Water Quality Evaluation and Limits

Transcription

1 Cooling Tower Institute Annual Conference February 7-10, 2010 Houston, TX, USA Water Quality Evaluation and Limits Robert J. Ferguson French Creek Software, Inc. Kimberton, PA 19442

2 Part 1: Evaluation of Water Quality

3 Evaluation of Water Quality Simple Indices and Rules of Thumb Langelier Saturation Index Ryznar Stability Index Practical (Puckorius) Scale Index Stiff Davis Index Ion Association Model Saturation Ratios Simultaneous evaluation of relevant scales

4 Solubility Product (Ca)(CO 3 ) = Ksp 3 Where: (Ca) is the calcium activity (CO 3 ) is the carbonate activity Ksp is the solubility product for calcium li carbonate at the temperature under study.

5 Saturation Ratio SR = [Ca][CO 3 ] Ksp

6 Saturation Ratio [Ca][CO 3 ] SR = Ksp what you ve got what you can have

7 Saturation Ratio If Undersaturated SR < If Saturated SR = 1.0 If Supersaturated SR > 1.0 SR = [Ca][CO 3 ] Ksp

8 Saturation Ratio If Undersaturated SR < If Saturated SR = 1.0 If Supersaturated SR > 1.0 Scale will tend to dissolve Scale will not tend to dissolve or form Scale will tend to form SR = [Ca][CO 3 ] Ksp

9 The Concept of Saturation Calcium sulfate Barium sulfate SL= (Ca)(SO 4 ) SL= (Ba)(SO 4 ) Ksp Calcium carbonate Ksp Calcium li fluoride SL= (Ca)(F) 2 Ksp Magnesium hydroxide SL= (Ca)(CO 3 ) Calcium li phosphate h Ksp SL= (Ca) 3 (PO 4 ) 2 Ksp Strontium sulfate SL= (Mg)(OH) 2 SL= (Sr)(SO 4 ) Ksp Ksp Solubility relationships for common scales

10 Interference WithEvaluation: SimpleIndices M Alkalinityfor EstimatingCarbonate In a simple carbonic acid system ANC = 2.0 * [CO 3= ] + [HCO 3 ] +[OH ] [H + ]

11 RefiningEvaluations: Carbonate Calc s Correct M Alkalinity for non Carbonate In a simple carbonic acid system ANC = 20* 2.0 [CO = 3= ] + [HCO 3 ] +[OH ] [H + ] In an industrial environment ANC = 2.0 * [CO 3= ] + [HCO 3 ] +[NH 3 ] + [PO 4 ] + [B(OH) 4 ] + [OH ] [H + ]

12 Ion Association Model free versus total analytical l AnalyticalValues or Free Ions? All ions are not free to form scale Analytical values are the total of free and bound ions

13 Ion Association Model free versus total analytical l Ions in waters are not totally available Ca CO total = [Ca +II ] free + [CaSO 4 ] + [CaHCO +I 3 ] + [CaCO 3 ] + [Ca(OH) +I ] + [CaHPO 4 ] + [CaPO 4 I ] + [CaH 2 PO 4 +I ] +...

14 Ion Association Model free versus total analytical l Ions in waters are not totally available Mg total = [Mg +II ] + [MgSO 4 ] + [MgHCO +I 3 ] + [MgCO 3 ] + [Mg(OH) +I ] +[MgHPO 4 ] + [MgPO I 4 ]+[MgH 2 PO +I 4 ]+[MgF +I ]

15 Ion Association Model free versus total analytical l Ions in waters are not totally available Na total = [Na +I ] + [NaSO I I 4 ] + [Na 2 SO 4 ] + [NaHCO 3 ] + [NaCO I 3 ] + [Na I 2 CO 3 ] + [NaCl]+[NaHPO 4 ]

16 Ion Association Model free versus total analytical l Ions in waters arenot totally available K +I I I total = [K ] + [KSO 4 ] + [KHPO 4 ] + [KCl]

