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Journal of Crystal Growth 290 (2006) 192–196

Effect of solvent on crystallization behavior of xylitol

Hongxun Haoa,b,, Baohong Houa, Jing-Kang Wanga, Guangyu Lina

aSchool of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P.R. ChinabPostdoctor Station of Tianjin Economic–Technological Development Area, Tianjin 300457, P.R. China

Received 8 July 2005; received in revised form 12 December 2005; accepted 22 December 2005

Available online 3 February 2006

Communicated by M. Uwaha

Abstract

Effect of organic solvents content on crystallization behavior of xylitol was studied. Solubility and crystallization kinetics of xylitol in

methanol–water system were experimentally determined. It was found that the solubility of xylitol at various methanol content all

increases with increase of temperature. But it decreases when increasing methanol content at constant temperature. Based on the theory

of population balance, the nucleation and growth rates of xylitol in methanol–water mixed solvents were calculated by moments method.

From a series of experimental population density data of xylitol gotten from a batch-operated crystallizer, parameters of crystal

nucleation and growth rate equations at different methanol content were got by the method of nonlinear least-squares. By analyzing, it

was found that the content of methanol had an apparent effect on nucleation and growth rate of xylitol. At constant temperature, the

nucleation and growth rate of xylitol all decrease with increase of methanol content.

r 2006 Elsevier B.V. All rights reserved.

PACS: 81.10.Aj; 81.10.Dn; 82.60.Lf 

Keywords:  A1. Crystallization kinetics; A1. Growth rate; A1. Nucleation rate; A1. Population balance equation; A1. Solubility; B1. Xylitol

1. Introduction

Solvent has a strong influence on the crystallization

behavior of crystalline materials. Solvent can influence the

solubility and the crystallization kinetics (crystal nucleation

and growth rate) of crystalline materials in solutions  [1,2].

However, the role played by solvent–solvent interactions in

enhancing or inhibiting crystal nucleation and growth is

still not completely resolved. It has been showed that

favorable interactions between solute and solvent on a

specific face lead to reducing the interfacial tension, causinga transition from a smooth to a rough interface and a

concomitant faster surface growth  [3,4].

Xylitol is an important sweetener which is one kind of 

sugar substitute. The main branches of xylitol use are food

production, perfumery, pharmaceutics and chemistry. In

industrial manufacture, xylitol is crystallized from solution

as the final step. So, crystallization is a key step since, in

many respects, it determines the yield and quality of the

target product   [5–7]. In order to choose proper solvents

and to design an optimized crystallizer, it is necessary to

know the effect of solvents on xylitol solubility and

crystallization kinetics. However, it was found that no

one has studied the effect of solvent on crystallization

behavior of xylitol. In this paper, the solubility and

crystallization kinetics of xylitol in methanol–water systemwere experimentally determined. The effects of methanol

content on solubility, nucleation and growth rate of xylitol

were discussed.

2. Theory

The nucleation rate per unit mass of solvent is

represented by the empirical expression [8]

B 0 ¼ K b DC bM i T. (1)

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0022-0248/$ - see front matterr 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jcrysgro.2005.12.099

Corresponding author. State Research Center of Industrialization for

Crystallization Technology (SRCICT), School of Chemical Engineering

and Technology, Tianjin University, Tianjin 300072, PR China. Tel.:

+8622 27405754; fax: +8622 27374971.

E-mail addresses:   hhx73@hotmail.com, virgilhao@yahoo.com.cn

(H. Hao).

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The nucleation rate constant   K b  may depend on many

variables, in particular temperature, hydrodynamics, pre-

sence of impurities and, perhaps, crystal properties. The

magma density term   M T   in the kinetic expression is

included to account for the secondary nucleation effect.

The overall linear growth rate per unit mass of solvent is

expressed as [9]

G ¼ K g DC g. (2)

The overall growth rate constant  K g  would be expected to

depend on variables such as temperature, crystal size,

hydrodynamics and presence of impurities.

For a well-mixed batch crystallizer in which the particle

size only changes owing to crystal growth (i.e. both

agglomeration and breakage disruption are neglected)

and there is no change of crystallizer volume, the

differential number balance   [2]  can be expressed in terms

of population density  n(L) by

qnðLÞ

qt  þ

d½G ðLÞ nðLÞ

dL  ¼ 0, (3)

where G (L) is crystal linear growth rate,  n(L) is population

density and  L  is crystal size.

