Adventures in pH Control

Adventures in pH Control

Greg McMillan CDI Process & Industrial

Dave Joseph Rosemount Analytical

Photography & Video Recording Policy

Photography and audio/video recording is not permitted in any sessions or in the exhibition areas without

press credentials or written permission from the Emerson Exchange Board of Directors. Inquiries should

be directed to:

[email protected]

Thank you.

Greg McMillan

Principal Consultant

Email: [email protected]

33 years Monsanto-Solutia Fellow

2 years WU Adjunct Professor

10 years DeltaV R&D Contractor

BS Engineering Physics

MS Control Theory

Dave Joseph

Sr. Industry Manager

Email: [email protected]

24 years with Rosemount Analytical

BS and MS in Chemical Engineering

Member AIChE

Presenters

4

Key Benefits of Course

Recognize the opportunity/challenges of pH control Learn about modeling and control options Optimize hardware implementation Understand the root causes of poor performance Prioritize improvements based on cost, time, and goal Gather insights for applications and solutions

5

Section 1: Measuring pH

Brief theory of pH Inside a pH sensor The Smart pH sensor Diagnostics

6

Top Ten Signs of a Rough pH Startup

Food is burning in the operators’ kitchen Only loop mode configured is manual Operator puts his fist through the screen You trip over a pile of used pH electrodes Technicians ask: “what is a positioner?” Technicians stick electrodes up your nose Environmental engineer is wearing a

mask Plant manager leaves the country Lawyers pull the plugs on the consoles President is on the phone holding for you

The definition of pHThe definition of pH

pH is the unit of measurement for determining the acidity or alkalinity of a solution.

The mathematical definition of pH is the negative logarithm of the molar hydrogen ion concentration, pH = - log([H+])

pH is measured by various different sensors, most common and economical is the glass electrode/silver reference system.

pH measurement requires periodic maintenance to maintain accuracy.

H+

OH-

H+

H2O

OH-

pH Scale vs Moles/Liter Ion Concentration

pH Hydrogen Ion [H+] Hydroxyl Ion [OH-]

0 Acidic1234567 Neutral891011121314 Basic

1.00.10.010.0010.00010.000010.0000010.00000010.000000010.0000000010.00000000010.000000000010.0000000000010.00000000000010.00000000000001

0.000000000000010.00000000000010.0000000000010.000000000010.00000000010.0000000010.000000010.00000010.0000010.000010.00010.0010.010.11.0

pH Values of Acids and Bases

0

2

4

6

8

10

12

14

1E00 1E-01 1E-02 1E-03 1E-04 1E-05 1E-06 1E-07 1E-08 1E-09 1E-10 1E-11 1E-12 1E-13 1E-14

Hydrogen Ion Concentration (Mole/Liter)

pH

4.0 % Sodium Hydroxide

0.04% Sodium Hydroxide

Milk of Magnesia

0.84% Sodium Bicarbonate

Water @ 25ºC

0.00001% Sulfuric Acid

0.0001% Hydrochloric Acid

0.01% Sulfuric Acid

0.1% Hydrochloric Acid

4.9% Sulfuric Acid

What is pH? – technical stuff

pH = - log([H+]) Kw = [H+]*[OH-] = 1.0x10-14 at 25ºC

pH + pOH = pKw

pH is measured using the Nernst equation E(mV) = Ex + 2.3(RT/F)*log aH+

~ Ex – (S)*pH in simple form

Where Ex = calibration constant 2.3(RT/F) ~ slope (S) in mV/pH units aH+ = activity of hydrogen ion ~ [H+]

Theoretical Response of a pH Sensor (25ºC)

-500

-400

-300

-200

-100

0

100

200

300

400

500

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

pH

mV

Slope of 59.16

mV/pH Unit

Zero mV

at 7 pH

12

pH Sensor Basics

•The pH electrode produces a

potential (in mV’s) proportional

to the pH of the solution.

pH Sensitive

Glass

Fill

Solution

Shield

Glass

Body

Ag/AgCl

Internal

Wire

• The reference electrode potential

must remain stable regardless of

process or time effects

• Internal element- Ag\AgCl

• Electrolyte fill - KCl/AgCl

• Liquid Junctions

13

Inside the pH Glass Membrane...

Glass Matrix

(unaffected)

Leached Layer

Dissolving

Core Glass

Inner Zone

Outer Zone

Alkali Metal Ions

Hydrogen Ions

Anionic Sites

H+

M

H+ H

+

H+

S

S S S

M

M

M M

H+

H+

H+

H+

H+

M M

MLeached Layer

(not to scale)

14

The reference electrode

pH17

AgCl/KCl

Fill

Solution

Ag/AgCl

Internal

Wire

Liquid

Junction

The Reference Cell maintains a

stable potential regardless of the

process pH or changes in the

activities of other ions in solution.

The Liquid Junction completes the

electrical circuit between the pH

measuring electrode and the

reference cell via the process

solution.

The sum of all potentials…

Assuming a preamp with low leakage current, the pH sensor

Ex =

Eoutside of glass (in process solution)

- Einside of glass (in glass fill solution)

- Emeasurement wire (in glass fill solution)

+ Ereference wire (in reference Ag/KCL solution)

+ Ejunction potential (sum of all interface potentials)

Glass fill solution typically formulated to cancel out effects so that 7 pH is 0 mV at any temperature.

