Application of Mixed Bed Ion Exchange to Water Purification in Nuclear Reactors MICHAEL STVETZ AND C. H . SCHEIBELHUT Argonne National Laboratory, Lemont, I l l .
I
N A recycling nuclear reactor water system heat is withdrawn
from fuel elements and steam may be used subsequently for doing work such as driving a turbine. Water erosion and corrosion always present a problem. Therefore it is important to use very pure water as a coolant and moderator in a nuclear reactor system. There are numerous references (7, 8) in the literature t o nuclear reactors cooled and moderated with light and heavy water, and several references (3, 10) t o water purification in specific reactors. In a recycling water system, the purity of water can be held under much better control than in a single-pass water system, and the activity induced in the water and its dissolved constituents ( 1 1 ) is confined to the recirculatory water loop. Water of high purity minimizes induced specific water activity and thus reduces the amount of shielding required on the water piping system external to the reactor core during reactor operation, and also reduces the deposition of irradiated substances from the water to piping walls. This allows greater accessibility to piping, pumps, etc., when the nuclear reactor is not operating. Maintaining low total dissolved solids ( T D S ) in the water eliminates catalytic corrosiveness-e.g., halide and copper ions on structural aluminum-and reduces water decomposition in the reactor core. Water with low total dissolved solids has few ions t o capture neutrons, hence offers increased pile reactivity and neutron economy. Optimum heat transfer needs to be maintained by preventing deposit of scale on heat generating fuel surfaces; hence low total dissolved solids must be maintained in the water. Finally, water of high purity allows control of p H t o near neutral, thus minimizing corrosion of specific metal surfaces. Any nitrogen from the air dissolved in reactor cooling waters undergoes oxidation to nitric acid in the intense pile radiations and water tends t o become acid (IO). It takes only a small amount of acid to alter the p H of water appreciably, as is shown in Table I. Thus t o obtain a p H of 6, the specific resistance of the bulk reactor water must be 2.4 megohm-cm. The bypass purification flow rate must be sufficient and of high enough quality to offset the nitric acid formation and introduction of corrosion products into the water system. The experimentally verified cleanup formula for a closed-loop water system can be derived to show that: &f M,e-.ft'" '
where
M, M f
5
. v
= = = = =
mass of impurities in bulk system a t start mass of impurities in bulk system a t any time flow through bypass cleanup unit time of cleanupvolume of bulk system
industrial rate for deionization of water in separate cation and anion resin beds is about 5 gallons per minute per square foot; higher specific flow rates result in ion leakage. These kinds of results have been obtained with strong acid and strong base polystyrene resins such as IR-120 and H C R and IRA-410 and SAR (Dowex l),respectively. The more thermally stable resins Dowex 2 and IRA-400 are more desirable for use in water a t higher temperatures. Volume ratios of cation t o anion resin are not critical factors in determining effluent quality except after breakthrough. Volume resin ratios of cation-anion used are about 1 to 2 or equal ion exchange capacities for cation and anion resin. The resins for this work were purchased generated and were specified as 6 meq. per dry gram for the cation resin (5 meq. per dry gram is more typical commercially), 3 meq. per dry gram for the anion resin, or not less than 40,000 grain-equivalents as calcium carbonate ( 2 gram-equivalents per liter) per cubic foot of cation resin and not less than 20,000 grain-equivalents as calcium carbonate (1 gram-equivalent per liter) per cubic foot of anion resin. Moisture contents of resins were determined within an hour by toluene distillation (6). Resin capacities were titrated as indicated by Kunin ( 9 ) . Since cation resins are manufactured in the acid (hydrogen) form, their exchange capacity is complete; however, cation resins are usually sold in the sodium form. Anion resins in the chloride form need about 20 pounds of sodium hydroxide per cubic foot of resin and good rinsing with high purity water for proper generation to the (hydroxyl) free base form. The generated cation and anion resins were shipped from the manufacturer mixed and were kept moist in sealed plastic bags until used. Mixed resin beds must be progressively filled, allowing 1 or 2 inches of free water during loading. This eliminates gas inclusions which cause water flow to channel and reduce availability of full resin bed capacity. Unless each resin is generated to its highest ion exchange capacity, the resulting mixed bed effluent will not exceed a few megohm-centimeters. I n use, cation resins will pick up 1 gram-equivalent per liter and anion resins will pick up 0.5 gram-equivalent per liter before mixed resin bed effluentquality falls below the magnitude of megohm-cm. (specific resistance) quality. If the ion exchange capacity of a given volume mixed resin bed is known in gram-equivalents and the quality of a given volume of water t o be purified in specific conductance, the resin bed ion load can be calculated. For example, the following "rule of thumb" can be used for practical purposes, since reactor waters are very dilute solutions of usually known ions and the waters are nearly neutral.
The exponent f t j u should be dimensionless. Although water may be purified by distillation, mixed resin beds offer high flow rates and high effluent quality most economically. As is known commercially, mixed ion exchange resin columns can yield water quality of 15 megohm-cm. or better and a near neutral pH. Experiments (1) indicate use of mixed resin beds a t unit flow rates up t o 100 gallons per minute per square foot (beds 3 feet deep) a t 3 t o 192 p.p.m. influent concentration without affecting resin capacity for producing pure water with a specific resistance of 1 0 6 ohm-cm. or better. T h e normal
Table I.
