(178) Sekmakas, K., Gaske, J. E. (to DeSoto, Inc.), U. S. Patent 3,450,791 (1969). (179) Sekmakas, K., Gaske, J. E. (to DeSoto, Inc.), U. S.Patent 3,597,495 (1971). (180) Sergeyeva, L. M., Belov, I. B., Lipatov, Yu. S., Todosiichuk, T. T., Kogan, 2. Ye., Kalaus, A. Ye., Polym. Sci. USSR, 12,2339 (1971). (181) Slagel, R. C., Bloomquist, A. E., Can. J . Chem., 45, 2625 (1967). (182) Smith, 0. W., Weigel, J. E., Trecker, D. J. (to Union Carbide Corp.), German Patent 2,103,870 (1971). (183) Solomon, I). H., “The Chemistrv of Organic Film Formers,” Chapter 2, Wiley, New York, N.”Y., 1967. (184) Stauffer ChemicalCo., Dutchpatent 69 03312 (1969). (185) Suzuki, H., J . Polyrn. Sci., Part A-l,9,387 (1971). (186) Suzuki, I., Ichikawa, K., Ohmura, J., Iwashita, H. (to A a h i Chemical Industry Co., Ltd.), British Patent 1,252,314 (1971). (187) Taft, D. D., Mohar, A. F., J . Paint Technol., 42, No. 550, 615 (1970). (188) Tarakanov, 0. G., Kondrat’eva, L. N., Polym. Sci. USSR, 13,642 (1972). (189) Toyo Rubber Chemical Industry Corp., French Patent 2,065,863 (1971). (190) Trapasso, L. E., U. S. Patent 3,627,735 (1971). (191) Tremco Manufacturing Co., British Patent 1,161,178 (1969). (192) Trojan Powder Co., Chemical Department, Division of Commercial Solvents Corp., Form 101, “DMPA,” 1968. (193) Tuzar, Z., Beachell, H. C., J . Polym. Sci., Part B , 9, 37 (1971). (194) Ulrich, H., J . Elastoplast., 3, 97 (1971); Ulrich, H., “Advances in Urethane Science and Technology,” Vol. 1, p 33, S. L. Reegen and K. C. Frisch, Ed., Technomic Publishing Co., Stamford,Conn., 1971.
(195) Union Carbide Corp., Technical Bulletin, “Niax Caprolactone Polyols,” 1971. (196) Unitex, Ltd., BritishPatent 1,098,487 (1968). (197) Unsworth, A. K., Paznt 0 2 1 Colour J., 160,313 (1971). (198) UrethanePZast. Prod., 1 (lo), 10 (1971). (199) Veba-Chemie, A.-G., Product Bulletin 22-ME-871-7,1971. (200) Vogelsang, G. .K., “Protective Organic Coatings for General External Aircraft Application,” Air Force Materials Laboratory Technical Report No. 67-63, Wright-Patterson Air Force Base, Ohio, 1967. (201) Whittaker, R. E., Polymer, 13,169 (1972). (202) Wleczorrek, W., Chzm. Pezntures, 32,223 (1969). (203) Wieczorrek, W., Farbe Lack, 75,318 (1969). (204) Wieczorrek, W., Deut. Farben-Z., 24,569 (1970). (205) Williamson, W. I. (to Imperial Chemical Industries, Ltd.), BritishPatent 1,077,390 (1967). (206) Winkelmann, H. D., Thoma, W., Rinke, H., Oertel, H. (to Farbenfabriken Bayer, A.-G.), German Patent 1,962,602 (1971). (207) Witco Chemical Corp., “Castomer P-0002,” 1972. (208) Wittcoff, H., Paintindia, 21 (l), 32 (1971). (209) Wooster, G. S. (to bllied Chemical Corp.), U. S. Patent 3,436,361 (1969). (210) Wooster, G. S., Delgado, F. h4. (to Allied Chemical Corp.), U. S. Patent 3,639,355 (1972). (211) Yagfarov, M. Sh., Gubanov, E. F., Polym. Sci. USSR, 12, 1310 (1971). (212) Zaganiaris, E., Tobolsky, A., J . Appl. Polym. Sci., 14,
+
1997 (\ 1- 9 _7-.~ ) )
(213) Zapp, R. L., Serniuk, G. E., Rlinckler, L. S., Rubber Chem. Technol., 43,1154 (1970). (214) Zwolinski, L. M., Kaplan, M., Bailey, M. E., J . Cell. Plast., 6 , 79 (1970). RECEIVED for review June 11, 1973 ACCEPTED September 7, 1973
Sorption of Sulfur Dioxide, Hydrogen Sulfide, and Nitrogen Dioxide by Ion-Exchange Resins Ayalur S. Vaidyanathan and Gordon R. Youngquist” Department of Chemical Engineering, Clarkson College of Technology, Potsdam, N . Y . 13676
Sorption of sulfur dioxide, hydrogen sulfide, and nitrogen dioxide by various polymeric ion-exchange resins has been studied experimentally in a batch system. Uptake curves suggest that a complex diffusion process controls sorption rates. Regeneration is difficult in most cases.
