December 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY literature Cited
(1) Baker, E. M., and Mueller, A. C., IND.ENG.CHEW,29, 1065
(1937).
(2) Bauermeister, H. O., unpublished M.S. thesis, Illinois Institute
of Technology, Chicago, Ill., 1941. (3) Bisesi, C. H., Ibid., 1942. (4) Brodkey, R. S., Ibid., University of California, Berkeley, Calif., 1950. (5) Bromley, L. A,, Chem. Eng. Progress, 46,221-7 (1950). (0) Bromley, L. A., unpublished M.S. thesis, Illinois Institute of Technology, Chicago, Ill., 1943. (7) Dwight, H. B., “Tables of Integrals and Other Mathematical Data,” New York, Maomillan Co., 1934. (8) Gilkison, T. M., unpublished M.S. thesis, Armour Institute of Technology, Chicago, Ill.. 1938.
(12) (13) (14) (15)
2969
Katq D. L., Hanson, G. H., Kemp, H. S.,and Ogdyke, E. G., Petroleum Refiner, 25, 419 (1946). Langen, E., Forsch. Gebiete Ingenieurw., 2 , 359-69 (1931). RlcAdams, W. H., “Heat Transmission,” Xew York, McGrawHill Book Co., 1942. Nusselt, W., 2. Ver. deut. Ing., 60, 541-6, 569-75 (1916). Peck, R. E., and Bromley, L. A., IND. ENG.CHEY.,36, 312-16 (1944). Peck, R. E., and Reddie, W. A., Ibid.,43,2926-31 (1951). Pierce, B. O., “A Short Table of Integrals,” Boston, Mass., Ginn and Co., 1929.
RECEIVEDfor review January 14, 1952. ACCEPTED June 30, 1952. Presented in part before the Division of Industrial and Engineering ChemAtlantic istry ‘at the 122nd Meeting of the AMERIcls C H E m c h L SOCIETY, City, N. J., September 1922.
Gas Absorption Absorption of Carbon Dioxide from Air by Sodium and Potassium Hydroxides I
HAROLD A. BLUM’, LEROY F. STUTZMAN,
AND
WAYNE S. DODDS
Norfhwerfern Technological Inrfifufe, Evansfon, 111.
B
EFORE 1937, several investigators made studies on the absorption of carbon dioxide from air by a caustic solution. Their work is reviewed by Sherwood (91). A brief survey of this review and subsequent investigations will give a background for the present study. Many investigators approached the problem of absorption of carbon dioxide by caustic solutions with a correlation using the over-all gas transfer coefficient, combining in this term the effects of gaseous diffusion, liquid diffusion, and chemical reaction. Hatta ( 5 ) found KOto be independent of the normality or other conditions in the liquid and concluded that the controlling resistance was in the gas film. Jenny’s findings (10) disagreed with Hatta, in that the liquid film was important since K , was lower than that for ammonia in the same apparatus. Jenny found that the normality between 1.0 and 2.0 N did not affect K , but that the normality below 1.0 N d i d affect Kg, Tepe and Dodge (94)used a countercurrent flow system and found that K,a was a maximum a t 2.0 N caustic, decreased linearly with sodium carbonate normality, increased as the 0.28 power of the liquid rate, increased as the sixth power of the absolute temperature, and was unaffected by the gas rate. They concurred with Jenny that the gas film resistance was negligible. Spector and Dodge (93),with the same system found that Koa varied as the 0.5 power of the absolute tower pressure, increased as the 0.35 power of the gas rate below a rate of 500 pounds per hour per square foot, and increased as the 0.15 power of the gas rate near a rate of 100 pounds per hour per square foot. Solutions of potassium hydroxide gave 20 to 30% greater values of Koa than solutions of sodium hydroxide. The investigators concluded that the resistance in the gas film was significant but not predominant. Their data were represented by equations of the form log Koa = 0.20 log L
-K
(1) The attempt to analyze the problem through the processes that occur in the liquid phase was begun by Brunner ( I ) , who pictured a double film in the liquid, in which the reacting gas 1 Present
address, Atlantic Refining Co., Dallas, Tex.
