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How to Reduce Sugar Losses in C Curing Final Molasses ...thaisugar.org/.../2017/05/How-to-Reduce-Sugar-Losses-in-C-Curing-Final-Molasses-v3.pdf• Massecuite must be reheated before
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Although the losses of sucrose in final molasses contribute the highest percentage of loss, sucrose final molasses losses should not be analyzed as an isolated phenomenon. Molasses quantity & purity highly depends on external variables such: Cane quality, juice purification, boiling scheme procedures & purge.
Molasses losses program must be addressed evaluating current operational procedures and equipment conditions. Cane quality and proper design / performance for each process step from cane milling to sugar boiling are essentials to maximize sugar recovery.
• Final molasses is a by-product on sugar manufacturing process, obtained after an exhaustion process of sugar cane evaporated juice (syrup) by successive stages of intermediate molasses crystallization.
Microbiological Control: Additional to sucrose looses microorganisms generates complications on later process stages: - High molecular weight polysaccharides - Organic acid formation and sucrose inversion - High viscosity in juices and molasses viscosity - Color increases - Poor quality sugar
• Exhaustion degree in vacuum pans • Exhaustion in cooling crystallizers • Sugar dilution in massecuite heaters and centrifuges • Bleed through of sugar crystals in the centrifuge
meshes • Presence of impuri t ies interfer ing sucrose
crystallization and measurement (polysaccharides & salts)
It is difficult to determine whether a molasses is completely exhausted or not. Exhaustibility means that more sucrose can be potentially be recovered from a molasses. Molasses purity is not a good exhaustion indicator since exhaustion highly depends of impurities presence.
Reducing sugars lowers the sucrose solubility but ash compounds of certain alkali metal slats increases it. The sucrose solubility largely determines the final purity of molasses ( DeCloux 2000).
Pan operation and solubility It is obvious the strong effect on temperature in solubility Vacuum pans must be operated in super saturation 1,1 – 1,25 known as metastable zone where sugar crystals will grow but no new crystals will appears. Vacuum pan operating temperature must be kept so high to ensure good heat transfer and evaporation but so low to get a steady state super saturation condition ( 1,25 – 1,3) to avoid spontaneous crystallization , or crystal melt.
Pan operation and solubility Crys ta l l i za t ion under vacuum invo lves simultaneous processes of mass and heat transfer. In sugar factories operating at sea level condition, recommended vacuum pan operating temperature is in the range 140 – 150 F. Vacuum pan must operate: • High efficiency vacuum systems that minimize
• Massecuite must be reheated before centrifugals and some of additional crystallized sucrose “melts” and returns from crystal to mother liquor again.
• It is a good practice to experimentally measure the purity of final molasses “exhausted” from massecuite before crystallizers to calculate actual improvement in exhaustion by cooling massecuite.
Most appropriate final crystal sugar size for C massecuite is in the range 0, 25 – 0, 40 mm. When complete sugar seeding technique is employed (operating vacuum pan in a saturation level avoiding new crystals appearance). Calculate required seed crystal ( size & amount) is required for proper crystallization. ( see appendix)
Crystal Size Distribution • Mead and Chen (1977) stated that crystal uniformity is a
desired quality because” with a mixture of crystal sizes, the pore volume of sugar is reduced and drainage of molasses is slower and less complete in the centrifugal operation”.
Exhaustion Related Factors Once appropriate amount of sugar nuclei (seed) and super
saturation are defined, there is still other additional consideration to obtain a good exhaustion from molasses. Crystal growth rate depends of : - sucrose diffusion velocity from mother molasses into crystalline
structure - Velocity of sucrose incorporating crystal
Lower velocity is the limiting step in exhaustion. Diffusion velocity is affected by viscosity and the second one is interfered by the presence of impurities on crystal surface.
Viscosity role in molasses exhaustion is complex, The higher sugar crystal content the higher crystallization velocity but same time the higher crystal amount the higher viscosity in massecuite. Can be expected lower molasses purity in high brix massecuite.
• Massecuite viscosity affects Crystallizer and centrifugal equipment performance and became on limiting factor for sugar mill process.
• Gillette found an upper limit for sugar crystal amount in low purity massecuite boiling, beyond this limit, any additional crystals tends to hold up massecuite flow.
• Massecuite feed to centrifugal must be kept at temperature between 55-58 C to equilibrate exhaustion and viscosity.
Viscosity and Impurities • Massecuite viscosity is function of brix, temperature and
presence of particular impurities like polysaccharides (starch and dextran).
• Sahadeo concluded the target purity in molasses can be increased by 0,2 for each increase in dextran level of 1000 mg/kg dry solids.
• Miller and Wright indicated a greater effect of dextran, and suggest that every 1000 mg dextran / kg dry substance increases the expected molasses purity by one unit.
Exhaustion Related Factors: Viscosity • Viscosity is in this way a limiting factor for both brix and
crystal content.
• High brix and crystal content values generates a sharp viscosity increases , this requires massecuite reheating after crystallizers and additional water in centrifugals to make centrifugal separation possible and loosing sugar recovered by cooling of massecuite.
• Generally a change of 10 C will double or halve the consistency. An increase in brix of only 2 units leads to a doubling of the consistency. Broadfoot et al (6).
• Normally acceptable viscosity value for massecuite is in the range 400 – 600 poise a temperature 55 C.
• It is common practice use on tensoactive agents added to massecuite in crystallizers in order to reduce viscosity and enzymatic treatment to hydrolyze starch and dextran.
• It is highly recommend a frequently viscosity measure in molasses and massecuite to detect any departure from normal parameters.