R e c e n t l y the study of diffusion and sorption of gases and vapors by polymers has assumed greater importance. Polymers are subject to exposure in diverse environments both domestically and industrially and a knowledge of their resistant and deterioration qualities in a given atmosphere is of great significance. I n addition, polymers have increased application as sorbents for small-scale removal operations like chromatography. Since these polymers are man-made, they could be custom built with specific predetermined structures 288 Ind. Eng. Chem. Prod.
Res. Develop.,
Vol. 12, No. 4, 1973
and properties to suit a particular operation. N a n y attempts have also been made to alter or control the sorptive properties of polymers by modifying their pore structure, during manufacture (Dicky, 1955; Gordon, 1961). Ion-exchange resins have been found to possess specific sorptive properties (Holm, 1962), which may be amenable to alteration or modification by suitable modification of the active sites (Duda, 1963). The chief drawback in using conventional resins as sorbents was their almost negligible porosi-
ties and surface areas. However with the introduction of macroreticular ion-exchange resins, resins with high porosities and surface areas have been made available. The possibility of using ion-exchange resins as sorbents for the removal of industrial pollutants like SO2, oxides of nitrogen, and H2S has created some interest in recent years and some experimental investigations (Cole and Shulman, 1960; Duda, 1963; Gupta, 1969; Holm, 1962; Layton and Youngquist, 1969; Pollio and Kunin, 1967; Thompson, 1972) have been carried out to determine the rate and equilibrium characteristics of a few gas ion-exchange resin systems. The earlier studies (Cole and Shulman, 1960; Duda, 1963) were carried out with conventional ion-exchange resins having negligible porosities and surface areas. Duda's work (1963) on monomethylamine-sulfonated polystyrene-divinylbenzene ion-exchange resins indicated that sorption occurs in stagesdiffusion into the void volume of the dry polymeric phase followed by penetration of the aggregates of sulfonated sites Kith swelling and reaction. This type of two-stage diffusionsorption mechanism \\-as supported by the studies of Layton aiid Youngquist (1969) on the SOz-Amberlyst 21 system. Further they showed that each stage could be represented by a simple diffusion-in-a-sphere model. The works of Gupta (1969) and Youngquist and Garg (1972) seem to lend support to this two-stage diffusional model. Other studies (Pollio and Kunin, 1967; Thompson, 1972) on sorption in fixed beds showed that the macroreticular ion-exchange resins have fair sorption capacities for sorbates like SO,and H,S.
