EVALUATION OF FOAMING/ANTIFOAMING IN WTP TANKS EQUIPPED WITH PULSE JET MIXERS AND AIR SPARGERS

Abstract

The foaminess of Hanford waste samples [i.e., pretreated AN-104 Hanford sample--post ion exchange and AN-104 blended with Submerged Bed Scrubber (SBS) recycle, then evaporated to 5 M Na] and waste simulants AP-101, AN-104, and AZ-101 was determined using an air-sparged foam column (3.1 cm inside diameter x 60 cm height). The Hanford waste samples AN-104 post ion exchange solution and AN-104/SBS recycle solution, and the simulants AP-101 and AN-104, were mixed with different weight percents of AY-102/ C-106 sludge to determine the effect of insoluble particles. The AN-104 Hanford samples have shown a tendency to foam when subjected to air sparging. The percent foaminess, defined as [100 (volume fraction of gas entrained liquid and foam)--(volume fraction of gas entrained liquid)], increased to a well-defined maximum with the increase of the amount of solid particles added to solution. For example, at a sparge rate of 4.4 ft3/min/ft2, the foaminess of an air-sparged AN-104/SBS recycle sample with no added insoluble solid particles was 80 percent vs. 95 percent for 12 wt. percent insoluble solids concentration in the sample. A more dramatic foaminess was observed for an AN-104 Hanford sample that was pretreated in ion exchange columns. The foaminess for this sample (no insoluble solids) at a nominal WTP flux of 2.2 ft3/min/ft2 was 358 percent as compared to 1048 percent when the sample was mixed with AY-102/C-106 sludge to a 6 wt. percent insoluble solids. For AN-104 simulant with 6 wt. percent insoluble solids sparged at air flux of 2.2 ft3/min/ft2, the foaminess was less than 2 percent. The measured surface tension of this simulant was 68.63 dyne/cm as compared to that of the AN-104 Hanford sample (post ion exchange), which was 71.59 dyne/cm. A 20-day air sparging experiment was performed to examine the effect of CO2 on the pH or chemical composition of the slurry and to determine how rheology may be impacted by air sparging. The experiment showed that air sparging has no significant effect on pH or chemical composition of the slurry. The rheology is also not impacted by air sparging. The primary effect of air sparging is the removal of water by the dry air passing through the column and exiting in a saturated condition. This effect can be mitigated by adding water back to the column or vessel during sparging. Therefore, an initial charge of 350 mg/L antifoam (Dow Q2-3183A) followed by small batch additions of 70 mg/LQ2-3183A every 24 hours is recommended for use in WTP tanks equipped with air spargers and pulse jets based upon the testing done in this study. However, this recommendation is based upon a limited set of antifoam degradation data developed for the WTP evaporator R and T program. Therefore, additional investigation into refining the kinetic behavior of Q2 antifoam under radiation dose is recommended.