A REVIEW OF THE CRYSTALLIZATION OF SUGAR

Abstract

E.T. WmTE Chemical Engineering Department The University of Queensland BRISBANE, Qld Australia 4072 Fax: 617 33654199 Email: tedw@cheque.uq.edu.au

A brief review is given on the relations necessary to model a sugar crystallizer. These include relations for solubility, the nucleation limit, and growth rates.

1. Introduction

Sugar crystallization has been extensively studied and there is now a good deal of knowledge on this system. World wide sugar production is over 100 million tonnes per year. This corresponds to 10 billion crystals, or on average, 3 billion every second. These are created, grown and consumed at this rate.

Overall cane and beet can be considered as a mixture of water (W), sugar (sucrose, S), water insolubles (fibre, F) and impurities (I) which includes all the other water soluble species (e.g. salts, reducing sugars). In processing, the fibre can be removed by mechanical means and the water by evaporation. Crystallization remains the preferred means of separating sugar from the impurities. As the sugar crystal is extremely selective as to what it builds into its lattice, theoretically absolutely pure crystals can be produced by crystallization. In practice however there is a significant carry-over of impurity with the adhering liquor film and often there is some included mother liquor within the crystals.

Sugar has a very high solubility in water (solutions range from 10 to 30% water content) so solutions are very viscous. This is a significant constraint in processing. Having prepared sugar crystals, it is necessary to separate them from the mother liquor (molasses). This is carried out in high speed

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B. Sen Gupta and S. Ibrahim (eels.), Mixing and Crystallization, 329-336. © 2000 Kluwer Academic Publishers.

330 E.T. WHITE

centrifuges. For efficient separation it is necessary for the crystals to be large and of reasonably uniform size. The size and size distribution of the material is also important in the later flow of the crystalline product for bulk handling.

2. Criteria

In a crystallization process, the information required for modelling, and thus decision making, is,

1. Phase information (solubility), including morphology. 2. Nucleation information, including the secondary nucleation limit. 3. Growth kinetics as a function of conditions.

Decisions have to be made concerning:

• Recovery, particularly how much product is lost (not recovered). In general, this decision has strong economic consequences.

• Quality. This includes purity, size, spread of sizes, crystal shape, etc. Quality has commercial consequences.

• Operating conditions (concentrations, temperatures and pressures, agitation level)

• Equipment design.

Some of these will now be considered in more detail.

3. Solubility

Sugar belongs to the monoclinic system (three axes of unequal lengths, two axes at right angles, the third inclined at 103.5°). The crystals formed can show a large number of pairs of faces. The eight common pairs found in industrial crystals are shown in Figure 1. From pure solutions, refined sugar shows a slight elongation in the b-axis direction. From typical industrial molasses the crystals are reasonably equi-dimensional. Most impurities have little effect on the shape, though dextrans and raffinose will cause serious elongation. Vavrinecz (1965) gives an atlas of crystal shapes.

r------ - - ------- --{ I C

I I I I I I

.... ";::::::::::::: , /

Figure 1: The faces on a sugar crystal. (Broadfoot, 1980)

A REVIEW OF THE CRYSTALLIZATION OF SUGAR

5 80~~~.-~~~~~~~~~,-~~" :;::;; :l '0 tJ)

.= Iii 75 Ol :l tJ)

~ ~ 70

~ :c :l

~ 65~~~~~~~~~~~~~~~~~

3.5

3.0

2.5

331

~ >-..c Figure 2: The ~ solubility of sugar in en pure water.

~ :c :l '0 en

30 40 50 60 70 80 Temperature,OC

While crystallization can be carried out by cooling, the recovery is limited (Figure 2). So for significant recovery it is necessary to evaporate water. Sugar will decompose (caramelise) at high temperatures (above 70-80°C), so evaporative crystallization is carried out under vacuum, typically at about 65°C. Since the crystallization is faster and the vacuum required is softer at higher temperatures, the highest practical operating temperature is required.

The fraction of the dissolved solids that is impurity (V[I+S]) typically is 10% initially and rises to about 50% at the end, so the effect of impurities becomes very important. Figure 3 shows the effect of impurities (typical for Australian canes) on the solubility at 65°C. The ratio of sugar to water S/W is not greatly affected by the impurity level or temperature and this becomes a convenient measure of concentration. Figure 4 shows the ratio of S/W to the pure solution value S/Weqm as the impurity content increases for one molasses (Maudarbocus and White, 1978).

Thus from the phase and other information, the crystallization of sugar will be undertaken primarily using vacuum evaporative crystallization at temperatures about 65°C to prevent sugar degradation. The morphology of the crystals formed is acceptable.

