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is an incentive for the study of the higher alcohols and their esters. A number of these alcohols, both secondary and tertiary, will soon be available and much interesting work can then be done. The tertiary alcohols are of particular scientific interest because they contain a very reactive hydroxyl group and a very inert hydroxyl-hydrogen. For this reason they should be classed by themselves. No doubt syntheses could be effected from them which are not possible in the case of other alcohols. The preparation of alcohols directly from paraffin hydrocarbons would be an advance of prime importance. The higher fatty acids, on account of their extensive use in the free condition or as esters, are worthy of detailed study. New, cheap synthetic methods for these compounds from hydrocarbons would result in a revolution in many of our industries. The history of industrial organic chemistry leads us to believe that this problem will be solved. How long will it be before the chemist makes fats and soap from petroleum? I have stressed the importance of finding new synthetic methods. The success obtained in the case of acetaldehyde, acetic acid, and methanol make us confident of the future. I would place formaldehyde and acetic anhydride in the first rank among the compounds which should be studied from this point of view. It is possible to take up one class of compounds after another and point out, in each case, opportunities for research. But I have accomplished my purpose if I have caused attention to be centered on the desirability of going back to a branch of organic chemistry that has received but scant attention in recent years. Emphasis has been put upon the kind of research which will be of value in developing potential industries. To one who has studied the relationship between pure and applied science in the industries based on organic chemistry it is clear that the advances in both branches of the science have
Vol. 18, No. 3
resulted from the cooperation between the workers in the two fields. Some of us are apt to think that all the credit should be given to the investigator who shuts himself off from the world and its problems. Where would he be without the results of industrial development and without the problems that naturally spring up in the study of largescale operations and the technical use of the results of pure science? It is only necessary to study the dyestuff industry to see what an incentive it has been in the development of pure organic chemistry. Let us recognize this fact and busy ourselves with the pure science which has great possibilities ahead from the standpoint of its applications. There will be many by-paths discovered which will lead to new fields for research. The formulation of the fundamental problems springing from industrial processes furnishes us with such subjects for research as catalysis in all its phases, the effect of structure on chemical reactivity, the energy relations involved in the interaction between molecules, molecular rearrangements, the nature of the bonding between atomsin fact the fundaments of organic chemistry. Why not investigate these problems with the very compounds which have been or can be used for the good of the world; advance will be the more rapid if this is done. America is taking a commanding position in chemical research. Should we not turn our attention to the study of one of our great natural resources-petroleum-and build up the chemistry of the products obtainable from it. We have lagged behind. H. S. Davis has been studying recently the literature of the diolefins. Up to 1910 but 3 per cent of the published papers were produced by American chemists; only 9 per cent appeared in the English language, whereas Russia furnished 45 per cent of the whole. A study of the work subsequent to 1910 would probably yield similar results. We have a fruitful field before us. Let us grasp the opportunity!
A Comparison of Gas Absorption and Rectification' By W. G. Whitman and G. H. B. Davis DEPARTMENT OP CHEMICAL ENGINEERING, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASS.
HE processes of gas absorption and of vapor recti-
T
fication are similar in many ways, since both are cases of interaction between liquid and vapor. The major difference is the presence of inert gas in most absorption problems and its absence in rectification. Recently Murphree2 has developed a mathematical treatment of 'rectification based upon the equations developed for gas absorption. This paper presents the results of a series of experiments in a plate column on the absorption of carbon dioxide from flue gas by sodium carbonate lye and compares them with other results obtained when the same column was used for rectifying a carbon dioxide-water vapor mixture by means of the lye. The absorption and rectification apparatus, shown diagrammatically in Figure l, is a 15-plate column with one boiling. cap per plate. A lye consisting of a mixture of sodium carbonate and bicarbonate is fed to the top plate at a controlled rate and temperature, and discharged from the bottom. Gas, which may be a mixture of carbon dioxide and an inert gas, a mixture of carbon dioxide and steam, or pure steam, is fed into the base of the column and discharged a t the top. Samples of lye and gas for analysis can be drawn from the entering and exit streams and from the five plates indicated in the figure. 1 2
Received December 23, 1925. THISJOURNAL, 17, 747 (1925).