molecule dissolves, passes through the liquid film, reacts instantly at an interface (or infinitely thin film) with the liquid reactant molecule, which has diffused through a second film to meet it. The product (assumed nonvolatile) diffuses back into the main body of the liquid. Weber and Nilsson (85), Hatta ( 5 ) , and Davis and Crandall ( 8 ) extended and developed this concept further. Hatta ( 5 ) found when using air-carbon dioxide mixtures and caustic solution that the gas resistance was of little importance above a carbon dioxide content of 38% and that the liquid resistance controlled the transfer rate. I n this range the rate of absorption was proportional to the residual base concentration of the liquid. Under this situation it was logical to analyze the problem from the liquid standpoint. I n the absorption of carbon dioxide by a caustic solution, many reactions can be suggested that might influence the chemical kinetics. Hatta ( 5 ) believed the following reactions to be the important ones:
+ OH- = HCO; + OH- = HzO + C0;-
COz HCO,
(3) He believed that Equation 3 was very rapid compared with Equation 2 and that both were rapid and irreversible. He developed a n equation based on these assumptions. An adaptation of his equation and a similar equation by Davis and Crandall(2) was discussed by Sherwood (21).
The equation was partially supported by Hitchcock ( 7 , 8) in batch experiments, in which the carbon dioxide pressure was 1 atmosphere and the normality of the base was not greater than two. Above a normality of two, the rate of absorption falls off and this equation does not hold. This fact was likewise noted in the batch experiments of Mitsukuri (16) and Ledig and Weaver (15). Jenny (10) thought that the reactions should not have been considered instantaneous but to have taken place through given
INDUSTRIAL AND ENGINEERING CHEMISTRY
2970
time and space intervals, resulting in a film thickness for the reaction zone. If the chemical reaction is extremely slow, the absorption is a process of diffusion alone and can be handled as in ordinary gas absorption. Also, if the chemical reaction is rapid and irreversible, the resistances to diffusion control the rate, as shonn by Equation 4. Between these two extremes, there is a need for a knowledge of the chemical kinetics involved.
Vol. 44, No. 12
From this equation and their Equation 7 , they developed the following :
CONSTANT GAS OUT
(7)
'
They assumed a second-order reaction, in which the concentrstion of t'he reactant. in the liquid phase ( c i ) did not change appreciably during the process. This equat'ion reduces to one of physical diffusion in packed towers when k ~ ~approaches c i zero. When krrc; is relatively great, the "chemical group" heroines approximately
bz "
3' OIAMETCR
SUPPLY
Liauio CYLlNkR
PUMP
TANK
(M2)'
ABSORPTION
OYEWLOW TANK
THERMOMETERS
COLUMN
@
a VAPOR LINES
0
SAMPLING V
0
a a
MANOMETERS C O L U M N PRESSURE
hd) 1'2
so that the rate of absorption will he proportional to the quaye root of the concentration of the reactive liquor component. They used the data of Tepe and Dodge ( 2 4 ) and the reaction velocity constants of Payne and Dodge ( 2 7 ) . The constant was calculated and an average value mas found t o be 0.01'3 & 23c7,
Figure 1. Packed Glass Absorption Tower Equipment Flowsheet VAPOR 0 , U T L E T
'3(
E
N
VAPOR VAPOR LIQUID LIQUID
INLET OUTLET INLET OUTLET
A
It'
DROP
ORIFICE PRESSURE DROP ORIFICE U P S T R E A M
PRESSURE
1
w1 CO,
SURGE
TANKS
CYLINDER
Figure 2.
