Absorption of Chlorine by Water in a Packed Tower* balance, the basic differential equation expressing the material balance may be written in numerous forms:
The solubility of chlorine in water has been computed from Arkadiev's equations for partial pressures of chlorine from 0 to 5000 mm. of mercury over a temperature range from 0" to 110" C., using the data of Yakovkin and a p chart for hydrocarbons of approximately the same critical ratio as chlorine. These solubility values are used for calculating absorption coefficients in an experimental coke-packed tower operating between 38" and 50" F., at gas velocities between 0.78 and 1.93 pounds per minute per square foot, and over a range of liquid velocities from 32 to 1 1 1 pounds per minute per square foot. The results show that under these conditions absorption is controlled by the resistance of the water film and that the coefficient, KLa,is proportional to the 0.8 power of the water velocity.
dW = KGa(pG - pL)Adh = KLa(C5 - CL)Adh 9
If the solubility curve is linear, integration of this equation yields one of the following form: 9
= Koa(Apu)V = KLa(ACM)V
Should the solubility curve be other than straight, or should the absorption be nonisothermal, graphical integration would become necessary. To obtain a value of p L or CG,and hence of A p , or AC,, the equilibrium relation between the concentration in the gas and liquid phases must be known a t the temperature of the system. In connection with the work on chlorine absorption to be described, a knowledge of the p-C-t relations of chlorine solutions is required. EXPERIMENTAL data concerning the solubility of chlorine in water are meager. The only available figures are those showing solubilities a t a total pressure of one atmosphere and a few a t higher pressures (4). From various observations, as well as from a theoretical point of view, it appears that a t low partial pressures the solubility is a great deal higher than would be predicted by Henry's law. The obvious explanation is that the chlorine combines chemically with the water, so that only part of the chlorine is held in true solution.
F. W. ADAMS2 AND R. 0 . EDMONDSS Massachusetts Institute of Technology, Cambridge, Mass.
A
LTHOUGH the subject matter of this article is suitable for use in design calculations for a plant producing a chlorine water bleach, it is of more immediate interest as an addition to the present knowledge of a phase of chemical engineering in which almost no satisfactory data have been reported. The particular operation referred to is the absorption, or scrubbing, of relatively insoluble gases from gas mixtures. Being more important industrially, the simpler and closely related problem of the absorption of soluble gases has been the subiect of considerable investigation, but the controlling factors do not seem to be the same in both cases. This is readily understood if the physical aspects of the two limiting cases of absorption are studied. According to Whitman's two-film theory of gas absorption, the over-all absorption coefficients are analogous to over-all heat transfer coefficients and can be based on either the liquid film or the gaseous film resistance to diffusion. Like the heat
0
40
80
80
20
TEMPERATURE
1 1 1 100
'C
FIGURE 1
With this in mind, Arkadiev (1) in 1918 evolved equations for calculating the solubility from experimental data obtained in 1898 by Yakovkin (6). Yakovkin, in a series of painstaking and thorough experiments, found that the hydrolysis constant for chlorine in water was a function of temperature only, and he obtained by separate methods a series of coinciding values for it from 0" to 85" C. so that a curve (Figure 1)of the hydrolysis constant versus temperature could be drawn. He also found experimentally that the unhydrolyzed chlorine in water was always proportional to the amount of chlorine in a second liquid phase-in this case, carbon tetrachloride. The concentration of chlorine in carbon tetrachloride was, in turn, proportional to the concentration in air. Hence the concentration of unhydrolyzed chlorine in water is proportional to the concentration of chlorine in air. Both the present calculations and those of Arkadiev are based on these experimental determinations.
1 Absorption and Extraction Symposium. Several of the papers presented a t this symposium appear on pages 270-318 of our March issue. Others will be printed in subsequent issues. * Present address, Mellon Institute of Industrial Research, Pittsburgh, Pa. a Present address, Carbide and Carbon Chemicals Corporation, South Charleston, W. Va.
447
INDUSTRIAL AND ENGINEERING CHEMISTRY
448
-5 10 30 50 100 150 200 250 300 350 400 450 500 550
600
650 700 750 800 900 1000 1200 1500 2000 2500 3000 3500 4000 4500 6000
00 c. 0.488 0.679 1.221 1.717 2.79 3.81 4.78 5.71
... ... ... ... ... ... ... ... ... ... ...