16

Double Junction Combination pH Electrode - Circuit Diagram

W

W

W

W

W

WW

W

W

W

Em

R10R9R8

R7

R6

R5

R4

R3

R2

R1

Er

E5

E4

E3

E2

E1

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

pH fill solution

Ii

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 may

migrate into porous

reference junction

while electrolyte ions

migrate out

Measurement

becomes slow

if glass gets coated

Dehydration, loss of active sites,

chemical attack, and premature

aging reduces efficiency and

makes sensor dramatically slowGel layer is used as a term

for the glass surface that

has water molecules

17

Life Depends On Process Conditions

25ºC 50ºC 75ºC 100ºC

Process Temperature

Months

>100% increase in life

from new glass designs

for high temperatures

High pH conditions decrease glass life at any temperature

Degraded accuracy and response time is also common

Leads to unreliable feedforward control

18

New Glass preserves response time

0

50

100

150

200

0 50 100 150 200

minutes

mV

New Glass

Other

Glass electrodes get slow as they age

High temperatures cause accelerated aging

New glass formulations can resist this effect

After 120 hours exposure at 140ºC

Review: pH Measurement loop

Analyzer

(not part of the sensor)

2. Reference

electrode

Liquid

junction

3.Temperature

element

1. Glass

electrode

4. Solution

ground

What is a SMART sensor?

SMART sensors store calibration data on an embedded chip. SMART sensors record the initial calibration data of the sensor

and all data from the last 5 calibrations They allow trending the performance variables of the sensor to

determine how healthy the sensor is and what work is needed on it before venturing out into the field.

Trended diagnostics enable Plantweb users to take action before the reading is compromised without any intimate knowledge of how a sensor works or what conditions the sensor may have been exposed to.

Results are reduced maintenance and increased measurement uptime.

Smart pH sensor used

in “ Smart ” mode

Smart pH sensor used

in “ Smart ” modeOR

SMART loop: instrument-cable-sensorSMART loop: instrument-cable-sensor

VP8 (or cable)

4-wire Models 56 and 1056

are smart-enabled

2-wire, FF Model 1066 is

smart-enabled

Model 6081pH Smart-enabled

Wireless Transmitter

SMART pH SensorsSMART pH Sensors

Plug and Play- Factory pre-calibrated- Calibrate in lab instead of in field- Can restore to factory values

SMART technology- Automatically trend diagnostics- Capture intermittent sensor problems- SMART signal superimposed on mV signal (like HART)

Simple Migration path- Compatible with previous analyzers- Compatible with previous sensors

Calibration historyCalibration history

Advanced diagnostics Last 5 calibration data

sets for troubleshooting

Calibration data set – diagnosticsCalibration data set – diagnostics

Calibration Data

Time stamp between calibrations

Calibration method

Slope

Offset

Temperature at the time of calibration

Glass impedance

Reference impedance

Current readings!

Calibration HistoryCalibration History

Plug & Play ConveniencePlug & Play Convenience

Conventional approach: Field calibration with buffers

SMART approach: Cal in the lab, Plug & Play in the field

Conventional sensor Field Equipment Smart Sensor Field Equipment

Siemens Water Technologies, WISiemens Water Technologies, WI

• Application: spent caustic, pH ~10-12

• pH sensor: 3500HTVP and 396PVP

• User comment:

“The SMART is somewhat fool proof.  I do like the backward compatibility with it,

because initially we had the wrong probes hooked to the wrong boards, and

everything still worked.  The SMART features obviously didn't, but the probes

themselves all functioned fine. “

Key Indicators of Sensor Performance Key Indicators of Sensor Performance

Plantweb pH measurements provide a complete

view of the operational parameters:

pH reading

raw sensor output

temperature

reference impedance

Glass impedance

RTD resistance

29

Diagnostics - Broken GlassDiagnostics - Broken Glass

Broken Glass Fault pH Glass electrode normally has high impedance of 50-500 Megohm Recommended setting of 10 Megohm will detect even hairline cracks Glass can be cracked at the tip or further back inside the sensor (and

not easily visible)

150 M-

- -

Reference

ElectrodeGlass Electrode

Solution

Ground

3K 0-5 M-

Broken

Glass!

30

Diagnostics - Coated SensorDiagnostics - Coated Sensor

Coated Sensor Fault (Ref Z Too High) pH Reference electrode normally has low impedance of 1-10 KilOhm Reference coating slowly builds up around the junction Setting of 20 KilOhm should not generally cause false alarms

Coated

Sensor!

- -

Reference

ElectrodeGlass Electrode

Solution

Ground

150 M-3K40k-

31

- -

Reference

ElectrodeGlass Electrode

Solution

Ground

60K 1500 M-

Dry Sensor Fault (Glass Z Too High) pH Glass electrode normally has impedance of 50-500 Megohm When sensor is dry there is no continuity between the electrode(s) and

the solution ground so impedance reading is very high Recommended setting of 1000 Megohm will not cause false alarms

Dry

Sensor!

Diagnostics - Non-Immersed SensorDiagnostics - Non-Immersed Sensor

More Advanced DiagnosticsMore Advanced Diagnostics

pH parameters slope, reference offset, glass, and reference impedances change little over bulk of operational life.

When parameters start to change, they indicate that more frequent calibrations will be necessary.

Diagnostics are at their most powerful when they can be compared to the original properties of the sensor.

Example: a pH slope of 54 may not indicate a problem, but a sudden drop in slope from 58 to 54 may indicate a 9 month old sensor will not last much longer.

Trending the electrode slope, reference offset, and reference impedance will show the first sign of problems.

SMART and Trending DiagnosticsSMART and Trending Diagnostics

SMART pH sensors automatically record their

initial conditions and the last 5 calibrations to

make trending easier.

