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 thesemisynthetic antibiotic ampicillin (AMPI) crystallization. The tested impurities phenylglycineand 6-aminopenicillanic acid are the building blocks of AMPI and will therefore be present inany AMPI manufacturing process, as well as the tested AMPI degradation products. The negativeimpact increases with an increase in the impurity concentration. This is shown by an increasein 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 witha 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 crystallizationprocess is argued to be the blocking of the crystal surface by the impurities.

1. Introduction

Increased product purity demands for pharmaceuticaland fine-chemical products together with environmentallegislation force pharmaceutical industries to investi-gate new concepts to optimize existing processes or todevelop new processes for new products. In these newprocesses, the reduction of the number of process unitoperations and waste material streams is paramount.In this context, biotechnological operations, such asenzymatic reactions applied in an aqueous environment,are becoming increasingly important for the productionof pharmaceutical products such as penicillin deriva-tives.1 These new synthesis routes imply the applicationof appropriate separation techniques, which play animportant role in the design of cost-effective unit opera-tions.

Crystallization is a suitable technique for the recoveryof pharmaceutical products of relatively low solubility,such as â-lactam antibiotics. Crystallization is thenconducted in multicomponent systems where the pres-ence of solutes, other than the targeted product, maybe very influential upon the kinetics of crystallization.Foreign molecules such as degraded products andbyproducts, additives, and other components may in-terfere with the nucleation as well as the growthprocess.2 The crystal growth may be disrupted becauseof the incorporation of the impurities into the crystal

lattice or because of their adsorption at the surface ofthe crystal.3 The structure, size, and morphology of thefinal product may consequently be modified by thepresence of impurities.4

In this paper, the impact of impurities will be reportedby accurately investigating the crystallization kineticsof the process, by means of the induction time, desu-persaturation rate, and growth rate.5-7 The effects ofimpurities upon the semisynthetic antibiotic (SSA)crystallization are accurately analyzed and can be usedfor the development of realistic, mechanistic nucleationand growth models, in the presence of impurities.

1.1. Impurities. We investigate in this paper theinfluence of 6-aminopenicillanic acid (6APA), phenyl-glycine (PG), and ampicillin (AMPI) degradation prod-ucts on the crystallization of the SSA AMPI from anaqueous solution. 6APA contains the basic â-lactamstructure, which is the primary building block for thesynthesis of AMPI. 6APA is manufactured by theenzymatic hydrolysis of penicillin G, which is a bulkantibiotic product produced via fermentation. Coupling6APA with activated PG produces AMPI with PG as aside product.1 Structures of the main components of thereaction mixture are given below.

AMPI has a limited chemical stability at and below pH5. Under such conditions, degradation products areformed and the main degradation product identified isampicillin penilloic acid (Aoic).8,9 Because such condi-tions are representative of the process conditions, AMPI-

* To whom correspondence should be addressed. E-mail:M.Ottens@tnw.tudelft.nl.

† 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: SA-

SOL Center for Separation Technology, Private Bag X6001,Potschefstroom 2520, RSA.

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

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

degraded products were selected as a potential crystal-lization contaminant.

2. Experimental Part

2.1. Batch-Crystallization Experiments. Ampicil-lin trihydrate was crystallized from freshly preparedsolutions. The experiments were conducted at pH 5.0and T ) 25 °C with different initial supersaturationsof AMPI S0 and different levels of contaminations with6APA (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 ina 2-L jacketed reactor comprising three baffles and twointer-MIG II impellers, with a stirring speed of 300 rpm.The temperature was set at 25 °C. Pure material wasfirst dissolved under acidic conditions at pH 1.9 for 20min, using HCl (2 M). The pH was subsequently raisedto a value of 5.0 using ammonia in water (12 M). Thefinal solubility of AMPI of 18.9 mmol L-1 is higher thanthe solubility of AMPI in pure water (15 mM)10,11

because of the presence of the salt (NH4Cl). The tem-perature and pH were kept at the set values. A laserprobe (TU Delft, Delft, The Netherlands) was insertedinto the reactor to monitor the changes of turbidity bylaser light reflection. The temperature, pH, and lasersignal were constantly recorded using a Biodacs system(Applikon, Schiedam, The Netherlands; see Figure 1).

