Adsorptive Separation of Water-Acetonitrile Mixtures

Tuesday, November 9, 2010: 9:10 AM
250 A Room (Salt Palace Convention Center)
Ravi Kumar1, Tom Van Assche2, Gert Desmet2 and Gino V. Baron2, (1)Praxair Inc., Tonawanda, NY, (2)Department of Chemical Engineering, Vrije Universiteit Brussel, Brussels, Belgium

Acetonitrile is a commonly used chemical with applications in pharmaceutical, analytical and synthetic domains. Most applications actually involve a mixture of acetonitrile and water, including Reversed Phase-HPLC. Acetonitrile, a byproduct of acrylonitrile production, has seen a large rise in cost due to decreasing supply since 2008. The lack of a direct production method for acetonitrile resulted in falling supply and a dramatic increase in price. The problem grew to such proportions that it became known as "The Great Acetonitrile Shortage" [1]. It is therefore of great importance to recuperate acetonitrile after its use, from both economic as ecologic point of view. The separation of both molecules is a challenge since both are highly polar and have small kinetic diameters. The conventional method to separate water and acetonitrile requires a pressure swing or azeotropic distillation due to the presence of an azeotrope. These techniques are quite expensive in both capital costs as well as operating costs. The use of pervaporation in this separation problem is quite promising, but suitable membranes are difficult to synthesize in a defect-free manner and the capital cost are quite high.

The use of an adsorption column to preferentially adsorb water out of these water-acetonitrile mixtures could provide a straight-forward and more cost-effective method to produce high grade acetonitrile. Such a device is also suited to be used with smaller waste streams, as encountered in HPLC, where distillation units are simply too large. Such a separation device has to be used in bulk separation mode as the amount of water is typically 50 wt.%. This is a significant difference with more conventional adsorption columns where the concentration of the adsorbents is often no more than several wt.%.

The present work deals with adsorption and separation of water and acetonitrile on solid adsorbents. In a first screening round, 29 sorbents of different classes including zeolites, alumina, Metal-Organic Frameworks and activated carbons were evaluated for their selectivity in the adsorption of water/ acetonitrile mixtures, by performing batch adsorption experiments. Composition of the liquid mixtures was determined using GC-MS and Karl-Fisher titration. These results from these experiments were implemented in numerical evaluation tool to rank the performance of the sorbents to separate water and acetonitrile. This screening allowed the selection of a distinct number of sorbents showing outstanding preferential adsorption of water. Only 3 of the 29 sorbents showed a preference towards acetonitrile, while most sorbents have a preference towards water ranging from very poor to excellent. Surprisingly even aluminum-deficient hydrophobic zeolites showed a slight preference towards water. A strong correlation between the Si/Al ratio and water adsorption capacity for a low water content mixture was found for all zeolites with different structures. The adsorption enthalpy (kJ/mol) of acetonitrile is in fact quite strong [2], but due to the large molar volume of the acetonitrile compared to water, it is energetically more favorable to adsorb water for most sorbents.

The selected sorbents were further investigated by assessing their competitive water-acetonitrile isotherms at 296K. Two sorbents were identified showing excellent separation behavior. The first sorbent shows a quasi-rectangular isotherm and a very large Langmuir constant. Average adsorption enthalpies were in excess of 100 kJ/mol. The second sorbent shows a lower Langmuir constant, but a slightly higher water saturation capacity of about 0.25 g/g. These two sorbents were studied in more detail. The kinetic uptake curves of water out of water-acetonitrile mixtures were determined for both powders as pellets at room temperature. Diffusion constants were obtained via fitting with a numerically solved diffusion model. Additional isotherms at 307K, 313K and 333K allowed to evaluate the temperature dependency of capacity and selectivity. Separation was tested in breakthrough mode using 30 cm columns packed with pellets and powder. Experimental breakthrough curves, obtained at different flow rate, feed composition and column temperature, were compared to simulated breakthrough curves. The model accounts for velocity variation due to the large amount adsorbed. Adsorption parameters obtained in the independent isotherm experiments were used in the model. Breakthrough of water was accurately predicted by the model, as shown in Figure 1

Figure  SEQ Figure \* ARABIC 1: Experimental breakthrough curve (dots) of water in 30wt.%water-70wt.%acetonitrile on a 30 cm column (diameter ½ inch) packed with 1 mm sorbent pellets at a flow rate of 0.6 ml/min compared to simulation (dotted line).

In conclusion, it is shown in the present work that adsorption processes using porous solids are promising for the removal of acetonitrile from waste stream in small or even larger scale processes.

References

[1] Lowe D., (2009), The Great Acetonitrile Shortage, Available at http://pipeline.corante.com, Retrieved August 26, 2009

[2] Janchen J., van Wolput J.H.M.C., van Well W.J.M, Stach H., (2001), Adsorption of water, methanol and acetonitrile in ZK-5 investigated by temperature desorption, microcalorimetry an FTIR, Thermochimica Acta, 379, pp. 213-225


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