After the column has been operating about half an hour a set of lye samples is drawn a t one time and the various gas samples analyzed within 5 minutes. The rate of lye flow is recorded both before and after taking the samples, and the gas flow read from a n orifice meter. This procedure is then duplicated several times in order to check the results. The lye samples can be analyzed later in the laboratory. Analysis of the lye is made with standard hydrochloric acid and double indicators (phenolphthalein and methyl orange). The gas is analyzed with an Orsat when it consists of air and carbon dioxide, and the partial pressure of carbon dioxide calculated from the analysis and the total pressure a t the sampling point, correcting for the vapor pressure of water a t that point in the column. When the gas contains only steam and carbon dioxide, the sample is blown through a 100-cc. glass buret with stopcocks until condensation ceases, then the cocks are closed and the lower cock is reopened under water with the buret in a vertical position. The volume of uncondensed gas represents carbon dioxide saturated with water vapor at the temperature of the water, and the calculation of the original pressure of carbon dioxide in the column can be made as before. Gas Absorption
A flue gas containing about 15 per cent carbon dioxide and a t 60" C. was fed a t a rate of from 60 to 220 liters (wet
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
March, 1926
gas at standard conditions) per minute. The entering lye was approximately 1.7 normal in alkali and was about 35 per cent converted into the form of bicarbonate. The lye flow was about constant a t 1.25 liters per minute. Table I shows the complete data on one run and certain of these data are plotted in Figure 2. Obtained In R u n 7 Bicarbonate in equilibrium Bicarbonate in liquor Normality Temp. Pressure Gas anal. with gas" Sample % ' of sodium C. Mm. HIZ % COz % noint 59.4 1.67 67.2 14.4 65 60.5 Bottom 1.67 56.5 66.1 14.0 60.5 40 4 52.1 65.5 1.67 13.4 28 7 60.0 47.7 1.67 64.4 12.8 15 60.0 10 41.5 1.67 63.0 12.0 6 60.0 13 1.67 3 4 . 4 6 1 . 4 1 1 . 0 TO " fi0.n 0 -r 1.24 liters per minute. Barometer = 765 mm. Lye flow Entering gas flow = 186 liters per minute (standard conditions). Calculated from data by McCoy, Am. Chcm. J . , 39, 437 (1903); and by Byrne and Carlson, unpublished M. S. thesis, M. I. T. 11920).
amount of absorption per hour divided by this potential difference. Table 11-Calculation of Coef3cients for R u n 7 Grams Cot Concentrationsi in Rate coefficient absorbed grams COz per cc. x 10' per plate per hour Plate W/e CQ-CL KaV CQ CL 1 25 24.7 21.8 2.9 8600 2 27 24.6 21.5 3.1 8700 24.5 21.2 3 27 3.3 8200 24.4 20.8 3.6 10000 4 36 24.3 20.3 5 38 4.0 9500 24.2 19.8 6 46 4.4 10500 24.1 19.2 4.9 8400 7 41 24.0 18.6 5.4 7600 S 41 2.3.9 18.1 9 38 5.8 6600 23.7 17.5 6.2 7400 10 46 23.6 16.9 11 57 6.7 8500 23.4 16.1 12 57 7.3 7800 23.2 15.4 7.8 8100 13 63 23.0 14.5 14 68 8.5 8000 22.7 13.6 9.1 7800 15 71
Table I-Data
-
-
-LYE I5
A
14
265
Although the coefficients vary somewhat from plate to plate, there is no regular trend to the variation, indicating that absorption follows the equation suggested above. Table I11 gives the results of ten absorption runs calculated in a similar manner. The average coefficient per plate has been figured from the average absorption per plate divided by the average absorption potential. A coefficient calculated in this manner differs slightly from one obtained by averaging fifteen individual coefficients from fifteen plates, but the discrepancy is within the limits of experimental error.
The a b s o r p t i o n of carbon dioxide by carbona t e-b i c a r b o n a t e lye may be t r e a t e d as a case of diffusion through a surface film of liquid a t the interface. h c t u a l l y , this assumption is not quite correct and the problem would be more rigidly Rim 1 treated as one of dif2 3 fusion through gas and 4 liquid films in s e r i e ~ . ~ 5 6 This point will be cons79 s i d e r e d later, as the simple equation 10
Av. temp. C. 63.5 60.5 60.5 60.0
50.0 55.0 60.0 42.2 27.8 84.3
T a b l e 111-Results of Absorption R u n s Av. grams COz K a V at absorbed Av. ( C a - C L ) Gas flow gas flow Hour/plate Gram/% KoV Liters/min. 150 39.3 5930 67.4 8830 4940 86.7 36.3 6500 as.5 7540 133 8000 7410 211 60.0 6290 192 56.3 6150 5440 52.0 6910 159 7130 45.6 186 8250 7400 132 35.5 3790 4040 26.4 2190 74.3 3110 30.5 10520 183 9530
-
Rate of absorption = rate coefficient X absorption potential
Figure 1-Diagram of P l a t e Absorption Tower w i t h S a m p l i n g P o i n t s (g)-Gas sampling point
-
(J.v )-Lye
s)av.
sampling point
b a s e d on liquid film diffusion alone is suffic i e n t l s adequate for the present purposes,
(
= grams carbon dioxide absorbed per hour KaV = over-all rate of absorption coefficient per plate CQ= average concentration of lye in equilibrium with gas, expressed as grams carbon dioxide per cubic centimeter CL = concentration of lye on plate, expressed as grams carbon dioxide per cubic centimeter
The complete method of computing coefficients for each plate from the data of Run 7 is indicated in Table 11. The absorption rate per plate, in the second column, is the product of the rate of lye flow and the difference in lye composition between successive plates, the data for individual plates being obtained by interpolation on Figure 2. The equilibrium composition, CG, is the average for the gas entering and leaving the plate, while the composition CL is that of the lye on the plate. C G - C L is the absorption potential, and the rate coefficient KaV for each plate is the 8
Lewis and Whitman, THXS JOURNAL, 16, 1215 (1924).