OUTLET
,a; A!"rn
OF
GEAR BLOWER
Packed Steel Absorption Tower Equipment Flowsheet
For a case of a transfer involving both diffusion and a firstorder chemical reaction, the rate of which was intermediate between the rapid and the slow, Hatta ( 6 ) presented the following equation:
(14). TVhiIe there is some support for the ajsumptioii that the reactions described here are the controlling ones, there is 110 universal agreement concerning mechanism or the kinetics of the process.
Scope of the Investigation
It was assumed that the concentration of the component transferring from the gas phase was zero or negligible in the main body of the liquid or, in other words, that the chemical reaction took place in the liquid film. Van Krevelen and Hoftijzer (2.3) believed Equation 2, a secondorder reaction, was controlling. By analogy to an equation by Gilliland and Sherwood (S),correlating kg in wetted wall columns, they developed an equation for kl for packed towers.
The investigation \l-as divided into two phases : 1. To determine the effect of the important variables on the transfer of carbon dioxide from air into sodium hydroxide, sodium hydroxide-sodium carbonate, potassium hydroxide, and potassium hydroxide-potassium carbonate solutions. 2 . T o correlate the data in a manner useful for design. The variables studied in the firqt phase and the range of the variables were: liquid rate, 13 to 185 pound moles/hour X square feet; gas rate, 2.9 to 18 pound moles/hour X square feet; mole fraction of carbon dioxide in gas, 0.03 to 0.28; packing size, 1/4-, 3/8-, 1;2-inch Raschig rings; height of packing, 2.87 to
December 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
4.33 feet; concentration of base, 0.07 to 3.90 N ; type of base, potassium or sodium hydroxide; concentration of carbonate, 0.00 t o 0.44 N (entering) and 0.02 to 0.65 N (average).
Experimental Procedure Two columns were used for the investigation, one of glass and the other of steel. The glass tower, Figure 1, was 4 feet high with an inside diameter of 2.8 inches and was packed to a height of 2.82. feet with '/*-inch Raschig rings, which were supported on a wire screen. The liquid was distributed across the column from eight small holes in a copper coil located slightly above the packing. Air, which was obtained from the building compressed-air line, entered a t the bottom and passed up through the tower. Its rate was measured in a calibrated gas rotameter. The gas samples were taken by means of an Orsatt apparatus, the inlet sample at a point between the rotameter and the tower, and the gas outlet sample downstream from the tower. The liquid solution, made up and stored in the holding tank, was transferred by a centrifugal pump to the constant head tank, then fed to the column through a small globe valve. The inlet liquid sample was obtained from the constant head tank and the outlet sample from the sampling tube in the base of the column. Based on the results obtained on the glass tower, runs 115 to 186 were made in the steel tower. The steel column shown in Figure 2 was made from a section of 4-inch carbon steel, schedule 40 pipe 84 inches long and was packed to various heights. It was operated countercurrently.
2911
The liquid, made up in a holding tank and transferred to the tower by a centrifugal pump entered a t the top of the tower and was distributed over the packing by a drilled l/rinch copper coil placed slightly above the packing and was measured by weighing the effluent. Air, obtained from the room and forced up through the system with a gear blower, was measured with a calibrated orifice and a manometer. I n both columns provisions were made for continuously observing the inlet and outlet temperatures of the gas and liquid streams and for measuring the pressure drop across the column. Thirty minutes were allowed for steady state to be reached, after which the data were taken and samples were obtained. Gas and Liquid Analyses. A 100-ml. gas sample was drawn into an Orsat apparatus, measured, then transferred into a n evacuated bottle containing 50 ml. of a standard barium hydroxide solution. After sufficient agitation, the contents of the bottle was titrated with standard hydrochloric acid to a phenolphthalein end point to determine the remaining hydroxide (18-20). With this result and with the values of the pressure and temperature a t which the gas sample was measured, the gas composition was calculated. A measured liquid sample was pipetted into a bottle containing 50 ml. of standard barium hydroxide solution and back-titrated with hydrochloric acid. The purpose of the barium hydroxide in this analysis was to precipitate the carbonate which had been formed in the column. Experimental Data. A total of 186 runs was made in the ranges of study as given in Table I. Gas temperatures were held between 72" and 92' F., while most of the runs were between 75" and 85" F. Liquid temperatures varied between 65' and 95" F. with most of the runs between 75" and 85" F. The pressure on the tower for all runs was essentially atmospheric. Material balances showed an average deviation of 10% and a maximum deviation of 37%. This deviation is within the maximum experimental error as calculated by the method used by Sherwood and Reed ($2).
