Impurity Effects on the Crystallization Kinetics of ... - ACS Publications

The Netherlands, DSM Research, P.O. Box 18, 6160 MD Geleen, The Netherlands, and Laboratory for Process Equipment, Leeghwaterstraat 44, 2628 CA Delft,...
0 downloads 0 Views 254KB Size
Download PDF
7932

Ind. Eng. Chem. Res. 2004, 43, 7932-7938

RESEARCH NOTES Impurity Effects on the Crystallization Kinetics of Ampicillin M. Ottens,*,† B. Lebreton,†,‡ M. Zomerdijk,† M. P. W. M. Rijkers,§ O. S. L. Bruinsma,| and L. A. M van der Wielen† Department of Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands, DSM Research, P.O. Box 18, 6160 MD Geleen, The Netherlands, and Laboratory for Process Equipment, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands

Impurities have a clear negative influence on the nucleation and growth rate kinetics of the semisynthetic antibiotic ampicillin (AMPI) crystallization. The tested impurities phenylglycine and 6-aminopenicillanic acid are the building blocks of AMPI and will therefore be present in any AMPI manufacturing process, as well as the tested AMPI degradation products. The negative impact increases with an increase in the impurity concentration. This is shown by an increase in the induction time of crystallization measured and reported in this paper. The growth rate G (m s-1) is related to the supersaturation ratio S of AMPI. The growth rate is determined with a previously developed adapted single-crystal growth rate analysis method (Ottens, M.; et al. Ind. Eng. Chem. Res. 2001, 40, 4821-4827). The mechanism for retardation of the crystallization process is argued to be the blocking of the crystal surface by the impurities. 1. Introduction Increased product purity demands for pharmaceutical and fine-chemical products together with environmental legislation force pharmaceutical industries to investigate new concepts to optimize existing processes or to develop new processes for new products. In these new processes, the reduction of the number of process unit operations and waste material streams is paramount. In this context, biotechnological operations, such as enzymatic reactions applied in an aqueous environment, are becoming increasingly important for the production of pharmaceutical products such as penicillin derivatives.1 These new synthesis routes imply the application of appropriate separation techniques, which play an important role in the design of cost-effective unit operations. Crystallization is a suitable technique for the recovery of pharmaceutical products of relatively low solubility, such as β-lactam antibiotics. Crystallization is then conducted in multicomponent systems where the presence of solutes, other than the targeted product, may be very influential upon the kinetics of crystallization. Foreign molecules such as degraded products and byproducts, additives, and other components may interfere with the nucleation as well as the growth process.2 The crystal growth may be disrupted because of the incorporation of the impurities into the crystal * To whom correspondence should be addressed. E-mail: [email protected]. † Delft University of Technology. ‡ Current address: Genentech, 1 DNA Way (MS# 75), South San Francisco, CA 94080. § DSM Research. | Laboratory for Process Equipment. Current address: SASOL Center for Separation Technology, Private Bag X6001, Potschefstroom 2520, RSA.

lattice or because of their adsorption at the surface of the crystal.3 The structure, size, and morphology of the final product may consequently be modified by the presence of impurities.4 In this paper, the impact of impurities will be reported by accurately investigating the crystallization kinetics of the process, by means of the induction time, desupersaturation rate, and growth rate.5-7 The effects of impurities upon the semisynthetic antibiotic (SSA) crystallization are accurately analyzed and can be used for the development of realistic, mechanistic nucleation and growth models, in the presence of impurities. 1.1. Impurities. We investigate in this paper the influence of 6-aminopenicillanic acid (6APA), phenylglycine (PG), and ampicillin (AMPI) degradation products on the crystallization of the SSA AMPI from an aqueous solution. 6APA contains the basic β-lactam structure, which is the primary building block for the synthesis of AMPI. 6APA is manufactured by the enzymatic hydrolysis of penicillin G, which is a bulk antibiotic product produced via fermentation. Coupling 6APA with activated PG produces AMPI with PG as a side product.1 Structures of the main components of the reaction mixture are given below.

AMPI has a limited chemical stability at and below pH 5. Under such conditions, degradation products are formed and the main degradation product identified is ampicillin penilloic acid (Aoic).8,9 Because such conditions are representative of the process conditions, AMPI-

10.1021/ie0307028 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/20/2004

Ind. Eng. Chem. Res., Vol. 43, No. 24, 2004 7933

Figure 1. Experimental setup for AMPI crystallization experiments, with the adapted single-crystal growth rate analysis (ASCGRA) technique.

degraded products were selected as a potential crystallization contaminant.