Samples were carefully extracted at various timeintervals. A fraction of the samples was filtered usingnylon membranes (0.2 µm; Gelman Sciences, Ann Arbor,MI), and the filtrate was appropriately diluted forsubsequent reverse-phase chromatography analysis.Another fraction of the samples was used for crystalgrowth rate determination. The growth rate was deter-mined according to the so-called ASCGRA method,whereby the size of several crystals, extracted from thecrystallization vessel at various time intervals, wasmonitored as a function of time under a light microscope(Leica Q500IW, Leica, U.K.).10

Scanning electron microscopy (SEM) pictures wereacquired to characterize the final product of eachexperiment using a scanning electron microscope JSM-5400 (JEOL, Yamagata, Japan). Samples were coatedwith 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-phasechromatography using a Waters high-performance liq-uid chromatography system comprising a Waters 996PDA detector, a Waters 910 Wisp injector, and a Waters590 pump. The reverse-phase column was a Zorbax SB-C18 column (4.6 × 75 mm with a pore size of 3.5 µm;Hewlett-Packard, Palo Alto, CA). The buffer was com-prised of 8 mmol L-1 tetrabutylammonium bromide, 10mmol L-1 Na2HPO4, and 15% (v/v) acetonitrile and waspH-adjusted to 6.6 with H3PO4. The elution profile wasisocratic, and the absorbance was measured at 230 nm.In combination with the saturation concentration, thusthe supersaturation could be calculated.

3. Theory

3.1. Induction Time. A useful lumped parameter tomonitor the nucleation mechanism is the induction time,which is defined as the period of time that elapsesbetween the achievement of supersaturation and theappearance of crystals having a “detectable” size. Theinduction time depends not only on the initial super-saturation but also on the detection method. If, forinstance, a concentration measurement is used, theinduction time depends on conversion, whereas for lightreflection, the detection depends on the crystal surfacearea produced. The relationship between the inductiontime and the (initial) supersaturation ratio S0 is givenby eq 110

with the supersaturation ratio defined as follows:

with C as the solute concentration in mol m-3 and CSas the saturation concentration of the solute (solubility)in mol m-3. The type of detector determines i, forinstance, i ) 2 in the present study, wherein laserreflection is used. Because the exponential term willdominate, a plot of ln(tind) vs [ln(S0)]-2 for the differentcrystallization experiments will yield B/3 as the slope.If the nucleation is considered as the primary nucle-ation, the factor B in eq 1 can be described according toTavare12 and Mullin:13

The surface energy γ can thus be calculated. Therelationship between the induction time and supersatu-ration described in eq 1 provides essential informationfor the nucleation kinetics. Initially, the nucleation willbe primary, and a method to determine the nucleationshould be applied in the time span where no largecrystals are formed (crystal density is approximatelyzero) and secondary nucleation can be neglected. Sec-ondary nucleation will become important as the crystaldensity and size increase during the course of the

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

tind ∝ [(S0 - 1)n]-1/i+1 exp{ B(i + 1)[ln(S0)]

2} (1)

S ) C/CS (2)

B ) 16πν2γ3

3(kT)3(3)

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

crystallization (i.e., at a substantial drop in the super-saturation).

To differentiate between the impurity influence onnucleation and growth is a genuine problem not resolvedin the literature because nuclei need to grow to adetectable size before they can be measured. Theinduction time method applied in this work to measureprimary nucleation kinetics is widespread in the litera-ture.13 Improvements need to be made in the experi-mental approach to determine nucleation kinetics ingeneral more accurately, but that falls beyond the scopeof the present paper. Note that in our approach theassociated growth rate is determined by an independentgrowth measurement under the microscope and is notbackcalculated from the evolving crystal size distribu-tion (CSD).

3.2. Growth. The growth rate model for G, in m s-1,used in this paper to analyze the influence of impurityis given by12

with kg as the overall mass-transfer coefficient and Sas the supersaturation ratio. The parameters of thegrowth rate relationship (eq 4) are determined fromcrystal growth rate experiments by plotting ln(G) vs ln-(S - 1), where the slope gives the power n and the yaxis intercept the overall mass-transfer coefficient kg.

4. Results and Discussion

4.1. Pure System. The kinetic data for pure AMPIaqueous crystallization are derived from the data usedfor a previous paper.10 The growth rate parameters ineq 4 were determined as exponent n ) 2.26 and crystalgrowth 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 innumerical value compared to those in the original paperbecause of a more accurate reprocessing. The new dataserve as the reference state to which the data from theimpure crystallizations are to be compared.

