Kecovery ot
Sulfur Dioxide from Waste Gases Equilibrium The partial vapor pressures have been measured at 35", 50°, 70°, and 90"C. over a long range of concentrations and compositions of sulfite-bisulfite solutions of sodium and methylamine. Suitable interpolation formulas are given to represent the vapor pressures as a function of temperature and solution composition. Sufficient data for a number of other sulfite-bisulfite solutions are reported to show the effect of the nature of the solution on the temperature coefficient of the vapor pressure of sulfur dioxide. Theoretical considerations show that the solutions having the greatest capacity for absorbing sulfur dioxide from dilute gases in a cyclic process and requiring the smallest amount of steam for regeneration are those containing sulfites of bases with an ionization constant of lo-' and solutions of salts of weak acids saturated with the free acid, for which
LI
Yressures over SulfiteBisulfite Solutions' H.F.JOHNSTONE, H.J. READ, AND H. C. BLANKMEYER University of Illinois, Urbana, Ill.
I
N A PREVIOUS paper in this series the equilibrium par-
tial vapor pressures over solutions of the ammonia-sulfur dioxide-water system were reported (8). The application of this system to a cyclic method of recovery of sulfur dioxide from dilute waste gases, involving absorption and subsequent regeneration of the solution with release of the dissolved gas by heating, with or without a vacuum, was also described. The possibility of using other sulfite-bisulfite solutions, especially those in which other hydrolytic equilibria may enter, suggests the importance of equilibrium vapor pressure data for such systems. I n this paper the data are reported for the sodium and methylamine systems for the complete range of temperatures and compositions likely to be encountered in practice. I n addition, partial vapor pressures over a few solutions of a number of other systems are reported. These are sufficient to indicate the effect of the nature of the base on the hydrolytic equilibrium, especially so far as affects the temperature coefficient of the vapor pressure of sulfur dioxide. A brief discussion of the theory of hydrolytic equilibrium of sulfite solutions will also be given.
K,K,
Vapor Pressure Data The vapor pressure measurements were made at 35", 50", 70°, and 90" C. by the dynamic method, using nitrogen as the inert gas. The apparatus and procedure were the same as those previously described. The sulfite-bisulfite solutions were made by adding a predetermined quantity of sulfur dioxide to standard solutions of the base. I n this way a constant concentration of the base was maintained for a given series while the ratio of sulfur dioxide to the base was varied. The sulfur dioxide was obtained from cylinders of refrigeration-grade material. The methylamine was a commercial grade kindly furnished by the Commercial Solvents Corporation. The ethanolamines were also commercial grades and 1
Previous papers in this series of articles appdared in IND. ENO.CHEM..
27, 687,659 (1935),29, 286, 1396 (1937).
101
=
1044.
used without further purification. Analyses of all the solutions were carried out on samples taken when the saturators were filled. Sulfur dioxide was determined iodometrically, and this analysis was frequently checked by gravimetric total sulfur determinations. The volatile bases were determined by evolution from causticized solutions into standard acid. The nonvolatile nitrogen bases were determined by the standard Kjeldahl method. Sodium was determined gravimetrically as sodium sulfate (15). Water was calculated by difference. I n all cases the solutions were stabilized against oxidation by addition of 0.1 per cent hydroquinone. For most solutions this inhibitor prevented the formation of more than traces of sulfate even at high temperatures. I n the case of the ethanolamines, however, the autoxidation was rapid. The vapor pressure data obtained for the sodium and methylammonium sulfite-bisulfite systems, respectively, are shown in Tables I and 11. Each series of solutions contains a constant concentration of base, the most concentrated solutions studied being nearly saturated with the sulfite at normal temperatures.
. INDUSTRIAL AND ENGINEERING CHEMISTRY
102
It is evident that the vapor pressure of the amine was too low to be measured for all solutions except those containing the lowest ratios of sulfur dioxide t o the base, and for these it was measurable only a t 90". Comparison with the data on the ammonia system on this point will emphasize the greater
VOL. 30, NO. 1
The effect of temperature on the vapor pressure of sulfur dioxide followed closely the usual equation:
+
where a, b
=
Iog pso, = a b/T (2) constants, and T = absolute temperature, ' K.
FIGURE1. PlRTIAL PRESSURE O F SULFUR DIOXIDEOVER SOLUTIONS METHYLAMMONIUM SULFITE-BISULFITE
OF
SODICX
ASD
(Le/O C = 8.0 moles sodium per 100 moles water (Riuht) C = 20.0 moles methylamine per 100 moles water
solubility of the gaseous amine in sulfite-bisulfite solutions. Even a t a ratio of bisulfite to sulfite of 80 to 1, the ammonia vapor pressure was appreciable for all concentrations of solutions a t 90" whereas the amine has no vapor pressure even for concentrated solutions at this temperature a t bisulfite-sulfite ratios one-tenth as great.
Interpolation Formulas As indicated in the previous paper ( 8 ) ,the vapor pressure of sulfur dioxide over sulfite-bisulfite solutions can be represented over a limited range of practical importance by an equation of the type: pso, = M
where M S
(2s- C)Z
C -s constant, depending on the temperature and system considered total concentration of dissolved SOI, moles/100 moles water total concentration of base, moles/100 moles water
= a =
C =
It is evident that between a p H range of 4.5 and 6.0, in which the first ionization of sulfurous acid is complete and that of all but the weakest bases is also complete, the group 2s - C represents the bisulfite-ion concentration, and C - S represents the sulfite-ion concentration. The values of M for each system were calculated from the vapor pressures determined experimentally and the analyses of the solutions. Since differences in concentrations are involved in the calculations, it is evident that the precision of the calculated quantity varies with the relative values of C and S,regardless of the precision of the vapor pressure measurements. Accordingly, weighted values were used in findi n g the best values of the constant.