• Predictive maintenance with reference

Impedance trending

• Determine optimum calibration frequency

and predict probe life with pH slope trending

Max pH error per calibration cycle^mV/ pH Change in Slope59 58.9 58.8 58.7 58.6 58.5 58.4 58.3 58.2 58.1 58 57.9 57.8 57.7 57.6 57.5 57.4 57.3

p̂H 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.70.1 # 0.02% # 0.03% # 0.05% # 0.07% # 0.09% # 0.10% # 0.12% # 0.14% # 0.15% # 0.17% # 0.19% # 0.21% # 0.23% # 0.24% # 0.26% # 0.28% # 0.30%0.2 # 0.03% # 0.07% # 0.10% # 0.14% # 0.17% # 0.21% # 0.24% # 0.27% # 0.31% # 0.34% # 0.38% # 0.42% # 0.45% # 0.49% # 0.52% # 0.56% # 0.59%0.3 # 0.05% # 0.10% # 0.15% # 0.20% # 0.26% # 0.31% # 0.36% # 0.41% # 0.46% # 0.52% # 0.57% # 0.62% # 0.68% # 0.73% # 0.78% # 0.84% # 0.89%0.4 # 0.07% # 0.14% # 0.20% # 0.27% # 0.34% # 0.41% # 0.48% # 0.55% # 0.62% # 0.69% # 0.76% # 0.83% # 0.90% # 0.97% # 1.04% # 1.11% # 1.19%0.5 # 0.08% # 0.17% # 0.26% # 0.34% # 0.43% # 0.51% # 0.60% # 0.69% # 0.77% # 0.86% # 0.95% # 1.04% # 1.13% # 1.22% # 1.30% # 1.39% # 1.48%0.6 # 0.10% # 0.20% # 0.31% # 0.41% # 0.51% # 0.62% # 0.72% # 0.82% # 0.93% # 1.03% # 1.14% # 1.25% # 1.35% # 1.46% # 1.57% # 1.67% # 1.78%0.7 # 0.12% # 0.24% # 0.36% # 0.48% # 0.60% # 0.72% # 0.84% # 0.96% # 1.08% # 1.21% # 1.33% # 1.45% # 1.58% # 1.70% # 1.83% # 1.95% # 2.08%0.8 # 0.14% # 0.27% # 0.41% # 0.55% # 0.68% # 0.82% # 0.96% # 1.10% # 1.24% # 1.38% # 1.52% # 1.66% # 1.80% # 1.94% # 2.09% # 2.23% # 2.37%0.9 # 0.15% # 0.31% # 0.46% # 0.61% # 0.77% # 0.92% # 1.08% # 1.24% # 1.39% # 1.55% # 1.71% # 1.87% # 2.03% # 2.19% # 2.35% # 2.51% # 2.67%1 # 0.17% # 0.34% # 0.51% # 0.68% # 0.85% # 1.03% # 1.20% # 1.37% # 1.55% # 1.72% # 1.90% # 2.08% # 2.25% # 2.43% # 2.61% # 2.79% # 2.97%

1.1 # 0.19% # 0.37% # 0.56% # 0.75% # 0.94% # 1.13% # 1.32% # 1.51% # 1.70% # 1.90% # 2.09% # 2.28% # 2.48% # 2.67% # 2.87% # 3.07% # 3.26%1.2 # 0.20% # 0.41% # 0.61% # 0.82% # 1.03% # 1.23% # 1.44% # 1.65% # 1.86% # 2.07% # 2.28% # 2.49% # 2.70% # 2.92% # 3.13% # 3.34% # 3.56%1.3 # 0.22% # 0.44% # 0.66% # 0.89% # 1.11% # 1.34% # 1.56% # 1.79% # 2.01% # 2.24% # 2.47% # 2.70% # 2.93% # 3.16% # 3.39% # 3.62% # 3.86%1.4 # 0.24% # 0.48% # 0.72% # 0.96% # 1.20% # 1.44% # 1.68% # 1.92% # 2.17% # 2.41% # 2.66% # 2.91% # 3.15% # 3.40% # 3.65% # 3.90% # 4.15%1.5 # 0.25% # 0.51% # 0.77% # 1.02% # 1.28% # 1.54% # 1.80% # 2.06% # 2.32% # 2.59% # 2.85% # 3.11% # 3.38% # 3.65% # 3.91% # 4.18% # 4.45%1.6 # 0.27% # 0.54% # 0.82% # 1.09% # 1.37% # 1.64% # 1.92% # 2.20% # 2.48% # 2.76% # 3.04% # 3.32% # 3.60% # 3.89% # 4.17% # 4.46% # 4.75%1.7 # 0.29% # 0.58% # 0.87% # 1.16% # 1.45% # 1.75% # 2.04% # 2.34% # 2.63% # 2.93% # 3.23% # 3.53% # 3.83% # 4.13% # 4.43% # 4.74% # 5.04%1.8 # 0.31% # 0.61% # 0.92% # 1.23% # 1.54% # 1.85% # 2.16% # 2.47% # 2.79% # 3.10% # 3.42% # 3.74% # 4.06% # 4.38% # 4.70% # 5.02% # 5.34%1.9 # 0.32% # 0.65% # 0.97% # 1.30% # 1.62% # 1.95% # 2.28% # 2.61% # 2.94% # 3.28% # 3.61% # 3.94% # 4.28% # 4.62% # 4.96% # 5.30% # 5.64%2 # 0.34% # 0.68% # 1.02% # 1.37% # 1.71% # 2.05% # 2.40% # 2.75% # 3.10% # 3.45% # 3.80% # 4.15% # 4.51% # 4.86% # 5.22% # 5.57% # 5.93%