Correlation and Interpretation of Data The analysis of the data presented in the previous section has been done by calculating the over-all gas transfer coefficients and studying the effects of the variables on them, and by developing an equation based on the assumed mechanism of the reaction and correlating the data with it. Calculation of K,a's were made using a log mean driving force and assuming a zero back pressure of carbon dioxide. The equation used for this calculation is
Koa =
Figure Curve NO.
Raschig Ring Size,
In.
3. Ksa vs. L in Steel Tower
h
4.33 4.30 4.38 4.33 4.30 4.30 4.30 4.30 3.02
0 4.3-12.6 3.4-9.7 7.5-18.8 2.9-11. 1 12.0-13.2 9.1-10.2 8.6-11.2 9 .O-11.5 4.5-9.5
uar. 0.03 0.12 0.06
0.05 0.05 0.05 0.04 0.04 0.04
NsaoH (Entering) 0.08 0.52-0.56 0.27 0.38 0.61 1.87 3.90 1.98 1.51
Nu
(9)
The data are plotted in Figures 3, 4, and 5 as K,a (over-all gas transfer coefficient) versus liquid rate. The curves show that K,a is independent of gas rate. It can also be seen that the transfer coefficient increases with the liquid rate, but that this effect becomes less as the liquid rate increases. K,a increases with the normality of the sodium hydroxide up to 1.51 N and then is not affected by a further increase in the base concentration. The effect on Koa of the average partial pressure of carbon dioxide was a function of the normality of the base. At high normalities (above 1.51 N ) , K,a decreased as the average partial pressure increaAed up to a value of 0.06 atmosphere. A further increase in partial pressure had no noticeable effect on K,a. At lower normalities (Le., 0.88 N ) , K,a decreased as the partial pressure increased throughout the range of the experimental data (Le., 0.05 to 0.20 atmosphere). As the average carbonate concentration increased, Koa decreased. The fact that potassium hydroxide was a better ab-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2972
Vol. 44, No. I2
It was assumed that the amount of carbon dioxide transferred per unit area of tower cross section was proportional to the concentration of ions and nonions that tend t o remove carbon dioxide from the solution. From the left-hand side of Equations 10 and 12, the following relationship may be written: Sa
m
(OH-)a ( C 0 # (C0;)C ( H z O ) ~
(13)
The concentration of the water does not change appreciably; therefore, it can be considered constant. The concentration of the carbon dioxide cannot be experimentally determined by ordinary analysis, but its concentration may be expressed as a function of the physical conditions as follows:
1
1
(C02)6 = P (ionic strength, p ’ ) , (inert gas rate, G ’ ) , (liquid rate, L ) , (packing size), (temperature), (absolute pressure), (partial pressure of carbon dioxide), (viscosity of solution, p ) , (packing height, h ) , etc.
The ionic strength was assumed to be the principal factor, since it is known that carbon dioxide a t a given temperature is less soluble in aqueous salt solutions than in pure water. When exponents are assumed to be equal to one, the following equation resulted:
(OH-)( CO-)
Xa = h’h
(14)
bom
K’ was calculated from the experimental data using this equation, such values being tabulated in Table I and plotted as a function of liquid rate, L, in Figure 6. Experimentally, it was found that this equation fits the data best when m = 1.09, also IC’
=
0.0176
(15)
Figure 4. Ksa vs. L in Glass Tower Raechig ring size,
G 7.6 6.2-10.3 8.6 8.6
7.0 9.0 7.0
8.6
inch; h = 2.87 feet Uav.
