of industrial columns for effecting separations, so that the mass transfer coefficients likewise vary. Perhaps the property that has the most effect is the mass rate of flow which appears in the Reynolds number. Certainly it changes when there is a substantial transfer of material between the two phases in absorption or stripping and even under conditions of constant molal overflow in distillation processes, the mass rate of flow changes because of differences of the molecular weights of the substances being separated. As a practical expedient, however, mass transfer coefficients are evaluated at mean conditions in a column. 

Akita and Yoshida (1974) evaluated the liquid-phase mass-transfer coefficient based on the oxygen absorption into several liquids of different physical properties using bubble columns without mechanical agitation. Their correlation for kL is... 

Akita and Yoshida (1973) correlated the volumetric mass-transfer coefficient kLa for the absorption of oxygen in various aqueous solutions in bubble columns, as follows ... 

The bubble column can withstand high gas velocities and still maintain high mass transfer coefficients. This column is particularly attractive for reactions that do not require large amounts of gas absorption or require well-mixed liquids. 

The height of an absorption column depends on the feed conditions, the product purity specifications, the solvent used and the extent of separation through the absorption equilibrium relationship, but also on the rate of separation. If the rate of mass transfer of the gaseous component from the gas phase into the liquid phase is slow, then the column needs to be longer to ensure that the required amount is removed. The rate of mass transfer depends on the mass-transfer coefficient, normally denoted kG or k. The value of the mass-transfer coefficient depends on the components in the gas feed and on the solvent used and is often determined experimentally. The type of packing used in the column will also have an impact on the column height as for distillation. 

The actual value of k a was measured by absorption of carbondiox-ide from air into a buffer solution of potassium-carbonate and bicarbonate. Care was taken that the mass transfer coefficient itself was not enhanced by the chemical reaction, although the composition of the buffers used guaranteed a substantial driving force for mass transfer over the whole length of the column. Literature about the subject is abundant and here referred to (11, 12, 13). 

Gas absorption is a function of the gas and liquid mass transfer coefficients, the interfacial area, and the enhancement due to chemical reaction. The gas-liquid interfacial area is related to the Sauter mean bubble diameter and the gas holdup fraction. The gas holdup fraction has been reported to vary with radial position (7-11) for column internal diameters up to 0.6 m. Koide et al" Tl2), however, found that the radial distribution of gas holdup was nearly constant for a column diameter of 5.5 m. Axial distribution of average gas holdup has been reported by Ueyama et al. (10). The average gas holdup... 

The reported study on gas-liquid interphase mass transfer for upward cocurrent gas-liquid flow is fairly extensive. Mashelkar and Sharma19 examined the gas-liquid mass-transfer coefficient (both gas side and liquid side) and effective interfacial area for cocurrent upflow through 6.6-, 10-, and 20-cm columns packed with a variety of packings. The absorption of carbon dioxide in a variety of electrolytic and ronelectrolytic solutions was measured. The results showed that the introduction of gas at high nozzle velocities (>20,000 cm s ) resulted in a substantial increase in the overall mass-transfer coefficient. Packed bubble-columns gave some improvement in the mass-transfer characteristics over those in an unpacked bubble-column, particularly at lower superficial gas velocities. The value of the effective interfacial area decreased very significantly when there was a substantial decrease in the superficial gas velocity as the gas traversed the column. The volumetric gas-liquid mass-transfer coefficient increased with the superficial gas velocity. 

The absorption rate depends upon the gas-liquid interfacial area, the gas residence time, and the gas-liquid mass-transfer coefficient. The gas residence time is lower and the gas-liquid mass-transfer coefficient is higher (due to higher Reynolds number) in a three-phase fluidized-bed system compared to a bubble-column with no solids present. It is the first factor (i.e., gas-liquid interfacial area) that plays an important and complex role on the mass-transfer rate in the three-phase fluidized-bed system. 

Bubble columns are convenient for catalytic slurry reactions also (67). It is therefore important to know how the hydrodynamic properties of the gas-in-liquid dispersion is influenced by the presence of suspended solid particles. In the slurry reactor absorption enhancement due to chemical reaction cannot be expected. However, if particle sizes are very small, say less than 5 yum, and if, in addition, the catalytic reaction rate is high a small absorption enhancement can occur ( 8). Usually the reaction is in the slow reaction regime of mass transfer theory. Hence, it is sufficient to know the volumetric mass transfer coefficient, kj a, and there is no need to separate k a into the individual values. 

The values of interfacial area and of overall mass-transfer coefficient increase with decreasing distance S between the spray nozzle and gas inlet, whatever the nozzle type, column dimensions, and flow rates. Indeed the spray provides a large interfacial area in the vicinity of the nozzle, where there is intensive circulation. Then a decreases quickly away from the nozzle, as a result of both coalescence of droplets and collection of liquid on the column walls, kaa and a are approximately proportional to (P7, H12, Mil) for absorption and desorption pro-... 

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