Radar level measurement in reaction tanks

Measurements of level serve, alongside the monitoring of important process parameters such as temperature and pressure, to optimise processes and cut production costs. Nevertheless, when complicated processes are to be improved, experience and the readiness of the sensor manufacturer and plant operator to cooperate are indispensable needs.

The exact determination of the content of a vessel is of decisive importance in numerous processes. As a result, radar technology has asserted itself as one of the standard measurement methods in very many industries in recent years. The introduction of the two-wire technique and continual improvements in signal processing have been mainly instrumental in the spreading of this method of measurement. Radar is now used in to determine the level of widely different vessels, whether simple storage tanks, batch vessels with a whole spectrum of products or complex reactors with stirrers and accessories.

Despite all the progress in sensor signal processing, however, applications in which radar technology is at its limits crop up time and again. These are the cases where the know-how of the sensor manufacturer and cooperation with the customer are of essential importance. Only such joint efforts can enable the utmost to be brought out of the sensor and make optimal adjustment to the application possible.

Applications in the chemical industry

In the production, diverse basic substances are mixed in a reactor, where a chemical reaction is initiated by heat and applied pressure. During the course of this, the polymerisation of the liquid brings about changes in the physical properties of the mix. These changes are a problem, making an exact level measurement extremely difficult. When a differential pressure sensor is used, the different densities can cause large measurement errors, or make its use very complicated.

The use of a capacitive measurement probe is made impossible by the change in electrical properties such as the dielectric constant, and besides this, the probe would be subject to great mechanical stress because of the stirring and the strong product movement. The use of ultrasound as measuring principle can be excluded because of the high process temperature and the changed gas composition, which would cause a change in the velocity of the sound waves.

Radiometric level measurement, an unpopular but generally reliable measuring principle, was often the only possibility left in the past. For the operator, this meant a high service and maintenance expenditure, alongside the high cost of the sensor. Furthermore, safety zones have to be created around the radiometric emitter, warning notices must be posted and a safety officer for radiation protection must be determined and trained. When a certain life-span of the emitter is reached, it must be disposed of which is an extra cost that should not be underestimated.

The problems and solutions in measuring levels

Problems can arise with this measurement method in polymerisation reactions. During these, the density of the basic material changes. The basis of the radiometric measurements which are in use is the damping of radioactive rays which are passed through the medium. As the measurement is made through the walls of the vessel, base damping overlays the absorption by the medium.

A scintillation counter determines the gamma radiation that arrives at the opposite side of the vessel. When the density of the medium changes, the damping will also change and so introduce an error in the measurement.

The problems were repeatedly discussed with manufacturers of level sensors, but even the new measuring principle "radar" did not seem suitable for this application. That was before companies such as Vega introduced new standards for this type of measurement by with a generation of radar sensors which utilised the user-friendly two-wire principle.

To get a feeling for the limits of these sensors, a test measurement system installed at Wacker Chemie in Germany. Technicians installed a Vegapuls 54 radar sensor with a horn antenna set-up of 150 mm diameter and a transmission frequency of 5.8 GHz. The service technicians used mainly standard parameters in the parameterization of the sensor, which worked on the pulse radar principle. No application-specific software was required.

The display of the echo curve on the PC was particularly helpful during the starting-up phase. It enabled the reflection signals to be monitored over the whole process, and so the reliability of the measurement principle to be judged.

The measurement was found to reliable over the whole measuring range, no measured value variations were found, not even during the polymerisation. The influence of the stirrer paddles was very small, and this was reduced by a software-false echo storage. The only disadvantage was the very poor reflection properties of the product, measurements very near to the antenna system were not possible, a minimum distance of 40 to 50 mm had to be maintained.

Although this measurement system worked distinctly more accurately than the radiometric measurement, it hindered the utilisation of the whole volume of the vessel - a solution that was not completely satisfactory.

Process optimisation with exact level measurement

Sensors from a new radar generation, working at a frequency of 26 GHz, a far higher transmission frequency than that of previous sensors, appeared to be the optimal solution for applications requiring the shortest possible minimum distance. They allowed a distinctly smaller antennas to be used to obtain the same focussing of the radar signals as at 5.8 GHz, and besides this, the accuracy of the sensors had in the meantime been clearly increased.

An exact and reliable determination of the content of a vessel allows, as in the case discussed, an optimisation of the course of the process. The process cycles of the polymerisation were optimised and, because of the better utilisation of the volume of the vessel, the product output increased. Overall, a worthwhile expenditure, as an exact level measurement optimizes the course of the process and enables the production of a high quality product at a favorable cost.

Powerful sensors are the prerequisite for the certainty of measurements, but the working together of people is the decisive point for the solution of such problems.