2009-M-14-Jansen

Jansen TM Proc Aust Soc Sugar Cane Technol Vol 31 2009 ______________________________________________________________________

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RAW SUGAR QUALITY FROM A REFINER’S PERSPECTIVE

By

TERRY M JANSEN

Manildra Harwood Sugars [email protected]

KEYWORDS: Raw Sugar, Quality, Colour, Ash, Filterability, Starch, Dextran.

Abstract RAW sugar quality is a critical factor as it determines raw sugar value and affects the performance of refineries. Refiners are increasingly demanding high quality raw sugar to optimise their own operations. Raw sugar producers need to understand the impact of quality issues on their customer’s (refiners) production. This paper examines the significance of raw sugar quality characteristics and their impact on sugar refining performance. The raw sugar quality criteria such as pol, colour, ash, dextran, filterability, grist and starch are very important to a refiner as they all have an effect on the efficiency and performance of the refinery. Although not a specific quality item, the affining quality of raw sugar and resultant impurities in the affined sugar are of particular interest to the refiner as they are key components in maintaining optimum production rates. Operational experiences on the affects of raw sugar quality at Manildra Harwood Sugars refinery are discussed.

Introduction

The challenge of the raw sugar producer is to achieve the following: • Produce marketable raw sugar of consistently high quality. • High sugar recovery. • Maximise processing rate with available equipment. • Reduced operating costs including labour and materials. • Maximise and utilise the value of by-products and wastes including bagasse,

mill mud and molasses. In many if not most instances, raw sugar producers have some form of factory

limitation in which compromises are necessary to simultaneously optimise the outcomes for all of the above criteria.

However, for producers of raw sugar, emphasis on quality is of increasing commercial importance as marketing and the sale of raw sugar become more competitive. Many raw sugar exporters are now supplying very high quality raws (high pol, low colour and low ash) traded at competitive prices.

The above goals for a refiner are similar. In raw sugar refining, raw sugar is the prime feedstock and good refinable quality raw sugar is just as important for the refinery as high quality sugar cane is for the raw sugar producer.


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Specifications for raw sugar exports While many raw sugar payment systems are based primarily on pol, there are a

number that also include penalties and bonuses for certain other quality parameters. The Australian industry has produced quality specifications for a range of bulk raw

sugar ‘brands’. In order to provide a tangible financial incentive for the production of premium quality raws, Queensland Sugar Limited (QSL) (2008) operates a raw sugar quality scheme that pays a premium value if all standard quality criteria are exceeded and applies a discount for deliveries that fall short of the specification. This scheme has proven successful in encouraging sugarcane millers to produce raw sugar within ‘brand’ quality ranges (Table 1).

Table 1—Typical brands and quality parameters of bulk raw sugar exported by QSL (source: www.queenslandsugar.com).

Polarisation (degrees)

Colour (ICUMSA)

Moisture (%)

Starch(ppm)

Dextran (ppm)

Brand JA 97.50–97.95 <3500 0.35 to 0.45 <80 <40

Brand 1 98.75–99.05 <2500 0.25 to 0.35 <70 <30

Brand IHP 99.15–99.35 <1500 0.20 to 0.25 <70 <30

Similar brands have been developed internationally and the main specifications are listed in Table 2.

Table 2—Typical analysis for various brands of raw sugar exporters (Rein, 2007).

Extra High Pol Very High Pol High Pol Low Pol Brand QHP V-VHP IHP VHP Brand 1 HP JA LP

Country1 AU BR AU SA AU SA AU SA Pol (oZ) 99.60 99.65 99.30 99.30 98.90 98.90 97.85 97.80 Moisture (g/100g) 0.12 0.06 0.18 0.10 0.29 0.24 0.60 0.35

RS (g/100g) 0.07 0.11 0.14 0.16 0.27 0.50 0.51 1.10 Ash (g/100g) 0.11 0.12 0.18 0.15 0.25 0.17 0.45 0.20 Colour (IU) 650 450 1100 1500 1800 1800 3300 2200 Starch (ppm) 40 250 50 110 60 110 80 110 Dextran (ppm) 20 350 20 90 22 90 19 90