The values of b for each solution studied are reported in the fourth column of Tables I and 11. Since the effect of temperature on constant ICI is the same as that on psoZ, the general equations derived from the data by the method of least squares are as follows: Sodium solutions: log M = 4.519 - 1987/T Methylamine solutions: log M = 5.390 - 2308/T Iog M = 5.865 - 2369/T Ammonia solutions:
(3)
(4)
(5 1
With these values of the constants, pSo2 is given in millimeters of mercury. Equation 5 is reproduced from the previous paper (8) in order t o compare the data for the three systems. The constants indicate (a) that a t a given temperature, solution concentration, and composition, the vapor pressure of sulfur dioxide over the sulfite-bisulfite solution of sodium is considerably less than that for the simple or substituted ammonium solutions; and (b) that the effect of temperature on the sulfur dioxide vapor pressure is greatest for the ammonium solutions. The significance of these differences will be discussed later. For the design of absorption and stripping columns it is necessary to know the vapor composition, and some interpolation formula for the partial pressure of water is required. It has been shown that Raoult's simple law holds with surprising accuracy for the sulfite-bisulfite solutions of the ammonia system. Expressed in terms of C and S, this becomes: loop, (6) 100 c s where P , = vapor pressure of pure water at temperature considered PHpO
=
+ +
INDUSTRIAL AND ENGINEERING CHEMISTRY
JANUARY, 1938
The same equation is now found to be valid for the sodium sulfite-bisulfite solutions for all concentrations up to saturation and for temperatures up to 90' C. Deviations are, in general, less than 2 per cent and are well within experimental precision. I n the case of the methylamine solutions, deviations of the vapor pressure of water from Raoult's law are apparent a t the highest concentrations where the mole per cent of water is only 60. Even here, however, the error introduced by assuming perfect agreement is less than 10 per cent; and since the discrepancy disappears entirely at moderate and low solution concentrations, the use of the simple law for all sulfite-bisulfite solutions is warranted. A comparison of the vapor pressures calculated by means of Equations 3 , 4 , and 6 with the experimental values is shown in Tables I and 11.
103
FIGURE 2. C.4PACITY O F SODIUM SULFITE-BISULFITE SOLUTIONS FOR ABSORBING SULFURDIOXIDE FROM GASESCONTAINING 0.3 PER CENT SULFUR DIOXIDE
Capacity of Sodium and Methylamine Solutions for Absorbing Sulfur Dioxide
(Regeneration to 1 mm. of sulfur dioxide a t t r o )
The capacity of a solution for absorbing sulfur dioxide in a cyclic system is determined by the difference between the concentration of sulfur dioxide in the solution leaving the scrubber and that leaving the regenerator for a constant concentration of a base. I n order to compare the capacities of the various solutions under several operating conditions, vapor pressure curves were calculated for rounded values of C
does not exist in the case of the two systems under consideration. The farther the regeneration is carried, however, the more steam is required for stripping the sulfur dioxide from the solution; there must, therefore, be an economic limit. Complete regeneration is desirable, on the other hand, since it affords greater capacity and also more complete removalof sulfur dioxide from the waste gases. From these arbitrarily chosen operating conTABLE I. EQUILIBRIUM PARTIAL VAPOR PRESSURES OVER AQUEOUS ditions, the capacities of the solutions in pounds SOLUTIONS OF SODIUN SULFITE-BISULFITE of sulfur dioxide per 100 pounds of extractor -7 * * -.w Temp. entering the scrubber were calculated from the c.N a + Coefficient, -9O0 C.-70° C.-50a C.-35' Solution SO2 Ions -bag2 Pgo2 PHZO Pgo2 PHZO Pa02 p8Oa PHlo equation: K-3 K-5
K-6 KB-1 KB-4 KB-5 KB-6 KC-2 KC-3 KC-4
KC-5
M o l e s / f 00 moles HzO 7.22 7.77 Calculated 7.58 7 . 7 8 Calculated 6.80 7.75 Calculated 5.38 5 . 8 7 Calculated 5.06 5.85 Calculated 4 . 4 0 5.87 Calculated 5 . 6 1 5.88 Calculated 3 . 5 4 3.98 Calculated 3.61 4.00 Calculated 3.30 4.01 Calculated 3 . 7 4 3 99 Calculated
Mzllimeters of mercury 2000 2000 2077 2004 1910 1493 2016 1549 1950 1845 1942
8.5 452 11.3 457 25.0 458 38.0 455 4.7 472 5.0 476 8.8 475 6.8 472 3.2 476 3.2 474 0.94 478 0.82 477 17.8 472 14.7 471 3.3 490 3.1 489 4.0 490 3.7 488 1 . 0 2 490 1 . 3 2 490 6.8 488 6.8 488
