Modeling flow and residence time distribution in an industrial-scale reactor with a plunging jet inlet and optional agitation

The quantified residence time distribution (RTD) provides a numerical characterization of mixing in a reactor, thus allowing the process engineer to better understand mixing performance of the reactor. Many reactors are mixing-limited and/or mass-transfer limited and micro-mixing can be the critical element in contrast to RTD which addresses the macro-mixing. This paper discusses computational and experimental studies to investigate flow patterns in an industrial-scale (110 m(3)) continuous reactor employing a three-stage agitation system with a plunging jet inflow. In operation, the agitation and liquid level are adjusted based on throughput and the specific product being produced. The objective of this study was to quantify RTD under different operating conditions of liquid level, throughput and agitation. Flow in the reactor was modeled with computational fluid dynamics (CFD). A unique coupling of different modeling approaches was developed. The volume of fluid (VOF) method was used to model the plunging jet with transient simulations, while the multiple reference frames (MRF) model was used to model the stirred tank with steady-state simulations. To account for the effect of gas entrainment due to the plunging jet impingement, the two modeling approaches were interfaced by using the plunging jet modeling result as an input boundary condition for the reactor flow simulation. Without mechanical agitation in the reactor, the plunging jet was a dominant feature of the hydraulics. An unexpected anomaly in CFD-predicted flow patterns was tested against and found to be in good agreement with laboratory-scale flow experiments, including local instability when the agitator was not adequately submerged. The reactor RTD was obtained from stochastic particle tracking which tracks residence times of massless tracers through the reactor. The random walk model was used for dispersion due to turbulent eddies. A large number of tracers were used to account for the random effects of turbulence and to ensure statistically significant results. The predicted mean residence times were consistent with the bulk reactor space times. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.