Lime and Sugar 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. Their innovative approach to manufacturing also enables British Sugar to create a range of co-products from power generation and bioethanol, to animal feed and much more, allowing British Sugar to become one of the most efficient sugar processors in the world. Sugar beet  processing is a complex and demanding process. To consistently produce highest quality product and maintain the lead in efficiency, requires continuous innovation and process enhancements.

Milk of lime measurement

One area, recently improved, 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, 125kg of limestone is consumed 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.

Being such a versatile product, lime is not just used to purify raw materials, it is also often used to clean and neutralise wastewater produced by sugarbeet. The resulting by-products are useful too, as it contains a mixture of lime and organic residues, which are commonly re-used in agriculture as a soil additive and conditioner, rich in organic matter. Most of the 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 into water and it is during this process that an accurate, reliable level measurement ensures consistent production. Originally a differential pressure transmitter was used with a modified back-pressure bubbler system, to try to maintain a clear level measurement. However, maintenance was unpleasant and running costs were still high, from the use of compressed air, accuracy compromised by tube blockages, build up and, of course, the changing density as the lime was added into the water. The exothermic reaction of mixing the alkali and water also produces heavy condensation inside the tank.

The process area requires necessarily high level of safety as the lime is harmful and an irritant to both skin and eyes in particular.  Tank overflows and any consequential clean-up are unpleasant and hazardous. So, any form of level control that can increase reliability and reduce exposure of maintenance personnel in this area of the plant is of interest, and British Sugar were keen to look for a new measurement solution.

VEGA 80 GHz radar level sensor installed very close to the edge of the milk of lime mixing vessel

Less than perfect position

As a result, British Sugar has installed a new 80 GHz radar from VEGA, which is successfully working and following the liquid level, despite the turbulent surface and steamy environment producing high levels of condensation; as well as build up and splashing on the antenna from the milk of lime solution. All outer surfaces of the device are either plastic or stainless steel in construction with a fully encapsulated PTFE antenna system, which means minimal corrosion and a longer operating life for the sensor in this highly alkaline environment.  Another challenge was the ideal mounting position: to minimise splashing and build up on the sensor, it was better situated quite close to the vessel wall – but not an ideal position for a contactless level sensor as the proximity of the wall and any build up will interfere and compromise both performance and accuracy. The unit installed, uses 80 GHz frequency, which means its DN80/3” flange only has a 3 degree beam angle. The result of this is that it handles the situation with ease, the more focused sensor signal beam bypasses the vessel wall and any build up, following the level confidently. VEGA 80Ghz radar possesses some unique performance characteristics, unrivalled 120dB sensitivity, combined with the enhanced focussing and market leading signal processing software means it creates a formidable package. These abilities enable it to cope with challenges like mounting close to vessel edges (through valves or long nozzles), coping with heavy condensation and build up on the sensor face to deliver a reliable measurement, even with a turbulent or agitated process surface.

Benefits

British Sugar at Newark are very happy with the results from this application, an improved reliability level measurement, better quality milk of lime and less maintenance time lost working on instruments or clean ups in the lime mixing plant. This new high frequency VEGA 80GHz contactless radar measuring principle is taking big strides in level measurement and moving into new areas of application for liquids and bulk solids. The technology is changing the way people perceive contactless level measurement, tackling old challenges in a new way by measuring level more reliably.