3.6 5.4 11.5 18.2 2.2 2.4 4.1 3.3
190 203 202 202 206 204 210 210 1.5 190 1.5 210 0 51 211 0 . 3 9 212 9.0 208 7.1 209 1.4 219 1.5 217 2.0 220 1.6 217 0 . 4 6 225 0 . 6 3 217 3.7 218 3.7 218
for several temperatures. The curves for C = 8 for the sodium system and C = 20 for the methylamine system are shown in Figure 1. These are approximately the highest concentrations that can be used for these systems because of the limitations of solubility. For purposes of comparison, an arbitrary set of operating conditions was chosen. The solution was assumed to leave the scrubber 90 per cent saturated with respect to gases containing 0.3 per cent sulfur dioxide. Regeneration was carried to the point where the vapor pressure of sulfur dioxide was 1 mm. a t the temperatureof regeneration. It will be recalled that the regeneration of the ammonium sulfite-bisulfite solutions was limited by the volatility of the ammonia, which increased as the sulfur dioxide was removed from the solution. This restriction
0.85 1.18 2.6 4.0 0.57 0.53 0.91 0.71 0.34 0.34 0.16 0.09 1.78 1.54 0.51 0.32 0.47 0:78 8 5 : 5 0 . 3 9 87.0 0:28 85.7 o : i 4 8 6 . 3 0.76 1.7 85.4 0 . 7 1 1.4 1.61 79.9 2.35 80.0 79.4 5.1 79.7 8.0 1.1 8 0 . 3 80.3 1.1 79.9 1.8 82.7 1.4 0.71 75.5 0 . 6 8 82.9 0.30 84.3 0.17 8 3 . 4 82.5 4.0 82.5 3.1 0.84 85.8 0.64 85.6
6400 A S
Capacity =
+
+
(7) 1800 MhC 64S,, where A S = change in concentration of SO2 in passing through scrubber, moles/100 moles H20 S, = concentration of SO2 in solution entering scrubber, moles/100 moles H20 Mb = equivalent weight of anhydrous base (31 for both sodium and methylamine)
32.6 36.3 37.6 36.2 35.6 36.5 37.8 37.6 36.9 37.7 37.2 37.9 37.6 37.5 38.6 38.9 38.6 38.8 37.4 38.9 38.6 38.8
Graphs of these results against the concentration of the base (C) for various temperatures of absorption and regeneration are shown in Figure 2 for the sodium solutions and in Figure 3 for the methylamine solutions. I n all cases the
PARTIAL PRESSURES OVER SOLUTIONS TABLE 11. EQUILIBRIUM O F SULFUR DIOXIDE, METHYLAMIYE, 4 N D W.4TER Solution FAA-1
FAA-2 FAA-4 FBB-1 FBB-2 FBB-3 FBB-4 FE-1 FE-3 FE-4
Temp. Coefficient
CHa"2 Moles/100 moles Hz0 1 8 . 3 5 22.00 Calculated 19.84 2 1 . 9 8 Calculated 1 9 . 5 5 21.95 Calculated 11.85 12.57 Calculated 1 0 . 3 7 12.46 Calculated 11.38 12.48 Calculated 1 0 . 8 4 12.46 Calculated 6.69 7.33 calculated 6.96 7.34 Calculated 6.86 7.34 Calculated
-bSOz
2485 2445 2500 2488 2219 2314 2385 2533 2100 2088
--- --
90' C. -70' C.1-50' C.-35' C.PSOz PCHaNH2 pH20 p.302 pE?O PSOz PHlo PSOz pHzO
-
M i l l i m e t e r s of m e r c u r y
6 . 0 0 . 5 6 347 8.0 , . 374 19.7 , , 347 .. 370 19.8 .. 349 16.2 16.6 .. 362 25.0 ,. 419 23.3 422 4 . 0 0136 424 4.5 428 , . 422 13.9 14.2 .. 424 .. 425 7.7 .. 425 7.1 8.0 , , 463 7.9 .. 460 , , 465 19.4 .. 460 15.4 , , 461 11.4 11.5 , . 460
..
2.5 3.4 7.9 8.4 6.5
7.1 10.5 9.9 1.7 1.9 6.1 6.0 3.3 3.0 2.3 3.3 9.2 6.6 5.1 4.9
153 166 150 164 154 165 185 187 182 190 183 188 187 187 202 202 205 204 201 204
0.88 1.31 2.8 3.2 2.4 2.7 3.3 3.8 0.67 0.73 2.4 2.3 1.2 1.2 0.89 1.29 3.8 2.5 2.3 1.9
58.8 65.5 56.8 64.9 59.2 65.0 72.8 73.9 70.0 74.9 71.5 74.2 68.3 74.5 77.0 80.6 75.8 80.5 77.7 80.5
0.36 0.59 1.27 1.45 0.93 1.21 1.72 1.73 0.32 0.32 1.03 1.04 0.54 0.52 0.41 0.57 1.85 1.13 1.00 0.84
26.4 29.8 26.7 29.5 27.7 29.7 42.7 33.6 31.9 34.1 27.6 33.8 28.7 33.9 36.6 36.7 36.9 36.5 37.6 36.5
INDUSTRIAL AND ENGINEERING CHEMISTRY
104
VOL. 30, NO. 1
PARTIAL VAPORPRESSURES OVER VARIOUSSULFITE-BISULFITE SOLUTIONS TABLE 111. EQUILIBRIUM Solution
Bze 6.61
M
NHa
0
Same Triethanolamine Same
R-2 R-3
R-4 RM-1
,
..
40.05 40.37 39.80
UA-1
Same Monoethanolamine Same Same Urea Aniline
UA-2 UA-4
Same Same
24.87 9.69
V-1
NH,
3.97
W X-1
KOH NHI
Y-1
Same
RM-2
RM-3
S
Y-2 Y-3 Z-1
.
Base
20.63 20.15 20.59 24.60 16.30
11.47 6.94
S E 15.96 ,
,
.
Temp. Coefficient. Remarks -bsoz pH Satd. with Na2SOs; 11. 83.% NaCl 2302 Satd. with MgSO, 2432 *.
PgOi
..
15.50 ........ 1 2 . 2 9 Rapid sulfate formation 14.85 Same 17.96 Same 19.89 Same 1 7 . 9 1 Same 5.85 .. .... .. 6 . 4 9 Enough lactic acid to make homogeneous soln. 5 . 1 6 Same 1.52 No lactic acid: two layers 0 . 9 2 42.317 AlClr-
fiH.8 _--.. . . .. . . .
12.55 21.18
- CONCENT RATION
OF
C&NH$AOLES
PER
7 - 7 0 ' C.--50° PgOi Pbase PHnO PSOz
C-. Pbaas PHiO
0.08 6 9 . 8 0 . 3 4 0.06 7 7 . 6 0 . 3 4
4.4 5.4
3.2 2.7
426 455
2.0 2.2
.. .. .. ..
437
4.34
439 428
0.84 2.87
...
21.4
..
479
0.52 0.43
.. .. .. .. ..
