Sucrose Crystal Growth - ACS Symposium Series (ACS Publications)

Chapter 6

Sucrose Crystal Growth Theory, Experiment, and Industrial Applications 1

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D. Aquilano , M. Rubbo , G. Mantovani , G. Vaccari , and G. Sgualdino 1

Dipartimento di Scienze della Terra, Universitá di Torino, Via Valperga Caluso, 37-10128 Torino, Italy Dipartimento di Chimica, Universitádi Ferrara, Via Borsari, 46-44100 Ferrara, Italy

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The face-by-face (R,σ ) isotherms on sucrose crystals growing from pure solutions allow us to determine the activation energies and, to some degree, the growth mechanisms for each of the F faces. Furthermore, when crystals grow from doped solutions, two other phenomena can be qualitatively interpreted: 1. the specificity of impurity adsorption as a function of the concentration, and 2D-epitaxy, when it does occur, both affecting the growth rate of each face, 2. the coloring matter effects, due both to adsorption and liquid inclusion capture under critical kinetic conditions. This effects are strongly dependent on the growth rate of the faces. By means of our experimental method (twin+single cry stal kinetics) steady state growth morphology can be predicted as a function of supersaturation and temperature.

Sucrose c r y s t a l i s monoclinic and polar c r y s t a l due to the c r y s t a l l i n e arrangement of polar molecules: i t generally grows from polar solvent, water i n p a r t i c u l a r , and t h i s means that both k i n e t i c and s t r u c t u r a l problems are rather complicated. I t i s for these reasons that we have been trying for many years to give a contribution to the solution of a part of these problems s t a r t i n g from two considerations: 1. I t i s necessary to know a l l possible configurations of the surface structure of a l l faces of the c r y s t a l because we cannot obtain useful information from a c r y s t a l considered as a whole. We studied the t h e o r e t i c a l growth morphology by means of the theory of Hartman and Perdok (1-2) and we stated

0097-6156/90/0438-072$06.00/0 © 1990 American Chemical Society

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


6. AQUILANO ET AL.

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Sucrose Crystal Growth

the character of the d i f f e r e n t faces (F and S) (3). 2. We need measurements both at equilibrium and during k i n e t i c experiments i n pure medium and i n the presence of s p e c i f i c impurities. Unfortunately measurements about equilibrium forms are f a i r l y d i f f i c u l t to make whereas k i n e t i c measurements are easier to carry out. Therefore a p a r t i c u l a r method was chosen (4). We worked on a s t a t i s t i c a l population of c r y s t a l s i n order to minimize the dispersion and on simultaneous measurement of a l l faces i n order to compare t h e i r growth rate under the same conditions of supersaturation and temperature. Therefore c l a s s i c a l (R,o ) isotherms were obtained. Experimentally we grew at the same time and i n the same solution a single c r y s t a l and twin. Whereas growth rate measurements of the forms { hOL } are r e l a t i v e l y simple (thanks to the fact that the b axis i s a binary axis) (Figure l b ) , the k i n e t i c measurements of the p { 110} and p{ 110} forms are more d i f f i c u l t . We solved the problem by measuring the advancement rate along the b axis of the twin and the single c r y s t a l , respectively. The R(p') value i s calculated by means of the equation shown i n Figure 1, whereas the R(p) value i s given by the following equation: 1

RP - ( Ab

single

-

A b twin

12)

