Crystallization as a Separations Process - American Chemical Society

Chapter 14 Kinetics of the Ethanolic Crystallization of Fructose M. R. Johns, R. A. Judge, and Ε. T. White

Downloaded by CORNELL UNIV on May 16, 2017 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0438.ch014

Department of Chemical Engineering, University of Queensland, Brisbane 4072, Australia

Batch crystallization studies of D-fructose from aqueous ethanolic solutions demonstrate that crystal growth rate is dependent on supersaturation (possibly to the 1.25 power), ethanol content and temperature. It appears that solution viscosity also has an effect. Growth rates of up to 1 μm/min were measured. No nucleation occurs provided the supersaturation is kept below a value equivalent to 35°C of subcooling. There is a size spread effect, but it decreases with high ethanol contents. The results indicate that a practical process is feasible to grow large fructose crystals by the addition of ethanol to aqueous fructose solutions. High fructose corn syrup (HFCS) has emerged i n recent years as an alternative n u t r i t i o n a l sweetener to sucrose. However, the use of HFCS has been confined to those applications suited to l i q u i d syrups, i n p a r t i c u l a r the beverage and canning sectors of the market. The manufacture of fructose as a c r y s t a l l i n e product would open up further market opportunities f o r the sweetener. One company i n the USA i s producing c r y s t a l l i n e fructose i n commodity quantities ( i ) , but at a higher price than sucrose. Two methods are employed i n d u s t r i a l l y to produce c r y s t a l l i n e fructose , aqueous c r y s t a l l i z a t i o n and alcoholic crystallization. Yields of fructose c r y s t a l l i z e d from water syrups are only of the order of 50%, due to the very high water s o l u b i l i t y of the sugar, while the high v i s c o s i t y of the concentrated solution r e s u l t s i n long c r y s t a l l i z a t i o n times, t y p i c a l l y 50 hours or more (2). The second process requires the addition of lower alcohols (eg. ethanol) to a concentrated fructose syrup, generally 90% t o t a l s o l i d s or more, at temperatures of 50°C to 80°C and then cooling to cause crystallization. Fructose y i e l d s are from 70 to 80% and the t o t a l time involved i s 8 to 12 hours (3). However, large quantities of

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Myerson and Toyokura; Crystallization as a Separations Process ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

14

JOHNS ETAL.

Kinetics ofthe Ethanolic Crystallization ofFructose

alcohol are required and alcohol recovery must be included for economic operation. Recently, Edye et al. (4) described a fermentation process which used a mutant s t r a i n of Zymomonas mobilis to produce high concentrations of fructose and ethanol when grown on a concentrated sucrose medium. Johns and Greenfield (5) proposed ethanolic c r y s t a l l i z a t i o n as a means of recovering the fructose from the broth. The k i n e t i c behaviour of fructose c r y s t a l l i z a t i o n from ethanolic solution has not been previously reported, and t h i s work investigates these c r y s t a l l i z a t i o n k i n e t i c s .


