Evaluation of Degassed After-Cation-Exchange Conductivity ...

Evaluation of Degassed After-Cation-Exchange Conductivity Techniques

Evaluation of Degassed After-Cation-Exchange Conductivity Techniques Nigel J. Drew

ABSTRACT After-cation-exchange conductivity (ACC) systems are the universally used method for rapid detection of anionic impurity ingress into the steam/water circuit of power plant. In some utilities, ACC is measured after degassing the sample. It is generally considered that carbon dioxide is the least aggressive contaminant of feedwater that is normally present and causes elevation of the measured ACC. The elevation of ACC by carbon dioxide can be particularly noticeable when the oxygen scavenger is carbohydrazide, or the water contains dosed amines or neutral organic compounds in the make-up water. Ingress of carbon dioxide could be considerable in the case of tube leakage in some parts of the boilers of gas-cooled nuclear stations. The elevated ACC then delays unit start-up. There are two widely used techniques for degassed ACC systems: gas stripping with nitrogen and heating to near boiling. Membrane gas-exchange systems are now also beginning to appear on the market. British Nuclear Fuels Plc. and British Energy arranged for an evaluation of these three types of system to determine if they would be sufficiently effective for use in their power stations. The results of the evaluation are summarised here and measured values are compared with theoretical calculations for ACC and degassed ACC.

In feed and boiler waters, the effect of the anions on conductivity is masked by the presence of the alkali. For many decades, the use of after-cation-exchange conductivity (ACC) has been the solution to this problem. A typical system is shown in Figure 1 and a brief example of the improvement in sensitivity available by replacement of sodium as a counter ion with hydrogen ion is shown below. Measurements are corrected to a sample temperature of 25 °C. Pure water has a conductivity of 0.055 µS · cm–1. 10 mg · kg–1 of sodium chloride in pure water raises –1 conductivity to 0.077 µS · cm .

1 mg · kg–1 of ammonia in a sample would have a con–1 ductivity of 6.721 µS · cm but with the addition of –1 of sodium chloride this rises to 10 µg · kg –1 6.743 µS · cm .

The cation exchange column (with R = resin + sulpho-

nate group) replaces cations, thus: + + Na + R-H Na-R + H

(1)

and NH4+ + R-H NH4-R + H+

(2)

The resulting solution containing HCl from 10 µg · kg–1

of sodium chloride and 1 mg · kg–1 of ammonia has an –1 ACC of 0.098 µS · cm .

INTRODUCTION Feedwater, boiler water and steam in generating plant should contain as little as possible of any ionic species known to initiate or enhance corrosion, such as chloride, sulphate, acetate and formate. To ameliorate the effects of any traces of these ions alkalising agents are added to ensure that the resulting water (or salts deposited) remains alkaline, typically in the range of pH 9 to pH 9.8 at 25 °C in the feedwater. The alkali added may be ammonia, amine (such as morpholine) or, in drum boilers, sodium phosphate or sodium hydroxide. The presence of ionic impurities in water is readily detectable by measuring the electrolytic conductivity of the water, a technique that is robust and responds rapidly. Identification of the individual ions requires more complex techniques, such as ion chromatography, with each analysis taking some time to be completed.

© 2004 by PowerPlantChemistry GmbH. All rights reserved.

PowerPlant Chemistry 2004, 6(6)

Figure 1: A typical after-cation-exchange conductivity system.