sin y

12

This technique allowed us to give a very precise measurement on two complementary forms (p and p) and distinguish t h e i r growth rates unambiguously. In t h i s way two objectives ensue: 1. We obtain the growth mechanism of the most important f l a t and stepped faces of the c r y s t a l . 2. We are able to foresee the global c r y s t a l morphology i n a steady state for a l l temperature and supersaturation values. In Figure 2 we see the t y p i c a l curve (R,O ) found for F faces. The continuous curve represents the normal trend of our experimental (R,0) isotherms. Part (a), which represents the parabolic law at very low 0 i s followed by a sudden increase i n the growth rate due to the exponential contribution of the two-dimensional nucleation i n between the steps, part (b), and at the end by the l i n e a r law, part (c). From our isotherms we can calculate the a c t i v a t i o n energy f o r c r y s t a l l i z a t i o n and we are able to choose the growth mechanism. In the past, many i n t e r e s t i n g data i n the l i t e r a t u r e have given only the average value f o r t h i s energy because the c r y s t a l was considered as a whole. From other calculations, Schliephake (5) determined the a c t i v a t i o n energy for surface d i f f u s i o n and Smythe (6) and Kucharenko (7) for volume d i f f u s i o n but i t was impossible to distinguish i f a certain face advances under surface d i f f u s i o n , volume d i f f u s i o n or both of them. In the l e f t portion of Figure 3 the values of two important complementary forms are represented, f


CRYSTALLIZATION AS A SEPARATIONS PROCESS

SINGLE

-

r

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(a)

(c)

U

R , P

(b

2

- b,)/2 1

slnY/2

(b) Figure 1. Cross section of single and twin c r y s t a l of sucrose: a) single c r y s t a l : view along | 001| d i r e c t i o n ; b) single cryst a l : view along | 010 | d i r e c t i o n ; c) twin c r y s t a l : view along |001 | d i r e c t i o n .

a * Figure 2. Typical (R,a ) growth isotherm f o r F faces.


6. AQUILANO ET AL.

AG

cr


Kcal./mole


20 *—

A G SURFACE DIFFUSION

Schlitphakt

, Alboit - VanHook V a l c i c Bennema Y

15-

Smythe

10 'Smythe A G VOLUME DIFFUSION - Kucharenko

5-

FACE BY FACE

CRYSTAL AS A WHOLE

Figure 3. Activation energy for the c r y s t a l as a whole (on the r i g h t : b i b l i o g r a p h i c a l data) and for d i f f e r e n t faces (on the l e f t : our data).


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that i s p and p , and for one face which i s not polar, the (101)-d face. It must be observed that the average of our values i s i n a very good agreement with the mean value obtained by Albon (8), V a l c i c (9), Bennema (10), Smythe (11) and VanHook (12). As a further comment to the advantage of the face by face method we can see i n Figure 4 the morphology of the c r y s t a l seen along the c-axis. We can observe the dramatic v a r i a t i o n of the shape when the supersaturation i s moving from very low values to the highest ones and when the temperature of c r y s t a l l i z a t i o n increases by only 10°C. Above we have taken into consideration the c r y s t a l l i z a t i o n from pure water solution only and we have not considered any interpretation founded on s t r u c t u r a l consideration. Now the c r y s t a l l i z a t i o n i n impure solution w i l l be examined and only a f t e r t h i s an i n t e r p r e t a t i o n which takes into account the interaction between the c r y s t a l structure and the growth medium may be attempted. In Figure 5 we can see as a f i r s t example a comparison between a sucrose c r y s t a l grown i n pure solution (b), and one grown in the presence of small amount (2-8 grams per one hundred grams of water) of r a f f i n o s e (a). It i s clear the dramatic v a r i a t i o n i n the growth morphology (13). As far as the e f f e c t of glucose and fructose on sucrose c r y s t a l morphology i s concerned, we observed that i n the presence of such low amounts as the ones quoted above for r a f f i n o s e , we were not able to observe evident e f f e c t s . On the contrary, by increasing the amounts of these impurities present i n sucrose solution to 150 grams per 100 grams of water the s i t u a t i o n changes dramatically. In p a r t i c u l a r , as f a r as glucose i s concerned, Figure 6 shows that c r y s t a l stops growing along the -b axis under the highest concentration of impurity. In p a r t i c u l a r the blocking involves faces p so favoring the disappearance from the morphology of the faces o, q, w on the l e f t pole. Moreover, such effect becomes more and more important as we increase the amount of glucose, that i s from zero to 150 grams per one hundred grams of water. As f a r as fructose i s concerned, phenomena of t o t a l blocking of one or the other pole of the c r y s t a l were not observed but we noted a macroscopic slowing down of (111)-© and ( O l l ) - q faces on the right pole which normally do not appear (Figure 7). With the purpose of confirming t h i s e f f e c t , already s u r p r i s i n g i n i t s e l f , we grew twins of the f i r s t type i n the presence of various amounts of fructose. In Figure 8 the sequence of these twins grown i n the presence of d i f f e r e n t amount of fructose, from 10 to 150 grams per one hundred grams of water, i s shown. Figure 9 shows a c r y s t a l grown i n the presence of glucose and fructose, each of them at a concentration of 100 grams per one hundred grams of water (b) i n comparison with a c r y s t a l grown i n pure solution (a). It i s quite clear that the c r y s t a l grown i n the presence of impurities does not show the small 1

