- The document summarizes key points from a presentation on advanced pH measurement and control. It discusses challenges in pH control and new technologies for high temperature glass electrodes, wireless transmitters, and online diagnostics. - Titration curves are essential for pH system design but can be deceptive due to nonlinearity. Factors like temperature, solvent concentration, and CO2 levels can also impact measured pH. - Accurate pH measurement requires selecting the proper glass and reference electrodes for the process and using techniques like middle signal selection and online diagnostics to detect errors.

1. ISA Saint Louis Short Course Dec 9-10, 2010 Advanced pH Measurement and Control - Day 1

2. pH Challenges and Opportunities

5. Extraordinary Sensitivity and Rangeability pH Hydrogen Ion Concentration Hydroxyl Ion Concentration 0 1.0 0.00000000000001 1 ` 0.1 0.0000000000001 2 0.01 0.000000000001 3 0.001 0.00000000001 4 0.0001 0.0000000001 5 0.00001 0.000000001 6 0.000001 0.00000001 7 0.0000001 0.0000001 8 0.00000001 0.000001 9 0.000000001 0.00001 10 0.0000000001 0.0001 11 0.00000000001 0.001 12 0.000000000001 0.01 13 0.0000000000001 0.1 14 0.00000000000001 1.0 a H = 10  pH pH = - log (a H ) a H =  c H c H  c OH = 10  pKw Where: a H =  hydrogen ion activity (gm-moles per liter) c H =  hydrogen ion concentration (gm-moles per liter) c OH =  hydroxyl ion concentration (gm-moles per liter)  = activity coefficient (1 for dilute solutions) pH = negative base 10 power of hydrogen ion activity pK w = negative base 10 power of the water dissociation constant (14.0 at 25 o C) Hydrogen and Hydroxyl Ion Concentrations in a Water Solution at 25 o C

6. Nonlinearity - Graphical Deception Reagent  Influent Ratio Reagent  Influent Ratio Despite appearances there are no straight lines in a titration curve (zoom in reveals another curve if there are enough data points - a big “IF” in neutral region) For a strong acid and base the pK a are off-scale and the slope continually changes by a factor of ten for each pH unit deviation from neutrality (7 pH at 25 o C) As the pH approaches the neutral point the response accelerates (looks like a runaway). Operators often ask what can be done to slow down the pH response around 7 pH. Yet titration curves are essential for every aspect of pH system design but you must get numerical values and avoid mistakes such as insufficient data points in the area around the set point 14 12 10 8 6 4 2 0 pH 11 10 9 8 7 6 5 4 3 pH

7. Nonlinearity - Graphical Deception Strong Acid and Weak Base pk a = 10 Slope moderated near each pK a pK a and curve changes with temperature! Weak Acid and Strong Base pk a = 4 Multiple Weak Acids and Weak Bases pk a = 3 pk a = 5 pk a = 9 Weak Acid and Weak Base pk a = 4 pk a = 10

8. Double Junction Combination pH Electrode W W E m R 10 R 9 R 8 R 7 R 6 R 5 R 4 R 3 R 2 R 1 E r E 5 E 4 E 3 E 2 E 1 outer gel layer inner gel layer second junction primary junction solution ground Process Fluid silver-silver chloride internal electrode silver-silver chloride internal electrode potassium chloride (KCl) electrolyte in salt bridge between junctions 7 pH buffer I i High acid or base concentrations can affect glass gel layer and reference junction potential Increase in noise or decrease in span or efficiency is indicative of glass electrode problem Shift or drift in pH measurement is normally associated with reference electrode problem Process ions try to migrate into porous reference junction while electrolyte ions try to migrate out Nernst Equation assumes inside and outside gel layers identical Measurement becomes slow from a loss of active sites or a thin coating of outer gel W W W W W W W W

9. Life Depends Upon Process Conditions 25 C 50 C 75 C 100 C Process Temperature Months >100% increase in life from new glass designs for high temperatures High acid or base concentrations (operation at the extremes of the titration curve) decrease life for a given temperature. A deterioration in measurement accuracy and response time often accompanies a reduction in life. Consequently pH feedforward control is unreliable and the feedforward effect and timing is way off for such cases.

