Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 391-395
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Stability of Ion-Exchange Resins. 4. Parametric Testing To Predict Resin Breakage under Plant Conditions Michael C. Skriba" and William M. Aivino Westinghouse R&D Center, Pittsburgh, Pennsylvania 15235
Robert Kunin 13 18 Moon Drive, Yardley, Pennsylvania 19067
A test was developed to determine the stability of ion-exchange resins in terms of their resistance to the combined effects of mechanical and osmotic shock. The test as presently designed has a high degree of flexibility in exploring parameters of interest in an ion-exchange system. The following parameters were studied: particle size, pressure drop, Freeze-thaw cycles, resin type, and resin mixtures. Under the conditions of the test, the gel resins are, on the average, considerably weaker than the macroporous resins. Resin stability increases with decreasing particle size for gel resins but is not as discernible for the macroporous resins. Freeze-thaw cycling of the resins does not slgnficiantly affect macroporous resin stability. Increased pressure drop results in increased resin deterioration and is more marked for the gel resins. There appears to be a critical pressure above which catastrophic resin failture occurs. This pressure varies with resin type and may vary from lot to lot of the same resin.
Introduction Traditionally, development of bead type ion-exchange resins has been along the lines of enhancement of the chemical performance properties of the resins. Some of these developments have included increased exchange capacity, higher selectivity coefficients, increased kinetics of loading and stripping, and greater resistance to fouling. Although the physical stability of resins has received some attention (Kunin, 1972; Golden and Irving, 1972; Ball and Ray, 1976),its measurement to predict resin breakage has not been of paramount importance because ion-exchange systems were generally confined to low-flow, fixed-bed operations in which the resin beads are treated relatively gently from a physical standpoint. That this condition exists is apparent from the multitude of chemical tests available and the paucity of physical tests which have been published. Contrast the list of tests shown in Table I. It is quite apparent that the chemical nature of the resins has been of primary concern and it is right that it should be since resins are functional chemical systems. However, breakage of these resins in service not only is a serious problem from the standpoint of cost (current cost of anion exchange resins is about $100200/ft3), but also from the standpoint of product contamination by the fines, disposal of contaminated resin (both radioactive and toxic chemical) when broken, high pressure drops in the beds, and a whole host of other operating problems. Abrams (1976) has shown, for example, that only a small amount of cross contamination by fines during the regeneration phase of condensate polishing can upset the desired bed performance when it is returned to service. Westinghouse encountered servere resin breakage problems in developing a high flow rate process for recovering uranium using a continuous counter-counter ion-exchange system (Brooke, 1977). In the course of trying to resolve the problems, it was apparent that a test was needed which would have the capability of simulating the stresses imposed on resins by process conditions. Not only did we wish to find a physically stable resin for use, but we wished to know how that stability would vary with 0196-4321/81/1220-0391$01.25/0
Table I. Testing Methods for Bead Type Ion-Exchange Resins Chemical Methods total ionic capacity salt sdittine caDacitv exchange capacity load k d elition properties (load and strip curves) Anaconda test equilibrium isotherms ionic preference basicity/acidity Physical Methods Jabsco pump test-Dow Chemical Company-Analytical Method No. 14 Chatillon test (crush strength)-Dow Chemical Companyanalytical method no. 25B screen impact test osmotic shock tests
changes in plant operating conditions. Accordingly, a test was developed at the Westinghouse R&D Center which would allow testing of resin under varying mechanical and osmotic conditions either individually or in combination.