1


6. AQUILANO ET AL. R

110

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Sucrose Crystal Growth T - 40°C c

T - 30°C

*U0 l

iIo

no


3.67

3.28

0.011

3.5

0.020

83

2.05

0.024

0.0335

3.05

2.34

2.46

0.040

2.16

3.54

0.060

3.51

0.087

3.13

4^.114

Figure 4. Effect of temperature and supersaturation

on c r y s t a l

morphology: | 001 | projection.

a

c (a)

(b)

Figure 5. Morphology of sucrose c r y s t a l grown i n pure solution (b) and i n the presence of raffinose (a): | 100 | projection.


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\

f

•

i \. ...... J (a)

(b)

;

{''

\

\ (c) Downloaded by UNIV OF IOWA on June 21, 2016 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0438.ch006

•

(d)

Figure 6. Morphology of sucrose crystals grown i n the presence of d i f f e r e n t amount of glucose ( a - 150; b - 100; c • 50; d 0 grams per 100 grams of water). Dashed l i n e s represent the i n i t i a l stage.

1

f

Figure 7. Appearance of the o and q faces on the right end of a sucrose c r y s t a l grown i n the presence of fructose (100 grams per 100 grams of water).

(a)

(O

(b)

(d)

Figure 8. Sucrose twins ( f i r s t type) seen perpendicularly to the a face grown i n the presence of d i f f e r e n t amount of fructose (a - 150; b - 100; c - 50; d - 10 grams per 100 grams of water)•


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AQUUANO E T AL.

f

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f

faces of the l e f t pole but the o and q faces of the right pole, that i s the opposite to the s i t u a t i o n for pure s o l u t i o n . Above we have outlined the importance of the structure of the c r y s t a l when we want to make a c o r r e l a t i o n between the k i n e t i c data and the surface of a l l the faces. So we considered the PBC's analysis as a necessary t o o l to obtain the maximum of information on a l l s i t e s of each c r y s t a l surface (3). The PBC's analysis s p e c i f i c a l l y allows us to determine the p o l a r i t y of the complementary forms. As an example we consider the complementary interface q and q (Figure 10). The two opposite interfaces show complementary behavior with respect to the hydrogen bond (HB) pointing toward the mother s o l u t i o n . The q interface exposes 3 HB donors and 4 acceptors whereas the opposite s i t u a t i o n i s set up on the q face. We fixed the r a t i o K between the number of donors and the number of acceptors over one unit c e l l . Hence for q face K • 0.75 and f o r the q face K = 1.33.


