Separation stability in binary mixtures with negative Soret numbers

In the present work, by using a paralelepipedic thermogravitational microcolumn, the temperature gradient influence on the stability of the flow was or has been examined, emphasizing mixtures with negative Soret coefficients. Experiments and numerical analysis were conducted for DCMIX2 Toulene-Methanol binary subsystem. This binary subsystem has a broad range of negative Soret values for low concentrations of Methanol which was analysed. Two different concentrations have been studied in order to confirm existence of temporal stability windows of those mixtures. Experiments were compared with numerical simulations conducted in open source software OpenFOAM, for both cases.


Introduction
Thermal diffusion phenomenon plays an important role in many technological and biological separation processes [1,2,3]. Thermodiffusion experiments are constructed in order to obtain Soret coefficients, which is linked with therrmodiffusion coefficients obtained by thermogravitational technique [1] and molecular diffusion coefficients obtained by pure diffusion experiments like those in Sliding Symmetric tubes, Open Ended Capillary tube or Taylor dispersion technique [4,5,6]. In our case, liquid binary mixture is placed between two parallel closed walls maintained on different temperature levels. Due to this gradient, the Soret effect appears and horizontal separation is provoked. If the Soret coefficient of the mixture is positive for denser component, denser component goes to cold wall and less dense component migrates toward hot wall. Due to natural convection, less dense component rises to the upper part of column, while denser goes to the bottom. For negative Soret coefficients, behavior will be opposite, hence creating unstable stratification of density inside column. Sketch of thermogravitational column can be found on figure 1.

Experimental procedure
This section narrates the experimental procedure and techniques followed for the determination of thermophysical properties of the binary mixtures. It is important to mention that thermogravitational microcolumn has dimension of d=0.51mm, b=3mm and h=30mm, making volume of column sufficiently small to operate with expensive or rare mixture such are biological fluids. Optical digital interferometry is used in determination of transport properties inside fluid, in our case, thermodiffusion coefficient. Further information about complete experimental procedure can be found in [7].

Thermophysical properties
Work is conducted with two different concentrations of Toulene-Methanol binary mixture, for two different Soret negative numbers. Table 1  Where c is concentration,  is kinematic viscosity, βT is thermal expansion coefficient, βc is mass expansion coefficient, D is molecular diffusion coefficient, DT is themodiffusion coefficient and ST is Soret coefficient. As it is possible to see from table 1, mixture 1 has higher negative Soret number.

Results
Stability windows for both mixtures is obtained numerically. This is possible to observe on figure 2. Its clearly shown that for mixture 1, that length of stability window was different for different temperature gradient applied, where higher temperature gradient resulted in longer stability window. For temperature different of 5K, theorical value was not reached at all. Experimentally, mixture 1 was observed for ∆ = 8 ∆ = 20 . Theoretical values were reached, but stability windows were not observed. This is very likely due to high negative Soret number. Behavior is shown on figure 3.
For mixture 2 and ∆T = 15K, both theoretical value and stability window were observed experimentally. However, length of window inside in experiment was significantly shorter then in simulation. On figure 4, it is possible to compare transient separation in simulation and experiment for this case.

Conclusions
This work brought new information about separation stability in binary mixture with negative Soret numbers. Stability windows are experimentally observed for first time in thermogravitational microcolumn, showing good coincidence with simulations.
Although, both theoretical value of separation and stability windows are reached in experiment, they are significantly shorter then in numerical simulations. This is probably due to imperfections in experimental setup, which are slightly different temperature gradient, orientation of thermogravitational column and slight differences in concentrations.
Future work will include ternary mixtures, where separation process will be much more complex.