Flow, level & control
VEGA 80 GHz radar level sensor installed very close to the edge of the milk of lime mixing vessel.
Effective level control in milk of lime mixing vessels
Here, Doug Anderson, VEGA, describes a level measurement solution that is benefitting British Sugar and resulting in better quality milk of lime
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Doug Anderson, marketing manager, VEGA
s the sole processor of the UK’s beet sugar crop, British Sugar works 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, it is the leading producer for the British and Irish food and beverage markets. British Sugar’s innovative approach to manufacturing also enables it 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. Consistently producing the highest quality product and maintaining the lead in efficiency requires continuous innovation and process enhancements.
Milk of liME MEASurEMEnt One area, recently improved, at British Sugar’s Newark production site is the
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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 a
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
October 2018 Instrumentation Monthly
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