Results of Large-Scale Testing on Effects of Anti-Foam Agent on Gas Retention and Release

Abstract

Testing SummaryThe U.S. Department of Energy (DOE) Office of River Protection's Waste Treatment and Immobilization Plant (WTP) will process and treat radioactive waste stored in tanks at the Hanford Site. The waste treatment process in the pretreatment facility will mix both Newtonian and non-Newtonian slurries in large process tanks. Process vessels mixing non-Newtonian slurries will use pulse jet mixers (PJMs), air sparging, and recirculation pumps. An anti-foam agent (AFA) will be added to the process streams to prevent surface foaming but may also increase gas holdup and retention within the slurry.Some gas retention tests that were carried out in nonprototypic systems-bubble columns and impeller-mixed vessels-indicated trends that posed process and flammable-gas concerns . Both types of nonprototypic results indicated that the presence of AFA in a chemical simulant of Hanford Tank 241-AZ-101 high-level waste (HLW) might increase gas retention by a factor of 10 or more over that in clay without AFA, the simulant on which WTP design studies were based (see Section 1.2). In addition, the increase over clay holdup was greater at lower simulant yield stress, implying that the 30-Pa simulant results, which had been used for WTP design, might not bound gas retention.The work described in this report addresses gas retention and release in simulants with AFA through prototypic testing and analytical studies. This test program was established to determine whether the AFA has as strong an effect in a large-scale prototypic mixing system as it did in the small-scale nonprototypic tests. Gas holdup and release tests were conducted in a 1/4-scale replica of the lag storage vessel operated in the Pacific Northwest National Laboratory (PNNL) Applied Process Engineering Laboratory using a kaolin/bentonite clay and an AZ-101 chemical simulant with non-Newtonian rheological properties representative of actual waste slurries. Additional tests were performed in a smallscale mixing vessel in the PNNL Physical Sciences Building using liquids and slurries representing major components of typical WTP waste streams to address the fact that simulants delivered to the WTP will come from other tanks in addition to 241-AZ-101. Analytical studies were directed at discovering how the effect of AFA might depend on gas composition, and a model was developed for predicting the effect of AFA on gas retention and release in the WTP, including the effects of mass transfer to the sparge air.The prototypic gas retention and release tests performed in this test program indicate that gas holdup with AZ-101 simulant with AFA is higher than it is in clay, but not to the extent that initially raised WTP design concerns. In addition, the trend to a higher increase in holdup with decreasing simulant yield stress was not seen in the prototypic system. The work at PNNL was part of a larger program that included tests conducted at Savannah River National Laboratory (SRNL) that is being reported separately. SRNL conducted gas holdup tests in a small-scale m...

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

“…Each particle species, with some level of dissolved organic species adsorbed on their surfaces, will have different interactions at bubble/liquid interfaces and different interactions at the solid/liquid interface with antifoaming chemicals. Previous studies elucidated key mechanisms, and the study by Stewart et al (2007) (see Section 7) specifically evaluated what simulant would comprise a bounding simulant.…”

“…While there have been extensive previous studies for resolving the performance of antifoaming agents in WTP vessels where antifoam agents will be used (see, for example, Josephs andCalloway 2001 andStewart et al 2007), there has been less attention given to some of the as-received wastes. There have only been measurements on actual waste samples from AN-102, AN-104, and AW-101 (Crowder 2001(Crowder , 2003(Crowder , 2004, and these samples were pretreated in various ways before foaming tests.…”

“…The key scaling relationship from these studies is a gasbubble holdup model that shows that the steady-state holdup is the ratio of the gas-generation superficial velocity to the effective bubble-rise velocity. Following the nomenclature of Stewart et al (2007), this relationship is…”

“…Hydrogen is known to be only a portion of the gas that is generated within the waste, and for predictions using holdup correlations, we follow Stewart et al (2007) and assume that hydrogen is 25 vol% of the total generated gas. In a study of the retained gas composition in Hanford tank waste, Mahoney et al (1999) concluded that the gas retained in the convective layer of the waste was less than or equal to 25 mol%, which is equivalent to 25 vol%, hydrogen.…”

“…In the most recent study by Stewart et al (2007), gas-holdup tests were conducted in scaled lag-storage vessels filled with either AZ-101 chemical simulant with AFA or with a mixture of kaolinite and bentonite clay. In these tests, the gas-generation superficial velocity was varied and its scaling behavior quantified for scaled lag-storage vessels that were well mixed with both air spargers and PJMs.…”