NMR microimaging of fluid flow in model string-type reactors

Abstract

Magnetic resonance microimaging (MRM) was employed to obtain quantitative velocity maps of water flowing in the channels possessing unconventional cross-section shapes formed by a bundle of parallel fibers within a tubular string-type reactor. The maps obtained demonstrate the presence of large amounts of an almost stagnant liquid in the stretched corners of the cross-sections representing distorted triangles or squares. This fact together with the irregularity of the filaments packing in the model string-type reactor was demonstrated to lead to a broad residence time distributions (RTDs) for liquid flow. Next, the pulsed field gradient NMR (PFG NMR) technique was employed to compare transport of water with that of butane gas in the same model string-type reactor. The experimentally measured average propagators (travel distance probability density functions) have demonstrated that Taylor dispersion can lead to much better RTDs for gas as compared to liquid in channels with sub-millimeter equivalent diameters. The PFG NMR data were compared with the RTD obtained using the conventional tracer time-of-flight transient response method. It is concluded that due to the differences in the quantities actually measured by the two techniques, and the significant differences in the measurement length scales (microns to 1-2 cm for NMR/MRM, tens of centimeters for transient response methods), there is no reliable way of directly comparing these results. The information obtained by NMR/MRM and more conventional techniques such as time-of-flight should be considered as complementary. In particular, NMR/MRM can reveal the reasons for the observed overall reactor performance by providing access to the transport processes on short length scales inside the reactor and by revealing structure-transport interrelations. ᭧

Residence time distribution on flow characterisation of multichannel systems: Modelling and experimentation

“…RTD has also been used to predict the chemical reaction conversions by combining chemical kinetics and RTD analysis [32] or to determine optimal design parameters of tubular reactors [33]. Besides conventional RTD models for CSTRs (Continuous Stirred Tank Reactor) and PFRs (Plug Flow Reactor), some recent applications in multichannel systems in the aim of flow and mixing characterisation attracts some attention too [34][35][36][37][38]. In particular, recent works by Wibel et al [39] investigated the impact of inlet/outlet volumes and uneven flow distribution on the RTD characters of micro devices consisting of an array of parallel microchannel, by both CFD simulation and experimental characterization.…”