180 203
0.73 0.81
3422
4.86
34.9
5.91 5.40
2.8 11.8
2435 2320 2753
5.44 4.77 5.26 1.54
5.1 453 1.80 201 0 . 6 4 31.1 398 1 2 . 6 176 4.72 9.2 437 3.32 179 1.18 Vapor pressure of SO2 too high t o measure
3633 3656
4.25 4.28
179
3575
4.53
30.7
17.5
5: is
113.7 5.2
..
27.m
..
10.24 5.86
.. .. ..
100 MOLES
OF
,. ..
.. .. .. .. 3.0 2.5
509
8.0
6.8
225
1.71 2.2
.
395 482
32.6 2.48
.. ..
175 216
6.8 1.01
1.6
439
,
..
.. .
.. . 429 .. .
3.83 0.34 1.65 .
..
423 431
4.6
2.7
453
2.0
,
. 196 ... 01..4216 2.84 .... i185 1.76 .. SS2.4 .. 177 1 . 1 5
0.48
200
0.82
., .. .. .. .. .. .. .. 0.12
--35' p8Oz
78.5
0.54
77.9 75.4
0.04 0.43
C.P b u e PHaO
0:04
.. ..
31.4 33.2 33.1 35.8 31.7
7 9 . 6 0.33 6 2 . 4 2.29 73.8
.. .. .. ..
81.7 3.0 8 5 , 5 2.7
0.24 1.07
40.7 34.8
81.6
0.57
1.08
37.4
66.0 85.0
1.67 0.47
78.5
0.49 0.12
..
..
74.3
0.81 0.64 7i:io.gg 70.0 0.36 79.0
0.33
.... .. .. .. .. .. ..
..
35.8 33.6
..
29.5 37.8 32.7
..
33.8 33:s 28.8 35.3
effect of the regeneration temperature on the capacity of the ammonia solutions. Because of the possibility of stripping the sulfur dioxide from the solution to a greater extent than is the case with the ammonia solutions, both of the systems being considered have relatively greater absorbing capacity than those of the ammonia system. Even the dilute sodium solutions have a capacity many times that of water, indicating that the use of these solutions as solvents makes it necessary to circulate only a small fraction of the quantity of liquor that is required for simple water absorption. A second advantage in the use of these solutions having low volatility of the base is the possibility of securing more efficient removal of sulfur dioxide from the gases. This is shown in the efficiency curves of Figure 4. With these solutions and with steam regeneration to 1 mm. of sulfur dioxide at the regeneration temperature, a n efficiency of over 95 per cent removal from gases containing 0.3 per cent sulfur dioxide can be reached. This was impossible with the ammonia solutions.
FIQURE 3. C A P A C I T Y O F METHYLAMMONIUM SULFITE-BISULFITE SOLUTIONS FOR ABSORBINGSULFUR DIOXIDEFROM GASESCONTAINING) 0.3 PER CENTSULFUR DIOXIDE (Regeneration t o 1 mm. of sulfur dioxide a t t r o )
0.23 0.95
221 11.9 220 8 . 9
,.
WATER
1.4
196 194
7.6 7.9
17.9 5.86 9.1 6.2
-
196
55.3 40.7
...
33.3 13.3 23.4 10.2
concentration of sulfur dioxide in the original waste gases is considered to be 0.3 per cent by volume. These curves are to be compared with the corresponding ones for the ammonia solutions (9). I n the latter case a definite maximum was observed at certain concentrations of ammonia which depend on the operating conditions. This concentration was found to coincide with that which requires the smallest amount of steam for regeneration and, therefore, represents the optimum concentration of solution for absorbing sulfur dioxide from dilute gases. The capacity of the sodium or methylamine solutions increases with concentration and is limited only by the solubility of the salts. It is of interest to note also that the capacity of these solutions increases with the temperature of regeneration, contrary to the
C
-
Pbsse pan0
3582 2873
2130 Satd. with caproic acid 2582 (Calcd. vapor pressure of caproic acid) 7 . 0 2 21 22 Satd. with valeric acid 3585 (Calcd. vapor pressure of valeric acid) 6 . 9 5 20.15 Same as Y-1 2900 7.04 1 8 . 8 1 Same as Y-1 3252 7.02 21.22 Satd. with oleic acid 2279
Same Same Same
,---90' C.
