4.5Influence of dextran concentrations and molecular fractions on the rate of sucrose crystallization in pure sucrose solutions

There are different methods to perform the crystallization experiment and follow the growth rate of sucrose crystals during the crystallization process. In laboratory isothermal crystallization experiments (as described under 3.2.3.1) the influence of dextran on sucrose growth was studied without interference of other factors.

The effect of different dextran concentrations and molecular weight fractions on crystallization rate in sucrose solutions will be discussed ahead to demonstrate the complexity of the problem and to elucidate the methodology of performed crystallization experiments. Based on first experiments the influence of temperature on crystallization rate in presence and absence of dextran will be balanced, too. All presented results finally demonstrate the sensitivity of the crystallization process on dextran concentration in the system. From this, the consequences of dextran presence for industrial crystallization units can be derived.

Figure 35 shows the dry substance contents of the mother liquor ( wDS ,ML ) during the

crystallization (ϑ = 60°C) for a pure sucrose solution and after addition of 500, 1500 and 2000 mg/kg DS dextran T40. The results indicate that the dry substance of mother liquor decreases during the crystallization process. The figure shows that concentrations of up to 500 mg/kg DS of dextran T40 in the sucrose syrup have only little effect on the dry substance decrease. However, concentrations between 1500 and 2000 mg/kg DS of dextran with the same molecular weight however give effect on the exhaustion of mother liquor.

The growth rates of sucrose crystals calculated from the data shown in Figure 35 during the crystallization process for pure sucrose solution after addition of dextran T40 of different concentrations (500, 1500 and 2000 mg/kg DS) at 60°C are shown in Figure 36. Only small differences in the crystallization rates of control (without dextran) and with addition of 500 mg/kg DS of dextran T40 were observed. On contrary, the admixture of higher concentrations of T40 (1500-2000 mg/kg DS) causes a serious retardation of crystallization rate and thus represents a danger for crystallization process economy, in particular if industry-relevant supersaturations above 1.1 are regarded. For example, at SS 1.12 the crystallization rate with 2000 mg/kg DS T40 is almost 15 % lower compared to the control experiment. On the

other hand, at supersaturation 1.13 (as indicated in the diagram), the growth rate is reduced by 7.4 %, 9 % and 18 % after dextran T40 at concentrations of 500, 1500 and 2000 mg/kg DS, respectively, was added.

Control 500 mg/kg DS 1500 mg/kg DS 2000 mg/kg DS

Figure 35: Dry substance content of mother liquor in isothermal crystallization experiments at 60 °C in pure sucrose solution and after addition of 500, 1500 and 2000 mg/kg DS of dextran T40

Figure 36: Growth rate of sucrose crystals in isothermal crystallization experiments at 60 °C in pure sucrose solution and after addition of 500, 1500 and 2000 mg/kg DS of dextran T40

Figure 37 illustrates the relationship between the dry substance content and the time of the crystallization process for pure sucrose solutions (control) and after addition of dextran T500 (500, 1500 and 2000 mg/ kg DS) at 60 °C. An increase of molecular weight of dextran from 40.000 (T40) (s. experiments above) to 500,000 (T500) at 60°C caused an increase of crystallization time. A relevant effect of dextran T500 was observed with concentrations higher than 500 mg/kg DS.

Figure 38 shows the effect of dextran T500 at concentrations of 1500 and 2000 mg/kg DS on the growth rate of sucrose crystals at 60°C. This figure indicates that the crystallization rate regarding a supersaturation of 1.13 in the sugar solution without dextran (control) is the same as the crystallization rate at supersaturations of 1.134, 1.135 and 1.138 in presence of 500, 1500 and 2000 mg/kg DS of dextran T500, respectively. Also, a stronger effect of dextran T500 than dextran T40 can be observed at 60°C. As it can be seen from Figure 38 that the crystallization rates at 60°C at a supersaturation of 1.13 are reduced by 7 %, 10 % and 20 % in presence of 500, 1500 and 2000 mg/kg DS dextran T500, respectively.

Time (min)

Control 500 mg/kg DS 1500 mg/kg DS 2000 mg/kg DS

Figure 37: Dry substance content of mother liquor in isothermal crystallization experiments at 60 °C in pure sucrose solution and after addition of 500, 1500 and 2000 mg/kg DS of dextran T500

Figure 38: Growth rate of sucrose crystals in isothermal crystallization experiments at 60 °C in pure sucrose solution and after addition of 500, 1500 and 2000 mg/kg DS of dextran T500

The results from Figure 37 and Figure 38 clearly demonstrate that the reduction of growth rate at technologically relevant supersaturations is even more than described in Figure 35 and Figure 36 (dextran T40). That means, if in a sugar house a thick juice contains 1500 mg/kg DS dextran T500, the crystallization period will be extended by up to 20 %.

Further experiments on the influence of high dextran concentrations were carried out with dextran T2000. The mother liquor dry substance contents in the presence of 500, 1500 and 5000 mg/kg DS of dextran T2000 are shown in Figure 39. The time till a supersaturation of 1.05 was reached increased by 21 %, 36 and 41 % after 500, 1500 and 5000 mg/kg DS dextran T2000 addition, respectively.

The growth rate of sucrose crystals at 60 °C and a supersaturation of 1.13 were 2.7, 2.4, 1.8 and 1.3 g/(m2 min) for control and after dextran T2000 addition of 1500 and 5000 mg/kg DS, respectively (see Figure 40). Here the crystallization rates were reduced by 12, 33 % and 51 % after addition of 500, 1500 and 5000 mg/kg DS dextran T2000