Experimental Section Test Apparatus. The Resin Strength Test (RST) apparatus is shown as an artist's conception in Figure 1. The first manually operated unit and the improved present unit are shown in Figures 2 and 3, respectively. A detailed description of the test unit is given elsewhere (Skriba and Alvino, 1979); however, the general operation of the apparatus is presented in the following section. The apparatus is a flow through cell in which a resin sample (typically 30 to 50 mL wet settled bed (WSB)) can be subjected to two different chemical solutions and high hydraulic pressure drops. As a result, the resin beads undergo simultaneous osmotic and hydraulic stresses. Because of their swelling properties, these stresses and strains can be very high. The cell in our unit was a piece of extra strong glass pipe (1 in. diameter) and 6 in. long. The glass used enabled visual observation of the resin beads while under test. Support for the resin bed is pro@ 1981 American Chemical Society
992 Ind. Eng. Chem. Prod. Res. Dev.. Vd. 20, No. 2. 1981
i ‘1
Figure 3. Present version of resin strength
Figure 2. Resin strength test apparatus initial manual model.
vided by a slotted disk bottom screen which is covered with glass beads to approximately a in. deph. The glass beads prevent the resin from being driven into the disk slots and blinding the flow cell. High bed pressure drops are simulated through the use of a bed ram driven by an actuating cylinder. The ram is drilled to allow solution flow through it and the force on the ram is controlled by adjusting the pressure to the actuating cylinder. The rate of application of the ram force is controlled by use of a throttling needle valve on the air supply line. This allows simulation of either a hydraulic shock force application or a gradual application such as would be applied by a pump startup. Applying the hp force separately in this manner then allows separate control over the rate of application of chemical or osmotic shock. Solutions of the desired chemical makeup are passed through the cell in whatever order desired. The solutions are fed by a gear pump through a flowmeter and into the flow cell, returning to the original container to conserve solutions. By varying the pumping rate, solution velocity in bed volumes per minute can be held to whatever value is desired to simulate the process in question while AI’ is set by the ram. After the resin has reached equilibrium with the solution being pumped through the cell, the inlet and outlet valves
t e a t apparatus
are switched to the alternate solution to induce osmotic shock. This process is repeated as many times as is desirable in whatever mechanical/flow sequence desired. The unit used in our tests was automated. Solenoid valves were substituted for the manual valves and a countdown type timer was used to control the number of cycles. A time delay was built into the outlet valve which approximated the solution travel time from the inlet valve to the outlet valve. This reduced the cross contamination of the testing solutions. Also for the extended cycle tests, provisions for backwash were added since significant resin degradation occurred and fines had to be removed to ensure smooth operation of the RST apparatus. To indicate when backwashing was necessary, pressure gauges were installed to measure the pressure drop across the bed. In all of our tests a IO-min cycle time was used; i.e., 5 min on each of the two solutions. These solutions were 20% (wt) H2S0, and deionized (DI)H 2 0 at room temperature. The exact pressure and number of cycles the resin beads were subjected to are reported in the text under the appropriate section. The resin beads were received in the chloride form and the initial loading of the glass cell was with resin in this form. After the test, the resin was in the sulfate form and was analyzed as such. It was not converted back to the chloride form of the as-received resin. Resin Identification. The materials used in thii study are classified as strong base anion-exchange styrenedivinylbenzene resins. Both gel and macroporous resins were tested. The gel resins were further subdivided on the basis of their osmotic shock stability and classified as “poor” and “good resins (Alvino, 1980). Resin Stability Evaluation. A detailed description of the analytical procedures used to measure bead attrition is described elsewhere (Alvino, 1980). The three parameters used to measure bead attrition are sphericity ( S ) , perfect beads (PB),and whole perfect beads (WPB). The first two parameters are a measure of bead fragmentation
