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The following is a report written by Dr. Vadim Kochergin, Audubon Sugar Institute, LSU AgCenter from data collected during the 2008 sugarcane harvest and can be found in The Sugar Bulletin: March 2009, Volume 87, Number 6, Pages 16-20. This research project was funded in 2008 by the Dedicated Research Funding Committee of the American Sugar Cane League.
Lead PI - Dr. Vadim Kochergin, Audubon Sugar Institute, LSU AgCenter
1. Effect of graining on the uniformity of C-sugar
A set of samples has been taken from a mill that tested two types of seeding procedures. In the fist test, a relatively large quantity of powdered sugar was used. In the second test, finely ground sugar from another mill was used for seeding. Time-delayed samples were taken; the attempt was made to follow the same batch of C-massecuite all the way to centrifugal station. The samples of massecuite were evaluated for particle size and uniformity using laser particle size analyzer. The data are reported in Figures 1 and 2. A coefficient of variation (CV) was measured to evaluate the uniformity of crystals in the massecuite.
In both cases, uniformity of sugar was improved during the water phase, which implies that some fine crystals could have been dissolved. This should not have been happening, however, if normal supersaturation was maintained during water phase. It is noteworthy that increase in CV (higher non-uniformity) was happening during graining in both sets of tests. The CV has not changed much during crystallization. Lower CV in the C-sugar is due to reduction in fine crystals across the centrifugals. When less sugar powder was used for graining a larger crystal population was grown with much better uniformity. Although molasses purity was not measured during the tests, a large difference in crystal size across the centrifugals indicates that more fine crystals were lost to molasses in case of large seed quantity.
The data in Figure 3 graph shows results of another test, where the Nutsch molasses purities were measured from several points around the mill. A relatively large charge of powder ( 2-2.5 lbs of powdered sugar per 1000 cu. ft. of C-massecuite) was used for graining. Although excellent purities were observed in the crystallizers, it was not possible to effectively purge the massecuite in the C-machines.
Studies of seed slurry preparation continued throughout the season in several sugar mills. Louisiana mills use either a milled powdered sugar or a 10XX grade powdered sugar obtained directly from the suppliers. It has been demonstrated that the median crystal size of powdered sugar varies from 15 to 20 micron with generally higher variability in crystal size compared with milled sugar (7-8 micron ) The milling time of over three hours usually gives acceptable results.
Operators in most Louisiana mills tend to use larger amounts of seed slurry compared with data available from the literature. Most mills use about 1 lb. of powdered sugar per 1000 cu. ft. of final C-massecuite. Use of smaller quantities requires more consistent operating procedure in the graining pans.
2. Effect of C-sugar crystal size and uniformity on molasses purity
Although it is generally known that non-uniform crystal reduce purging efficiency of the C-centrifugals, no quantitative data were readily available to establish the target parameters providing minimal losses of sugar in molasses. Samples of C-massecuite were filtered through a Nutsch chamber and the purity was compared with purity of final molasses. In addition, C-massecuite sugar was evaluated for crystal size and uniformity using laser diffraction particle size analyzer. The samples were taken regularly throughout the season in a sugar mill. The mill employed consistent seeding procedure, and settings of C-centrifugals (RPM, water usage) remained mostly unchanged throughout the season. The results (Figure 4-see related files below) show that purity rise across centrifugals was unacceptable (above 3 points) until the crystals in C-massecuite reached over 250 microns.
A variation of median crystal size throughout the season is shown in Figure 5 (see related files below). Note that the seeding procedure was modified around day 15 of the season resulting in larger crystal size. There is still enough variability in crystal size that leaves room for optimization of molasses exhaustion and centrifugal performance.
3. Technical Assistance during the grinding season
Technical assistance has been provided to most Louisiana mills that were experimenting with grinding seed slurries or validating crystal size of various massecuites during the season. Several hundred samples have been processed for crystal size analysis. Complete crystal size profiles across the boiling house have been taken at various mills from both continuous and batch pans. The results provided fast feedback to the mill operators on the status of the boiling house operation.
4. Mathematical modeling of boiling house operation (Dr .M. Saska)
A mathematical model of boiling house operation has been developed by Dr. M.Saska. The model has been used to investigate the influence of molasses return with C-magma on the capacity of C-massecuite. Several scenarios have been considered with various molasses recycle with C-magma. An important conclusion is that relatively high values of molasses recycle (around 20%), which has been observed in some mill operations, result in increase of massecuite volume throughout the boiling house. The effect is particularly pronounced in the C-boiling, where massecuite volume may be increased by as high as 10%. This is critical when the syrup purities are low and mill performance is limited by C-boiling and crystallization steps. Detailed information on the modeling studies is available from Dr. Saska.
5. Molasses recycle and purity of final molasses
An experiment has been performed toward the end of the season when the mill was running consistently. Because of the high syrup purities (87.5-87.9%) the purity of final molasses was also increased. The mill was not limited by the raw grade boiling or crystallization. In an attempt to reduce molasses purity wash water was reduced from 4.5 to 3.5 gpm in one (out of two in operation) C- centrifugals. The results are shown in the graphs in Figures 6 and 7 (see related files below). As expected, lower water usage resulted in reduced molasses purity and reduced magma purity (increased molasses recycle). The mill operators reported darker color in the resulting C-massecuites. A certain increase in viscosity in B-sugar boiling was reported; no measurements were taken. This method may be used at high syrup purities provided that crystal size and uniformity of C-massecuite allow adequate purging in the C-centrifugals. Note: No additional sugar can be recovered from the recycled molasses.
CONCLUSIONS
1. Analysis of samples of C-massecuites in most mills shows high level of non-uniformity with a large number of small crystals. There is still significant room for improvement in seeding/graining process.
2. Development and implementation of consistent and repeatable graining procedures will help improving the boiling house efficiency. Following parameters need to be improved in many mills: purity and quantity of footing for the grain charge, purity of syrup added during the grain development, quantity of seed.
3. A trend has been observed that growing relatively small crystals in C-massecuite (in the range of 180-220 micron) generally results in higher losses. Small crystals are typically less uniform and have typically higher values of coefficients of variation (CV). The median crystal size of C-massecuites should be in the range of 250-260 microns.
4. Steps should be taken for reduction in crystal size variability in C-massecuite. This in turn will minimize sugar losses in C-centrifugals.
5. Molasses recycle with C-massecuite should normally be reduced, especially with low purity syrup. It is desirable to reduce molasses recycle by improving crystal size distribution rather than by increasing washing.
6. Depending on mill configuration and syrup purities operational strategies should be developed targeting reduction of molasses purity.