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Using the cation exchange column has nearly doubled the conductivity from the presence of sodium chloride and eliminated the background contribution from ammonia addition. When measuring direct conductivity, variations in large background conductivity will make the small contribution from sodium chloride undetectable. The after-column technique is particularly sensitive in the detection of condenser tube leaks when measuring condensate. Another contaminant of boiler water and condensate is carbon dioxide. There are several sources of this, such as absorption from air in the condenser at start-up, decomposition of any organic compounds in the feed such as those deliberately introduced, e.g., amines, carbohydrazide (for oxygen removal), and from polysaccharides and humic acids in make-up. Dissolved carbon dioxide alone will promote corrosion of mild steel. However, when carbon dioxide and excess alkali (e.g., ammonia) are both present, the high temperature pH remains protectively alkaline [1]. [2] calculates the effect of anions with ammonia on pH (at 100 °C) and puts them in the following order: Chloride > sulphate, formate >> acetate >> carbon dioxide This order of the effect on pH and thus potential for corrosion is the reason why many operators would prefer ACC not to include the contribution from carbon dioxide. Currently, there are three techniques available to measure degassed conductivity directly: Heating the sample to boiling or near boiling point Gas stripping with a "bubbler" system

except when investigating the effect of sample flow rate variations.

Gas Stripper Another manufacturer loaned a gas stripping system. This comprised a cation exchange column with downward flow, followed by a ~ 0.6 x 0.05 m gas stripper that used a counter-current flow of nitrogen applied through a stainless steel sinter. Sample from the DGCC measuring cell had to be returned to the system for liquid height control of the gas stripper. Water and sample flows were 160 and 3 –1 700 cm · min respectively, except when investigating the effect of sample flow rate variations.

Membrane Gas Stripper A prototype system was supplied on a single panel with a cation exchange column and a polysulphone-membrane 2 gas exchanger of an approximate active area of 1.38 m . Two conductivity cells measured ACC and DGCC, being connected to a dual channel conductivity analyser with "acid" temperature compensation selected. Being a prototype, optimum operating conditions were not well established until the effect of sample flow rate variations had been investigated. Optimal sample water and gas flow 3 –1 rates were then found to be 150 and 250 cm · min respectively. The manufacturer is subsequently considering fitting a second membrane to operate with lower gas flow rates and some results from an early 2-membrane system are included here.

Diffusion of dissolved gases out of the sample through

a membrane. Systems based upon these three methods were evaluated by British Nuclear Fuels Plc. (BNFL) Nuclear Sciences and Technology Services and the results are reported here. This paper attempts to look at the generic merits of each method, without reference to the particular manufacturer of the systems. Systems that extract and measure the carbon dioxide concentration and then calculate its effect on conductivity were not examined.

SYSTEMS TESTED Heating A widely used "reboiler" system was loaned by the manufacturer. It consisted of a single cation exchange column with downward flow, a vented reboiler operating near 100 °C and three conductivity cells connected to a single transmitter. The conductivity of the incoming sample was measured at sample temperature, as was the ACC value, with the degassed conductivity (DGCC) measured near 100 °C. The optimum sample flow rate was found to be in 3 –1 the range of 170 to 200 cm · min and the system was operated near the mid-point of this range during all tests

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TEST PROCEDURES The three measurement systems were supplied with sample water from a recirculating de-ionising system as shown schematically in Figure 2. Solutions of known concentration of sodium chloride and sodium carbonate were applied using a flow dilution technique, in which concentrated solutions prepared volumetrically were continuously added from a motorised syringe into known flows of sample water. Flow rates were calibrated by gravimetric methods. Solutions of organic acids were prepared and applied in the same fashion. However, when adding acids, ammonia solution was simultaneously applied by a second motorised syringe, such that the concentration of ammonia in –1 the sample would be 2 mg · kg . After-cation-exchange conductivity (ACC) and degassed conductivity (DGCC) were measured on the systems' internal instrumentation (where fitted) and on a reference instrument. All values quoted here have been "normalised" as if taken by the reference instrument; see Figure 2. Test solutions were devised and applied as in Table 1 to ascertain the following:



i. Did ACC values match theoretically calculated values for the concentrations applied? ii. Did degassing affect the values for mineral and light organic acids? iii. Did degassing remove carbon dioxide from water? iv. What was the combined effect of degassing on solutions containing both acids and carbon dioxide? v. Did variations in sample temperature affect the degassing system?

Figure 2: Test apparatus.