0.065 0.056 0,055
0.055 0.12 0.06 0.12 0.025
iYsaoH (Entering) 0.07 0 48-0 53 0 74-0 77 1 51 2 97 1 94 2 02
1.20
sorber than sodium hydroxide can be seen by comparison of Figures 3 and 5. There was no noticeable effect of packing size on K,a. From the observations of the variables affecting the over-all gas transfer coefficient, i t can be seen that the correlation of K,a is complex. I n view of this, a n attempt was initiated t o find another method of correlating the data. Since Koa is independent of the gas rate, the principal resistance t o transfer must be in the liquid phase; a useful correlating factor might be found by placing more emphasis on what goes on in t h a t phase. An assumption was made t h a t the rate of transfer is dependent on the chemical kinetics. The chemical reactions t h a t are believed to occur are given by the following equations:
+ CO1 = HCO; OH- + HCO, = HOH + COT CO; + COz + HOH = 2 HCO, OH-
(10) (11) (12)
Equation 11 is believed t o be a rapid one since it is a reaction between ions (4). Therefore, the remaining two reactions involving nonions would control the rate. Equation 12 can be considered irreversible if OH- is present.
0.0
50
100
I50
L (LB MOLS/HR. SP FT)
200
Figure 5. Kga vs. L with Potassium Hydroxide Solution Curve No. 1 2 3
NKOH (Entering) 0.193 0.363
0.704
Ua..
0.05 0.06 0.045
December 1952
INDUSTRIAL A N D ENGINEERING CHEMISTRY Table 1.
R~~ No. 2 3 55 9
2973
Data on Rate of Absorption of Carbon Dioxide from Air"
(Sodium hydroxide solution, 2.75-inch glass tower, I/r-inch Raschig rings, 2.87 feet packing depth) Na Lb. Kga, Lb. Moles T ~ , DeviaL' Lb 6" Lb. NOHMolks HzO Moies Air in Mhes (Av.), K' K' tion, ole %$e (Enter- NOHO F. pl.o( (Exptl.) (Calcd.) % Hr. x Sq. Ft. Hr. X Sq. Ft. Fract. Fract. my) (Av.) NcoH * x Atm. 0.44 75 0.21 0.080 0.608 0 . 2 5 2 0 . 2 4 8 0.32 2 0.0695 0.0583 0 . 6 3 6.2 23
6
37 91 35 60 104 47 56 31
10
11 12 42 43
6.1 6.1 13.1 13.1 13.1 13.1 8.6 8.9
0.0611 0.0611 0.0485 0.0489 0.0674 0.0643 0.0609 0.0748
0.468 0.0374 0.0438 0.0406 0.0536 0.0568 0.0406 0.0585
0.49 0.49 0.47 0.47 0.47 0.47 1.51 1.51
0.34 0.40 0.34 0.38 0.38 0.34 1.32 1.19
0.16 0.09 0.13 0.09 0.09 0.13 0.20 0.32
0.102 0.155
0.084
0.102 0.170 0.113 0.198 0.179
0.67 1.19 0.52 0.82 1.02 0.67 1.39 0.95
76 76 73 73 73 73 72 72
0.535 0.508 0.508 0.488 0.486 0.508 1.676 1.761
0.367 0.736 0.327 0.490 0.834 0.443 0.449 0.283
0.367 0.780 0.343 0.548 0.870 0.450 0.515 0.310
0 - 6
- 5 -11 - 4 - 1 1. - 5
-
" Complete data filed with American Documentation Institute.