2.1 # 0.36% # 0.71% # 1.07% # 1.43% # 1.79% # 2.16% # 2.52% # 2.89% # 3.25% # 3.62% # 3.99% # 4.36% # 4.73% # 5.10% # 5.48% # 5.85% # 6.23%2.2 # 0.37% # 0.75% # 1.12% # 1.50% # 1.88% # 2.26% # 2.64% # 3.02% # 3.41% # 3.79% # 4.18% # 4.57% # 4.96% # 5.35% # 5.74% # 6.13% # 6.53%2.3 # 0.39% # 0.78% # 1.18% # 1.57% # 1.97% # 2.36% # 2.76% # 3.16% # 3.56% # 3.97% # 4.37% # 4.78% # 5.18% # 5.59% # 6.00% # 6.41% # 6.82%2.4 # 0.41% # 0.82% # 1.23% # 1.64% # 2.05% # 2.47% # 2.88% # 3.30% # 3.72% # 4.14% # 4.56% # 4.98% # 5.41% # 5.83% # 6.26% # 6.69% # 7.12%2.5 # 0.42% # 0.85% # 1.28% # 1.71% # 2.14% # 2.57% # 3.00% # 3.44% # 3.87% # 4.31% # 4.75% # 5.19% # 5.63% # 6.08% # 6.52% # 6.97% # 7.42%2.6 # 0.44% # 0.88% # 1.33% # 1.77% # 2.22% # 2.67% # 3.12% # 3.57% # 4.03% # 4.48% # 4.94% # 5.40% # 5.86% # 6.32% # 6.78% # 7.25% # 7.71%2.7 # 0.46% # 0.92% # 1.38% # 1.84% # 2.31% # 2.77% # 3.24% # 3.71% # 4.18% # 4.66% # 5.13% # 5.61% # 6.08% # 6.56% # 7.04% # 7.53% # 8.01%2.8 # 0.48% # 0.95% # 1.43% # 1.91% # 2.39% # 2.88% # 3.36% # 3.85% # 4.34% # 4.83% # 5.32% # 5.81% # 6.31% # 6.81% # 7.30% # 7.80% # 8.31%2.9 # 0.49% # 0.99% # 1.48% # 1.98% # 2.48% # 2.98% # 3.48% # 3.99% # 4.49% # 5.00% # 5.51% # 6.02% # 6.53% # 7.05% # 7.57% # 8.08% # 8.60%3 # 0.51% # 1.02% # 1.53% # 2.05% # 2.56% # 3.08% # 3.60% # 4.12% # 4.65% # 5.17% # 5.70% # 6.23% # 6.76% # 7.29% # 7.83% # 8.36% # 8.90%

3.1 # 0.53% # 1.05% # 1.58% # 2.12% # 2.65% # 3.18% # 3.72% # 4.26% # 4.80% # 5.34% # 5.89% # 6.44% # 6.98% # 7.53% # 8.09% # 8.64% # 9.20%3.2 # 0.54% # 1.09% # 1.64% # 2.18% # 2.74% # 3.29% # 3.84% # 4.40% # 4.96% # 5.52% # 6.08% # 6.64% # 7.21% # 7.78% # 8.35% # 8.92% # 9.49%3.3 # 0.56% # 1.12% # 1.69% # 2.25% # 2.82% # 3.39% # 3.96% # 4.54% # 5.11% # 5.69% # 6.27% # 6.85% # 7.44% # 8.02% # 8.61% # 9.20% # 9.79%3.4 # 0.58% # 1.16% # 1.74% # 2.32% # 2.91% # 3.49% # 4.08% # 4.67% # 5.27% # 5.86% # 6.46% # 7.06% # 7.66% # 8.26% # 8.87% # 9.48% # 10.09%3.5 # 0.59% # 1.19% # 1.79% # 2.39% # 2.99% # 3.60% # 4.20% # 4.81% # 5.42% # 6.03% # 6.65% # 7.27% # 7.89% # 8.51% # 9.13% # 9.76% # 10.38%3.6 # 0.61% # 1.22% # 1.84% # 2.46% # 3.08% # 3.70% # 4.32% # 4.95% # 5.58% # 6.21% # 6.84% # 7.47% # 8.11% # 8.75% # 9.39% # 10.03% # 10.68%3.7 # 0.63% # 1.26% # 1.89% # 2.53% # 3.16% # 3.80% # 4.44% # 5.09% # 5.73% # 6.38% # 7.03% # 7.68% # 8.34% # 8.99% # 9.65% # 10.31% # 10.98%3.8 # 0.65% # 1.29% # 1.94% # 2.59% # 3.25% # 3.90% # 4.56% # 5.22% # 5.89% # 6.55% # 7.22% # 7.89% # 8.56% # 9.24% # 9.91% # 10.59% # 11.27%3.9 # 0.66% # 1.33% # 1.99% # 2.66% # 3.33% # 4.01% # 4.68% # 5.36% # 6.04% # 6.72% # 7.41% # 8.10% # 8.79% # 9.48% # 10.17% # 10.87% # 11.57%4 # 0.68% # 1.36% # 2.04% # 2.73% # 3.42% # 4.11% # 4.80% # 5.50% # 6.20% # 6.90% # 7.60% # 8.30% # 9.01% # 9.72% # 10.43% # 11.15% # 11.87%

