(242d) A Convective-Diffusive Formulation for Crossflow Filtration of Nuclear Defense Wastes

Crossflow filtration (CFF) is a solid-liquid separation technique that will serve as a key enabling technology in the disposition of nuclear defense wastes stored at the Hanford Site in the U. S. state of Washington. Studies evaluating technical sufficiency of CFF for separation of nuclear defense wastes observe a persistent loss of filter performance that continues throughout >100-hour batch processing times and that is not recoverable through filter back-flushing. Loss of performance, when coupled with the time-intensive and operationally complex chemical cleaning operations needed to restore filter performance, increase the time and cost associated with waste disposition (and could be rate limiting). Attempts to model fouling of CFF elements by Hanford waste solids using crossflow correlations available in the literature and semi-empirical have met with limited success. Literature models predict a rapid achievement of filtration steady-state not observed in Hanford prototypic CFF; and semi-empirical approaches cannot be extended to different operating conditions and provide no insight into the underlying mechanisms effecting performance loss. These models focus on convection driven fouling of >10 µm particles. However, Hanford wastes exhibit gel polarization when filtering at high-solids loadings, suggesting that diffusion or a diffusion-like mechanism controls foulant mass transport to and from the filter. The current study develops a convective-diffusive framework for evaluating filter fouling dynamics of Hanford waste systems. Application of this convective-diffusive framework to filter fouling dynamics measured for a dilute (~5-wt% solids) high-level waste slurry suggests that relaxation from a gel-polarized state dominates the earlier filter performance loss while the longer-time losses appear to be controlled by a diffusive-like mass transport of solids within the filter membrane.