Impedance spectroscopy as a process analytical technology (PAT) tool for online monitoring of sucrose crystallization

With electrical impedance spectroscopy (EIS), a method is presented which is possibly suitable for the independent determination of the content of dissolved and crystalline sucrose and the concentration of salts during the crystallization of sucrose from thick juice. Therefore, a sensor using this measurement principle could be an ideal tool for process analytical technology (PAT). We present a first, promising feasibility study on the model system water-sucrose-NaCl. The impedance spectra were recorded with an immersion probe. Spectra of samples of known composition were fitted with an equivalent electrical circuit. Resistance and capacitance of the suspension as well as the constant phase element describing the double-layer at the solution-electrodes interface are related to the sample composition by semi-empirical equations. The temperature dependence is expressed by the viscosity and the relative permittivity of aqueous sucrose solutions. In principle, the found equations allow to calculate the composition from spectra of unknown samples via the fitting to the equivalent circuit, although some improvements of the measurement equipment are required. After a successful validation of the method with technical sucrose suspensions and typical industrial processing regimes, crystallization processes can be monitored with a single sensor, even if incoming and outgoing mass flows should not be known.