2. Experimental Part 2.1. Batch-Crystallization Experiments. Ampicillin trihydrate was crystallized from freshly prepared solutions. The experiments were conducted at pH 5.0 and T ) 25 °C with different initial supersaturations of AMPI S0 and different levels of contaminations with 6APA (C6APA ) 0, 3.6, and 10 mM), PG (CPG ) 0, 10, and 30 mM), AMPI degradation products (CAoic ) 0, 1.5, and 8 mM) and combinations thereof. Batch-crystallization experiments were conducted in a 2-L jacketed reactor comprising three baffles and two inter-MIG II impellers, with a stirring speed of 300 rpm. The temperature was set at 25 °C. Pure material was first dissolved under acidic conditions at pH 1.9 for 20 min, using HCl (2 M). The pH was subsequently raised to a value of 5.0 using ammonia in water (12 M). The final solubility of AMPI of 18.9 mmol L-1 is higher than the solubility of AMPI in pure water (15 mM)10,11 because of the presence of the salt (NH4Cl). The temperature and pH were kept at the set values. A laser probe (TU Delft, Delft, The Netherlands) was inserted into the reactor to monitor the changes of turbidity by laser light reflection. The temperature, pH, and laser signal were constantly recorded using a Biodacs system (Applikon, Schiedam, The Netherlands; see Figure 1). Samples were carefully extracted at various time intervals. A fraction of the samples was filtered using nylon membranes (0.2 µm; Gelman Sciences, Ann Arbor, MI), and the filtrate was appropriately diluted for subsequent reverse-phase chromatography analysis. Another fraction of the samples was used for crystal growth rate determination. The growth rate was determined according to the so-called ASCGRA method, whereby the size of several crystals, extracted from the crystallization vessel at various time intervals, was monitored as a function of time under a light microscope (Leica Q500IW, Leica, U.K.).10

Scanning electron microscopy (SEM) pictures were acquired to characterize the final product of each experiment using a scanning electron microscope JSM5400 (JEOL, Yamagata, Japan). Samples were coated with gold for 3 min using an ion sputter JFC-1100E (JEOL, Yamagata, Japan). 2.2. Reverse-Phase Chromatography. Samples (containing AMPI) were analyzed by reverse-phase chromatography using a Waters high-performance liquid chromatography system comprising a Waters 996 PDA detector, a Waters 910 Wisp injector, and a Waters 590 pump. The reverse-phase column was a Zorbax SBC18 column (4.6 × 75 mm with a pore size of 3.5 µm; Hewlett-Packard, Palo Alto, CA). The buffer was comprised of 8 mmol L-1 tetrabutylammonium bromide, 10 mmol L-1 Na2HPO4, and 15% (v/v) acetonitrile and was pH-adjusted to 6.6 with H3PO4. The elution profile was isocratic, and the absorbance was measured at 230 nm. In combination with the saturation concentration, thus the supersaturation could be calculated. 3. Theory 3.1. Induction Time. A useful lumped parameter to monitor the nucleation mechanism is the induction time, which is defined as the period of time that elapses between the achievement of supersaturation and the appearance of crystals having a “detectable” size. The induction time depends not only on the initial supersaturation but also on the detection method. If, for instance, a concentration measurement is used, the induction time depends on conversion, whereas for light reflection, the detection depends on the crystal surface area produced. The relationship between the induction time and the (initial) supersaturation ratio S0 is given by eq 110

tind ∝ [(S0 - 1)n]-1/i+1 exp

{

}

B (i + 1)[ln(S0)]2

(1)

with the supersaturation ratio defined as follows:

S ) C/CS

(2)

with C as the solute concentration in mol m-3 and CS as the saturation concentration of the solute (solubility) in mol m-3. The type of detector determines i, for instance, i ) 2 in the present study, wherein laser reflection is used. Because the exponential term will dominate, a plot of ln(tind) vs [ln(S0)]-2 for the different crystallization experiments will yield B/3 as the slope. If the nucleation is considered as the primary nucleation, the factor B in eq 1 can be described according to Tavare12 and Mullin:13

