A nonlinear frequency response method for estimation of single solute and competitive adsorption isotherms

The knowledge of adsorption isotherms is crucial for design and optimization of chromatographic processes. However even relatively simple isotherms characterizing single solute adsorption systems can be determined only experimentally. Although there are different experimental methods for determination of adsorption isotherms, it is not so easy to choose the right one. In some cases, competitive isotherms can be derived theoretically from the single solute isotherms of the compounds involved. Unfortunately, these theoretical isotherm models are often not able to describe the equilibrium behavior at higher concentrations, because the investigated system is then far from ideal. Frequency response (FR) techniques, which were in the last few decades widely used for investigation of linear and nonlinear systems in mechanical and electrical engineering, can be used to study different phenomena in chemical engineering as well. So far the FR method was mostly used for analysis of adsorption kinetics and characterization of different materials by applying very small excitation amplitudes in order to keep the assumption of the system linearity. However, application of available tools for the analysis of the nonlinear FR can provide valuable information about kinetic and equilibrium parameters of the systems studied. The nonlinear FR method for determination of adsorption isotherms, which is presented in this work, analyses the nonlinear FR of a chromatographic column subjected to sinusoidal inlet concentration changes using the concept of higher order FR functions (FRFs). Single solute and competitive isotherms can be estimated by this method, in which inlet and outlet concentration changes during time are analyzed in the frequency domain. Simulations of the nonlinear FR of a chromatographic column were used to address some aspects related to the practical application of the new method before its experimental realization. In order to verify this method experimentally, single solute and competitive isotherms for selected case studies were estimated applying the nonlinear FR method and classical methods, which were used as reference ones, and compared. Single solute isotherms determined with different methods were found to be in a better agreement than competitive isotherms. The thesis presents the first systematic and successful application of the nonlinear FR method. Observed discrepancies leave room for further improvement.