Europe’s largest production facility for ammonia and nitrogen fertiliser in Sluiskil, The Netherlands, produces up to 4000 tonnes of nitric acid per day to make nitrogen fertilisers. The plant has an automated filling station for transferring the nitric acid onto tanker trucks where the driver uses a memory card to enter the quantity and concentration into the station’s control system. The automatic filling requires both concentration and mass flow measurement.
These functions are usually implemented with mass flow meters that utilise the Coriolis effect. These meters measure the mass flow and the density, which is proportional to the concentration, by means of phase shifts in the vibration of the measuring tube and changes in the resonance frequency. The filling station at Sluiskil is equipped with such a Coriolis measuring system, however, nitric acid is a challenging fluid even for robust measuring methods and rugged instruments. Sluiskil experienced frequent process disruptions and failures of the measurement systems which had a negative impact on the performance of the plant. As a result, the production engineers started to look for alternative methods in order to ensure the continuous availability of the filling station. Unexpectedly, they found the solution in ultrasonics.
Acoustic process analytics
Ultrasonic flow measurement with transducers clamped to the outside of pipes is a well-established technology, particularly in the chemical industry, where aggressive and toxic fluids are often encountered. Much less common, however, is the application of this technology in the field of process analysis eg, for concentration measurement. For flow measurement, the difference of the transit times of an ultrasonic signal in and against the direction of flow is determined and used to calculate the flow velocity. Concentration measurement on the other hand uses the signal’s transit time to determine the specific speed of sound for the fluid. The speed of sound in a fluid depends on the fluid’s adiabatic compressibility and density, and also on its temperature. It is a material-specific quantity, so the mass flow can be calculated from the volume flow and the density. Determining the actual mass flow, therefore merely requires combining these two ultrasonic measurement functions. As clamp-on transducers are not in direct contact with the fluid, ultrasonic technology is particularly suitable for the measurement of challenging materials. Because of positive experiences with Flexim’s Fluxbus ultrasonic flow meters, the Sluiskil engineers presented their idea to Joost van Parreeren, director of the Benelux branch of Flexim.
Unlike ultrasonic volume flow measuring equipment, which can be set up in less than half an hour, concentration and mass flow required more extensive measurement preparation. Until now, there was little data on sound velocity as a property of the characteristics of this type of material. It was therefore necessary first to determine a set of curves representing the relationship between concentration and the speed of sound at various temperatures. If a good measuring effect was observed in the concentration range of interest, the method would be deemed suitable for use in the process. Laboratory tests demonstrated that, in the relevant range of measurement, it was possible to determine the concentration of nitric acid by measuring the speed of sound. The Flexim laboratories in Berlin set about determining the characteristic concentration curves for nitric acid and the corresponding coefficients were transferred to the Piox S transmitter.
However, initial installation of ultrasonic measuring equipment in the process resulted in disappointment. It seemed the effort had been in vain.
Achieving success through quick temperature measurement
“The first measurement results came as a very unpleasant surprise,” recalls van Parreeren. Naturally, the initial reaction was to question the accuracy and reliability of the ultrasonic technology. “However,” van Parreeren however insisted, “ultrasonic measurement is not imprecise.” Closer inspection revealed that the difficulties did not originate from the sound velocity measurement.
The tank plant is equipped to deliver nitric acid at various levels of concentration, with 60 and 68% being most common, the lower concentration achieved by dilution with water. The dilution process is however exothermic, causing the temperature of the liquid to rise and it was found that the inability to measure this change in temperature accurately was the weak link in the system. The Pt 100 clamp-on probes that were initially used measured the temperature at the outer surface of the pipe, which made them unable to respond quickly enough to rapid temperature changes of the liquid. The manufacturer of the measuring equipment advised the plant engineers to refit the measuring point with a built-in thermo-element.
The installation of a special T-type precision thermo-element proved to be successful and using the correct temperature data, the Piox S transmitter delivered the correct mass flow results. “Everybody at Sluiskil is delighted with an excellently adapted measuring system and the successful implementation of genuine multiparameter measurement,” says van Parreeren. “At a comparatively low cost, the system measures the concentration in addition to the volume and mass flow without being worn out by the aggressive fluid.”
Further use of this adapted ultrasonic flow measurement technology is planned by by the engineers at Sluiskil.
For more information contact Peter Jones Electronic Equipment, +27 (0)11 608 2944, [email protected], www.peterjones.co.za
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