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Mettler-Toledo Ingold, Inc. Address 36 Middlesex Turnpike
Bedford, MA 01730 Phone 781-301-8800 Customer Service 800-352-8763
Fax 781-301-8920 Internet http://www.mtpro.com
RELIABLE PH MEASUREMENTS IN CHEMICAL PROCESSING USING
PRESSURIZED LIQUID-FILLED ELECTRODES
Mark Barnett Maintenance Supervisor
INEOS Phenol 4770 Rangeline Road Theodore, AL 36582
Kenneth M. Queeney Product Manager
Mettler-Toledo Ingold, Inc. 36 Middlesex Turnpike Bedford, MA
01730
KEYWORDS
pH Electrode, Liquid Electrolyte, Applications, Chemical
Industry
ABSTRACT
The Chemical Processing Industry places a high demand on process
pH electrodes; provide accurate and reliable measurements under
difficult chemical and physical application conditions. A chemical
manufacturer has years of experience evaluating several types of
process pH electrodes for pH control. For most of the critical
applications in challenging environments, pressurized liquid-filled
electrodes have demonstrated superior results over other
technologies. Process pH electrodes are available in three major
categories of reference electrolyte; liquid-filled, gelled
electrolyte, and solid polymer electrolyte [1]. Liquid-filled
electrolytes such as 3M KCl with saturated AgCl have been in use
for decades and have a strong track record. In recent years, gelled
and solid electrolytes have become increasingly popular in on-line
applications. This is due in part to their compact, rugged designs,
and non-refillable electrolyte. However, for demanding
applications, the pressurized liquid-filled electrode is more
resistant to reference junction fouling which is a major cause of
electrode drift and failure. The specific installation requirements
for use of pressurized liquid-filled pH electrodes will be
reviewed. Plant experience in specific process applications
illustrating improved process control through accurate pH
measurement is also discussed.
Copyright by ISA – The Instrumentation, Systems, and Automation
Society Presented at the ISA 2004, 5-7 October, Reliant Center
Houston, Texas
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INTRODUCTION pH is defined as the negative base 10 logarithm of
the hydrogen ion activity [2]: pH = - log10 (H+) (1) The
relationship between H+ and OH- is governed by the dissociation
constant of water: H+ + OH- H2O where, Kw = 10-14 (2) H+, the
active component of acid, and its hydroxide ion counterpart, OH-,
the active component of caustic, have substantial use in
practically all chemical manufacturing facilities. Since the two
ions are inversely related in aqueous solutions, it is obvious why
pH measurement is the most common analytical measurement performed
[3]. pH plays a critical role in many stages of a chemical plant.
At some measurement points, its purpose is to detect breakthrough
of potentially corrosive chemicals. Water discharge is closely
monitored by regulatory agencies, as uncontrolled pH discharges can
wreak environmental havoc in a short period of time. In chemical
reactors, it may directly impact the properties, purity, or yield
of a product. In the most demanding applications, pH monitoring and
control may prevent runaway reactions and their catastrophic
results. pH is most often measured by potentiometric means [4]. In
simplistic terms, a millivolt signal is developed between a
reference electrode and a glass pH measurement electrode. This
voltage varies proportionately to the pH of the solution. Measure
the voltage, and you have an indication of the pH. In theoretical
presentation, the relationship between pH and the resulting voltage
is straightforward. This paper addresses problems that arise in pH
measurement in chemical processes, and a specific reference
electrode design to minimize these problems.
ANATOMY OF POTENTIOMETRIC pH MEASUREMENT OVERVIEW Most process
pH measurements today are performed with a “combination” pH
electrode [4]. The anatomy of a combination electrode is
illustrated in Figure 1. The measured voltage is the sum of several
voltages developed both internally and at the surfaces of the
electrode [5]. Ideally: E1 is constant because the silver/silver
chloride wire is in equilibrium with a constant electrolyte
(typically KCl) saturated with AgCl. E2 is constant, serving as a
salt bridge to complete the electrical circuit E3 is constant
because its silver/silver chloride wire is also in equilibrium with
a constant electrolyte (typically pH 7 buffer) saturated with AgCl.
E4 is a constant potential since the pH of the inner solution is
fixed and buffered. E5 is a variable potential developed at the pH
selective glass/gel interface which responds specifically to the pH
of the measured solution.
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The above “ideal” conditions are not real. While the reference
electrode and the pH measuring electrode are physically combined
into one sensor, the electrodes still function independently, and
characteristic problems remain. This paper focuses on the problems
associated with the reference electrode E1 and E2. Specifically,
the thrust of this paper revolves around the chemical and physical
activity which takes place at the reference junction.