RUN N0.7
30
I
I
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266
The best representative line on Figure 3 has a slope of 0.5 and the gas velocity effect a t constant temperature may
therefore be expressed as KaV = constant
x
(gas ve1ocity)o.s
The corrected coefficients in the last column of Table I11 are then calculated for a standard gas velocity of 150 liters per minute by multiplying the observed coefficient by
1-01. 18, To. 3
where C, is the concentration in the solution a t the gasliquid interface. The average ratio kaV/KaT' from this work is 2.4 (= ~ L / K L )I.n any particular case, therefore, a value of C, may be calculated from this ratio and the values of CGand CL. The individual gas film coefficient, ICG, is related to the liquid film coefficient, k L , by the relationship
d / l j O / g a s velocity
The effect of temperature is observed in Figure 4, where the coefficients, corrected to standard gas velocity, are plotted against temperature on semilogarithmic paper. The coefficient from Run 10 a t 84.3' C . , while calculated in Table I11 and recorded on the plot, is not given much weight in determining the best representative line because the original data for this specific run were incomplete, and the results were entirely dependent upon the accuracy of a single gas analysis. The best representative line on this temperature plot s h o m that the rate coefficient doubles with every 24' C. rise in temperature.
where P G = the partial pressure of carbon dioxide in the gas and P , = the partial pressure of carbon dioxide a t the interface-in equilibrium with C,. If C , has been determined P , is then known as above from CG,CL,and the ratio ~ L / K L by the equilibrium relationship. PG is, of course, determined from CGin the same way. It is therefore possible to calculate the ratio of liquid film to gas film coefficients, k ~ / k from ~ , the data obtained in this work, and to compare the ratio so estimated with that obtained by other investigators. This has been done in several cases, giving an arerage ratio k ~ / = k ~19.
Rectification
The results of four runs in which a mixture of steam and carbon dioxide was rectified by passing up through lye in the same column are given in Table IV. The method of calculating and expressing results is the same as in the absorption runs. The corrected coefficients for these runs are plotted on Figure 4 on the same scale as was previously employed.
Run A
Table IV-Results of Rectification R u n s Av. Av. grams COz K a V at temp. absorbed Av ( C c - C L ) Gas flow gas flow C. Hour/plate Gram/cc. K a V Liters/min. = 150
B
93.1 100.5
D
96.0
C
103.0
0.00173 0.00148 0,00195 0.00158
69.6 96.6 146.8 125.0
40,300 65,300 75,000 79,100
183 274 305 228
36.500 48,300 52,900 64,500
It is highly significant that the rectification coefficients are of the same order of magnitude as those for absorption and are only two and a half times as great as t h e extrapolated absorption results a t the same temperature. As a matter of fact, it would be expected that the rectification coefficients should be greater than those for a b s o r p t i o n . I n absorption t h e c a r b o n d i o x i d e must diffuse first through a film of i n e r t noncondensable GAS MLOCITY gas and then through a surface film of liquid, so that the problem is really one f; two-film diffusion and the calculated coefficient is an overall value for the two films. I n rectification there is no inert noncondensable gas and the calculated coefficients therefore apply to diffusion through a single (liquid) film. Lewis and Whitmans distinguish between individual coefficients for single films and over-all coefficients for two films in series by designating the individual one as IC and the over-all as K . Accordingly, the values obtained here for absorption should be termed KaV, while those for rectification are kaV. The same authors derive algebraically from their basic absorption equation the following expression: k L = - CR-CL _
KL
Ci-CL
BOO00
6 0 000 40 000
20000
> 10000
*d
a000 6 000 4 CQO
2000
20
30
40
50
60
70
80
SO
100
110
Lewis and Whitman3 calculate a ratio of from 15 to 30 for gas bubbles and wetted wall columns. More recently a value .of 12 has been found for a drop of water falling through a gas.4 It is therefore evident that the ratio compares favorably with those obtained in other ways for similar types of absorption reactions. This close comparison also justifies the theory offered to explain why coefficients for rectification are higher than those for absorption in the presence of an inert gas. Conclusions
1-The absorption of carbon dioxide from a gas mixture by a carbonate-bicarbonate lye in a plate column can be adequately treated by the general absorption equations. 2-The over-all rate coefficient for absorption is proportional to the square root of the gas velocity through the column. 3-The coefficient for absorption doubles with every 24' C. increase in temperature. 4-The same absorption equation governs the rate of rectification when steam and carbon dioxide are passed up through hot lye. The coefficients in this case are about 2.4 times as great as those for absorption. This discrepancy is predicted by the general theory, since the presence of a film of inert gas during absorption interposes a gas-film resistance which is absent during rectification. 4
Whitman, Long, and Wang, accepted for publication in THIS JOURNAL.
