(501c) Dynamic Simulation with Digital Twins of Heat Exchangers

Felix Rößler, Ingo Thomas, Pascal Freko, Sebastian Rehfeldt, Harald Klein

The share of renewable energies in German power supply is increasing due to the “Energiewende”. Therefore, volatility of the energy market increases. This requires major industrial energy consumers to adapt their mode of operation in terms of flexibilization as a measure of power grid stabilization. The high potential of flexibly operated air separation units (ASU) is investigated within the project “Kopernikus SynErgie FlexASU”, which is funded by the German Federal Ministry of Education and Research (BMBF).

The development of a detailed dynamic simulation model of an ASU, which is called digital twin, is a major step towards flexible plant operation. The digital twin needs to be capable of accurately simulating the whole operating range of an ASU, including start-up, load change and shutdown procedures. In order to guarantee the required simulation accuracy, the pressure-driven approach according to Thomas et al. 2020 is used for dynamic simulation [1]. Kender et al. 2019 introduced a digital twin of an ASU without argon production based on the pressure-driven approach and presented simulation results of a warm plant start-up [2].

One crucial part of an energy efficient ASU operation is thermal integration, which is realized by a network of heat exchangers. Therefore, detailed heat exchanger models are required for a digital twin of an ASU in order to ensure an accurate prediction of the dominant time scales of thermal integration. In this work the dynamic simulation of heat exchangers with the pressure-driven approach according to Thomas et al. 2020 is presented. A detailed, generic heat exchanger model is described, which consists of process stream as well as metal heat capacity modules. This digital twin of a heat exchanger enables the simulation of different types, e.g. plate-fin, coil-wound or shell and tube heat exchangers. Design correlations for pressure drop and heat transfer are included. This allows for a physically correct prediction of heat exchanger behavior and dynamic simulations close to reality. In addition, a lot of heat exchangers contain condensing or evaporating process streams. Therefore, phase change phenomena occur, which lead to discontinuous design correlations and numerical instabilities. This work addresses a stable numerical treatment of the discontinuous set of model equations. Furthermore, the impact of highly transient plant operation scenarios on different heat exchangers is presented based on dynamic simulations.

[1] Thomas, I.; Wunderlich, B.; Grohmann, S.: Pressure-driven dynamic process simulation using a new generic stream object. Chemical Engineering Science, 215, 10.1016/j.ces.2019.115171, 2020.

[2] Kender, R.; Wunderlich, B.; Thomas, I.; Peschel, A.; Rehfeldt, S.; Klein, H.: Druckgetriebene dynamische Simulation einer gesamten Luftzerlegungsanlage (Pressure-driven Simulation of a Complete Air Separation Unit). Jahrestreffen der ProcessNet-Fachgruppe Fluidverfahrenstechnik (German annual conference on fluid process engineering), oral presentation, Potsdam/Germany, 2019.