If McCabe  DL law holds, the crystal growth rate is size-

independent; Eq. (3) can be changed into  [10,11]

qnðLÞ

qt  þ G 

  d½nðLÞ

dL  ¼ 0. (4)

The   j -th moment of population density   n(L) can be

defined as

m j  ¼

Z   10

nL j  dL   ð j  ¼ 0; 1; 2; . . .Þ. (5)

The moment equations for crystal obtained by moment

transformation of the population balance Eq. (4) with

respect to size are

dm0dt

  ¼ B 0, (6)

dm j 

dt  ¼  j m j 1 G    ð j  ¼ 1; 2; 3; . . .Þ. (7)

By replacing the derivatives with differentials, the

nucleation and growth rate can be calculated from

B 0 ¼Dm0

Dt  , (8)

G  j ; j 1  ¼Dm j 

 j m j 1 Dt. (9)

From crystal nucleation and growth rate, magma density

and supersaturation data of different moment, the kinetics

parameters of Eqs. (1) and (2) can be calculated by

nonlinear least-square method.

3. Experiments

3.1. Materials

A crystal of xylitol, with a melting/decomposition point

of 94.6 (70.5) 1C, was prepared by recrystallization from

aqueous solution. Its purity is higher than 99.9 mass%. Itwas dried in vacuo at 501C for 48 h and stored in a

desiccator. Methanol used for experiments was of analy-

tical reagent grade. It was dehydrated with molecular

sieves, and its purity was higher than 99.8 mass% checked

by gas chromatography. Distilled, deionized water of 

HPLC grade was used.

3.2. Measurement of solubility

Solubility of xylitol was measured by the last crystal

disappearance method. Laser monitoring observation

technique was used to determine the disappearance of the

last crystal in solid + liquid mixtures. The laser monitoring

system consists of a laser generator, a photoelectric

transformer and a recorder. The equilibrium cell is a

cylindrical double-jacketed glass vessel. A constant desired

temperature was maintained by circulating water through

the outer jacket from a thermostat. The uncertainty in

temperature was 70.05 K. The cell has a perforated rubber

cover plate to prevent the solvent from evaporating,

through which a mercury thermometer with an uncertainty

of  70.05 K was inserted into the inner chamber of the

vessel. The mixtures of solute and solvent in the vessel were

stirred with a magnetic stirrer. The masses of the solvent

and solute were weighed using an analytical balance withan accuracy of 70.0001 g.

In experiments, known masses of xylitol solid and

solvent were transferred into the equilibrium vessel. The

solid + liquid mixtures were maintained at a fixed

temperature for about 1 h. Then, the solid + liquid

mixtures were heated slowly at rates less than 2 K/h with

continuous stirring. This procedure was repeated until the

last crystal disappeared completely. This process lasts more

than 6 h. In the early stage of the experiment, the laser

beam was blocked by the undissolved particles of xylitol in

the solution, so the intensity of laser beam penetrating the

vessel was low. The intensity increased gradually along

with the increase of the amount of xylitol dissolved. When

the last portion of xylitol disappeared, the intensity of laser

beam penetrating the vessel reached a maximum. The

temperature was recorded as the saturation temperature of 

xylitol and the solubility expressed in g solute/g solvent was

calculated. The same solubility experiment was conducted

two times. The confidence of the solubility values is about

96%.

3.3. Measurement of kinetics

A series of batch crystallization experiments were carried

out in a 500 ml double-jacketed glass crystallizer. The

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supersaturation was produced by decreasing the tempera-

ture. The solution was mechanically agitated using an four-

blade, pitch-type impeller located in the center of the

crystallizer. The impeller rotation speeds were maintained

at 330 rpm. Temperature control within the crystallizer was

achieved by pumping constant temperature water con-

tinuously through the hollow draft tube at the maximumpossible rate. The temperature difference between the

solution and the constant temperature water was about

0.1 1C and, in this way, the crystallizer temperature in all

runs was controlled to within 70.1 1C. In a typical run, a

hot filtered solution of xylitol in methanol–water-mixed

solvents was charged into the crystallizer. The solution was

initially maintained about 10 1C above the saturation

temperature and the concentration of initial solution was

measured. Initial supersaturation was achieved by slow

cooling. When the temperature is between saturation

temperature and supersaturation temperature, pre-sized

accurately weighed seeds of uniform size were charged into

the crystallizer. Slurry sample (5 ml) was withdrawn from

the crystallizer using a micropipette. About 15 samples

were taken over a period of about 4 h, the time interval

between samples ranging from 10 min at the beginning up

to a maximum of 30 min at the end of the run. The samples

were subsequently used for crystal size distribution (CSD)