0: PORE DIAMETER (MICRONS)
2 0.6k
1 I
-
I
u 0.42
-
a
+ E u E a
0.2-
0
,
ll,,,,l
, , , , ,1
, ,
100
IO
,
,,,,(,I
: _ _ _I _ _I _ _ _ _ _ _ _ _ _ _ _ _ _ I _I - _ _ - - - - - I '
10000 2 0 0 0 0
1000
ABSOLUTE PRESSURE (PSIA)
Figure 1 . Cumulative pore volume of ion-exchange resins 1.0
-
I
I
o
I
I [ I I PRESSURE INTERVAL I l l 30 rnrn Mq
1
I
2'4
(mi#
1
I
[
I
I
1
1
I
- 0 00-
b -6000-13300
b-133 00 -31s 00
MI Mm
Present Work
In this study the rate and equilibrium loading characteristics of the follou-ing systems were investigated using interval sorption techniques: (1) the Duolite A6-sulfur dioxide system a t 35' and (2) the -4mberlyst 26-hydrogen sulfide system a t 25 and 35". Studies were also undertaken to determine the suitability of the commercially available macroreticular ion-exchange resins for the removal of NO,. Four different Xmberlyst resins in addition to the Duolite -16 resins were investigated. The -1mberlyst resins were supplied by the Rohm and Haas Co. while Duolite A6 resin originated from the Diamond Shamrock Corp. Kuri and Kunin (1967), on the evidence of electron micrographic studies of these Xmberlyst resins, concluded that "the macroreticular resins are agglomerates of randomly packed microspheres wibh a continuous noiigel structure permeated with holes and channels." As regards the Duolite -46 resin having high porosit'y and surface area, it is expected t,o have a complex structure similar to that of a sponge. The resins were washed with methanol and dried a t 140°F before being used. Pure grade gases supplied by bhe Xlatheson Co. were used. The experimental procedure corisist'ed of exposing a known quantity of uniform-sized ion-exchange resin particles to the sorbate gas a t the required pressure and following the sorbate uptake gravimetrically. The temperature of the system was maintained at the desired level with *0.2". Two types of apparatus mere used in t,liis study: (i) a Calin electrobalaiice with a n accuracy of 10-7 g and (ii) a quartz spring balance with a n accuracy of 10-5 g. The details have been described elsewhere (Gupta, 1969; Laytoii and Youngquist, 1969; Vaidyanathan, 1972). The volume of the sorbate gas reservoir is large enough to ensure a constant pressure during the entire course of a run, till a n equilibrium value is reached. Once equilibrium is reached, the weighing unit is isolated from the rest of the system whose pressure is adjusted to the
Figure 2. Sorption rate curves for Duolite A6-sulfur dioxide a t 35" desired value. Thus, by increasing (or decreasing) the pressure in steps, the equilibrium isotherm in the desired pressure interval was obtained. Results and Discussion 1. Duolite A6-Sulfur Dioxide System. Duolite A6 is a tertiary amine having a matrix which is a phenol-formaldehyde condensate. These are always in t h e form of granules, as contrasted to the spherical polystyrene excliangers.oThey have high porosities and a pore size range of 200-400 A as shown in the mercury porosimeter data of Figure 1. The rate and equilibrium characteristics of the Duolite .46-S02 system a t 35" are presented in Figures 2-4. h perusal of the rate curves brings out the following interesting features. a. Rate of Sorption. All the uptake curves in Figure 2 are characterized by a very low rate of sorption. In all t h e cases equilibrium is reached only after very long intervals, of the order of 10,000 min or more. The initial rapid uptake accounting for about lOyo of the total weight gained lasts anywhere between 0 aiid 25 min. S o orderly increase or decrease of rate is observed either in the beginning ( t + 0) or as the sorption proceeds. b. Influence of t h e External Pressure a n d of t h e Size of Pressure Increments. Variation in the external pressure has little or no effect on either the rate or the shape of the uptake curves, nor do they exhibit any definite trend with change in the pressure increments. Thus, the overall rate process, i t appears, is pressure independent. This could hapInd. Eng. Chem. Prod. Res. Develop., Vol. 12, No.
4, 1973 2 8 9
I
I
I
I
I
I
I
30
40
50
60
70
PRESSURE INTERVbL
PRESSURE INTERVAL 0-561.0 426.0 mmHq
-
-4zso--2es.o
0.8
9-2Scr.O-
0.6
11.0
(PVMP RUWNINOI
,0 . 4 0.2 0 0
IO
20
30
40
50
70
60
80
90
0
10
20
80
90
ti'' (rninP
Figure 3. Desorption rate curves for Duolite A6-sulfur dioxide at 25"
L
I
,I
0
I
pen if the macropore diffusion is rapid followed by very slow gel penetration which is independent of the external pressure. This appears to be the case for this system. c. Shape of the Uptake Curves. The shapes of t h e sorption curves in Figure 2 clearly indicate t h a t the process follows what is known as the "anomalous diffusion" behavior in the polymer. The flat rate curves suggest a rapid macropore diffusion with little or no sorption in the macropores followed by gel diffusion. This gel diffusion, which is very slow, appears to proceed in two stages: (i) a relatively faster penetration initially, which slows up considerably after X t / J f m = 0.15-0.20; (ii) a second stage, which is not so pronounced in some cases but evident all the same, characterized by a very slow approach to equilibrium. This second stage is expected to be accompanied by swelling and relaxation of stresses in the gel mat,rix. This phenomenon of a twostage sorption is often encountered in studies on diffusion in polymers, especially for organic vapors and water vapors (Park, 1968). Given the complex nature of the sorption process, it is difficult to determine clearly the rate mechanism. The simplest explanat,ion would be to assume that a quasi equilibrium is rapidly reached a t the polymer surface which is followed by the establishment of the same equilibrium concentration of the sorbate throughout the polymer by diffusion. The second stage of sorption then results when the rigid polymer undergoes a n elastic expansion on absorption of the penetrant until the elastic force due to swelling increases the chemical potential of the sorbate within the material to a value equal to the chemical potential outside it. Hence no further sorption 290 Ind.