Water pH and Resistivity us. Nitric Acid Concentration Hydrogen Ion Nitrate Ion Concentration, Concentration, P.P.M. P.P.M. 10 -7 0
10-6
10 -6 10-4 10-
Pure water.
1020
0.06 0.62
6.2 02
Calcd. Specifio Resistance at
25' C., Ohm-Cm.
:240 1000
18 100 OOOa 2 400 '000 24,000
2,400
May 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
I
I
1
I
I
I
carbon dioxide solution in the water from the atmosphere. Megohm-cm. water is unbuffered and can giveerratic p H readings. Oxidation of organic matter in a reactor usually results in carbon dioxide. The p H and carbon dioxide solubility relations in water are shown in Figure 1.
I
I
1021
7
RESISTIVITY
24
3-
I
I
HzCOs, C
I
I
Con, P.P.M.
I
*o.
t
I
-
or 1s -4
I
I
sistance,
H, UC
a
pH
OhmCm.
0 . 5 X 10-1
2220
0.0031
1 . 6 6 X 10-4
3.81
10-2 0 . 5 X 10-2 10-3 0.5 x 10-3 10 - 4 0 . 5 x 10-4 Equilibrium with 0 . 0 3 7 0 10-6 Con in air Pure water 0
440
2.2
0.0067 0.0091 0.021 0.031 0,066 0,091
6.7 4.5 2.1 1.56 6.6 4.5
X 10-6 x 10-5 X 10-6 X 10-6 X 10-8 x 10-6
4.17 4.35 4.68 4.81 5.18 5.35
0.44
0.21
2 . 1 X 10-8
6.68
1.2
x
7.00
18.0
220 44 22
4.4
0
...
1.0
10-7
0.02 X 1 n6 0.04 0.06 0.12 0.16 0.38 0.56
The resistivity of the cycling reactor water should be recorded before and after it passes the bypass resin column (see Figure 2). Commercial industrial conductivity recorders (such as those of Industrial Instruments, 17 Pollack Ave., Jersey City 5, N. J.), which use nickel cells with constants of 0.1, have been found reliable t o 1 megohm-cm.; however, above several megohmcentimeters with 0.01 cell constants, somewhat varying resistivities were obtained, depending on the rate of water flow past the electrodes. This was believed to be due to an indeterminate leaching phenomenon. Platinum metal electrodes are recommended for exact work, where such deviations are t o be eliminated. The resistivity of the inlet water to the resin bed indicates the quality of dissolved ionized solids in the bulk reactor water, while the resistivity of the resin bed effluent indicates the absorptive ability of the resin bed. Figure 3 shows the pure water (H,O) resistivity and p H variance with temperature; the resistivity varies appreciably. Heavy water (DtO) has a specific resistivity about three times greater than light water of the same purity ( 2 ) and its dissociation constant ( l a ) a t 26" C. is 1.95 X lo+" The p D of pure deuterium oxide is therefore about 7.4 vs. a p H of 7.0 for pure water.
Normality = 10 X specific conductance (micromhos cm.-l) based on average salt equivalent conductances ( 4 ) . In the case of deionization of 30,000 gallons (125 X 106 grams) PH of water of 330,000 ohm-cm. specific resistivity, 3.75 gram-equivaAs noted in Table I, if water resistivity is above 1 megohmlents of mixed ion exchange resin capacity will be used; where N = em., the pH, being related t o water resistivity, is so near neutral 10 X 3 x 10-6 gram-equivalent per liter and there are 125,000 that its corrosive aggressiveness is small. When bulk water liters. resistivity falls below 1 megohm-em., the p H record of resin Mixed resins have a fair capacity for picking up nondissolved column influent is a better index of when to replace a fresh resin (nonionized) matter from water and their ability to do so is bed. valuable. However, mixed resin beds should be flanked by 100mesh screens and be protected by a prefilter, so as not to plug ANALYTICAL CONTROL the beds with particulate matter. A postfilter is desirable t o prevent fine resin fragments from entering the main water reIf reactor coolant water is of megohm-em. quality, other probcirculatory system. A characteristic of resin beds used in conlems usually do not occur. However, it may be advisable to have junction with demineralization of reactor cooling waters is the monthly spectrochemical determinations of all metal elements in concentration of radioactive ions a t the top of the bed. When these "hot" resin beds no longer pass 10000-=8 m e g o h m - c m . ( s p e c i f i c resistance) quality water, then the resin may be ashed and the ash may be stored for decay of radioactive elements. New resin may then be replaced in the bypass columns. Present developments indicate that the use of mixed resins in electroregenerative cells will replace the mixed resin bed. Instruments and associated analytical procedures required for proper control of reactor water quality deserve special attention. The most important factor in measuring water pH and resistivity is that these properties must be measured in the flowing stream. Recorders for p H and resistivity are preferred. Samples removed from the flow system for measI I l l 0 I I I l l I 1 I I 25 I 5 O I 15 20 25 30 I 5 ID 15 20 25 30 I 5 10 15 20 25 30 I 5 O I 15 urement in the laboratory do not give APRIL YAY JUNE JULY ANUS7 true readings, not only because of conFigure 2. pH and conductivity of deuterium oxide during loop test, 1951 trtmination of water by handling and Spot readings a t 2 P . h i . daily glassware but also because of rapid
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1022 I
I
I
I
I
I
I
I
Vol. 4’1, No. 5
I
80 - 7
’
-4
-e
I
I IEMPERAIVRE.