10. New High Temperature Glass Stays Fast Glass electrodes get slow as they age. High temperatures cause premature aging

11. New Design Eliminates Drift after Sterilization New Old #1 Old #2

12. Solid Large Surface Reference Junction

13. High Temperature Glass with Removable Reference Junction

14. Low Flow Assembly for Low Conductivity Streams

15. Horizontal Piping Arrangements 5 to 9 fps to minimize coatings 0.1 to 1 fps to minimize abrasion 20 to 80 degrees The bubble inside the glass bulb can be lodged in tip of a probe that is horizontal or pointed up or caught at the internal electrode of a probe that is vertically down pressure drop for each branch must be equal to to keep the velocities equal Series arrangement preferred to minimize differences in solids, velocity, concentration, and temperature at each electrode! 10 OD 10 OD 20 pipe diameters 20 pipe diameters static mixer or pump flush drain flush drain throttle valve to adjust velocity throttle valve to adjust velocity AE AE AE AE AE AE

16. Vertical Piping Arrangements 5 to 9 fps coating abrasion 10 OD 10 OD 0.1 to 1 fps hole or slot Orientation of slot in shroud throttle valve to adjust velocity throttle valve to adjust velocity Series arrangement preferred to minimize differences in solids, velocity, concentration, and temperature at each electrode! AE AE AE AE AE AE

17. What is High Today may be Low Tomorrow A B A B A B pH time Most calibration adjustments chase the short term errors shown below that arise from concentration gradients from imperfect mixing, ion migration into reference junction, temperature shifts, different glass surface conditions, and fluid streaming potentials. With just two electrodes, there are more questions than answers.

22. Temperature Comp Parameters Solution pH Temperature Correction Isopotential Point Changeable for Special pH Electrodes pH / ORP Selection Preamplifier Location Type of Reference Used Ranging AMS pH Range and Compensation Configuration

23. Impedance Diagnostics On/Off Reference Impedance Warning and Fault Levels Reference Zero Offset Calibration Error Limit Glass Electrode Impedance Warning and Fault Levels Glass Impedance Temp Comp (Prevents spurious errors due to Impedance decrease with Temperature) AMS pH Diagnostics Configuration

24. Live Measurements and Status Calibration Constants from Last Calibrations Buffer Calibration Type & Buffer Standard Used Sensor Stabilization Criteria Zero Offset Beyond this Limit will create a Calibration Error If you want to know more about Buffer Calibration, hit this button… AMS pH Calibration Setup

25. Tips on doing Buffer Calibration Temperature Range and Values of Selectable Buffers in the Xmtr Explanation Buffer Calibration Parameters AMS pH Buffer Calibration Tips

26. AMS pH Standardization Steps

27. AMS Temperature Standardization Steps

28. AMS pH Overview Dashboard

29. AMS pH Auxiliary Variables Dashboard

30. AMS pH Diagnostics Dashboard

31. AMS pH Diagnostics Descriptions

32. Calibration Record in Electrode History Time Stamp Calibration Method Slope mV/pH Offset (mV) Glass Impedance, (mOhm) Reference Impedance, (kOhm) Temperature ( o C) 1-Latest 120days Auto-cal 56.0 11 820 13 25 2 90days Auto-cal 57.5 10 760 10 28 3 60days Auto-cal 57.1 9 670 15 26 4 30days Manual 58.4 10 550 10 27 5-Oldest 2days Manual 59.0 3 480 3 30 Factory Calibration 0.0 Auto-cal 59.3 2 500 2 25

33. Wireless pH Transmitters Have Latest Advances Better resolution and diagnostics than smart wired transmitters

34. Wireless pH Transmitters Eliminate Ground Spikes Wired pH ground noise spike Temperature compensated wireless pH controlling at 6.9 pH set point Incredibly tight pH control via 0.001 pH wireless resolution setting still reduced the number of communications by 60%