Eng.
Chem. Prod. Res. Develop., Vol. 12, No. 4, 1973
Figure 5. Sorption rate curves for Amberlyst A26-hydrogen sulfide a t 25"
occurs. On standing, however, the elastic forces sIowly relax, the chemical potential of the sorbate within the material decreases, and further sorption occurs to reestablish equilibrium. The process continues until the elastic forces have decayed away completely and a final true equilibrium sorption has been attained. On this basis the rate of sorption in the second stage is a measure of the initial unrelaxed force within the particle. On the basis of the above explanation it appears that diffusion of SO, in Duolite A6 is non-Fickian, possibly accompanied by swelling and stress relaxation. One possible method of determining whether it is Fickian would be to carry out rate studies with part'icles of different sizes a t same pressure intervals and temperature to see whether the different rate curves superimpose when each time axis is divided by the respective (radius)2. If they do not', the process is nonFickian. However it is doubtful whether the surface area and pore structure will be the same for differently sized particles, especially for those available commercially, and hence this test may fail to yield conclusive results. Moreover, the coupling of molecular diffusion with polymer relaxation can produce a wide variety of sorption curves (Odani, 1967; Park, 1968). I Kaddition ~ to the complex behavior associated with the relaxation of a polymer which is below its glass transition temperature, the rate data may be further complicated by such effects as strongly concentration-dependent diffusion coefficients and reaction or immobilization of the diffusing species. Figure 3 illustrates the desorption rate curves for the Duolite A6-S02 system a t 35". The desorpt,ion curves, unlike the sorption curves, show a definite trend with decreasing pressure. *As t'he absolute pressure decreases progressively from 561.0 mm, the rate of desorption appears to fall progressively. The desorption curves are characterized by (i) a fast initial desorption rate, accounting for about 15-40% of the total weight loss and (ii) a very slow desorption rate accounting for the rest of the weight lost. The rapidity with which the SO2 is desorbed in the initial stages seems to indicate that this portion of the desorbed SOz could have come from the macropores. The s l o ~tailing portion then should represent desorption from the matrix. But, as earlier pointed out, the sorption rate curves do not indicate any significant macropore sorption. Hence it is possible that these molecules have desorbed from the gel matrix adjacent to the macropores. The shape of the desorption curves seems to suggest that the process could be Fickian, a t least in the initial stages. When no more sorbate was found removable by decrease in the pressure, the sample was heated to 50" and maintained a t that temperature with heat tapes wound around the sam-
PRESSURE Emm ti9
0.6
"F-
" OP
1
0
IO
1
20
I
30
I 40
I x)
I
60
I
70
I
80
tI4
Figure 6. Sorption rate curves for Arnberlyst A26-hydrogen sulfide at 35"
- -
BO'nERII AT 25.C e BOTHERM m U*C
0
2 9 0.2s
10
100 P
1
90
tY(minY2
700
(rnrnng)
Figure 7. Sorption isotherms for Amberlyst A26-hydrogen sulfide
ple tube and the evacuation was continued. The sample was found to retain as much as 0.1216 g of SOz/g of the resin a t the end of heating and evacuation, thereby suggesting the possibility of reaction in the matrix. This amounts to 1.9 mequiv/g of resin where as the exchange capacity of the A6 resin is reported to be 6.5 mequiv/g. However, it should be borne in mind that even at 561.00 mm the total weight gain was only 0.275 mg of S02/mg of the resin which will amount to only 4.4 mequiv/g of resin. Hence no conclusion could be arrived at regarding possible reaction within. However, it could reasonably be inferred that there is a strong force, either physical or chemical, that binds the SO2 molecule rather well with the matrix. The sorption and desorption equilibrium isotherms for the Duolite A6-SOz system (Figure 4) are clearly nonlinear in nature. Both the sorption isotherm and the desorption isotherm are convex upward, similar to the isotherms obtained by Layton (1969) for the Amberlyst A21-SO2 system. Since the rate curves for the SOz-Duolite A6 system are complex, i t is doubtful whether any existing model could give a quantitative interpretation of these rate curves. Sorption is probably accompanied by swelling and stress relaxation the character of which is complex and largely unknown and thus does not lend itself to accurate description. No definitive rate model has been developed to date which accounts for swelling and stress effects. Further, since the sorption rates and regeneration are poor, the Duolite A6 ion-exchange resin is not likely to be useful for SOz removal. 2. Amberlyst A26-Hydrogen Sulfide System. Amberlyst A26 is a hard, spherical, strong base (quarternary ammonium), a n ion-exchange resin having high ion-exchange capacity and good thermal stability up to 60' in the hydroxide