B)

16πν2γ3 3(kT)3

(3)

The surface energy γ can thus be calculated. The relationship between the induction time and supersaturation described in eq 1 provides essential information for the nucleation kinetics. Initially, the nucleation will be primary, and a method to determine the nucleation should be applied in the time span where no large crystals are formed (crystal density is approximately zero) and secondary nucleation can be neglected. Secondary nucleation will become important as the crystal density and size increase during the course of the

7934

Ind. Eng. Chem. Res., Vol. 43, No. 24, 2004

Figure 2. SEM pictures of the needle-shaped final products resulting from AMPI batch crystallizations. The experiments were conducted with a starting relative supersaturation of 1.8 (see the Experimental Part). The experiments were conducted for pure AMPI (A) and in the presence of 10 mM 6APA and 30 mM PG (B). Magnification: bar ) 10 µm.

crystallization (i.e., at a substantial drop in the supersaturation). To differentiate between the impurity influence on nucleation and growth is a genuine problem not resolved in the literature because nuclei need to grow to a detectable size before they can be measured. The induction time method applied in this work to measure primary nucleation kinetics is widespread in the literature.13 Improvements need to be made in the experimental approach to determine nucleation kinetics in general more accurately, but that falls beyond the scope of the present paper. Note that in our approach the associated growth rate is determined by an independent growth measurement under the microscope and is not backcalculated from the evolving crystal size distribution (CSD). 3.2. Growth. The growth rate model for G, in m s-1, used in this paper to analyze the influence of impurity is given by12

G ) kg(S - 1)n

(4)

with kg as the overall mass-transfer coefficient and S as the supersaturation ratio. The parameters of the growth rate relationship (eq 4) are determined from crystal growth rate experiments by plotting ln(G) vs ln(S - 1), where the slope gives the power n and the y axis intercept the overall mass-transfer coefficient kg. 4. Results and Discussion 4.1. Pure System. The kinetic data for pure AMPI aqueous crystallization are derived from the data used for a previous paper.10 The growth rate parameters in eq 4 were determined as exponent n ) 2.26 and crystal growth mass-transfer coefficient kg ) 8.16 × 10-8 m s-1. The aspect ratio (crystal length divided by its width) was found to be AR ) 14.9. The data differ slightly in numerical value compared to those in the original paper because of a more accurate reprocessing. The new data serve as the reference state to which the data from the impure crystallizations are to be compared. 4.2. Systems with Impurities. 4.2.1. Solubility. No significant change in the solubility of AMPI upon the addition of 6APA, PG, or degradation products was measured. Therefore, changes in the crystallization behavior upon the addition of impurities are solely due to kinetic effects (nucleation and growth). 4.2.2. Morphology. The influence of the impurities on the crystal shape and morphology is shown in Figure 2. The surface of AMPI crystals becomes more irregular and shows some asperities. X-ray diffraction patterns showed that both final products were pure, and redis-

Figure 3. Experimental desupersaturation curves from data sets 6, 9, 12, and 18 (see, for example, Table 1). Influence of the 6APA single-contaminant concentration (initial supersaturation ratio of approximately 2.45). Influence of the PG single-contaminant concentration and multicomponent impurity effect (combination of PG, 6APA, and Aoic).

solution of these materials did not indicate the presence of extra components in the crystal lattice within the detection limit of the assay. 4.2.3. Aspect Ratio. AR of the crystals can be obtained from both the SEM pictures and the image analysis using the light microscope at various measurements. The light microscope image analysis uses large numbers of crystals and, therefore, gives a better average value for AR. The average value was 14.9. 4.2.4. Desupersaturation. Experimental desupersaturation data for the impure systems are shown in Figure 3. Depending on the concentration of the impurity and the initial supersaturation of AMPI, a distinct difference in the induction time is present (delay). This indicates that the presence of impurities delays the onset of crystallization and/or crystal growth. 4.2.5. Nucleation. The results of all contaminated AMPI experiments regarding nucleation kinetics are shown in Table 1. Influence of 6APA. The presence of impurities proved to have a marked effect on the nucleation rate. A series of crystallization experiments at increasing 6APA concentration and at different supersaturations of AMPI showed an increase in the induction time tind (see Figure 4A). Increased levels of C6APA increase the value for γ, as the slope becomes steeper, indicating an increase in the surface energy to create an AMPI crystal surface from solution at increasing 6APA concentration. This supports the hypothesis that 6APA molecules are adsorbed at the AMPI crystal lattice, leaving less space or a higher energy barrier for AMPI adsorption and incorporation. Indeed, 6APA molecules are structurally similar to a portion of the AMPI molecules (see the reaction scheme in the Introduction section) and can easily incorporate the lattice or crystal surface at different sites. There seems to be no evidence for direct incorporation of contaminants into the crystal lattice, so possible effects are likely to occur by adsorption at the crystal surface. Influence of PG. Figure 4B shows several crystallization experiments with a variable PG concentration

