Application note for Chlor Alkali processes

1. INGOLD
Leading Process Analytics
Precise and Reliable pH Measurement
Application Note
in the Chlor-Alkali Process
For the purpose of yield on one side, and equipment
integrity on the other, accurate pH control is of the
utmost importance throughout the chlor-alkali mem-
brane process. Elevated temperatures and the pres-
ence of oxidants typically make the measurement of
pH problematic as they affect the performance of pH
electrodes. The new InPro 4850 i pH sensor uses a
pNa glass membrane in its reference electrode. This
greatly increases measurement reliability as the
sensor is unaffected by the chlor-alkali process
conditions.
Background
Chlorine is one of the most important building blocks in chem-
istry and is used in a large number of manufacturing proce-
dures. Production of chlorine from the chlor-alkali process is
through the electrolysis of brine (a saturated sodium chloride
solution). Although three production processes exist: mercury
cell, diaphragm cell and membrane cell, the latter process is
nowadays preferred for environmental and energy consump-
tion reasons and will eventually replace the other two tech-
niques.
Process
In the membrane cell process, saturated brine enters an elec-
trolysis cell where, at the anode, chloride ions are oxidized to
chlorine gas which is collected at the top of the cell. The sodi-
um ions in the brine diffuse through an ion exchange mem-
brane and enter the cathode side of the cell. Here, water from
the solution is hydrolyzed, forming hydrogen and hydroxide
ions. The sodium and hydroxide ions combine to form caustic

2. soda. Both the hydrogen and caustic soda are sold as by-products. the dechlorination treatment the pH is lowered to 2 or less, which
Precise and Reliable pH Measurement
The depleted brine from the anode compartment is resaturated allows the remaining chlorine to become gaseous and be ex-
with fresh brine and is circulated back to the electrolysis cell. tracted. Chlorate in the brine can be converted to chlorine under
acidic conditions or is reduced at high pH.
The costly ion exchange membranes are typically made of per-
fluorinated polymers and although both sides are exposed to a The chlor-alkali process is very tough on conventional pH elec-
chemically aggressive environment, they can last for several years trodes. It exposes them to high temperatures, and clogging and
with correct treatment. poisoning from a variety of compounds. This is particularly true
in the anode side of the electrolysis cell. Here, chlorine diffuses
pH value is extremely important throughout the chlor-alkali pro- through the electrode’s diaphragm and attacks the reference sys-
cess, particularly in the electrolysis cell. On the anode side the re- tem. This results in incorrect pH measurement and shorter sensor
action takes place under acidic conditions through the addition of lifetime. Up until now, the most reliable pH electrodes for chlor-
hydrochloric acid. Though very low pH (< 3) leads to higher yield, alkali use have been those with pressurized reference electrolyte.
it has a detrimental effect on the life of the cell membrane, leading The overpressure prevents chlorine diffusing into the electrolyte
to regular and expensive membrane replacement. Chlorate (ClO3–) and poisoning the reference. Such electrodes require special in-
formation during the process is unavoidable and undesirable. They stallation accessories such as a pressurizable housing, and de-
reduce the solubility of salt and negatively impact chlorine yield. mand regular cleaning and calibration if measurement accuracy
Over pH 4, chlorate formation increases significantly. To maintain is to be maintained.
a balance between chlorine yield and membrane life, pH of the
anolyte is usually controlled in the range 3 – 4. Reliable pH measurement with pNa reference
The InPro 4850 i from METTLER TOLEDO is a dual-membrane pH
Impurities in the brine will negatively affect electrolysis and mem- electrode that has been designed to provide long-term accurate
brane performance. Therefore, before entering the cell the brine measurement in chlor-alkali processes. The main difference in
goes through an extensive purification process where undesirable measuring technology between the InPro 4850 i and conventional
components are removed via precipitation filtration. By adding pH electrodes is the presence of a sodium-reference (pNa) system.
various salts as precipitants and increasing the pH carefully in The electrode features a sodium-sensitive glass membrane which
steps to pH 10 – 12, impurities such as sulfates, carbonates and is charged by the sodium ions in the brine. The sodium concentra-
hydroxides of calcium, barium, magnesium and other metals are tion in the brine is used as a reference. The pNa reference system
allowed to precipitate. Following filtration the brine passes through is hermetically sealed; there is no diaphragm, therefore no oxi-
an ion exchanger for polishing to remove other impurities. dants can enter the electrode and attack the reference system. The
electrode also features a high-alkali resistant pH membrane glass
The depleted brine leaving the membrane cell contains some dis- for pH measurement. It is the amalgamation of pH measurement
solved chlorine and chlorate, both of which need to be removed. In and pNa reference that is one reason that the InPro 4850i is
highly suited to chlor-alkali processes.
1
The new InPro 4850 i dual-membrane pH electrode:
1 pH glass membrane
2 Sodium glass membrane
2
2 METTLER TOLEDO Application Note