1AU Australia, BR Brazil, SA South Africa Typical sugar refinery

A typical Australian refinery processes Very High Pol (VHP) (99.2o Z) raw sugar. Raw sugar of approximately 1600 colour (ICUMSA) is decolourised through several stages to produce a refined white sugar of 25 colour (ICUMSA). There are different types of


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refineries but generally they have several features in common. The refinery process consists of affination, clarification (carbonatation or phosphatation), filtration, decolourisation (activated carbon or ion exchange), crystallisation and recovery. The diagram below (Figure 1) shows the process flow for the Manildra Harwood Sugars (MHS) refinery.

Fig. 1—Process flow for MHS refinery.

Raw sugar quality criteria There are a number of criteria that can be used to define raw sugar quality, most of

which are of critical concern to the refiner as they all have an effect on the efficiency and performance of the refinery or the quality of the refined sugar product. These key quality criteria are pol, colour, ash, dextran, filterability, grain size/distribution (grist), starch and insoluble solids. In addition, both moisture content and temperature are critical factors influencing the storage quality of bulk raw sugar.

Pol Pol has long been the primary parameter in establishing the quality of the raw sugar

and it’s value to the refiner and is the key commercial measure of sugar content. The higher the pol of a raw sugar, the lower the impurity load on the refinery and the higher the refined sugar yield per tonne of melt raw sugar. The amount of molasses produced and the pol in the molasses are dependent on the nature and quantity of the impurities. The exhaustion of


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molasses in the refinery is much more difficult than in a raw sugar mill. The Harwood Refinery was designed to process VHP raw sugar and the recovery house target is a final molasses purity of 55. In practice this target is very difficult to achieve due the nature and viscosity of the recovery syrups. Small reductions in pol lead to significant increases in impurity loading on the recovery house. For example, a small reduction in pol from, say, 99.2o Z to 98.9o Z, will increase the load on the recovery station by 30%. In the refinery recovery house a larger proportion of the impurities are preferentially included within the sugar crystals, as they have a greater propensity to co-crystallise. Refineries that process VHP sugars contain a higher proportion of impurities such as polysaccharides and oligosaccharides, the latter leading to elongation of the crystals (Rein, 2007). Figure 2 shows a typical Harwood last stage recovery boiling. This photograph was taken when processing ‘fresh’ raw sugar with very low dextran levels.

Fig. 2—Typical 3rd crop (boilout) massecuite showing crystal elongation.

In practice, there are additional factors that can influence the optimum sugar pol for any given refinery. One is the commercial value that a refinery places on premium pol raw sugar. A further factor is the makeup of the final product mix from the refinery. This is the relative outputs of refined crystal, liquid sugar, speciality (soft) brown sugars and syrups (Rein, 2007). The strategy of many refineries is to divert molasses impurities into the production of relatively high value speciality syrups and soft brown sugars rather than produce a low value.

Colour Most refiners regard colour as the most important raw sugar quality parameter. The

removal of colour is the key function of a refinery and is a high cost procedure, regardless of the methods employed. In effective processing, having knowledge of the total colour and the nature of the colorants in the raw sugar is very important, as well as the distribution of colorants between the syrup film and the crystal.

Colour in raw sugar is not a single compound but a heterogeneous complex mixture. Considerable research has been undertaken to identify and have a better chemical understanding of colorants, their origins and best methods for removal. In summary, sugar colorants are normally characterised by:


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• Indicator Value (IV) is used as a measure of pH sensitivity of different types of colorants (IV = colour at pH 9/colour at pH 4).

• Low molecular weight (LMW) colour. The cane plant pigments supply around 30% of all raw sugar colorant compounds (Paton, 1992). These include flavonoids, chlorophylls and phenolic compounds. Phenonolic compounds may undergo polymerisation (enzymatically or thermally) and/or oxidation to produce more highly coloured materials. These have a high indicator value (IV) as they are usually ionised at high pH levels and are more easily removed in the refinery.

