Non-steady-state density effects in liquid thermal diffusion columns

The Furry, Jones and Onsager column theory is reformulated to determine the effect of the buoyancy-driven convection induced by the density composition dependence on the separation of a binary liquid mixture (the 'forgotten effect'). The transport equation is obtained in terms of correction factors depending on the separation parameter and is applied to the standard column coefficients. From this equation, the column transient operation is carefully analysed. It is found that, contrary to what happens at the steady state, the forgotten effect greatly affects the approach to equilibrium. The excellent agreement obtained between the theory developed and the experimental data opens up the possibility of determining accurate values of ordinary and thermal diffusion coefficients of liquid mixtures from column separation measurements.