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 2, 1981 393
Table 11. Resin Strength Test a-Repeatability poor gel good gel S
PB
WPB
S
av stddev
75 74 76 78 5.6
Unit No. 1 65 90 49 92 44 87 88 78 94 47 35 91 65 48 90 62 47 91 66 52 91 13.9 14.5 2.1
1 2 3 4 5 6 7 av stddev
71 72 84 69 76 69 68 73 5.6
Unit No. 2 63 45 90 51 37 86 89 75 92 63 44 89 58 44 82 70 48 92 62 42 91 65 48 89 12.1 12.5 3.7
1 2 3 4 5 6 7
83 73 80 88
78 66 55
PB 97 96 95 97 97 96 97 96
Effect d Particle Size on Strength
WPB 87 88
.79
83 91 87 86 88 87 2.4
1
50
Pmr Gal Rosin 70 prig 10 Cycles
40
o Sphericity A Perfect Bead 0
+ I6
96 95 96 93 95 96 93 95 1.4
87 82 89 83 78 88 84 84 3.7
“ Strong base, gel type anion-exchange resin. Test conditions: 65 psig on resin bed ( 3 5 mL wsb); 10 cycles of 20% H,SO,/DI water at room temperature. S = sphericity; P B = perfect beads; WPB = whole perfect beads (expressed as weight percent passing test); WPB = S x PB. and cracking while the third parameter is a measure of the total attrition. Results and Discussion Test Repeatability. As we were initially using the method to distinguish the relative stability of candidate resins and needed to test many samples, we were concerned about the reproducibility of data from such a new test. Two sets of the test apparatus were constructed and run by two different operators on two large samples of gel type resin. These samples had been shown by previous osmotic shock testing to be of “poor” stability and “good” stability. The samples were each run seven times on each of the two test units. The results shown in Table I1 indicate relatively good agreement between the two test units since the averages are within one standard deviation. It is interesting to note that the ion-exchange beads behave very much like natural materials in that if the beads are weak, there is a very high degree of scatter in the data (range for WPB of 35 to 78 on the poor resin). The stronger beads have a much more narrow range (82 to 91). This performance can be contrasted to other individual bead tests such as the crush strength, where the range is always large regardless of the average strength. It is expected that if the sample size in the RST was made larger by using a larger cell, the variability would decrease, but as of this time, no tests have been done on exploring cell size. Our conclusion as a result of this test series was that the RST apparatus and procedures gave sufficiently reproducible results to be used in our parametric testing program. Particle Size Effects. The uranium extraction plant we were supporting was designed to operate at a very high linear flow rate to make uranium recovery economical. The effect of resin particle size on the process therefore became critical from the standpoint of hydraulic pressure drop and kinetics. Obviously, we were concerned with how the strength of the beads varied with size. Golden and Irving (1972, 1976),in their exhaustive work on crush strength, had shown conclusively that the crush
Whole Perlecl Be&
+M
+28
Mesh Cuts
Figure 4. Resin stability as affected by particle size. Table 111. Particle Size Effects” strong gel +30
sphericity (st dev) perfect beads (st dev) whole perfect beads (st dev) number of data points
” Test conditions:
88
(12) 97 (1.7) 86 (13)
4
-30 90 (8) 99 (0.5) 89 (8)
macroporous +28 98 (0.94) 100 (0.13) 98 (1.0)
5
-28 97 (3.6) 100 (0.30) 97 (3.8)
70 psi, 1 0 cycles, 20% H,SO,/DI.
