Using the spectrophotometric method, as a new method, the influence of dextran on the sucrose solubility and metastable zone width has been studied. In agreement with the literature the experimental data show that the dextran has a negligible effect on the sucrose solubility. The results also show that this impurity decreases the sucrose metastable zone width. The study of the nucleation kinetics performed, using Nyvlt’s approach, shows that the dextran accelerates the nucleation and that the nuclei are formed in the solution by instantaneous nucleation. The presence of dextran in the system causes a decrease in the growth rate of sucrose. The growth process of sucrose is governed by a Birth and Spread mechanism. The kinetic parameters of sucrose growth in aqueous solutions without and with dextran were estimated.
Crystallization is an important process in industrial operations. In sugar manufacturing, crystallization is a crucial step that determines the quality of the final product, which requires control of its fundamental parameters. The presence of impurities (non-sugars) in the sugar solution may influence the sucrose crystallization. In fact, impurities in supersaturated solutions significantly affect the nucleation, growth rate, morphology, and also the agglomeration rate of the crystals [
The presence of dextran in the sugar juice causes certain problems at the level of the sugar treatment in particular in the crystallization process, in fact the presence of this impurity causes a significant increase in the viscosity, elongated crystals, lower evaporation rates, longer wash and separation cycles in centrifuges, and loss of sugar to molasses [
Consequently, the aim of this work is to study the effect of dextran on the fundamental parameters of sucrose crystallization in particular the solubility, the metastable zone width, the nucleation rate, and the growth rate.
According to Nývlt’s approach [
On the other hand, Nývlt [
The supersaturation rate during cooling is given by the following relation:
The following equation gives the relation between the mass of formed nuclei and the number of nuclei formed:
The increase in the mass of crystals with time, in the case of isothermal seeded crystallization, may be expressed by the following expression [
From Eq. (
One can assume constant initial seed area ac which leads to the following expression:
Sucrose and dextran employed are “of analytical” quality to avoid any other impurities which may influence the measurements. Distilled water is used as solvent.
The experimental setup used for the determination of the sucrose solubility and metastable zone width consists of a jacketed reactor with a mechanical stirrer, a cryothermostat for temperature control, and a digital thermometer for temperature measurement (accuracy ±0.1°C) (see Figure
Experimental setup.
The experimental setup is identical to that presented in Figure
To determine the nucleation kinetics of sucrose, water-sucrose solutions were prepared by dissolving known masses of sucrose in 100 ml of distilled water at temperatures higher than those of the solubility. The metastable zone widths for different cooling rates (0.2, 0.5, 1, and 1.5°C/min) were measured. Based on Eq. (
The experimental setup is identical to that presented in Figure
Preliminary tests showed that the concentrations of dextran studied do not influence the refractive index of sucrose and therefore the refractive index measured in the impure mediums corresponds to that of sucrose.
The influence of dextran on the sucrose solubility and the metastable zone width of its aqueous solutions has been studied. Three concentrations of this impurity (500, 1000, and 1500 ppm) and for different concentrations of sucrose were selected for the measurements. Figure
Saturation temperature
system |
|
|
|
|
MSZW (°C) |
---|---|---|---|---|---|
Water-Sucrose | 220 | 0 | 32.7 | 22.9 | 9.8 |
230 | 0 | 37.9 | 28.9 | 9.0 | |
240 | 0 | 42.6 | 35.2 | 7.4 | |
|
|||||
Water-sucrose-dextran | 220 | 500 | 32.8 | 23.1 | 9.7 |
1000 | 32.9 | 23.9 | 9 | ||
1500 | 33 | 25 | 8 | ||
230 | 500 | 37.8 | 29 | 8.8 | |
1000 | 37.9 | 29.8 | 8.1 | ||
1500 | 38.1 | 31 | 7.1 | ||
240 | 500 | 42.7 | 35.5 | 7.2 | |
1000 | 42.6 | 35.8 | 6.8 | ||
1500 | 42.9 | 37.1 | 5.8 |
Sucrose solubility in water for different concentrations of dextran.
