PAT monitoring and PBE modelling of batch cooling solution crystallization in the presence of impurities

International audienceHindering effects of impurities on the crystal growth are usually assumed to result of the adsorption of impurity species on the crystal surface. In the presence of impurities the growth rate does not depend on supersaturation only, but also on the concentration of impurities and on the time a given particle spent in contact with inhibiting species (unsteady-state adsorption mechanisms). Few kinetic models describe such phenomena. Moreover, published models are derived from data obtained from specific experiments performed on singles crystals, which makes their application to real industrial crystallizers rather questionable. Indeed, for process engineering purposes, the available kinetic inhibition models accounting for the effect of impurities (e.g. Cabrera-Vermilyea or Kubota-Mullin's approaches), have to be evaluated in industrial situations where complex and distributed features of the crystallizing suspensions are involved (e.g. during batch solution crystallization). Population Balance Equations (PBE) modelling offers an invaluable simulation tool for such evaluation. With this aim in view, a comprehensive modelling approach based on in situ continuous and dispersed phase measurements, and specific PBE simulation was developed to represent and better understand the effect of impurities on the development of batch crystallization processes. The cooling solution crystallization of Ammonium Oxalate (AO) in water in the presence of various concentration of Nickel Sulphate was selected as a model system during this study. In situ measurements of supersaturation were performed using ATRFTIR spectroscopy and the CSD was assessed thanks to in situ image acquisition followed by off-line image processing. The experimental results were simulated after estimating crystallization kinetic parameters, including parameters of models describing the inhibiting adsorption of impurity on the growing crystal surfaces. Nonlinear optimization techniques were used to fit the experimental data to the simulated ones

Acoustic Emission: a new in-line and non-intrusive sensor for monitoring batch solution crystallization operations

International audienceAcoustic emission (AE) is shown to provide complex and in-depth information on both the liquid and the dispersed phases during batch cooling solution crystallization processes. Despite its complexity, such information might be highly valuable for process monitoring and control purposes owing to its non-intrusive features, its relative cheapness, and the very wide scope of its potential applications. Basic crystallization phenomena such as the onset of nucleation and the development of crystal growth, several key-process variables like the concentration of solid in suspension, and overall data describing the average particle size and the content in impurities of the crystallization medium are evaluated from real experimental data obtained at the lab-scale. AE is not claimed here to allow replacing "usual" particle sizes sensing technologies like image analysis or FBRM; it is rather suggested that the large amount of information contained in the acoustic data is quite interesting and deserves further investigation

Modélisation de cristallisations discontinues en solution en l'absence et en présence d'impuretés.

National audienceThe batch solution cooling crystallization of ammonium oxalate (AO) was performed in pure water and in the presence of dissolved impurities (NiSO4). The solute concentration was measured using in situ ATR FTIR spectroscopy and the crystal size distribution was estimated from in situ image acquisition and processing, using an immersed video acquisition probe. The experimental supersaturation trajectories show that the development of the crystallization is significantly inhibited by the presence of impurity. Such impurity effects, which were widely investigated at the scale of one single mono-crystal, were neither observed nor described in the case of a whole set of dispersed crystallizing particles. After identification of the nucleation and growth kinetics of AO in pure medium, the inhibition parameters were determined according to Kubota-Mullin's model. A mechanism of unsteady-state adsorption of impurity species at the growing crystal surface was identified, which allowed to satisfactorily represent the experimental results. A dynamic simulation model was thus proposed and developed using Population Balance Equations (PBE), which were solved thanks to a numerical method of characteristics.La cristallisation discontinue en solution de l'oxalate d'ammonium (OA) dans l'eau, en présence et en l'absence d'impuretés (NiSO4), a été conduite par refroidissement. La mesure de concentration en soluté a été réalisée grâce à la spectroscopie ATR FTIR in situ et le suivi granulométrique par analyse d'images acquises à l'aide d'un capteur vidéo in situ. Les trajectoires de sursaturation en présence de sulfate de nickel montrent que la cristallisation est considérablement inhibée par la présence de l'impureté. Ces effets d'impuretés, largement étudiés dans la littérature à l'échelle d'un monocristal n'ont jamais été observés et décrits sur l'ensemble d'une population de cristaux en suspension. Après avoir identifié les cinétiques de nucléation et de croissance de l'OA en milieu pur, les paramètres d'inhibition sont déterminés sur la base du modèle de Kubota-Mullin. Un mécanisme transitoire d'adsorption de surface de l'impureté a été identifié, dont la simulation permet de rendre compte des résultats expérimentaux. Un modèle de simulation dynamique par les équations de bilans de populations (EBP) a été développé et résolu à l'aide d'un algorithme numérique fondé sur la méthode des caractéristiques