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1

1

We w i l l now consider the sequence of the growth rates i n pure solution of a l l faces as determined from l i t e r a t u r e and our experimental data (Figure 11). If we neglect the behavior of nonpolar faces (a,d,c,r) we must observe that a l l faces belonging to the l e f t pole grow more slowly than those belonging to the right pole. At the same time we observe that the K r a t i o increases from the l e f t to the right pole according to the experimental sequence. K-ratio of a given face, roughly depends on whether the surface dipole moments point outward or inward the face. So, for a donor 0Hbond, the dipole moment vector points outward and, for an acceptor, inward; hence, when the number of donor exceeds the number of acceptors (per unit c e l l ) we can e a s i l y assume that over the unit c e l l there i s a r e s i d u a l component of the dipole moment pointing outward the c r y s t a l surface. In other words a l e f t pole surface (K < 1) behaves as a dipole layer showing negative charges as the outermost ones; the opposite occurs for the right pole (K > 1). Therefore K r a t i o works as an indicator of the hierarchy of p o l a r i z a t i o n of the outermost layers of each face. This means only that, i f sucrose c r y s t a l s might grow from vapor phase, we w i l l continue to observe the p o l a r i t y e f f e c t s (both f o r equilibrium and growth forms). When considering now the water solution growth we must take into account that a sucrose molecule, with i t s surrounding water solvating molecules, behaves as a dipole surrounded by a given number of dipoles associated to i t ; hence t h i s complex dipolar aggregate, when "landing" onto a dipole layer (as mentioned above) adsorbs more or less strongly according to whether the outermost charges are negative or p o s i t i v e . This i s a f i r s t step which a f f e c t s the growth anisotropy between the two poles. A second step a f f e c t i n g the growth anisotropy i s that concerning the desorption of water molecules (those adsorbed as " f r e e " molecules onto free surface



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Figure 9, Sucrose c r y s t a l grown i n pure solution (a) and i n the presence of glucose and fructose (100 grams per 100 grams of water each).

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Figure 10, Complementary interface q and q .


AQUILANO ET AL.


GROWTH RATE IN PURE MEDIUM a « c < p < q < o < r < d « p

< q

f

s

o

f

r i g h pole

l e f t pole


l

donors HB

n

HB

acceptors

P

0,5

P

f

« 2

q

0.75

q

f

- 1.33

o

1

o

f

- 1

Surface d i f f u s i o n i s the rate determining step: e.g. AG (p) 21 Kcal/mole cr s

Volume d i f f u s i o n i s the rate determining step: e.g. AG ( p ) 10 Kcal/mole cr f

s

Surface desolvation i s more d i f f i c u l t on the l e f t pole. Figure 11. Sequence of growth rate of d i f f e r e n t faces and K values f o r the d i f f e r e n t faces of the right and l e f t pole.



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s i t e s and those "sandwiched" between sucrose surface and landing sucrose molecules). For t h i s step K-ratio i s no longer working: i n fact the rate of desorption depends on desorption energy of water molecules and, i n turn, on the interaction between water, running onto the surface, and a l l accessible atomic s i t e s . A good way to solve t h i s problem seems to be (Saska, M., Audobon Sugar I n s t i t u t e , personal communication, 1989.) the determination of the a c c e s s i b i l i t y of the Van der Waals-sized surface atoms for a spherical probe representing the solvent molecule (14). By t h i s way we can consider i n the adsorption mechanisms, not only those atoms involved i n sucrose-sucrose H-bonds (across the c r y s t a l solution i n t e r f a c e ) , but also a l l the remaining ones that may provide good s i t e s for surface-water contacts. 1

