Nuclear magnetic resonance in chemical engineering: Principles and applications

Abstract

In recent years chemical engineers have shown an increasing interest in non-invasive measurement techniques; Nuclear Magnetic Resonance (NMR) is perhaps the ultimate technique of this kind. Over the past 10 years notable developments have been made in both spectrometer hadrdware our ability to understand and manipulate nuclear spin interactions, and it is now possible to address research areas in catalysis, materials science, mass transfer and flow visualisation which are of real interest to chemical engineers. This review is divided into two sections. Part I identifies three broad categories of magnetic resonance measurements: spectroscopy, diffusion measurement and imaging, and outlines the basic principles underlying these experiments. A summary of the various nuclear spin interactions and the chemical information they yield is given. In progressing to the introduction of diffusion measurements, the application of magnetic gradients is discussed and the basic Pulsed Gradient Spin Echo (PGSE) and related measurement techniques are presented. The principles of NMR imaging are then described in the context of the two popular experimental schemes: projection—reconstruction and spin-warp imaging. The principles and application of parameter-selective imaging experiments are outlined and limitations on attainable resolution are noted. Extension of NMR imaging to the study of flow phenomena is also discussed. Part II of the review reports a number of examples of NMR methods applied to problems of direct relevance to chemical engineers. This literature survey starts with an overview of applications of NMR spectroscopy in the fields of catalysis, adsorption, measurement of phase equilibria and the consideration of NMR as a quality control technique. The use of PGSE methods to study diffusion phenomena is discussed, with particular emphasis being placed on how theoretical models in combination with NMR experiments are being used to gain insight into transport processes occurring within porous media. Recent developments in NMR imaging and their application to the study of ceramics processing, polymers, porous media, catalysis, food processing, filtration processes, and transport within reactors and packed columns are also presented. Finally, the state-of-the-art in NMR flow imaging studies is discussed and the ability of NMR to study two-phase flow phenomena is highlighted.