(452d) CFD Simulation of a Pulse Jet Mixed Vessel

Pulse Jet Mixed (PJM) vessels have been used to process nuclear waste at Sellafield and are part of the flowsheet for the pretreatment and vitrification of Hanford high level tank waste. PJM vessels contain a series of downward directed pulse tube mixers that periodically discharge fluid into the vessel and are then refilled with vessel fluid, with the cycle being repeated until the process mixing/blending objective is met. During the drive phase, positive air pressure drives the fluid from the narrowed tips of the pulse tubes. These high intensity jets are discharged onto the vessel bottom, travel radially inward and merge to create a central up wash plume that mixes the vessel contents as it dissipates. During suction, negative air pressure pulls the vessel fluid back into the tubes. The air to operate the pulse tubes is supplied from outside leaving the black cell free of moving parts and maintenance. An off normal incident known as “overblow” can occur if a drive to suction phase timing mismatch causes drive air to be discharged into the vessel fluid. Large bubbles travel upward through the vessel into the headspace causing intense fluid agitation. Many of the Hanford vessels will process slurries with fast settling solids while others are relatively dilute. In this study we modeled the turbulent velocity field in a pure fluid during normal PJM operation and in the presence of “overblow” to gain preliminary insight into vessel operations, modeling strategies and design of experiments for model validation.

A Computational Fluid Dynamics (CFD) model was developed for a 1 m diameter pilot scale PJM vessel containing 4 pulse tubes. The vessel fluids were water and a more viscous 30 cP Newtonian liquid of the same density, chosen to bound slurry viscosity. To account for fluid-air interactions, fully transient, 3-D Volume of Fluid (VOF) simulations of the RANS equations were carried out with ANSYS Fluent using the realizable k-e turbulence model with enhanced wall treatment. For normal operation, the pressure profile in the air line feeding the pulse tubes was set to mimic typical cyclic operations. During overblow, pulse tube operation ceased once gas was discharged into the vessel but the evolution of the velocity field was tracked until all bubbles had escaped into the vessel head space. A mesh independence study was conducted and the effect of air compressibility was studied.

The results for normal operation show extremely complex flow patterns throughout the vessel. During drive, mixing layer vortices form between the central up wash plume and the pulse tube bodies and travel upward to the surface. There is weak downward flow close to the vessel walls. During suction, fluid is pulled downward from all vessel locations. There are numerous regions of flow reversal. Increased viscosity impacts the upper regions of the central up wash plume. The mean velocity, deformation and dissipation fields will be discussed in detail. Several overblow scenarios will be presented and contrasted. The effect of air compressibility and bubble expansion on fluid agitation will be shown. Application of the results to Hanford waste treatment plant process operations will be discussed.