Batch cooling solution crystallization of ammonium oxalate in the presence of impurities: Study of solubility, supersaturation, and steady-state inhibition

International audienceCompared with equilibrium data in pure solvent, variations of solubility are frequently observed in the presence of dissolved impurities. It is also well-known that impurities can inhibit the crystallization processes and lead to supersaturation barriers below which the growth of crystals is canceled. However, many papers dealing with the inhibiting effects of impurities in solution crystallization are rather unclear about the appropriate way of expressing the supersaturation in impure media. Indeed, as suggested in the present work, the latter can be defined either with respect to the solubility in pure solvent or in reference to the solubility of the impure solvent. Setting the correct reference for computing supersaturation in impure solutions is obviously a key issue for understanding and modeling the dynamics of elementary crystallization phenomena (i.e., primary and secondary nucleation, crystal growth, etc.) The present case study aims at clarifying this point. Solubility data were acquired by in situ ATR FTIR measurements of the concentration of ammonium oxalate in pure water and in impure water containing various concentrations of nickel sulfate dissolved as impurity. Supersaturation thresholds were observed and analyzed, according to the theoretical framework of the Kubota-Mullin impurity adsorption model. The experimental thresholds are attributed to the solubility drifts resulting from the pollution of the pure solvent by impurities. Finally we propose that the impurity-dependent solubility should be considered as the appropriate reference for defining supersaturation

Suivi en-ligne des procédés de cristallisation en solution en milieux pur et impur par émission acoustique

National audienceMany mechanical and physical processes are associated with the generation of acoustic waves produced by the rapid release of energy from localized sources within a material. Acquiring and processing these waves allows non-destructive control of phenomena such as the extension of cracks in a structure under stress, plastic deformation, phase transitions, etc. Few works were devoted in past to acoustic emission (AE) during crystallization processes. Moreover, most available studies are purely descriptive and do not provide results enabling to quantitatively monitor basic crystallization phenomena (i.e., nucleation, growth, agglomeration, etc.) New results dealing with the monitoring of batch solution crystallization operations are presented here. The acquired acoustic signals are interpreted thanks to the joint monitoring of the continuous phase (Measurement of the solute concentration thanks to ATR FTIR spectroscopy) and of the dispersed phase, thanks to the video in situ acquisition of images of crystals in suspension. It is shown that acoustic emission sensors "perceive" the development of the crystallization process and, in particular, that an early detection of the nucleation of crystals is allowed. Analysing the parameters of acoustic waves is shown to allow separate monitoring of basic crystallization phenomena which remain to be identified. As far as online monitoring of industrial crystallization processes is concerned, the measurements thus obtained open promising perspectives.De nombreux processus mécaniques et physiques s'accompagnent de la génération d'ondes acoustiques dues à une libération rapide et localisée d'énergie au sein des matériaux. L'acquisition et le traitement de ces ondes permet un contrôle non-destructif de nombreux phénomènes tels que la fissuration sous contrainte, la déformation élastique, le changement de phase, etc. Peu de travaux ont été consacrés par le passé à l'émission acoustique (EA) pendant les procédés de cristallisation et l'essentiel des travaux disponibles, purement descriptif, ne fournit pas de résultats susceptibles de permettre une évaluation quantitative indirecte des processus élémentaires de la cristallisation (i.e., nucléation, croissance, agglomération, etc.). On présente ici des résultats nouveaux obtenus pendant le suivi de cristallisations discontinues en solution. Les signaux acoustiques recueillis sont interprétés grâce à la mesure, en parallèle, de la phase continue (mesure de concentration par spectroscopie ATR FTIR) et de la phase dispersée, par acquisition d'images de la suspension à l'aide d'une sonde vidéo in situ. On montre que les capteurs d'émission acoustique "perçoivent" le déroulement de la cristallisation et, en particulier, qu'elle permet une détection extrêmement précoce de la nucléation des cristaux. Une analyse des composantes des signaux acoustiques montre également que l'EA permet de suivre séparément plusieurs processus élémentaires de la cristallisation qui restent à identifier clairement. Les mesures ainsi obtenues ouvrent des perspectives prometteuses quant au suivi en-ligne des opérations de cristallisation industrielle

Acoustic on-line monitoring of solution crystallization process in pure and impure media