= 1% = 2% = 3% = 4% = 5%Change in Process

Beginning Slope - Ending Slope = Slope Change

Slope Change * Maximum Change in Process pH = Maximum pH Deviation

Beginning Slope - Ending Slope = Slope Change

Slope change * Max change in pH = Max mV deviation per

calibration cycle

(Max mV deviation / Beginning Slope) = Max pH error

59mV/pH - 55mV/pH = 4mV/pH * 6pH = 24mV / 59mV/pH = 0.41pH

For a typical application ranging from 4 to 10 pH the error from assuming 59

slope instead of 55 could be 0.41 pH units

>>> need to recalibrate

Using DiagnosticsUsing Diagnostics

Instruments ship with the diagnostics turned off

When enabled, default setpoints will generally be ok

Few false alarms when correctly configured

Some problems may not be detectable with online diagnostics

When in doubt, check with buffers

35

Section 2: Modeling and Control

Virtual plant and embedded process models Online identification of titration curve Minimization of project capital cost Cascade pH control Batch pH control Linear reagent demand control Elimination of split range control Model predictive control

36

Embedded Process Model for pH

37

Figure 3-1b: Weak Acid Titrated with a Strong Base

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

0.000 0.500 1.000 1.500 2.000

Reagent / Influent

pH Calculated pH

Weak Acid and Strong Base

pka = 4

Figure 3-1c: Strong Acid Titrated with a Weak Base

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

0.000 0.500 1.000 1.500 2.000

Reagent / Influent

pH Calculated pH

Strong Acid and Weak Base

pka = 10

Figure 3-1e: Weak 2-Ion Acid Titrated with a Weak 2-Ion Base

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

0.000 0.500 1.000 1.500 2.000

Reagent / Influent

pH Calculated pH

Multiple Weak Acids and Weak Bases

pka = 3

pka = 5

pka = 9

Titration Curves can Vary

Slope moderated

near each pKa

pKa and curve

change with

temperature!

Figure 3-1d: Weak Acid Titrated with a Weak Base

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

0.000 0.500 1.000 1.500 2.000

Reagent / Influent

pH Calculated pH

Weak Acid and Weak Base

pka = 4

pka = 10

38

Nonlinearity can cost big money

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

pH measurement error may look smaller on the flatter portion of a titration curve but the

associated reagent delivery error is larger

39

Slope

pH

Titration Curve Matched to Plant

40

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

Modeled pH Control System

41

Start of Step 2

(Regeneration)

Start of Step 4

(Slow Rinses)

One of many spikes of recirculation pH

spikes from stick-slip of water valve

Tank 1 pH for Reagent Demand Control

Tank 1 pH for Conventional pH Control

Influent pH

Conventional vs. Reagent Demand

42

Feed

Reagent

Reagent

ReagentThe period of oscillation (4 x process dead time) and filter time

(process residence time) is proportional to volume. To prevent

resonance of oscillations, different vessel volumes are used.

Small first tank provides a faster response

and oscillation that is more effectively filtered

by the larger tanks downstream

Big footprint

and high cost!

Traditional System for Minimum Variability

Major overlooked

problem is reagent

Deliver delay from

dip tube design

43

Reagent

Reagent

Feed

Reagent

Traditional System for Minimum Reagent Use

The period of oscillation (total loop dead time) must differ by more

than factor of 5 to prevent resonance (amplification of oscillations)

The large first tank offers more cross neutralization of influents

Big footprint

and high cost!

44

Tight pH Control with Minimum Capital

Influent

FC

1-2

FT

1-2

Effluent AC

1-1

AT

1-1

FT

1-1

10 to 20

pipe

diameters

f(x)

*IL#1R

eag

ent

High Recirculation Flow

Any Old Tank

Signal

Characterizer

*IL#2

LT

1-3

LC

1-3

IL#1 – Interlock that prevents back fill of

reagent piping when control valve closes

IL#2 – Interlock that shuts off effluent flow until

vessel pH is projected to be within control band Eductor

45

Linear Reagent Demand Control

Signal characterizer converts PV and SP from pH to % Reagent Demand– PV is abscissa of the titration curve scaled 0 to 100% reagent demand– Piecewise segment fit normally used to go from ordinate to abscissa of curve– Fieldbus block offers 21 custom space X,Y pairs (X is pH and Y is % demand)– Closer spacing of X,Y pairs in control region provides most needed compensation– If neural network or polynomial fit used, beware of bumps and wild extrapolation

Special configuration is needed to provide operations with interface to:– See loop PV in pH and signal to final element– Enter loop SP in pH– Change mode to manual and change manual output

Set point on steep part of curve shows biggest improvements from: – Reduction in limit cycle amplitude seen from pH nonlinearity– Decrease in limit cycle frequency from final element resolution (e.g. stick-slip)– Decrease in crossing of split range point– Reduced reaction to measurement noise– Shorter startup time (loop sees real distance to set point and is not detuned)– Simplified tuning (process gain no longer depends upon titration curve slope)– Restored process time constant (slower pH excursion from disturbance)

46

Cascade Control to Reduce Downstream Offset

M

AT

1-2

Static Mixer

Feed

AT

1-1

FT

1-1

FT

1-2

Reagent

10 to 20

pipe

diameters

Sum

FC

1-1

Filter

Coriolis Mass

Flow Meter

f(x)

AC

1-1

AC

1-2

PV signal

Characterizer

RSP

f(x)

Flow Feedforward

SP signal

characterizer

Trim of Inline

Set Point

Enhanced PID

Controller

Linear Reagent

Demand Controller

Any Old Tank

47

Full Throttle Batch pH Control

Batch Reactor

AT

1-1

10 to 20

pipe

diameters

Filter

Delay

Sub Div

Sum

Dt

Cutoff

Past

DpH

Rate of

Change

DpH/Dt Mul

Total System

Dead Time

Projected

DpHNew pH

Old pH

Batch pH

End Point

Predicted pHReagent

Section 3-5 in New Directions in Bioprocess Modeling and Control

shows how this strategy is used as a head start for a PID controller

48

Linear Reagent Demand Batch pH Control

Batch Reactor

AC

1-1

AT

1-1

10 to 20

pipe

diameters

f(x)

