in Wiley Online Library (wileyonlinelibrary.com).The batch cooling solution crystallization of ammonium oxalate was performed in water at various constant cooling rates. Measurements of the solute concentration were obtained using in situ attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy, and final estimates of the crystal size distribution (CSD) were computed; thanks to in situ image acquisition and off-line image analysis. The crystallization process was then simulated using population balance equations (PBEs). Estimates of the nucleation and the growth parameters were computed through model/experiments fitting. According to the cooling rate, the PBE model allowed distinguishing between two distinct crystallization regimes, separated by an ''intermediate regime.'' The respective contributions and shortcomings of solute concentration measurements and granulometric data to the identification of nucleation and growth kinetic parameters are analyzed and discussed. It is shown in particular that no real separate estimation of nucleation and growth parameters can be obtained in the absence of CSD data. PBEs Crystal growth ratePBEs are widely used as a modeling framework in the engineering of dispersed media, with applications including crystallization, powder technologies, polymerization processes, biotechnologies, etc. [26][27][28][29] In most published

a b s t r a c tIn situ ATR-FTIR spectroscopy coupled with in situ image acquisition measurements were used to determine the solubility curve and to investigate the metastable zone width (MSZW) of ammonium oxalate monohydrate (AO) aqueous solutions under polythermal conditions. The experimental data allowed estimating the MSZW and induction time of the system and the results were compared with published literature values for the same system [1]. ''Pseudo'' induction time estimation techniques based on polythermal methods showed that AO aqueous solutions exhibit two nucleation regimes depending on the cooling rates.Even though they are based on rough and questionable assumptions, induction time and MSZW estimation methods are often considered as essential for the development of industrial crystallization processes. However, both the relevancy and the physical meaning of the results provided by these methods are uncertain. In the actual industrial context where many advanced measurement techniques and modeling tools became available, the present paper intends to call into question the outcome of the notions of MSZW and induction time.

Acoustic emission (AE) which has been successfully applied for monitoring a rather wide variety of solids elaboration processes was almost never evaluated in the field of industrial pharmaceutical crystallization. Few papers reported that solution crystallization processes give rise to acoustic emission signals that could be related to the development of the basic crystallization phenomena. This study is intended to demonstrate new perspectives opened up by the possible use of acoustic emission (AE) as a non-intrusive and non destructive sensor for monitoring solution crystallization with a particular focus being put on the presence of impurities in real industrial processes. The wealth of acquired AE information is highlighted and it is suggested that such information could allow the design of innovative multipurpose sensing strategies. It is shown notably that AE provides a very early detection of nucleation events, much before the onset of the so-called "nucleation burst". It is also shown that AE brings new insight into the effect of impurities on both the development of the crystallization process and the quality of the crystallized product.