Steam Requirements for Regeneration The quantity of steam required for stripping the sulfur dioxide from the solutions also depends upon the conditions of operation. The shape of the vapor pressure curve is also a n important factor, the extent of regeneration for a given steam ratio depending upon the concavity of the equilibrium curve. For comparison, the minimum steam requirements have been estimated for several operating conditions on the basis of the attainment of equilibrium between the effluent vapors and influent solution to the regenerator. These values are shown in Figures 5 and 6 plotted against solution concentration. The ordinates show the minimum quantity of in pounds per pound of sulfur dioxide, required for the recovery of sulfur dioxide from gases containing 0.3 per
JANUARY, 1938
INDUSTRIAL AND ENGINEEhING CHEMISTRY
105
under consideration. This condition is favorable to steam regeneration because of the greater ratio of sulfur dioxide to water vapor in equilibrium with the saturated solvent. I n order to determine whether any of the solutions proposed for smelter gases would be preferable to the solutions already studied, the effect of temperature on the vapor pressures of a number of these solutions was measured. The same technic was used as that described for the previous work. The concentration of sulfur dioxide in the solution was adjusted so that it would correspond to a solution leaving the absorber a t about 40" C. The results are shown in Table 111. The data are insufficient to determine the capacity of the solutions, but they indicate the relative quantities of steam required for regeneration. This can be seen best by comparing the ratio of P S O ~a t 90" C. to that a t 35". The temDerature coefficient. -bsozi is the exponential term from the iogarithmic relation F~~~~~ 4, MAXIMUMEFFICIENCY OF SOLUTION~ OF SODIUM of Equation 2. AND METHYLAMMONIUM SULFITE-BISULFITE IN REMOVINQSULFUR DIOXIDE FROM GASESCONTAINING 0.3 PERCENTSULFUR A nuhber of the solutions studied are excluded from pracDIOXIDE tical considerations because of decomposition. This is par(Regeneration to 1 mm. of sulfur dioxide at tr') ticularly true of those containing formates and thiocyanates. (The f;ormer have been proposed as a n addition agent to promote the cent sulfur dioxide. T h e a c t u a l r e m o v a l of sulfur dioxide from quantity of steam required will exsolutions, 6.) The decomposition ceed the minimum quantity by an in these solutions was so rapid that amount which will depend on the I d C . ABSORPTION the vapor pressures could not be extent of the regeneration and on measured. the efficiency of t h e s t r i p p i n g The ethanolamines, w h i c h a r e column. b e i n g u s e d commercially for reThe curves may be compared covering hydrogen sulfide and carwith similar curves for the ambon dioxide by a cyclic process . monia system (9). A comparison and have been suggested for reof the three systems indicates that, moving sulfur dioxide from gases for the same operating conditions, (S), would require about 25 per both the sodium and methylamine cent less steam than the ammonia solutions require more steam for s o l u t i o n s . Unfortunately, here regeneration than do the ammonia also the spontaneous oxidation of solutions; the sodium solutions rethe sulfite into sulfate, even in the quire the most because of the absence of air, takes place rapidly limitation of solubility and smaller and renders the solutions useless. effect of temperature on the vapor Urea solutions, which are very pressure of sulfur dioxide. Neither weak bases and should have high the sodium nor methylamine solu5.C.ABSORPTION I tions show any optimum concentration in so far as steam requirements are concerned, in distinction - from the ammonia solutions. I n 0 4 e both of the former cases also the C -CON C E N T RATION SODIUM-MOLES PER I quantity of steam required is less FIGURE 5. MINIMUM STEAMREQUIREMENTS FOR a t the lower temperatures of regenREGENERATION OF SODIUM SULFITE-BISULFITE Soeration than a t the higher. This LUTIONS (Gases contain 0.3 per cent sodium dioxide) is also contrary to the case of the ammonia solutions. which can be traced to the volatility of the base. Optimum conditions of temperature coeffi- ; operation for the sodium and methylamine solutions, therecients, have such fore, exist a t the economic balance of the cost of steam and low capacities for the cost of maintaining vacuum, obviously somewhat below absorbing s u 1 f u r the normal boiling point of the solution. dioxide that they c a n n o t b e considered. Equilibrium Vapor Pressures over Other Aniline soluSystems tions, which have A number of suggestions for the improvement of the steam been suggested for 1 regeneration process for the recovery of sulfur dioxide have the removal of sulappeared in the patent literature. For the most part these fur dioxide f r o m are based on substituting other bases for ammonia, or adding smelter fumes (d), FIGURE6. MINIMUMSTEAMREto the ammonia solutions a compound that gives a higher would require only QUIREMENTS FOR REGENERATION OF acidity when the solution is heated. Most of these proposals about one-third as METHYLAMMONIUMSULFITE-BISULhave been made for removing sulfur dioxide from smelter much steam as the FITE SOLUTIONS gases where the concentration is higher than in the gases ammonia solutions (Gases contain 0.3 per cent sulfur dioxide)
T
I
-
1 i
'
INDUSTRIAL AND ENGINEERIPV-G CHEMISTRY
106
require when applied to dilute gases. Ordinarily these solutions would separate into two layers in the absorber, but a modification to maintain homogeneity consists in adding a definite proportion of lactic acid. The high cost of these chemicals is unfavorable to the practical development of this scheme.
VOL. 30, NO. 1
lytic reactions that take place in the solutions. A brief discussion of these equilibria will indicate the type of solvent most suitable for the cyclic process. Because of the general application of the underlying principles, this discussion should be of value in choosing solvents for the absorption of other acid gases, a subject of particular interest to the chemical industry a t present. An exact representation of ionic equilibria requires the use of the thermodynamic principle of chemical activity. Because data of this nature for the system being considered are almost entirely lacking, and since it is desired to make only general comparisons, the simple classical theory of electrolytes will be applied. The vapor pressure of sulfur dioxide over a sulfite-bisulfite solution is directly proportional to the concentration of unionized sulfurous acid present in the solution due to hydrolysis. The proportionality factor has been shown by Johnstone and Leppla (10) to be strictly constant over a long range of concentration in pure sulfurous acid solutions. Deviations due to secondary salt effects, of course, may be expected in high sulfite-bisulfite concentrations, but the constancy of the factor M , already considered, indicates that this influence must be relatively small. An expression for the vapor pressure as a function of the solution composition can be derived as follows : Let s be the total concentration of sulfur dioxide in the solution in moles per 1000 grams of water, K1 the ionization constant of sulfurous acid, K z that of the bisulfite ion, and kl the proportionality constant; then s = H2SOS
IH
+ HSOs- + so3--
(8)
FIGURE 7. EFFECTOF HYDROQEN-ION CONCENTRATION ON VAPOR PRESSURE OF S ~ F U R DIOXIDEOVER SULFITE-BISULFITE SOLUTIONS 5 MOLESOF SULFURDIOXIDE CONTAININQ PER 1000 GRAMS OF WATER
The addition of aluminum chloride to the ammonia solutions in order to decrease the steam requirements has been patented by the Imperial Chemical Industries (4). Recently a process using a solution of basic aluminum sulfate for the recovery of sulfur dioxide from smelter gases was described by Applebey ( 1 ) . These solutions would require the smallest quantity of steam for regeneration of any solution studied, the ratio of vapor pressures indicating a requirement about one-fourth of that specified for the ammonia solutions. For dilute gases, however, the capacity of the particular solution studied as a solvent for sulfur dioxide renders its use for this purpose unlikely. From theoretical considerations, regeneration of the sulfite-bisulfite solutions in contact with a partially soluble organic acid should not only increase the capacity of the solution, but, if the acid has the proper physical properties, it should greatly decrease the steam requirements. Although the acids chosen for the study are not suggested for practical development a t present, the results in Table I11 indicate that the theoretical predictions have been verified.