Master Reagent Demand

Adaptive PID Controller

Static Mixer

AC

1-1

AT

1-1

10 to 20

pipe

diameters

Secondary pH

PI Controller

Signal

Characterizer

Uses Online

Titration Curve

FT

1-1

FC

1-1

FQ

1-1

FT

1-2

Online Curve

Identification

Influent #1

Reduces injection and mixing delays and enables some cross

neutralization of swings between acidic and basic influent. It is

suitable for continuous control as well as fed-batch operation.

Influent #2

49

Conventional Fine and Coarse Valve Control

Neutralizer

AC

1-1

AT

1-1

PID Controller

Large

(Coarse)

Small

(Fine)

ZC

1-1

Integral only Controller

(CV is Implied Fine

Control Valve Position)

CV

ZC speed of response must

be slow and tuning is difficult

Must add feedforward for fast

and large influent disturbance

50

Advanced Fine and Coarse Valve Control

manipulated variables

Small (Fine)Reagent Valve SP

NeutralizerpH PV

Small (Fine)Reagent Valve SP

cont

rolled

va

riab

le

MPC Large (Coarse)Reagent Valve SP

cont

rolled

va

riab

le

null

Model Predictive Controller (MPC) setup for rapid simultaneous

throttling of a fine and coarse control valves that addresses

both the rangeability and resolution issues. This MPC can

possibly reduce the number of stages of neutralization needed

51

Key Points

More so than for any other loop, it is important to reduce dead time for pH control because it reduces the effect of the nonlinearity

Filter the feedforward signal to remove noise and make sure the corrective action does not arrive too soon and cause inverse response

The effectiveness of feedforward control greatly depends upon the ability to eliminate reagent delivery delays

If there is a reproducible influent flow measurement use flow feedforward, otherwise use a head start to initialize the reagent flow for startup

The reliability and error of a pH feedforward is unacceptable if the influent or feed pH measurement is on the extremities of the titration curve

Use a Coriolis or magnetic flow meter for reagent flow control Every reagent valve must have a digital valve controller (digital positioner) Except for fast inline buffered systems, use cascade control of pH to reagent flow

to compensate for pressure upsets and enable flow feedforward Linear reagent demand can restore the time constant and capture the investment

in well mixed vessels, provide a unity gain for the process variable, simply and improve controller tuning, suppress oscillations and noise on the steep part of the curve, and speed up startup and recovery from the flat part of the curve

52

Key Points

Changes in the process dynamics identified online can be used to predict and analyze changes in the influent, reagent, valve, and sensor

New adaptive controllers will remember changes in the process model as a function of operating point and preemptively schedule controller tuning

Use inline pH control, mass flow meters, linear control valves, and dynamic compensation to automatically identify the titration curve online

Use gain scheduling or signal characterization based on the titration curve to free up an adaptive controller to find the changes in the curve

Batch samples should be taken only after all the reagent in the pipeline and dip tube has drained into the batch and been thoroughly mixed

Use a wide open reagent valve that is shut or turned over to pH loop based on a predicted pH from ramp rate and dead time to provide the fastest pH batch/startup

Use online titration curve identification and linear reagent demand pH control for extremely variable and sharp or steep titration curve

Use an online dynamic pH estimator to provide a much faster, smoother, and more reliable pH value, if the open loop dead time and time constant are known and there are feed and reagent Coriolis mass flow meters

Use linear reagent demand model predictive control for interacting systems and

constraint or valve position control

53

Section 3: Practical Considerations

Causes and Effects of Drift Common Problems with Titration Curves Effect of Measurement Selection and Installation Options to improve accuracy and maintenance Effect of piping design, vessel type, and mixing pattern Implications of oversized and split ranged valves Online Troubleshooting

Reference Liquid Junction is a Porous “Membrane”– Diffusion Rate Must Remain Constant to Eliminate Drift– Coating, Pressure (flow) changes, chemical reactions interfere

Drift

Process Inside Sensor

Reference

KCl outH2O in

Other Process

Constituents in

Concentration Gradient Through

Reference Junction (Membrane)

Gradient Through Reference When CleanGradient Through Reference When Coated

Difference in the Gradient between Clean and Coated Causes Offset

55

High Today may be Low Tomorrow

A

BA

BA

BpH

time

With just two electrodes, sometimes there are more questions than answers.

Calibration adjustments chase short term effects such as:

–Imperfect mixing

–Ion migration into reference junction

–Temperature shifts

–Different glass surface conditions

–Fluid streaming potentials…

56

Drift effects on Feedforward control

pH

Reagent to Feed

Flow Ratio

4

10

6

8

Feedforward

Reagent Error

Feedforward

pH Error

Sensor Drift

pH Set PointInfluent 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

Normal Condition: inlet pH is 5 and setpoint is 7

Sensor drifts to 4.5 causes overfeed of reagent and outlet to be pH 9

57

Common Problems with Titration Curves

Insufficient number of data points were generated near the equivalence point Starting pH (influent pH) data were not plotted for all operating conditions Curve doesn’t cover the whole operating range and control system overshoot No separate curve that zooms in to show the curvature in the control region No separate curve for each different split ranged reagent Sequence of the different split ranged reagents was not analyzed Back mixing of different split ranged reagents was not considered Overshoot and oscillation at the split ranged point was not included Sample or reagent solids dissolution time effect was not quantified Sample or reagent gaseous dissolution time and escape was not quantified Sample volume was not specified Sample time was not specified Reagent concentration was not specified Sample temperature during titration was different than the process temperature Sample was contaminated by absorption of carbon dioxide from the air Sample was contaminated by absorption of ions from the glass beaker Sample composition was altered by evaporation, reaction, or dissolution Laboratory and field measurement electrodes had different types of electrodes Composite sample instead of individual samples was titrated Laboratory and field used different reagents

58

Horizontal Piping Arrangements

AE

AE

AE

AEAEAE

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 keep

the velocities equal

Series arrangement preferred to minimize differences in solids,

velocity, concentration, and temperature at each electrode!

