REGENERATION O'F CARBOXYLIC CATION EXCHANGE RESINS WITH CARBON DIOXIDE ROBERT K U N I N AND BASIL VASSlLlOU Rohm t 3 Haas Go., Philadelphia, Pa.
A study of the regeneration of the sodium form of various carboxylic cation exchange resins using pressurized COz solutions demonstrates that the regeneration efficiency of carbonic acid depends upon the acidity of the resin. Because of its low acidity, Amberlite IRC-50 is suited ideally for this unusual regeneration technique. Because of the much higher selectivity of carboxylic acid resins for calcium ions than for sodium ions, the regeneration efficiency of pressurized COZsolutions for the calcium form of the resin is much lower. Using saturated solutions of COz a t modest pressures ( 1 00 to 300 p.s.i.), one obtains as eluates solutions of NaHC03 suitable for use as the regenerant for the weak base anion exchange resin of a deionization system, thereby minimizing materially the cost of deionizing brackish waters.
HE primary limitation in the use of ion exchange systems Tfor the deionization of waters in the brackish regions of salinity is that of regeneration cost. The use of weak electrolyte ion exchange systems based upon carboxylic cation exchange and weakly basic anion exchange resins permits one to deionize brackish water a t almost 100% regeneration efficiency for both resins (5). T h e use of these resins has made possible the use of ion exchange techniques for the deionization of waters a t salinities above 500 p.p.m. or, in some cases, above 1000 p.p.m. However, in most instances, even this regeneration cost is prohibitive when such regenerants as NH3 and H2S04 are used. To reduce the regeneration costs further, cheaper regenerants must be obtained or methods must be devised for the recovery of a t least portions of the waste regenerants at low cost. It has been demonstrated (3) in the beet sugar industry that the cost of the anion exchange resin regeneration can be minimized by the recovery of the waste ammonia regenerant, using limr. O n a large scale, the cost of the regeneration then reduces to the cost of an equivalent amount of lime. I n a large installation such as a beet sugar refinery, limestone is burned at the site. resulting in additional savings. Since several noteworthy technical advances have been made in the burning of limestone. the use of lime prepared on the site as a regenerant for anion exchange resins becomes attractive economically. Since there are technical difficulties associated with the direct use of lime slurries as a regenerant, the indirect use of lime (use of ammonia coupled with a recovery system based upon reaction Ivith a lime slurry) is the method of choice a t the present time. even though some attempts (3) a t the direct use of lime slurries have looked promising. Few advances have been made in the direction of minimizing the cost of the rrgeneration of the cation exchange resin. Although sulfuric acid is relatively inexpensive, this cost is still appreciable. Since COZ is available from the burning of limestone. from the stack gases of a fuel-burning operation. or from a sewage plant, the possible use of this acidic gas as an acid regenerant for the cation exchange resin comes to
mind. Although this gas would be too weakly acidic for the regeneration of strongly acidic cation exchange resins, the use of COZ as a regenerant for weakly acidic cation exchange resins is not unreasonable, particularly where some waste COZ is available. The use of saturated COz solutions as regenerants for weakly acidic cation exchangers was suggested by Gray and Crosby (2). Since there are very few data available for such systems, the regeneration efficiency of the COz-saturated solution was studied for three carboxylic cation exchange resins differing in acidity. Theory
The regeneration reaction of any carboxylic resin can be written as: RCOONa COz HzO @ RCOOH NaHC03
+
+
+
This is a reversible reaction and the degree of completion depends upon the concentration of the COZ in the solution, which is a function of the solubility, increasing with pressure and decreasing with temperature. The degree of regeneration may also be improved by removing the N a H C 0 3 generated. The regeneration efficiency of the COz solution will also depend upon the acidity of the resin. It is therefore obvious that the lower the acidity of the resin. the greater will be the regeneration efficiency, because the equilibrium value of the reaction depends upon the relative affinities of the carboxylic group of the resin for hydrogen and sodium ions. Experimental Procedure
Ion Exchange Resins. Three carboxylic cation exchangers were selected for this study: Amberlite IRC-50, a carboxylic resin based upon a methacrylic acid cross-linked copolymer having an apparent p K of 6.1, and BD-129 B and B-0681, experimental resins based upon a n acrylic acid cross-linked copolymer having apparent pK's of 5.3 and 5.8. The acidities and other properties of these resins are well described by Fisher and Kunin (7). The resins were conditioned prior to the elution study by exhaustion with I N NaOH, followed by regeneration with 1 N HCl. After the samples had been VOL. 2
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rinsed with water, they were transformed to the Na+ form with 1N N a O H and rinsed with water. The samples were wetscreened to obtain the 20- to 40-mesh fraction, which was used for the elution study. In the one experiment involving the calcium form of the resin, the sodium salt of Amberlite IRC-50 was treated with 10 bed volumes of 1N CaCll and rinsed with water. Apparatus. A pressurized ion exchange column was constructed of sight-glass tubing capable of sustaining pressures as hi& as 320 p.s.i. For safety purposes, this tube was surrounded by a Plexiglas tube (Figure 1). The remainder of the apparatus is described i n Figure 2. The C O P solutions were prepared in an autoclave first loaded with water and COZ was passed in a t the desired pressure until saturation was achieved. The flow of solution was maintained by means of gas pressure applied to the autoclave from a C o t gas cylinder adjusted to the designated pressure. The flow rate was regulated a t the outlet side of the column of resin. Elution Details. T h e desired volume of the resin in the sodium form was charged to the column and classified by backwashing with deionized water. The bed was permitted to settle and then washed with deionized water a t a pressure equal to that of the COz solution. Flow during this rinse was maintained with Nz pressure. The COz solution was then passed through the column a t the desired flow rate. Samples of the effluent were collected periodically and analyzed for alkalinity by titration to the methyl orange end point with standardized HzSO,. Since the course of the elution proceeds according to the reaction,
1/4" SST PIPE
Figure 1. Schematic drawing of pressurized ion exchange column
RCOONa
+ C 0 2 + H,O
-P
RCOOH
+ NaHC03
this analysis is all that is required to characterize the elution. Variables Studied. T h e variables investigated in this study of the elution efficiency of COSincluded : Effect of acidity of the cation exchange resin Effect of COn pressure Effect of flow rate of the COZ solution Effect of a neutral salt background ' n the regenerantacos solution Effect of the valence of the exchangeable cation in the cation exchange resin
Results
Table I summarizes the conditions employed during the course of the above experiments and data from the analyses of the effluent solutions. Figure 2. system
Schematic drawing of pressurized ion exchange
Bed Height Flow R a t e
h.24" u: l/2 p o l / f t 3 / m r
-1 i
$
02
f
z LL
O
01
0''
I
2
3
4
5
6
7
8
9
10
I1
12
I3
14
I5
6
B E 0 VOLUMES OF R E G E N E R A N T
Figure 3. COz regeneration of various carboxylic cation exchange resins 2
I h E C PRODUCT R E S E A P C H AND DEVELOPMENT
-
'0
I
2
3
4
5
6
7
E
B E D V O L U M E S OF R E G E N E P A N T
Figure 4. Effect of pressure on COPregeneration of Amberlite IRC-50
Table 1.