(mid4
Figure 8. Sorption and desorption rate curves for Amberlyst A21-nitrogen dioxide at 35"
form and 90" in the chloride form. It has large pores (400700 A) and relatively large surface area (25-30 mz/g). Figure 1 gives mercury porosimeter data which give an indication of the pore size distribution. Figures 5 and 6 represent the sorption rate curves of the Amberlyst A26-hydrogen sulfide system a t 25 and 35O, respectively. Figure 7 gives the sorption isotherms a t these temperatures. a. Rate of Sorption. All the rate curves, excluding curves 1 and 2 of Figure 6, are Characterized by their slow approach to equilibrium. To reach equilibrium for the system a t 25" took around 10,000 min while a t 35" the time required was somewhat less. b. Influence of Pressure and the Size of Pressure Increments. The uptake curves show no gradual or orderly dependence either on absolute pressure or on the magnitude of pressure steps. This indicates that the overall process is pressure independent. c. Shape of the Uptake Curves. The curves in both Figures 5 and 6 indicate non-Fickian sorption of the type discussed earlier. The rate curves are characterized by a relatively fast initial uptake which soon falls off. X quasi equilibrium may be assumed to have been reached. The rate once again increases, albeit very slowly, and sorption proceeds to equilibrium. One or more quasi equilibria may be encountered before equilibrium is reached. This behavior is attributable to non-Fickian diffusion in the polymer matrix accompanied by swelling and stress relaxation. As has been pointed out earlier, this "stepwise" increase in sorbed species is attributable to slow relaxation of the elastic force which, in this particular case, appears to occur in stages. The uptake curves for the i121-HzS system show a remarkable similarity to those for the Duolite A6-SOz system. I n both the cases the rate curves are similar in shape and are independent of both the absolute pressure and the pressure increments. I n both the cases the rate of sorption is very low, often requiring as many as 8-10 days to reach apparent equilibrium. The two-stage behavior for rate curves of the A21-H2S system is more pronounced than it is for the Duolite A6-SOz system. So are its "stepwise" increase in sorbed species. This suggests that swelling and stress relasation are more pronounced in the case of the h21-H1S system. The equilibrium sorption isotherms a t 25 and 35" for the Amberlyst A21-hydrogen sulfide system are given in Figure 7 . At 25O, A21 sorbs as much as 0.47 g of HzS/g of the resin, and a t 35O, it sorbs 0.24 g of H2S/g of resin, both a t a pressure of 400 mm. Both the isotherms are convex upward and are "favorable." Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 4, 1973
291
PRESSURE 7 mm ng SORPTION LOADINO OIOBT g NO, DESORPTION LOADING 0,2357 NO2
0.4
0.2 __-*
0
J
I
1.0
0
10.0
40.0
t"(minY2
Figure 9. Sorption and desorption rate curves for Amberlyst A26-nitrogen dioxide at 35"
Figure 1 1 . Sorption and desorption rate curves for Amberlyst A29-nitrogen dioxide at 35'
PRESSURE
8 rnm ~g
SORPTION LOMINO 0.4192
1
0.2 0
0.1
I
10
x,
tu2(minP
Figure 10. Sorption rate curve for Amberlyst A27-nitrogen dioxide at 35'
The rate curves for the Amberlyst A26-hydrogen sulfide system are complex and indicate a diffusional process that is non-Fickian in nature as in the case of the Duolite A6-SO2 system. It is doubtful whether such a process could be quantitatively interpreted by any known physical model. Notwithstanding its high equilibrium loading capacities, especially a t higher pressure, it is doubtful whether Amberlyst -426 resin could be used as a sorbent for the removal of H2S because of its poor rate characteristics. 3. Ion-Exchange
Resin-Nitrogen
Dioxide Systems.