strength of the beads increased linearly with increasing bead size (the square of bead diameter). Resin purchase specifications,however, for recent condensate polishing and military applications were putting increasingly lower limits on the allowable beads over +16 mesh as it was generally felt that the largest beads disappeared the fastest in many applications. The poor gel resin shown in Table I1 was screened into 3 fractions; +16 mesh, -16 +20, and -20 +28 (US. Standard) mesh so that sufficient material was available to each fraction to do a resin strength test. We wished to determine if any portion of the lot was weaker than the others, so the 3 fractions were tested under the same conditions. The results of the tests are shown in Figure 4. While this test series was not as meticulously done as Golden and Irving’s work in that strength vs. individual bead diameter was not determined, it is quite clear that the effect is exactly the opposite. The small beads were considerably stronger than the large beads when it came to resisting breakage under combined mechanical and osmotic stress. The next basic question to be resolved was to determine if the same relationship holds for stronger beads. We tested four different lots of the strong gel resin (the same one as in Table 11) and five lots of the strongest macroporous resin we found (and eventually used in the plant). Both of these resins, however, were prescreened by the manufacturer to a much tighter mesh cut than the first resin. Due to constraints in the test program, only the +30, -30 mesh cuts of the strong gel and the +28, -28 mesh cuts of the macroporous resin were tested. The results of the tests are shown in Table 111. Statistically there is no difference in the larger and smaller cuts of the two resins, but in the case of the strong gel, the larger beads still appear to be weaker. In the case of the macroporous resin, however, the trend is reversed. Our conclusions, based on these limited data, are that the larger beads in the bed will break more readily than the smaller beads if the bed is stressed sufficiently. As the
394
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20,No. 2, 1981 c
o
c I
lT-------r--T r,
150 Cvcle Data
’ ” M
v 0 0
&/I
40
’
50
toi I - Satisfactory Lot 2. Unacceptable lot 3 - Very Bad 1
1
1
’
1
70 80 W 1W Bed Pressure lpsig)
60
’
1
110 120
Figure 6. Effect on increased test severity on macroporous resins. 12
n
-
Table IV. Effect of 50% Mixturesa Poor Cei Resin 10 Cycles m H ~ S O ~I D
68-
5
-
20
I
do MI 80 1W Awlled Pressure i o Resin ipsigl
resin
I
120
Figure 5. Effect of load on resin breakage.
strength of the resin increases, it appears that this effect becomes less pronounced-it is suspected that if the test were to be made more severe for the stronger resins, the trend would be reestablished. Bed Pressure Drop Effects. It is assumed that the higher the pressure drop is across an operating bed, the greater the resin breakage will be. This assumption is rational but does not provide all the information that is desired. For a proper design we need to know how the rate of breakage changes with increases in pressure drop. Increases in pressure drop were simulated for the three resins previously tested by increasing the ram pressure and holding all other parameters constant. It should be noted that the same lots of the various resins were used since we had already shown significant lot-to-lot variability in the strength of the resins from each individual manufacturer (Skriba and Alvino, 1979). The results of the tests (Figure 5 ) show that the relative strength of the three different resins still holds true. The weakest resin deteriorates very rapidly with increasing pressure drop and does so with a high degree of data scatter. The stronger gel resin also deteriorates, but not as rapidly. The curious part of the strong gel curve is that there appears to be a critical pressure at about 70-80 psig where the rate of breakage accelerates markedly. The strong macroporous resin, however, shows only a small decline in strength with pressure drop and for this set of tests, no critical pressure. It should be noted that the breakage rates are linear with pressure drop except at the critical pressure for the strong gel. After the plant had been running for some time, however, we started experiencing a very high rate of resin breakage. At this time, our standard test was 70 psig at 150 cycles. We suspected that we had received some bad lots of resin, but our tests had indicated that they were all good since they had passed our standard test. Further testing by increasing the severity of the test from 70 psi to 120 psi (the apparatus limit), however, showed that even the strong macroporous resin had a critical pressure for some lots and that the initial pressures used in the test cell did not provide enough stress to adequately model the stresses in the plant. Figure 6 illustrates the results of the more severe testing program. Three lots were explored; lot 1had performed satisfactorily in the plant, lot 2 showed an unacceptably high breakage rate, and lot 3 broke very rapidly. All had essentially the same response at the 70 psi standard test, but when the test pressure was increased out to 120 psi there was a change in the resin responses. Lot 1continued its linear decline out to 120 psi, but lot 2 started to show
a
S
PB
WPB
macroporous 99 100 99 strong gel 90 96 86 weak gel 77 60 47 macrolweak gel 95 96 92 (expected av) 88 (80) (73) macro/strong gel 94 98 92 (expected av) (94) (98) (92) stronglweak gels (actual) 86 88 16 (expected av) (84) (78) (66) Test conditions: 70 psi, 10 cycles, 20% H,SO,/DI.