10 OD10 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

59

Vertical Piping ArrangementsA

E AE AE

AE

AE

AE

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!

60

Options for Maximum Accuracy Select best glass and reference electrolyte for process A hemi-spherical glass electrode and flowing junction reference offers

maximum accuracy, but in practice maintenance prefers:– A refillable double junction reference to reduce the complexity of installation – often

the best compromise between accuracy and maintainability.– A solid reference to resist penetration and contamination by the process and

eliminate the need to refill or replace reference particularly for high and nasty concentrations and pressure fluctuations – takes the longest time to equilibrate and is more prone to junction effects.

Use smart digital transmitters with built-in diagnostics Use middle signal selection of three pH measurements

– Inherent auto protection against a failure, drift, coating, loss in efficiency, and noise (see February 5, 2010 entry on http://www.modelingandcontrol.com/ )

Allocate time for equilibration of the reference electrode Use “in place” standardization on a sample with the same temperature and

composition as the process. If this is not practical, the middle value of three measurements can be used as a reference. The fraction and frequency of the correction should be chosen to avoid chasing previous calibrations

Keep process fluid velocity constant at the highest practical value for clean and responsive electrodes

61

Wireless pH Lab Setup

Wireless pH measurements offer

• Best sensor technology for a wide range of process conditions

• Reduced electrical noise from ground issues

• Predictive diagnostics using smart pH sensors

• Convenient platform to establish specific solution temperature compensation,

develop inferential measurements of process concentrations, and relocate the sensor for best results considering velocity, mixing, delay, &

bubbles

62

Wireless pH 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%

63

Wireless Bioreactor Adaptive pH Loop Test

64

Mistakes in pH System Design

M

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

65

Mixing Pattern and Vessel Geometry

Stagnant

Zone

Stagnant

Zone

MFeed

Reagent

AT

1-3

Short

Circuiting

Stagnant

Zone

Plug

Flow

66

Oversized Reagent Valves

dead band

Dead band

Stick-Slip is worse near closed position

Signal

(%)

0

Stroke

(%)Digital positioner

will force valve

shut at 0% signal

Pneumatic positioner

requires a negative %

signal to close valve

The dead band and stick-slip is greatest near the closed position so valves that

ride the seat from over sizing or split ranged operation create a large limit cycle

Dead band is 5% - 50%

without a positioner !

Limit cycle amplitude is operating point dependent and can be estimated as:

stick-slip (%) multiplied by valve characteristic slope (pph/%) and by titration curve slope (pH/pph)

67

Control Valve Rangeability and Resolution

pH

Reagent Flow

Influent Flow

6

8

Influent pHB

A

Control BandSet point

B

Er = 100% * Fimax * ----

Frmax

Frmax = A * Fimax

B

Er = ----

A

Ss = 0.5 * Er

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

Er = allowable reagent error (%)

Frmax = maximum reagent valve capacity (kg per minute)

Fimax = maximum influent flow (kg per minute)

Ss = allowable stick-slip (resolution limit) (%)

68

Key Points

The pH measurement error may look smaller on the flatter portion of a titration curve but the associated reagent delivery error is larger

The control system should schedule automated maintenance based on the severity of the problem and production and process requirements

pH measurements can fail anywhere on or off the pH scale but middle signal selection will inherently ride out a single electrode failure of any type

Equipment and piping should have the connections for three probes but a plant should not go to the expense of installing three measurements until the life expectancy has been proven to be acceptable for the process conditions

A series installation of multiple probes insures the electrodes will see the same velocity and mixture that is important for consistent performance

69

Section 4: Summary

Extraordinary Sensitivity and Rangeability Deceptive and Severe Nonlinearity Extraneous Effects on Measurement Difficult Control Valve Requirements

Look at the titration curve

pH Control is difficult because of nonlinearity:– Large Amounts of chemical cause little change initially.– Small Amounts cause huge changes near equivalence point.

Titration Curves are essential for pH system modeling

0.90000

0.99000

0.99900

0.99990

0.99999

1.00000

1.00001

1.00010

1.00100

1.01000

1.10000

0123456789

1011121314

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1

pH

ml of base added

Equivalence

Point

Not a

pipette!

Rules of thumb: multiple stages

When the process pH must be changed by more than 2 units:

Use Multiple Stages!

Remember that 2 pH units is a factor of 100 in concentration.

Can you accurately dilute a concentrated acid by a factor of 500 in one step?

pH Hydrogen Ion [H+] Hydroxyl Ion [OH-] 0 Acidic 1 2 3 4 5 6 7 Neutral 8 9 10 11 12 13 14 Basic

1.0 0.1 0.01 0.001 0.0001 0.00001 0.000001 0.0000001 0.00000001 0.000000001 0.0000000001 0.00000000001 0.000000000001 0.0000000000001 0.00000000000001

0.00000000000001 0.0000000000001 0.000000000001 0.00000000001 0.0000000001 0.000000001 0.00000001 0.0000001 0.000001 0.00001 0.0001 0.001 0.01 0.1 1.0

Rules: Mixing

If the sensor does not see a representative sample of the process, it won’t measure correctly.

