Pressure-driven dynamic simulation of start up and shutdown procedures of distillation columns in air separation units

Abstract

Changing power generation from fossil energy sources to renewables poses several challenges to the energy grid. Major energy consumers such as cryogenic air separation plants need to adapt in terms of operational agility and flexibility. Within this project, detailed dynamic models of all air separation unit (ASU) components are presented, which are an important tool to gain a deeper understanding of flexible plant operations. The main focus of this work, which is an extended version of Kender et al. (2018), lies in the dynamic modeling of the cryogenic distillation columns. Hence, a full-order stage-wise model (FOSM) is presented. All flows are pressure-driven, i.e. they result from a quasi-stationary momentum balance. Using this approach, specific scenarios for start-up or load change procedures, such as zero flow condition and reverse flow, can be simulated. To cope with the complexity of this modeling task, an innovative simulation framework is introduced. As case studies, the warm start up and a plant shutdown procedure are presented to demonstrate the ability to simulate the full operating range of an ASU. Warm start up means to initialize the model inventory at ambient conditions with zero flow. The shutdown procedure starts at steady state operation. The feed inlet valves are closed and the simulation is continued until a state of cold stand-by is reached.