PAUL COHEN Westinghouse Bettis Plant, Pittsburgh 30, Pa.
Membrane Electrodialysis of Simulated Pressurized Water Reactor Coolant Membrane electrodialyzers operate smoothly at high pressure conditions required for closed-cycle purification of pressurized water reactor coolant
WHEN this work was done in 1952, experience with membrane electrodialyzers was limited to simple synthetic solutions a t atmospheric pressure. For purification of primary coolant in pressurized water reactors, experience had to be obtained a t the low-concentrationhigh-pressure conditions of such plants. T o this end tests were performed with an Ionics, Inc., electrodialyzer in a high pressure dynamic test loop, simulating a reactor system. Test Equipment
T h e loop, constructed of 1-inch Schedule 80 stainless steel pipe with a Westinghouse 30-A canned motor pump, was pressurized by an electrically heated boiler (Figure 1). The normal purification system for the loop consisted of a column, 1 inch in inside diameter and 36 inches long, containing 300 cc. of MB-1 resin. For separate removal of solids the ion exchanger was preceded by a high pressure filter containing two disks of 15-micron Micro-Metallic filter elements. Provision was made for sampling the water, adding oxygen to control the corrosion rate, and measuring conductivity of the loop water and purification system effluent by high pressure conductivity cells. The purification stream was cooled to less than 120' F. Makeu p water was degassed, cooled, demineralized, and pumped into the system by a high pressure chemical feed pump. A membrane electrodialyzer cell consists of an alternating series of anion and cation selective membranes separated by solution-filled spacers with electrodes at each end of the alternating series. When an electric current is transferred through such a unit, ionized solids are transferred from alternate compartments; the solution in one set of compartments becomes more dilute and in the other set, more concentrated. Flow is through manifolds connecting alternate spacer paths. I n the cell used a series of 15 cell pairs was utilized; each contained a CR-61 cation-selective membrane, a diluting compartment, an AR110 anion-selective membrane, and a concentrating compartment. The spacers and membranes were 9 inches
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square. T h e corners were trimmed to permit insertion into the autoclave, 12 inches in inside diameter. The membranes were 0.03 inch thick; the spacers, 0.04 inch thick. The spacers were cut to define a path having a projected area of 0.12 square foot. The total thickness of a cell pair was 0.14 inch. The 15 cell pairs occupied a space approximately 9 inches square by 2.1 inches high for a transfer area of 1.8 square feet, equivalent to 18 square feet of effective area per cubic foot (later and larger units increase this space utilization to 50 sq. feet per cu. foot). Platinum foil electrodes were supported by plastic end plates that compressed the entire assembly through staybolts. I t was necessary to coat the entire unit with beeswax to prevent stray currents. To reduce parasitic currents further, the cell was operated with the low conductivity effluent stream surrounding the unit. The assembly was bolted to the head of an autoclave 12 inches in inside diameter, through which were brought the three streams (feed, product, and waste) and the two electrical leads. The hydraulic and electrical connections for the membrane unit are shown in Figure 1. Method of Operation Loop Operating Conditions 500 1500 5 to 7 6.0 to 7.0
Temp., F. Pressure, p.s.i. O1 concn., cc./liter PH Purification, hr. Ion exchanger (1st period) Electrodialyzer (2nd period) Purification flow rates, gal./ min. Ion exchange (1st period) Electrodialyzer (2nd period) Resistivity of loop water, megohm-cm. 1st period (ion exchange) 2nd period (electrodialyzer) Feed water Loop flow rate, gal./ min.
INDUSTRIAL AND ENGINEERING CHEMISTRY
522 304
0.045 0.0434
0 . 7 to 1 . 3
0 . 5 to 3 . 0 Demineralized and degassed 15.5
Make-up rate, electrodialyzer operation (waste stream), gal. ihr.
0.11
The loop and electrodialyzer were filled and vented. Circuiation was started a t low pressure, and all circuits were purged. The initial gas concentration of the loop was gradually reduced by degasification of the coolant in the pressurizer, M-hich was periodically vented, to eliminate inert gases which interfere with oxygen analyses. The loop temperature was increased 50" F. per hour, with oxygen additions, until operating conditions were achieved. .4t start-up both the ion exchanger and filter were placed on stream; the filter was kept on stream throughout the test. At 552 hours the ion exchanger was cut off, and the electrodialyzer placed on stream. The voltage was adjusted to 250 volts and the waste stream flow was adjusted to achieve a level of purification corresponding to that obtained in the ion exchanger portion of the run. Degassed, and demineralized water was added to make up the loss in the electrodialyzer waste stream. At the applied voltage (250 volts) the cell current remained essentially constant throughout the test a t 20 ma. No mechanical difficulties were experienced with the electrodialyzer. -4t the end of the test, samples of water? resin eluate, and filtered solids were submitted for analysis. Test Results
The salient data of the electrodialyzer operations are presented in Figure 2. Purity of loop water increased continuously throughout the run, except for a short period (June 7 and 8), when it first increased and then decreased. No explanation can be offered, except that because of difficulty with the loop heaters the loop temperature decreased to 400' F. for a few hours. During ion exchange, loop water resistivity ranged betlieen 0.7 and 1.3 megohm-cm. Because the ion exchanger purification rate was the same as the electrodialyzer purification rate at 100% efficiency, it may he asked why loop water purity increased during electrodialyzer operation.