The measured MSZWs as a function of cooling rate in aqueous solutions without and with dextran are illustrated in Figure
Plot
Based on data analysis and Nyvlt’s theory, the linear dependence expressed by Eq. (
The slopes of the curves, shown in Figure
Correlation equation between
System | Correlation |
R2 |
---|---|---|
water-sucrose |
|
0.9987 |
water-sucrose-dextran (500 ppm) |
|
0.9981 |
water-sucrose-dextran (1000 ppm) |
|
0.9999 |
water-sucrose-dextran (1500 ppm) |
|
0.9969 |
Nucleation kinetic parameters estimation.
System | m | k |
---|---|---|
water-sucrose | 3.53 | 4.55×106 |
water-sucrose-dextran (500 ppm) | 3.39 | 9.58×106 |
water-sucrose-dextran (1000 ppm) | 3.25 | 2.17×107 |
water-sucrose-dextran (1500 ppm) | 3.21 | 4.83×107 |
After estimating kinetic parameters for given systems, the estimated nucleation rates as a function of supersaturation based on Nyvlt’s theory, as given by Eq. (
Nucleation rate as a function of supersaturation in aqueous solutions without and with dextran.
Depending on Nyvlt’s approach evaluated, the nucleation orders m were found between 3.21 and 3.53 for different systems. These values are in good agreement with published data for organic materials. Moreover, the apparent nucleation order values (< 4) reveal that nuclei in the solution are formed by instantaneous nucleation [
According to the results obtained, the presence of dextran accelerates the nucleation. This acceleration can be explained by the fact that the increase of the concentration of this polysaccharide reduces the amount of free water molecules available for solvation of sucrose and, hence, the collisions probability between the solute molecules becomes more important.
The effect of dextran on the kinetics of sucrose growth has been studied. Three concentrations of this impurity (500, 1000, and 1500 ppm) and for different supersaturations were selected for the measurements. In order to estimate the growth kinetic parameters, the variation of the mass as a function of time for different supersaturations is plotted as shown in Figure
Estimation of growth kinetic parameters.
System |
|
R2 | |
g |
---|---|---|---|---|
water-sucrose |
|
0.9928 |
|
1.0 |
water-sucrose-dextran (500 ppm) |
|
0.9969 |
|
1.0 |
water-sucrose-dextran (1000 ppm) |
|
0.9951 |
|
1.0 |
water-sucrose-dextran (1500 ppm) |
|
0.9999 |
|
1.0 |
Evolution of mass sucrose crystal over time as function of supersaturation in aqueous solutions without and with dextran.
The results show that the orders obtained from the experimental data for the different systems are found close to 1 indicating that the growth process of sucrose is governed by a Birth and Spread mechanism (B+S) [
Growth rate versus supersaturation in aqueous solutions without and with dextran.
The decrease in the sucrose growth rate in the presence of dextran can be explained by the fact that this polysaccharide causes an increase in viscosity [
In this work, solubility of sucrose in aqueous solutions without and with dextran and their metastable zone width have been investigated using the spectrophotometric method, as a new method. In agreement with the literature, the addition of dextran causes a slight decrease in sucrose solubility. The results also show that the presence of dextran causes a decrease in the sucrose metastable zone width.
The nucleation study of sucrose, using Nyvlt’s theory, shows that the presence of dextran accelerates the nucleation of sucrose and that nuclei in the solution are formed by instantaneous nucleation.
The kinetics of sucrose growth in aqueous solutions without and with dextran was also studied. The results show that this impurity reduces crystal growth rate and that the growth process of sucrose is governed by a Birth and Spread mechanism (B+S).
Overall seed area,
Concentration, Kg.
Solubility concentration, Kg.
Supersaturation, Kg.
Maximum supersaturation, Kg.
Linear growth rate, m.
Order of growth, dimensionless
Nucleation rate,
Mass nucleation constant,
Number nucleation constant, dimensionless
Crystal growth rate coefficient,
Characteristic length, m
Solid mass, kg
Apparent nucleation order, dimensionless
Number of formed nuclei, dimensionless
Cooling rate, °C.
Critical nucleus radius, m
Rate of solute consumption per unit seed area, kg.
Correlation coefficient, dimensionless
Temperature, °C
Time, s
Supercooling, °C
Metastable zone width or MSZW, °C.
Volume shape factor, dimensionless
Area shape factor, dimensionless
Density, kg.
Initial relative supersaturation, dimensionless
Relative supersaturation, dimensionless.
The data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.