...
OF CHLORINE IN WATER TABLEI. SOLUBILITY
.... .... .... ..
200 c. 0.438 0,575 0.937 1.210 1.773 2.27 2.74 3.19 3.63 4.06 4.48 4.88 5.29 5.71 6.12 6.62 6.90 7.29 7.69 8.46 9.27 10.84 13.23 17.07 21.0
.... .... .... ....
.... .... ....
100 c. 0.451 0.603 1.024 1.354 2.08 2.73 3.35 3.95 4.64 5.13 5.71 6.26 6.85 7.39 7.97 8.52 9.09 9.65 10.21
C i h ~ separates ~ o
... ... ... ... ... ... ...
...
VOL. 29, NO. 4
30° C. 0.424 0.553 0.873 1 106 1I573 1.966 2.34 2.69 3.03 3.35 3.69 3.98 4.30 4.60 4.91 6.21 5.60 5.80 6.08 6.68 7.27 8.42 10.14 13.02 15.84 18.73 21.7 24.7 27.7 30.8 I
Solubility Grams per Liter 40° C. 5OO'C. 6OoC. 0.383 0.412 0.398 0.492 0.632 0.512 0.743 0.821 0.781 0.912 1.025 0.962 1.228 1.424 1.313 1.482 1.599 1.754 1.706 1,856 2.05 1.914 2.34 2.09 2.10 2.31 2.61 2.28 2.53 2.86 2.47 3.11 2.74 2.64 3.36 2.94 2.80 3.61 3.14 2.97 3.84 3.33 3.13 4.08 3.52 3.29 4.32 3.71 3.44 4.54 3.89 3.69 4.07 4.77 3.75 4.99 4.27 4.04 5.44 4.62 4.36 4.97 5.89 4.92 5.67 6.81 5.76 8.05 6.70 8.38 7.14 10.22 8.48 10.03 12.32 9.83 11.70 14.47 11.22 16.62 13.38 12.54 18.84 15.04 13.88 20.7 16.75 15.26 23.3 18.46
Arkadiev developed an equation expressing the ratio of concentration of chlorine in water to that in air as the sum of two terms. The first was the ratio of unhydrolyzed chlorine to the chlorine in air as determined by Yakovkin, and the second was a term to account for the hydrolyzed chlorine,
7OoC. 0.369 0.470 0.704 0.863 1.149 1.382 1.580 1.764 1.932 2.10 2.25 2.41 2.56 2.69 2.83 2.97 3.10 3.23 3.37 3.63 3.88 4.37 5.09 6.26 7.40 8.52 9.65 10.76 11.91 13.01
900 c. 0.339 0.431 0.642 0.781 1.034 1.227 1.396 1.553 1.700 1.831 1.966 2.09 2.21 2.32 2.43 2.55 2.66 2.76 2.87 3.08 3.28 3.67 4.23 5.17 6.05 6.92 7.79 8.65 9.49 10.35
80' C. 0.361 0.447 0.671 0.815 1.085 1.294 1.479 1.642 1.793 1.940 2.08 2.22 2.35 2.47 2.69 2.72 2.84 2.96 3.08 3.30 3.53 3.95 4.58 5.63 6.61 7.54 8.53 9.52 10.46 11.42
1000 c. 0.326 0.415 0.627 0.747 0.987 1.174 1.333 1.480 1.610 1.736 I.854 1.972 2.08 2.19 2.29 2.41 2.60 2.60 2.69 2.89 3.07 3.43 3.96 4.78 6.59 6.38 7.16 7.94 8.72 9.48
1100 c. 0.316 0.402 0.598 0.722 0.950 1.137 1.276 1.413 1.542 1.661 1.773 1.880 1.986 2.09 2.19 2.28 2.37 2.47 2.56 2.74 2.91 3.26 3.74 4.49 5.25 6.97 6.72 7.42 8.13 8.84
Starting with c/70.9 moles of chlorine, we obtain as products, /3/70.9 moles of HC10, /3/70.9 equivalents of C1-, and /3/70.9 equivalents of Hf. From the law of mass action the hydrolysis constant is defined by the equation: k(a/70.9) = (p/70.9)*
(4)
where k is a function of temperature only. Let K = (70.9)%, so that Kcr = 0s
(5)
Combining this with Equation 3
c=cr+%yE It was experimentally shown by Yakovkin (6) that a,the concentration of unhydrolyzed chlorine in water, was proportional to the concentration of chlorine in a second liquid phase, TEMPERATURE 'C
FIGURE 2
using Yakovkin's hydrolysis constant. Unfortunately the terminology employed was such as to make the practical application of the results difficult. No account was taken of the deviation of chlorine-air mixtures from the perfect gas laws a t low temperatures, and no correction for pressure was made in a simplification of the second term of Arkadiev's equation.