CONDUCTlVllY CELL
Figure 1. The test loop was pressurized by an electrically heated boiler
Figure 2. Loop water purity increased almost continuously during test run
This is believed to be due to aging of the loop and decreasing corrosion fro& the oxygen added to the loop, a phenomenon observed in other tests. The loop water p H was constant from 6.3 to 7.1, with most readings between 6.5 and 6.8. Although no direct determinations were made on the p H of the electrodialyzer effluent, it is inferred from the essentially neutral p H of the loop water and experience with acid effluent from ion exchanger purification that it was close to neutral. Three measures of the performance of the unit are of interest: demineralization efficiency, current efficiency, and power efficiency. The demineralization efficiency, F, may be computed from flow rates and observed conductivities. The fraction of salt removed from the process stream, by assuming salt concentration proportional to conductance, is given by: F = 100 (1
- 0.96) x (specific resistance of influent) (specific resistance of product)
where 0.96 is the ratio of product to influent flow rates. The demineralization efficiency at 20 ma. is reported here as a function of influent salt concentration, expressed as specific conductance. Within the precision of the data influent salt concentration has no apparent effect on demineralization efficiency in the range 0.57 to 1.03 micromhos per cm.
'
Influent Concn., Micromhos/Cm.
Demineralization Efficiency, %
1.03 0.90 0.76 0.64 0.57
72 68 68
.
74 66
The current efficiency, p, ' is defined as the equivalent of salt transferred per cell (anions cations) per faraday of electricity consumed. An approximate value can be obtained from the current
+
Tim, days
and from the conductivity and flow data. Influent Concn., Micromhos/Cm.
Current Efficiency, %
1.03 0.90 0.76 0.64 0.57
0.68 0.54 0.45 0.35
0.27
As would be expected because the current was constant, current efficiency decreased with decreasing concentration of salts in the influent water. The values presented would appear to be excessively low. However, Winger and others (3) applying similar equipment to influent solutions of 1570 p.p.m. of sodium chloride, observed a decrease in current efficiency from 90% at 400 p.p.m. product concentration to 7.5% at 25 p.p.m. Waiters, Weiser, and Mofek ( 2 ) observed current efficiencies as low as 24% in the demineralization of 100 p.p.m. tap water and sodium sulfate influent solutions. Rosenberg and Tirrell (7) have shown a t higher concentrations that current above a polarization limit of 1000 to 2000 (ma./ sq. cm.)/(meq./cc.) is ineffective in demineralization, as it is carried by hydrogen and hydroxyl ions rather than mineral content. The corresponding density for the subject tests is approximately 20,000 (ma./sq. cm.)/(meq./cc.). The unit at 0.042 gallon'per minute, 250 volts, and 20 ma. operated a t a constant input very close to 2 kw.-hr. per 1000 gallons of influent, varying little with influent concentration. For a given unit, the power requirement per unit volume of water treated, P, is given by an equation of the form
p = : K- E A C P
where K is a constant for the apparatus, E is the cell voltage, and AC is-ihe concentration change. In view of the relationships found between influent concentration and demineralization and
current efficiencies, the uniformity of the current observed and corresponding uniform value of P is to be expected. I n the system used there was no independent control of the quantity of salt throughput, so that flow and concentration could be varied independently only by running the ion exchanger and the membrane electrodialyzer simultaneously. This was not done, because there was no interest in the performance of the membrane electrodialyzer at lower duties. Summary and Conclusions
A membrane electrodialyzer can purify the coolant of a pressurized water reactor a t influent concentrations as low as 0.5 to 3.0 megohm-cm. specific resistance. Such a unit can be operated without difficulty at 1500 p.s.i. Under conditions of constant flow, purification efficiency is independent of influent concentration; current efficiency decreases with decrease of influent,concentration. Acknowledgment
The author acknowledges the assistance of Richard Esper, Vennard Thompson, and Wilbur Singley, Jr., Bettis Plant, Westinghouse Electric Corp., in the experimental part of the program. Literature Cited (1) Rosenberg, N. W., Tirrell, C. E., IND. ENG.CHEM.49. 780 (1957). (2) Walters, W. R., Weiser, D. W., Morek, L. J., Zbid., 47, 61 (1955). (3) Winger, A. G., Bodamer, G. W., Kunin, R., Prizer, C. J., Harmon, G. W., Ibki., 47, 50 (1955).
RECEIVED for review April 7, 1958 ACCEPTEDOctober 28, 1958 Division of Industrial and Engineering Chemistry, Nuclear Technology Sub: division, 133rd Meeting, ACS, Sari Francisco, Calif., April 1958. Work carried out unaer prime contract to the u. S , Atomic Energy Commission, VOL. 51, NO. 1
JANUARY 1959
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