4.2. Systems with Impurities. 4.2.1. Solubility. Nosignificant change in the solubility of AMPI upon theaddition of 6APA, PG, or degradation products wasmeasured. Therefore, changes in the crystallizationbehavior upon the addition of impurities are solely dueto kinetic effects (nucleation and growth).

4.2.2. Morphology. The influence of the impuritieson the crystal shape and morphology is shown in Figure2. The surface of AMPI crystals becomes more irregularand shows some asperities. X-ray diffraction patternsshowed that both final products were pure, and redis-

solution of these materials did not indicate the presenceof extra components in the crystal lattice within thedetection limit of the assay.

4.2.3. Aspect Ratio. AR of the crystals can beobtained from both the SEM pictures and the imageanalysis using the light microscope at various measure-ments. The light microscope image analysis uses largenumbers of crystals and, therefore, gives a betteraverage value for AR. The average value was 14.9.

4.2.4. Desupersaturation. Experimental desuper-saturation data for the impure systems are shown inFigure 3. Depending on the concentration of the impu-rity and the initial supersaturation of AMPI, a distinctdifference in the induction time is present (delay). Thisindicates that the presence of impurities delays theonset of crystallization and/or crystal growth.

4.2.5. Nucleation. The results of all contaminatedAMPI experiments regarding nucleation kinetics areshown in Table 1.

Influence of 6APA. The presence of impuritiesproved to have a marked effect on the nucleation rate.A series of crystallization experiments at increasing6APA concentration and at different supersaturationsof AMPI showed an increase in the induction time tind(see Figure 4A).

Increased levels of C6APA increase the value for γ, asthe slope becomes steeper, indicating an increase in thesurface energy to create an AMPI crystal surface fromsolution at increasing 6APA concentration. This sup-ports the hypothesis that 6APA molecules are adsorbedat the AMPI crystal lattice, leaving less space or ahigher energy barrier for AMPI adsorption and incor-poration. Indeed, 6APA molecules are structurallysimilar to a portion of the AMPI molecules (see thereaction scheme in the Introduction section) and caneasily incorporate the lattice or crystal surface atdifferent sites. There seems to be no evidence for directincorporation of contaminants into the crystal lattice,so possible effects are likely to occur by adsorption atthe crystal surface.

Influence of PG. Figure 4B shows several crystal-lization experiments with a variable PG concentration

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

G ) kg(S - 1)n (4)

Figure 3. Experimental desupersaturation curves from data sets6, 9, 12, and 18 (see, for example, Table 1). Influence of the 6APAsingle-contaminant concentration (initial supersaturation ratio ofapproximately 2.45). Influence of the PG single-contaminantconcentration and multicomponent impurity effect (combinationof PG, 6APA, and Aoic).

7934 Ind. Eng. Chem. Res., Vol. 43, No. 24, 2004

(10 and 30 mM). Increasing the impurity concentrationof PG, again as for 6APA, increases the induction time.The value of γ is practically the same as that for thepure system because the slopes of the curves are almostthe same.

We do see higher induction times as the PG concen-tration increases from 0, 10 to 30 mM, i.e., 1500, 2400,and 3600 s at approximately S0 ) 2.45. We see animpact similar to that for 6APA, which again can beexplained by the similarity in the structures of PG andAMPI.

Influence of Degradation Products (Aoic). Figure4C shows the same trend as that observed before with6APA and PG as contaminants: the induction timeincreases upon an increase in the levels of degradationproducts. 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 thatthe induction time for multicomponent impurity AMPIcrystallization is higher than that for the pure system.

Longer induction times are obtained upon addition ofthese impurities (see Table 1).

4.2.6. Growth. Influence of 6APA. Figure 5 showsthat the introduction of the 6APA impurity apparentlyhas a distinct influence on the growth rate as deter-mined from the ln(G) vs ln(S - 1) plots. Although inthe pure system there is also some variation of thegrowth rate relationship with initial supersaturation S0,the addition of the impurity resulted in lowering of thegrowth rate by approximately a factor of 2 (the exactfactor depends on the supersaturation; see Table 2). Atlower S values (S < 2.4), the presence of 6APA lowersthe growth rate of AMPI crystals, conforming to thegeneral literature findings.4

However, increasing the 6APA impurity level from 3.6to 10 mM does not decrease the growth rate furthersignificantly. This might be due to the adsorption of6APA molecules at the limited number of suitablegrowth sites, blocking growth on the AMPI crystals.Performing experiments in the range of 0-3.6 mM

