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Fully automated pH measurement

Economical and safe process control
Fully automated pH measurement

Processes with highly corrosive, solvent-based, abrasive, fibrous or sticky media are inevitably maintenance and cost-intensive. Fully automated solutions seem to be a logical consequence. However, conventional systems have often failed when it comes to connecting the process. A complete system for fully automated pH measurement consisting of the Ceramat remote calibration probe, the Protos modular process analysis system and the Unical control unit has meanwhile proved its process suitability even under extreme conditions.

Dr. Dirk Steinmüller

Processes with difficult media are a challenge for every automatic calibration probe. Classic retractable probes made of metal or plastic with dynamically loaded O-rings are wear-prone and maintenance-intensive and have tended to represent an obstacle to the broad acceptance of fully automated pH measurement systems. The aim has therefore been to develop a remote calibration probe which is virtually wear-free and manages without dynamically loaded seals.
Remote calibration probe
The heart of the Ceramat WA 150 automatic remote calibration probe is its ceramic interior and the associated seal between the calibration chamber and the process. The wear-prone O-ring seal is omitted. Impurities which get into the sealing mechanism no longer cause a probe failure. This opens up a broader field of system applications, including processes with solid-containing media or strong impurities where conventional seals have regularly failed. This is illustrated by the following three examples.
Typical applications
The process medium in a phosphorylation process during herbicide production temporarily contains phosgene as well as organic solvents. It is therefore extremely aggressive and toxic. Neither Hastelloy nor plastic retractable probes have been able to provide a viable solution. Up to now, the Knick ceramic process gate has shown absolutely no signs of disturbances such as corrosion, leakage or component swelling.
The combination of corrosion-resistant and high-quality but stationary plastic (Peek or PVDF) and a moving ceramic part located inside the Ceramat remote calibration probe has also proved advantageous in the chlor-alkali electrolysis process. The media to be measured at the residue filter contain active chlorine (sodium hypochlorite, pH 8.5). Here, conventional probes began to corrode after 2 weeks, even if made of the special Hastelloy material.
A third typical application is the production of dye pigments. After operation under rough process conditions, the open measuring chamber exhibits no abrasion or deposits from the pigment-containing media which have a tendency to precipitate. This is a further design advantage compared to conventional probes. The pH sensors are not chambered in a guiding tube as is usually the case. They are shadow-free in suspensions and can be cleaned automatically and very efficiently.
Easy maintenance under process conditions
Seals in conventional retractable probes are doubly stressed when used in solvent-based media: chemically by swelling and etching and mechanically by the displacement mechanism. The ceramic process gate is not subjected to mechanical stress and the seals can be replaced quickly and easily under process conditions. This considerably increases the system availability. If required, the complete drive unit can be replaced or serviced on the site at full process pressure. The complete unit can also be mounted overhead.
The superiority of the hard, highly polished ceramic also reveals itself in fibrous media, e.g. during pulp processing. Even after 200,000 strokes, the Ceramat probe does not show any recognizable wear. When the sensor is retracted, the fibres are smoothly cut. A classic ball valve, on the other hand, attracts fibres, develops leaks and may even shear off a tightly fixed pH glass electrode. This is prevented in the Knick probe by a double locking mechanism.
Control unit
In addition to the sampling unit, the control unit and the measuring system itself must of course also be designed for rough process conditions. Suitable media are required for automatic cleaning and calibration. The piston-free expansion pumps used for this purpose operate virtually without wear. The modular design allows easy replacement on the site. The system automatically identifies the number and types of pumps. Human error is impossible. At the same time, the use of separate pumps ensures redundancy and avoids entrainment of different media, as can occur when a single pump is used with common hoses. Only 25 ml of calibration solution is required for each calibration cycle. This is less than would be needed for a typical manual calibration.
Safety
In critical processes, safety and increased process availability play the most important role alongside reduced costs for maintenance and manual sampling. A complete system of the kind described here must therefore not only be easy to maintain but also meet safety requirements, including explosion protection if required – also for non-electrical apparatus according to the 94/9 EC directive. The following safety measures, among others, have consequently been implemented in the Knick system:
  • Service (maintenance) switch with override function and indication of safety status
  • Redundant safety pressure monitoring
  • Rinse medium pressure monitoring with alarm function
  • Flooding sensor in the housing with alarm function
  • Fully encapsulated electronic components to prevent corrosion and protect against humidity
  • Identification and signalling of false media (cleaning/calibration solution, process medium)
  • Software with electronic signature according to FDA 21 CFR Part 11 (Audit Trail) for applications in the pharmaceutical industry
  • Maintenance and failure messages in plain text with complete storage of data history
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