I
L. F. WIRTH, Jr., C. A. FELDT, and K. O D L A N D N a l c o Chemical Co., Chicago 38,111.
Effects of Oxidants on Ion Exchangers Chlorine attack can be prevented by using a reducing agent
O x D m i w AGENTS present in water in small amounts have varied effects on ion exchange materials. Cation exchange resins are oxidized by chlorine; in addition to limiting the operating usefulness of the cation resins in demineralizers, this can result in serious fouling of strongly basic anion resins, as well as contamination of demineralized water. By elimination of chlorine with Na2S03 or S02, the life of cation exchange resins is extended materially, and difficulties such as anion resin fouling and poor water quality are reduced or eliminated. While the water supply is a primary source of organic contaminants, and adequate pretreatment for organic removal is necessary, fouling of anion resins by organics present as raw water contaminants is probably not the most serious. Replacement of anion resin to correct a poor water quality condition should not be made without first examining operating conditions of the demineralizer and the condition of the cation exchange resin. Influent and effluent tests for chlorine should be made to determine the quantity of chlorine and extent of reduction. Samples of cation exchange beds should be taken representing the top 6 inches, as well as lower levels. For a mixed bed, samples of the separated cation resin should be obtained. Evaluation of these samples will indicate if the cation resin is contributing to the difficulty and, further, if replacement is necessary to avoid rapidly contaminating the new anion exchange resin. While chlorine present in water to be demineralized is primarily responsible for cation resin degradation, nitrates have also been suspected. At the present time, the authors are not aware of any problem caused by an oxidant which cannot be corrected through the use of a reducing agent. All styrenetype cation exchangers commonly commercially available are affected by chlorine when operated in the hydrogen cycle, regardless of cross linkage or method of manufacture. Warm waters are more likely to cause difficulty than
638
cold waters. Highly mineralized waters, and the presence of heavy metals which may act as catalysts, are known to increase the rate of attack. When sodium cycle cation exchangers follow hot process lime treatment operating a t temperatures ranging from 210' to 290' F., small amounts of oxygen can cause a breakdown of cross linkage of styrene type resins. Deaeration and catalyzed Na2S03 feed have been very effective in promoting long resin life, with plants on record having operated at temperatures as high as 290' F. for a t least 5 years with very little change in water content and exchange capacity. Physical degradation of conventional resins usually takes place after extended periods of use; however, the strain-free products currently available have minimized this condition. Care should be taken to reduce oxygen to less than 0.1 p.p.m. to assure reasonable resin life. A 10% cross linked resin will assure maximum resin life while maintaining good operating salt efficiency. Field Observations
Within the last year several problems occurring with old demineralizers were studied. I n one case, a process utilizing demineralized water was affected periodically, and the cause of difficulty was traced to a degraded cation exchange resin which was contributing breakdown products into the effluent. This contaminant was passing through the weakly basic anion resin into the demineralized water . I n another case, fresh, strongly basic anion resin was placed in two units of a large demineralizer as part of a systematic replacement procedure which has been carried out for several years. Under this arrangement, old anion resins, which in this case were Type 11, were being operated in parallel and on the same cation effluent as new Type I1 resins. The new beds were installed about a month apart. The first to be installed bccamc irreversibly fouled to the
INDUSTRIAL AND ENGINEERING CHEMISTRY
extent that replacement was necessary in a few months time. Prior to replacement, a thorough evaluation of the situation a t the point of operation revealed several interesting and enlightening factors. The cation exchangers had undergone severe degradation through chlorine attack. These resins were several years old at the time the difficulty occurred. Sodium sulfite feed for removal of chlorine improved cation resin operation and, although these degraded cation beds were not replaced a t the time, further degradation was stopped and the second anion bed continued operation without further fouling. Type I1 anion resins that have undergone a large change in salt splitting capacity (basicity-i.e., where salt splitting capacity is approximately 50% of total capacity-apparently have little affinity for the breakdown products of the cation exchanger. Analytical Techniques
In evaluating these problems, some analytical techniques not commonly used in routine water evaluation have been employed. Surface tension measurements taken at various points throughout the demineralizer have bcen helpful. For example, in one case the surface tension of a raw water supply was 68 dynes but dropped to below 60 dynes when measured on the effluent from a degrading cation resin. Surface tension increased to 70 dynes on the effluent of a new Type I1 strongly basic anion exchanger. The drop from 68 to 60 dynes indicates the presence of a surfactant contributed by the cation exchanger, while the increase from 60 to 70 dynes indicates the sorption of the surfactant by the anion exchanger. Another helpful test is the methyl green anionic detergent test (5). This test has been used to show either the presence or absence of a sulfonate type product through the development of a colored complex with methyl green.
