Absorption of Carbon Dioxide D. H.KILLEFFER 60 East 42nd Street. New York, N. Y.
C
ARBON dioxide is concentrated from dilute mixtures by absorption in carbonate solutions and regeneration by boiling. The over-all reaction is :
+ COz + HzO
NazCOa
2NaHCOa
This apparently simple reaction has probably been studied more diligently by more chemists than any other of similar type. Yet in its progress from left to right, the greater part of the carbon dioxide it seeks to absorb is wasted, and in the reverse direction heat is wasted in large amounts in boiling water from the solution to secure the evolution of even a substantial part of the absorbed carbon dioxide. In both directions the rate is slow. Numerous methods of increasing the efficiency of the process and many other alkaline absorbents have been suggested with more or less helpful results (a copious bibliography was given by Martin and Killeffer, 8). The numerous explanations offered from time to time to account for the slowness of this reaction leave much to be desired, and none of them to date has supplied the key to accelerating it. Certainly viscosity is not the whole answer to this problem, since viscosity cannot be the ruling factor in a solution spread in thin layers on coke or other packing, even if it should answer some questions about thick layers of caustic solutions. Riou and his eo-workers (3) studied viscosity but could not relate it to absorption rate. However, their work suggests that surface tension, instead of viscosity, might be the key to the rate of absorption of carbon dioxide. They were unable to show a relation between viscosity and rate, but the materials they added to carbonate lyes to change viscosity included several recognized surface-tension depressors. With additions which depressed surface tension, absorption rate increased. Sketchy checks made on some of the addition agents used by Riou, and on a few others, have shown that even 1 per cent additions of some of the recognized surface-tension depressors greatly increase the slope of the absorption curve of sodium carbonate-bicarbonate solutions. Among others, such dissimilar addition compounds as formaldehyde, methanol, phenol, soap, sodium lactate, and the ethanolamines have brought absut pronounced increases in absorption rate. Experiments were made in a stirred solution in a glass cylinder and in a small absorption tower, and all were stopped soon enough to avoid the approach of equilibrium-i. e., while the absorption curve was still a straight line. The solutions were twice normal with respect to sodium and contained a mixture of carbonate and bicarbonate. This figure approaches commercial practice where maximum concentration (of sodium) is about 2.4 normal. The gas used was approximately 50 per cent carbon dioxide in air, which has a gas film of varying concentration and yet which gives a readily measurable rate. Results in a cylinder with a well-defined limited contact surface were roughly checked, using a single pass of both liquid and gas countercurrent through a 3-inch absorption tower packed with 3/s-inch stoneware Raschig rings. Addition of 1 per cent formaldehyde (2.5 per cent of 40 per cent solution) increased absorption rate by 115 per cent; 2 per cent methanol, 20 per cent; and 0.5 per cent “Tergitol 3” (a sodium
alcohol sulfate), 50 per cent. Soap, ethanol, phenol, sodium lactate, and others also increased absorption rates, as did triethanolamine and isopropanol. None of these additions should be sufficient to change viscosity appreciably, but all should materially influence surface tension. These results at least suggest that the surface tension will bear investigation. Viscosity certainly does not explain the rates of absorption of carbon dioxide by carbonates, hydroxides, and organic amines a t one time without such important changes of constants that it ceases to be a rule and becomes a mere empiricism altered to fit. Kor does it explain these results or those of Riou. The success of the McKee-Winter (Macmar) process ( I ) also bears out the importance of film resistance in the liquid interface. By adding ammonia to the gas in the absorbing tower, they achieve an increase of about 100 per cent in absorption rate, although the amount of ammonia is too small to affect equilibrium considerably. The carbon dioxide is apparently carried through the interface as ammonium carbonate, and this is decomposed by the alkali in the solution. In other words, the bottleneck of the absorption (passage of carbon dioxide through the interface) is avoided by substituting two rapid transfers of ammonium carbonate and ammonia, This presumably does not influence the effect of viscosity on the transfer of carbon dioxide in or into the body of the liquid, but it works practically on a commercial scale. The effectiveness of the organic amino alcohols also fits in with the surface tension idea. Ethanolamine solutions are very viscous (one of their principal drawbacks in use on account of the cost of pumping) but have, presumably, relatively low surface tensions as compared with carbonate lyes. There is probably a difference in the driving force of absorption between the two as well, but this does not account for the closer approach to equilibrium with ethanolamines as compared with alkali carbonates. It is hoped that the investigation here suggested will prove attractive to workers in this field.
Literature Cited (1) McKee and Winter, U. 9. Patent 2,043,109 (June 2, 1936). . , 632 (1937). (2) Martin and Killeffer, IXD.ENG.C H ~ M29, (3) Riou and co-workers, C m p t . rend., 174, 1017, 1463 (1922) ; 184, 326 (1927); 186, 1543, 1727 (1928).
RECEIVED January 19, 1937. Presented as discussion before the Bbsorption and Extraction Symposium held under the auspices of the Division of Industrial and Engineering Chemistry of the American Chemical Society at Columbia University, New York, N. Y . , December 28 and 29, 1936.
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