17 Ion Association Model free versus total analytical l Ions in waters arenot totally available Fe +II +III +I +II total = [Fe ] + [Fe ] + [Fe(OH) ] + [Fe(OH) ] + [Fe(OH) I 3 ] + [FeHPO4+I] + [FeHPO 4 ] + [FeCl +II ] + [FeCl 2 +I ] + [FeCl 3 ] + [FeSO 4 ] [FeSO 4 +I ] + [FeH 2 PO 4 +I ] + [Fe(OH) 2 +I ] + [Fe(OH) 3 ] + [Fe(OH) I 4 ] + [Fe(OH) 2 ] + [FeH 2 PO +II 4 ]

18 Ion Association Model Ions in waters are not totally available Ba total = [Ba +II ] free + [BaSO 4 ] + [BaHCO +I 3 ] + [BaCO 3 ] + [Ba(OH) +I ] +...

19 Ion Association Model Ions in waters are not totally available Sr SO total = [Sr +II ] free + [SrSO 4 ] + [SrHCO +I 3 ] + [SrCO 3 ] + [Sr(OH) +I ] +...

20 Evaluation of Water Quality State of the Art Scale Evaluations Correct alkalinity for non carbonate contributions Account for ion pairing, common ion effects Use free ion activities for index calculations Model ph and acid requirements differently for closed and open systems

21 Evaluation of Water Quality: Corrosion

22 Reason to Use Computer Modeling Classic predictions (Simple Indices) Lack accuracy as TDS, ph, and alkalinity increase. Do not account for all scales State of the art calculations (Ion AssociationModels) State of the art calculations (Ion Association Models) Correction for non carbonate alkalinity Use Ion Association Model Free Ion Concentrations Provide Options for Closed or Open Systems

23 Practical Applications Maximizing Cycles with and wthout acid feed with and without scale inhibitors Optimizing Water Reuse Evaluating Disposal Optimizing Treatment Troubleshooting

24 Part 2: Limits and Limitations

25 Goals and Targets You have a goal, a target You have best available technologies How do you achieve your goals

26 Limits and Limitations Know your limits and limitations They are like Debts and Obligations Ignore them and trouble follows

27 Limits and Limitations: Goals Determine Maximum Cycles With Different Make up water sources By blending sources With pretreatment Determine Maximum Reuse Water R.O. Concentrate Process Waters Produced Waters

28 Types of Limits Limits and Limitations Mechanical limits to cycles of concentration. Untreated limits based upon mineral scale solubility Limits with ph control Limits with scale inhibitors Limits with other pretreatment methods

29 Limits and Limitations: Types of Limits Mechanical limits to cycles of concentration Maximum cycles with no blowdown Dift Drift and Leaks limit it maximum cycles

30 Limits and Limitations: Types of Limits Untreated limits based upon mineral scale solubility Historically, i simple indices and other rules of thumb Derived from the basic solubility product relationships e.g. Langelier Saturation Index < 0.2 [Ca] [SO 4 ] < 50, SiO 2 < 120, SiO 2 < 150, SiO 2 < 180 For further info : Water Treatment Rules of Thumb: Myths or Useful Tools, AWT 2003

31 Limits and Limitations: Types of Limits Untreated limits based upon mineral scale solubility State of the art Ion Association Model Saturation Ratios Calcite Saturation Ratio < 1.5 Silica Saturation Ratio < 1.2 Gypsum Saturation Ratio < 1.0 Anhydrite hd Saturation Ratio < 1.0 Barite Saturation Ratio < 1.0 Celestite SaturationRatio < 1.0 For further info : Computerized Ion Association Model Profiles Complete Range of Cooling System Parameters, IWC 1990

32 Limits and Limitations: Types of Limits Limits with ph Control Same limits for Simple indices and other rules of thumb Same limits for Ion Association Model Saturation Ratios Notes: Limited to carbonate and phosphate h based scales May aggravate sulfate based scales At high cycles, may cause mystery scales

33 ph Control & Acid Feed Can Increase Sulfate Scale Potential ti at Higher Concentration ti Ratios Rti ph controlled using 98% Sulfuric acid