• High molecular weight (HMW) colour. The factory-produced colorants are much higher in molecular weight and are responsible for about 70% of raw sugar colour (Paton, 1992). They are categorised as melanoidins, alkaline degradation products (ADP) and caramels. They have a low IV, are insensitive to pH and less ionisable. These colorants are considerably harder to remove as they persist through the refining process and exhibit preferential inclusion into the growing sugar crystals Lindeman and O’Shea, 2004).

For refiners, the emphasis is on the colour in the affined sugar (raw sugar after affination) (Wright, 1988) compared with the total raw colour levels on which quality criteria are based. This affined colour is determined by the efficiency of the affination process and the distribution of the colorants (particularly HMW colour) in the raw sugar crystal. This is because affination can only remove colorants that are concentrated on the syrup film surrounding the crystal. Of the greatest interest to the refiner is the amount of impurities in the affined sugar i.e. the crystal colour. With high affined sugar colours, often the first response is to increase the amount of affination wash. This sends more sugar to the recovery house with increased losses. Higher affined sugar colour increases the loading on decolourisation processes, which in turn leads to reduced capacity and increased costs.

The effect of colour in the affined sugar will vary for each refiner and is dependent on the decolourisation capabilities of the refinery. Australian sugars are often regarded as difficult to refine due to HMW colour. However, it has been shown that, while Australian raw sugars exhibit slightly above average levels of HMW colorants, levels in affined sugar were among the lowest (Lindeman and O’Shea, 2001). Table 3 summarises the characteristics and removal processes of different colorants.

As the above table demonstrates, different decolourisation processes are more effective in removing different types of sugar colorants. In MHS refinery’s case, cane colorants derived from cane tops and trash that carry through into raw sugar are of a particular concern. They are included in the sugar crystal and are difficult to remove in phosphatation/ion exchange decolourisation process. Of further concern is the move to co-generation and thermal efficiency initiatives in raw sugar mills. These processes lead to cane juice exposed to higher temperatures through the evaporator station. This increases the propensity of HMW colour formation, in particular, caramels, which again are difficult to remove in the MHS decolourisation process.

Ash Affination is the main ash removal process in a phosphatation/ion exchange refinery. The

amount of ash removed in affination is dependent on a number of factors; one of the most important being the quantity and composition of the impurities in the adhering molasses film.


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Table 3—Characteristics of different groups of colorants (Rein, 2007).

Cane colorants ADP Caramels Melanoidins Melanins

Low molecular weight High molecular weight

Origin

Flavonoids, other phenolics

extracted from cane stalk. Iron complexes with

phenols

Reaction of hexoses at high

pH (>8; optimum at pH

11)

Thermal decomposition of

sucrose at temperatures above 120oC

In Maillard reactions involving

amino acids and invert

sugar at high RDS levels

Enzymic browning of

phenols. Also from reaction of phenolics

+ amino acids

Degree of ionisation

Ionised at high pH. Neutral at

low pH

Uncharged. Cationic below pH 5, anionic above pH 6.

Slight charge. Ionised at pH 7.2

Cationic below pH 5, anionic above pH 6.

Negatively charged

Polarity Least polar Polar Intermediate Polar

Indicator value

Phenolics and flavonoids

5 – 14 1.5–3 1–2 1–1.2 Low

Removed by

Plant pigments removed in juice

clarification. Phenolics by carbonatation

and GAC

Phosphatation and ion

exchange (styrenic)

Granular activated carbon

(GAC)

Completely removed by ion

exchange GAC

Affinity for sugar crystal

Included in crystal

Largely removed in affination

Included in crystal

Largely removed in affination

Readily included in

crystal

Ash impacts on the refining processes in a number of ways: • Ash components such as potassium and sodium increase the solubility of

sucrose thus leading to increased sugar loss in molasses. • Accumulation of high levels of chlorides in lower grade syrups can lead to

stress corrosion cracking of centrifugal baskets. • High sulphate levels can produce increased scaling in evaporators and pans. • High levels of sulphate ions can lead to reduction in decolourisation

performance of acrylic and styrene resins although this effect is often quite small (Donovan, 1993).