1.0

0.9

c 0.8 0 ., c> 0.7 II ..; 0.6 ~

0.5

0.4

0.0 0.1

W

0.2 0.3

II(S+I), by wt. 0.4 0.5 0.6

Figure 3: Effect of impurity on solubility at 65°C

332

1.0

,J \ ~ \ ~

0.9 \

~ ~ 0.8

0.7 a

\ \ \

'-.. --2 3 Itw, bywt.

4 5

E.T. WHITE

Figure 4: Effect of impurity on solubility

It is useful now to consider quantities in the crystallization of sugar as shown in the following table,

Corresponding juice Syrup to crystallizer End of crystallization

S 100 100

10

W 500

50 3

I 10 10 10

Product Xtl.

90

The figures are for 100 units of sugar. Sugar juice is far more dilute than a saturated solution, so most of the water in the juice is evaporated off prior to crystallization to give syrup as a feed. So much crystal is produced and so little liquor remains that for flow the crystallization cannot be done in a simple single step. In Australia, generally three steps (boilings or strikes) are used. The syrup is crystallized to give the A boiling and the molasses is centrifuged off for a second B boiling. Both the A and B crystal is final product. The molasses centrifuged from the B boiling is further crystallized (C boiling) to give the seed for the A and B boilings. The molasses from the C boiling is then cooled for final sugar recovery.

If a crystal is grown from a seed of 5 J..I.m to a product of say 1 mm, the volume of the crystal has increased by a factor of 8 million. Thus 10 tonne of product would have come from about 1 g of seed. Handling this range of quantities in a single operation is not practical, so the crystals are grown in a series of graining stages for the C boiling and the C boiling crystals are used as the seeds for the A and B boilings.

A REVIEW OF THE CRYSTALLIZATION OF SUGAR 333

4. Nucleation

Sugar is the classical case of a material with a large metastable zone. In this region below the SNT (secondary nucleation threshold) crystals will growth but will not nucleate even in the presence of other sugar crystals (secondary nucleation). Figure 5 shows data on the magnitude of this threshold (Broadfoot, 1980). For sugar, the growth rates in this region are substantial, so it becomes an ideal place to operate a crystallization. As no further nucleation will occur, the extent of seeding controls the product size. The secondly nucleation threshold is a fuzzy not a 'hard' limit, so practice is to operate below the limit but as close to it as control will allow. If the SNT is exceeded, nucleation will occur. Figure 6 gives some measures of nucleation rates for pure solutions at low crystal contents. In practical terms, these are massive.

~

o .. 0.4

~ -~ 0.3

::::;

W I-0« 0:: 20 Z o i= 0« w ...J U 10 :::> z ai

7S0C 23% crystal content

• BaHerham elal. (1975) • Broadfooland Wrighl (1972)

- O'SHT=O.11 +3.6[V(S+I)f

•

0.6 0.7 0.8 0.9 SOLUTION PURITY, SI(S+I) by wt.

60 - 70·C 0.3 - 2 % crystal conlenl pure sucrose solulions

as NT = 0.11

00'" BaHerham elal. (1975) • Broadfool and Wrighl (1972)

o

0.0 0.1 O'-O'SNT 0.2

1.0

0.3

Figure 5.: Secondary nucleation limit for sugar .

Figure 6. Nucleation rates.

334 E.T. WHITE

5. Growth rates

The growth rate of a crystal in practice is defined as the change of size with time (e.g. in J.l.m /min units), with size usually taken as the volume equivalent size (i.e. the diameter of a sphere with the same volume as the crystal). This measure of size is convenient for models using mass balances. A lot of data exists for the crystallization of sugar from pure solutions (Wright, 1971), however impurities at the levels encountered in practice have a marked effect, so only the results for industrial conditions will be illustrated.

The driving force for crystallization is the sugar concentration in excess of the solubility value. Many measures of this can be used but it has become traditional to use the relative supersaturation a = (S/w)/(S/W)eqm -1.

The effect of growth rate on supersaturation follows the classical BCF theory, with the growth rate showing an initial square law behaviour at low supersaturations, and becomes a straight line at higher values. As the linear region covers the usual industrial conditions, it is usual to fit this region only and take the growth rate as depending linearly on (a-0.005), i.e. G = ko (a-0.005). An overall equation summarising Australian data is given by Wright (1971). Figure 7 shows the dependence ofG for a = 0.1 on solution purity and temperature. A supersaturation of 0.1 is rather high for A and B boilings and low for C. Typically growth rates for A boilings are below 5 J.I.ffi /min and for C below 1. The substantial effect of temperature and impurities can be seen.

