( T o p ) FLWE-GAB WATERS C R ~ B E R S CARBON DIOXIDE AB~ORBE~S (Rear Center) AND LYEBOILER (Right,)
WITH
(Center) CARBONDIOXIDE ABSOHBSHOWING LYE C I R C ~ A T I X G PUMPS (Lejt)
ERS
(Right) GAS FEED TO MACMARPROCESSCARBON DIOXIDEABSORBERS, SHOWIXG FLcE GAS BLOWER (Center) TO A s s o a ~ ~ n s , AMMONIA SUPPLY CYLINDEH (Right Foreground), LYEHEATEXCHANGERS (Left),ARD LYECIRCCLATIXG PUMPS (Right,) Cmulcsy. Internolional Carbo-Ice. Ltd.
632
from Power Plant Flue Gas ARBON dioxide can be economically recovered from gases of low concentration by a newly developed method using ammonia to assist in the absorption process. Not only is recovery from such sources as power plant flues made feasible a t low cost, but the product is obtained in a purity satisfying the most exacting requirements. This development has proceeded through the laboratory and the pilot plant and has finally been put to use in commercial production. The possibilities opened by cheap production of pure carbon dioxide from flue gases of plants operated for power production are especially important in the solid carbon dioxide industry where supplies are needed a t consuming markets to avoid losses in handling and shipping this important refrigerant. Already its practical economy has been shown, and where off-peak or incremental power a t low prices is available, it is probable that it may be able to compete successfully with present production from byproduct gas.
C
Absorption Process Recovery of carbon dioxide from gas mixtures, while an apparently simple problem of absorption and desorption, has nevertheless attracted the attention of inventors and investigators for more than half a century (5, 14, 32, 48, 64). The commercial process by which carbon dioxide is obtained from flue gas in high concentration is represented by the changing equilibrium in this reaction:
K&oa
+ COz + HzO
2KHCOa
(1)
As practiced, potassium or sodium carbonate is used, and within the past few years organic bases have been employed with success. Because the concentration of carbon dioxide in flue gas from simple combustion is limited by the oxygen content of the air supplied to the fire (bo), the progress of this reaction towards the right is also limited. This lin-itation is less serious than the economic one imposed by the low speed of the reaction a t practicable temperatures, which either necessitates the provision of very large absorbers or prevents the close approach to equilibrium in practicable equipment (25, 40). To explain this slowness of reaction, many hypotheses have been advanced; they are based principally on the complexities of subsidiary reactions and on the belief that some or all of them are slow. Other theories are based on viscosity effects (26, 40, 4). The fundamental reactions involved are those suggested by this chain of transformations :
efforts were partially successful, but none fully met the new needs imposed on the process by the requirement of the industry of solid carbon dioxide for widely distributed production of low-cost gas. The methods employed for increasing the speed of absorption, and hence the extent of conversion of carbonate to bicarbonate in commercial equipment of fixed size, include reactions between carbon dioxide and solid carbonate in the presence of steam (1, 51), application of pressure to the gas in the absorber ( 2 , 3, 4, 7, 22, 23, 32, 37, 46, 47, 48), use of alkaline compounds other than carbonate in the absorbing solution (8-13, 21, 26, 41, 49, 63), and solution of carbon dioxide in water by pressure methods (7, 24). On the other hand, addition agents such as phenols (4S), formaldehyde, sucrose, dextrose, and others have been employed to accelerate the carbonation of the alkaline solution (4).Use of ammonia as the absorbent has also been suggested (3, 6, 16, 17, 19, 30, 39, 62). The presence of negative ions other than carbonate, such as borate, chromate, and phosphate (9,12, 1 3 , 2 l , 26, 32, 49), apparently have the effect of favoring the evolution of carbon dioxide from the bicarbonate solution and hence more complete stripping of the lye. Vacuum has also been suggested as a means of removal of carbon dioxide from the carbonated lye (28, 29). One process has been designed to utilize for absorption a solution of sodium carbonate so concentrated that sodium bicarbonate is precipitated and this, after separation, is made to yield its carbon dioxide from a hot brine suspension, allowing carbonate crystals to be returned to the absorber (18).
