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Binary chromatographic retention times from perturbations in flowrate and composition

Heslop, M.J. and Buffham, B.A. and Mason, G. (2008) Binary chromatographic retention times from perturbations in flowrate and composition. Adsorption, 14 (1). pp. 143-155. ISSN 0929-5607

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Abstract

This work is a theoretical and experimental investigation of the binary retention time (t step) when the disturbance is made to a chromatographic system by adding a small flow of one of the pure components. The established theory is for addition of a pulse: in this case, the retention time (t pulse) depends on the two binary isotherm gradients, and should be independent of the choice of pulse gas. From the column material balance, the value of t step also depends on the column pressure drop and perturbation gas-the value of t step should always be greater for the more-adsorbed component. The theory has been validated from results on the nitrogen-argon-5A zeolite system at 25, 54 and 81 °C. For a 50% mixture at 25 °C with a column pressure drop of 0.1 bar, the values of t step are 257 and 254 seconds for the nitrogen and argon perturbations. The values of t step are different because addition of the perturbation flow causes a very small increase in average column pressure (about 0.5 mbar), which causes the binary isotherm gradients to be measured in (slightly) different directions along the isotherm surface. The intention is to determine the value of t step for the case of a zero change in the average column pressure: experimentally, this would require a column with a zero pressure drop. The material balance shows that t step for a column with a zero pressure drop is obtained from a simple weighted function of the values of t step for the two pure-component perturbations. Accurate determination is essential because the 'zero pressure drop' values are used to determine binary adsorption isotherms which are, of course, at a fixed pressure.