Dextran Dextran is a natural polysaccharide produced by bacterial degradation of sucrose in

sugar cane or juice. Significant quantities of dextran in raw sugar affect the following: Clarification/filtration Dextran has a deleterious effect on raw sugar mill clarification by inhibiting

coagulation and thus allowing fine suspended matter to carry over into clarified juice. The presence of fine insoluble matter has an adverse effect on filterability in the refinery. Donovan (1993) stated that the presence of suspended solids resulted in poor filtration rates which affected production rates very noticeably.


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Viscosity The impact on viscosity is one of the most important of the dextran effects on sugar

processing. It significantly increases viscosity in syrup streams resulting in reduced heat transfer coefficients of pans with reductions in boiling rates.

Crystallisation Dextran causes both a reduction in crystallisation rates (high viscosity) as well as

crystal elongation. In particular, the centrifugal purging characteristics of recovery massecuites are reduced due to high syrup viscosities and elongated and fine grain. The poor purging characteristics of recovery boilings can lead to high impurity recycle and subsequent increased recovery massecuite production. At times it may be impossible to boil the final massecuite and intermediate recovery syrups may need to be discarded.

Filterability Filterability is an indication of the filtering characteristics of raw sugar when it is

processed through the clarification stage of a carbonatation refinery. Anything that impedes the normal rate of filtration is a concern as it greatly increases operating costs by reducing throughput and hence refining capacity. The typical operational response to poor filterability is to reduce liquor brix (Donovan, 1993). This is costly in terms of increased energy costs and reduced production. Poor filterability can result from one or a combination of the following impurities:

• Mud, bagacillo and turbidity. Particulate impurities of 5 μm or less have the greatest adverse effect.

• Phosphorous, silica and other inorganic impurities. Phosphorus is not generally a problem with modern high standards of clarification. However, colloidal and soluble silica appear to be one of the most critical impurities. Salts of Mg2+, Fe3+ and Al3+ have been shown to impede filtration (Watson and Nicol, 1975).

• Dextran, starch, gums, proteins and other high molecular weight polysaccharides can also reduce filterability. Although starch can cause serious filtration problems in carbonatation refineries, it does not affect the filterability test and so is analysed separately. HMW polysaccharides reduce filterability through a viscosity effect. Protein levels are normally low in raw sugar.

Starch

Starch is a naturally occurring polysaccharide found in the stalk, leaves and tops of sugar cane. Starch molecules have a high partition co-efficient to sugar crystals and consequently attach themselves to the growing crystals more easily than other impurities. In carbonatation refining it interferes with the precipitation and coagulation of calcium carbonate crystals. This results in poor filterability characteristics (Donovan, 1993).

It is thought that starch has very little effect in refineries which operate with phosphatation or direct filtration (SRI, 2007). However, South African literature (Murray et al., 1976) states that starch modifies the precipitation and coagulation of calcium phosphate, resulting in high phosphate levels in clarified liquor.

It has been observed at MHS refinery that processing raw sugar with higher starch levels, increases clarified liquor turbidity and phosphate levels.


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Insoluble solids Insoluble solids in raw sugar are defined as the insoluble matter that is caught on a

20–25 μm filter. High levels of insoluble solids put extra load on clarification. It also causes reduced purging efficiency of affination fugals by blocking the backing screens. Refineries that run continuous affination fugals may need to clean backing screens on a daily basis where there is a high level of insoluble solids in the feed sugar.

Grain size (Grist) Refiners prefer raw sugar of uniform grain size (narrow distribution) with a minimum

of conglomerates. . The absence of fines in raw sugar will create less dust problems during bulk handling. Large uniform crystals give excellent drainage during affination fugalling. In contrast, a small crystal size with its associated larger crystal surface area requires more fugal wash to achieve the same level of colour and impurity reduction. Sugar with wide crystal distribution sizes tend to pack down in the affination fugal with smaller channels for molasses drainage reducing the performance of the fugals. Conglomerates in raw sugar crystals make it very difficult to wash the molasses film from them effectively. Molasses that is trapped is carried through to other decolourisation stages of the refinery, increasing the load on these processes.

