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  • Robust Bayesian Estimation of Kineticsfor the Polymorphic Transformation of

    L-Glutamic Acid CrystalsMartin Wijaya Hermanto, Nicholas C. Kee, Reginald B. H. Tan, and Min-Sen Chiu

    Dept. of Chemical and Biomolecular Engineering, National University of Singapore, Singapore, Singapore 117576

    Richard D. BraatzDept. of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801

    DOI 10.1002/aic.11623Published online November 4, 2008 in Wiley InterScience (www.interscience.wiley.com).

    Polymorphism, in which there exist different crystal forms for the same chemicalcompound, is an important phenomenon in pharmaceutical manufacturing. In this arti-cle, a kinetic model for the crystallization of L-glutamic acid polymorphs is developedfrom experimental data. This model appears to be the first to include all of the trans-formation kinetic parameters including dependence on the temperature. The kinetic pa-rameters are estimated by Bayesian inference from batch data collected from twoin situ measurements: ATR-FTIR spectroscopy is used to infer the solute concentration,and FBRM that provides crystal size information. Probability distributions of the esti-mated parameters in addition to their point estimates are obtained by Markov ChainMonte Carlo simulation. The kinetic model can be used to better understand the effectsof operating conditions on crystal quality, and the probability distributions can beused to assess the accuracy of model predictions and incorporated into robust controlstrategies for polymorphic crystallization. 2008 American Institute of Chemical EngineersAIChE J, 54: 32483259, 2008

    Keywords: pharmaceutical crystallization modeling, polymorphism, Bayesian inference,Markov Chain Monte Carlo

    Introduction

    Polymorphism, in which multiple crystal forms exist forthe same chemical compound, is of significant interest to

    industry.15 The variation in physical properties such as crys-tal shape, solubility, hardness, color, melting point, andchemical reactivity makes polymorphism an important issuefor the food, specialty chemical, and pharmaceutical indus-tries, where products are specified not only by chemical com-position but also by their performance.2 Controlling polymor-phism to ensure consistent production of the desired poly-morph is important in those industries, including in drugmanufacturing where safety is paramount. With the ulti-mate goal being to better understand the effects of processconditions on crystal quality and to control the formation ofthe desired polymorph, this article considers the estimation

    Additional Supporting Information may be found in the online version of thisarticle.

    N. C. Kee is also affiliated with Dept. of Chemical and Biomolecular Engineer-ing, University of Illinois at Urbana-Champaign, Urbana, IL 61801.

    R. B. H. Tan is also affiliated with Institute of Chemical and Engineering Scien-ces Singapore, Singapore 627833.

    Correspondence concerning this article should be addressed to M.-S. Chiu [email protected]

    2008 American Institute of Chemical Engineers

    AIChE JournalDecember 2008 Vol. 54, No. 123248

  • of kinetic parameters in a model for polymorphic crystalliza-tion. Such a process model can accelerate the determinationof optimal operating conditions and to speed process devel-opment, when compared with time-consuming and expen-sive trial-and-error methods for determining the operatingconditions.

    In this article, a kinetic model of L-glutamic acid polymor-phic crystallization is developed from batch experiments within situ measurements including ATR-FTIR spectroscopy toinfer the solute concentration and FBRM to provide crystalsize information. Kinetics of polymorphic transformationhave been estimated by various procedures.69 A commonlyused method to estimate model parameters in nonlinear pro-cess models is weighted least squares,1012 which has beenapplied to polymorphic crystallization.1316 Althoughweighted least squares methods are adequate for many prob-lems, Bayesian inference is able to include prior knowledgein the statistical analysis, which can produce models withhigher predictive capability. Although Bayesian inference isnot within the standard toolkit for chemical engineers, therehave been many applications to chemical engineering prob-lems over the years including the estimation of parameters inchemical reaction,17 heat transfer in packed beds,18 microbialsystems,1921 and microelectronics processes.22

    Quantifying uncertainties in the parameter estimates isrequired for assessing the accuracy of model predictions.23,24

    When weighted least squares methods are used for parameterestimation, the widely used approaches to quantify uncertain-ties in parameter estimates are the linearized statistics and like-lihood ratio approaches.25 In the linearized statistics approach,the model is linearized around the optimal parameter estimatesand the parameter uncertainty is represented by a v2 distribu-tion. This model linearization can result in highly inaccurateuncertainty estimates for highly nonlinear models,25 and thisapproach ignores physical constraints on the model parame-ters. The likelihood ratio approach, which is the nonlinear ana-logue to the well-known F statistic, takes nonlinearity intoaccount but approximates the distribution25 and ignores con-straints on the model parameters. This article applies a Bayes-ian inference approach that not only avoids making theseapproximations but also includes prior information during theestimation of parameter uncertainties.