Ind. Eng. Chem. Res., Vol. 43, No. 24, 2004 7935

Figure 4. Induction time of AMPI crystallization as a function of the supersaturation of AMPI and as a function of (A) the 6APA concentrations (C6APA ) 0, 3, 6, and 10 mM), (B) the PG concentrations (CPG ) 10 and 30 mM), (C) the degradation product (Aoic) concentrations (CAoic ) 1.5 and 8 mM), and (D) the multicomponent impurity concentrations (C6APA ) 10 mM, CPG ) 30 mM, and CAoic ) 1.5 mM). Circles correspond to the pure AMPI system. See Table 1 for more experimental conditions. Table 1. Impurity Effect on the Induction Time of AMPI Crystallization C6APA, mM

CPG, mM

CAioc, mM

S0

tind, s

figure

expt

0.0 3.6 10.0 0.0 0.0 10.0 0.0 0.0 10.0

0.0 0.0 0.0 10.0 30.0 10.0 0.0 0.0 30.0

0.0 0.0 0.0 0.0 0.0 0.0 1.5 8.0 1.5

1.72, 2.37, 2.45, 3.29 1.93, 2.48, 3.40 1.85, 2.45, 3.72 2.44, 2.91 2.49 2.56 2.41 2.22 2.35, 3.20

13500, 1650, 1500, 540 5805, 1650, 600 22200, 2400, 690 2400, 1620 3600 1650 2100 4050 3270, 1020

4A-D 4A 4A 4B 4B 4A,B 4C 4C 4D

1-4 5-7 8-10 11 and 12 13 14 15 16 17 and 18

(10 and 30 mM). Increasing the impurity concentration of PG, again as for 6APA, increases the induction time. The value of γ is practically the same as that for the pure system because the slopes of the curves are almost the same. We do see higher induction times as the PG concentration increases from 0, 10 to 30 mM, i.e., 1500, 2400, and 3600 s at approximately S0 ) 2.45. We see an impact similar to that for 6APA, which again can be explained by the similarity in the structures of PG and AMPI. Influence of Degradation Products (Aoic). Figure 4C shows the same trend as that observed before with 6APA and PG as contaminants: the induction time increases upon an increase in the levels of degradation products. From examination of the induction time alone, the impact of Aoic is larger than that of 6APA and PG. Influence of Multiple Contaminants (6APA, PG, and Degradation Products). Figure 4D shows that the induction time for multicomponent impurity AMPI crystallization is higher than that for the pure system.

Longer induction times are obtained upon addition of these impurities (see Table 1). 4.2.6. Growth. Influence of 6APA. Figure 5 shows that the introduction of the 6APA impurity apparently has a distinct influence on the growth rate as determined from the ln(G) vs ln(S - 1) plots. Although in the pure system there is also some variation of the growth rate relationship with initial supersaturation S0, the addition of the impurity resulted in lowering of the growth rate by approximately a factor of 2 (the exact factor depends on the supersaturation; see Table 2). At lower S values (S < 2.4), the presence of 6APA lowers the growth rate of AMPI crystals, conforming to the general literature findings.4 However, increasing the 6APA impurity level from 3.6 to 10 mM does not decrease the growth rate further significantly. This might be due to the adsorption of 6APA molecules at the limited number of suitable growth sites, blocking growth on the AMPI crystals. Performing experiments in the range of 0-3.6 mM