Massecuite Viscosity Reduction Numerous methods have been used or continue to be used to control or reduce massecuite viscosity: • Increase massecuite Purity • Crystal account • Re-cycled molasses addition to massecuite at pan
discharge. • Heating massecuite in minglers • Resistance heating of massecuite (using electrodes and
Centrifugal Operation Vacuum pan and crystallizers exhausted
sucrose may be “spoiled” by miss operating condition or equipment fails in centrifugal separation. Control washing water Temperature, pressure, impurities Water is to reduce viscosity not to dissolve sugar. Usually, purity increase in fugals should not be more than one to three units.
Centrifugal Operation Purity increase in centrifugals must be
tested at lab by extracting molasses from massecuite using Nutsch filter before process. Check centrifugal screen condition to minimize crystal sugar pass across the screens. Routine inspection of centrifugal baskets and correct choice of the screens is mandatory.
Color Removal and Water Requirements • Excessive use of wash water in centrifuges is generally
associated with the need to produce low-color sugar and this is further accentuated when processing low quality materials.
• The use of new technologies such as color precipitants in the pans (Sarir 2016) has shown that it is possible to reduce the amount of washing water in centrifuges by up to 30% while preserving or even improving color levels in sugar.
• Most common boiling schemes are: two boiling system, three boiling system, double magma, and Very High Pol (VHP) scheme. VHP produces the lowest sugar color.
• Any modification in the boiling scheme should be evaluated taking into account the impact on the quantity and purity of the final molasses generated and not only steam consumption or sugar color
• Currently, many raw sugar factories are adopting double magma or VHP boiling schemes in order to improve the quality of their sugar.
• Color reduction, however, sometimes is not high enough and process modification involves high capital investment.
• The boiling process is energy demanding and evaluation of overall sugar recovery must be undertaken to evaluate the effect of boiling scheme modifications (Madho and Davis, 2008).
• Reduced recirculation of final molasses to the seed will improve not only the quality of the raw sugar but also the quality of the material for the first (A) and second strikes (Jullienne, 1989).
• The quality improvement of these strikes will be due to a lower recirculation of non-sugars from the low grade massecuite (3 rd strike).
It is long time recognized (9) sucrose solubility decreases in presence of reducing sugars and soluble non organic salts increases.
It is well expected, molasses exhaustion be affected by reducing sugar and ash presence in factory process.
A complete technical report from Hawaiian Planters Experiment station (HST, 1968 pag 169) for a constant viscosity values demonstrates, final molasses purity drops for high values in reducing sugars / ash ratio.
According this consideration, many sugar researchers developed mathematical models and equations trying to predict final molasses purity as reducing sugar / ash ratio (RS/A) function.
Reducing sugar ( glucose , fructose) are high reactive and under certain conditions reacts to form complex organic compounds, colorant and carboxylic acids.
Is important to preserve as much possible reducing sugar incoming with sugar cane juice and those from normal sucrose inversion.
This can be done by a synergized plan by approach in global reduction of impurities incoming factory and additional process and equipment to remove them.
Improve sugar cane harvesting procedures to reduce trash content, almost 40% of trash impurities ( soil, roots, foils, tops) are transferred to cane juice affecting purification process and increases ash and non sugars levels in juice and molasses.
The list below outlines some useful practices to obtain good molasses exhaustion, however there is not universal solution to achieve low purity molasses, here are some considerations:
• Ensure an adequate massecuite boiling scheme, to even up purity drops and sugar quality.
• Operate vacuum pans in metastable saturation zone to avoid spontaneous crystallization or melting of crystals. Ensure proper vacuum system.
Example: a C Massecuite of 2000 cubic feet it is elaborated, desired final C sugar crystal size of 300 microns ( 0,03 cm), final massecuite concentration 98 Brix, crystal recovery 45% and initial sugar seeding slurry 11 microns, calculate required seed slurry quantity. Data: Crystalline sucrose density: 1, 59 gr/cm3 C 98 Brix Massecuite density: 96 pounds / cubic feet Weight of C massecuite: 2000 cubic feet * 96 pounds / c.f.3 = 192,000 pounds (8,71 x10e7 grams) Weight of crystal in massecuite = 192,000 * 0,45 = 86,400 pounds (3,92 * 10e7 grams) Weight of one final C sugar crystal = Final crystal volume * sugar crystal density Weight of one final C sugar crystal = (0,03) ^3 * 1,59 = 4,29 x 10e-5 grams Amount of sugar crystals on final C massecuite = Weight of crystal in C massecuite/ weight of one final C sugar crystal Amount of sugar crystals on final C massecuite = (3,92 x 10e7) / (4,29 x10e-5) = 9,14 x 10exp 11
Crystal Seed Calculation For a complete seeding procedure, the amount of crystal in final massecuite it’s assumed almost the same in seeding slurry. Weight of crystals on seeding slurry = amount of crystals on seeding slurry * weight of one slurry crystal Weight of one slurry crystal = 1,59 gr/cm3 *(0,0011 cm)3 =2,11 x 10 e-9 grams Weight of crystals on seeding slurry = 9,14 x 10e11 * 2,11 x 10 e-9 = 1,934 grams Seeding slurry is prepared milling 2 kilograms of refined sugar in 4 liter of Isopropyl alcohol on a ball mil for 8-12 hours, then, 2,6 grams of seeding slurry contains 1 gram of sugar crystal. Required seeding slurry = 1,934 * 2,6 = 5,028 grams Traditionally, the amount of seeding slurry is corrected by a safety factor of 20-25% to compensate looses or melting of fine nuclei during seeding or boiling. According this the required amount of slurry is: 6,28 grams de slurry per 2000 c.f. of final C massecuite Is evident form above the amount of seeding slurry depends strongly of initial slurry crystal size, it’s recommend consistent preparation of slurry and to check often the size of initial crystals.