and solution concentration analysis. Malvern Mastersizer

(MAM5005) by Malvern Instruments Limited was used to

measure the CSD of xylitol which was sampled out. The

size analysis was performed twice on the same sample. The

population density was calculated by

ni  ¼   M T DV i 

k v rc   ¯ L3

i  DLi , (10)

where  DV i  is volume percentage of crystals falling into the

i th size range   k v   is volume shape factor;   rc   is density of 

xylitol; M T is magma density;  DLi  is width of  i th size range;

and   ¯ Li   is average size of crystal in  i th size range.

After running the crystallizer for about 5 h, the entire

contents were removed and filtered, the product crystals

were air-dried and size-analyzed.

4. Results and discussion

4.1. Solubility

Solubility is basic thermodynamics data involved in

crystallization processes. The solubility of xylitol in

methanol–water system at different methanol content   X 

is presented in Fig. 1. From the figure, it can be seen that

the solubility of xylitol increases with raising temperature

in methanol–water system at various methanol content. It

was also found that the presence of methanol has anapparent effect on the solubility of xylitol. The solubility of 

xylitol decreases quickly with increase of methanol content.

The temperature dependence of xylitol solubility in

methanol–water system at different methanol content is

described by the empirical Eq. (7)

C n ¼ A þ Bt þ Ct2, (11)

where C * is the solubility of xylitol, t  is Celsius temperature

and A, B , C  are parameters. The values of parameters  A, B ,

C   and the mean square error (MSE) are listed in  Table 1.

The MSE is defined as

MSE ¼Xni ¼1

ðC ni   C n

c Þ=C n

i 

2=N 

( )1=2, (12)

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Fig. 1. Solubility of xylitol in methanol–water system.

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where   N    is number of experimental points;   C i * is

solubilities calculated from Eq. (11); and C c* is experimental

values of solubility. The calculated results show satisfac-

tory agreement with the experimental data.

4.2. Crystallization kinetics

In all experiments, the typical CSD of final xylitol crystal

is shown in   Fig. 2. The typical SEM photograph of final

xylitol crystal is also given in Fig. 3. From Fig. 3, it can be

seen that the shape of xylitol crystal is rounded, and

breakage and agglomeration of it is neglectable. From aseries of experimental CSD data of xylitol, the population

density of it can be calculated by Eq. (10). Then, the crystal

nucleation and growth rate of xylitol at different conditions

can be determined by moments method. The values of the

parameters of Eqs. (1) and (2) for xylitol in methanol–-

water system at different methanol content are presented in

Table 2. The relationship of nucleation and growth rate

with supersaturation is depicted in Fig. 4. From Table 2, it

can be seen that the nucleation rate parameters   K b   and

growth rate parameter   K g   decrease with increase of 

methanol content, but the nucleation rate parameters   b

and i  and growth rate parameters  g increase with increasing

methanol content. So, we can conclude that the existence of 

methanol has an obvious effect on nucleation and growth

of xylitol. The values of parameters  b  (6–7.1) and  i  (4.7–6)

of nucleation rate are especially high. The reason maybe is

that the crystallization of xylitol is a high-viscosity and

high-magma-density process, and the secondary nucleation

rate is very large. From Fig. 4, it also can be seen that the

nucleation and growth rate of xylitol decrease with the

increase of xylitol content at same supersaturation. As we

know, the solubility of xylitol in methanol–water system

decreases with increase of methanol content. This resulted

in the decrease of the xylitol molecules per unit volume

solution with the increase of methanol content. So, thecollision probability of xylitol molecules in solution

decreases with the increase of methanol content  [12]. This

is maybe the reason that the growth and nucleation rate of 

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Table 1Values of parameters  A,  B ,  C , and the mean square error of Eq. (11)

Methanol content

X  (%)

A   10B    103C    102MSE

0.0 1.704   0.412 1.87 3.40

33.3 1.474   0.527 1.87 2.06

50.0 1.485   0.601 1.71 1.78

66.7 1.789   1.05 2.27 2.88

Fig. 2. Typical CSD of xylitol crystal.

Fig. 3. Typical SEM photograph of xylitol crystal.

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