Run 1
-7 3
4
5 6 7
8 a
Regeneration of Carboxylic Cation Resins by Using COZ (Bed height, 24 inches) Flow Rat?, CO2 Concn. of Gal./ Pressure, .\‘aCl, Cu. Ft./ Resins P.S.I. P.P.M. Min.
I\niberlite IRC-50 ( S a ) B-0681a BD-129 B“ Amberlite IRC-50 (Na) .4mberlite IRC-50 ( N a ) Amberlite IRC-50 i N a ) Amberlite IRC-50 ( N a j Amberlite-IRC-50 (Ca)
220 220 220 290 150 150
220 290
... . . .. ... ... ..
5000
...
l/2 ’ i 2
‘/L
‘iz
PRESSURE (pptl
li2
1
Figure 5. Effect of pressure on peak concentration during COz regeneration of Amberlite IRC-
‘/z
‘/z
Exffrimmtal carboxylic ion exchange resins based on acrylic acid.
50 04
Discussion of Results
Considering the shape of the elution curves described in Figure 3 and the pKa values of the resins (Amberlite IRC-50 = 6.1, B-0681 = 5.8, BD-129 B = 5.3), one may conclude that the resin with the higher pKa value or the lower acidity can be regenerated more readily. This is in agreement with the general theory presented previously. Because of its lower acidity, Amberlite IRC-50 is the more attractive resin from the viewpoint of regeneration efficiency. The effect of pressure (Figure 4) is that which one would anticipate from the effect of pressure on the solubility of C O Q in water. Figure 5 illustrates the effect of pressure on the elution peak concentration. The effect (Figure 6) of flow rate, at constant pressure, is that normally anticipated of diffusional processes. One of the more important results Jvas the negligible effect which was noted on the addition of a neutral salt (NaC1) (Figure 7) to the COz solution. This enables one to employ untreated water for preparing the regenerant solution and for rinse purposes. Because of the much higher selectivity (4)of the carboxylic acid resin. .4mberlite IRC-50, for calcium ions than for sodium ions, the regeneration efficiency of the COZ solution for the calcium form of the resin is much lower. O n passing the same volume of C O z solution through the Ca-resin bed as in the case of the sodium form, one achieves but a 50% regeneration of the resin.
Bed Height
I
h = 24”
B E D V O L U M E S OF R E G E N E R A N T
Figure 6. Effect of flow rate on Cog regeneration of Amberlite IRC-50
Flow Rate Bed H e i g h t Pressure
u = 1/2 g a l / f t 3 / m $ n h = 24’ P = 220 p i 1
c
z
W
3 iL ii W
z
COP
IN 5000 ppm NaCl SOLUTION
10
0 0
I: z 0 LI
0
z
Conclusions
This study demonstrates that pressurized C O S solutions are adequate as regenerants for the sodium form of some carboxylic cation exchange resins. The availability of cheap sources of C O P will enable one to improve the economics of deionization systems based upon weak electrolyte ion exchange resins. When a saturated COz solution is used as the regenerant, a solution of N a H C 0 3 is obtained as a product of the regeneration reaction. This solution, when degassed, can be used as a regenerant for the weak base anion exchange resin in a deionization system, further minimizing the cost of the deionization. Acknowledgment
T h e authors acknowledge the invaluable assistance of Albert G. Coffman, Jr., in carrying out the experimental portion of this study.
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B E D V O L U M E S OF R E G E N E R A N T
Figure 7. Effect of salt background on Arnberlite IRC-50
CO:, regeneration of
literature Cited (1) Fisher, S., Kunin, R., J . Phys. Chem. 60, 1030 (1956). (2) Gray, K., Crosby, H., U. S. Patent 2,656,245 (Oct. 20, 1953). (3) Kunin, R., Amber-Hi-Lites (Rohm & Haas Co.), No. 57 (May 1960). (4) Kunin, R., “Ion Exchange Resins,” p. 40, Wiley, New York, 1958. (5) Kunin, R., Vassiliou, B., Proceedings, First International Symposium on Fresh Water from the Sea, Athens, Greece, Ida? 1962. RECEIVED for review October 15, 1962 ACCEPTED November 30, 1962 VOL. 2
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