This model, applied t i l l now to a, c, d, and p, p forms, does distinguish, at least q u a l i t a t i v e l y , the fast growing p from the slow growing p face. A l l that reasoning i s i n good agreement with another consideration which ensues from our isotherms. When the c r y s t a l l i z a t i o n temperature i s lower than 45°C a l l faces belonging to the l e f t pole are dominated by the surface d i f f u s i o n whilst a l l faces belonging to the right pole are dominated by volume d i f f u s i o n . This fact c l e a r l y emerges when considering the c r y s t a l l i z a t i o n energy for p and p as an example. F i n a l l y we are induced to argue that surface desolvation i s more d i f f i c u l t on the l e f t pole and that water, together with the anisotropy of surface p o l a r i z a b i l i t y , i s an important agent responsible for the difference i n the growth rate between the two complementary forms. Now we can consider the e f f e c t of the impurities i n a q u a l i t a t i v e way. As far as the raffinose e f f e c t i s concerned we have shown that the raffinose molecule i s able to poison both the kinks and the surface of the p faces (13): this i s due to the geometry of t h i s molecule i n which the sequence fructose-glucosegalactose i s able to enter the kinks as a sucrose molecule and to be adsorbed onto the surface. The galactose portion hinders the entering of other sucrose molecules. For t h i s reason, small concentrations of raffinose are s u f f i c i e n t to block the right end growth. The s i t u a t i o n i s completely d i f f e r e n t for fructose and glucose molecules. The previously described mechanism does not occur because no effect i s shown when the concentration i s small. The fact that the morphological effect increases continuously when the concentration of these impurities r i s e s leads us to consider alternative hypotheses: A. Both molecules can form a 2-D epitaxy with residual water molecules as for KC1, NaBr, as shown i n previous work (4); B. Glucose and fructose molecules, as their concentration increases, can give o r i g i n to a competition i n the surface adsorption with the sucrose molecules. Another effect must be taken into consideration: fructose 1

f

f


6. AQUILANO ET AL.


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increases the solution v i s c o s i t y more than glucose (at the same concentration). Then i t would be expected that a l l forms growing through volume d i f f u s i o n mechanism are more affected by fructose than by glucose. In p a r t i c u l a r p w i l l reduce i t s advancement rate with respect to p. Hence the consideration that fructose acts on the right pole better than glucose and that the l a t t e r acts on the l e f t pole better than fructose i s i n favor of the B hypothesis and i s i n good agreement with the considerations we made when geometrical a f f i n i t y of raffinose was taken into consideration.


1

The phenomena described and discussed above have a remarkable importance from the i n d u s t r i a l processing point of view both concerning sugar beet and cane. The fact that the various faces of sucrose c r y s t a l show d i f f e r e n t growth rates a f f e c t s not only i t s habit but also the i n c l u s i o n of either inorganic or organic impurities present i n s o l u t i o n . In the case of inorganic impurities, their i n c l u s i o n inside growing sucrose c r y s t a l s causes an increase of the ash content. The i n c l u s i o n of organic compounds, and i n p a r t i c u l a r of the coloring matters which abound i n i n d u s t r i a l solutions, causes an increase of the c r y s t a l color showing even p r e f e r e n t i a l l o c a l i z a t i o n (15-19) (Vaccari, G.; Mantovani, G.; Morel du B o i l , P.G.; Lionnet, G.R.E. Proc. 20th I.S.S.C.T. Congress, 1989, i n press.), so decreasing the c h a r a c t e r i s t i c s of the f i n a l product from the trading point of view. The i n c l u s i o n of coloring matters inside sucrose c r y s t a l s obviously causes also an increase of the processing costs since needing onerous operations of washing and/or r e c r y s t a l l i z a t i o n . The formation of needle shaped c r y s t a l s , which, as pointed out above, characterizes the presence of raffinose i n beet processing, can also occur as f a r as cane i s concerned due to the presence of organic non-sugars and i n p a r t i c u l a r dextrans. It i s interesting to emphasize that, i n the case of cane sugar, the elongation of c r y s t a l s does not occur along the b-axis, as i n the presence of r a f f i n o s e , but along the c-axis. From the i n d u s t r i a l point of view, t h i s p a r t i c u l a r habit modification causes the formation of fragments due to the breakage of the f r a g i l e needles. These fragments not only cause losses of sugar during the centrifuging step but also d i f f i c u l t i e s inside the centrifuges owing to clogging phenomena which hinder the separation between s o l i d and l i q u i d phases. As far as the influence of glucose and fructose on sucrose habit i s concerned the modifications we have pointed out and discussed above influence, though not remarkably, the trading c h a r a c t e r i s t i c s of the f i n a l product which must p r e f e r e n t i a l l y show a stout compact shape. Again concerning the e f f e c t of glucose and fructose on sucrose c r y s t a l habit, the amounts of these compounds we have taken into consideration i n the experiments we have discussed above are not to be considered i l l o g i c a l . In f a c t , i n the cane sugar processing, high amounts of reducing sugars, that i s glucose and



