Edited by Jeanette G. Grasselli
D. F. Pensenstadler M. A. Fulmer Westinghouse Research and Development Center Pinsburgh, Pa. 15235
Pure Steam Ahead! The purity of steam and water is very vital to the power production industry because the chemical environments within modern steam generators and turbines often cause turbine failures. Even trace (pph) concentrations of corrosive impurities can cause excessive deposit formations, as shown in Figure 1,leading to corrosive attack or distress. Chemical analysis of these deposits has identified such inorganic species as chlorides, sulfates, and caustic. In order to maintain product integrity and improve reliability for this industry, Westinghouse initiated a program in 1977 to monitor the chemical environment in operating systems and to determine the operating conditions in which corrosive ionic impurities enter the steamlwater cycle of fossil fuel power plants. (See FOCUS,ANALYTICAL CHEMISTRY, 1980,52, 1409-10 A). T h e analytical approach taken involved the use of ion chromatography, a widely accepted method for cation and anion analyses. T o make ioa chromatography a useful quantitative analytical tool for the power production industry, it was necessary to adapt the technique for analysis a t a low ppb level by preconcentrating samples. This adaptation will be described later. The Westinghouse steam purity analysis program involves sampling various locations throughout the steamlwater cycle of a power plant and using on-site ion chromatographic instrumentation for analysis of these samples. In typical field tests in operating power plants, a temporary ion chromatographic laboratory is set up for on-site sample analysis. Whereas condensate samples can he collected directly into sample containers, steam samples require heat exchangers to 0003-2700181IO35 1-859ASO 1.0010 @ 1981 American Chemical Society
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I Figure 1. Example Of deposits on steam turbine blades
condense the steam prior to sample collection. Since ion chromatography normally has a detection limit of approximately 100 pph for common ions (with the standard 100-pL injection loop), it was necessary to improve the detection limit by a factor of 100 to get to the 1-10 ppb range required for steam purity measurements. The detection limit was improved to about 20 ppb simply by increasing the capacity of the 100-pL injection loop to 500 pL (0.5 mL). However, further improvement of the detection limit necessitated replacement of the injection loop by a concentrator column, which is now available commerciallv. A 10-mL sample of steam condensate is passed through the concentrator col-
umn, which contains an ion exchange resin. All of the cations or anions (depending on concentrator resin type) are retained on the concentrator column, hut the volume of water retained, 0.5 mL, is 5% of the input volume. T h e concentrator column contents are thus concentrated by a factor of 20, decreasing the detection limit to 1 ppb. For those unfamiliar with the ion chromatographic process, a schematic is shown in Figure 2. A sample is injected into an eluant stream, pumped through a separating column where the sample species are resolved by interaction with an ion exchange resin, and then DumDed throueh a sumressor column where the ions of the.eluant are suppressed. T h e species of in-
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7. JUNE 1981
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ses At Once .always been the Key SO,'- is now' receivtention..,3Nolein the .below that Dionex both CI- and SO,*-minute run. for monitoring surface eralized feed water, boiler iished condensate in both and nuclear plants.. ader si
concentrat,ons
terest are then detected by a conductivity cell. Either cation or anion analyses can be performed by utilizing separator and suppressor columns suitable for one or the other. In an anion exchange process, the eluant is often a solution of NaHC03. The anions in the injected sample interact with the resin in the separator Na+X- * column: Resin-N+HCO; Resin-N+X- Na+HCOT, where Xis an analyte anion. As the analyte anions flow through the separator column in a background of HCO;, Xand HCO; compete for the resin-N+ exchange sites. Separation of ions depends on their relative affinity for the exchange sites: The greater the affinity for the resin, the longer an anion is retained. After the separation process takes place, the analyte anions and the eluant enter the suppressor column, where the following reactions take place: Resin-SOiH+ Na+HCO;, * Resin-SO;Na+ + H2C03; and Na+XResin-SO;H+ Resin-SO;Na+ H+X-. The analyte ions, which could, for instance, be sulfate and nitrate, thus enter the conductivity detector as strong acids, sulfuric and nitric. The eluant is converted to a solution of carbonic acid, a weak electrolyte. Thus, the analyte anions can be easily detected against the carbonic acid background. Without the suppressor column, the analyte signal would be swamped by the strong conductivity signal from the concentrated bicarbonate eluant. For anion analyses, the analytical or