An ultimate limit to the recovery of sugar is the solubility of sugar in the final molasses and in principle, water could be evaporated off until one of the impurities started to crystallize. However as the impurity level becomes high, the growth rates become uneconomically slow. Also the moisture content of the solution become so low, and thus the viscosity so high, that the material will not circulate in an evaporative crystallizer (pan). The critical value of the viscosity depends on the design of the pan but is in the order of20 Pa s.

In a batch pan, already running at the highest allowable temperature, for optimum operation the supersaturation must be kept as high as possible, consistent with the SNT (Figure 5). In multistage continuous pans, there is some advantage in apportioning the feed rates to the various compartments (Broadfoot and Wright, 1978).

A REVIEW OF THE CRYSTALLIZATION OF SUGAR 335

20

c 10 E 6 -E 4 :::i. ~ 2 0r-

o 1 II b 0.6 - 0.4 ca

C) 0.2

0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

1/(5+1), by wt.

Figure 7: Gro\\oth rate of sugar crystals (from equation of Wright (1971)

There is a serious aspect of growth not yet considered. This is Growth Rate Dispersion (GRD) and is the phenomenon where two sugar crystals of the same size in the same solution can grow at vastly different rates. Differences of a factor of 10 have been found. Thus by nature some crystals are fast growers and others slow. This is thought to be related to differences in dislocation densities within crystals. GRD has a marked effect on the spread of sizes from a sugar crystallizer, both batch and continuous. This is an active area of present research and new methods have been proposed to quantify it (White et aI., 1998)

6. Modelling

All the above information can be fitted to equations and incorporated into a model. These models are widely used and accepted, particularly in the Australian industry. A model can be used to predict the performance of a sugar crystallizer, to design one or to devise advanced control schemes (Wilson et aI., 1991). Models are available for evaporative and cooling crystallizers, operated either batchwise or continuously (e.g. Wright and White, 1968, 1969; Wright, 1971; Broadfoot and Wright, 1975; Broadfoot, 1980; Maudarbocus and White, 1978).

336 E.T.WHITE

The models are based on material balances (for sugar, water, impurity and crystal), the solubility and crystallization rate equations considered above and the population balance. The population balance is an accounting for crystal numbers and allows for the shape of the crystal size distribution (e.g. the size spread). Its use is vital for an accurate sugar crystallization model. Recent work has been to incorporate GRD into these models.

7. Conclusions

A brief review of mechanisms affecting sugar crystallization has been given. A substantial amount of research has been carried out and there is an acceptable level of understanding, sufficient to give models which are acceptable for industrial use. The data given are values averaged over Australian practice. It is not known what variations there are from molasses to molasses or from the materials in one country to another. Despite the amount of work, there is still need to refine the data and lower the level of uncertainty associated with the correlations. GRD remains an area still to be finally resolved.

References

Batterham, R.I, T.E. Norgate, F. Sweett and R.N. Taylor (1975) "Nucleation in sugar boiling", Proc. Qld. Soc. Sugar Cane Technol., 42, 211.

Broadfoot, R. (1980) "Modelling and optimum design of continuous sugar pans", PhD thesis, The Univ. of Queensland.

Broadfoot, R and E.T. White (1975). "Performance charts for continuous pans". Proc. Qld Soc. Sugar Cane Technol., 42, 235.

Broadfoot, R. and E.T. White (1978) "Sugar crystallization as a continuous flow process", Int. Sugar 1., 80, 135.

Broadfoot, R and P.G. Wright (1972) "Nucleation studies", Proc. Qld. Soc. Sugar Cane Technol., 39, 353.

Charles, D.F. (1960) "The solubility of pure sucrose in water", Int. Sugar 1.,62,126. Maudarbocus, S.M.R and E.T. White (1978) "Computer model of a cooling crystallizer", Proc.

Qld. Soc. Sugar Cane Techno!., 45, 45. Vavrinecz, G. (1965) "Atlas of sugar crystals", Bartens, Berlin. White, E.T., D.L. Mackintosh, B.K. Butler, H. Zhang and M.R. Johns (1998) "Modelling GRD

in sugar crystall-ization", Proc Aust. Soc. Sugar Cane Techno!., 20, (in press). Wilson, 0.1., P.L. Lee, E.T. White and RB Newell (1991) "Advanced control of a sugar

crystalliser",1. Process Control, 1, 197. Wright, P.G. (1971) "A model of industrial sugar crystallization", PhD thesis, The Univ. of

Queensland Wright, P.G. and White, E.T. (1968). A mathematical model of vacuum pan crystallisation. Proc.

Internat. Soc. Sugar Cane Techno!., 13th Congr., 1697. Wright, P.G. and White, E.T. (1969). Size distribution studies in sugar crystallization. Proc. Qld

Soc. Sugar Cane Technol., 36, 299.