how to reduce sugar losses in c curing final molasses...

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How to Reduce Sugar Losses in C Curing

Final Molasses Losses Control

E.M. Sarir, B.R.Pabon, Carbo-Solutions International, USA

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Introduction •  Target in sugar processing is to maximize sucrose

recovery and impurities removal. •  Sugar quality must fit customer quality requirements. •  Most sugar technician’s work must be oriented to sugar

losses reduction.

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•  Losses in a sugar factory can be classified in two main groups:

•  Determined sugar losses ( sucrose lost in bagasse, filter cake and final molasses).

•  Undetermined losses: Sugar losses with and uncertain explanation and difficult to quantify as: sugar leaks and entrainments, or sugar inversion

•  Major sugar loss is present in final molasses.

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Sugar Losses




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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.

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Introduction




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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.

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Introduction




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•  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.

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Final Molasses




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•  After two boiling exhaustion stages, C massecuite is made, using B molasses purity 50 - 55 %.

•  Final molasses are separated from C massecuite by means of centrifugation (curing).

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Final Molasses




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•  B molasses are concentrated under vacuum in pans; once desired super saturation level is reached, Crystallization is initiated.

•  Crystallization is essential on molasses exhaustion.

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C Massecuite Boiling




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•  Very fine sugar seed slurry is added inside vacuum pan •  Nuclei to start crystallization, slurry is a sugar + isopropyl

alcohol mixture, containing a known amount of very small sugar nuclei ( 8-12 microns).

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•  Nuclei grows from sugar in the mother liquor by crystallization induced by evaporation of molasses.

•  C massecuite crystal grows to the final desired size ( 250 -300 um).

•  Brix is tight to 97- 98% •  Massecuite is discharged to cooling

crystallizers to recover additional sugar from mother liquor.

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•  Massecuite it’s gravity discharged to continuous high speed centrifugal to separate (purge) the sugar crystals from final molasses.

•  Final molasses, is a dark, high density and viscous liquid, with majority of soluble non- sugar impurities.

•  Sucrose content of 30-40% but recovery of this sucrose is difficult by traditional exhaustion methods.

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Sugar Cane Molasses Typical Composition

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Molasses Quantity

•  Sugar losses in final molasses are not exclusively related to molasses sucrose content but also molasses quantity.

•  Molasses quantity in the same way of molasses composition depends of many external & internal factors.

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Factors Impacting Molasses Quantity

•  Cane variety •  Cane Maturity •  Cane residence time and trash •  Juice & syrup purification performance •  Boiling scheme & Massecuite brix •  Added Impurities & melasigenic compounds •  Centrifugal washing & operation •  Microbiology •  It is normally accepted final molasses being 3-4% of

processed sugarcane .

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Molasses Quantity

•  The certain & quick determination of the amount of final molasses produced is mandatory for a sucrose loss reduction program.

•  Types of molasses scales •  Calibration & Maintenance •  Precise determination of inventories in storage tanks and

sugar materials in process

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Molasses Quantity control •  Daily control for molasses production

•  Factory reducing sugar, ash & acidity profile •  Increases of microbiological growing across the whole

production process •  Added water control •  Sugar reprocessing

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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




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Molasses Purity •  Molasses Purity depends on:

•  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)

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Molasses Exhaustion

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.

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Molasses Exhaustion

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).

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Saturation and crystal grow

•  Unless a sugar solution is supersaturated there is not crystal appear or crystal grow.

•  Saturation degree is the d r i v i n g f o r c e f o r crystallization.

Is essential to determine actual oversaturation condition of sucrose solution at vacuum pan operating temperature and massecuite purity.

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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.

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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

temperature oscillations during boiling

•  Low water and energy demand

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Cooling Crystallization •  A sugar solution becomes in to super saturation zone both by

concentration increases or temperature decrease.

•  When sugar crystals in vacuum pan reaches desired size, massecuite is gravity discharged to stirred crystallizers.

•  PURITY DROP PROFILE SINCE PANS TO CENTRIFUGALS




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Cooling Crystallization •  Vertical or horizontal vessels provided with mechanical stirrer

to supply motion and additional exhaustion stage by cooling massecuite

•  Cooling forces saturated sucrose in mother liqueur surrounding crystals pass to crystalline structure.




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Cooling Crystallization

•  Massecuite must be in continuous motion to ensure a high concentrate sugar film around crystals and favoring sugar exhaustion.

•  Cool Massecuite increases its viscosity and flow properties decrease.




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Cooling Crystallization

•  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.




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Crystal Content •  Sucrose transfer from saturated solution to crystal ( exhaustion)

is favored by a high crystal “population” (available crystallization surface area).