Moisture The storage and handling quality of raw bulk sugar during storage is determined in a

large part by its moisture content. There is an optimum range for the moisture content of raw sugar that can be acceptably stored and handled. If the moisture level is too low, dust problems occur at transfer points and the risk of potential sugar dust explosions increases. When the sugar moisture is too high, micro-organisms in the molasses film surrounding the crystal cause deterioration of the sugar, reducing the pol and increasing the amount of impurities. High moisture can also cause handling problems due to caking and sticking. The dilution indicator (DI) is a factor used to forecast the keeping quality and handling quality of raw sugar (BSES, 1984).

DI = moisture / (100 – (pol + moisture)) x 100

DI values between 30 and 40 are satisfactory, above 50 cause deterioration and below 25 cause serious dust problems.

Temperature The temperature at which raw sugar is stored influences its rate of colour formation.

It is generally accepted that for every 10oC rise in temperature the rate of colour formation increases by a factor of approximately three (SRI, 2007). Australian raw sugar is often stored for extended periods (up to 12 months or more), therefore an upper limit exists for the temperature of good quality sugar entering storage.

Bulk sugar is highly insulating so there is little temperature change from when the sugar leaves the drier to when it is in storage. Substantial overcooling can lead to reabsorption of water from the atmosphere into the sugar with subsequent problems associated with high moisture. Conclusion

In many refineries, and certainly in the case of the MHS refinery, production rates are well above the original design throughput. Refiners require raw sugar that will allow their factories to operate at this peak capacity. Small differences in the level of impurities in raw


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sugar can cause large differences in the way the refinery is operated and its overall capacity and efficiency. It is the impurities in the affined sugar that affect production rates. Impurities in the raw syrup (molasses film after affination) impact on the recovery house exhaustion. The MHS refinery will generally not slow melt rate to achieve better recovery house exhaustion.

Of the impurities contained in affined sugar, colour is the most important to the refiner (although the extent of this will depend on the decolourisation configuration and capacity of the plant). Manildra Harwood refinery has a relatively weak decolourisation process, therefore, given the choice between raw sugars with similar total colours, the preference will be the raw sugar with the lowest crystal colour.

It is important for raw sugar producers to understand the affect that impurities have on refinery operations. Refiners operate under similar economic and market pressures to those experienced by mills. Refiners require raw sugar that will allow their factories to operate at optimum efficiency, performance and capacity. Acknowledgements

The author thanks the Production Staff at Manildra Harwood Sugars for their support and assistance. ‘Cane Sugar Engineering’ by Professor Peter Rein was used as a reference book on aspects of overseas raw sugar quality and refining processes.

REFERENCES BSES (1984) ‘The Standard Laboratory Manual for Australian Sugar Mills’. Volume 1 p. 3.

(BSES: Brisbane) Donovan M (1993) Raw sugar quality—The effect on the refiner. Proceedings of the Sugar

Industry Technologists 52, 117–134. Lindeman PF, O’Shea MG (2001) High molecular weight (HMW) colorants and their impact

on refinability of raw sugar. A study of Australian and overseas raw sugars. Proceedings of the Australian Society of Sugar Cane Technologists 23, 322–329.

Lindeman PF, O’Shea MG (2004) Colorant removal during clarification and decolorisation process. Proceedings of the Australian Society of Sugar Cane Technologists 26, (CD-ROM).

Murray JP, Runggas FM, Shepherd GS (1976) Influence of raw sugar quality on phosphatation process. Proceedings of the South African Sugar Technologists Association 50, 179–183.

Paton NH (1992) The origin of colour in raw sugar. Proceedings of the Australian Society of Sugar Cane Technologists 14, 8–17.

Queensland Sugar Limited (2008). Sugar quality management. http://www.queenslandsugar.com/page.cfm?pageid=192 (accessed 2 December 2008).

Rein P (2007) ‘Cane Sugar Engineering’ (Bartens: Germany). SRI (2007) Handbook for Chemical Supervision Course. Watson JA, Nicol WM (1975) Raw cane sugar quality in relation to refining requirements.

Sugar Technologists 64, 77–88. Wright PG (1988) Colour as a quality problem – crystal quality. Sugar Research Institute

Technical Conference.

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