Theoretical Considerations of Hydrolytic Equilibria The discussion of sulfite-bisulfite systems as solvents for the cyclic process of sulfur dioxide recovery, involving regeneration by heating, seems to show that, besides the availability and cost of the chemicals, the important properties of the system are those that determine the capacity of the solvent, the shape of the equilibrium vapor pressure curves, and the effect of temperature on the vapor pressure of sulfur dioxide. All of these properties are dependent on the hydro-
or
The symbols indicate the molal concentration of the various species, in moles per 1000 grams of water. Using the values of K 1 = 0.013 and kl = 0.795 atmosphere per mole per 1000 grams of water (found by Johnstone and Leppla for 25' C.) and K z = lo-' (the value of the second ionization constant found by Kolthoff, 11), the variation of SO, with pH for 5-molal solutions of total sulfur dioxide is plotted in Figure 7. The curve indicates that such solutions would be in equilibrium with gases containing 0.3 per cent sulfur dioxide a t approximately p H = 5.0. For more concentrated solutions the pH would be correspondingly higher. I n a cyclic process for recovering sulfur dioxide from dilute gases involving regeneration of the solvent by stripping with steam, the basic equilibrium may be represented by SO2 (g)
whereBc-1
+ HzO + Bz-' F? HSOs- + Bz
(13)
substance capable of acting as a proton acceptor
= any
According to the Brdnsted theory B"-' is a base and B' is an acid (14). This is a simpler and more general concept than the older theories of hydrolysis, which would require the existence of several types of reactions. The degree of completion of either of the opposing reactions would depend, of course, on the ionization constant of sulfurous acid and on the equilibrium constant for the reaction: Bz-1
+ H + +Be
(14)
INDUSTRIAL AND ENGINEERING CHEMISTRY
JANUARY, 1938
A large number of substances act as conjugate acids and bases according to Equation 14. For the present discussion three examples are sufficient: (A) The sulfite ion is the base, as in the absorption by alkali sulfites:
+H
SOa--
HS03-
+
(15)
(B) A partially ionized base of the type MOH is present: MOH
+H
*
a H20
+M+
+ H + a HA
KbH'C KbH' where Kb = ionization constant of the base K , = ionization constant of water
(17)
Using Equation 18, the relation between s and H Anow becomes:
It is evident that (-4)is a particular example of (C), but because of their interest both will be considered. CASE A. ABSORPTION O F SULFUR DIOXIDE B Y ALKALI SULFITESOLUTIOXS.Let c = M = total alkali concentraper 'Oo0 grams Of water. Then from the tion, in principle of electroneutrality, H' M - = HSOs2S03-OH(18)
+
+
+
This condition is favorable to absorption of the sulfur dioxide but unfavorable to stripping, and reduces the capacity of the solution cycled for absorbing the gas. OF SULFUR DIOXIDEBY SOLUTIOSS CASEB. ABSORPTION O F PARTIALLY IONIZED B A S E S O F THE TYPE MOH. Again let c = total concentration of base, and cy = fraction of base present as ions, then
(16)
(C) The ion of a weak acid is present: A-
107
M + = ac
k'bH &C
=
[H*+K,,
=
K,
+
( H +)*
+ KbH+ - O H - ] [-KIH'
(22)
+ KiH + KiKz + 2K1K2 +
1
(23)
Curves for several values of Kb are plotted in Figure 8 for 5 molal solutions. The corresponding vapor pressure curves a t 25" C. are shown in Figure 9.
Substituting the values of Equations 8 to 10:
+ c - OH-] [ ( H + ) ' + KiH+ + KiKzl KiH + 2K1K2 ki(H+)'[H-+ c -OH-] P802 = KiH + 2K1Kz s =
[H+
(19)
+
+
Equation 20 is a more general formula for vapor pressure than Equation 1. When H+ and OH- are negligible compared to c, and (H+)2is negligible compared to KIH+ KlK2
+
H+
=
2s - c (K)
KZ
and Equation 20 reduces to the form of Equation 1. (Small letters refer t o concentrations in moles per 1000 grams of water.) Values of s were calculated for corresponding values of H-- for c = 5 from Equation 19. These are plotted as pH vs. s/c in Figure 8. The calculated equilibrium vapor pressures for these solutions a t 25" C. from FIGURE 9. PARTIAL PRESSURE OF SULFUR DIOXIDEOVER VARIOUS Equation 20 are shown by the solid curve designated SOLUTIOSs WITH TOTAL BASECONCElrTR.4TION OF 5 M O L H S PER 1000 GRAMSOF WATER as "alkali sulfite-bisulfite" in Figure 9. It is evident from Figure 8 that a 5-molal solution of sodium sulfite-bisulfite is by no means buffered a t pH 5 ; consequently, The conclusions to be drawn from these curves are as folthe vapor pressure curve is abruptly concave in this region. lows: (I) Solutions of ammonium sulfite-bisulfite (Kb = 1.8 x 10-5) are similar to those of the strong alkali, as far as the reversible absorption of sulfur dioxide is concerned, except for secondary salt effects dependent upon the size of the ions. (2) The most thoroughly buffered solution a t pH 5, requisite for high capacity and ease of regeneration, is obtained by using a weak base, for which the ionization constant is between 10 *and CASE c. ABSORPTION O F SULFUR DIOXIDE BY SOLUT I O I ~ SCONTAIKIKG THE IONS OF A WEAKACID. Consider the case of the absorption of sulfur dioxide by the salt of a weak acid, the free acid of which is completely soluble a t the given concentration of the salt; that is, there will be no phase separation if the acid is entirely displaced by sulfur dioxide. Let c be the molal concentration of the salt; then the concentration of the acid anion is :
-
%-RATIO
SULFUR
DIOXIDETO BASE
FIGURE8. NEUTRALIZATION CURVES FOR VARIOUS SOLUTIONS WITH TOTAL BASECOXCENTRATION OF 5 MOLESPER lo00 GRAMS OF WATER where K,, = ionization constant of weak acid
INDUSTRIAL AND ENGINEERING CHEMISTRY
108
