(186a) Parametric Effects of Antifoam Composition, Simulant Properties and Noble Metals on the Gas Holdup and Release of a Non-Newtonian Waste Slurry Simulant

Gas holdup tests were performed in a small-scale (60 L) mechanically-agitated mixing system at the Savannah River National Laboratory (SRNL) with a simulant of waste from the Hanford Tank 241-AZ-101. The simulant had 17 – 25.3 wt. % solids and was chemically similar to the real waste but without radioactivity. It also featured fine particle-stabilized foaming like the real waste. Air was sparged just below a Smith turbine agitator to break up large bubbles into small bubbles representing radiochemically generated hydrogen bubbles. Additions of 350 mg/L DOW Corning Q2-3183A antifoam agent (AFA) controlled the surface foaming; however, mixing tests indicated that this AFA increased bulk gas holdup in the waste simulant by a maximum factor of four. Also counter-intuitively, the holdup increased as the non-newtonian simulant shear strength decreased from 30 Pa to 3 Pa (apparent viscosity also decreased). Such results raised the potential of increased flammable gas retention in Hanford Waste Treatment and Immobilization Plant (WTP) vessels mixed by air sparging and pulse-jet mixers (PJMs) during a Design Basis Event (DBE). Additional testing showed that trace noble metals, which might catalyze reactions with the antifoam, resulted in only a slight reduction in gas holdup.

Investigation was initiated into an alternate antifoam that results in a lower gas holdup than with Q2-3183A. The two major components of Q2-3183A, Polypropylene glycol (PPG) and Polydimethyl siloxane (PDMS), were tested individually in a bench scale (4 L) mixing system. In water, PPG resulted in higher holdup than PDMS by a factor of 2-15, depending on superficial gas velocity. In AZ-101 simulant, PDMS and PPG had similar gas holdups, which in turn were lower than for Q2-3183A by about a factor of two. Two alternate antifoams, Dow Corning 1520-US and AF7500, which did not contain PPG, were also tested and found to have decreased gas holdup by a factor of 1.5-3 compared to Q2-3183A. Testing of Dow Corning 1520-US in the small scale mixing system confirmed the results of the bench scale testing of this antifoam. These alternate AFAs however required much higher concentrations in the range 1624-2000 mg/L to control foaming.

The possible significant effect of the surfactant property of antifoams on gas holdup was evaluated by measuring the surface tension of AZ-101 simulant at various Q2-3183A concentrations. The surface tension was found to rapidly decrease by 40% in the concentration range 0-100 mg/L and then slowly decrease thereafter. Thus a possible mechanism is that reduced surface tension at low AFA concentration increases gas holdup by stabilizing small bubbles. At high concentrations, the antifoaming property of the AFA is increased, enhancing bubble coalescence (higher rise velocity) and thus lower holdup.