E5
E3
E2
E4
E1
REFERENCEELECTROLYTE
INNER BUFFER
REFERENCE JUNCTION
E MEASURED
E5
E3
E2
E4
E1
pH METER
E MEASURED
E5
E3
E2
E4
E1
REFERENCEELECTROLYTE
INNER BUFFER
REFERENCE JUNCTION
E MEASURED
E5
E3
E2
E4
E1
pH METER
E MEASURED
FIGURE 1 – COMBINATION pH ELECTRODE
REFERENCE ELECTRODE The junction between the reference
electrolyte ameasurement circuit. One function is to complete
blocked either with suspended solid in the samplereadings will
become erratic. Another function isprocess fluid into the reference
electrolyte. As composition surrounding the inner reference wire be
When solutions of dissimilar composition are in contat the
interface. For many solutions found in csignificant. 1 molar HCl in
contact with saturated KC14 millivolts, or nearly 0.25 pH [2].
Liquid junctioelectrode comes to equilibrium with the outside solut
Figures 2 A and B illustrate that junction potentials athen the
reference electrolyte pressure “P1”, processpotential E1. As the
composition of sample changesis added to the first.
Where: E1 = the potential developed at the reference electrode
silver/silver chloridewire. E2 = the “junction potential” E3 = the
potential developed at the “pH cell” silver/silver chloride wire.
E4 = the pH potential developed at the inner surface of the pH
glass bulb E5 = the pH potential developed at the outer surface of
the pH glass bulb E Measured = sum of E1 through E5
ANATONY
nd the sample medium is a critical link in the the electrical
circuit [6]. If this junction becomes , or precipitate reactions
with the electrolyte, the to act as a physical barrier to prevent
ingress of stated above, it is necessary that the chemical
constant to provide a stable E1.
act with one another, a liquid potential is developed hemical
processing industry this voltage may be
l electrolyte produces a liquid junction potential of n
potentials will result in measurement drift as the ion.
re additive. If the process pressure “P2” is greater media will
permeate the junction creating junction , a different junction
potential is developed, which
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Referenceelectrolyte
(P1)
Samplesolution
(P2)
P1 < P2JUNCTION
∆E1
glass
Referenceelectrolyte
(P1)
Samplesolution
(P2)
P1 < P2JUNCTION
∆E1
glassglass
FIGURE 2 A and B– LIQUID JUNCTION POTEN The best means of
keeping the pores of the junction oelectrolyte [7]. This serves
both to clean the junction asample. The flow of electrolyte is
regulated by pressupressure. Such a pressurized liquid electrolyte
combin FIGURE 3 – LIQUID REFERENCE ELECTROLY
Referenceelectrolyte
(P1)
Samplesolution
(P2)
P1 < P2 JUNCTION
∆E1+∆E2
glass
Referenceelectrolyte
(P1)
Samplesolution
(P2)
P1 < P2 JUNCTION
∆E1+∆E2
glassglass
TIAL DEVELOPMENT
pen is to maintain a constant flow of fresh liquid nd provides a
uniform chemical interface with the rizing the electrolyte
reservoir above the process
ation pH electrode is shown in Figure 3.
ELECTRICAL CONNECTOR
ELECTROLYTE FILL HOLE
REFERENCE JUNCTION
pH GLASS
ELECTROLYTE RESERVOIR
ELECTRICAL CONNECTOR
ELECTROLYTE FILL HOLE
REFERENCE JUNCTION
pH GLASS
ELECTROLYTE RESERVOIR
TE COMBINATION pH ELECTRODE
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PROCESS pH APPLICATIONS WASTEWATER NEUTRALIZATION APPLICATION
The primary subject application is wastewater neutralization at
INEOS Phenol, the world’s largest producer of phenol and acetone.
At its Theodore, Alabama site, manufacture is achieved by oxidation
of cumene feedstock to cumene hydroperoxide, which decomposes to
acetone and phenol following reaction with sulfuric acid.