9
50 8
0 in
40 7%
,-
e
.-
30
0 c
6r 2
Cl 5
0)
c 0 e
15
4
1.37
5 50 0
:
. iM
r
2: 'CIC
s
e C
- 40
E io
z
- 0
-g i i
I
New Mat'l.
-
/
L
0:
----
I
0 'c1
0
30
5
r
W ~~~~
2
4
6
8
10
12
14
16
18
20
Totalized Feed, Kg./Cu. F t . , Anions A s COCO,
2
12 14 16 18 Totalized Feed, Kg, /Cu. Ft., Anions A s CaCO, 4
6
8
20
IO
Figure 1 . pH and conductivity values from Type II anion resins during first cycle of demineralizing test water produced by degraded cation resin
Figure 2. Lower conductivity curve shows flow sensitivity developed b y new Type I I anion resins in three cycles on test water containing cation resin degradation products
New resin i s quickly fouled; used resin does not remove contaminant
Upper conductivity curve indicates used resin is not Row sensitive
I n one case studied, the raw water supply was positive, but the cation effluent yielded a considerably stronger color than the raw water. The effluent from new Type I1 resin was negative, equal to a blank produced from a water known to be free of sulfonate. I n severe cases, it is also possible to detect breakdown products by fluorescence of the water under ultraviolet light. Ion exchange resins are known to fluoresce and when operated under favorable conditions fluorescence cannot be detected in demineralized water. A considerable quantity of breakdown product must be present to be detectable by this method. Cation exchanger beds which have undergone considerable degradation will show a greater change in the upper portion of the bed than in the lower portion. This is due to contact by larger amounts of chlorine in the upper bed levels, since some of the chlorine is removed by the resin as it passes down through the bed. The rate of attack by chlorine is said to be influenced by heavy metals present in the water. This is neither confirmed nor denied at this writing, since the work was done with no heavy metals present in the water. The only metals present would be those which are in the resins through initial processing. I n the case of study using field samples, heavy metals may have been present in the resin in varying amounts, depending on the water supplies on which the resins were operated. I n evaluating cation eschange resins, total capacities (on dry weight and wet volume bases) and water content are important. For several years, water content has been a guide to early indication of cation resin degradation. This may be misleading when evaluating
certain highly cross linked products. Dry weight and wet volume capacity . measurements then reflect more accurately the true condition of the resin. I n the case of conventional resins, as water content increases, the wet volume capacity decreases usually without a change in the dry weight capacity. A change in dry weight capacity becomes extremely meaningful, since this can be caused by introduction of chlorine into the resin through attack by excess chlorine. Dry weight capacity loss through desulfonation is small, although this is a factor which becomes more important when degradation is taking place a t higher temperatures in the hydrogen cycle. Breakdown products of cation resins that cause fouling of the strongly basic anion resins, or poor water quality in cases where the resins are fouled and can no longer retain this contaminant, appear to be styrene sulfonic acid polymer units which behave like anionic detergents. All of the commercially and commonly available styrene resins studied are affected by chlorine, and all such resins contained chlorine at the conclusion of testing. The rate of attack is essentially the same on all products, as measured by chlorine content of the resins after testing.