IT IS DESIRED to obtain values of c in equilibrium with partial pressures of chlorine from 0 to 5000 mm. of mercury a t temperatures ranging from 0" to 110" C. Introducing the correction factor, p , into the perfect gas equation, the molal volume of gaseous chlorine is given by:
y = - &RT
70.9~
The equation for the hydrolysis of chlorine is: C1,
+ HzO % C1- + H + + HClO
By definition:
PR
c=a+8
(3)
FIGURE 3
APRIL, 1937
449
INDUSTRIAL AND ENGINEERING CHEMISTRY
Table I contains the calculated values of c as obtained from selected values of temperature and partial pressure and from the corresponding values of K , y, and p as read from Figures 1 2, and 3. Figures 1 and 2 are obtained directly from the experimental data of Yakovkin (6). Figure 3 is an enlargement of a portion of a p chart for hydrocarbons presented by Cope, Lewis, and Weber ( 2 ) . The critical ratio, RT,/P,V,, for chlorine is 3.64 as compared to 3.78 for the data from which the chart was constructed. Charts of this type have had wide use in the past few years in solving problems dealing with mixtures of hydrocarbon gases a t high pressures. They have been found very successful for this purpose and have been extended to apply to gases other than hydrocarbons. Although the corrections given by this chart are supposed to hold only for pure chlorine, their application to chlorine-air mixtures should introduce only a small error if the case can be considered analogous to hydrocarbon mixtures. More exact formulation of the deviations of the mixtures from the perfect gas law using newly developed methods involving the internal pressures of the components would not seem to justify the increased calculation required, because the corrections are not large, Actual p-E-T data were found to coincide very closely with the curves of Figure 3, so that this p chart should prove entirely satisfactory. The data of Table I were plotted in Figures 4 and 5 to provide a more convenient interpolation.
I 0
I 50
I
1
1
1
I
1
100 150 200 PARTIAL PRESSURE MILLIMETERS OF MERCURY
250
FOR purposes of checking these curves, existing data on chlorine solubility (4)are compared in Table I1 with values read from the curves; in general the agreement is within about 2 per cent. No precise determinations were available in the desired ranges, but recalculation of a few rough experi-
300
-
FIQURE
4
carbon tetrachloride, when the two phases were present,
It was also shown that the latter, in turn, was proportional to
TABLE11.
the concentration of chlorine in air. Let y be the ratio of concentration of unhydrolyzed chlorine in water to concentration of chlorine in air:
LATED FROM
y = - apRT
Concn., G /L.Adams & Edmonds 9.90 9.68 9.28,8.72 8.89 7.24 7.26 5.68 5.80 4.58 4.55 3.84 3.80 3.14 3.24 2.63 2.72 2.10 2.15 1.49 1.23
--Ch t,
(7)
70.9~
COMPARISON OF CHLORINE SOLUBILITIES, CALCU-
INTERNATIONAL CRITICAL TABLES ( 4 ) ,WITH FIQUREB 4 AND 5
It was found that y varied only with temperature. Eliminating a from Equations 6 and 7
C. 10 12 20 30 40 50 60 70
I. C. T. (4)
p , Mm. H g
80 90
TABLE 111. DATAFOR ABSORPTIONOF CHLORINE IN WATERIN 1
2
KQa Run No. Recalcd.
4 5 6
3
4
K L ~ Recalcd.
tf O
If.