37. Inferential Measurements - Effect of MEA Solvent on pH

38. Inferential Measurements - Effect of PZ Solvent on pH

42. Control Valve Rangeability and Resolution pH Reagent Flow Influent Flow 6 8 Influent pH B A Control Band Set point B E r = 100%  F imax   F rmax F rmax = A  F imax B E r =  A S s = 0.5  E r Where: A = distance to center of reagent error band on abscissa from influent pH B = width of allowable reagent error band on abscissa for control band E r = allowable reagent error (%) F rmax = maximum reagent valve capacity (kg per minute) F imax = maximum influent flow (kg per minute) S s = allowable stick-slip (resolution limit) (%)

43. Diaphragm Actuator with Solenoid Valves Port A Port B Supply ZZZZZZZ Control Signal Digital Valve Controller Must be functionally tested before commissioning! SV Terminal Box

44. Size of Step Determines What you See Maintenance test of 25% or 50% steps will not detect dead band - all valves look good for 10% or larger steps

45. Effect of Step Size Due to Sensitivity Limit

46. Limit Cycle in Flow Loop from Valve Stick-Slip Controller Output (%) Saw Tooth Oscillation Process Variable (kpph) Square Wave Oscillation

47. Real Rangeability Minimum fractional flow coefficient for a linear trim and stick-slip: Minimum fractional flow coefficient for an equal percentage trim and stick-slip: Minimum controllable fractional flow for installed characteristic and stick-slip: C xmin  minimum flow coefficient expressed as a fraction of maximum (dimensionless)  P r  valve pressure drop ratio (dimensionless) Q xmin  minimum flow expressed as a fraction of the maximum (dimensionless) R v  rangeability of control valve (dimensionless) R  range of the equal percentage characteristic (e.g. 50) X vmin  maximum valve stroke (%) S v  stick-slip near closed position (%)

49. Reagent Savings is Huge for Flat Part of Curve pH Reagent to Feed Flow Ratio Reagent Savings Original set point Optimum set point 4 10 Oscillations could be due to non-ideal mixing, control valve stick-slip. or pressure fluctuations

50. Effect of Sensor Drift on Reagent Calculations pH Reagent to Feed Flow Ratio 4 10 6 8 Feedforward Reagent Error Feedforward pH Error Sensor Drift pH Set Point Influent pH The error in a pH feedforward calculation increases for a given sensor error as the slope of the curve decreases. This result Combined with an increased likelihood of Errors at low and high pH means feedforward Could do more harm than good when going from the curve’s extremes to the neutral region. Flow feedforward (ratio control of reagent to influent flow) works well for vessel pH control if there are reliable flow measurements with sufficient rangeability Feedforward control always requires pH feedback correction unless the set point is on the flat part of the curve, use Coriolis mass flow meters and have constant influent and reagent concentrations

52. Dynamic First Principal Modeling Integration of component mass balances in vessel volume Calculation of component normalities Interval halving search for pH that satisfies the charge balance all component mass flows* ( F i ) Calculation of mixture’s density mixture’s liquid density (  m ) signed ionic charges ( Z i ) component mass fractions ( X i ) molecular weights component normalities ( N i ) acid and base negative logarithmic base 10 acid dissociation constants ( pK ai ) water pK w pH component fractional molar densities ( w i ) For inline systems, mass flow ratios are used instead of integrators to compute mass and mole fractions  m   (w i   i ) Component liquid densities (  i ) *Note that all component mass flows in all streams (not just acids and bases) must be included in the component mass balances The titration curves are obtained by incrementing the reagent mass flow

53. Calculation of Normality z * d * x N = ------------- (2-1e) M N c = ------- (2-1f) z Where: c = molar concentration of diluted acid or base (gm-moles per liter) d = density of solution (gm per liter) M = molecular weight of pure acid or base (gm per gm-mole) n = number of gm-moles of pure acid or base x = weight fraction of pure acid or base in solution z = number of replaceable hydrogen or hydroxyl ions per molecule of acid or base N = grams-ions of replaceable hydrogen or hydroxyl groups per liter

54. Charge Balance Normalities from Dissociation 1 N 1  s  N (2-4a) 1 + P 1 (1 + 0.5  P 2 ) N 2  s  N (2-4b) (1+ P 2  (1+P 1 )) (1+0.33*P 3 *(2+P 2 )) N 3  s  N (2-4c) (1+P 3  (1+P 2  (1+P 1 ))) P 1 = 10 (s*(pH - pK 1 )) (2-4d) P 2 = 10 (s*(pH - pK 2 )) (2-4e) P 3 = 10 (s*(pH – pK 3 )) (2-4f)