34 Limits and Limitations: Types of Limits Treated limits based upon mineral scale solubility Historically, simple indices and other rules of thumb Derived from the basic solubility product relationships e.g. Langelier Saturation Index < 2.5 [Ca] [SO 4 ] < 10,000,000 SiO 2 < 250

35 Limits and Limitations: Types of Limits Treated limits based upon mineral scale solubility State of the art Ion Association Model Saturation Ratios Calcite Saturation Ratio < 150 Silica Saturation Ratio < 2.2 Gypsum Saturation Ratio < 5.0 Anhydrite hd Saturation Ratio < 3.0 Barite Saturation Ratio < 80 Celestite SaturationRatio < 12

36 Limits and Limitations: Comparison CALCIUM CARBONATE RULES OF THUMB vs SATURATION RATIO Index Untreated Treated Limit Stressed Comments Limit Inhibitor Limit Langelier Saturation Level Use alkalinity corrected for noncarbonate (e.g. NH 3, CN, PO 4, Si) alkalinity. Ryznar Stability Empirical i rearrangement of ph Index and phs used to calculate Langelier Saturation Index. Practical Scaling Index Interpretation similar to Ryznar. Index applicable to NH 3 or other alkali contaminated waters. Calculates a ph as if only carbonic acid based alkalinity present. Calcite Saturation Index corrects for ion pairing, Level noncarbonate alkalinity, activity effects. Reproducible results at the same index.

37 Limits and Limitations: Comparison CALCIUM SULFATE RULES OF THUMB vs SATURATION RATIO MINERAL FORM UNTREATED RULE OF SATURATION TREATED RULE SATURATION LEVEL THUMB LEVEL AT LIMIT OF THUMB AT LIMIT GYPSUM CaSO 4 *2H 2 O [Ca][SO 4 ] < 50,000 Ca as mg/l Ca, 0.96 (at 120 o F) [Ca][SO 4 ] < 10,000,000 Ca as mg/l Ca, 4.98 X Saturation (at 120 o F) SO 4 as mg/l SO 4 SO 4 as mg/l SO 4 ANHYDRITE [Ca][SO 4 ] CaSO 4 < 50,000 Ca as mg/l Ca, 0.99 (at 120 o F) [Ca][SO 4 ] < 10,000,000 Ca as mg/l Ca, 3.10 X Saturation (at 120 o F) SO 4 as mg/l SO 4 SO 4 as mg/l SO 4

38 Limits and Limitations: Comparison TREATED LIMITS COMPARISON SCALE FORMING SPECIE FORMULA MINERAL NAME TYPICAL SATURATION RATIO LIMIT STRESSED TREATMENT LIMIT Calcium carbonate CaCOCO 3 Calcite Calcium sulfate CaSO 4 *2H 2 O Gypsum Barium sulfate BaSO 4 Barite Strontium sulfate SrSO 4 Celestite Silica SiO 2 Amorphous silica Tricalcium phosphate Ca 3 (PO 4 ) ,000

39 Recommendations for Establishing Limits Simple Indices and Rules of Thumb Calculated from total analytical values Estimate carbonate from uncorrected M Ion Activity estimated for low T.D.S. solutions Accuracy drops quickly with ph and ionic strength Not recommended for ph > 7.6, TDS > 500

40 Recommendations for Establishing Limits State of the Art Scale Evaluations Correct alkalinity for non carbonate contributions Account for ion pairing, common ion effects Use free ion activities for index calculations Model ph and acid requirements differently for closed and open systems

41 Critical to Know Failure Points for maximizing cycles

42 Critical to Know Failure Points for maximizing cycles

43 Critical to Know Failure Points for maximizing cycles, optimizing blends

44 Comparing Treatments Untreated PBTC HEDP

45 Troubleshooting Operating Range

46 ph control Filtration i Lime Softening Reverse Osmosis Blending Water Sources Pretreatment

47 In Summary The first step is always defining your goals, e.g. maximize cycles, minimize water usage, g, maximize water reuse. Understanding d system limits is a critical first step in achieving these goals. Once limits are established, computer modeling can assist in determining the benefits of various treatment and pretreatment scenarios.