Assessment of target purity difference for a Louisiana sugar mill
Luz Stella Polanco
The main goals of a "raw" sugarcane factory are to have an efficient profitable operation with the required sugar quality and maximum sugar recovery. The loss of sugar to final molasses accounts for 45 to 47% of the total sugar losses. An estimation of the average cost of the manufacturing losses for the 2007 harvest season was approximately $9 million -the sugar lost to the final molasses accounts for approximately 50% of this cost (Salassi, 2008) . For final molasses, the "Target Purity" (or
... quilibrium purity) refers to the minimum level of sucrose that theoretically remains in solution for a fixed composition of non-sucrose substances. The "Target Purity Difference" (TPD), which is the difference between the actual "True Purity" and the "Target Purity," is the non-bias measurement of factory performance. TPD was used for this research to define the operational goals of the low-grade station, toward reaching the physical maximum molasses exhaustion. The approach to molasses exhaustion for the studied factory was focused on the low-grade station stages of the boiling house which include: 1) continuous cooling crystallizer's performance; 2) molasses consistency; 3) supersaturation; 4) crystal size distribution and crystal content; and 5) purity drop and goals by stage. The main recommendations are: 1) to increase the crystal content in the "C" massecuite at the exit of the continuous vacuum pan (CVP) by regulating the seed/massecuite ratio and Brix profile; and 2) to improve the flow pattern and cooling temperature control in the vertical crystallizers by modifying the internal arrangement (baffles and cooling coils arrangement) and placing more temperature probes. Complementary recommendations are: 1) implement procedures for seed preparation (slurry preparation by ball milling); 2) increase grain strike capacity according to the expected "C" massecuite rate; 3) grain batch pan automation; and 3) monitor and control crystal size distribution per stage according to standard desired values. The establishment of a routine to measure purity drop/rise and crystal size distribution specifying achievable goals per stage at the low-grade station is the best tool to achieve the ultimate goal (zero target purity difference).