•  High account of fine sugar crystals increases exhaustion.

•  High crystal account also consistency of massecuite and perturb crystallizers and centrifugals performance.

•  Must be obtained and equilibrium between massecuite crystal content and massecuite viscosity.




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Crystal Size

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)




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Crystal size




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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”.

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Crystal Size Impact •  During sugar boiling grains must be of right number and

uniform size free from false grains and conglomerates. Crystals must provide maximum surface for sugar adsorption from mother liquor.

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Cssecuite




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Crystal Uniformity and Content




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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.




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Exhaustion Related Factors

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.




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Table shows viscosity and density of sucrose solution as concentration function





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•  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.





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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.




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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).




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•  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.

Exhaustion Related Factors: Viscosity




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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

high electrical currents).

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Massecuite Viscosity Reduction •  Massecuite heaters •  Water or molasses addition •  Use of process aids ( Tensoactive chemicals, viscosity

reducers) •  Continuous centrifugal basket heating Wash water

addition in the basket.

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Dextran and Sugar Recovery

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Dextran and Sugar Recovery




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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.




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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.

SugarequipNutschFilterApparatus-SMRI




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Not only is it rework cost, there will also be chemical loss when recycling the sugar.

The dissolved sugar will end up in the run off and increase run off purity which will drop the exhaustion

It can be shown that 2 second extra wash will drop exhaustion by 1.44 units

Since 1 unit exhaustion will elevate the UDL (Undetermined loss) by 0.035%

1.44 units exhaustion ≡ 0.05 increase in UDL ( T de Beer 2014)

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Centrifugal Operation




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Centrifugal Screens




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Some factories renew C centrifugal screens routinely every 4 to 6 weeks, cascade the old C screen to B duty

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Screens •  A typical open areas for a batch

centrifugals screens is 20%. •  For a continuous screen the open area

ranges from 4% to 20% depending on the design of screen and the process duty.

•  The molasses flows through the sugar cake within the basket then through the open area of the screen to the liquid discharge.

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A Molasses Purity •  If A molasses purity is too high due high sugar crystal

dissolution, purity profile will be high making so difficult to get a low final molasses purity.

•  Excessive use of washing water in A sugar centrifugal must be controlled.

•  Centrifugal plough must remove all sugar from screens after purge cycle.

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Need a good clean screen after ploughing out the sugar. Any sugar remaining on the centrifugal screen is dissolved &

recycled with basket wash

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Notlikethis.Closeto20mmsugarle>onthisbasket

(Onemachinelikethiswillrecyclenearly40tonsugaraday)

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Evaluate Screen Dimensions

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Screen Performance Survey.. •  The performance of some commercial continuous

centrifugals Using various open area screens on B and C massecuites was monitored over five seasons at Umzimkulu Mill.

•  Three di f ferent screens f rom three di f ferent manufacturers, With a 10%,12%and 14%open area were tested.

•  In general final molasses brix was higher and apparent purity lower with large open area screens.

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Screen Performance Survey..




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Open area

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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.

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Color Precipitants •  Color precipitants can be added directly to raw syrups, or A,B

and C massecuites in vacuum pans. •  This is because they are liquid products of low viscosity and,

therefore, can be easily added to both syrup and massecuites using conventional metering pump

•  Results show high color removal and up 25% reduction in washing time

THELOWERWATERINCENTRIFUGALTHELOWERMOLASSESPURITY!!

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Color Precipitants

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Impact of Boiling Scheme •  Sucrose crystallization in vacuum pans can be performed

using different boiling schemes.

•  The choice of boiling procedure is determined by syrup purity and the sugar quality required.

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•  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

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Impact of Boiling Scheme




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•  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).

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Impact of Boiling Scheme




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Three Boiling Scheme

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CBA Boiling…

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Double Purge of C-Magma •  Hugot (1986) recommends double purge of C-magma when

syrup purity is low, adjusting the purity of the 2nd magma to 88.

•  He also mentioned that a 2nd magma purity ranging from

92 – 93 will significantly reduce ash content in raw sugar and will improve the filterability of the melted sugar at the refinery.

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•  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).

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Double Purge of C-Magma




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Double Purge of C-Magma




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Reducing Sugar/Ash Ratio

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.




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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.




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Most popular worldwide used equation was developed by SMRI in South African (11), it express Target Purity in molasses (TP)

TP = 39, 94 – 19, 6 log (Reducing Sugars / Ash)





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The Influence of (F+G)/A Ratio on Target Purity

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Improving Reducing Sugar/Ash Ratio

Minimizing reducing sugars destruction:

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.