Present Process Although all of the methods suggested from time to time have been aimed a t improvement in the efficiency of the process, the commercial operation is still based on the simple reaction noted in Equation 1. When potassium carbonate is the salt used in the absorbing liquid, its concentration is approximately 13 to 20 pounds of anhydrous carbonate per cubic foot of solution (3 to 4.6 N ) (25, 43). Under commercial conditions this solution enters the absorber containing approximately 40 per cent of the potassium in the form of bicarbonate and leaves the absorber after absorbing carbon dioxide from 18 per cent flue gas with 55 to 70 per cent of its potassium in bicarbonate form. The concentration of the washed raw flue gas is a critical factor in absorption. The gas commercially available to such an absorber from a coke fire may show a carbon dioxide content as high as 20 per cent, but in practice this is held between 17 and 19 per cent (42). After absorption the waste gas may be reduced to a 9 to 13 per cent carbon dioxide concentration (27, 36, 43, SO), even though the limits theoretically possible are only half of these values (20). The lower solubility of sodium bicarbonate limits the usable concentrations of a sodium carbonate lye to those which will not precipitate sodium bicarbonate in the absorbing towers. Sodium carbonate concentrations of 8 to 9 pounds per cubic foot (2.4 to 2.7 N ) are consequently used, and in practice the weak lye from the lye boiler may contain as little as 30 per cent bicarbonate as against 50 to 70 per cent after absorption from flue gas high in carbon dioxide. The spent gas from a soda absorber may contain 9 to 13 per cent carbon dioxide, and under special conditions somewhat lower concentrations.
Obviously if any one of these reactions proceeds slowly, the whole will be slow in proportion; if two or more are slow, the total decelerating effect will change as the product and not as the sum of the slow rates.
Increasing Absorption Efficiency Early attacks on the problem of increasing the efficiency of the commercial process were directed towards pushing the absorption equilibrium farther to the right. Later the limit of the evolution reaction to the left was studied with a view to more complete stripping of carbon dioxide from the lye before returning it to the absorption cycle. Many of these 633
INDUSTRIAL AND ENGINEERING CHEMISTRY
634
Obviously in none of these situations is the equilibrium of the reaction reached. Various modifications of absorbing equipment have been suggested from time to time to improve efficiency in this respect, but the fact remains that as much as two-thirds of the carbon dioxide in the flue gas entering the absorbers of commercial plants is still present in the waste vented from them. The various addition agents proposed to improve this situation had, in general, the common failing of adding odor or flavor to the gas; and since in most of its applications the gas comes in contact with food, foreign odors and flavors destroy its value (15, 31, 33, 36, 38).
VOL. 29, NO. 6
ratio of carbonate to bicarbonate. The zone of reaction 4 is followed by a cooler absorber section (100' F.) fed with lye further cooled from reaction 4 whose function is to wash out ammonia remaining in the flue gas. In a single tower system this is accomplished by recirculation of part of the exit lye from the tower through a cooler to its top. Here the cooled lye and the water condensed from the gas generated in the lye boiler clean up the ammonia from the waste
Macmar Process The Macmar process, recently developed by McKee and Winter (34), overcomes the defects of the alkali carbonate absorption to a remarkable degree a t very low cost without introducing odor or flavor into the purified gas. I n principle, this process consists in combining the carbon dioxide in the gas phase with ammonia in the presence of water vapor. This reaction occurs in the partly stripped flue gas and converts carbon dioxide, which is difficultly soluble, into carbonate of ammonia, which is readily soluble in alkali carbonate solution. I n this way the capacity of the absorbing system may be actually doubled and the concentration of carbon dioxide in the exit gas reduced to as little as a fraction of 1 per cent. The reaction involved is:
+ Hz0 + C02 e (NHhCOa
2"s
20 I
I
I
3
I
I
1
/
12
'
I
\
I
3
2
/
\
I
/
?
B4
T~PCMNRE-T.