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fructose are normal. During c r y s t a l l i z a t i o n step, which concerns products which become more and more impure, sugar solutions have a very high dry substance content which can reach 95% and consequently very low water amounts. Obviously, i n these conditions, the r a t i o between the various non-sugar constituents and water becomes very high and then reaches the high amounts we have taken into consideration. On the contrary, i n the case of beet processing, the effect of raffinose can be remarkable also i n the presence of very low amount, since r a f f i n o s e , through an even temporary adsorption mechanism, can very remarkably slow down the growth rate of some sucrose c r y s t a l faces causing the formation of needles we have shown and discussed above.

Literature Cited 1. Hartman, P.;Perdok, W.G. Acta Cryst. 1955, 8, 49-52 2. Hartman, P. in "Crystal Growth: An introduction" - Hartman, P., Ed.; North Holland: Amsterdam, 1973; pp 367-402 3. Aquilano, D.; Franchini-Angela, M.; Rubbo, M.; Mantovani, G.; Vaccari, G. J. Crystal Growth 1983, 61, 369-376 4. Aquilano, D.; Rubbo, M.; Vaccari, G.; Mantovani, G.; Sgualdino, G. in "Industrial Crystallization 84" - Jancic, S.J.; de Jong, E . J . , Eds.; Elsevier Science Pub. B.V.: Amsterdam, 1984, pp 9196 5. Schliephake, D.; Austmeyer, K. Zucker 1976, 29, 293-301 6. Smythe, B.M. Sugar Technol. Rev. 1971, 1, 191-231 7. Kucharenko, J.A. Planter Sugar Mfg. 1928, 80, 463-464; 484-485 8. Albon, N.; Dunning, W.J. Acta Cryst. 1960, 13, 495-498 9. Valcic, A.V. J. Crystal Growth 1975, 30, 129-136 10. Bennema, P. J. Crystal Growth 1968, 3-4, 331-334 11. Smythe, B.M. Australian J . Chem. 1967, 20, 1087-1095 12. VanHook, A. Zuckerind. 1973, 17, 499-502 13. Vaccari, G.; Mantovani, G.; Sgualdino, G.; Aquilano, D.; Rubbo, M. Sugar Tech. Rev. 1986, 13, 133-178 14. Connolly, M.L. Science 1983, 221, 709-713 15. Mantovani, G.; Vaccari, G.; Sgualdino, G.; Aquilano, D.; Rubbo, M. Ind. Sacc. Ital. 1985, 78, 7-14 16. Mantovani, G.; Vaccari, G.; Sgualdino, G.; Aquilano, D.; Rubbo, M. Ind. Sacc. Ital. 1985, 78, 79-86 17. Mantovani, G.; Vaccari, G.; Sgualdino, G.; Aquilano, D.; Rubbo, M. Zuckerind. 1986, 111, 643-648 18. Mantovani, G.; Vaccari, G.; Sgualdino, G.; Aquilano, D.; Rubbo, M. Proc. 19th I.S.S.C.T Congress, 1986, pp 633-669 ; Ind. Sacc. Ital., 1986, 79, 99-107 19. Vaccari, G.; Mantovani, G.; Sgualdino, G.; Aquilano, D.; Rubbo, M. Gazeta Cukrownicza 1988, 95, 1-10 RECEIVED June 19, 1990 Myerson and Toyokura; Crystallization as a Separations Process ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Sucrose Crystal Growth - ACS Symposium Series (ACS Publications)

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