(104c) Dynamic Relief Analysis on a Distillation Column Works As a Mitigation for an Undersized Flare System

Relief devices on a distillation tower were estimated to be undersized for several overpressure scenarios, e.g., cooling water failure, total electrical power failure, based on steady state simulations. A mitigation study was conducted for these undersized cases by adding a new relief device using steady state simulation relief studies as well. However, adequate protection to the distillation tower increased the relief disposal loading to the current plant flare, leading to an insufficient flare capacity. A further detailed analysis was conducted by checking the tower pinch point, and taking credits from reduced reboiler duty and tower feeding rate reduction as well at relief pressure, but the flare system still failed from its insufficient capacity. Considering steady state simulation as a conventional method for calculating relief loads is generally conservative and can easily lead to overly sized relief and flare system design, a dynamic simulation relief study was conducted for this distillation tower system. The dynamic models simulate the time-dependent tower operation and flow properties, e.g., flowrate, compositions, pressure temperature, and etc., in the distillation tower system, as well as fluid inventory hold-up in each of equipment including tower trays. Dynamic relief analysis provided a more accurate prediction of relief rate at dynamic operating duties from overhead condenser and bottom reboilers.

Key words: dynamic simulation, distillation tower, overpressure relief, flare system