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Minimizing reducing sugars destruction:

Reducing sugar must be protected from:

-  Long residence time equipments and processes

-  Low brix conditions

-  Bacterial growing: Process equipment must be sanitized and clean against microbiological contamination.

-  Excessive high temperature heating in juice and molasses ( up 220 F)

-  High pH values ( up 8,0).





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Reducing ash concentration

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.





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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.





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-  Optimizing juice liming procedures, using high purity milk of lime

-  Avoid excessive lime consumption in juice due failures in pH. meters ( over liming), fractioned liming and calcium saccharate are recommended.

-  Ensure proper juice clarification, avoid swept away muds in juice to evaporation and boiling process





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Improving juice purification


-  Milling rete control

-  Mixed juice flow control

-  Juice turbidity & sediments control

-  Juice temperature control

-  Flocculant and lime control in juice purification

-  Mud level inside clarifier control




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-  Syrup phosf lotat ion to remove ashes and turbidity, theoretical remove of 90% turbidity and 20% ash is possible (12).

-  Use of high performance mud filtration technologies as centrifuge decanters reduce mud recirculation to juice.





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- Increase of B and C magma sugar to reduce ash recirculation in to vacuum pans.

- C magma purity must B as minimum 82% and B magma up 88%, in this way those crystals used as nuclei for A and B sugar has less ash content.





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Do not add ash. •  Formation of organic acids that are neutralized with Ca(OH)2

will produce soluble ash specially with poor quality lime •  Contamination with sodium hydroxide from chemical cleaning

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Do not add ash. •  Poor quality water with high ash levels for lime & flocculant

dilution. •  Recycling neutralized sweet water from leaks and spillage •  These are examples of procedures that will increase ash and

increase the target purity of final molasses.

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Summary

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.




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Summary

•  Operating vacuum is a critical issue in crystallization

procedures.

•  Molasses purity drop control

•  Control of oversaturation

•  Brix seeding point

•  Seed slurry ( amount, size)




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•  Continuous measurements in size and CV for A, B, C sugar crystals and seeding slurry.

•  Cleaning, sanitizing and use of biocides and enzymatic treatments to avoid sucrose inversion and invert sugar degradation.

Summary




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•  Obtain high populations of crystals in massecuite, uniform size and shape crystals and low viscosity massecuite.

•  Development and implementation of consistent and repeatable graining procedures will help improving the boiling house efficiency.

•  To Conserve reducing sugars incoming the factory process.

Summary




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•  Improve operating controls to minimize impurities in juice and molasses

•  Set up additional purification systems for ash and impurities removal ( syrup flotation, high performance decolorants, color precipitants)

Summary




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•  Continuous laboratory test to establish a process ash and reducing sugars profile.

•  Measurements of molasses purity profile from vacuum pan discharge to centrifugals using Nutsch filter.

•  Measurements of massecuite and molasses viscosity and tensoactive agents performance.

Summary




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•  Measurement and control in water used in vacuum pans and centrifugals.

•  Continuous check in centrifugal screens and equipment.

•  High purity on B and C sugars ( without excessive washing water) to reduce ash recirculation on massecuite

Summary




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Appendix : Crystal Seed Calculation

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




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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.




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References 1- Meade-ChenJames,SugarCaneHandbook,1999Pag4572- Meade-ChenJames,SugarcaneHandbook,1999Pag4023- Gillette,LowGradesugarcrystallization,1948pag254- Sahadeo,P.LionnetGRE(1999)Ananalyticalsurveyoffinalmolassesfromfifteen

caneproducingcountries,Proc.ISSCT23,92-1035- Miller F,F.: Wright P,G. (1977) Experiments with molasses exhaustion, Proc.

QueenslandSocietySugarCanetechnologist24,371-3726- Broadfoot,RViscositylimitationofmassecuiteexhaustion,Proc.AustralianSociety

Sugarcanetechnologist,6,279-286,19847- Meade-ChenJames,Sugarcanehandbook,1999Pag3988- Rein,Peter,CaneSugarEngineering,2007,pag4639- Meade-ChenJames,Sugarcanehandbook,1999Pag39110- HawaiianPlantersExperimentstationHST,1968pag16911- Bruijn,J.;FitzGerald,J.R.;Koenig,S.andMacGillivray,A.W.(1972).Exhaustionof

SouthAfricanFinalMolasses.SASTAProc46,103-109.12- Technologyofphosflotationofsugarmelt,SugarTechnologyReviews1972,pag1613- SarirEmmanuel,CarboUA200714- Herzfeld,RubenZuckerIndustrieDeutsch,1892,pag42177-181

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