Experimental Methods The investigation was carried out using a seeded, batch c r y s t a l l i z a t i o n i n the absence of nucleation. Supersaturated solutions were prepared, seeded and maintained at a constant temperature while c r y s t a l l i z a t i o n proceeded. Samples were taken p e r i o d i c a l l y to give a solution for analysis and c r y s t a l s for s i z e analysis and c r y s t a l content determination. The c r y s t a l l i z e r was a s t i r r e d , 1 - l i t r e c y l i n d r i c a l glass vessel agitated with a c e n t r a l , 6-blade turbine impeller running at 500 RPM. The vessel was sealed to prevent alcohol evaporation. D-fructose (Boehringer-Mannheim GmbH, Mannheim) was dried at 70°C overnight i n a vacuum oven and stored i n a desiccator. Anhydrous ethanol (CSR L t d . , Sydney) was used. Weighed amounts of D-fructose, ethanol and d i s t i l l e d water were added to* the crystallizer, initially operated at 10 to 15°C above the crystallization saturation temperature by a heated water bath. When the fructose had completely dissolved, the temperature was reduced to give a supersaturated solution. A suitable quantity of seed c r y s t a l s was then added. The seed c r y s t a l s of D-fructose were obtained by b a l l m i l l i n g crystals produced by spontaneous nucleation from an aqueous ethanolic s o l u t i o n of fructose and allowing them to stand at room temperature i n s l i g h t l y supersaturated ethanolic solution u n t i l the desired c r y s t a l s i z e (20-40 microns) was achieved. They were then stored at 30 C i n saturated, anhydrous ethanol to prevent further c r y s t a l growth. A portion of t h i s s l u r r y was added to the c r y s t a l l i z e r as the seeds. At appropriate time intervals (several minutes), two known volume samples of the c r y s t a l l i z e r contents were taken. One was used to determine the c r y s t a l content, where the c r y s t a l s were recovered on a 0.45 um membrane f i l t e r , washed, dried i n vacuo at 60 C overnight and weighed. The other sample was also f i l t e r e d on a membrane f i l t e r to give a solution for solute analysis and c r y s t a l s for s i z e analysis. The c r y s t a l s were washed from the membrane by dipping i t i n an ethanol e l e c t r o l y t e solution containing 2% lithium chloride. S i z i n g was done using an electronic sensing zone s i z e r ( P a r t i c l e Data) with a 300 um o r i f i c e . The solution was analysed for ethanol by gas chromatography and for fructose by both HPLC (5) and vacuum evaporation to dryness.


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CRYSTALLIZATION AS A SEPARATIONS PROCESS

Results and Discussion


Properties of Ethanolic Fructose Solutions. Published information on the properties of aqueous ethanolic fructose solution i s very limited. As a r e s u l t , s o l u b i l i t y data from 25 to 60 °C was measured (Figure 1) and w i l l be published separately. The equilibrium fructose/water mass r a t i o for zero a l c o h o l , ranges from over 4 at 25°C to over 8 at 60°C (7,8). It can be seen that reasonable y i e l d s w i l l only r e s u l t with high alcohol additions (E/W at least 2). This i s the range used i n t h i s study. Aqueous ethanolic solutions have a wide range of v i s c o s i t i e s . These were measured at operating conditions using a Rheomat concentric cylinder viscometer. Limiting Supersaturation for Nucleation. Like sucrose, D-fructose solutions can tolerate a high degree of supersaturation without nucleating, even i n the presence of seed c r y s t a l s . This i s the metastable region on the Miers s u p e r s o l u b i l i t y diagram (9). To measure this l i m i t i n g supersaturation level before nucleation occurs, a batch of s t i r r e d , supersaturated s o l u t i o n containing large seed c r y s t a l s was slowly cooled (0.5 C/min) and observed. When nucleation occurred, the degree of subcooling was noted. The r e s u l t s are shown on Figure 2. It can be seen that 35 C of subcooling i s possible before nucleation occurs and t h i s i s independent of both the i n i t i a l fructose concentration (saturation temperature) and ethanol content. Provided subcoolings are kept above t h i s value, batch c r y s t a l l i z a t i o n studies can be c a r r i e d out without nucleation. Crystal Growth Rate Studies. Nine runs were undertaken (Table I ) , three at each of three temperatures, 25, 40 and 55°C. Figure 3 shows s i z e d i s t r i b u t i o n s of the samples i n the cumulative number "greater than" form, for run 5 at 40 C with an E/W content of 4.42. The t o t a l c r y s t a l number counts (> 10 um) were s u b s t a n t i a l l y constant, showing that there were no nuclei growing into the s i z e range. For the p l o t , a l l the d i s t r i b u t i o n s were scaled s l i g h t l y to give the same value at 10 um, to increase the accuracy i n evaluating the growth rate. The cumulative size d i s t r i b u t i o n s are seen to broaden (covering an increasingly wider range of sizes) as they grow, showing either s i z e dependent growth or growth dispersion. From the change i n the mean s i z e with time, an average growth rate can be calculated for each time i n t e r v a l . Figure 4 shows a plot of the average growth rate and the fructose concentration during run 5. From t h i s p l o t , the average concentration correspond ing to each growth rate was evaluated. The growth rates for the nine experiments plotted against solution concentration, C are shown i n Figure 5. The growth rates are quite substantial (up to 1 jun/min). These could be increased further (without nucleation) by operating at a higher supersaturation. As the E/W r a t i o increases, the value of C i s smaller and the dependence on C becomes steeper. Thus a process to grow large single c r y s t a l s of D-fructose i s f e a s i b l e . The order of the dependence of growth on supersaturation could not be evaluated. The solutions take a considerable time (up to a day) to reach equilibrium and f i n a l equilibrium s o l u b i l i t y values were not determind. As i t eventuated, s o l u b i l i t y values given i n


