On-line parameter estimation of dispersion coefficients with a Fast-Fourier-Analyzer.

The prior aim in chemical engineering is to design multiphase mass transfer reactors based on information found in small scale laboratory plants. The determination of the necessary column height is still subject to considerable uncertainty. This fact is primarily attributable to fluid dynamic effects within the contacting phases, generally named dispersion. Dispersion leads to an irreversible reduction of the concentration driving forces which is, in turn, important for the efficiency of mass transfer contactors. Several models have been developed to describe the influence of fluid dynamics on mass transfer operations. A traditional and widely used description is in terms of the one dimensional axial dispersion model. The parameters are determined by comparing experimental data of transient response experiments with model predictions and picking them so that the data and predictions match or fit in some sense. The best method to date to determine dispersion coefficients from tracer response measurements for an arbitrary input signal is a time domain analysis. An analytical solution of the axial dispersion model is fitted with discrete experimental data using a nonlinear regression procedure based on the convolution integral. A simpler and even more accurate way of the parameter estimation is possible, based on a frequency response analysis performed on-line with a Fast-Fourier-Analyzer. The residence time density function can be evaluated with arbitrary input/output signals from the measured frequency response function. (A)