55. Charge Balance Equations For strong acids and bases, the acids and bases are completely dissociated everywhere on the pH scale, which gives the following charge balance:  N bi  N ai  10  pH  10 (pH – pKw) = 0 (2-4g) i i For weak acids and bases with a single dissociation, the concentration of ions depend upon pH and are computed via equation 2-4a, which gives the following charge balance: 1 1  N bi ]   N ai ] + 10  pH – 10 (pH  pKw)  (2-4h) i (1 + P bi ) i (1 + P ai )

56. Charge Balance Nomenclature Where: N = concentration of an acid or base (normality) N 1 = concentration of ions from a single dissociation (normality) N 2 = concentration of ions from a double dissociation (normality) N 3 = concentration of ions from a triple dissociation (normality) N ai = concentration of an acid i (normality) N bi = concentration of a base i (normality) pK 1 = negative base 10 logarithmic first acid dissociation constant pK 2 = negative base 10 logarithmic second acid dissociation constant pK 3 = negative base 10 logarithmic third acid dissociation constant pK w = negative base 10 logarithmic water dissociation constant P 1 = parameter for an acid or base with one dissociation P 2 = parameter for an acid or base with two dissociations P 3 = parameter for an acid or base with three dissociations P ai = parameter for dissociation of an acid i with a single dissociation P bi = parameter for dissociation of a base i with a single dissociation s = ion sign (s  1 for acids and s  1 for bases)

57. Virtual Plant Knowledge Synergy Dynamic Process Model Online Data Analytics Model Predictive Control Loop Monitoring And Tuning DCS batch and loop configuration, displays, and historian Virtual Plant Laptop or Desktop Personal Computer Or DCS Application Station or Controller Embedded Advanced Control Tools Embedded Modeling Tools Process Knowledge

58. Virtual Plant Setup Advanced Control Modules Process Module for Bioreactor or Neutralizer Virtual Plant Laptop or Desktop or Control System Station Configuration and Graphics

59. Modeled pH Control System Signal characterizers linearize loop via reagent demand control AY 1-4 AC 1-1 AY 1-3 splitter AT 1-3 AT 1-2 AT 1-1 AY 1-1 AY 1-2 middle signal selector signal characterizer signal characterizer pH set point Eductors FT 1-1 FT 1-2 NaOH Acid LT 1-5 Tank Static Mixer Feed To other Tank Downstream system LC 1-5 From other Tank To other Tank

60. Titration Curve Slope is Key to Fidelity slope pH Acid Flow Ramp

61. Field Generation of Titration Curve Static Mixer Feed AT 1-1 FT 1-1 FT 1-2 Reagent 10 to 20 pipe diameters Filter Delay Div Curve Reagent to Feed Ratio Measured pH X,Y pair Div Transportation Time Delay Feed Flow Ramp in ROut mode Filter Y Axis (filtered pH) Synchronized X Axis (Reagent/Feed Ratio) Coriolis Mass Flow Meter Coriolis Mass Flow Meter Mass Hold Up = Density  Volume FC 1-1 AC 1-1

62. Axial Agitation Recommended for pH Control The agitation in a vessel should be a vertical axial pattern without rotation and be intense enough to break the surface but not cause froth baffles Vertical well mixed tank liquid height should be 0.5 to 1.5x tank diameter

63. Radial Agitation Detrimental for pH Control Stagnant Zone Stagnant Zone The stagnant zones introduce a large and variable dead time. The use of an external recirculation stream and an eductor ring can reduce stagnation

64. Horizontal Tanks and Sumps are Bad News Feed Reagent AT 1-3 Short Circuiting Stagnant Zone Plug Flow Multiple recirculation streams can help but this type of geometry is best used for attenuation of oscillations upstream or downstream of a pH control system. The insertion of an inline pH control system in the recirculation line could make this a viable system by putting a fast system in series with a large volume M

