Approach to Understanding Cohesive Slurry Settling, Mobilization, and Hydrogen Gas Retention in Pulsed Jet Mixed Vessels

摘要

The Hanford Waste Treatment and Immobilization Plant (WTP) is being designed and built to pretreat and vitrify a large portion of the waste in Hanfords 177 underground waste storage tanks. Numerous process vessels will hold waste at various stages in the WTP. Some of these vessels have mixing-system requirements to maintain conditions where the accumulation of hydrogen gas stays below acceptable limits, and the mixing within the vessels is sufficient to release hydrogen gas under normal conditions and during off-normal events. Some of the WTP process streams are slurries of solid particles suspended in Newtonian fluids that behave as non-Newtonian slurries, such as Bingham yield-stress fluids. When these slurries are contained in the process vessels, the particles can settle and become progressively more concentrated toward the bottom of the vessels, depending on the effectiveness of the mixing system. One limiting behavior is a settled layer beneath a particle-free liquid layer. The settled layer, or any region with sufficiently high solids concentration, will exhibit non-Newtonian rheology where it is possible for the settled slurry to behave as a soft solid with a yield stress. In this report, these slurries are described as settling cohesive slurries. The slurry rheology will also depend on the particle characteristics and their interactions in addition to the solids concentration. If a liquid region of the vessel exists with sufficiently few particles, this region will be Newtonian. As has been observed historically within the Hanford waste tanks, the non-Newtonian region with sufficient settled solids will retain hydrogen gas bubbles unless the mixing system can mobilize the settled solids and release the hydrogen gas. An External Flowsheet Review Team (EFRT) identified potential inadequate mixing of these vessels as a technical issue, and this issue is referred to as M3Inadequate Design of Mixing SystemsPulse Jet Mixers (PJMs). The purpose of this report is to define an approach to understanding cohesive slurry settling, mobilization, and hydrogen gas retention in pulsed jet mixed vessels. An overall approach to the hydrogen gas issue is presented, which illustrates the relationship between waste properties and PJM performance. This approach underscores the importance of quantifying how waste becomes inhomogeneous when mixing stops, the rate that solids settle, and the physical and rheological properties of the stratified waste. Previous work for the Hanford tank farms and the WTP project is being evaluated to determine where sufficient information exists and where needed information is uncertain or absent. A model of PJM performance for mobilizing settling layers is presented, and the performance limits are compared with the estimated strength of settling and stratified layers. The use of the settling and PJM performance models is demonstrated to evaluate how well PJMs mobilize slurries during and after off-normal events with settled or stratified layers and how the settling behavior of a slurry affects the capability of mixing systems to remobilize waste.