60
7
9 10
11
12 13 14 15 16 17 19 20 21 22 23 24 26 26 27 28 29 30
42
43 43 43 43 50 43 43 43
5
G
6
L
7
8
!kl??’ $2 Gas Gas
9
10
Inlet
Exit
AP
AP
A
Deviation, % -2.2 -4.2, +1.9 +0.3 +2.1 -0.7 -1.0 -3.1
-3.8
-2.3 +21.1
COKE-PACKED TOWER 11
Log Mean Ap
12 Inlet Ac
13
14
15
Exit Ac
Log Mean Ac
E ve
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INDUSTRIAL AND ENGINEERING CHEMISTRY
I
PARTIAL PRESSURE - MILLIMETERS OF MERCURY
FIQURE 5 mental values obtained by V. S. Titov in the Physico-Chemical Laboratory of Zemgora-Zemsoyuza at Moscow to verify Arkadiev's calculations (1) agrees with Figures 4 and 5 for the values given. Points obtained by Volkoff ( 1 ) and Richards (3) are also included in Figure 5. It would be highly desirable to obtain much more complete experimental verification of these solubility curves as portions of them are extrapolated from Yakovkin's data. I n 1928 Gilmour, Lockhardt, and Welcyng (S), using a gas containing 10 to 20 per cent chlorine, made a series of thirty runs in a 6-foot clay pipe tower, 6 inches in diameter and packed with 1-inch coke for 4 feet of its height. They calculated the absorption coefficients on a gas film basis, using the over-all coefficient Koa. Since no extensive solubility data were available, curves based on meager experimental values were used. Values of K,a have been recalculated, assuming the validity of the calculated solubilities and using graphical integration, although it was found that the values obtained do not differ appreciably from those calculated by the use of the logarithmic mean of the partial-pressure driving forces a t the top and bottom of the tower, Graphical integration was unnecessary to calculate the coefficients on a liquid basis since the solubility curve was practically straight in the range involved. In Table I11 column 3 gives the corresponding values of KLa. Columns 4 to 8 show, respectively, the values of temperature, gas velocity, liquor velocity, inlet gas analysis, and outlet gas analysis, These columns summarize the basic data involved in the calculation of the coefficients. Columns 9 to 15 show the calculated intermediate values used in obtaining the absorption coefficients. I N ORDER to obtain an empirical equation for predicting coefficient K,a under various operating conditions, an attempt was made to find KLa as a function of L, G, and T. I n Figure 6 KLa was plotted against L on logarithmic paper for all the
VOL. 29, NO. 4
runs, and a straight line drawn through the points. A slope of 0.81 was obtained from which the relation K,a = 0.0097 Lo.*' follows. The runs at high gas velocities are shown by circles containing crosses; those at low gas velocities are shown by black circles. The distribution of these points shows that in the range investigated KLa is independent of gas velocity. Higher coefficients are shown by the results a t 50" F. than at the lower temperatures, but the small temperature range investigated precludes any general conclusions as to the effect of temperature on the absorption coefficient. These results indicate that the absorption coefficient for chlorine in water is controlled by the resistance of the liquid film. Since KGa and K,a are over-all coefficients, they may be used interchangeably. It is common practice, however, to use Koa when gas film is controlling and KLa when liquid film is controlling, since this is the most logical and satisfactory way to prevent unnecessary confusion and recalculation of the coefficients. Although it is common practice to extrapolate Henry's law to obtain solubility relations a t pressures and concentrations for which data are not available, it can be seen that in this case such an extrapolation would lead to serious errors in the calcula-
0.6 (0
2 0 . 5 W -
0.4
u
$ 0.3
s
0 z
E o.2 LL
0
20.15 a
0.1
1.13 20
30
50
40
80 70 80 90 100
LIQUOR VELOCITY, LBS./MIN/
I50
I 200
SQ.FT,
FIQURE 6
tion of absorption coefficients. It would even allow the operating and equilibrium lines on a p-C plot to cross one another, an impossible situation in isothermal operation of absorption towers with steady-state conditions prevailing.
Nomenclature a
= area of
A
= = =
c
C G
=
active wetted surface in interphase contact,
sq. ft./cu. ft. cross-sectional area of tower, sq. f t . concn. of chlorine in water, grams/liter concn. of solute gas in solvent liquid, Ib./cu. f t . mass velocity of solute-free gas (inert carrier gas),
Ib./min./sq. ft.