FIGURE1 The feed to the tower is ordinary carbonate lye from the lye boiler, previously cooled to 120-140' F. This dissolves the ammonium carbonate formed in reaction 3, and in solution the following reaction liberates ammonia for further carbonate formation: (NH4)eCOa
+ K2COa
2KHCOs
+ 2"s
VS. PER CCNT CARBON DlOXlDC ABSORBED FOR FLUE GA5 CONTAINING 13 PER CEyT Cop,
TEMKMTURE
II
L
I20
I -2
I
I
I25 I
I
,
130
I I
1
I
I
I40
136
1
I
1
I
CARBON DlOXlOL
W
6
-2 s Y
B
4
-2
(3)
Since this reaction takes place in the gas phase, it proceeds rapidly. The preferred temperature is 130' F. I n the presence of liquid water the solid ammonium salt is rapidly removed from the reaction zone, and the only limit to completion of the reaction is the decomposition pressure in aqueous solution of the ammonium carbonate formed. By establishing an excess of ammonia in even a very short section of tower virtually all of the carbon dioxide can be forced to react. The loss of ammonia with the waste nitrogen is prevented by washing the waste gas with water condensed by cooling the carbon dioxide evolved from the lye boiler, after it has been washed with cooled lye (100' F.).
I
/
(4) The ammonia previously introduced is thus largely evolved from the lye, leaving the carbon dioxide combined as POtassium bicarbonate. Reaction 4 takes place best a t 130' to 140' F. with cooled lye direct from the lye boiler having the highest possible
I
I20
I
I
I25
I
130
I
140
135
I
TEMPERATURE*E
FIGURE 2
nitrogen. As this lye falls down the absorber column, warmer lye from the lye boiler is added to the stream to bring the next section up to the temperature for reaction 4. Preferably the absorber system is divided into three distinct sections or towers. The first section receives lye, cooled to about 100" F., from the second and also receives fresh flue gas washed free of dust, sulfur dioxide, etc. The second section takes gas from the first and lyedirect from the lye boiler plus that from the third section, the temperature is-adjusted to 130-140' F. Gas to the third section comes from the second, and its liquor supply is cooled lve (100" F.) from the second section and condensed "water (containing ammonia) from the purified gas coolers. The absorber system is similar to those now in use. It consists of Dacked towers down which lye trickles countercurreni to the flow of flue gas. Ordinarily absorption towers are packed with coke, but experience has shown that steel turnings (sifted free of particles under 0.25-inch mesh) are superior to coke. Bubbleplate towers also have high efficiency but the higher cost of their construction, as well as their higher back pressure in operation, have militated against their general adODtion.
4
Reaction in Gas Phase I n practice, ammonia present in the gas phase in the tower apparently acts in the manner of a cataIyst in carrying the carbon dioxide into the lye solution. As ammonium carbonate is formed, i t is dissolved in the lye whose alkalinity decomDoses it. freeing ammonia to reenter the vaDor-Dhase reaction zone. This cyclic process continuously repeats itself. The acceleration of the absorption by this method can apparently best be explained by the avoidance of the resistance offered to the solution of gaseous carbon dioxide in the aqueous lye solution by skin or surface tension effects u
I
^
JUNE, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
in the interface. It may also be explained by the greater driving force made available by the fact that the solubility of ammonium carbonate is greater than that of gaseous carbon dioxide. Figures 1 and 2 show approximate equilibrium conditions and indicate opt'mum over-all operation a t 130' F. in the absorbing tower. The boiling off of the carbon dioxide from the charged lye is conducted in the ordinary manner. Water condensed from the cooled pure gas contains ammonia and is returned to the top of the absorber with the cooled spent lye. Because the lye is brought to a higher carbon dioxide content in the absorbers, the amount of steam required for operating the lye boiler per pound of evolved gas is slightly lower than in standard practice.
Economy of Process The questions naturally arising concerning this process from an operating standpoint are: (1) Is the carbon dioxide produced contaminated with ammonia? (2) Is ammonia lost in the waste gas? A small amount of ammonia does remain in the lye after absorption to be evolved with the carbon dioxide in the lye boiler a t the higher temperature met there. However, the reaction in the lye boiler also involves a rather copious evolution of steam along with the carbon dioxide. Before compression of the gas most of this steam is condensed, carrying with it the greater part of the ammonia, with some carbon dioxide, in solution. This condensate, which removes all but traces of ammonia from the carbon dioxide, is returned to the top of the final absorption tower where it serves t o wash the final exit gas practically free from ammonia and returns it to the system. As the cooled carbon dioxide from the condenser, freed from all but slight traces of ammonia, is compressed in the compressor system, the residue of its moisture content is trapped out between the compressor stages, and in this condensate the residue of the ammonia is dissolved. This condensate, too, is returned to the system. There is a slight loss of ammonia from the system with the waste gas. However, this loss is so small (2 pounds per ton of carbon dioxide output) that it has been uneconomical to attempt its complete recovery. Obviously an additional small scrubber added to the system could recover this loss if, under market conditions prevailing a t some future date, it should become economical to use it.
