Scoping Study of Airlift Circulation Technologies for Supplemental Mixing in Pulse Jet Mixed Vessels

Abstract

At the request of the U.S. Department of Energy Office of River Protection, Pacific Northwest National Laboratory (PNNL) conducted a scoping study to investigate supplemental technologies for supplying vertical fluid motion and enhanced mixing in Waste Treatment and Immobilization Plant (WTP) vessels designed for high solids processing. The study assumed that the pulse jet mixers adequately mix and shear the bottom portion of a vessel. Given that, the primary function of a supplemental technology should be to provide mixing and shearing in the upper region of a vessel. The objective of the study was to recommend a mixing technology and configuration that could be implemented in the 8-ft test vessel located at Mid-Columbia Engineering (MCE). Several mixing technologies, primarily airlift circulator (ALC) systems, were evaluated in the study, first by reviewing the available literature and then performing tests with a simple Newtonian simulant (water and glass beads). The experimental study was performed in a 90-in. diameter tank with a maximum liquid operating level of ~108 in. The initial testing evaluated the ability of ALC configurations to lift, transport, and distribute solid particles to near-surface locations in the test vessel. This testing established that ALCs generated significant liquid flows (several hundred gallons per minute) at air flow rates of 5 to 20 standard cubic feet per minute (scfm), and solid particles were readily transported up the riser tube at air flow rates as low as ~5 scfm. The principal outcome of the Newtonian testing was that candidate ALC systems were identified and recommended for testing in a non-Newtonian fluid. The candidate ALC system had the following features: a nominally 10-in. riser tube diameter, 5-ft riser tube length, positioned 5 in. off the tank floor, and an air distributor centrally located at the bottom of the riser tube (tube inlet). An alternative technology, a Geyser Hybrid Pump (GHP), was also identified as a possible candidate. Both of the recommended systems were confirmed to be capable of lifting and dispersing very large solid particles up to approximately 6 mm in size.The non-Newtonian testing was conducted to determine if the recommended ALC configuration from the Newtonian testing was also suitable for mobilizing a Bingham plastic fluid (clay slurry). It was found that mobilizing a yield-stress fluid (initially, the simulant had Bingham parameters of ~22-Pa yield stress and ~34-cP consistency) in the upper region of the vessel required more mixing energy than transporting solid particles in a Newtonian fluid; the 10-in. diameter ALC did not mobilize fluid out to the tank wall in the upper region of the vessel. The GHP was also tested and found to be an improvement over the 10-in. ALC. Some alternatives to the initially recommended ALC were briefly investigated, including testing with a larger diameter ALC and devices that could supply a periodic burst of pressurized air. ALC devices that incorporated pulses of air were demonstrated in proof-of-princip...