International audienceWe show that acoustic emission (AE) is a very attractive non intrusive technique for monitoring crystallization processes. AE has been successfully applied in many fields of material sciences and it was also used in the pharmaceutical industry for monitoring various chemical engineering processes including fluidized bed granulation, fluidized bed coating, powder compaction, etc. However, few studies deal with the use of AE to monitoring crystallization processes. The objective of this work is to evaluate the potential for using acoustic emission to monitor polythermal batch crystallization in the absence and presence of impurities. The basic concept behind AE monitoring of crystallization processes is that the phase transitions occurring during crystallization in solution induce physicochemical changes in the suspension. Such changes release energy and therefore generate acoustic elastic waves propagating in the liquid medium. Furthermore, as crystal particles are generated, the elastic properties of the dispersed phase also change. The latter changes affect the acoustic emission caused by the particle collision impacts and inter-particles and/or particles-wall frictions. The elasticity of crystals and their kinetic energy are also affected by many other properties such as size, shape, hardness, density, uniformity of composition which obviously depend on the presence of impurities during the process. We report preliminary batch solution cooling experiments obtained with the model system Ammonium Oxalate/water (AO) in the presence of Nickel Sulfate as impurity. The experiments are monitored using AE, ATR FTIR measurement of supersaturation and CSD analysis performed thanks to in situ image acquisition. Complex but promising information is obtained thanks to AE monitoring

Etude expérimentale et modélisation par bilans de populations des cinétiques de nucléation de croissance d’opérations discontinues de cristallisation par refroidissement en absence et en présence d’impuretés.

Hindering effects of impurities on crystal growth are usually assumed to result in the adsorption of impurity species on the crystal surface. In the presence of impurities the growth rate does not depend on supersaturation only, but also on the concentration in impurities and on the contact time of a given particle with inhibiting species (unsteady-state adsorption mechanisms). Few kinetic models describe such phenomena. Indeed, for process engineering purposes, the available kinetic inhibition models accounting for the effect of impurities (e.g. Kubota-Mullin’s approaches), have to be evaluated in industrial situations where complex and distributed features of the crystallizing suspensions are involved (e.g. during batch solution crystallization). Population Balance Equations (PBE) modeling offers an invaluable simulation tool for such evaluation.With this aim in view, a comprehensive modeling approach based on in situ continuous and dispersed phase measurements, and specific PBE simulation was developed to represent and better understand the effect of impurities on the development of batch crystallization processes.Cooling solution crystallization of Ammonium Oxalate (AO) in water in the absence and presence of Nickel Sulphate at different concentrations was selected as a model system during this study. In situ measurements of supersaturation were performed using ATRFTIR spectroscopy and the CSD was assessed thanks to in situ image acquisition. The experimental results were simulated after estimating crystallization kinetic parameters, including parameters of models describing the inhibiting adsorption of impurity on the growing crystal surfaces. Primary and secondary surface nucleation mechanisms as well as growth of the main crystal dimension (length) were described in pure and impure media. The model roughly represents the effects of different cooling rates and impurity concentration on the supersaturation profiles and the CSD of the final particles.De façon évidente, la pratique industrielle de la cristallisation ne peut éviter la présence d’impuretés indésirables produites suite aux nombreuses réactions chimiques précédant les étapes de cristallisation. Même en quantités infimes, les impuretés présentes dans les jus mères peuvent affecter de façon considérable la cristallisation et la qualité du produit obtenu. Dans ce contexte, les technologies de mesures en ligne fournissent un apport considérable en permettant l’obtention d’informations riches, en temps réel et de façon quasi-continue sur l’évolution des phases liquide et dispersée. L’objectif du présent travail est la compréhension des effets des impuretés sur les produits de cristallisations discontinues. Des expériences sont effectuées, sur une installation-pilote, en vue d’étudier les effets des paramètres opératoires de cristallisation de l’oxalate d’ammonium monohydrate pur et en présence de sulfate de nickel (impureté) sur la taille et la forme des cristaux produits. Pour cela, deux techniques analytiques in situ, la spectroscopie ATR FTIR pour la mesure de sursaturation et l'analyse d’image in situ pour l’évaluation des distributions des tailles des cristaux, seront utilisées. A partir des données expérimentales obtenues, nous proposons des modèles cinétiques de nucléation (primaire et secondaire) et de croissance tenant compte de l'action des impuretés et décrivant l’adsorption de celles-ci à la surface des cristaux. Ces modèles sont ensuite exploités pour la mise en place de simulations fondées sur les équations de bilans de populations.L’originalité de l’approche adoptée réside dans l’emploi du modèle classique de Kubota – Mullin, modifié par l’ajout d’une variable temporelle permettant la prise en compte de la durée d’exposition de chaque cristal aux impuretés. Les résultats de simulation obtenus décrivent de façon satisfaisante l’évolution temporelle de la sursaturation et de la distribution de taille des cristaux

Mesure de la distribution de taille de cristaux aciculaires par analyse d'images d'une sonde video in situ

