(589a) A Graphical Method to Obtain Mixture Adsorption Equilibrium Data from Intermediate Plateaus of Breakthrough Curves: Alcohols on All Silica BEA Zeolite

Knowledge on mixture adsorption equilibria can be considered essential for an adsorptive separation application. Even though a variety of experimental measuring techniques is available, such data remains hard to come by [1]. While pure gas or vapor adsorption measurement techniques are well-established and often highly automated, measuring multicomponent equilibria remains more cumbersome. Breakthrough methods are amongst the most used techniques to obtain such mixture data [1].

Breakthrough curves are collected by applying a step in concentrations at the adsorbent column inlet and measuring the concentration (and velocity) changes at the outlet. After the column has equilibrated with the feed, mixture adsorption can be calculated: Typically, the adsorbed amounts in equilibrium with the feed are found by integrating the curves to complete breakthrough. The difference in feed mass and exiting mass is then allocated to adsorption phenomena. Such methods yield just one equilibrium point per breakthrough test, which is both time-consuming as labour-intensive. Remarkably, many breakthrough curves present themselves as a sequence of constant concentration (plateau) regions, separated by mass transfer regions. Beyond the obvious initial and final plateaus, intermediate plateaus then emerge. A typical example is the so-called ‘roll-up’ phenomenon, a temporary overshoot beyond the feed concentration, where the less strongly adsorbed component can be displaced by other components.

By replotting experimental time-dependent data as capacity curves (average column capacity), a simple graphical tool emerges to directly obtain mixture equilibrium information at the conditions of the intermediate plateau(s). No model fitting or pure component isotherms are required. This allows to obtain two equilibria points from one (binary) diluted breakthrough test, and even more when more complex (ternary, etc.) mixtures are used, provided that the intermediate plateaus are well-developed. The intermediate plateau gas composition is not controlled, with one species typically absent in the intermediate plateau region. This can be avoided by selecting a preloaded (non-regenerated) starting state of the column.

The technique is first demonstrated numerically (finite volume method, with various models including IAST) and discussed from theoretical point of view[2]. An example is given where 6 successive binary breakthrough tests yield 1 unary and 11 binary equilibrium points. Furthermore, the technique is demonstrated experimentally for the adsorption of various alcohols on an all-silica BEA zeolite. Pure component alcohol isotherms are S-shaped due to the hydrophobic behaviour of the zeolite[3], and this give rise composite (shock and dispersive) breakthrough waves. In this work, adsorption of methanol/ethanol and ethanol/isobutanol mixtures is studied, with and without preloading the zeolite column, to obtain multiple equilibria points from one breakthrough test. The method may offer practitioners an extra tool to verify their breakthrough tests with e.g. static pure component isotherm test, or increase the experimental output from their breakthrough tests.

References

[1] D. Shade, B.W.S. Bout, D.S. Sholl, K.S. Walton, (2022), Opening the Toolbox: 18 Experimental Techniques for Measurement of Mixed Gas Adsorption, Industrial and Engineering Chemistry Research, 61(6), 2367-2391.

[2] H.K. Rhee, R. Aris, and N. R. Amundson, (1970), On the Theory of Multicomponent Chromatography, Philos. Trans. R. Soc. A Math. Phys. Eng. Sci., 267(182), 419–455.

[3] B. Claessens, G.R. Wittevrongel, F. Rey, S. Valencia, J. Cousin Saint-Remi, G.V. Baron, J.F.M. Denayer, (2020), Capturing renewable isobutanol from model vapor mixtures using an all-silica beta zeolite, Chemical Engineering Journal, 412, 128658.