Don’t try to do all the neutralizing in a pipe! pH reagents can be more viscous than water

and require time to mix and react. Static mixers are good for first stage

treatment, especially in feedforward mode. Achieving a good setpoint will usually require

a downstream stabilization tank.

73

Mixing II

A system normally considered to be well mixed may be poorly mixed for pH control

To be “well mixed” for pH control, the deviation in the reagent to influent flow from non ideal mixing multiplied by the process gain must be well within the control band

Back mixing (axial mixing) creates a beneficial process time constant and plug flow or radial mixing creates a detrimental process dead time for pH control

The agitation in a vessel should be vertical axial pattern without rotation and be intense enough to break the surface but not cause froth

Rules: Holdup time

1. Use sufficient holdup time to balance throughput and efficiency.

2. Prevent short-circuiting by using baffles.

3. Locate tank exit lines to give the reagent the maximum time to react (tanks using heavy pH solutions should overflow, not exit the bottom).

improve performance provide a better location for a feedback

pH loop help prevent overshoot and oscillation

75

Holdup time II

Horizontal tanks are notorious for short circuiting, stagnation, and plug flow that cause excessive dead time and an erratic pH response

To provide isolation, use a separate on-off valve and avoid the specification of tight shutoff and high performance valves for throttling reagent

Rules: Minimize deadtime

Deadtime is the killer of all good control loops.

The response time of a pH sensor depends most on how clean the glass surface is.

Install the sensor in a flowing stream at about 5 feet per second velocity for a self-cleaning action.

Try to minimize extractive sampling since that is another delay and may not provide a representative sample.

77

Deadtime II

The actual equipment dead time is often larger than the turnover time because of non ideal mixing patterns and fluid entry and exit locations

The dead time from back filled reagent dip tubes or injection piping is huge

Rules: Keep your pH sensor clean

The biggest maintenance headache for pH sensors is usually just cleaning them off.

Some sensors are designed to resist coating by providing large reference areas.

Use a retractable sensor when the process cannot be shut down to clean the sensor.

Automatic retraction (and cleaning) devices are available to save on labor costs, but can be expensive.

Rules: Valve selection

Good control valves have a turndown ratio of about 10:1.

Don’t oversize pH control valves! Allow for some hysteresis and

stiction in your valves to prevent overshoot problems.

Don’t try to control too close to the desired setpoint.

pH control obeys the Uncertainty Principle

80

Valve Selection II

Set points on the steep portion of a titration curve require a reagent control valve precision that goes well beyond the norm and offers the best test to determine a valve’s actual stick-slip in installed conditions

Reagent valve stick-slip may determine the number of stages of neutralization required, which has a huge impact on a project’s capital cost

Extreme pH values

pH is a very sensitive measure of acid or base.

When there is a lot of acid or base (i.e. pH over 13 or under 1), there may be more appropriate methods.

Methods based on bulk measurements like electrical conductivity, near infrared, or refractive index may be more accurate since they are linear in concentration.

H+

OH-

H+

H2O

OH-

82

Key Points - Measurement

The time that glass electrodes are left dry or exposed to high pH solutions must be minimized for the best performance from the hydrated gel layer

Most accuracy statements and tests are for short term exposure before changes in the glass gel layer or reference junction potential are significant

The cost of pH sensor maintenance can typically be reduced by a factor of ten with realistic expectations and calibration policies

The first sign of coating on the glass measurement electrode is a large increase in its time constant and response time

The first sign of a non conductive coating on the reference electrode is usually a large increase in its electrical resistance

Non-aqueous and pure water streams require extra attention to shielding, process path length, and velocity to minimize pH measurement noise

83

Key Points - Measurement II

Slow references may be more stable for short term fluctuations from imperfect mixing and short exposure times from automated retraction

The fastest and most accurate reference has a flowing junction but requires regulated pressurization to maintain a small positive flow

The best choice might not be the best technical match to the application but the electrode that gets the best support from maintenance, operations, and vendor

For non abrasive solids, installation in a recirculation line with a velocity of 5 to 9 fps downstream of a strainer and pump may delay onset of coatings

For abrasive solids and viscous fluids, a thicker glass or flat electrode can minimize coatings, stagnant areas, and glass breakage

For high process temperatures, high ion concentrations, and severe fouling, use automatic retractable assemblies to reduce exposure

When the fluid velocity is insufficient to sweep electrodes clean, use an integral jet washer or a cleaning cycle in a retractable assembly

Conclusion

pH is a versatile and powerful analytical technique for characterizing your process

Understanding the nonlinear aspect of pH is key to successful implementation

There’s more to pH control than selecting the “best” pH sensor and tuning a PID loop

Rewards for proper pH management far outweigh the small cost of the installed field equipment

Where To Get More Information

“What’s the Real pH of that Stream?”– http://

www2.emersonprocess.com/siteadmincenter/PM%20Rosemount%20Analytical%20Documents/Liq_Article_61-2111_200503.pdf

Greg’s excellent book– http://

www.amazon.com/Advanced-Measurement-Control-3rd-Edition/dp/1934394432

Emerson Application data sheets– http://

www2.emersonprocess.com/en-US/brands/rosemountanalytical/Liquid/Documentation/ADS/Pages/index.aspx