Quantification of two-phase flow in liquid-solid risers

This study focuses on the fluid dynamics in the riser of liquid-solid circulating fluidized bed reactors. Liquid-solid circulating fluidized beds are used for reactions that utilize a liquid phase reactant and a highly active solid catalyst, and yield products in the liquid phase. A common feature of these processes is that the solid catalyst deactivates quickly, and needs to be continuously and rapidly recirculated between the principal reactor vessel (the riser) and the catalyst regenerator. Liquid and solids flow cocurrently upwards in the riser, at high fluxes and under highly turbulent conditions. The complex fluid dynamics thus generated is key in determining the performance of the riser as a chemical reactor.; In this work, liquid-solid fluid dynamics has been investigated in a 6 in. (15 cm) “cold-flow” circulating fluidized bed riser using non-invasive flow monitoring methods. A protocol has been developed to measure the solids circulation rate non-invasively, so that the exact operating conditions (solids fluxes) for a closed loop circulation system could be determined. The experimental study examines nine operating conditions, i.e. three liquid superficial velocities and three solids flow rates. Gamma ray Computed Tomography (CT) has been used to measure the time-averaged cross-sectional solids volume fraction distributions at several elevations. The time-averaged mean and “fluctuating” solids velocity fields have been quantified using the Computer Automated Radioactive Particle Tracking (CARPT) technique. Analysis of the time-series of tracer particle position and velocity data yielded solids dispersion coefficients, overall residence time and circulation time distributions, trajectory length and return length distributions, macromixing indices, Hurst exponents, Pox diagrams and other parameters that quantify the transient nature of the solids flow in the riser. Overall liquid backmixing has been assessed by the examination of the liquid residence time distribution obtained via conductivity measurements.; A two-fluid model coupled with kinetic theory of granular solids has been used to predict the overall flow pattern of liquid and solids in the riser. The predictions from a two-dimensional axisymmetric simulation for solids radial volume fraction and axial velocity distribution in the fully developed section of the riser have been found to compare favorably with the experimental data. The overall backmixing in the individual phases has been simulated. A three-dimensional simulation, using the same fundamental model, was performed for assessment of the transient flow behavior. Three-dimensional simulations lead to an even closer agreement of model predictions for the time-averaged quantities with experimental data, indicating the importance of the 3D characterization of the flow.; Finally, as part of the global objective of improving the CARPT technique, a novel method of estimating the resolution and sensitivity of the CARPT technique from theoretical considerations has been proposed. The methodology can be used for an a priori assessment of the optimal detector arrangement in any general system of interest in which CARPT needs to be implemented.*; *Originally published in DAI Vol. 62, No. 6. Republished here with corrected degree date.