Effective level control in milk of lime mixing vessel
As the sole processor of the UK’s beet sugar crop, British Sugar work in partnership with 3,500 growers and customers to deliver a world-class product that is made to the highest standards every day. Processing around eight million tonnes of sugar beet and producing up to 1.4 million tonnes of sugar each year, they are the leading producer for the British and Irish food and beverage markets.
Sugar beet processing is a complex and demanding process. To consistently produce the highest quality product and maintain the lead in efficiency, the organization requires continuous innovation and process enhancements. One recent improvement at its Newark production site is the production and mixing of milk of lime. This is an essential formulation for use in sugar production when purifying the juice from beet or cane.
Typically, 125 kilograms of limestone is used in the production of one ton of sugar, so it is a significant process input. Sugar beet is sliced up and passed through a diffuser to extract the sugar juice. Lime, produced from calcium limestone, is used in the next stage of the production process. It is converted into milk of lime and this is used to capture and remove impurities in the juice of sugar beet. A high quality lime-water mixture is required in the sugar process to ensure an efficient, high quality sugar juice purification stage.
Most sugar-processing plants have their own lime kilns, and they require good, high purity limestone to burn. This ‘high calcium’ limestone is converted into quicklime in lime kilns, at a temperature of 900°C. To obtain milk of lime, the ‘calcined’ lime is mixed with water, and it is during this process an accurate, reliable level measurement ensures consistent production.
Reduction of operating costs and maintenance effort
A differential pressure transmitter with a modified back-pressure bubbler system was originally used to try to maintain a clear level measurement. However, maintenance was unpleasant and running costs were still high due to the use of compressed air. Accuracy was constantly compromised by tube blockages, build up, and of course, from the changing density as the lime was added into the water. On top of all this, the exothermic reaction of mixing the alkali and water produces heavy condensation inside the tank.
The process area requires a necessary high level of safety as the lime is harmful and an irritant to both the skin and the eyes. Tank overflows and any consequential clean-up are unpleasant and hazardous. Any form of level control that can increase reliability and reduce exposure of maintenance personnel in this area of the plant is essential, and British Sugar was keen to look for a new measurement solution.
Buildup and difficult installation position are no problem with the high measurement frequency of 80 GHz
VEGA 80 GHz radar level sensor installed very close to the edge of the milk of lime mixing vessel.
Finding the ideal mounting position proved to be another challenge. To minimise splashing and build up on the sensor, it needed to be situated quite close to the vessel wall – not an ideal position for a contactless level sensor. The proximity of the wall and any build up can interfere and compromise both performance and accuracy. The VEGAPULS 64, however, uses 80 GHz frequency, which means its DN80/3” flange only has a 3 degree beam angle. The result: the radar sensor handles the situation with ease. A more focused sensor signal beam bypasses the vessel wall and any build up, following the level confidently.
VEGA 80 GHz radar possesses some unique performance characteristics, like an unrivaled sensitivity, using a dynamic range of 120dB. Combining the sensitivity with the enhanced focusing and market-leading signal processing software creates a formidable package. These abilities enable it to overcome challenges like mounting close to vessel edges or through valves and long nozzles and aimed at a turbulent or agitated process surface. This radar sensor can even make a reliable measurement when heavy condensation and build-up are on the sensor face. This application just happened to deal with all of these at once.
British Sugar at Newark are very happy with the results from this application. An improved, reliable level measurement ensures better quality milk of lime and less maintenance time lost working on instruments or clean ups in their lime mixing plant.
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