14. JOHNS ETAL.

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201 Kinetics ofthe Ethanolic Crystallization of Fructose 1

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ETHANOL CONTENT, mass ratio E/(E+W) Figure 1. S o l u b i l i t y of fructose i n aqueous a l c o h o l i c solutions, 25-60°C.

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SATURATION TEMPERATURE, °C Figure 2. Limiting nucleate.

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202

CRYSTALLIZATION AS A SEPARATIONS PROCESS

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PARTICLE SIZE, um Figure 3. Cumulative number s i z e d i s t r i b u t i o n s fructose c r y s t a l l i z a t i o n f o r Run 5.

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14.

JOHNS ETAL.

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Figure 5. Growth rate vs C for fructose c r y s t a l l i z i n g from aqueous ethanol. Also shown are the r e s u l t s of Shiau and Berglund (10) for aqueous s o l u t i o n .

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Figure 1 were not known with s u f f i c i e n t certainty to pinpoint a value. Further investigations, using an accurate evaluation of the final equilibrium fructose concentration, are i n progress to determine t h i s dependence.

Table I.


Run No.

Conditions for the Crystallization Experiments Temp. (°C)

E V

Typical F/W

25 40 55 25 40 55 25 40 55

4.62 6.09 8.80 3.29 4.42 7.40 2.48 3.48 5.94

2.2 3.2 4.6 2.4 3.7 4.9 2.7 4.0 5.4

1 2 3 4 5 6 7 8 9 * #

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tt q

92 130 112 49 37 21 11.4 9.8 10.4

0.07 0.05 0.05 0.14 0.10 0.12 0.19 0.14 0.16

K from equation 1, jm/min (g fructose / g s o l n )

1#

G

q from equation 2

Shiau and Berglund (10) investigated the c r y s t a l l i z a t i o n o f fructose from aqueous solutions (no alcohol) at 30, 40 and 50 C. They determined a dependence of growth rate on supersaturation ( i n the same units as Figure 5) at the 1.25 power, although there i s uncertainty on t h i s value. Their r e s u l t s have also been plotted on Figure 5. For further analysis of the data, t h e i r dependence was taken and f i t t e d to the data of Figure 5 to give the best growth rate constant, ic where o

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Values of K are plotted against E/W i n Figure 6 with temperature as a parameter. It might be tempting to f i t the effect of temperature at a given E/W with an Arrhenius p l o t , but t h i s would show the s u r p r i s i n g r e s u l t of an inverse dependence, i e . the growth rate i s higher at the lower temperatures. This i s no doubt due to the effect of v i s c o s i t y . At higher temperatures the increase i n fructose s o l u b i l i t y increases solution v i s c o s i t y , and thus the f i l m thickness increases and the d i f f u s i v i t y decreases. Further experiments are i n progress to evaluate these effects. It i s l i k e l y that the temperature dependence found by Shiau and Berglund (10) (for E/W = 0) could also be influenced by t h i s v i s c o s i t y e f f e c t , although they found a normal Arrhenius dependence on temperature. If the solution v i s c o s i t y has a marked effect on the growth rate, then mass transfer must play an important part i n the growth mechanism. q

Crystal Size Spread. Figure 3 shows that the s i z e range of the c r y s t a l s increases as the c r y s t a l s grow. In Figure 7 the standard deviation of the d i s t r i b u t i o n ,

Crystallization as a Separations Process - American Chemical Society

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