Seasonal sweetness

Originally built in 1925, British Sugar's Wissington factory in Norfolk is the largest sugar beet processing factory in Europe, with a daily output of crystal sugar reaching up to 1500 tonnes. But, unlike other processes such as those in the oil and chemical industries, that output is largely dependent on a seasonally fluctuating supply of feedstock.

British Sugar sources its beet for Wissington from around 1800 growers. During the peak harvesting period between October and February, these farmers deliver up to 900 lorry loads a day to the factory, which can process over 16,000 tonnes of beet a day. The sugar, produced in a series of evaporation and crystallisation stages, is stored in seven silos with a combined storage capacity of 97,000 tonnes. Some of the processed beet, however, is stored as thick juice in ten large tanks for later refining into crystal sugar during the late spring and early summer periods.

Because the factory operates 24 hours a day throughout the processing 'campaign' - which lasts on average 22 weeks - and a further 22-week juice refining period, there is only a small window in which plant modifications and updates can be carried out.

This tends to mean that developments have to take place in a piecemeal fashion, rather than as the one major refurbishment or revamp that is common in continuous process industries such as oil and petrochemicals, which can schedule lengthy shutdown periods often years in advance.

One of the most significant recent developments at Wissington followed a troublesome 2001/2002 campaign that typified the vagaries of the seasons. A wet spring in 2001 led to late sowing of the beet by the farmers, but the subsequent growing season produced a crop of unusually high purity.

Good news for British Sugar, you might think, but the factory in fact only averaged 75 per cent of its potential throughput for that season.

The problem centred on the purification stage of the process. Sugar 'juice' is initially extracted from the chopped-up beet by diffusion - basically, heating the beet in water at 70 degrees C. The raw juice produced by this process is then mixed with milk of lime and the mixture is carbonated by the addition of CO2. This causes calcium carbonate to be produced, which precipitates out taking most of the impurities in the juice with it. The precipitated CaCO3 is removed by filtration and sold on as LimeX, a soil-improving agent.

What engineers at Wissington discovered, however, was that the lime particles produced by the high purity beet were abnormally small and blinded the filters, bringing the factory to a stop. British Sugar scientists traced the problem to the high alkalinity ratio of the beet. The high purity was due to low amino nitrogen levels in the beet, but sodium and potassium levels were normal, making the alkalinity ratio unusually high.

Problem solved

After research done during the campaign, two techniques were identified that would enable Wissington, and all other British Sugar factories, to handle such high purity beet in the future. The first was simply to put in additional recycle with agitation, while the other was to add separately prepared precipitated calcium carbonate solution (PCC) to increase the porosity of the filter cake.

The £2million capital investment at Wissington to implement these process changes involved the building of a new 70ft high by 20ft diameter purification vessel. This tank provides for much greater agitation and longer retention times, which increases the size of the lime particles produced. In addition, a reaction vessel to produce PCC has been built on site so that PCC can be added to the purification process when the alkalinity ratio of the incoming beet reaches high levels.

Despite those problems, Wissington still produced over 1.2million tonnes of sugar that year, matching the crop estimates made at the beginning of the campaign.

On-going developments

Other on-going developments at Wissington have seen handling specialist Geo Robson redesign and optimise the handling system in the sugar screening plant. Incorporating screw conveyors, elevators, vibratory screens/pre-feeders and sugar holding bins, the system has increased the plant's capacity to deliver various grades of sugar to either the bulk outloading system or the bagging plant.

On the bulk handling front, B&W Mechanical Handling has installed several of its Samson feeders and conveyors in areas such as the lime kiln and refined sugar storage silos, and for handling some of the beet processing by-products such as shredded pulp.

Perhaps with the earlier filtration problems in mind, Bollfilter duplex filters have now also been installed at various points downstream from the main process precoat filters. These are to protect product quality by preventing contamination of sugar syrups and condensates if the main upstream plant should fail.

And to demonstrate the increasing sophistication of what is in reality an ages-old process, Emerson Process Management has been involved between the last few campaigns in upgrading the various flow measurements throughout the plant.

A total of 56 Rosemount 8732C magnetic flow transmitters, for example, have been installed on lines ranging from 50 to 150mm in diameter to monitor and control the flow of liquors through the recirculation stages of the process. Typically, these liquors contain up to 65 per cent sugar, but the efficiency of the process is dependent on crop quality.

Micro Motion Coriolis massflow meters have also been adopted for liquor density measurements across the plant. These measurements are critical to controlling the efficiency of the plant and British Sugar's instrumentation manager, John Fleming, commented: 'I was impressed at the ease of commissiong these units using the handheld interface. Our next step will be to establish maintenance and calibration records on the AMS software installed on our control system, using the same HART communications.'