Figure 3. Temp.,
c. 0
10 15
18
25 35 40 50 100
I
I
0
.C.
Variation of pH and specific resistivity of water with temperature A 0 Dorsey ( 6 ) Kohlrausoh, Calod. Exptl. Megohm-Cm. 84.0 71.4 43.2 ... 31.8 26.7 25.0 18 1 18.2 11.0 11.9 ... 8.7 5.4 5.9
..,
...
-___I_
*I
PH 7.47
...
-A’--
- ----
I
II COYDUCflVlrv
Figare 1,. Typical bypass installation for purification
... ...
7.00
LITERATURE CITED
...
(1) Caddell, J. R., and Moison, R. L., Technical Information
...
...
6 10
the water and chemical determination of particular anions. In addition, the curve of specific radioactivity and decay for the reactor water may be useful. A typical bypass insttallation for mixed resin purification as shown in Figure 4 may be used. ACKNOWLEDGMENT
The brief results and experiences reported here mere the cooperative effort of many individuals at the hrgonne Xational Laboratory and elsewhere. It is a natural desire of those interested in this subject to ask for more specific and extensive information. The authors are gratified to have these data released, and promise that additional information will be made available as fioonas security regulations permit.
Service, U. S. Atomic Energy Commission, Oak Ridge, Tenn., “Performance of Ion Exchange Resins at High Flow Rates,” DP4 (March 1952). (2) Chittum, T. P., and LaiVler, 1‘.K., J . Ani. Chem. Soc., 59, 2245 (1937). (3) Dahl, O., and Randers, G., Nz~cleonics,9, KO.5 , 5-17 (1951). (4) Daniels, F. H., ”Outlines of Theoretical Chemistry,” Wiley, New York, 1940. (5) Doreey, N. E., “Properties of Ordinary Water Substances in All Its Phases,” Reinhold, New York, 1940. (6) Furman, N. H., ed., “Scott’s Standard Methods of Chemical Analysis.” 5th ed., Vol. 2, p. 1342, Van Nostrand, New York, 1939 (7) Isbin, H. S., Nucleonics, 10,No. 3, 10-16 (1952). (8) Ibid., 11, No. 6 (1953). (9) Kunin, R., ”Ion Exchange Resins,” Wiley, New York, 1950. (10) McCorkle, W. H., Nucleonics, 11, No. 5, 21-6 (1953). (11) Sivetz, M., Atomics, 6, No. 5 (1950). (12) Wynn-Jones, W. S. K., Trans. Faraday Soc., 32, 1397 (1936). RECEIVED for review August 13. 1964.
ACCEPTEDDecember 23, 1954.
Antioxidant Sweetening of Gasolines C . M. BARRINGER Petroleum Laboratory, E. I. du Pont de Nemours & Co., Inc., P.O. Box 1671, Wilmington 99, Del.
T
HE use of p-phenylenediamine gasoline antioxidants as
catalysts for converting the mercaptans in gasoline to less reactive and less disagreeable compounds is termed “antioxidant” or “inhibitor” sweetening. Because of its simplicity, low cost, and other advantages, the use of this process is becoming more widespread in the refining industry, and today an appreciable percentage of gasoline production is sweetened in this way, generally with good results. The bulk of the information on antioxidant sweetening available at the present time is empirical. Although the results of a t least one investigation of the chemistry of the process have been published (6),users still rely principally on data obtained in plant experience, which is necessarily somewhat fragmentary and difficult to interpret. As the number of applications has increased, it has become evident that there are some cases where this type of sweetening is not too successful. A few instances have been encountered in which a successfully sweetened gasoline component has proved to be either unstable or “dirty” in automotive use. To increase the understanding of this process, a study has been undertaken of the chemistry of antioxidant sweetening, in which
an effort was made to separate and identify the individual reactants and the reaction variables which influence the course of sweetening. By conducting experiments under controlled conditions in simplified hydrocarbon-mercaptan systems, a considerable amount of information applicable to the more complex mixtures encountered in gasolines has been obtained. Because of the procedure followed as well as the simple nature of the mixtures sweetened, one should not attempt to correlate directly the laboratory sweetening times and effects reported here and those expected in full scale plant operation. The general conclusions that can be drawn concerning the mechanisms of the sweetening reactions and the use of these conclusions to answer questions arising in practice are considered to be more valuable. EXPERIMENTAL
Materials. Reagent grade hydrocarbons were used in the sweetening experiment as follows: IMethyl cyclohexane, Phillips Petroleum Go.