Again applying the principle of electroneutrality of the solution : s=
0
p++ - -- -1[ c
I
Koc H++&
2
3
OH
4
'+
1
(H ) KiH++ KiKz KiH+ 2K1Kz (25) +
d
+
7
6
a
e
W ._ - RATIO SULFUR DIOXIDE TOTOTAL BASE
VOL. 30, NO. 1
dioxide over several of these heterogeneous systems are shown in Figure 10. The decrease of the concavity of the curves as the buffer capacity of the solution is increased is evident. I n order to compare the capacities of the various solutions for absorbing sulfur dioxide, vapor pressure curves for the regeneration temperature, t,', are plotted as broken lines in Figures 9 and 10. The regeneration temperature is arbitrarily taken as that temperature a t which the vapor pressure of sulfur dioxide is twenty times that a t 25" C. This will depend on the temperature coefficient, which differs for each type of solution, but, in general, will lie somewhere near the normal boiling point. A comparison of the capacities is shown in Table IV. The much greater dissolving power of the properly buffered solutions for the cyclic system is due to the greater spread of the vapor pressure curves. The outstanding advantages of these solutions over the alkali or ammonium sulfite-bisulfite solutions is apparent. From a practical standpoint, a high carrying capacity of a solution is desirable not only to reduce the quantity of solution required for circulation, but also to permit the use of more dilute solutions when the circulation is maintained a t a minimum.
Effect of Temperature on Partial Vapor Pressure of Sulfur Dioxide
ID
The temperature coefficient of the vapor pressure of sulfur dioxide over the various solutions is more difficult to predict than the shape of the vapor pressure curves. Using the familiar van't Hoff equation, constant b in Equation 2 should be approximately equal to A H-1/4.575,where A H is the heat evolved in calories when one mole of sulfur dioxide is absorbed. From the Hess law, the heat evolved for the general reaction represented by Equation 13 is:
FIGURE 10. PARTIAL PRESSURE OF SULFUR DIOXIDE OVER VARIOUS SOLUTIONS SATURATED WITH WEAKACIDS WITH TOTAL BASECONCENTRATION OF 5 MOLESPER 1000 GRAMS OF WATER
Curves similar to those derived for weak bases could be calculated from Equation 25, showing the concentration of sulfur dioxide in solutions of varying p H for different values of K,. Corresponding vapor pressure curves could be found from Equation 20. It should be noted, however, that if Ka =KwIKb
(26)
where K , is the ion product constant of water, Equation 25 reduces to Equation 23. For the absorption of sulfur dioxide, a solution of a salt of a weak acid, therefore, is exactly similar to a solution of a weak base when the ionization constants of the acid and base are related by Equation 26. This important principle should not be overlooked in the choice of solvents for acidic gases. If we consider the case in which the free acid is of limited solubility in water so that separation of a solid or liquid phase is caused by the absorption of sulfur dioxide, an entirely different set of conditions exists. Here, the concentration of the acid anion is:
where K , = molal concentration of un-ionized portion of acid in solution I n general, K , is a function of temperature only. The relation between s and H+ now becomes:
+
+
K K * KiH K I K z ] (28) s = [ H + + c - " - -H" + - O H - ] [ ( H A I H + 2K1K2 +
+
It is evident from the curves in Figure 8 that these heterogeneous solutions are much more nearly buffered than any of those previously considered. Furthermore, solutions buffered a t any pH may be had by suitable choice of the product K,K,. An increase in K,, due to an increase in solubility with rise in temperature, considerably decreases the p H of the solution. The calculated curves for the vapor pressure of sulfur
AH
(29)
=AH4+AH1+AHP-AHs+AHd
where A H , A HI
= heat o f solution of SO2 in H20 = heat of first ionization of HnSOa A H , = heat o f protolytic reaction, Equation A H2 = heat of second ionization of H&Os
A Hd =
14
net integral heat of dilution of the ions with the heats of dilution of the original constituents of the solution positive, and those of the 6nal products negative
The values of A H , and A H I are, respectively, - 6260 and -3860 calories per mole (IO); AH2 is approximately 700 calories (7'). The other quantities depend upon the type of solution. Heats of dilution of salts vary considerably, even for similar salts, since they are specific properties of the nature of the electrolyte. The difference between the ammonium SOLUTIONS FOR TABLE Iv. CAPACITY OF SULFITE-BISULFITE ABSORBINGSULFUR DIOXIDE FROM DILUTE GASES ( p g o , leaving scrubber, 2.0 mm. a t 25O C.; psol leaving regenerator, 2.0 mm. at tro; concentration of solution, 5.0 molal in total b a a 4
Solution
Concn. of SO^ Leaving Scrubber
Moles/1000 G. H ~ OLeavlng Regenerator
4.95
4.35
0.80
4.95 4.88 4.50 2.95
4.35 3.82 2.32 0.91
0.60 1.06 2.18 2.04
4.88 4.50 2.95
3.82 2.32 0.91
1.06 2.18 2.04
1.50 3.10 4.02
0.13 0.38 1.35
1.37 2.72 2.67
Alkali sulfite-bisulfite Sulfite-bisulfite of base: Kb = K b = lo-' Kb = Ka = 10-9
Alkali salt of acid:
KO Ka Ko
=s
10-7
10-6 10-5 Alkali salt of uartiallv sol. acidK.K* = 10-4 K o K a = 2.5 X 10-5 KoKa = 10- 5 =
Capacity Moles
SO~/IO~)O 0. Hs0
JANUARY, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