7936

Ind. Eng. Chem. Res., Vol. 43, No. 24, 2004

Figure 5. Growth rate during AMPI crystallization as a function of the supersaturation of AMPI and as a function of the 6APA concentration. (A) C6APA ) 0 mM: circles, expt 1, S0 ) 1.72; triangles, expt 2, S0 ) 2.37; squares, expt 3, S0 ) 2.45; diamonds, expt 4, S0 ) 3.29. (B) C6APA ) 3.6 mM: expt 6, S0 ) 2.48. (C) C6APA ) 10 mM: diamonds, expt 9, S0 ) 2.45; squares, expt 10, S0 ) 3.47. (D) combined plot of the model lines of A-C. See Table 2 for more experimental conditions. Table 2. Impurity Effect on the Growth of AMPI Crystals C6APA, mM

CPG, mM

CDegr, mM

S0

na

kg,a nm s-1

figure

exp

GS)1.5,a,b nm s-1

GS)2.0,a,b nm s-1

0.0 3.6 10.0 0.0 0.0 10.0 0.0 0.0 10.0

0.0 0.0 0.0 10.0 30.0 10.0 0.0 0.0 30.0

0.0 0.0 0.0 0.0 0.0 0.0 1.5 8.0 1.5

1.72, 2.37, 2.45, 3.29 2.48 2.45, 3.72 2.91 2.49 2.56 2.41 2.22 2.35, 3.20

2.26 ( 0.26 3.02 ( 0.23 2.93 ( 0.24 3.76 ( 0.32 1.39 ( 0.17 3.59 ( 0.36 3.47 ( 0.35 3.19 ( 0.38 5.62 ( 0.84

81.6 ( 17.3 64.8 ( 8.9 65.5 ( 10.9 20.5 ( 3.2 162.5 ( 22.9 55.6 ( 7.6 57.0 ( 6.7 65.3 ( 8.9 9.0 ( 2.7

5A,D 5B,D 5C,D 5D 6A 6B 6C 6C 6D

1-4 6 9 and 10 12 13 14 15 16 17 and 18

17.0 ( 8.5 8.0 ( 3.4 8.6 ( 4.2 1.5 ( 0.9 61.8 ( 15.8 4.6 ( 2.5 5.1 ( 2.4 7.2 ( 3.5 0.2 ( 0.4

81.6 ( 17.3 64.8 ( 8.9 65.5 ( 10.9 20.5 ( 3.2 162.5 ( 22.9 55.6 ( 7.6 57.0 ( 6.7 65.3 ( 8.9 9.0 ( 2.8

a

Propagation of experimental errors calculated by propagation of experimental errors according to the general equation ∆y )

x(dy/dx)2∆x2+(dy/dz)2∆z2+.... b Values of the model growth rates G using the experimentally determined values for n and kg at different supersaturation values, S. Given for a quick comparison of the impurity effect on the value of G.

6APA might be beneficial elucidating the relationship between G and C6APA further. Influence of PG. Parts A and B of Figure 6 show the growth rate of the impure system at different PG contamination levels. The growth rate decreases when CPG ) 10 mM, conforming to expectations. However, at CPG ) 30 mM, the growth rate is a factor of 2-3 higher than that of the pure system (see Table 2). From the literature it is also known that the presence of an impurity may increase the growth rate,14 although not many examples are known. The mechanism responsible for this increase is the creation of more kink sites on the crystal surface. However, there is no clear trend in the PG data upon an increase of the PG concentration (first lower and then higher growth rates).

Influence of Degradation Products (Aoic). In Figure 6C, the growth rates are shown for AMPI crystallization with degraded products. Again, increasing the impurity concentration leads to slower crystal growth. The data for 1.5 and 8 mM Aoic are comparable. Influence of Multiple Contaminants (6APA, PG, and Degradation Products). In Figure 6D, a clear decrease in the growth rate is observed when the multiple contaminant level is increased. The combined effects of the single contaminants seem to be additive and influence further the growth kinetics, regarding the very low growth rate of experiments 17 and 18 (see Table 2). Using image analysis for growth rate determination will always cause a certain degree of scatter because

Ind. Eng. Chem. Res., Vol. 43, No. 24, 2004 7937

Figure 6. Growth rate during AMPI crystallization as a function of the supersaturation of AMPI and as a function of different types and concentrations of impurities: (A) PG as the impurity, CPG ) 10 mM; (B) PG as the impurity, CPG ) 30 mM; (C) AMPI degradation products as the impurity, CAoic ) 1.5 mM (squares, expt 15) and 8.0 mM (triangles, expt 16); (D) multicomponent impurity effect, combination of PG and 6APA with CPG ) C6APA ) 10 mM (squares, expt 14) and PG, 6APA, and Aoic with C6APA ) 10 mM, CPG ) 30 mM, and CAoic ) 1.5 mM (triangles, expt 17; diamonds, expt 18). See Table 2 for more experimental conditions and values of n and kg.

