Size Matters: Crystal Size Analysis for the Louisiana Sugar Industry

Franz Ehrenhauser, Aragon, Daira, Tishechkina, Iryna, cgaudet

Size matters when it comes to sugar crystals and the efficient processing of cane juice into refined sugar for human consumption. More than a third of the U.S. domestic cane sugar production, 1.4 million tons, is grown and processed into raw and white sugar in Louisiana. The basic processing steps at each of the 11 raw sugar factories in Louisiana are milling and extracting the juice from the cane, then clarifying and concentrating the juices to syrups. Out of these syrups raw sugar is crystallized in the raw sugar factory. The raw sugar is further processed into refined sugar for human consumption at one of two Louisiana refineries. Recovery during the crystallization is key to efficient operation and to maximize the return to factory and grower. To aid the sugar industry, the LSU AgCenter Audubon Sugar Institute provides educational and technical support and has since 1891, when the first Audubon Sugar School was held.

Since 2007, the Audubon Sugar Institute has been providing crystal size analysis based on laser diffraction. Before then, crystal size measurement was done manually by measuring sugar crystals under a microscope, which is time consuming and lacks reproducibility unless a large number of crystals are counted. The laser diffractometer is capable of measuring thousands of particles in suspension from the nanometer to the millimeter scale within a few minutes. As the instrument assesses diffractive properties, it measures a size-equivalent sphere, giving a better volume representation than the two-dimensional measurement under the microscope. During the grinding season more than 500 samples undergo analysis, and reports are sent to the industry to help them improve their process.

Seed Slurry

Tiny seed crystals are used to initiate crystallization from the syrup or molasses. As such the seed is the heart of every sugar crystal leaving the factory. The modern practice in sugar crystallization is to use full seeding, where every crystal is deliberately initiated with a seed crystal from seed slurry. Three slurry types are typically used – powdered sugar, ball-milled sugar and ball-milled powdered sugar. All are suspended in isopropanol. The ball-milled type involves putting the sugar in a mixer with lots of steel balls inside. A consistent quality and quantity of seed crystals is important to operate the sugar crystallization efficiently.

Table 1 gives an overview of the basic properties for analysis. D10%, the 10-percentile, describes the smallest crystals in the distribution, i.e. 10 percent of the mass of all crystals are below this size. D50% is a descriptor also called the median, and D90% describes the largest crystals in the distribution. The coefficient of variation (CV) describes the width of the distribution, where a smaller number means a more favorable, narrower distribution. The size of powdered sugar ranges from 3 microns to 50 microns (a human hair is between 40 microns and 60 microns thick) with a typical coefficient of variation from 0.62 to 0.76 and a seed content of 51 to 86 million crystals per gram. Ball-milled seed slurries have substantially smaller crystals with a D90% of only 19 microns providing at least 10 times more seed per gram of sugar than powdered sugar. Other important factors found to provide consistency for the seeding are proper (dry) storage of powdered sugar, proper mixing and sufficient ball milling time (minimum four hours for powdered sugar, five hours for granulated sugar).

C-sugar and Centrifugal Recovery

Typically, the seed slurry is used to produce so-called C-sugar. This C-sugar is not pure enough for the refinery, and it is used as seed for the commercially valuable, high purity A- and B-sugars. The crystal size and uniform distribution of the C-sugar are therefore important because affect production into commercial raw sugar. The size of the C-sugar is also relevant for overall sugar recovery because C-sugar is the last sugar crystallized before the syrup becomes blackstrap molasses (used for animal feed), and no further sugar can be obtained from it. Any losses to the blackstrap molasses are final. Figure 1 shows the variation of the measured C-sugars for the 2015-2016 season for all 11 raw sugar factories. The size ranges from 14 microns to 428 microns with an overall average median of 184 microns. In Louisiana, C-sugar size rarely exceeds 450 microns, with a typical 90-percentile season average of 405 microns. This seemingly arbitrary upper limit is most likely associated with a slowing down of crystal growth as the process proceeds.

While smaller crystals provide more surface area for crystallization, they also challenge the separation. The loss of sugar in C-station centrifugal machines, where the molasses-sugar mixture is separated, is particularly troublesome because these losses are unrecoverable losses to molasses. Figure 2 shows the purity rise (loss to molasses) across a C-sugar centrifugal machine in dependence of the median crystal size. The purity rise ranges from 3.5 to 6.5 with seemingly no direct correlation to the crystal size. However, the data exhibit no points in the lower left corner of the graph, where low purity rises (low loss to molasses) would be achieved with low crystal sizes. While there are many factors contributing to the purity rise, it is clear that a low purity rise is rarely achieved with small crystal sizes. As such, the factory should aim at an optimum crystal size where there is sufficient surface area for crystallization and the crystals are large enough to keep centrifugal purity rise to a minimum.

Sugar crystal size analysis can give quick and valuable insight to help factories maximize sugar recovery. While some general recommendations are possible, there is no one-size-fits-all answer when it comes to sugar crystallization. Every sugar mill is unique in its process and the seed slurry and C-sugar sizes need to be optimized to the individual mill’s need.

Franz Ehrenhauser and Daira Aragon are assistant professors, and Cy Gaudet and Iryna Tishechkina are research associates at the Audubon Sugar Institute.
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Left to right are Iryna Tishechkina, Daira Aragon, Cy Gaudet and Franz Ehrenhauser. Photo by Olivia McClure

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Table 1. Size distributions of seed slurries used in Louisiana.

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Figure 1. C-sugar crystal size in Louisiana during the 2015-2016 season with 10- and 90-percentile (D10% and D90%) and median value given. The error bars denote the standard deviation of the values.

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Figure 2. Purity rise across a C-centrifugal machine versus the median diameter of the C-sugar.

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Cilas 1180 Laser Diffractometric Particle Size Analyzer

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Samples for the sugar factories. Photo by Olivia McClure

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A-, B- and C-sugar (from left to right)

4/4/2017 4:55:16 PM
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