Caffeine degradation in a plasma-liquid reactor with the lateral liquid flow: Elucidating the effects of mass transport on contaminant removal

Abstract

This work identifies the phenomena relevant to designing and operating highly efficient plasma reactors for water treatment applications using a simple plasma-liquid system featuring laminar, unidirectional fluid flow in contact with the plasma phase. The reactor performance was established by treating caffeine under various combinations of plasma-liquid contact area and liquid flow velocity.The plasma-liquid contact area was the most significant contributor to caffeine degradation. A complex, nonlinear effect of fluid flow on caffeine degradation was observed for a constant plasma area. The experimental data suggested two caffeine transport modes disambiguated using a transient diffusion–reaction model with interfacial adsorption. Two distinct rate-limiting contaminant transport regimes were identified depending on the solution-plasma contact time: diffusion on the timescale of the plasma discharge period (milliseconds) and diffusion on a liquid flow timescale (seconds). A simple criterion for estimating the optimal contact time for pulsed plasma reactor operation was developed based on the modeling insights.Model-based lower and upper bounds for the maximum contaminant degradation rates were established and compared to the observed performance of plasma reactors reported in the literature. Most reactors with flat plasma-liquid interfaces performed within the predicted bounds, suggesting contaminant mass transport is the rate-limiting process in the overall degradation. Furthermore, the area-normalized degradation rate can be used to determine the rate-limiting process for a given reactor configuration and compare performance between different plasma reactors. These findings have significant implications for the design and scale up of plasma reactors for water treatment applications.