Figure 4. Induction time of AMPI crystallization as a function of the supersaturation of AMPI and as a function of (A) the 6APAconcentrations (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 0.0 0.0 1.72, 2.37, 2.45, 3.29 13500, 1650, 1500, 540 4A-D 1-43.6 0.0 0.0 1.93, 2.48, 3.40 5805, 1650, 600 4A 5-7

10.0 0.0 0.0 1.85, 2.45, 3.72 22200, 2400, 690 4A 8-100.0 10.0 0.0 2.44, 2.91 2400, 1620 4B 11 and 120.0 30.0 0.0 2.49 3600 4B 13

10.0 10.0 0.0 2.56 1650 4A,B 140.0 0.0 1.5 2.41 2100 4C 150.0 0.0 8.0 2.22 4050 4C 16

10.0 30.0 1.5 2.35, 3.20 3270, 1020 4D 17 and 18

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

6APA might be beneficial elucidating the relationshipbetween G and C6APA further.

Influence of PG. Parts A and B of Figure 6 showthe growth rate of the impure system at different PGcontamination levels. The growth rate decreases whenCPG ) 10 mM, conforming to expectations. However, atCPG ) 30 mM, the growth rate is a factor of 2-3 higherthan that of the pure system (see Table 2). From theliterature it is also known that the presence of animpurity may increase the growth rate,14 although notmany examples are known. The mechanism responsiblefor this increase is the creation of more kink sites onthe crystal surface. However, there is no clear trend inthe PG data upon an increase of the PG concentration(first lower and then higher growth rates).

Influence of Degradation Products (Aoic). InFigure 6C, the growth rates are shown for AMPIcrystallization with degraded products. Again, increas-ing the impurity concentration leads to slower crystalgrowth. The data for 1.5 and 8 mM Aoic are comparable.

Influence of Multiple Contaminants (6APA, PG,and Degradation Products). In Figure 6D, a cleardecrease in the growth rate is observed when themultiple contaminant level is increased. The combinedeffects of the single contaminants seem to be additiveand influence further the growth kinetics, regarding thevery low growth rate of experiments 17 and 18 (seeTable 2).

Using image analysis for growth rate determinationwill always cause a certain degree of scatter because

Figure 5. Growth rate during AMPI crystallization as a function of the supersaturation of AMPI and as a function of the 6APAconcentration. (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) combinedplot 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 0.0 0.0 1.72, 2.37, 2.45, 3.29 2.26 ( 0.26 81.6 ( 17.3 5A,D 1-4 17.0 ( 8.5 81.6 ( 17.33.6 0.0 0.0 2.48 3.02 ( 0.23 64.8 ( 8.9 5B,D 6 8.0 ( 3.4 64.8 ( 8.9

10.0 0.0 0.0 2.45, 3.72 2.93 ( 0.24 65.5 ( 10.9 5C,D 9 and 10 8.6 ( 4.2 65.5 ( 10.90.0 10.0 0.0 2.91 3.76 ( 0.32 20.5 ( 3.2 5D 12 1.5 ( 0.9 20.5 ( 3.20.0 30.0 0.0 2.49 1.39 ( 0.17 162.5 ( 22.9 6A 13 61.8 ( 15.8 162.5 ( 22.9

10.0 10.0 0.0 2.56 3.59 ( 0.36 55.6 ( 7.6 6B 14 4.6 ( 2.5 55.6 ( 7.60.0 0.0 1.5 2.41 3.47 ( 0.35 57.0 ( 6.7 6C 15 5.1 ( 2.4 57.0 ( 6.70.0 0.0 8.0 2.22 3.19 ( 0.38 65.3 ( 8.9 6C 16 7.2 ( 3.5 65.3 ( 8.9

10.0 30.0 1.5 2.35, 3.20 5.62 ( 0.84 9.0 ( 2.7 6D 17 and 18 0.2 ( 0.4 9.0 ( 2.8a 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 differentsupersaturation values, S. Given for a quick comparison of the impurity effect on the value of G.

7936 Ind. Eng. Chem. Res., Vol. 43, No. 24, 2004

the selectivity in objects chosen for analysis. Withrespect to scatter, however, the presented data arecomparable to those of other efforts in the field ofcrystallization.

4.3. Mechanism for the Influence of Impurities.In general, the impact of impurities acts on the meta-stable zone width. However, this is certainly not truefor all impurities.13 In our investigation, it is clearlyfound that the thermodynamics, i.e., solubility, are notsignificantly influenced by the impurities. Therefore,one of the other possibilities of impurity action isapplicable, i.e., retardation of growth and nucleation.

