Scaling machine learning-based chemical plant simulation: A method for fine-tuning a model to induce stable fixed points

Idealized first-principles models of chemical plants can be inaccurate. An alternative is to fit a Machine Learning (ML) model directly to plant sensor data. We use a structured approach: Each unit within the plant gets represented by one ML model. After fitting the models to the data, the models are connected into a flowsheet-like directed graph. We find that for smaller plants, this approach works well, but for larger plants, the complex dynamics arising from large and nested cycles in the flowsheet lead to instabilities in the cycle solver. We analyze this problem in depth and show that it is not merely a specialized concern but rather a more pervasive challenge that will likely occur whenever ML is applied to larger plants. To address this problem, we present a way to fine-tune ML models such that solving cycles with the usual methods becomes robust again.