Simultaneous Laser Doppler Velocimetry and stand-off Raman spectroscopy as a novel tool to assess flow characteristics of process streams

Abstract

Flow characteristics of process streams are important in industrial chemical plants. Online measurement of physical and chemical properties of such streams like velocity, turbulence, chemical composition, and concentration, plays a key role in adjustment and optimization of industrial processes. In transient processes with steep changes in the concentration and velocity (e.g. mixing of fluid with different viscosities or multiphase flows) it is important to monitor process parameters at the same time and position to be able to iterpret them correctly. In this work, a novel method for simultaneous measurement of velocity, composition, and concentration relying on two well-known methods, Laser Doppler Velocimetry (LDV) and Raman spectroscopy is presented and tested. Both techniques were combined using the same laser as light source, thus making sure sampling from exactly the same position at the same time is achieved. Experiments on mixing of water and ethanol streams in a custom-built T-junction geometry were performed using LDV to obtain velocity and Raman spectroscopy to measure concentration using the suggested method. Results are compared against Computational Fluid Dynamics (CFD) simulations using models for mixing of miscible, multi-species liquids at different flow regimes. CFD predicts turbulent diffusion to be the dominant phenomena in mixing in the T-junction since the turbulent diffusion coefficient (∼0.02 m2/s) is much higher than the molecular diffusion coefficient (∼10−8 m2/s). A mean deviation of 8% between model and experiment for velocity and 10% for concentration evaluation was observed, which suggests the feasibility of this technique for simultaneous monitoring of process streams.