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Optimising colour and turbidity control in sugar refining

In industries like edible oils and sugar refining, the process of making the product can be optimised and/or the product quality can be heightened, through the use of colour measurement.

In the past, this has either been a laboratoray measurement or has not been done because the processing is such that it never needs it, according to AMS Instrumentation and Calibration’s analytical product manager Peter Sims.

“What that normally means is that the product is over processed and the production is not optimised but it is maximised,” said Sims. “Optimising a process maintains product quality while lowering production costs, which in turn minimises the energy and effort used.”

For colour measurement there are some important examples. In edible oils, colour measurements are important both after the bleaching and filtration processes, and as a final quality check after deodorisation just prior to tank storage.

Applications will normally need to be correlated to the respective colour scales such as AOCS (American Oils Chemists’ Society), APHA (American Public Health Association) and Hazen. It is also needed in the control of chlorophyll.

The concentration of chlorophyll varies in oilseeds but it is an important measurement to know because large amounts of chlorophyll reduce the shelf life of vegetable and other edible oils.

Oils with higher concentrations of chlorophyll generally require an extra processing step where the oil is heated to break down the chlorophyll. Monitoring oil in the pipeline before it is sent to the bleaching process allows the highly concentrated chlorophyll oil to be diverted through the extra heating process, while oil with a low concentration of chlorophyll moves into the bleaching process. Using an optek dual channel colorimeter, chlorophyll concentrations < 30 ppb (parts per billion), can be measured in real-time and in-line with high precision and repeatability.

In the sugar refining industry, similar measurements can lead to greater control of the process. In the manufacture of granular or liquid sugar, multiple steps are undertaken to convert raw sugar to pure sugar. Within these steps, opportunities present themselves where automatic monitoring of certain parameters allows the real-time ability to optimise manufacturing performance; ensuring the highest product quality and the greatest overall yield. In-line measurements provide dynamic inputs to the process control system in the refining and bleaching processes.

“As we understand the sugar process, the sugar liquor that results from dissolving the washed sugar crystals in water will still contain some solids, colour, and other impurities that needs to be removed,” said Sims. “After adding a coagulant, the sugar liquor is passed through pressure leaf filters to remove the remaining solids from the liquor. While we could apply a scattered light turbidity sensor at the outlet of the leaf filters so that the sugar liquor stream can be monitored in real time to ensure filter performance and alarm when there’s a filter break, this is of lower importance at this site, so is best suited for a later date.”
The resultant filtered sugar liquor is then passed through columns of granulated activated carbon (GAC) to remove the liquor’s colour. In general, the less colour the liquor has, the better the quality of the product. For process control of the evaporation process, getting the right colour can enhance the process and improve throughput.

The sugar refining  industry uses a standardised measurement for the measurement of sugar colour called ICUMSA, which is multi-faceted.

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Simplifying it, ICUMSA is a measurement of the liquid colour at a particular wavelength, which must be corrected for turbidity. The Brix and density reading at that point and the optical path length of the colour sensor, are the final components of ICUMSA.

“The ICUMSA level can be around 4000-7000 when dark and 30-100 when clear,” said Sims. “By installing a dual-beam absorption sensor in the clarification stages, it is possible to measure the outlet sugar liquor’s ICUMSA colour, thus providing real-time monitoring of the colour removal process. And because the sensor can be configured to specific wavelength combinations, and use short or long optical path lengths, even the darkest solution or the slightest ICUMSA colour changes will easily be detected. For this measurement, an ICUMSA of about 200 was the aim.

“As you can imagine, the process has quite large lines. We had a client who produced short runs in a plant that was fairly old, where this application was installed in a suitable line that enabled a good measurement.”

Retrofitting into an old plant with short runs needed some deal of review and evaluation. One of the big objectives was to not create too much pressure drop in the lines. Through a series of steps an armature (flow cell) was selected that optimised the measurement, the pressure drop and the cost.

“This is an analyser and for all the right reasons it is in-line,” said Sims. “That created additional discussions initiated by AMS about service and validation. One of the optek sensor’s advantages is its validation capability. For biotech types of application it is ideal and external sophisticated. For colour and turbidity, a range of optical filters provide confidence in the measurement and save returning the sensor system for re-calibration.

The right installation is also necessary because it is no use having an on-line analyser if the whole sugar refining process has to shut if there happens to be a problem, or there is a need to validate the system.”

AMS advised that by adding a simple bypass, a drain and an upstream ‘tap’ where clean water could fill the line, the user would ensure that the system installed would give them long life and service capability without shutting their process unnecessarily. The installation went well, and after a training session the system was put online.

“Since the installation, some 18 months ago, the system has been rigorously tested and checked,” said Sims. “Repeatability and reliability of the measurement has been well proven and the output signals are used for control of the process. It was always envisaged that, once proven, there would be an expansion of this type of measurement. This remains the aim.”

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