Studies were also undertaken to determine the suitability of the commercially available macroreticular ion-exchange resins for the removal of nitrogen dioxide. The rate and loading characteristics of the following resins were studied: (1) Amberlyst -421, (2) Amberlyst A26, (3) Amberlyst A27, (4) Amberlyst A29, and (5) Duolite A6. I n all of the above systems, except for A26 and A27, a pressure of 8 mm of nitrogen dioxide was used. For A26 and A27 a pressure of 7 mm was used. Sorption was allowed to proceed for a t least 24 hr (1440 min) before it was terminated. All the rate data were taken a t a temperature of 35". The rate data are shown in Figures 8-12 and equilibrium values are given in Table I. a. Rate Characteristics. All the ion-exchange resins studied exhibited excellent uptake rate characteristics, Amberlyst A27 being the best of all. ;1fractional weight gain of about 0.2 was achieved within 1 min, except in the cases of Amberlyst -427 and Duolite -46, where it was about 0.4 and 0.1, respectively. I n most of the cases, equilibrium was almost achieved within 100 minutes except for Duolite A6 which showed a fractional weight gain of about 0.6 only and 292
Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 4, 1973
Figure 12. Sorption and desorption rate curves for Duolite A6-nitrogen dioxide at 35"
Amberlyst A26 which had a fractional weight gain of about 0.9. b. Equilibrium Loading Characteristics. All the samples tested showed excellent loading characteristics. Duolite led the field with a loading capacity of 0.419 mg of NOz/mg of resin while Amberlyst A29 showed the poorest capacity of 0.187 mg of NOz/mg of resin. These resins, thus, have excellent loading capacities in addition to having very good rate characteristics. c. Reversibility. Desorption was carried out on all the samples except Amberlyst A27 by continuous evacuation. I n general, the desorption characteristics appear very poor. Duolite A6, which sorbs most, retains most of what it had sorbed, retaining as much as 87%, and Amberlyst A29 which sorbed the least, retained the least, retaining only about 58%. However, the effect of increased temperature on desorption was not studied. The irreversible nature of sorption suggests a kind of bonding, either physical or chemical, within the matrix. It is possible that the NOz reacts a t the exchange sites. The shape of the uptake curve suggests that either macropore diffusion is rapid with no macropore sorption or macropore diffusion controls with a fast gel uptake. I n either case the uptake curves could be fitted in with a model for Fickian diffusion in a sphere (Crank, 1957)
If 7 (= Dt/R2) is plotted against time and a straight line results, then a simple diffusion-in-a-sphere model is adequate. Figure 13 shows the plots of 7 us. t . The fit is very good up to a value of MZ/Xm= 0.75. The values of D/R2 obtained from
Table I5 Sorbent
(I
Pressure, mm
loading, mg of NO*/mg of resin Sorption Desorption
Exchange capacity, mequiv/g
9/R2, min-1
A21 8 0.2731 0.2198 0.00800 A26 7 0.3087 0.2357 0.00225 A27 7 0.2136 0.02170 A29 8 0.1869 0.1092 0.00500 Duolite A6 8 0.4192 0.3628 Conditions: sorbate, NOz; sorption and desorption temperature, 35" (continuous evacuation). TIME (min)
c:'
I
Figure 13. Comparison of sorption rate curves for nitrogen dioxide with a simple diffusion-in-a-sphere model