nonlinear decline at that pressure. Lot 3 showed a very rapid decline in the strength after its critical pressure of 70 psi was passed. These data, coupled with the data shown in Figure 5, would strongly indicate that a critical pressure exists not only for each resin type but for each lot of resins within a type. We now had a set of “plant calibrated” resins in that we had a set of lots which broke at given rates in the plant. Changing the test conditions in the RST to 120 psi and 150 cycles allowed us to break the resins in roughly the same relative rates in the lab. The 120 psi, 150 cycle set of conditions therefore became our standard acceptance test. Our conclusion from these data is that increasing system pressure drop should increase resin breakage in a linear fashion with a slope that can be predetermined in the laboratory. The exception to this conclusion is if the resin in the system has a “critical pressure”, increasing the hp above that pseudo-pressure can dramatically increase resin breakage. Mixtures of Resins. The appearance of the critical pressure for each batch of resin led naturally to the question of the effect of adding a bad batch of resin to a column filled with good resin. A common belief expressed in discussions with resin and equipment manufacturers is that resins are like apples, i.e., a bad batch can spoil the lot, so one short test series was conducted using the same three resins as used in the previous parametric tests. The resins were tested individually and then 8s 50% by volume w.s.b. mixtures. The results are shown in Table IV. The first set of numbers are the results of subjecting the individual resins themselves to the test, and as can be seen, there is a wide discrepancy in their relative strengths. In addition, there is a structural difference in the resins since the strongest resin is a macroporous resin and the other two are gels. A first hand, simplified analysis would be that the mixed resins would behave in a proportionate fashion, so the numbers in parentheses are the expected average breakage of the resin blends. In the case of the two stronger resins, that did occur. But when the weak resin is added to the stronger resins, the degradation rate is noticeably less than expected. It would appear that the stronger resin assumes a higher degree of the load and imparts a greater strength to the mixture than its proportional makeup. This indicates that a bad batch of resin
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 2, 1981
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Table V. Effect of Freeze/Thaw Cycles no. of cycles
sphericity
0 1
99’ 99 99
5
Test conditions: macroporous resin (plant archive sample); freeze overnight in freezer of refrigerator; thaw in day at room temperature ; RST: 70 psi, 150 cycles, 20% H,SO,/DI. a
10
20
30
40 x1 b3 Numbor d Cycler
70
80
W
Figure 7. Cell pressure drop as function of percent add back and number of cycles.
mixed into good resin at less than 50% by volume would degrade the performance of the bed as a whole, but would not degrade as rapidly as if it were used by itself. These results from such a short test are only indicators, or course, and for more definitive results should be expanded into different proportions and alternate resins. Of particular interest would be a test sequence which would test a mixture at a pressure higher than the critical pressure of one of the components of the mixture and determination of the response as a function of the volume proportion of the weaker resin. One other point should be noted in the results of the tests. The mixture of macroporous and gel resins seemed to provoke no unusual breakage of the gel resin. There had been concern expressed by some resin vendors in private communications that the macroporous resin would quickly “grind away” the gel resin since the macroporous resin is “harder” and rougher in surface texture. This phenomena did not occur as indicated by the higher than expected whole perfect bead values for the macroporous/gel mixtures. Visual examination of the fines showed no different appearance than the normal breakage encountered in other tests run in the same equipment. Under the conditions of long term operation in operating limits such as a power plant, breakdown by rubbing attrition may take place, but under the conditions of the resin strength tests (combined mechanical/osmotic shock) there was no such apparent effect. Another set of mixtures were tested as a result of