Systematic experimental and theoretical investigation of the operating behaviour of natural circulation evaporators at low operating pressures and temperature differences for multi-component systems (OptiNUV)

• Applicants: Dr.-Ing. Katharina Jasch; Professor Dr.-Ing. Jens-Uwe Repke
• Subject Area: Process Systems Engineering
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 566007043

Evaporation, rectification and desorption are among the most widespread unit operations in all areas of the process industry, whereby the energy input for rectification and desorption usually takes place via an evaporator at the bottom of the column. The operating range of the column as well as the quality of the thermal separation depend essentially on the operating range of the evaporator. Natural circulation evaporators are one of the most widely used forms of evaporator because of their simple, pumpless design and their low operating and investment costs, but they are usually operated at larger driving forces of the temperature. Due to increasing energy scarcity and resource efficiency, operation at lower driving temperature differences is a good option in order to be able to use low-temperature heat process technology. At lower pressures and temperature differences, however, instabilities become apparent. These instabilities or oscillations cause vibrations and pressure surges, which result in mechanical fatigue of the equipment or control problems in the long term. The main objectives of the project are the further development of engineering-scientific fundamentals through the development of an extended understanding of fundamental phenomena in natural circulation evaporators for mixtures at low pressures and low driving temperature differences as well as the study of robust and optimal operating concepts of natural circulation evaporators in flexible operation. Systematic experiments for pure substances and two mixtures (narrow- and wide-boiling) are intended to further close current knowledge gaps regarding the relationships between instabilities, operating and material parameters, especially at low pressures and temperature differences. The investigations will focus on relevant static and dynamic instabilities in a single-tube evaporator, such as Ledinegg instabilities, geysering or density-wave oscillations. In addition, a dynamic, pressure-driven, robust and simultaneously solved model of a natural circulation evaporator including instabilities and a phase relaxation is developed. Furthermore, it is investigated whether the initially prioritised homogeneous modelling approach is sufficient or whether a heterogeneous approach is required to describe the operating behaviour. With the help of the clarifying experiments and the validated model, an analysis of the instabilities with different boiling behaviour of the mixtures is carried out. Finally, multidimensional stability maps are drawn to visualise the operating behaviour as a function of the varied parameters with the intention of supplementing previous maps with additional information regarding mixture properties and stability behaviour in the lower operating range of natural circulation evaporators. At the end of the project, a dynamic model of a natural circulation evaporator is available, which has been validated with the help of extensive experimental data.

• DFG Programme: Research Grants