In the HYSYS dynamic simulation, the conversion rate at the reactor outlet continued to decrease over time as the process ran.

After reading your description, it seems to be two common but easily overlooked issues in dynamic simulation. The conversion rate decreases over time; generally, one should first check whether there have been any changes in the feed composition, temperature, or catalyst activity during the process, or it could be that there are issues with the settings of the reaction kinetics parameters. The pressure in the circulation loop keeps rising; most likely, there is an imbalance in density or flow rate within the loop, or the performance curve of the compressor/pump does not match that of the loop. It is recommended to check the convergence strategy of the circulation loop as well as the valve opening settings. These two types of problems are often related to the time step and iteration accuracy in the model. If it’s inconvenient to attach screenshots, please provide additional details such as the type of reactor, the loop structure, and exactly which node experiences the pressure rise. This way, we can all analyze the situation together and more easily identify the root cause. Of course, these are just some suggestions based on common experience; the actual troubleshooting should take into account real data and your own processes.