the selectivity in objects chosen for analysis. With respect to scatter, however, the presented data are comparable to those of other efforts in the field of crystallization. 4.3. Mechanism for the Influence of Impurities. In general, the impact of impurities acts on the metastable zone width. However, this is certainly not true for all impurities.13 In our investigation, it is clearly found that the thermodynamics, i.e., solubility, are not significantly influenced by the impurities. Therefore, one of the other possibilities of impurity action is applicable, i.e., retardation of growth and nucleation. Adsorption to the AMPI crystal surface of 6APA, PG, and/or degraded products is believed to be responsible for the retardation of the AMPI crystallization kinetics. The crystal structure of ampicillin trihydrate was resolved by X-ray diffraction analysis by James et al.15 Such crystal structure represents the alternating layered structure of AMPI molecules with the three water molecules in between. The influence of the impurities 6APA and PG can easily be understood by blocking the surface of the AMPI crystal because these molecules are the same as part of the AMPI molecule and can therefore easily fit at the surface. The degraded AMPI product Aoic is also structurally very similar to AMPI (see the Introduction section) and may act in the same way as 6APA and PG. A mechanism based on adsorption was proposed by Kubota et al.3 This model related the impurity concentration to the decrease in the growth velocity by means of adsorption isotherms. We, however, do not see a clear

influence of the impurity concentrations on the growth only. Their model, if used in our case, should be adapted to incorporate the decrease in the nucleation rate. 4.4. Implication on Process Design. A peak broadening of the CSD of AMPI and a shift to higher average crystal sizes will be observed at increasing 6APA impurity levels because of slower nucleation kinetics. This will influence the process design of an AMPI production process. In general, a broad CSD complicates solids handling and further processing and should therefore be prevented. Because 6APA and PG are present during AMPI crystallization in the production processes (see the reaction scheme in the Introduction section), measuring pure AMPI crystallization kinetics will lead to a flawed process design. This paper provides more correct kinetic data for process design under industrial practice. Although the paper discusses batch crystallization, the obtained kinetic data can be used when designing a continuous crystallization process. In industry, generally AMPI is crystallized in a batch process, without seeding. An AMPI solution at low pH is neutralized with a base. The size of the crystals is tuned by optimizing the titration profile. To understand and further improve upon this so-called fed-batch operation, the data obtained in this study are of importance. Primary nucleation kinetics together with the induction time measurements can be used to understand how the solution during the fed-batch operation leaves and re-enters the metastable zone in order to generate the required number of nuclei. The

7938

Ind. Eng. Chem. Res., Vol. 43, No. 24, 2004

growth rate kinetics, on the other hand, provide the knowledge to calculate the optimum titration profile, while keeping the solution in the upper part of the metastable zone at constant supersaturation and thus optimizing the AMPI production without generating a new explosion of nuclei, with the latter being detrimental for the size distribution as well as the filtration of the product. The knowledge of the effects of the relevant impurities on the crystallization kinetics provides further relevance of these data. 5. Conclusions This paper reports AMPI crystallization kinetics and shows and quantifies the effect of inherent impurities thereon. The presence of impurities during crystallization of the SSA AMPI has a marked influence on the induction time, the nucleation rate, the growth rate, and the crystal morphology of the product. The impurities investigated are the building blocks of AMPI, 6APA, and PG as well as the main degradation product Aoic. They are most likely adsorbed at the AMPI crystal surface, leaving less space or a higher energy barrier for AMPI adsorption and incorporation and resulting in slower nucleation and growth rate, thereby reducing crystal formation. Such an impact has been quantified in this paper, and such knowledge can be used to fine-tune the final product characteristics, i.e., by using the impuritydependent crystallization kinetics during process design and the appropriate population balance based crystallization reactor models.10,16 Symbols AR ) aspect ratio C ) liquid-phase concentration, mol m-3 G ) growth rate, m s-1 k ) Boltzmann constant ) 1.38 × 10-23 J K-1 kg ) growth rate coefficient, m s-1 S ) supersaturation ratio T ) temperature, K t ) time, s Greek Letters γ ) interfacial free energy, J m-2 ν ) molecular volume, m3 Subscripts AMPI ) ampicillin Aoic ) ampicillin penilloic acid 6APA ) 6-aminopenicillanic acid PG ) phenylglycine 0 ) initial S ) saturated