= height of tower, f t . = hvdrolysis constant on a molal basis K = (iO.S)*k Koa = over-all coefficient, lb./min./cu. ft./mm. H
h k
KLU
=
ovfy$l coefficient, lb./min./cu. ft./unit IC.)
L
A 8 (Ib./cu.
= mass velocity of solute-free solvent, lb./min./sq. f t .
INDUSTRIAL AND ENGINEERING CHEMISTRY
APRIL, 1937
P P R t t'
O
-T
O
v
v w Y
a
B Y
f
e
= partial pressure of solute gas (chlorine) mm. Hg = partial pressure of solute gas (chlorine), atm. = gas conztant C. = temp., = temp., F. = temp., K. = molal volume, liters vol. of tower, cu. ft. = weight solute absorbed, lb. = ratio of ofconcn. of total chlorine in waterto concn. of = chlorine in air = concn. of unhydrolyzed chlorine in water, grams/liter in water, @;rams/1iter Of = = ratio of concn. of unhydrolyzed chlorine in water t o concn. of chlorine in air gas law = p5jRT = coefficient of deviation from the = time, min.
i
451
Subscripts: c = critical state G = gas L = liquid M = logarithmic mean of the driving force R = reduced conditions
Literature Cited (1) Arkadiev, V.9 J. Buss. Phw-Chern. Soc.9 50, 205 (1918). (2) Cope, Lewis, and Weber, IND. ENQ.CHEM.,23, 887 (1931). (3) Gilmour, Lockhardt, and Welcyng, Mass. Inst. Tech., School of Chem. Eng. Practice, Bangor Sta. Report, 1928. (4) International Critical Tables, Vol. 111, p. 256, New York, McGraw-Hill Book Co., 1928. ( 5 ) Yakovkin, A., J . Ruse. Phus.-Chem. SOC.,32, 673 (1900). R R E ~ I YNovember ~D 25, 1936.
Countercurrent Extraction of Benzoic Acid between Toluene and Water' PERFORMANCE OF SPRAY AND PACKED COLUMNS The performances of columns of the spray and of the packed type for solvent extraction were studied using the system toluene-benzoic acid-water. Capacities of the columns were evaluated in terms of an extraction coefficient, K W a , based on the water phase. H.E. T. P. and H.T. U. for typical runs were also calculated. Data for the holdup of the discontinuous phase in such columns are included. The capacity of the spray column depended on the rates of feed of both the discontinuous and the continuous solvents, on the flow ratio,
and especially on the size of the drops of the former produced by the entrance nozzle. In the packed column, capacity depended upon the rate of feed of the discontinuous phase and only slightly upon that of the continuous, and varied inappreciably with the drop size produced by the nozzle. In general the dependence of holdup on these variables was similar to the capacity. For present conditions the capacity of the packed column was intermediate between that of the spray for the smallest and largest drop size studied. A spray column may be either more or less effective than a packed, depending upon the drop size produced by the nozzle and the nature of the packing.
F. J. APPEL AND J. C. ELGIN Princeton University, Princeton, N. J. LTHOUGH theoretical aspects of liquid-liquid solvent extraction operations are becoming well developed, few of the data required for their application t o the design of actual extraction equipment are yet available. This is particularly true of the rate of extraction and the capacity of column contacting equipment. Extracting acetic acid between water and isopropyl ether, Elgin and Browning (8) studied the capacity and operation of a spray-type extraction column as affected by the more important variables determining the performance of such equipment. A theoretical analysis of the spray column was also developed. Qualitatively, this was found to inter-
A
pret the observed data satisfactorily, and the latter were concluded to be characteristic of the behavior of the spray column. In the present communication additional data on the spray column are reported, using the extraction of benzoic acid between toluene and water, and the work is extended to include the capacity of a packed column with this system. The effect of rates of flow of the two solvents and the drop size of the discontinuous phase at entrance have been more thoroughly investigated, and holdup of the dis1 Absorption and Extraction Symposium. Several of the papers presented at this symposium appear on pages 270-318 of our March issue. Others will be printed in subsequent issuea.