a problem which occurred in the plant. For some unknown reason (chemical attack is suspected) the entire bed of resin in the plant deteriorated badly in terms of breakage strength, Values for sphericity after testing dropped from the high 8O%/low 90% range for new resin to the high 50%/low 60% range for plant resin. A decision was needed quickly as to wheter we could add new resin to take advantage of the “strong neighbor” effect shown in Table IV or whether the ion exchangers would have to be shut down completely and refilled with new resin. There was not sufficient time available for a detailed investigation so three tests were conducted in which plant resin was mixed with 25%,50%, and 75% new resin, then tested in the RST. To get more information from the tests, cell pressure drop was monitored until the first backwash. The results of these tests are shown in Figure 7. The sphericity values increased from 74% at zero addition to 75% at 25%, 82% at 50%, and 90% at 75% addition. The increase in sphericity and the decrease in cell pressure drop as a function of cycles led us to believe that the plant should see little to no effect on resin breakage until the new resin addition exceeded 50% of the bed volume. New resin was added to replace the breaking old resin, and, as near as can be determined by plant records, when the new
resin addition was between 50% and 75% of the exchangers bed volume, the breakage rate dropped rapidly. This confirmed that the “strong neighbor” effect does operate in a plant environment. This also gave u9 some confidence that the RST can be used for some predictive situations to answer the “what if” types of questions. Freeze/Thaw Test. Some of our resin had been stored in an area which was relatively unprotected during the winter and it was feared that some of the resin may have frozen. The general concensus based on manufacturers stated precautions was that freezing should hurt gel resins but not the macroporous type we were using. A new archive sample of resin being used in the plant was pulled, put in the freezer overnight, and then thawed by sitting at room temperature. This was then tested in the RST. Another aliquot was repeatedly frozen overnight and thawed through the day for five freeze/thaw cycles. The results shown in Table V indicate no effect on the macroporous resin of freezing/thawing under the relatively mild conditions of testing being used at the time. Even though the test conditions were mild (70 psi vs. 120 psi), it is safe to state that the freeze/thaw cycles did not cause catastrophic damage, so our conclusion was that extraordinary measures were not needed to assure freeze protection for the resin under consideration. Gel resins might not have fared as well.
Conclusions We feel that physical stability criteria are needed as the next logical step in the evolution of particulate ion-exchange materials. It is our conclusion that the test we have developed may form the basis for setting those criteria, especially when combinations of stress are involved in the process. I t remains to be shown whether the test can be extended to other ion-exchange areas such as condensate polishing service, but it is certainly of use as a predictive tool for single-bed systems. Literature Cited Abrams, I. M. Proc. 37th Int. Water Conf. 1078, 185-174. Alvino, W. M. Ind. Eng. Chem. prod. Res. D e V . 1980, IO, 271-276. ASTM Methods: D1782-78, Operating Performance of Particulate Cetlon-Exchange Materlals; D2187-77, Physical and Chemical Ropertles of Partlcw late Ion Exchange Resins; D2687-77, Sampling Particulate Ion Exchange Materials; D3087-78, Operating Performance of Anion-€xchange Meterleis for Strong Acld Remova); 03375-75. Column Capacity of Particular Mlxed Bed Ion Exchange Materials. Ball, M.; Ray, N. J. Effluent Water TreatmentJ. Fib 1078. 73-81. Brooke, J. N. Min. Congr. J . Aug 1977, 38-41. Davles, V. R. “Condensate Demineralizatlon Using Deep Mixed Beds Of Ion Exchange Resins”, Rohm and Haas Publlcatlon IE 249178, May 1979. Golden, L. S.; Irving, J. Chem. Jnd. Nov 4, 1972, 837-844. Golden, L. S.; Irving, J. “Resin Struture-Mechanical Strength” In “The Theory and Practice of Ion Exchange”. Society of Chemical Industry Conference, July 1976. CambrMge, England, pp 6.1-6.7. Kunin, R. “Ion-Exchange Resins”, R. E. Krieger Publishing Co.: Huntingdon, NY, 1972. Skriba, M. C.; Ahrlno, W. M. hoc. 40th Int. Water Conf. 1979, 273-275.
Received for review November 21,1980 Accepted February 17,1981