and the sodium ion has already been seen to cause a difference of about 18 per cent in the value of coefficient b. Since the heats of dilution are small compared to the other heat quantities involved, for a general comparison we may neglect this term and consider the effect of A H,. The heats of ionization of most weak electrolytes are small (6,1.2). The value of constant b, therefore, will usually not vary much from -2360, the value given by neglecting the terms A H , and A H d . I n one case, however, the heat of ionization is known to be large-namely, that of water. Case B, the absorption of sulfur dioxide by solutions of partially ionized bases of the type MOH, involved the ionization of the base and the formation of water from the ions (Equation 16). In this case if A H d is neglected as well as the heat of the ionization of the weak base, taking A H , as -13,850 calories (IS), the value of b should approach -5390. Although none of the solutions studied shows such a large negative value of the coefficient, evidently because some of the heat effects omitted are not negligible, it is true that the value of the constant for the systems of weak bases, such as the ethanolamines and the basic aluminum chloride, reaches -3700. This represents an increase in the ratio of the vapor pressure a t 100” C. to that at 25O, of from 98 for the alkali sulfite system to 1120 for the weak base. I n other words, the effect of temperature on the vapor pressure is 115 times as great for the latter as for the former solutions. Another important conclusion can be drawn in regard to the effect of temperature from a consideration of the heats of reaction. Although homogeneous solutions of salts of weak acids should not differ greatly in this respect from the alkali sulfite-bisulfite solutions, the heterogeneous systems, in which the free acid forms a new phase, may show entirely different temperature coefficients, for an additional term, the heat of solution of the free acid, must be subtracted from the summation of Equation 28. A number of free acids show considerable increase in solubility, and many reach complete miscibility at moderate temperatures. Systems containing these acids are expected to give large temperature coefficients of the vapor pressure of sulfur dioxide. This is verified by the experimental data reported in Table 111.
Conclusions From a practical standpoint the general conclusions to be reached from this study are as follows: 1. The use of sulfite-bisulfite solutions of sodium or methylamine for the cyclic method of recovering sulfur dioxide from dilute gases requires a larger quantity of steam for regeneration of the solution than is required by the ammonia system. Because of the nonvolatility of the base, regeneration may be carried farther with the former solutions, and consequently more complete recovery of the sulfur dioxide may be accomplished. The sodium system is limited by the low solubility of the sulfite.
109
2. From theoretical considerations, the solutions with the greatest capacities for the reversible absorption of sulfur dioxide from gases containing 0.3 per cent are those containing sulfites of weak bases having ionization constants of salts of weak acids having ionization constants of or heterogeneous mixtures of solutions of salts of weak acids, saturated with the free acid, for which K,K, = 10-4.4. These solutions are the ones which are most nearly buffered in the range desired, and consequently the vapor pressure curves are most nearly linear. Evidence indicates that the effect of temperature on the vapor pressure of sulfur dioxide is greater for the sulfite soIutions of the weak bases than for any other homogeneous solvent of this type. With the proper selection of the weak acid, however, the temperature effect may even be larger for the heterogeneous system mentioned above. Although the practical application of these principles derived from theoretical considerations cannot be given a t present, they point the way to the development of the solvent of the desired properties; furthermore, they indicate the value of a similar study of solvents for the absorption of carbon dioxide, hydrogen sulfide, and other acidic gases.
I n a later paper of this series a description of a cyclic system involving chemical regeneration of the solvent will be given.
Literature Cited (1) Applebey, M. P.,J.SOC. Chem. Ind.,56, 139T (1937). (2) Boswell, M. C., and Beal, G. P., U. S. Patent 2,047,819(July 14, 1936). (3) Bottoms, R.R.,IND. ENG.CHEW,23,501 (1931);U. S. Patent (to Girdler Corp.), 1,783,901(Dec. 2, 1930),Reissue 18,958 (Sept. 26,1933);1,834,016(Dec. 1, 1931). (4) Clark, A. M. (to Imperial Chemical Industries), U. S. Patent 1,908,731(May 13,1933). (5) Gumlich, O.,and Richter, A. (to I. G. Farbenindustrie), German Patent 553,910(June 16,1932). (6) Harned, H.S.,and Embree, N. D., J. A m . Chem. SOC., 56, 1050 (1934). (7) International Critical Tables, Vol. V , p. 178, New York, McGraw-Hill Book Co., 1929. (8) Johnstone, H.F.,IND. ENG.CHEM.,27,587 (1935). (9)Ibid., 29, 1396 (1937). (10) Johnstone, H. F., and Leppla, P. W., J.A m . Chem. Soc., 56, 2233 (1934). (11) Kolthoff, I. M.,2. anorg. Chem., 109,73 (1920). (12) Kraus, C. A., “Properties of Electrically Conducting Systems,” p. 149,New York, Chemical Catalog Co., 1922. (13) Lewis, G. N., and Randall, M., “Thermodynamics,” p. 486, New York, McGraw-Hill Book Co., 1923. (14) Taylor, H. S., “A Treatise on Physical Chemistry,” p. 1005, New York, D.Van Nostrand and Co., 1931. (15) Treadwell and Hall, “Analytical Chemistry,” Vol. 11, p. 58, New York, John Wiley & Sons, 1935. RECEIVSD July 12, 1937. Presented before the] Division of Industrial and Engineering Chemistry at the 94th Meeting of the American Chemical Society, Rochester, N. y., September 6 to 10, 1937. Published by permission of the Director of the Engineering Experiment Station, Universlty of Illinois. This paper contains part of the results obtained on the ao6perative research project, Case 34, with the Utilities Research Commission of Chicago.