Wastewater comes from the process at a temperature ranging from 40
to 60 oC at 15 psig. Typical sample composition is water-based
containing sulfuric acid with pH 2.5 to 3.5, and organics
concentrations of 2000 ppm to 1% phenol, acetone, methanol and
cumene. Although there are no suspended solids in the sample, the
cumene has a consistency of diesel fuel and has a tendency to coat
onto electrodes, particularly when concentrations run high. The
incoming sample is neutralized using 5% caustic (NaOH) to a pH 7. A
schematic of the installation is shown in Figure 4. The main
process runs through a 3” diameter pipe. A sample side stream is
diverted through a 1” pipe for pH measurement. It is felt that the
increased liquid velocity experienced in the smaller line
contributes to the extended life. This high speed flow prevents the
opportunity for coatings to adhere to the sensor. The side stream
has isolation ball valves
FIGURE 4 – PROCESS FLOW SCHEMATIC OF SIDE STREAM WITH ISOLATION
VALVES
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on either side of the measurement port to allow for sensor
maintenance without shutting down the main process line. Before
going to the organic removal treatment stage, the sample is cooled
using a heat exchanger. Improper pH neutralization will result in a
corrosive liquid that will shorten the useful life of expensive
down-line equipment. This sample stream has presented measurement
challenges for years. Several types of electrode design have been
evaluated escalating from standard single junction, to double
junction, and even a triple junction reference electrode. A review
of more than 3 years operational history indicated that process
upsets played a key role in killing electrodes; with cumene being a
likely culprit, fouling even the triple junction. At best,
satisfactory performance was maintained for 2 months. This was
deemed unsatisfactory. Based on success stories coming from related
plants in Belgium and Germany, the Alabama site implemented the use
of Ingold liquid-filled combination pH electrodes. A photo of this
pressurized, liquid reference electrolyte electrode and housing is
shown in Figure 5. The particular electrode selected has features
deigned for difficult applications, in addition to the flowing
electrolyte. The electrode has two separate liquid junctions rather
than the traditional one, adding increased surface area and reduced
likelihood of plugging. The electrode also uses an internal
“silver-ion trap” which
Protective Cable Cap
Pressure Regulator
Pressurized Electrolyte Reservoir
Process Connection
Process Piping
Protective Cable Cap
Pressure Regulator
Pressurized Electrolyte Reservoir
Process Connection
Process Piping
FIGURE 5 – DETAIL OF PRESSURIZED LIQUID-FILLED ELECTRODE
INSTALLATION
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allows for use of an outer electrolyte free of silver ions. This
eliminates problems associated with silver precipitates forming at
the junction; particularly silver sulfide. Application of this
measurement system has been so successful that present maintenance
requires a two-point calibration only on a quarterly basis. Two
times a week a “walk-by” is performed to check electrode pressure,
liquid electrolyte level and color (to assure no process ingress).
A performance “check” is performed once a week. Rather than the
traditional check with buffers, the history trends of the past week
are carefully reviewed. The electrodes are physically checked only
if anomalies are revealed, such as sluggish response curves or
deviations from periodic grab samples. ADDITIONAL APPLICATIONS
Throughout the Theodore plant there are a total of 14 installations
using liquid-filled pH electrodes. Many are located on direct
processing equipment, exposed to hotter sample and higher pressure
than the wastewater application. Due to the success of the
electrode performance and lifetime, the sample bypass configuration
and electrode are identical throughout. COST SAVINGS Electrode
lifetime has been extended from 2 months to over one year resulting
in electrode replacement cost savings of approximately $1,250/year.
Maintenance labor costs to replace these electrodes add
approximately $900/year savings. Reduced time for checking
electrodes to assure they were operational adds another $900/year
savings. Total material and labor costs savings of over $3,000 per
measurement loop, results in plant-wide savings in access of
$42,000/year.
CONCLUSION Many styles of reference electrolyte systems
commercially available represent a compromise of long term
electrode life and performance accuracy for convenience. A common
negative comment of the liquid-filled reference electrode is that
it requires periodic refilling. Often, as a result of this
requirement, it has obtained the reputation of high maintenance. In
many difficult applications, this is far from the truth. The
pressurized liquid-filled electrodes have demonstrated significant
cost savings per loop, and the confidence that the reading in the
control room is an accurate indication of the solution pH is even
more valuable. At this manufacturing site, the liquid-filled pH
electrode has earned its nickname as “the problem solver”.
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REFERENCES 1. Meier, Peter, Lohrum, Albert, Amman, Jurgen,
Practice and Theory of pH Measurement, Mettler-
Toledo AG, Urdorf, Switzerland, 1989, p. 19. 2. Bates,Roger G.,
determination of pH, Theory and Practice, John Wiley and Sons,
Inc., New York,
1964, pp.17, 41.
3. Bach, Hans, Baucke, Friedrich G.K., Krause, Dieter, editors,
Electrochemistry of Glasses and Glass Melts: Including Glass
Electrodes, Springer, Berlin, New York, 2001, p.21.
4. McMillan, Gregory K., Editor-in-Chief, Process/Industrial
Instruments and Controls Handbook, 5th Edition, McGraw-Hill, New
York, 1999, p. 6.23.
5. Westcott, C. Clark, pH Measurements, Academic Press, New
York, 1978, p. 11. 6. Dreyfus, Robert H., "D 1293-99, Standard Test
Methods for pH of Water", 2002 Annual Book of
ASTM Standards, Vol. 11.01 Water (I), ASTM, West Conshohocken,
PA, 2002, pp. 128-136. 7. Dreyfus, Robert H., "D 6569-00, Standard
Test Methods for On-Line Measurement of pH", 2002
Annual Book of ASTM Standards, Vol. 11.01 Water (I), ASTM, West
Conshohocken, PA, 2002, pp. 906-911.
CONCLUSION