Literature Cited (1) Bruggink, A.; Roos, E. C.; de Vroom, E. Org. Process Res. Dev. 1998, 2, 128-133. (2) Chayen, N. E.; Radcliffe, J. W.; Blow, D. M. Control of nucleation in the crystallization of lysozyme. Protein Sci. 1993, 2, 113-118. (3) Kubota, N.; Yokota, M.; Mullin, J. W. Supersaturation dependence of crystal growth in solutions in the presence of impurity. J. Cryst. Growth 1997, 182, 86-94. (4) Black, S. N.; Davey, R. J. Crystallisation of amino acids. J. Cryst. Growth 1988, 90, 136-144. (5) Lebreton, B.; Zomerdijk, M.; Ottens, M.; Rijkers, M.; van der Wielen, L. A. M. Effects of impurities upon crystallization kinetics of β-lactam antibiotics. Presented at the AIChE Annual Meeting, Dallas, TX, 1999. (6) Ottens, M.; Lebreton, B.; Zomerdijk, M.; Rijkers, M. P. W. M.; Bruinsma, D.; van der Wielen, L. A. M. Crystallization kinetics of semi antibiotics in the presence of impurities. In AIChE Separations Technology Topical Conference; Bryan, P., Serbezov, A., Eds.; 2001 AIChE Annual Meeting, Reno, NV, Nov 4-9, 2001; AIChE Publication No. 150; AIChE: New York, 2001; pp 268274; ISBN 0-8169-9762-4. (7) Bruinsma, O. S. L.; Ottens, M.; Lebreton, B.; Zomerdijk, M.; Rijkers, M. P. W. M.; van der Wielen, L. A. M. Kinetics and Impurity Effects in Ampicillin Crystallization. Presented at the ISIC15 Proceedings of the 15th International Symposium on Industrial Crystallization, Sorrento, Italy, Sept 2002; ISIC: 2002; Vol. 2, pp 701-706. (8) Hou, J. P.; Poole, J. W. Kinetics and mechanism of degradation of ampicillin in solution. J. Pharm. Sci. 1969, 58, 447-454. (9) Robinson-Fuentes, V. A.; Jefferies, T. M.; Branch, S. K. Degradation pathways of ampicillin in alkaline solutions. J. Pharm. Pharmacol. 1997, 49, 843-851. (10) Ottens, M.; Lebreton, B.; Zomerdijk, M.; Rijkers, M. P. W. M.; Bruinsma, O. S. L.; van der Wielen, L. A. M. Crystallization Kinetics of Ampicillin. Ind. Eng. Chem. Res. 2001, 40, 4821-4827. (11) Rudolph, E. S. J.; Zomerdijk, M.; Ottens, M.; van der Wielen, L. A. M. Solubilities and partition coefficients of semisynthethic antibiotics in water + 1-butanol systems. Ind. Eng. Chem. Res. 2001, 40 (2), 398-406. (12) Tavare, N. S. Industrial crystallization. Process simulation analysis and Design; The Plenum Chemical Engineering Series; Plenum Press: New York, 1995. (13) Mullin, J. W. Crystallization, 4th ed.; Elsevier Butterworth Heinemann: Oxford, U.K., 2001; ISBN 0 7506 4833 3. (14) Sangwal, K.; Mielniczek-Brzo´ska, E. Effect of Fe(III) ions on the growth kinetics of ammonium oxalate monohydrate crystals from aqueous solution. J. Cryst. Growth 2001, 233, 343-354. (15) James, M. N. G.; Hall, D.; Hodgkin, D. C. Crystalline Modifications of Ampicillin I: the Trihydrate. Nature 1968, 220. (16) Randolph, A. D.; Larson, M. A. Theory of particulate processes. Analysis and techniques for continuous crystallization; Academic Press: New York, 1971.

Received for review September 4, 2003 Revised manuscript received September 15, 2004 Accepted October 4, 2004 IE0307028