Dynamic models for safety shutdown of distillation columns
Abstract Pressure relief valves are commonly used to protect distillation columns from overpressure in emergency situations like loss of cooling. An alternative safeguarding method, which may eliminate the need for relief altogether, is safety shutdown of the energy inputs to the column. Keeping the holdup enclosed in the pressure system is especially advantageous in hazardous, corrosive or reacting systems. Furthermore, the time required for restarting operations may be shorter if the holdup is not relieved to a flare. Shutting down the heat input leads to a rapid collapse of the pressure profile and fast draining of low boiling liquid from the internals to the hot column bottom. During the dynamic draining process, the mixture's vapor pressure continuously changes and may increase considerably. The maximum pressure rise during the shutdown process must be considered in the mechanical design pressure of all connected equipment. During shutdown, the flow regime leaves the stable hydraulic operating range. Experimental data from an industrial valve tray column is analyzed. A standard hydraulic model is unable to predict the pressure dynamics. A literature sieve tray weeping model is adapted to valve trays and fitted to plant data. Simulation results show good dynamic agreement with experimental data.