Adsorption to the AMPI crystal surface of 6APA, PG,and/or degraded products is believed to be responsiblefor the retardation of the AMPI crystallization kinetics.The crystal structure of ampicillin trihydrate wasresolved by X-ray diffraction analysis by James et al.15

Such crystal structure represents the alternating lay-ered structure of AMPI molecules with the three watermolecules in between. The influence of the impurities6APA and PG can easily be understood by blocking thesurface of the AMPI crystal because these molecules arethe same as part of the AMPI molecule and cantherefore easily fit at the surface. The degraded AMPIproduct Aoic is also structurally very similar to AMPI(see the Introduction section) and may act in the sameway as 6APA and PG.

A mechanism based on adsorption was proposed byKubota et al.3 This model related the impurity concen-tration to the decrease in the growth velocity by meansof adsorption isotherms. We, however, do not see a clear

influence of the impurity concentrations on the growthonly. Their model, if used in our case, should be adaptedto incorporate the decrease in the nucleation rate.

4.4. Implication on Process Design. A peak broad-ening of the CSD of AMPI and a shift to higher averagecrystal sizes will be observed at increasing 6APAimpurity levels because of slower nucleation kinetics.This will influence the process design of an AMPIproduction process. In general, a broad CSD complicatessolids handling and further processing and shouldtherefore be prevented.

Because 6APA and PG are present during AMPIcrystallization in the production processes (see thereaction scheme in the Introduction section), measuringpure AMPI crystallization kinetics will lead to a flawedprocess design. This paper provides more correct kineticdata for process design under industrial practice. Al-though the paper discusses batch crystallization, theobtained kinetic data can be used when designing acontinuous crystallization process.

In industry, generally AMPI is crystallized in a batchprocess, without seeding. An AMPI solution at low pHis neutralized with a base. The size of the crystals istuned by optimizing the titration profile. To understandand further improve upon this so-called fed-batchoperation, the data obtained in this study are ofimportance. Primary nucleation kinetics together withthe induction time measurements can be used tounderstand how the solution during the fed-batchoperation leaves and re-enters the metastable zone inorder to generate the required number of nuclei. The

Figure 6. Growth rate during AMPI crystallization as a function of the supersaturation of AMPI and as a function of different types andconcentrations of impurities: (A) PG as the impurity, CPG ) 10 mM; (B) PG as the impurity, CPG ) 30 mM; (C) AMPI degradationproducts as the impurity, CAoic ) 1.5 mM (squares, expt 15) and 8.0 mM (triangles, expt 16); (D) multicomponent impurity effect, combinationof 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.

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

growth rate kinetics, on the other hand, provide theknowledge to calculate the optimum titration profile,while keeping the solution in the upper part of themetastable zone at constant supersaturation and thusoptimizing the AMPI production without generating anew explosion of nuclei, with the latter being detrimen-tal for the size distribution as well as the filtration ofthe product. The knowledge of the effects of the relevantimpurities on the crystallization kinetics provides fur-ther relevance of these data.

5. Conclusions

This paper reports AMPI crystallization kinetics andshows and quantifies the effect of inherent impuritiesthereon. The presence of impurities during crystalliza-tion of the SSA AMPI has a marked influence on theinduction time, the nucleation rate, the growth rate, andthe crystal morphology of the product. The impuritiesinvestigated are the building blocks of AMPI, 6APA, andPG as well as the main degradation product Aoic. Theyare most likely adsorbed at the AMPI crystal surface,leaving less space or a higher energy barrier for AMPIadsorption and incorporation and resulting in slowernucleation and growth rate, thereby reducing crystalformation. Such an impact has been quantified in thispaper, and such knowledge can be used to fine-tune thefinal product characteristics, i.e., by using the impurity-dependent crystallization kinetics during process designand the appropriate population balance based crystal-lization reactor models.10,16

Symbols

AR ) aspect ratioC ) 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 ratioT ) temperature, Kt ) time, s

Greek Letters

γ ) interfacial free energy, J m-2

ν ) molecular volume, m3

Subscripts

AMPI ) ampicillinAoic ) ampicillin penilloic acid6APA ) 6-aminopenicillanic acidPG ) phenylglycine0 ) initialS ) saturated

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

Accepted October 4, 2004

IE0307028

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