INTEGRATION OF THE PROCESS OF FRUCTOSE CRYSTALLIZATION BY
ADDITION OF ANTI-SOLVENT
C. Crestani, A.T.C.R. Silva, A. Bernardo, C. B. B. Costa, M. Giulietti.
Department of Chemical Engineering, Federal University of São Carlos, São Carlos,
Brazil (giulietti@ufscar.br).
Keywords: (anti-solvent, crystallization, fructose).
Sometimes crystallization processes can be more complex than usual. Crystallization of fructose requires the addition of an anti-solvent to increase process yield, improving its economic feasibility. It occurs because of the high solubility of fructose in water. The use of a second solvent allows working at lower temperatures and, in the case of fructose, reduces significantly the metastable zone width. The second solvent must be miscible with the first one and the solute must be slightly soluble in it. The solubility of fructose in mixed water/ethanol solvent was obtained through the polithermic method
The experimental data were adjusted using the equation proposed by Nývlt et al.
[1]
[2]
.
, which has three adjustable parameters. These constants were calculated for each concentration of the mixed solvent, turning possible the comparison of the solubility of fructose in different concentrations of ethanol in the mixed solvent. The constants were obtained by varying the temperature from 29°C to 60°C so the parameters are representative over this range. Figure 1 represents the solubility of fructose at different mass fractions of water/ethanol solvent. According to the results, the higher the concentration of ethanol in solvent, the lower is the solubility of fructose, being the saturation temperature higher for the same concentration of fructose. Besides the addition of ethanol to the system decreases the metastable zone width, reducing induction time.
Figure 1.Solubility of fructose in different mass composition of the mixed solvent. E is the mass of ethanol and W the mass of water. Solubility is expressed as the molar fraction of fructose at saturation in the ternary mixture.

Crystallization experiments were conducted in a jacketed crystallizer, starting with aqueous solutions of fructose with different concentrations. Each solution was cooled at a constant rate and ethanol was introduced to the system when temperature was 2°C lower than the aqueous solution saturation one. Different concentrations of solvent and different mass fractions of fructose were used in the experiments. In all runs, final temperature was fixed to 30°C. Some results are showed in Table 1 and Figure 2.
Table 1. Results of the crystallization batches. T sat
stands for the saturation temperature, F for the mass of fructose, E for the mass of ethanol and W for the mass of water.
Experiment
Mass fraction of ethanol in solvent
(E/(E+W))
Mass fraction of fructose without ethanol
(F/(F+W))
T sat without ethanol
(°C)
Calculated
Yield (%)
Experimental
Yield (%)
1
2
3
4
5
6
7
0.80
0.86
0.869
0.881
0.894
0.869
0.881
0.894
0.869
0.881
50.5
55.0
60.0
50.5
55.0
60.0
50.5
58.56
62.93
68.36
70.63
73.73
76.86
78.04
57.10
61.64
63.71
68.37
71.41
75.17
75.98
8
9
0.90
0.894
55.0
60.0
80.35
83.68
75.30
76.70
In Table 1, calculated yield was obtained based on the fructose solubility (Figure 1) at each solvent composition. It represents the relation between the maximal mass of fructose that can be obtained in crystal phase and the total one. Experimental yield is the relation between the experimental mass of crystallized fructose and the total one.
Results of crystallization batches show that, for each solvent composition, higher fructose initial mass fraction results in higher yield, but saturation temperature is higher too, which implies in higher energetic demands of the process.
Figure 2.Yields of crystallization at different solvent compositions. F, E and W are the same of Table 1.

Figure 2 shows the experimental yields for each composition of solvent for different initial fructose mass fractions without ethanol. It is evident that the higher yields are achieved with the solvent most concentrated in ethanol and that, with higher ethanol concentration, yield becomes near independent on fructose mass fraction. It indicates, therefore, that it would be interesting to conduct the crystallization process with highly concentrated solvent and, based on the fact that fructose solution becomes highly viscous if sugar fraction is increased, it would be advantageous to work with fructose solutions with not so high mass fractions.
Therefore, the addition of ethanol generated conditions to effective crystallization of fructose by cooling, with yields above 70% of crystallized mass. However, the addition of an anti-solvent to the process generates a negative economic factor, making essential the study of separation and reuse of the anti-solvent. So ethanol has to be distilled and recycled to the process. On the other hand, an aqueous solution of not crystallized fructose is generated as a by-product and should also be recycled to the process. The proposed integrated process is schematically shown in Figure 3. Line 1 in the process flowdiagram is a fresh aqueous solution of fructose. This solution could be a byproduct of calcium gluconate production from inverted sucrose. The solution of fructose is concentrated in an evaporator to the inlet concentration of the crystallizer. Concentrated fructose aqueous solution (line 5) is added to the crystallizer together with ethanol (line
11). Crystallized fructose is separated from the suspension, and the remaining solution, composed of water, ethanol and a few amount of fructose (line 7) is distilled, recovering hydrous ethanol as distillate and aqueous fructose solution at the bottom. Hydrous ethanol is recycled to the crystallizer. Two options are evaluated for column bottom stream destination: upstream or downstream to the evaporator.
Figure 3. Integrated system of fructose crystallization and ethanol and fructose recovery system.

The flowdiagram showed in Figure 3 is a schema that turns possible the synthesis of the process. This process was synthesized with material and energy balances
[3,4]
. Fructose crystallization is the center of the process, but the other operations involved in the integrated system are equally important to the global process. Because of this, the process flowdiagram was simulated and different operational conditions were tested to improve the performance of the process. For the evaporation process, two ways of feeding the recovered aqueous fructose of the distillation (line 12) were evaluated. This stream may be fed upstream to the evaporator, causing losses of a little amount of ethanol present in the bottom stream of the distillation column or it may be fed downstream to the evaporator. In this case, the evaporated solution should be more concentrated and, therefore, more viscous. Concentrated fructose aqueous solutions are too viscous, and it may impair process operability. For the distillation process, the composition of the streams could be modified, which turns possible the assessment of required energy and ethanol losses. The lower is the required energy, higher is the concentration of ethanol in the bottom of the column, which implies on higher losses of ethanol in the recycling operation. Besides, with a study of the influence of fructose on vapor-liquid equilibrium of water/ethanol, the distillation could be refined, turning possible an optimization of this process.
Therefore, each process of the integrated system is important. Each one was studied separately and the integration of the system was made by simulation, being possible several analyses of the process. The experiments were made by batches in laboratory and the data were used in the synthesis of a continuous process. Process optimization studies are possible, searching for the most profitable process schema.
References:
[1] Nývlt, J.
The kinetics of industrial crystallization . Elsevier Science Publishers:
Amsterdam, 1985 .
[2] Nývlt, J.; Hostomsky, J.; Giulietti, M. Cristalização . EdUFSCar/IPT: São Carlos,
SP, 2001 .
[3] Seider, W. D.; Seader, J. D.; Lewin, D. R. Product & Process Design Principles ,
Second Edition. John Wiley and Sons, Inc, 2003 .
R. , Second Edition. John Wiley and Sons, Inc, 2005 .