65. Vessel Time Constant and Dead Time For a vertical well mixed vessel:  V  dp  (5-3a) F i  F r  F c  F a F a  N q  N s  D i 3  (5-3b) 0.4 N q  (5-3c) (D i  D t ) 0.55  dp  L  U  ) (5-3d)  V  p   dp (5-3e) F i  F r For a vessel with proper geometry, baffles, and axial patterns, the equipment dead time from mixing is approximately the turnover time For a vessel with proper geometry, baffles, and axial patterns, the continuous equipment time constant is the residence time minus the dead time

66. Equipment Dynamics Nomenclature Where: D i  impeller diameter (meter) D t  tank inside diameter (meter) F i  influent mass flow (kg per minute) F r  reagent mass flow (kg per minute) F c  recirculation mass flow (kg per minute) F a  agitation mass flow (kg per minute) L  distance between inlet and outlet nozzles (meter) N q  agitator discharge coefficient (0.4 to 1.4) N s  agitator speed (revolutions per minute)   average fluid density (kg per cubic meter)  dp  process dead time from mixing (minutes)  p  process time constant from mixing (minutes) U  average bulk velocity (meters per minute) V  vessel liquid volume (cubic meters)

67. Reagent Injection Delay For reagent dilution or after closing of a control valve to a dip tube or injection tube:  V  dp  F r Where: F r  reagent mass flow (kg per minute)   average fluid density (kg per cubic meter)  dp  process dead time from mixing (minutes) V  injection volume (cubic meters) The delivery delay from an empty or back filled reagent pipe, injection tube, and dip tube is the largest source of dead time in a pH loop Use isolation valves close coupled to the injection point coordinated with the action of the control valve to reduce reagent holdup between process and reagent control valve

68. Static Mixer - Good Radial but Poor Axial Mixing Oscillations and noise will pass through a static mixer un-attenuated and the poor dead time to time constant ratio leads to more oscillations The extremely small residence time of a static mixer greatly reduces the magnitude of the dead time and the volume of off-spec material “ The Future is Inline” due to a much lower cost, smaller footprint, and faster response if you can address oscillation and noise issues

69. Attenuation of Oscillations by a Volume For a single vessel volume: T o A o  A i  (5-3j)    p Where: A i  amplitude of input oscillation into volume (reagent to influent ratio) A o  amplitude of output oscillation from volume (reagent to influent ratio) T o  period of oscillation (minutes)  p  process time constant from mixing (minutes) Back mixed volumes attenuate oscillations in concentration that must be translated to pH via a titration curve to see the effect on the pH trend The average pH measurement upstream and downstream will differ due To the nonlinearity. The upstream pH fluctuations must be translated to concentration fluctuations (changes in reagent to influent ratio) and then attenuated per Equation 5-3j and translated back to pH via titration curve

70. Translation of Oscillations via Titration Curve pH Reagent Flow Influent Flow 6 8 The effect of fluctuations in influent concentration or mixing uniformity on measurement error and the effect of pressure fluctuations and control valve resolution on the ratio of reagent to influent is less on flat portions of the titration curve but high acid or base concentrations at the curve extremes may attack glass and wetted materials of construction and increase reference junction potentials

71. Everyday Mistakes in pH System Design Mistake 7 (gravity flow) AT 1-1 AT 1-3 AT 1-2 Mistake 4 (horizontal tank) reagent feed tank Mistakes 5 and 6 (backfilled dip tube & injection short circuit) Mistake 11 (electrode in pump suction) Mistake 8 (valve too far away) Mistake 9 (ball valve with no positioner) Mistake 10 (electrode submerged in vessel) Mistake 12 (electrode too far downstream) Mistake 3 (single stage for set point at 7 pH) Influent (1 pH) Mistake 1: Missing, inaccurate, or erroneous titration curve Mistake 2: Absence of a plan to handle failures, startups, or shutdowns M

72. Inline and Tank Control System Performance The Inline system has the fastest recovery time and lowest cost but due to the lack of back mixing, the signal will wildly fluctuate for set points on the steep part of the titration curve. A signal filter (e.g. 12 sec) or reagent demand control attenuate the oscillations but an offset from the set point is seen downstream The tank responses shown above assumes a negligible reagent delivery delay and are thus faster than typical. In most systems the reagent injection delay is so large due to a dip tube, the period of oscillation is 10x slower than what is seen in an inline system