    In this article, the parameters in a kinetic model for L-glu-tamic acid polymorphic crystallization process are determinedby Bayesian estimation. The probability distribution over pro-cess model parameters is defined through the Bayesian posteriordensity, from which all parameter estimates of interest (e.g.,means, modes, and credible intervals) are calculated. However,the conventional approach to calculate these estimates ofteninvolves complicated integrals of the Bayesian posterior den-sity, which are analytically intractable. To overcome this draw-back, Markov Chain Monte Carlo (MCMC) integration2628

    was applied to compute these integrals in an efficient manner.MCMC does not require approximation of the posterior distri-bution by a Gaussian distribution.20,29,30 This posterior distribu-tion for the estimated parameters can be used to accuratelyquantify the accuracy of model predictions and can be incorpo-rated into robust control strategies for crystallization process.24

    This article is organized as follows. The next sectiondescribes the experimental procedure to obtain measurementdata for parameter estimation. A short review of Bayesian

    theory and MCMC integration is discussed next. This is fol-lowed by the description of the L-glutamic acid crystalliza-tion model and the results of the parameter estimation.Finally, conclusions are given.

    Experimental Methods

    The crystallization instrument setup used was similar tothat described previously.31 A Dipper-210 ATR immersionprobe (Axiom Analytical) with ZnSe as the internal reflec-tance element attached to a Nicolet Protege 460 FTIR spec-trophotometer was used to obtain L-glutamic acid spectra inaqueous solution, with a spectral resolution of 4 cm21. Thechord length distribution (CLD) for L-glutamic crystals in so-lution were measured using Lasentec FBRM connected to aPentium III running version 6.0b12 of the FBRM ControlInterface software.

    Calibration for solution concentration

    Different solution concentrations of L-glutamic acid (99%,Sigma Aldrich) and degassed deionized water were placed ina 500-ml jacketed round-bottom flask and heated until com-plete dissolution. The solution was then cooled at 0.58C/minwhile the IR spectra were being collected, with continuousstirring in the flask using an overhead mixer at 250 rpm. Ta-ble 1 lists the five different solution concentrations used tobuild the calibration model.

    The IR spectra of aqueous L-glutamic acid in the range11001450 cm21 and the temperature were used to constructthe calibration model based on various chemometrics meth-ods such as principal component regression (PCR) and partialleast square regression (PLS).32 The calculations were carriedout using inhouse MATLAB 5.3 (The Mathworks) codeexcept for PLS, which was from the PLS Toolbox 2.0. Themean width of the prediction interval was used as the crite-rion to select the most accurate calibration model. The noiselevel was selected based on the compatibility of the predic-tion intervals with the accuracy of the solubility data. Thechemometrics method forward selection PCR 2 (FPCR 2)33

    was selected because it gave the smallest prediction interval;using a noise level of 0.001, the prediction interval (0.73 g/kg) was compatible within the accuracy of this model withrespect to solubility data reported in the literature.13

    Solubility determination and feedbackconcentration control experiments

    The commercially available L-glutamic acid crystals wereverified to be pure b-form using powder X-ray diffraction(XRD) and were used for the determination of the b-formsolubility curve. Pure a-form crystals obtained using a rapid

    Table 1. Glutamic Acid Aqueous Solutions Used forCalibration

    Concentration (g/g of water) Temperature Range (8C)

    0.00837 35210.01301 48130.01800 57320.02300 64340.02800 6445

    AIChE Journal December 2008 Vol. 54, No. 12 Published on behalf of the AIChE DOI 10.1002/aic 3249

  • cooling method outlined previously13 were used to determinethe a-form solubility curve in similar fashion as the b-formin a separate experiment. For each polymorph, the IR spectraof L-glutamic acid slurries (saturated, and with excess crys-tals) were collected at different temperatures ranging from 25to 608 C. T

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