Heavy Water Displacement in Molecular Sieve Drying Beds at Various Humidities

TL;DR

This study demonstrates that heavy water vapor ($D_2O$) can displace pre-adsorbed light water in a fixed-bed molecular sieve ($4A$), enabling passive capture of tritiated water in fusion-relevant systems. Using a mass-transfer-zone framework and sigmoid fits, the authors show that the displacement kinetics scale with heavy-water humidity, yielding linear relationships between the sigmoid parameters ($k$ and $x_0$) and $D_2O$ dew point or concentration. Experimental data collected with a $5.2$ cm/s superficial velocity and varying $D_2O$ humidity reveal a near one-to-one displacement, with higher humidity producing faster MTZ evolution; nonlinearity emerges at very low concentrations. The findings support the design of passive tritium-detritiation processes using saturated dryer beds, guiding operation at ppb–ppm tritium-equivalent concentrations for effective environmental mitigation.

Abstract

Tritium plays a critical role in nuclear fusion power plant designs and dryer beds are an essential tool for managing tritiated water vapor. A series of tests were performed to investigate the ability of a saturated dryer to preferentially adsorb heavy water vapor. The design of passive tritiated control systems is feasible by utilizing a dryer's ability to preferentially trap heavier isotopologues of water. This work investigates this displacement phenomenon and the effect of the heavy water humidity on the dryers performance. Significant displacement was observed when a humid stream of heavy water was diverted through a dryer pre-saturated with light water, as indicated by changes in the partial pressures of $D_2O$ and $H_2O$. After the capture of heavy water in the bed, the subsequent rise in $D_2O$ partial pressure depended on the humidity of heavy water in the gas stream. Higher humidity values lead to faster and steeper mass transfer profiles in the dryer, which could be empirically fit with sigmoid curves.

Figures (9)

• Figure 1: Example of packed dryer bed used in this study
• Figure 2: Idealized stoichiometric front
• Figure 3: Stoichiometric of Realistic Breakthrough Curve
• Figure 4: Photograph of the Experimental Setup
• Figure 5: The dryer loop comprises three Mass Flow Controllers (MFC), a heated nickel bed, two Dew Point Sensors (DP), a dryer, vacuum pumps, RGA, and Compressed Dry Air (CDA), hydrogen, deuterium, and nitrogen gas supplies