Pilot Plant Results The improved performance effected by the new process is shown in Table I; the data cover a number of test runs on a pilot plant scale (capacity, half a ton of carbon dioxide per day), using flue gas of various concentrations. These experimental data are plotted graphically in Figure 3, and for comparison a curve is drawn in to approximate average performance of plants employing the standard alkali process under similar conditions. The advantage of using
635
ammonia as a supplementary absorbent is obvious a t gas concentrations below 14 per cent and especially below 10 per cent.
TABLE I. Gas flow cu. ft./dal/
P E R F O R M A N C E DAT-4
Flue Gas& COSin entrance
COz, in exit gas
Recovery
%
%
%
gas
Turmericb Test Sec. 10 30 30
Temp.C of Lye to Tower F.
191,000 16.2 9.4 42.0 135 15.2 9.3 209 000 38.8 120 14.2 7.0 188:OOO 50.7 130 8.0 14.0 42.9 130 199,500 10.0 5.4 198,000 46.0 30 132 7.6 4.1 209.000 46.0 15-instantaneous 123 15.9 15 l48;OOO 7.7 51.6 134 9.3 10 165,000 3.0 67.6 125 8.2 48.7 16.0 Instantaneous 99 000 127 7.8 47.3 14'.8 10 132 106:OOO 14.4 6.8 52.8 30 105 000 130 14.3 45 6.5 54.5 108'000 131 13.8 2.9 79.0 30 135 111:ooo 13.5 5.3 60.7 30 98,000 130 13.3 1.6 88.0 N. T. 135 95,000 13.3 15 5.1 61.7 129 99 000 15 5.5 56.3 12.6 132 107'000 12.2 2.9 76.2 12s 100:000 15 (N. T.) 12.2 5.3 56.6 129 Instantaneous 107 000 12.1 4.9 59.5 133 102:ooo 30 12.0 15 3.4 71.7 131 98,000 2.8 10.3 30 72.8 128 102 000 4.2 10.0 60 58.0 130 101'000 15 2.9 65.5 129 104'000 8.6 15-30 2.7 65.8 136 7.9 100:000 0.5 4.5 15 88.9 135 92,500 49,600 15.2 1.5 90.1 130 a The percentage figures are based on the average hourly chemical analyses for the duration of the run. These concentrations of COz in flue gas cover the range of industrial stack gases which are as follows: from natural gag, COz; from coal, l0-15%: from fuel oil, 11-14%. 8-; e turmeric test oonsists of placing a strip of damp turmeric paper in the exit as flow and measuring the time of exposure before color change. 15-secon3 exposure is equivalent to 0.005% NHs' 30-second exposure i; N. T. signifies no trade of ammonia. equivalent to 0.0025% "a;. C These figures are approximately 30' 6. above those for standard alkali process using potassium carbonate.
..
..
la
The pilot plant absorbers in which the results shown in Table I were obtained were two towers in series, each 2.5 feet in diameter and 50 feet high. Each was packed with steel turnings sifted free of particles passing 0.25-inch mesh through 40 feet of its length. The density of packing was 15 to 20 pounds per cubic foot, but no figures are available on its exposed surface. The total volume of packed tower was 393 cubic feet. The lye used originally contained 14.0 to 16.0 pounds of potassium carbonate per cubic foot (specific gravity 1.18-1.20). The feed of liquor to the absorption system was 8 gallons of spent lye from the lye boiler per minute, and 15 gallons per minute of lye were recirculated from the foot of each tower. The spent lye from the lye boiler showed an average of 35 per cent of its alkalinity as bicarbonate, varying between 28 and 41 per cent. The liquor rate was held constant during the runs shown in Table I, and the gas rate varied as shown. The output of this absorption system of half a ton of pure carbon dioxide per day corresponds to 786 cubic feet of packed tower per ton per day. I n a production Dlant with an output of 15'tons per day, 750 cubic feet of similarly packed tower per ton per day suffice; only three of the four towers shown in the photographs are used. These performance figures compare with values given in the literature for coke-packed towers using carbonate lye absorbent of 1200 to 1300 cubic feet per ton per day (27, 43). The difference is partly due to the greater apparent efficiency of the steel turnings as packing but largely to the effect of ammonia on the absorption.