Solution

Nominal ACC –1

[µS · cm ]

Chloride –1

[µg · kg ]

Carbon dioxide

Formate

–1

–1

[µg · kg ]

[µg · kg ]

Acetate [µg · kg–1]

1

0.14

10

2

0.31

25

3

1.2

4

0.14

25

5

0.32

100

6

0.54

250

7

0.11

10

8

0.45

50

9

0.15

20

10

0.32

50

11

0.62

12

1.5

13

0.83

14 Table 1:

100

10

250

100

250

0.055

250

50 –1

2 000 µg · kg

NH3

Test solutions and their nominal conductivity.


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THEORETICAL VALUES ACC is an important measurement in power plant, hence the opportunity was taken to compare the theoretically calculated conductivity to the measured value for each solution applied. During the experimental programme, theoretical values were calculated using a computer programme written by the Central Electricity Generating Board (CEGB) in the UK. The programme had not been verified against more recent data for ionisation, equilibrium and mobility constants but used the values accepted in the 1980s. Table 2 compares the values calculated from the CEGB programme and those from the EPRI code CCHEM v. 0.91 (2002) quoted in [2]. It can be seen that for calculation of the ACC values presented here, the two methods differed only in the last significant figure. (The two calculation methods exhibited more significant disagreements for mixtures of ammonia and salts, but this was not relevant to the work reported here.)

RESULTS Comparison of ACC with Theoretical Values All three systems gave excellent agreement between the calculated and measured ACC values as shown in Table 3. All indications were that the uppermost values (above –1 1 µS · cm ) were consistently lower than the calculated value by some 3 to 4 %. Background conductivity values –1 when pure water was applied were near 0.070 µS · cm

20 µg · kg–1

Concentration

and any excess in background from 0.055 µS · cm–1 has been deducted from the measured values in Table 3.

Effect of Degassing on Acids All three systems exhibited negligible differences between measurements of ACC and DGCC with acids (in the absence of carbon dioxide). Results are shown in Table 4, where a positive difference between the ACC and DGCC value would indicate that some of the anion species had been removed, and a negative value that some contamination had been added. Notice that when adding formic –1 or acetic acids to the sample, 2 mg · kg of ammonia had also been added prior to the ion exchange column.

Effect of Degassing on Carbon Dioxide Table 5 shows the results when the sample contained sodium carbonate, and hence carbon dioxide after the column. Concentrations are given for the resulting carbon dioxide after the cation exchange. Each system removed some carbon dioxide and hence DGCC was lower than ACC. These reductions are given as percentages of the reduction in conductivity that would be achieved if all carbon dioxide were removed. The equation used was: ACCmeasured – DGCCmeasured Reduction in = 1003 · ––––––––––––––––––––––––– conductivity ACCtheory – DGCCtheory

The results indicated the difficulty of removing all the carbon dioxide from pure water in the absence of any other acids, a situation that would represent an unlikely extreme on power plant.

50 µg · kg–1

60 µg · kg–1

Species

EPRI

CEGB

NNC

EPRI

CEGB

NNC

EPRI

CEGB

NNC

HCl

0.249

0.250

0.250

0.603

0.604

0.604

n/a

0.724

0.724

H2SO4

0.191

0.192

0.192

0.451

0.453

0.453

0.539

0.542

0.542

Formate

0.190

0.191

0.191

0.450

0.452

0.452

0.537

0.539

0.540

20 µg · kg–1

Concentration

50 µg · kg–1

250 µg · kg–1

Species

EPRI

CEGB

NNC

EPRI

CEGB

NNC

EPRI

CEGB

NNC

Acetate

0.145

0.145

0.145

0.321

0.323

0.323

1.371

1.380

1.380

20 µg · kg–1

Concentration

200 µg · kg–1

1 000 µg · kg–1

Species

EPRI

CEGB

NNC

EPRI

CEGB

NNC

EPRI

CEGB

NNC

CO2

0.123

0.124

0.123

0.474

0.476

0.472

1.149

1.153

1.140

Table 2:

(3)

A comparison of methods of calculating after-cation conductivity. All data are stated in µS · cm–1.