3. Precise and Reliable pH Measurement
Other pH / pNa electrodes are available, but all suffer from the same These tools mean sensor maintenance or replacement can be con-
inherent problem. As both the pH and the reference side of the sen- ducted as and when required, rather than by a fixed and perhaps
sor use glass membranes, a double high impedance output is gener- over-enthusiastic schedule.
ated. This signal is extremely sensitive to electrical interference and
causes the pH measurement to be very unstable. Even approaching Plug and Measure
or touching the sensor or cable can be enough to cause a change in InPro 4850 i can be pre-calibrated using two METTLER TOLEDO
the measuring signal. Additionally, the high impedance limits the pH buffer solutions with fixed sodium concentrations. Pre-calibra-
distance between the sensor and the transmitter. tion is achieved using an ISM-equipped transmitter or iSense Asset
Suite software* running on a standard computer / laptop. Pre-cali-
The InPro 4850 i offers a unique solution to this problem: it outputs bration can be made in a convenient location away from the pro-
a digital signal. An integrated microprocessor in the sensor head cess, and calibrated sensors can be stored until they are required.
digitizes the measurement data which it exchanges with the con-
nected transmitter. Not only is the digital measurement signal Upon connection to an ISM transmitter, a pre-calibrated sensor is
more precise than an analog signal, impedance problems are to- instantly recognized and the transmitter configures itself appropri-
tally eliminated. Further, the signal is unaffected by moisture, ately. Plug and Measure functionality means a sensor can be in-
cable length or electrical interferences from adjacent equipment. stalled and be ready to measure in under a minute, therefore sub-
stantially increasing production capacity by avoiding long interrup-
The combination of diaphragm-less sodium reference for mea- tions, and allowing operators to concentrate on more critical tasks.
surement precision, and digital transmission for signal integrity
means that the InPro 4850 i dual-membrane pH electrode provides iSense – a powerful software tool for ISM sensors
exceptional accuracy and durability with prolonged intervals be- iSense Asset Suite is
tween calibrations. a user-friendly soft-
ware tool which
Intelligent Sensor Management allows verification
The InPro 4850 i features METTLER TOLEDO’s and calibration of
Intelligent Sensor Management (ISM) technology. ISM improves ISM pH and oxygen
sensor handling, enhances reliability and reduces sensor lifecycle sensors under labo-
costs. A few of ISM’s features are outlined below. ratory conditions in a main-
tenance shop.
Predictive diagnostics
The microprocessor in the head of ISM sensors constantly moni- Conclusion
tors process conditions and sensor “health”. Using current and The InPro 4850i dual-membrane pH electrode removes the barri-
stored historical data, the sensor outputs a number of useful diag- ers to repeatable, accurate pH measurement and extended sensor
nostic tools which are displayed on the transmitter: life. By combining a sealed reference system with the digitized
• Dynamic Lifetime Indicator (DLI) signal of Intelligent Sensor Management along with ISM’s other
Max. °C
Days of operation
The DLI estimates in real-time the remain-
ing lifetime of the sensor. By knowing when DLI
Dynamic Lifetime Indicator
ACT MAX
significant advantages, the InPro 4850i provides unequalled mea-
suring performance, low maintenance and exceptional durability.
Adaptive Calibration Timer Max. Temperature/ODI
a sensor is going to fail, a replacement can be
fitted before process safety is compromised. * Pending
• Adaptive Calibration Timer (ACT)
TTM CAL
Max. °C
Days of operation
DLI ACT
Predicts the time until the next calibration Time to Maintenance
will be required, based on process conditions Adaptive Calibration Timer
Dynamic Lifetime Indicator
MAX
For more information, visit:
Calibration History
4www.mt.com/InPro4850i
Max. Temperature/ODI
and probe status.
Mettler-Toledo AG
Process Analytics TTM
Time to Maintenance
CAL
Calibration History
www.mt.com/pro
For more information
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CH-8902 Urdorf
Switzerland
© 09/2011