Experimental Data Since the problem of organic fouling of anion exchangers was recognized some 10 years ago, considerable efforts have been made to determine the nature of foulants found in natural waters. I n most of the reports on organic fouling (7, 3, 4, 6),the emphasis has been on water-borne organics, such as humic acid, tannins, lignins, and other products from plant decay. Un-
doubtedly these are the most common foulants. Removal of organic material by pretreatment has become increasingly more important, because of the demand for higher quality water and longer resin life. Several procedures for cleanu p of fouled resins have been suggested
(7,3). The authors have suspected for some time that the products of cation resin degradation, being organic substances of large molecular size, could also affect the operation of an anion resin. Other investigators (4) have also suggested that the cation resin could be a source of foulant. I n a series of recent field investigations this situation has definitely been found to exist. I n normal plant operation it is difficult to determine the actual amount of organic material present in the feed water. I t is even more difficult to distinguish between the substances resulting from breakdown of the cation exchanger and the organics originally present in the water. T o avoid this difficulty these studies were conducted on filtered Chicago tap water, which contains only trace amounts of organics. A solution was prepared by recirculating 30 gallons of acidified demineralized water through a degraded polystyrene-type cation resin a t temperatures of 110' to 112' F. for a period of 8 days. During this time approximately 1.3 pounds of free chlorine were added per cubic foot of resin. The chlorine content in the cation influent was kept at an average of approximately 15 p.p.m. ; the maximum chlorine content was kept below 35 p.p.m. Although no metallic cations were added to the system, the cation exchanger was regenerated regularly during the recirculation period to VOL. 53, NO. 8
AUGUST 1961
639
IO 9
8 I
a 7 0 c
.-
c
1 6 cn
5 4
3
1
2
3 4 5 6 7 8 9 T i t r a t i o n Volume, ml. N / 5 0 NaOH
1
0
Figure 3. Titration curve of cation effluent test solution after removal of mineral acids shows strong (pK = 3.5) and weak (pK = 6.5) organic acids
ensure complete decationization of the water in the polyethylene tank. At the end of the recirculation period, the water was pumped through the cation exchanger and discharged into a second tank for use in operation of the anion columns. Final analysis of the recirculated water showed an acidity of 430 p.p,m. as CaC03, consisting primarily of Sod- and C1-, plus a small amount of a weak acid. The methyl green test indicated the presence of 4.4 p.p.m. of sulfonates, as sodium lauryl sulfate. Conductivity was approximately 3600 micromhos. A portion of this water was diluted by a factor of 5, but the original total acidity was maintained by addition of HC1 and HzS04. Figure 1 illustrates the rapid fouling of a fresh Type-I1 anion exchange resin in its first cycle of operation on this diluted supply. The old Type-I1 material, on the other hand, exhibits a low p H immediately, indicating the failure of this material to sorb the high molecular weight organic acids. I n successive cycles, low p H and high conductivity were experienced with both resins throughout the run. When the undiluted test water was used, fouling was so rapid that the low p H and high conductivity of the effluent water were experienced from the very beginning of the run. Once the new resin had become fouled, the quality of the water produced by the resin in successive cycles was poorer than that produced by a used resin which had lost 50Yo of its salt splitting capacity. This is typical of fouled resins. When the usual clean-up procedures for organic fouled anion exchangers are employed in the field, only about two cycles of good quality water are obtained.
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2 4 6 8 IO 12 14 16 18 Totalized Feed, Kg. / C u . F t . , Anions A s CaCO,
Figure 4. pH and conductivity values from new Type II anion resins show fouling during first cycle of demineralizing test waters produced b y degraded cation resins A and B (standard cross linkage) and C (high cross linkage)
When fouling of fresh anion resins by degrading cation resins has been experienced, severe flow sensitivity is evident. Figure 2 shows the difference in flow sensitivity of fouled fresh resin and an old Type-I1 resin. Figure 3 illustrates the titration curve of the effluent collected during demineralization tests of this solution with new and used Type-I1 resins. Although the solution contained no sulfates or chlorides, it exhibited the high conductivity and low p H characteristic for the anion effluent. The solution was concentrated by a factor of 5 by slow evaporation before titration. Approximately two thirds of the total acidity was due to a fairly strong acid having a pK of approximately 3.5. The remaining acidity was caused by a weak acid having a p H of 6.5. I t should be emphasized that the acidity was caused entirely by organic acids, since no mineral acid was present. Since all the preceding tests were conducted on foulants obtained from a field-degraded cation resin, it was decided to degrade three styrene-type resins commonly available commercially by chlorination of fresh resins and to use the water containing the degradation products for operation of the anion columns. Three jacketed columns were prepared for recirculation of chlorinated water through samples of standard cross linked and highly cross linked styrene type cation exchangers. Each resin was prepared especially for this cycling operation by treatment with ethyl alcohol, hot HC1, and hot water rinses. T h e treament was continued until no organic material, as determined by color throw, was leached from the resin upon prolonged contact with ethyl
INDUSTRIAL A N D ENGINEERING CHEMISTRY
20
alcohol; 50-ml. tapped samples of each resin were placed in the jacketed columns. A 5-gallon container of demineralized water, acidified with 85 p.p.m. of HC1 and 85 p.p.m. of "03, both as CaC03, was connected to each of the three columns. Chlorine was added to each container a t the start and during the test to maintain an average of 15 p.p.m. chlorine, but not more than 35 p.p.m. The columns were operated on the chlorinated, acidified supplirs, collecting each effluent in a reservoir which, when filled, was used for repeating the cycle. The recirculated water was tested periodically, and when free chlorine was depleted to zero, samples of the recirculated liquor were collected for analytical tests. The batchwise recirculation was carried out for approximately 19 passes. Each unit was then put on a continuous recirculation basis. Total contact was 30 days for each column. During this time, chlorine additions amounted to 3.92, 4.72, and 4.92 pounds per cubic foot of resin, respectively, for standard cross linked resins A and B, and for highly cross linked resin C. Table I lists the characteristics of each of the three resins tested before and after chlorination. The characteristics of the recirculated waters, analyzed periodically during the recycling operation, are reported in Table 11. T h e recirculated waters then were deioniied by new Type-I1 anion resins. The results of the first cycle on each resin are shown in Figure 4. Again, there is an indication that organic acid discharge from the degrading cation resins rapidly fouls the anion resins. I t should be noted that these accelerated aging tests are not necessarily a true indication of relative stability of the cation resins.