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EPRI

values calculated by EPRI CCHEM v. 0.91

CEGB

values calculated by CEGB programme MIXT2

NNC

values calculated using a programme recently derived by B. Handy of NNC Limited, courtesy of NNC Limited



Species

Concentration

Calculated ACC

–1

[µg · kg ]

[µS · cm ]

0.055

0.061 to 0.079

10.3

0.141

0.146

25.7

0.316

0.309

102.7

1.234

1.169

25.7

0.143

0.146

102.7

0.324

0.320

256.7

0.549

0.548

10.3

0.113

0.136

51.3

0.463

0.443

20.4

0.148

0.159

51.0

0.329

0.333

10.3 / 256.7

0.625

0.610

102.7 / 256.7

1.484

1.420

51.3 / 256.7

0.847

0.856

Chloride

Carbon dioxide

Formate

Acetate

Formate and CO2 Table 3:

Comparison of calculated and measured after-cation-exchange conductivity (ACC) values.

Species

Concentration

Difference ACC – DGCC (measured)

–1

[µS · cm–1]

[µg · kg ]

Chloride

NH3

Reboiler

Gas stripper

Membrane

10.3

–0.003

–0.007

–0.014

25.7

+0.003

–0.002

–0.003

102.7

+0.019

+0.005

+0.014

–0.001

+0.001

–0.001

10.3

–0.009

+0.019

+0.003

51.3

–0.085

+0.013

+0.010

20.4

–0.011

+0.009

–0.005

51.0

–0.004

+0.008

+0.004

2 000

Formate *

Acetate * Table 4:

–1

[µS · cm ]

Pure water

Chloride / CO2

Measured ACC

–1

The effect of degassing on acids in the absence of carbon dioxide. * 2 mg · kg–1 of ammonia had been added prior to the ion exchange column.

Solution applied

Efficiency of reduction in degassed conductivity value [%]

–1

–1 –1

25.7 µg · kg 102.7 µg · kg 256.7 µg · kg Table 5:

Reboiler

Sparger

Membrane

CO2

58

70

40; with 2 membranes: 48

CO2

46

70

59

CO2

28

70

63

The efficiency of degassing solutions containing carbon dioxide.


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Solution applied

Efficiency of reduction in degassed conductivity value [%] Reboiler

Sparger

Membrane

256.7 µg · kg CO2 and 10.3 µg · kg–1 chloride

28

70

58; with 2 membranes: 64

256.7 µg · kg–1 CO2 and –1 102.7 µg · kg chloride

44

89

83

256.7 µg · kg–1 CO2 and 51.3 µg · kg–1 formate

11

84

82

–1

Table 6:

The efficiency of degassing when other anions are present.

Effect of Degassing on Other Species Table 6 shows the results when both "strong" acids and carbon dioxide are present in the degassing system. Not surprisingly, the efficiencies have increased because water has poor buffering capacity and carbon dioxide solubility reduces with reduced pH. Calculated pH values with chloride present were 6.5, 5.8 and 5.55 for concentrations –1 of 10, 50 and 100 µg · kg respectively.

The membrane diffusion system showed slight evidence of small increases in degassing efficiency with increasing sample temperature. However, like the gas stripping system, the largest effect was related to the gas/water flow ratio. The membrane system was tested with only some 35 % of the flow used for the gas stripper, i.e., over a range of 1.0 to 1.7 for the gas/water flow ratio. The prototype membrane system had significant potential for further development to improve degassing efficiency.