Theoretical Discussion PCRCENT CARBON DlOliDC IN FLUC GAS
FIGURE 3
The reason for the increased economy of this process probably lies in avoiding the decelerating effect of the
INDUSTRIAL AND ENGINEERING CHEMISTRY
636
fdms in both the gaseous and liquid phases in ordinary absorption. To account for the slowness of absorption reactions generally, the theory of the existence of such films of nearly equal concentrations of the absorbed constituent has been widely accepted. That the driving force of absorption (which depends upon the concentration difference) is relatively small across these two films is put forward to explain the otherwise unaccountable slowness of such reactions. The truth of this view seems further confirmed by the effectiveness of ammonia in the present case. Here the reaction takes place in the gas phase entirely, and the subsequent absorption of the reaction product and regeneration of the ammonia take place through simple solution of the highly soluble ammonium compound and its reaction in solution. Obviously this mechanism avoids the slow transfer of carbon dioxide through a gaseous film and through a liquid film, and the increased reaction velocity with gaseous ammonia indicates that no such decelerating effect is there encountered.
Commercial Importance The commercial importance of this new process becomes evident as the carbon dioxide content of the flue gas supplied to the absorption plant drops below the maximum ordinarily attainable from coke fires. It is evident from the curves in Figure 3 that a t 12 per cent concentration and below, the use of ammonia as a supplementary absorbent is highly valuable. Indeed, since this is the range into which fall the flue gases from normal operation of power plants, the new process may be seriously considered as placing these producers of enormous tonnages of gaseous carbon dioxide into the category of potential commercial sources of this material. By thus permitting the economical recovery of carbon dioxide from power plant stack gases in locations where populations are normally large the ammonia-alkali absorption process immediately simplifies the problems of producing solid carbon dioxide. Because of the relatively low recovery from even specially controlled coke fires, the cost of independent production of carbon dioxide has been high, and supplies needed for making solid carbon dioxide have been drawn largely from by-product sources seldom ideally located with respect to consuming markets. The necessity for additional handling of the refrigerant with attendant evaporation losses, as well as freight costs from plant to market, have placed a handicap on the expansion of markets necessary to lowered costs. If, on the other hand, power plants located near markets can produce solid carbon dioxide as cheaply with the new process (utilizing flue gas and either or both off-peak or incremental power) as seems probable, these aspects of the industry’s problem are solved and new expansion is indicated.
Literature Cited (1) AI, Jan (to N.-V. de Bataafsche Petroleum Maatschappij), U. S. Patent 1,831,731 (Nov. 10, 1931). (2) Allen and Michalske, U. S.Patent 1,934,472 (Nov. 7, 1933). (3) Arnold, E. E. (to Nitrogen Corp.), Ibid., 1,611,401 (Dee. 21, 1926).
(4) Behrens and Behrens, German Patents 162,655 (Sept. 27, 1905) and 173,130 (July 7, 1906). (5) Beins, H., U. S.Patent 152,269 (June 23, 1874).
