(298e) Using ATR-FTIR to Track the Crystallization of Multiple Solutes

In-situ measurements of concentration play a large role in developing and implementing control schemes for crystallizers [1, 2]. In recent years Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) technology has been widely used to track the concentration profiles of a single solute during crystallization processes [3-5]. However, if the parent solution is complex and contains multiple potentially crystallizing solutes (e.g. nuclear waste) multiple solutes must be tracked simultaneously. This can be a challenging task if the characteristic infrared absorbance peaks for the individual solutes are overlapping. In this work a supervised learning approach was taken to construct a correlation between the infrared absorbance spectrum and the concentrations of multiple solutes in the solution. Furthermore, to avoid over-fitting and establish robust correlations, the training set was carefully selected and projection-based regression techniques [3, 6] were applied—of these, Partial Least Squares Regression (PLSR) performed the best.

This methodology was used to monitor crystallizations of complex electrolytic solutions representative of nuclear waste stored at Hanford, WA [7-9]. In particular, four solutes (carbonate, sulfate, nitrate, and nitrite) were tracked during batch cooling crystallizations. By monitoring a number of crystallizations, operating zones for crystallizing particular salts were identified. Furthermore, in future work, multi-solute concentration monitoring will be used to facilitate a direct design approach [10] to control the crystallization of clean (non-radioactive) sodium salts from nuclear waste mixtures.

1.         Nagy, Z.K. and R.D. Braatz, Advances and New Directions in Crystallization Control.Annual Review of Chemical and Biomolecular Engineering, Vol 3, 2012.

2.         Zhou, G.X., et al., Direct design of pharmaceutical antisolvent crystallization through concentration control. Crystal Growth & Design, 2006. 6(4): p. 892-898.

3.         Togkalidou, T., et al., Solute concentration prediction using chemometrics and ATR-FTIR spectroscopy. Journal of Crystal Growth, 2001. 231(4): p. 534-543.

4.         Lewiner, F., et al., On-line ATR FTIR measurement of supersaturation during solution crystallization processes. Calibration and applications on three solute/solvent systems. Chemical Engineering Science, 2001. 56(6): p. 2069-2084.

5.         Dunuwila, D.D. and K.A. Berglund, ATR FTIR spectroscopy for in situ measurement of supersaturation. Journal of Crystal Growth, 1997. 179(1-2).

6.         Xiaobo, Z., et al., Variables selection methods in near-infrared spectroscopy. Analytica Chimica Acta, 2010. 667(1–2): p. 14-32.

7.         Herting, D.L., Clean Salt Process - Final Report, E. Management, Editor. 1996.

8.         Nassif, L., et al., Pretreatment of Hanford medium-curie wastes by fractional crystallization. Environmental Science & Technology, 2008. 42(13): p. 4940-4945.

9.         Rajbanshi, A., B.A. Moyer, and R. Custelcean, Sulfate Separation from Aqueous Alkaline Solutions by Selective Crystallization of Alkali Metal Coordination Capsules. Crystal Growth & Design, 2011. 11(7): p. 2702-2706.

10.       Fujiwara, M., et al., First-principles and direct design approaches for the control of pharmaceutical crystallization. Journal of Process Control, 2005. 15(5): p. 493-504.