The Shell Rheinland Refinery in Germany recently decided to operate a Knick automatic pH measuring system, available from Gas Alarm Systems , instead of their usual system with permanently installed sensors to measure and analyse the pH levels within their flue-gas desulfurisation plant.
After the fuel extraction process at the Shell refinery, fractions of heavy oil are left which are then used to operate an oil-fired power station that supplies the site. In the associated flue-gas desulfurization plant, the waste-water treatment is monitored at three points of measurement with pH values averaging between two and ten. The deposits occurring in this pH range place large amounts of stress on the points of measurement and require expensive and regular maintenance. Automation systems fail as metallic retractable fittings corrode, moving parts become blocked due to severe incrustation, and conventional plastic fittings fail due to the mechanical and thermal loads.
The new, robust Knick automatic pH measuring system, comprising the Ceramt WA 150 sensor lock-gate, the Protos 3400 analyser, and the Unical 9000 automated cleaning and calibration system not only cleans regularly it automates calibration and permits access for maintenance while the plant is still in operation.
The wet scrubbing process is usually used in flue-gas desulfurization plants, whereby an alkaline solution, often lime water, is used to absorb the sulphur dioxide. It is sprayed from above in the absorber tower or scrubber while the flue-gas is channelled upwards in a counter flow. The reaction creates a lye, from which gypsum can be extracted through precipitation and flocking. The left over excess water is then neutralised by adding sulphuric acid to bind it with the excess lime.
These process conditions pose challenges to the pH measurement points. The 2%-15% of solids content and the alkaline scrubbing solution together encourage the formation of clogging, deposits, and incrustation on the fittings, with the scrubbing solution in particular shortening the life of the electrodes.
Due to this heavy build-up of deposits, the pH sensors have to be cleaned often, making an automated pH measuring system from Knick a much more efficient and simple solution.
The Ceramat WA 150 sensor lock-gate combined with the Unical 9000 automatic cleaning and calibration system facilitates the total automation of such a tricky pH measuring task. These systematic solutions increase reliability and minimise maintenance costs by the service switch with check-back of the actual probe position and the separate media routing, which requires minimum buffer consumption and prevents backmixing of the media. The system is connected to the process control technology through a Profibus PA interface.
The Ceramat WA 150 sensor lock-gate aids the regular and automatic cleaning and calibration of the sensor. The sensor can remain in the rinsing chamber to protect and extend its life, needing to be only briefly inserted into the process medium to carry out measurements at a duration and rate of ten seconds per minute.
The probe is also easy to maintain. The pneumatic drive can be replaced on site under full process pressure, and the rinsing chamber and all relevant seals can be accessed without special tools. The Ceramat WA 150 sensor lock-gate comprises an almost indestructible, rotating ceramic element, which due to its hardness, level sealing surfaces do not wear even in the most abrasive of media.
The Ceramat WA 150 also features a corrosion-resistant, carbon fiber reinforced, fixed PEEK plastic housing, as well as lapped and polished sealing surfaces for high and constant tightness.
The Unical 9000 automatic cleaning and calibration system, powered via the Protos analyser, serves as an electro-pneumatic controller for both the insertion and retraction of the sensor, and the pumps for the buffer and cleaning solutions.
Calibration of the sensor is fully automatic and takes place by means of preset or plant-specific program flows, which significantly reduces maintenance costs. The Unical 9000 has a reliable pump system with three separate pumps with separate media routing for buffer, cleaning, and rinsing solutions. This prevents contamination of the media thus avoiding the costly purging of the lines, while increasing measuring accuracy and economic use of the media.
All process data taken by the Knick automatic pH measuring system is shown on the display of the Protos 3400 analyser. Protos enables the integration of different measuring functions, with three slots available for the various measuring and add-on modules.
The SmartMedia card aids the saving of up to five parameter sets. These memory cards can be used to almost limitlessly expand the capacity of the integrated measurement recorder and the logbook. A Profibus PA interface facilitates the digital connection to the control technology.
The Protos 3400 analyser’s user interface is clearly structured, using neat icons and plaintext menus and explanations in six available languages. The graphical representation in the form of a sensor network diagram, allows all important sensor data such as slope, zero point, reference impedance and glass impedance to be viewed and understood at a glance.
Furthermore, Protos facilitates the definition of individually accessible operating levels protected by pass codes. As a result every user only has access to the device functions relating to their duties, which in turn benefits operational and functional security.
Shell chose to initially operate the Knick automatic pH measuring system on a test basis in parallel with its current system with permanently installed sensors, and after a few hours the test values differed significantly from each other as the permanently installed sensors became polluted, while the use of the Ceramat WA 150 sensor lock-gate provided reliable results.
Furthermore, in long-term operation with conventional maintenance intervals, the sensor consumption proved to be disproportionately higher than the savings from the cheaper installation.
Since the conversion, the availability of the system has reliably been at almost 100%, in contrast to approximately 5% to 10% with the previous system.
In the automatic pH measuring system, Knick has developed a complete solution characterized by low operating costs, maximum availability, minimal maintenance expenditure and optimal process management.
The Knick system is also recommended for use in other applications in the chemicals, pharmaceutical and foodstuffs industries as it's measuring accuracy and reliability during operation, resistance to contamination and suitability for problematic media all increase process safety and open up applications for first time automation.