VOL. 29, NO. 6
(6) Bird, E. H. (to Koppers Co.), IZSd., 1,654,782 (Jan. 3, 1928). (7) Blau, Herman, U. S. Patent 1,817,667 (Aug. 4, 1931). (8) Bottoms, R. R. (to Girdler Corp.), Ibid., 1,783,901 (Dec. 2. 1930); Reissue 18,958 (Sept. 26, 1933). (9) Ibid., 1,834,016 (Dec. 1, 1931). (10) Ibid., 1,964,808 (July 3, 1934). (11) Ibid., 2,031,632 (Feb. 25, 1936). (12) Bragg, G. A. (to Koppers Co. of Del.), U. 5. Patent 1,920,626 (Aug. 1, 1933). (13) Ibid., 1,924,178 (Aug. 29, 1933). (14) Cabot, S.,U. S.Patent 359,996 (March 29, 1887). (15) Carmichael, H. G., Ibid., 2,044,279 (June 16, 1936). (16) Clancy, J. C. (to Nitrogen Corp.), Ibid., 1,425,577 (Aug. 15, 1922). (17) Claus, C. F., British Patent 15,173 (1888). (18) Drewsen, Viggo (to W. Va. Pulp & Paper Co.), U. S.Patent 1,619,336 (March 1, 1927). (19) Gaus, Wilhelm, et al. (to I. G. Farbenindustrie A.-G.), Ibid., 1,897,725 (Feb. 14, 1933). (20) Goosman, J. C., Refrig. Eng., 18, 155 (1929). (21) Hackhofer, Ernest, and Beuther, A., U. S. Patent 2,044,116 (June 16, 1936). (22) Hasche, R. L., et al. (to American Smelting & Refining Co.), U. 8. Patent 1,794,377 (March 3, 1931). (23) Ibid., 1,798,733 (March 31, 1931). (24) Henderson, W. N. (to Solvay Process Co.), U. S. Patent 1,698,722 (Jan. 13, 1929). (25) Hitchcock and Cadot, IND. ENQ.CHEM.,27, 728 (1935). (26) Horvitz, G. J., U. S.Patent, 1,916,980 (July 4, 1933). (27) Howe, H. E., IND.EXQ.CHEM.,20, 1091 (1928). (28) Jones, C. L., U. S. Patent 1,864,068 (June 21, 1932). (29) Knowles, A. E., Ibid., 963,586 (July 5, 1910). (30) Krase, H. J., et al. (to Arthur B. Lamb, Trustee), Ibid., 1,673,877 (June 19, 1928) ; British Patent 335,913 (1929) : Krase and Hetherington, IND.ENG.CHEM.,19, 208 (1927). (31) Littler, H. G., Chemistry & I n d u s t r y , 52, 533 (1933). (32) Luhmann, Eduard (to Surther Maschinenfabrik), U. S.Patent 491,365 (Feb. 7, 1893). (33) McKee, R. H., U. S. Patent 1,937,832 (Dec. 5, 1933). (34) McKee and Winter (to Macmar Corp.), Ibid., 2,043,109 (June 2, 1936). (35) Martin, J. W., Am. SOC.Refrig. Engrs., Refrigerating Data Book, 1934-36. (36) Metzger and Backus (to U. S. Industrial Alcohol Co.), U. S. Patent 1,982,223 (Nov. 27, 1934). (37) Mount, W. D., Refrig. Eng., 23, 291 (1932). (38) Nathan, Leopold (to Hansena A.-G.), U. S. Patent 1,968,899 (Aug. 7, 1934). (39) Parnell, E . W., et al., British Patent 46 (1886). EXQ.CHEM.,24, 630 (1932). (40) Payne and Dodge, IND. (41) Perkins, G. A. (to Carbide & Carbon Chemicals Corp.), U. S. Patent 1,951,992 (March 20, 1934). (42) Pratt, J. H., IND.ENQ.CHEM.,24, 631 (1932). (43) Reich, G. T., Chem. & M e t . Eng., 38, 136 (1931). (44) Riou et al., Compt. rend., 174, 1017, 1463 (1922); 184, 326 (1927): 186. 1543. 1727 (1928). (45) Rogers,‘D. A,’(to Atmospheric Nitrogen Corp.), U. S. Patent 1,968,655 (July 31, 1934). (46) Shoeld, Mark (to Koppers Co. of Del.), Ibid., 2,002,357 (May 21, 1935). (47) Smith, W. L., Brazilian Patent 10,120 (June 19, 1931). (48) Stevens, Simon, U. S. Patent 68,321 (Aug. 27, 1867). (49) Tyrer, Daniel (to Imperial Chemical Industries Ltd.), Ibid., 2,031,802 (Feb. 25, 1936). (50) Ullmann, “Enzyklopadie der technischen Chemie,” 2nd ed., Vol. 6, p. 591 (1930). (51) Walker, W. (to Dan Rylands, Ltd.), U. S. Patent 496,546 (May 2, 1893). (52) Wallace, R. W . , et al., Ibid., 1,154,145 (Sept. 21, 1915). (53) Woodhouse, J. C. (to E. I. du Pont de Nemours & Co., Inc.), Ibid., 1,912,877 (June 6, 1933). (54) Zweitusch, O., I b i d . , 518,361 (April 17, 1894).
RECEIVED February 1, 1937.