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separator column thus contains a strong anion exchange resin, while the suppressor column contains a strong cation exchange resin in the hydrogen form. For cation analyses, a similar principle operates, except that the eluant is now a solution of a strong acid such as HCI, the analytical column contains a strong cation exchange resin, and the suppressor column contains a strong anion exchange resin in the hydroxide form. With the concentrator column playing such an important role in sample analysis, it was desirable to qualify it for field use as completely as possible. Following any analysis, a concentrator column is normally still filled with the eluant solution. T o test the effect of this prolonged storage, a concentrator column used for anion analysis was stored in eluant (.003M NaHCO3/.0024 M Na2C03) for two weeks and then rerun. The chromatogram showed surprisingly high chloride and sulfate peaks, 25 ppb and 10 ppb, respectively. It appears that prolonged exposure to eluant leaches ions out of the glass or frees them from deep resin sites. Similar results were also found for cation concentrator columns, where the eluant used was ,0075 N HCI. I t would therefore seem prudent to rinse all concentrator columns with fresh eluant prior to use if they have been stored in eluant for any appreciable length of time. Caution is also advised when using a concentrator column for a low-level sample after the column has previously contained a
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Figure 3. Concentrations of sodium (A),chloride (O),and sulfate (m), and pH in 3oiler main steam high concentration species (ppm level), e.g., a concentrated boiler blowdown sample. Unless it is thoroughly rinsed with fresh eluant, some contamination could result from ions of the previous sample that are being removed from deep resin sites. The effect of prolonged storage of water samples on concentrator columns was also studied. The aqueous samples do not show the same leaching effect as eluant storage. The recovery of all species after prolonged storage was found to be statistically similar to normal reproducibility levels. The efficiency of the concentrator column in removing and retaining all sample ions during loading was investigated. This test was conducted by pumping a 45-mL sample solution containing 15 ppb chloride and 180 pph sulfate through two concentrator columns connected in series. The chromatogram of the second concentrator column showed no peaks, indicating total removal and retention of all ions on the first column. The linearity of dilution from the ppm to ppb range of sodium, chloride, or sulfate was checked with progressively diluted standards. And to make certain that no unusual effects were being contributed by the concentrator columns, the linearity of peak height vs. concentration for chloride calibration in the 0-50 ppb range was determined. The accuracy of sample analyses is based, in part, on the accuracy of standard solution preparation. Water used
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in the preparation of standards must contain less than 1 ppb of ionic species, Normally, distilled and deionized water contains ions at too high a level to be useful for standards, and it was necessary to fabricate a system that would produce ultrahigh-purity water. The system involved recirculating prefiltered water through an organic removal cartridge, two mixed bed deionizers, a UV sterilizer and a 0.2.~filter. The system is capable of delivering 1-3 Llmin of Type 1reagent-grade water ( normal operation over a 4-h period. he first three points at each load vel represent analysis of water phase imples and show relatively high inial impurity levels. The remaining oink indicate analysis of steam samles and show the effectiveness of the rum in maintaining steam purity ,vels. All of these analyses have shown iat ion chromatography is a very viale analytical approach for the deteriination and quantification of ionic npurities in the voluminous steam ow typical of most power plants (2 x O6 to 1X lo7 Ib/h). Ion chromatogra-
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phy is a valuable on-site analytical tool, offering plant personnel direct feedback for detecting problems or improper operating conditions in the power production industry.
Acknowledgment The authors wish to acknowledge the assistance of Steven H. Peterson and James C. Bellows in the steam purity program; the efforts of S. L. Auderson and W. E. Snider in collecting and analyzing samples; and the many helpful discussions with Gerald L. Carlson. Also, the cooperation of Decker Power Plant (Austin, Tex.) personnel is greatly appreciated.