t h e slope range from 0.00225 min-' (for Amberlyst A26) to 0.0217 min-1 (for Amberlyst -427). Since the diffusion-in-asphere model would fit the data irrespective of whether i t is macropore controlled or gel diffusion controlled with very rapid macropore diffusion with little or no sorption in the macropores, it is difficult to assign these values of D/R2 to one or the other. The amount of NOz present on desorption also presents a fair idea of what could be happening within the matrix. The amount retained is approximately the same as the exchange capacity of the resins, thereby indicating possible reactions a t the exchange sites with exchange groups. In the case of Duolite -46, however, the difference in the value obtained (7.9 mequiv/g of resin) is significantly different from the literature value (6.2 mequiv/g of resin). I t is however possible a further increase in temperature might have released some more NO2 and consequently the amount of ? I T 0 2 retained might have been closer to the exchange capacity. Summing u p the studies on sorption of nitrogen dioxide by macroreticular ion-exchange resins, i t may be concluded that (1) the resins have excellent rate characteristics, (2) the resins have excellent equilibrium loading characteristics, and (3) the resins have very poor desorption characteristics. It appears t h a t these resins could be used when traces of NO2 have to be removed from the process stream. Reuse of the resin for repeated sorption purposes does not appear t o be feasible. Conclusions
Rate and equilibrium data were obtained for sorption of gases (SOz and H2S) by M R ion-exchange resins (Duolite
Amt retained, mequiv/g
4.7-5.0 4.14.4
4.8 5.12
2.7 6.2
2.38 7.9
A6 and Amberlyst A26) and these have been analyzed qualitatively. The nature of the uptake curves suggests that diffusion in these polymers is non-Fickian in nature. Further it appears that sorption is accompanied by swelling and stress relaxation in the matrix. Both of the ion-exchange resins studied showed considerable equilibrium loading capacities, especially at elevated pressures, but the slow rate to equilibrium which takes as many as 8-10 days, renders these resins unimportant for any commercial use as adsorbents for the removal of the respective sorbate gas. Studies were also undertaken to determine the suitability of the M R ion-exchange resins for the removal of NO,. Both rate and equilibrium loading characteristics were found excellent. B u t the resins are characterized by poor desorption characteristics that suggest reaction with the matrix, possibly a t the ion-exchange sites. Nomenclature
M , Weight gain at any instant t , g of sorbate/g of sorbent M , Weight gain at equilibrium, g of sorbate/g of sorbent XI Effective diffusivity, cm2/sec t
R T
Time, sec Radius of the particle, cm Dt/R2, dimensionless time
literature Cited
Cole, R., Shulman, H. L., Ind. Eng. Chem., 52, 10 (1960). Crank, J., "Mathematics of Diffusion," p 86, Oxford University Press, New York, N. Y., 1957. Dicky, H. D., J . Phys. Chem., 59, 695 (1955). Duda, J. Ph.D. Thesis, University of Delaware, 1963. Gordon, G. A., Sc.D. Thesis, Massachusetts Institute of Technology, 1961. Gupta, V. K., M.S. Thesis, Clarkson College of Technology, 1969. Holm, R. A., Ph.D. Thesis, University of Delaware, 1962. Kun, K. A., Kunin, R., Polym. Sci., Part C, No. 16, 1437 (1967). Layton, L. K., Youngquist, G. R., Ind. Eng. Chem., Process Des. Id.,
Develov.. 8. 317 119691.
Odani, H.,'J.'Polym. Sci:, 5, 1189 (1967). Park, G. S., in "Diffusion in Polymers,'' Chapter 5, J. Crank and G. S. Park, Ed., Academic Press, New York, N. Y., 1968. Pollio. F. X.. Kunin. R.. Ind. Ena. Chem.. Process Des. Develop.. I
,
1
,
6,516 (1967).
Thompson, R., M.S.Thesis, Clarkson College of Technology, 1972.
Vaidyanathan, ,4.S., Ph.D. Thesis, Clarkson College of Technology, 1972. Youngquist, G. R., Garg, S. K., Ind. Eng. Chem., Process Des. Develop., 11,259 (1972).
RECEIVED for review January 4, 1973 ACCEPTEDJuly 30, 1973 This research was supported in part by the Air Pollution Division of the Public Health Service under Grant A P 246.
Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 4, 1973
293