National audienceThe batch cooling crystallization of ammonium oxalate (AO) in water was investigated in order to design a dynamic model of the process in both pure and impure solvent. The solute concentration was measured using in situ ATR FTIR spectroscopy while the solid dispersed phase was monitored through in situ image acquisition, using an immersed CCD video probe. In order to evaluate the particle size distribution (PSD) an algorithm for the processing of video 2D images of AO crystals was developed. The algorithm is intended to enable reliable crystal size measurements; the specificity of the system under investigation being related to the acicular shape of AO crystals. In particular, a major difficulty arises from the many overlapping particles which have to be differentiated prior to their size measurement. After segmentation, the identification of particles was mostly based upon a refined method for the detection of corners followed by the reconstruction of the crystals perimeters. The efficiency of the method was evaluated through comparison between the automatic image processing measurements and measurements performed by a trained human operator. More than 80% of AO particles are satisfactorily detected in suspension, even fuzzy crystals, provided that their corners are unbroken.La cristallisation discontinue d'oxalate d'ammonium (OA) a été étudiée en vue de la mise au point d'un modèle dynamique du procédé en solvant pur ou contaminé par des impuretés dissoutes. La concentration en soluté a été mesurée par spectroscopie ATR FTIR in situ, tandis que la phase solide dispersée était suivie par une sonde vidéo CCD immergée permettant l'acquisition d'images. En vue d'évaluer la distribution des tailles de particules (DTP), un algorithme de traitement des images vidéo bidimensionnelles des cristaux d'OA a été mis au point. Cet algorithme vise à assurer une mesure fiable de la taille des cristaux ; la spécificité du système étudié ici étant essentiellement liée à la forme aciculaire des cristaux. En particulier, une difficulté majeure du traitement provient du grand nombre de particules superposées qui doivent être différenciées avant la mesure de leur taille. Après segmentation, l'identification des particules a été essentiellement basée sur une technique de détection des coins, permettant ensuite une reconstruction du périmètre des cristaux. L'efficacité de la méthode a ensuite été évaluée par comparaison du traitement automatique avec celui effectué manuellement par un opérateur humain. Plus de 80% des particules d'OA en suspension ont été évaluées de façon satisfaisante, même les cristaux présentant une image floue on été identifiés, à condition que leurs coins soient intacts

Recognizing overlapped particles during a crystallization process from in situ video images for measuring their size distributions

International audienceThis paper presents a method to recognize polygonal-shaped particles (i.e. rectangles, regular/irregular prisms) among agglomerated crystals from in-situ images during a crystallization process. The aim is to measure the particle size distributions (PSD); a key measurement needed for monitoring and controlling industrial crystallization operations . The method is first based on detecting the geometric features of the particles identified by their salient corners. A clustering technique is then applied by grouping three correspondent salient corners belonging to the same polygon. The efficiency of the method is tested on particles of Ammonium Oxalate (AO) monitored during batch crystallization in pure water. Particle size distributions are calculated, and a quantitative comparison between automatic and manual sizing is performed

Morphological and physicochemical properties of dip-coated poly {(2,5-diyl pyrrole) [4-nitrobenzylidène]} (PPNB) thin films: towards photovoltaic applications

A new material: conjugated poly {(2,5-diyl pyrrole) [4-nitrobenzylidène]}, that we called (PPNB), has been synthesized and characterized. The cyclic voltammetry has been used in order to estimate first oxidation (Ep) and reduction (En) potentials of our polymer. These values have been assigned, respectively, to the position of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and determination of the energy band gap which have been estimated to be 6.16, 3.89 and 2.27 eV respectively. Energy levels values of the HOMO and LUMO of the PPNB polymeric donor material were evaluated and the results are compatible with an electron transfer to C60 within an eventual junction, such values show that PPNB could be probed for applications in organic solar cells as donor material. PPNB Thin films have been deposited by dip-coating technique from Dichloromethane solvent with different polymer concentrations, and a dipping speed of 3.0 cm/min. For morphological characterization of the films scanning electron microscopy (SEM) was carried out. The samples, when observed by SEM, reveals that the films deposited are less dense, uniform. Cross-sectional SEM micrographs PPNB films show that thickness of the layers is homogeneous and has value of 35–40 nm. Optical characteristics of the polymer thin films were studied using UV-vis spectroscopy; absorption of wide range of wavelengths from 350 to 700 nm was observed. The optical band gap energy ranges between 1.9 eV and 1.94 eV. Based on these analyzes we realized heterojunction organic solar cells with the structure: ITO/Au/PPNB/C60/BCP/Al, the cells had a photovoltaique effect after J-V measuring, however the efficiency of photo generation under AM1.5 illumination was weak (about 0.02%) and needs to be improved