Consideration of some sampling problems in the on-line analysis of batch processes by low-field NMR spectrometry

Nordon, A. and Diez-Lazaro, A. and Wong, C.W.L. and McGill, C.A. and Littlejohn, D. and Weerasinghe, M. and Mamman, D.A. and Hitchman, M.L. (2008) Consideration of some sampling problems in the on-line analysis of batch processes by low-field NMR spectrometry. Analyst, 133 (3). pp. 339-347. ISSN 0003-2654

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Abstract

A low-field medium-resolution NMR spectrometer, with an operating frequency of 29 MHz for 1H, has been assessed for on-line process analysis. A flow cell that incorporates a pre-magnetisation region has been developed to minimise the decrease in the signal owing to incomplete polarisation effects. The homogeneous esterification reaction of crotonic acid and 2-butanol was monitored using a simple sampling loop; it was possible to monitor the progression of the reaction through changes in CH signal areas of butanol and butyl crotonate. On-line analysis of heterogeneous water-toluene mixtures proved more challenging and a fast sampling loop system was devised for use with a 5 L reactor. The fast sampling loop operated at a flow rate of 8 L min−1 and a secondary sampling loop was used to pass a sub-sample through the NMR analyser at a slower (mL min−1) rate. It was shown that even with super-isokinetic sampling conditions, unrepresentative sampling could occur owing to inadequate mixing in the reactor. However, it was still possible to relate the 1H NMR signal obtained at a flow rate of 60 mL min−1 to the composition of the reactor contents.