ION EXCHANGERS
Table I. Analyses of Cation Resins Used for Breakdown Studies
Total CapacityQ Water Meq./ MLlcq./ Content,a C1, Gram 311. % %
Resin
S, %
Standard cross linked
Table II.
Analyses of Water Recirculated through Cation Resins Present in Present in Standard Highly Cross Cross RecirLinked, Linked. culation, P.P.M. P .P,M . Resin B Resin C Days Constituents Resin A
Resin A Before After
5.00 4.42
1.90 1.59
53.4 57.2
0 6.3
13.7 13.2
6
0.0
75.5
75.5
c1--a
13.8 103
18.7 103
17.2 105
SOa-& c1-a
31.2 276
Sulfonate as Na lauryl sulfate
Resin B
Before After
4.75 4.31
1.78 1.41
54 57.6
0 7.2
14.5 12.5
21
Sulfonate as Na lauryl sulfate
Highly cross linked Resin C
a
0.0 75.5
0.0
c1-a
0
30
Before After Hydrogen form.
4.54 3.84
1.74 1.49
54 53.2
0 7.3
c14
14.7 12.4 a
No attempt was made to study the fundamentals of this degradation process. However, from the early work of Schmidt and others ( Z ) , the following facts can be pointed out. 0 The
c-c
H -CH, -$-CH,THE MOST.....
Sulfonate as Na lauryl sulfate .4sCaC03.
the degradation products contain chlorine. The tertiary hydrogen is the most, and quaternary carbon the least, vulnerable site of attack (below). I n the presence of oxygen, the tertiary hydrocarbon is transformed first to the hydroperoxide and then to the ketone, resulting in chain scission.
Probable Degradation Mechanism
aolvstvrene chains have a - numbir of 'weak links because of peroxide bridges, tail-to-tail additions, and chain transfers. a The links next a carbon with a double bond are weak points. @ Chlorine degrades polystyrene, and
I t is reasonable to assume that after a certain number of chain scissions, portions of the cation exchange resin consisting of styrene sulfonic acid polymer units become soluble and are
CH-CH,
tert-H
SOI-"
- q 'CH,-
0
*.THE LEAST VULNERABLE quaternary C
0.25
0.20 35 302
0.15 30 325
2.4
1.4
33.5 358
39.3 387
38.6 415
1.05
4.9
4.9
3.5
leached from the rein. These soluble units undoubtedly contain chlorine and weak acid groups. These observations should be helpful in preparing ion exchange resins of greater stabilitv. T h e degradation of anion exchange resins is shown in the lower portion of the reaction diagram (below). Although they have the same backbone as the cation exchange resins, the most vulnerable point of attack is a t the nitrogen. The quaternary nitrogen is progressively transformed to tertiary, secondary, and primary nitrogen and finally degraded to a nonbasic product. This has been the subject of several reports in recent years ( $ 4 ) .
e
Acknowledgment
The assistance of Alfred W. Oberhofer in the experimental work is acknowledged. References
CHAIN SCISSION
(1) Bacon, H. E., Lewis, W. J., Combustion 32,No. 1, 37 (1960). (2) Boundy, R. H., Boyer, R. F., "Styrene, Its Polymers, Copolymers and Derivatives,'' Chap. 13, Reinhold, New York, 1952. (3) Frisch, N. W.? Kunin, R., IND.ENG. CHEM.49,1365 (1957). (4) . , Frisch, N. W.. Kunin. R.. J . A m . Water Works A.rsoc.' 52, 875 (1960). (5) Moore, W. A , Kobelson, R. A , , Anal. Chem. 28, 161 (1956). (6) Wilson, A. L., J . Afipl. Chem. 9, 352 (1959). (7)' Wiith, L. F., Proc. Am. Power Conf. 16, 626 (1954).
RECEIVED for review September 26, 1960 ACCEPTED May 5, 1961
ANION EXCHANGE RESIN DEGRADATION Probable degradation mechanisms for ion exchange resins
Division of Water and Waste Chemistry, 138th Meeting, ACS, New York, September 1960. VOL. 53, NO. 8
AUGUST 1961
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