No attempt was made to study the effect on K' of temperature and pressure, these conditions being essentially the same for all runs. The average deviation for all 186 runs is 11.1% with a maximum deviation of 32%. A summary of the net and mean deviations is presented in Table 11. K' was found to be independent of gas rate and ring size. While there was an effect of partial pressure of carbon dioxide on K', this effect was small and varied with different solutions. It was possible to correlate the data without considering this effect. There was a noticeable trend with normality of the base. K' tended to increase slightly with normality up to above 1.5 N and then decrease as the normality was further increased. Potassium hydroxide, as indicated by the net deviation from the correlation, is a better absorber than the sodium hydroxide. The effect of the liquid rate on the concentration of carbon dioxide in solution can be explained on the basis of holdup. Jesser and Elgin (11) found that holdup is proportional to the liquid rate to the 0.6 power for l/*-inch Raschig rings. The amount of holdup or liquid inventory in a column is dependent on the liquid rate. If the chemical reaction controls and the films are negligible or nonexistent, the rate of transfer varies with amount of solution present. Effectively, this means that when the liquid rate is increased the size of the reactor is greater. Since K' is independent of gas rate, the assumption that the important resistance to transfer is in the liquid phase is supported. The mechanism developed in this study assumes that the gas or liquid films offer little resistance or are nonexistent. That the film might be nonexistent in the liquid phase is supported by the present study and by the work of Koch, Stutzman, Blum, and Hutchings ( l a ) ,in which the effect of packing size on the transfer rate was found to be negligible. These studies were on system in which the controlling resistance was in the liquid phase. In a system in which the controlling resistance was in the gas phase,
Table
II.
Hutchings, Stutzman, and Koch (9)found the Raschig ring size to affect the transfer rate, thus supporting the film theory in gas phase. The increase of K' with normality up to 1.5 N and the decrease with further increasing normality can be explained by two opposing forces. One is the removal of the carbon dioxide by reaction with the hydroxide ion, and the other the impedence preventing the carbon dioxide and hydroxide ion from making contact. This latter factor might be viscosity. Since neither the correlating equation nor the mechanism on which it is based refers specifically to sodium hydroxide, runs 175 to 183 were made, in which potassium hydroxide was the absorbent. A trend in the data indicated that potassium hydroxide solutions were better absorbers than the sodium hydroxide solution. It fits the correlation reasonably well. One explanation
Summary of Mean and Net Deviations for K' System
Over-all (180 runs) Glass column l/:-inch Raschig rings, 2.82 it. packing ht. (runs 1-114) Steel column */pinch Raschig rings, 4.30 ft. packing ht. (runs 115154)
Mean Net, Deviation, Denation,
%
1.4
9.1
3.3
17.4
J/tinch Raschig rings, 4.33 ft. packing ht. (runs 155163)
a/r-inch Raschig rings, 3.02 ft. packing ht. (runs 164171)
J/r-inch Raschig rings, 3.02 ft. packing ht. (runs 172174), sodium carbonate in entering solution */&oh Raschig rings 3.02 ft. packing ht. (runs 175183), potassium hydroxide solution a/r-inch Raachig rings 3.02-ft. packing ht. (runs 184Ma), otassium hyhroxide and carbonate in entering aofution
%
11.1
-
6.8
2.0
0.0
10.0
- 4.0
11.0
11.0
20.0
20.0
23.0
-23.0
L.
Figure 6.
LBMOLS /HR.r SQ.FT.
K'
YS.
L for All Runs
for its better absorption might lie with the viscosity effect. Another possible reason is that, the sodium ion being smaller than the potassium ion, it will to a greater extent form covalent bonds. In water this tendency is indicated by hydration and the subsequent lower mobility of the sodium as compared to the potassium ion ( 4 ) . If there is a greater amount of hydration With the potassium ion, it means less water in solution as free water; therefore, the effective concentration of the hydroxide would be greater. The previous work of Tepe and Dodge ($4) did not check this correlation. Using the data in the above-mentioned study, the
2974
INDUSTRIAL AND ENGINEERING CHEMISTRY
values obtained for K‘ were low. The low partial pressure they used mag account for this. For example, in run 20 in their work, I
