Mass Transfer of Oxygen in Sodium Cold Traps

Cold traps have been used extensively for controlling oxygen concentration in sodium systems. Cold traps utilize the positive temperature dependence of the solubility of sodium monoxide in sodium and the resultant precipitation of sodium monoxide at low sodium temperatures. Sodium monoxide precipitation is here assumed to be heterogeneous because of the strong effect of surface area on the precipitation rate shown in previous packless cold trap experiments. Growth of crystals from a super-saturated solution is said to proceed according to the equation where dm/dt is the rate of mass accumulation on the crystal, k is an over-all mass transfer coefficient including a diffusion step and a precipitation step, A is the crystal surface area, C is the bulk solute concentration and Ce is the equilibrium concentration at the temperature of the crystal. A packless cold trap of simple geometry and operating with a temperature gradient was used in this study. In the presence of the temperature gradient, the effective precipitation surface area was a function of the oxygen concentration in the sodium entering the cold trap. An equation describing the change in oxygen concentration with time was derived by performing a mass balance around the system using equation 1 and by accounting for the change in effective surface area with changing oxygen concentration. Eleven cold trapping runs were performed in which concentration was measured as a function of time. Tests were performed at various flow rates, initial oxygen concentrations and final oxygen concentrations. In general, the theoretical model fit the data quite well. Values of the mass transfer coefficients were calculated and were found to be a function of both temperature and flow rate. The dependence on sodium flow rate was proportional to Reynolds number to the 0.6 power. Variation of the mass transfer coefficient with flow rate implied that the precipitation process was diffusion controlled. An Arrhen