Influence Quantities The test programmes used by BNFL are derived from a Company Standard for Chemical Analysis Equipment [3]. This standard incorporates developments in instrument evaluation techniques since the 1960s, but in the 1990s was aligned with procedures and nomenclature in modern IEC standards [4–7]. In accord with the Company Standard, each system was subjected to the appropriate tests for the effects of influence quantities. These were: variation of sample flow rate, variation of gas flow rate (where appropriate), variation of sample temperature and electrical supply variations. Each system required some degree of optimisation, for example for sample flow rate. The results, in as much as they pertain to the method of degassing, are summarised below. The reboiler system could be affected if the electrical supply was reduced or sample flow increased to a point where the reboiler temperature was not maintained. Very low sample temperatures also reduced the efficiency of the reboiler system. The gas stripping system showed no marked effect with variations in sample temperature. However, degassing efficiency was significantly increased by decreasing sample flow rate and/or increasing gas flow rate. The values quoted in Tables 5 and 6 are therefore only indicative of efficiencies obtainable within the manufacturer's rated flow ranges. The sample used during flow variation tests con–1 –1 tained 257 µg · kg of carbon dioxide with 10.3 µg · kg of chloride. Efficiency rose from 61 % to 80 % as the gas/water flow ratio was increased from 2.7 to 6.7; the results in Tables 5 and 6 were taken with a ratio of 4.7.

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SUMMARY The measured effect for ions on the ACC was in excellent agreement with values calculated by two independent methods. The effect of dissolved carbon dioxide was also as predicted by calculation. This confirms that indicative impurity concentrations calculated from ACC values are correct. The three systems tested all significantly reduced the contribution to ACC from dissolved carbon dioxide (i.e., DGCC was lower than ACC). None of them achieved 100 % removal of carbon dioxide, although the presence of other acid species aided degassing. For on-line operation on power plant each system has its advantages and disadvantages. The advantage for the reboiler was that it required no supply of nitrogen. However the solubility of carbon dioxide is not zero even at 100 °C, so the efficiency may not be 100 % even at boiling point. A significant disadvantage was that the displaced carbon dioxide could be entrained in the sample and re-dissolve if the sample was cooled. Hence, measurement of the degassed conductivity at elevated temperature was essential, thus requiring highly accurate sample temperature compensation over a 75 °C range. The gas stripping system had the advantage of simplicity and within the conditions used for this evaluation showed the highest degassing efficiency. However, for this high degassing efficiency the nitrogen usage would amount to typically one cylinder per week.



Both the above systems were vented and, hence, provided a final sample having little pressure to operate a column rinsing down, which would be useful because these systems only had one column. The membrane system tested showed a similar efficiency to the gas stripper, but with much lower gas consumption and outlet sample was at pressure. The membrane itself took some time to rinse to low conductivity and initially indicated a slightly elevated conductivity with pure water. The system was a prototype and showed potential for further development.

REFERENCES [1]

Bursik, A., PowerPlant Chemistry 2003, 5(4), 225.

[2]

Bursik A., PowerPlant Chemistry 2002, 4(10), 597.

[3]

Magnox Company Standard M/CS/TCED/100, 1997. Magnox Electric plc., Berkeley, United Kingdom.

[4]

IEC Standard 770, 1984. International Electrochemical Commission, Geneva, Switzerland.

[5]


[6]


[7]


THE AUTHOR Nigel J. Drew (B.S., Ph.D., University of Nottingham, England) started his career in the power industry in 1978, with the South Western Scientific Services of the Central Electricity Generating Board. He worked with suppliers to assist with development of chemical instrumentation for gas analysis, oil in water, moisture in gases and water oil. During the last 25 years, the emphasis of Nigel Drew's work has changed to mainly chemical instrument evaluation (of water and gas instrumentation), and monitoring applications for decommissioned nuclear plants. His employers since 1990 have been nuclear power generation companies in the United Kingdom. He was a member of IEC SC65D during the drafting of the international standards for chemical analyzers for gas and water. He is a member of the Royal Society of Chemistry and a Chartered Chemist.

CONTACT Nigel Drew BNFL Nuclear Sciences & Technology Services Berkeley Centre Berkeley, GL13 9PB United Kingdom E-mail: [email protected]

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